Risks and Hazards of Particle Accelerator Technologies

Otto, Thomas

doi:10.1007/978-3-030-57031-6_2

Thomas Otto⁸

Part of the book series: Particle Acceleration and Detection ((PARTICLE))

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Abstract

In this section, the motivation and operation of particle accelerators are briefly introduced. Then, safety aspects of the key building blocks are treated. Magnets provide the steering forces for accelerated particles. Cryogenics provides the low temperatures required for the operation of superconducting magnets; radiofrequency technologies impart energy to accelerated particles. A byproduct of their operation is Non-ionising radiation. Another type of NIR is represented by lasers which find increasing use in accelerator applications. Finally, collimators shape the particle beams and protect sensitive elements, while dumps absorb the particles at the end of their course.

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2.1 Accelerators for Pedestrians

2.1.1 Why Particle Accelerators?

Fundamental research provided the first motivation to accelerate subatomic particles to ever higher energies. One approach to understand the need for high energies is the analogy to an optical microscope: to analyse the properties of subatomic particles, with dimensions less than a femtometre (1 fm = 10⁻¹⁵ m), probes with a wavelength of the same order of magnitude were required. The de Broglie wavelength λ_B of a relativistic particle with momentum p is:

$$ {\lambda}_B=\frac{h}{p}=\frac{hc}{\sqrt{E^2-{\left(m{c}^2\right)}^2}}\cong \frac{hc}{E} $$

(2.1)

The approximation is valid in the ultrarelativistic case where the total energy E is at least three times the rest-mass energy mc² of the particle. Inserting numerical values in this equation yields an energy of 1.24 GeV, which is ultrarelativistic for electrons, to achieve a De Broglie wavelength of 1 fm. For probing smaller structures, even higher energies would be required.

A second picture to grasp the need for high energies is the creation of new particles. Einstein’s relation of the equivalence of matter and energy is:

$$ E={m}_0{c}^2 $$

To generate a particle with rest mass m₀ in a relativistic collision, the total centre-of-mass energy of the colliding particles must be at least of the value indicated by Einstein’s relation. In CERN’s electron-positron collider LEP, operating in the 1990s, the particles had an individual energy of 45 GeV [2]. In a head-on collision this added to 90 GeV, enough to produce the Z-Boson, one of the mediators of the electro-weak interaction. Much higher particle energy is required when particles collide with a fixed target, for this reason the high-energy frontier of particle physics is explored by colliders.

Ever higher energies than achievable by LEP were necessary to test theoretical models of particle physics, a process which culminated so far with the construction and operation of CERN’s LHC at a centre-of-mass energy of 13–14 TeV [3]. It permitted the discovery of the Higgs boson, the last missing element in the Standard Model of particle physics. Its present objective is to find evidence of physical processes not described by the Standard Model, and to lay the experimental fundament to a new, more complete theory of particle physics.

2.1.2 The Particle Accelerator Family

The simplest accelerator was found in cathode ray tubes (in wide use for monitors in television sets and oscilloscopes until the breakthrough of flat screens). Electrons were emitted by a cathode (a heated piece of metal), accelerated by a static voltage gradient between cathode and anode, and deflected by transversal electric or magnetic fields. The energy of an electron with charge e traversing an electrical voltage difference U is E = eU, it is expressed in the unit eV (1 eV = 1.602 10⁻¹⁹ J). The accelerated electrons hit a phosphorescent screen, thus forming a visible image. The limitation of a static voltage accelerator is the voltage breakdown between anode and cathode.

2.1.2.1 Linear Accelerator

Linear accelerators (“linac”) overcome the limitation of the breakdown voltage by accelerating particles passing through a series of aligned accelerating structures. This allows adding up the energy gain. Linear accelerators are relatively simple and are the workhorse in the medical field, where they accelerate electrons to energies between 6 and 25 MeV for the generation of bremsstrahlung X-rays for diagnostics and therapy. An estimate places the number of medical linacs at 14000 worldwide, counting for approximately 30% of all accelerators.

Modern high-power accelerators are realised as linear accelerators. A recent, operating example is the Spallation Neutron Source SNS at Oak Ridge (USA). A liquid mercury (Hg) target is bombarded by a proton beam with an energy E = 1 GeV and a beam power P = 1.4 MW. Mercury atoms are shattered upon impact of a proton and 20–30 neutrons are released per collision. They are moderated and guided to experimental stations with spectrometric instruments. More power translates into more neutron flux, and the SNS plans an upgrade to E = 1.3 GeV and P = 2.8 MW [8]. The European Spallation Source ESS in Lund (SE), presently under construction, will use a proton driver with E = 2 GeV and P = 5 MW [5].

Similar instantaneous beam intensities as for spallation neutron sources are envisaged for beam-dump facilities. In these, the accelerated particle beam is projected onto a massive beam dump/ target. Detectors are placed downstream from the beam dump /target, where it is hoped that exotic, hitherto unobserved particles can be identified.

At the high-energy end, linear accelerators become very long. The International Linear Collider Study (ILC) projects a linear electron/positron collider which could be built in several stages, with a final collision energy of 500 GeV. At this energy, the two Linacs built in opposing direction would have a length of 31 km [7].

2.1.2.2 Cyclotron

In a linac, the accelerated particles pass each accelerating gap only once. In a circular accelerator, of which the simplest example is the cyclotron (Fig. 2.1), the accelerating gap is passed repetitively. Between the poles of a large electromagnet, two D-shaped, hollow electrodes are placed. Between the “Dees”, as these electrodes are called, a high-frequency alternating voltage is applied. In the centre of the slit separating the “Dees” an ion source is placed. Ions emitted by the source will be accelerated by passing the field gap between the “Dees”, while their path is bent by the magnetic dipole field. At every passage they gain kinetic energy, leading to the next orbit with a higher radius because magnetic flux density B, particle momentum p and orbit radius ρ are connected by the relation

$$ B\rho =\frac{p}{q}=\frac{pc}{qc}\approx \frac{E}{qc.} $$

Once they have passed the gap between the Dees, the particles are shielded by the Dee-walls from the electrical field which passes through the reverse polarity. At the outer radius of the “Dees”, the particles are extracted at their maximum energy by a septum magnet.

There are two limitations to the cyclotron principle:

At high energies, relativistic effects become important and the revolution frequency in the magnetic field is no longer matching the frequency of the electrical accelerating field. This can be overcome by introducing variable accelerating frequencies in so-called synchrocyclotrons.
The maximal radius of the particles’ orbit and thus the maximal energy is determined by the size of the magnetic poles. The presently largest cyclotron is located at TRIUMF in Vancouver (CA), its magnet has a diameter of 18 m and a magnetic flux density of 0.46 T. The mechanical problems of a large magnet size can be overcome by splitting it in several units. The Swiss Paul Scherrer Institute in Villigen (CH) operates its ring cyclotron with eight sectors and obtains a beam power of P = 1.2 MW at an energy of E = 590 MeV [6].

Smaller Cyclotrons are used in medical centres to produce radioisotopes from which radiopharmaceuticals are synthesised for diagnostics (SPECT and PET) and for targeted tumour therapy. In industry, cyclotrons are used to implant ions into materials to modify their physical properties, for example in highly integrated microelectronic circuits.

2.1.2.3 Synchrotron

To reach even higher particle energies, one resorts to repetitive acceleration in a synchrotron, another type of circular accelerator. Here, the average particle orbit is closed, the path of the particle oscillates around it. The magnetic bending field is produced by dipole magnets arranged along the particle orbit; their field extends only over the comparatively small volume of the beam line. The magnetic flux density is increased synchronously with the particle’s energy gain so that the average radius of the orbit remains constant.

Accelerators with the synchrotron principle are built from circumferences of a few 10 metres to 27 km (CERN’s LHC, [3]), and synchrotrons with a circumference of nearly 100 km are envisaged (CERN’s Future Circular Collider, [4]).

Synchrotrons are used predominantly in research applications, where particle beams are accelerated and collided either with fixed targets or with each other (see below, “Collider”), or produce synchrotron radiation (see below, “Storage Ring”). A few medical treatment facilities world-wide employ synchrotrons for accelerating protons or charged heavy ions for hadron therapy. For a few cancer types and sites, this modality of radio oncology is advantageous over conventional radiotherapy and requires accelerators of a size comparable to those of research centres.

2.1.2.4 Storage Ring

The first synchrotrons were built to accelerate particles to high energies and to make them collide against external or internal targets as soon as they reached the terminal energy.

There are two reasons to keep particles for extended times circulating in a synchrotron, which is then called a storage ring:

The generation of synchrotron radiation
The collision of particles with each other (see below, “Collider”)

Synchrotron radiation is a by-product of the acceleration and change of direction of electrons. The electromagnetic radiation emitted upon momentum change is in the near-UV range of wavelengths and is used for material and biological research. While one tries to minimise the emission of synchrotron radiation in high-energy accelerators, because of the induced loss of energy, one maximises it in special circular accelerators, the synchrotron light sources (Fig. 2.2). They prove invaluable in material research, from semiconductors to biological samples.

Two features of synchrotron light sources are:

the emission of synchrotron radiation is enhanced by making the particles follow undulatory paths in straight sections of the ring (so-called Undulators and Wigglers).
the energy emitted in form of synchrotron light must be continuously restored by operating the accelerating RF cavities.

In synchrotron light sources, the electrons may be stored in the synchrotron which accelerated them in the first place. This leads to an operation scheme where the ring is filled with a low-energy beam provided by a linac or a smaller synchrotron, the electrons are accelerated to the terminal energy, and then synchrotron light is produced and emitted. The light intensity will decrease over time because of beam loss. A more flexible arrangement is to separate the synchrotron for acceleration and the storage ring. As soon as the intensity of the emitted synchrotron light become too weak one can “top up” the filling of the storage ring with a small injection form the accelerator synchrotron. This mode is called “top-up operation”. The split between accelerator and storage ring has other advantages. For example, the magnets in the storage ring need to operate only in a very small range around the maximal energy and thus can have a better field quality.

2.1.2.5 Free-Electron Laser

Free-Electron Lasers (FEL) are a specific type of synchrotron light source, in which the undulating structure of the beam path emits coherent photons (i.e. Laser, Sect. 2.5) in the low-energy X-ray range. This is the only known source of X-ray lasers and opens a new research field exploiting the spatial and temporal coherence of the radiation. FEL are built either as a linear accelerator or in “racetrack” configuration, which one can imagine as a coiled-up linac in which the same accelerating cavity can be used during several passages.

2.1.2.6 Collider

A collider is the combination of two storage rings^{Footnote 1} in which the beams circulate in opposite directions and are brought to collide at defined locations, in the centre of large particle physics detectors. A collider combines the characteristics of an accelerating synchrotron and a storage ring. It consists of circular arcs which are connected by long straight sections in which the collision points are inserted. Special focussing magnets provide for the charged particle optics to bring the beams to collision in the smallest possible volume, to enhance the probability of a collision, and to bring them back to a stable orbit after the collision point.

2.1.3 Particle Acceleration from Source to Target

After this overview of different particle accelerator types, I describe the path of a particle from its source to the collision with another particle. This serves to introduce different types of hardware which are subject of the following sections.

Charged particles are generated in a particle source, for a proton accelerator, hydrogen atoms coming from an ordinary gas bottle are ionised in a plasma ion source. Electrons are emitted by a cathode, either by heating it, or by shining laser light (Sect. 2.5.1) onto it. The second method allows to impregnate a time structure on the flux of emitted electrons.

In their further parcourse, the charged particles are manipulated by electromagnetic fields, one can distinguish between

accelerating fields, exploiting the potential difference between two electrodes or in a time-varying radiofrequency field (Sect. 2.4) to exert an accelerating force on the charged particle, and
guiding fields generated by electromagnets (Sect. 2.2), changing the direction of the particle’s path.

In a simplifying manner one can say, that in a linear accelerator the technology defining the accelerator performance is radiofrequency (RF) technology, while in a circular accelerator it is magnet technology.

In a linear accelerator, the particle passes a series of RF cavities in which it draws kinetic energy from the electromagnetic field. The most efficient RF cavities are superconducting, requiring cryogenic technology (Sect. 2.3) to reach their operating temperature. Quadrupole magnets between the cavities keep the beam well focused. One the particle has reached the terminal energy of the accelerator, it is used to produce X-rays (in a medical linac), neutrons (in a neutron spallation source) or coherent X-rays (in a free electron Laser).

In a circular accelerator, the beam passes the same RF cavity repeatedly. It either spirals outward with increasing energy (Cyclotron) or it is kept on a unique orbit by magnets with increasing flux density (Synchrotron). To achieve the highest particle energies, strong magnetic fields are provided by superconducting magnets (Sect. 2.2.2) cooled to operating temperature by cryogenic technology (Sect. 2.3).

Having reached the highest possible energy of a synchrotron, the beam is sent on a target (Sect. 2.6.2), for production and subsequent investigation of other, exotic particles, for example antiprotons or unstable nuclei.

Alternatively, in a storage ring or a collider, the beam is left circulating at the highest energy. Here, dissipative processes make the beam size larger. These processes include scattering on rest-gas atoms in the evacuated beam line and electromagnetic and strong interaction during near-collisions with the opposing beam particles, To prevent excessive beam loss in sensitive equipment, collimators (Sect. 2.6.1) remove particles which have moved too far away from the ideal orbit. Once the beam intensity has become too low to produce a satisfying rate of collisions or intensity of synchrotron light, the beam is deflected by switches into large absorbers, so-called beam dumps (Sect. 2.6.3), where the energy is absorbed. If large deviations of the particle’s path from the nominal orbit are observed, an emergency beam dump is triggered automatically to prevent damage to accelerator elements by a massive impact of the particle beam.

2.2 Magnets

Magnetic fields guide and focus particles on their orbit in circular and linear particle accelerators (Fig. 2.3).

One can distinguish between dipole magnets, generating the field necessary for bending the beam, quadrupole magnets for focussing it, and higher-order multipole magnets for correcting either deviations from the ideal field shape by the simpler magnets, or non-linear ion-optical effects induced by the particle beam itself.

2.2.1 Normal Conducting Magnets

A normal conducting magnet is an electromagnet powered by a resistive current (running in “normal” conductors), operating at room temperature. It consists of coils to produce the magnetic induction, which are wound around an iron yoke with the purpose of shaping the magnetic flux lines to the desired multipole shape. As a rule of thumb, normal conducting magnets can be used for flux densities of up to B = 2 T, above this value, only superconducting magnets are economically viable.

Dipole magnets create a homogeneous magnetic field in the gap of their yoke, oriented at a right angle to the direction of the particle beam. The Lorentz Force deflects the beam in the direction orthogonal to both its original velocity and the magnetic field lines. A popular realisation of a dipole magnet is the C-type magnet (Fig. 2.4), because it leaves easy access to the vacuum chamber enclosing the particle beam.

The magnetic flux density in the gap of a C-type dipole magnet is given by

$$ N\cdot I=\eta \frac{B}{\mu_0}{l}_{\mathrm{air}} $$

Here, N is the number of turns of the electrical conductor, I is the electrical current, l_air is the gap-width between the magnet poles and η is an efficiency coefficient and usually close to one. The required magnetic flux density can be achieved by varying the number of turns or the electrical current.

The energy stored in the magnet is

$$ {W}_m=\frac{1}{2}L{I}^2 $$

with the inductivity given by L = ημ₀N²A/g . The shape factor A is related to the geometry of the magnet, in a dipole it corresponds approximately to the product of length and width of the magnet pole. Further information can be found in [9, 16].

As an example, the parameters of a dipole magnet in the Super Proton Synchrotron SPS are given in Table 2.1 and its layout is illustrated in Fig. 2.5. This magnet is a Window-frame magnet, for which the relation between current-turns, gap-width and magnetic flux density is, in first order, identical to a C-type magnet.

Table 2.1 Parameters of SPS dipole magnet type B2

Full size table

2.2.2 Superconducting Magnets

Normal conducting magnets with iron yokes are limited to a magnetic flux density of approximately 2 T because of saturation effects in the iron. Stronger magnetic fields can be produced in superconducting magnets. Superconductivity is a low-temperature phenomenon in many metals, where electrical resistance R vanishes once the material has a temperature T < T_c, the critical temperature.

Superconductivity was discovered by H. Kammerlingh Onnes in 1911 in mercury below the critical temperature of T_c = 4.2 K, after having liquefied helium three years earlier. It was soon discovered that superconductivity is also a function of magnetic flux density B and current density J, beyond a critical value B_c or J_c the material becomes normal conducting. The boundary between super- and normal conducting state can be described by the relation of the critical current J_c as a function of temperature T and magnetic flux density B and visualized as a hyperplane in the 3-dimensional phase diagram with coordinates T, B and J (Fig. 2.6).

The presently most used superconducting alloy for magnets is Nb-Ti, with a critical temperature of 9.2 K at zero field and zero current. To achieve a magnetic flux density of 8 T with sufficient operational margin for accelerator operation it is necessary to cool the magnet to a temperature of approximately 1.9 K.

Table 2.2 and Fig. 2.7 illustrate the LHC dipole magnet, the most powerful accelerator magnet currently in operation. Higher flux densities of up to 16 T can be reached by using the alloy Nb₃Sn, with a critical temperature of 18.2 K at zero field and current density.

Table 2.2 Parameters of LHC dipole magnet

Full size table

2.2.3 Safety Aspects of Magnets

The nature of hazards from normal and superconducting magnets are similar: exposure to strong magnetic fields, exertion of magnetic forces on metallic objects and electrical hazards. The hazards emerging from the use of cryogenics will be treated in Sect. 2.3.

Personnel is usually not exposed to these hazards during operation, because in these phases, access to accelerator buildings or tunnels is forbidden or restricted. However, the life cycle of accelerator magnets includes test phases, for example after fabrication, for diagnostic purposes or after maintenance and repair. These tests are conducted in accessible locations: in laboratories or workshops, or in the accelerator area outside of operation. Personnel may stay close to the magnet to perform measurements, or they follow other occupations in the vicinity of the tested magnet.

2.2.3.1 Magnetic Field Hazard

During magnetic testing, personnel may be exposed to the magnetic fringe field penetrating out of the magnet yoke. This is unavoidable, for example, when making electrical or magnetic measurements for which the magnet must be powered. For accelerator dipole magnets, a rule of thumb is that the dipole field falls over two gap-widths by a factor of 5 [15]. In the example of the SPS dipole magnet (Table 2.1), the fringe field in 10 cm from the pole face would amount to 400 mT, the magnetic field gradient in these first 10 cm would be 16 T/m. There is presently no evidence of adverse health effects from an occasional exposure to magnetic fields of this magnitude.

2.2.3.2 Health Effects of Magnetic Fields

The International Commission on Non-Ionising Radiation Protection (ICNIRP) [12] has the aim to protect workers and members of the public from harmful effects of static and dynamic electromagnetic fields and of non-ionising radiation (electromagnetic radiation with wavelengths greater than 100 nm [12]).

In 2009, ICNIRP published exposure limits to static magnetic fields [13]. They investigated three interaction mechanisms between magnetic fields and living tissues: magnetic induction, magneto-mechanical and electrical interactions. A thorough analysis of the available research literature showed:

No pronounced physiological effects have been found from exposure to fields of up to 8 T, except a small increase in systolic blood pressure.
No evidence of health effects of exposures of up to 8 T on other aspects of cardiovascular function, on body temperature, memory, speech, or auditory-motor reaction time and of any other serious health effects in human volunteers.
Magnetic fields of 2–3 T can cause transitory sensory effects including nausea, vertigo, metallic taste and phosphenes (light sensations induced in the retina and the optical nerve) when moving the head.
The few available epidemiological studies of workers in aluminium smelters, chloralkaline plants or as welders do not indicate strong effects of exposure of up to several tens of mT on cancer incidence and reproductive health.

Based on this evidence, ICNIRP recommends that occupational exposure of the head and trunk shall not exceed a spatial peak magnetic flux density of 2 T. Exceptionally, for specific applications and in controlled situations, exposure of up to 8 T can be permitted.

Members of the public shall not be exposed to more than 400 mT on any part of the body.

The electrical circuits of implanted medical devices (pacemakers, cardiac defibrillators, insulin pumps etc.) can be perturbed by external magnetic fields, leading potentially to health risks for the implanted persons. Studies have shown that magnetic flux densities of less than 0.5 mT have no adverse effects on implanted medical devices and on implants from ferromagnetic metals. This leads to the practical requirement to put warning signs at the limit of areas where the magnetic flux density may exceed 0.5 mT.

2.2.3.3 Magnetic Forces

While there is no evidence for biological effects of fringe fields from accelerator magnets, a more imminent danger results from the mechanical forces exerted by the magnetic field on metallic objects.

An external magnetic field exerts a force on the magnetic moment of a metallic object. The magnetic moment can be induced, as in diamagnetic and paramagnetic materials or permanent, as in ferromagnetic materials. The induced magnetic moment of a metallic object with volume V is related to the external magnetic flux density by

$$ \overrightarrow{m}=\frac{\chi }{\mu_0}{\overrightarrow{B}}_eV. $$

χ is the magnetic susceptibility, for ferromagnetic materials it can take on considerable values, which expresses the amplification of the external field within the ferromagnet. The force on the magnetic moment in an external field B_e is:

$$ \overrightarrow{F}=\nabla \left(\overrightarrow{m}\cdot {\overrightarrow{B}}_e\right) $$

As an example, consider the force on a stainless-steel spanner (χ = 100) in the inhomogeneous field close to the pole face of a dipole magnet. In an external field of B = 1 T, the induced magnetic moment in the spanner (V = 200 · 10 · 3 mm³ = 6 · 10⁻⁶m³) is 477 A m². The force on the spanner’s magnetic moment in an inhomogeneous magnetic field with gradient of 1 T/m is of the order of 500 N. The likely result is that the tool would be wrought out of the hands of the worker and stick to the pole face of the magnet. This calculation is approximate, neither the susceptibility χ is precise, it depends on material composition and thermal history, nor is the inhomogeneous magnetic field. It only demonstrates the order of magnitude of the expected forces. At a larger distance, the magnetic force on a metallic object would be smaller, but often large enough to mobilise it and convert it into a projectile flying in the direction of the magnet poles.

The examples and conclusions above are valid for workers on powered accelerator magnets. The situation is different for workers in the vicinity or inside the large, superconducting solenoid magnets employed in particle detectors at high-energy accelerators. There, magnetic flux densities may attain up to 4 T, and access should only be permitted to trained volunteers, following specific procedures to minimize the sensory effects described in the literature.

As a summary of the section on magnetic field hazards one can observe

Personnel is exposed to the fields from accelerator magnets only in exceptional situations, for example in test laboratories. The fringe field of dipole and higher order multipole magnets decays rapidly outside of the yoke and coils.
In this configuration, physiological effects can be excluded. They would be expected at magnetic flux densities exceeding 2 T. Long-term health effects have not been observed in groups which are regularly exposed to fields of up to 100 mT, like welders.
A limit of 0.5 mT is considered safe for persons with implanted medical electronic devices.
Finally, the mechanical forces on ferromagnetic, metallic objects can be considerable and can project them in the direction of the magnet. To avoid injury, such objects must be banned from magnetic test stands.

2.2.3.4 Electrical Hazards of Magnets

Accelerator magnets are electrically powered. To generate magnetic fields in accelerators one needs generally high current (I > 1 A) and in some cases high voltage (U > 1 kV). The potential danger from a powered accelerator magnet shall be illustrated at the example of the SPS magnets. Figure 2.8 shows an equivalent electrical scheme of an electromagnet, composed of an ideal inductivity L, a resistance R (the ohmic resistance of the coil and cables), and a stray capacity to ground C.

The following relations apply:

magnet voltage:

$$ U= RI+L\frac{dI}{dt}; $$

instantaneous power:

$$ P= UI=R{I}^2+ LI\frac{dI}{dt}; $$

stored energy:

$$ E=\frac{1}{2}L{I}^2\cdot \frac{dE}{dt}= LI\frac{dI}{dt}; $$

therefore power:

$$ P= UI=R{I}^2+\frac{dE}{dt}. $$

The dipole magnet of the CERN Super-Proton Synchrotron (SPS) (Table 2.2), powered by a continuous current of value I_peak for test purposes, stores an electromagnetic energy of W = 163 kJ. If one creates accidentally a short circuit between the terminals of this magnet, the circuit will drain the electrical power P = UI over the resistance of the short circuit R_sc with a characteristic time constant of $ \tau =\raisebox{1ex}{$L$}\!\left/ \!\raisebox{-1ex}{${R}_{sc}$}\right., $ which is in the order of a second. If this energy were dissipated entirely in the piece of metal causing the short, it would be enough to melt approximately 100 g of iron, giving an impression of the injury and material damage which can be created by an accidental electrical short circuit.

In the SPS, 784 dipole magnets of two types with slightly different parameters are connected in series. Resistance and inductance of the magnets add up to values of R_tot = 3.25 Ω and L_tot = 6.6 H. The stored energy of the full SPS dipole circuit amounts to 109 MJ. At peak current, the resistive voltage across R is 18 kV, to which the induced voltage over L_tot must be added. The SPS magnets are ramped at 1.9 kA/s, adding the induced voltage $ {V}_L=L\ \raisebox{1ex}{$ dI$}\!\left/ \!\raisebox{-1ex}{$ dt$}\right.=12.5\ \mathrm{kV}. $ Therefore, the insulation of the magnets must withstand 30 kV.

If the powering circuit of the magnet is suddenly opened, the induced voltage

$$ {U}_i=-L\frac{dI}{dt} $$

appears across the interrupter gap. The time derivative of the current is initially very high, and the high inductive voltage will result in an electrical arc across the gap. This electrical arc carries high electrical power, and its primary hazard is heat radiation and projection of fused metal, not electrical shock. More on arc flashes is found in Sect. 4.1.1.

2.3 Cryogenics

With cryogenics one designates the realms of science and technology operating at temperatures lower than 120 K, where gases such as nitrogen, oxygen or argon begin to liquify [27]. Hazards of cryogenics are related to very low temperatures and to oxygen deficiency (Fig. 2.9).

Cryogenic technology is essential for the function of a modern high-energy particle accelerator using superconducting magnets and radiofrequency cavities. Superconducting magnets are also used in Nuclear Magnet Resonance (NMR) imaging in medical diagnostics, and cryogenics has become a key technology with a certain economic impact.

The purpose of accelerator cryogenics is to cool magnets and RF cavities to temperatures well below the critical temperature for superconductivity T_c (see 2.2.2) to ascertain their functioning with enough operational margin. The cool-down is achieved in a heat-exchange process with helium as a cryogenic fluid, the only element apart from the explosive hydrogen which does not become solid other than under extreme pressures.

Due to the size of a modern high-energy accelerator, the cryogenic installations have industrial dimensions. The cryogenic plants at CERN are world-wide the largest of their kind.

2.3.1 Production of Low Temperatures

The physical principle of cooling an object is to transfer its thermal energy to a reservoir at a higher temperature by a heat exchange mechanism using mechanical work. The cryogenic fluid which is produced in a refrigeration plant can be transported to the object to be cooled where it absorbs part of the object’s thermal energy. In this process, the fluid may undergo a phase change from liquid to gaseous. Finally, the “warm” fluid is transported back to the refrigeration plant from where on the cycle repeats. This is the working principle of the refrigerator or the heat-pump.

The refrigeration plant makes use of a thermodynamic process to extract heat from the cryogenic fluid (Fig. 2.10). Heat is extracted from a low-temperature reservoir (T_c) and mechanical energy W is used to bring it to a higher temperature T_w where it is rejected to the environment. This is the principle of the heat pump, but in cryogenics, the interest is not the heat flow at the high temperature $ {\dot{Q}}_w $ but the heat flow extracted from the cold medium $ \dot{Q_c}, $ in order to lower its temperature further. The cyclic thermodynamical process with the highest efficiency is named after the French physicist Carnot [18]. Accordingly, any thermodynamical process to extract heat energy from a cold reservoir will need at least a mechanical energy W

$$ W>{\dot{Q}}_c\left(\frac{T_w}{T_c}-1\right) $$

The expression in parenthesis is called the Carnot factor ”.

It indicates that even in an ideal Carnot cycle one must inject $ \left(\frac{300}{4}-1\right)=74 $ W of process energy at room temperature (300 K) to remove 1 W of thermal energy at the cryogenic temperature of 4 K.

The Carnot cycle can only be approximated, and numerous technical cycles have been developed for application in real refrigeration plants, see for example [26]. All thermodynamic cycles used for cryogenics have in common that the temperature of the gas is reduced by first charging it isothermally with mechanical energy in a compression step and then letting the gas expand, for example in a nozzle (Joule-Thompson process) or more efficiently, by letting it perform work against the resistance of a turbine.

Due to the unfavourably low thermodynamic efficiency, the refrigeration plants for a particle accelerator have industrial dimensions. CERN is presently operating the world’s largest helium refrigeration plant, distributed over 8 sites, to cool the approximately 20 km of superconducting magnets in the LHC to a temperature of 1.9 K. These plants have a high engineering complexity and they are usually conceived and constructed in collaboration with specialised engineering firms.

Safety aspects concerning the construction of the compressors and refrigerators are taken account of by their manufacturer who constructs them following industrial standards. In the European Union, the mandatory application of the Machinery Directive (see 4.2.2) guarantees minimal health and safety standards across the member states and for imported products.

The noise level inside of a cryogenic service building can be overpowering and easily exceed the legal limits (Sect. 4.5). The manufacturers of compressors and turbines take all possible steps to reduce the emissions form their devices. Further attenuation of compressor noise is often impossible because additional insulating material would impede the heat exchange of the plant with its environment and lead to overheating. Cryogenic compressors and refrigerators are installed in specific buildings with isolated walls to reduce noise emission to the environment. The control rooms for these plants are preferably installed in a separate building, or at least isolated so well that the sound pressure level does not exceed a comfortable level.

2.3.2 Cryogenic Fluids

The most common cryogenic fluid used in particle accelerators is Helium. It has a low enough boiling temperature to remain liquid in the temperature range where most low-temperature superconductors operate. Another cryogen of technical importance is Nitrogen, whereas liquid Argon is used in some particle detectors. In the past, Hydrogen was used in bubble chambers, particle detectors which have been replaced by modern alternatives. Table 2.3 lists some physical properties of these fluids.

Table 2.3 Properties of cryogenic fluids used in accelerators and particle detectors. M molar mass, T_b boiling temperature at 1013 hPa, ρ_l liquid density at T_b, ρ_g gaseous density at T_b, ρ_STP gaseous density at standard pressure and temperature (1013 hPa and 0° C), ΔH_V enthalpy of evaporation. After [20]

Full size table

Cryogenic fluids have in common

a very low temperature in the liquid state, and
a high ratio of gaseous volume at ambient temperature to liquid volume ρ_l/ρ_STP = (650–800).

Because of its low enthalpy of evaporation, Helium must be thermally isolated from all heat transfer by conduction, convection, or radiation to remain in the liquid state. This can be achieved with super-insulated cryostats, combining vacuum insulation to interrupt convection, metallised mylar foils as radiation barrier and low-conduction mechanical supports from organic materials. If the insulation of a cryostat fails, two effects are the consequence:

the heat flow in the cryogen will lead to its partial evaporation, increasing rapidly the internal pressure in the cryostat (overpressure),
once the cryogenic fluid is released from the cryostat, persons may be exposed to cold and to oxygen deficiency.

2.3.2.1 Overpressure

In case the thermal insulation of a cryostat fails, for example by a degradation of the insulation vacuum, thermal energy is transported from the surroundings to the cryogenic vessel by a combination of conduction and convection. A part of the cryogenic fluid evaporates, and by the relation for an ideal gas pV = const., the pressure in the fixed-volume vessel increases. From Table 2.3, the conversion of liquid helium to gaseous helium at the boiling temperature leads to a pressure increase by a factor of 7.4. Further heating of the gas leads to higher pressure. Obviously, cryostats constitute pressure vessels, containers built and tested to withstand pressures higher than atmospheric pressure. The topic of pressure vessels is treated in more detail in Sect. 4.2. To avoid damage to the cryostat from the increasing internal pressure, they are equipped with so-called safety devices, in form of pressure relief valves. An international standard [22] describes the models and calculations to determine the correct dimension of the safety valves on cryogenic pressure vessels. For cryogenic liquids, these calculations are far from trivial because often the cryogenic fluid undergoes a phase change from liquid to gas during release, or a mixture of liquid and gaseous phases of the fluid is released.

2.3.2.2 Cold Burns

The exposure of skin to cold surfaces or cold fluids may lead to “cold burns”, named after the similarities of their effects on tissue to those by heat or flame. When exposed to temperatures well below 0 °C for longer than the temperature regulation mechanism of the body can cope, the skin develops in a first phase reddening and blisters, which may be painful but reversible (1^st degree). The cold may penetrate layers below the skin and lead to tissue damage by the formation of ice crystals, leading to irreversible effects, like tissue necrosis. These effects are well-known to high-altitude mountaineers and arctic explorers but may as well occur in the workplace when exposed to cryogenic liquids.

To prevent cold burns, one must prevent the exposure to extremely cold surfaces or fluids. Protection measures are:

Orientation of cold gas relief devices (safety valves) away from passageways.
Insulation of cryogenic vessels and pipes, so that no cold spots appear on the external surface.
Wearing correct personal protective equipment when working on cold cryogenic systems. This consists of long-sleeved trousers and jackets, temperature-insulating gloves which remain flexible at low temperatures and a face shield or at least safety goggles.
Never walk across the condensation cloud around a release of cryogenic fluids. The lack of visibility makes it impossible to avoid the jet of cold liquid and severe injury might result.

2.3.3 Oxygen Deficiency Hazard from Cryogenic Fluids

Everybody can feel the effects of a reduced oxygen concentration in air during a visit in the high mountains. After leaving a cable car at an altitude of nearly 3900 m above sea level^{Footnote 2} one may feel dizziness and headache. During longer stays at high altitude, for example when climbing a summit or staying overnight in a mountain refuge, tiredness and vomiting may add to the symptoms of what is called altitude sickness. This is the mildest form of the consequences of hypoxia or oxygen deficiency, the term used in the occupational safety context. It is caused by the lower partial pressure of oxygen, which is 21% at sea level, but only 13% at 3900 m altitude. The physiological effects of reduced oxygen concentration are listed in Table 2.4

Table 2.4 Physiological effect of reduced oxygen concentration [21]

Full size table

Cryogenic fluids experience a volume expansion by a factor of 700 to 800 upon evaporation, and they can displace or dilute breathable air, creating an atmosphere depleted of oxygen. Therefore, a risk assessment must evaluate the likelihood and consequences of oxygen deficiency at workplaces where cryogenic liquids are used.

2.3.3.1 Oxygen Deficiency from Nitrogen Gas

Liquid Nitrogen (LN₂) is employed in cryogenic shields, or to cool high-temperature superconductors. Once gaseous, it mixes perfectly with air of which it is the major constituent.

A first, coarse estimate of oxygen deficiency hazard in a closed room with volume V_o is obtained by assuming that the evaporated volume of the released LN₂, V_R,g, displaces an equivalent volume of air from the room and that the remaining air and cryogen mix perfectly (Fig. 2.11). In this case, the oxygen concentration of the mixture is:

$$ C\left({\mathrm{O}}_2\right)=0.21\cdot \left(1-\frac{V_{R,g}}{V_o}\right) $$

If the oxygen concentration shall not drop below 18%, the lowest concentration which is universally regarded as safe for workers, then the volume of the room must be more than 7 times the volume of the evaporated gas:

$$ {V}_0>7\ {V}_{R,g}\gtrsim 5000\ {V}_{R,l} $$

This relation makes use of the typical volume ratio between the cryogenics gaseous state gas at 300 K and its liquid volume. This can be expressed as a rule of thumb: “No oxygen deficiency hazard in a room with volume in m³ exceeding the cryogenic liquid volume in L by at least a factor of five”.

A more realistic estimate can be obtained from the well-mixed room approximation. Figure 2.12 shows the schematic of a room into which a substance S is released with a volume flow R (m³ s⁻¹) and with a relative concentration C_s,r (Volume-% or m³ m⁻³). Fresh air is supplied with a ventilation flow Q (m³ s⁻¹). It is assumed that pressure in the room remains constant. Therefore, no accumulation of air or gas can happen, and the outflow of the room is (Q + R).

The time dependent concentration of substance S in the room is denoted C_s(t). The rate equation for C_s(t) is:

$$ V\frac{d{C}_s(t)}{dt}=R{C}_{s,r}-\left(Q+R\right){C}_s(t) $$

If the release of substance S stops after time T, then the solution for the time-dependent concentration of s, C_s(t) is during the release:

$$ t\le T:{C}_s(t)=\frac{R{C}_r}{\left(Q+R\right)}\left[1-\exp \left(-\frac{Q+R}{V}t\right)\right]. $$

After the release, the concentration of S develops as:

$$ {\displaystyle \begin{array}{l}t>T:{C}_s(t)={C}_s(T)\exp \left[-\frac{Q}{V}\left(t-T\right)\right]\\ {}\kern4.5em =\frac{R{C}_r}{\left(Q+R\right)}\left[1-\exp \left(-\frac{Q+R}{V}T\right)\right]\exp \left[-\frac{Q}{V}\left(t-T\right)\right].\end{array}} $$

During a release of LN₂, the concentration of the released gas (the evaporated liquid) is 100%. One is interested in the remaining concentration of oxygen in air, which is $ {C}_{0_2}(t)=0.21\left(1-{C}_s(t)\right) $. It follows for times during the release:

$$ {C}_{O2}(t)=0.21\left\{1-\frac{R}{\left(Q+R\right)}\left[1-\exp \left(-\frac{Q+R}{V}t\right)\right]\ \right\} $$

$$ =\left(\frac{0.21}{Q+R}\right)\left\{Q+R\ \exp \left(-\frac{Q+R}{V}t\right)\right\} $$

For long times, the equilibrium concentration C_O2,eq is attained:

$$ {C}_{O_{2,\mathrm{eq}}}=0.21\left(\frac{Q}{Q+R}\right) $$

If a minimum concentration of 18% O₂ shall be preserved, one can evaluate the minimal ventilation flow Q_min required to cope with the worst-case release rate R_max of the cryogenic gas:

$$ \frac{0.18}{0.21}=\frac{Q_{\mathrm{min}}}{Q_{\mathrm{min}}+{R}_{\mathrm{max}}} $$

This equation can be solved for the result Q_min = 6 R_max.

A numerical example shall illustrate this relation: a liquid nitrogen cooled circuit loses 10 mL s⁻¹ LN₂, evaporating to 6.5 L s⁻¹ N₂-gas at STP. To retain an oxygen concentration above 18%, the ventilation must supply 140 m³ fresh air per hour. In a standard room one considers that air is exchanged between 2 and 3 times per hour. This means that the assumed leak rate of LN₂ of 10 mL s⁻¹ does not pose a problem in normally sized laboratories. It could however lead to oxygen deficiency in a small wall cabinet in which a leaking LN₂-dewar is stored.

The well-mixed room approximation gives more reasonable estimates than the simple displacement hypothesis if the released gas is indeed well mixed. In not too large rooms, the turbulence created during the outflow and evaporation of cryogenic liquid suffice to promote mixing in the room. Refinements of the well-mixed room approximation can be found in [23]. A strong ventilation system supplying fresh air provides an effective protection against local pockets of oxygen deficiency in a room even for stronger releases that in the example above.

2.3.3.2 Oxygen Deficiency from Helium Gas

The most important cryogenic fluid for superconducting magnets is liquid Helium (LHe). The oxygen deficiency hazard which may arise after a release of LHe cannot be described with the well-mixed room approximation because of its low density once in gaseous state. Below a temperature of 40 K the density of Helium gas exceeds the density of air, 1.2 kg m⁻³. The heat capacity of Helium is very low and little energy is required to warm it up. Thus, liquid Helium or cold He gas released to air rapidly warms up by radiation and by turbulent convection. The gas density diminishes proportionally to 1/T. Once the temperature of the outpouring gas exceeds 40 K, buoyancy drives the gas upwards. In closed rooms the warming-up Helium gas forms a layer under the ceiling. A quick estimate yields that the thickness of the helium layer in a room with surface area A attains 5.6/A m for each kg of Helium released. In a standard laboratory room with a height of 3 m and a surface of 4 m by 7 m, 7.5 kg Helium (60 litres of LHe) would displace half the air from the room and form a layer from the ceiling extending until a height of 1.5 metres.

In laboratories housing experimental Helium cryostats, simple preventive measures can be taken: exhaust fans placed at a small distance under the ceiling expel the helium gas from the room, at the same time a vent at floor level supplies fresh air.

To evaluate the oxygen deficiency hazard in accelerator tunnels, a series of Helium release experiments have been performed in CERN’s LHC [19, 25]. 1000 litres (125 kg) of liquid Helium were released into the LHC tunnel with three different mass flows: 100 g/s, 320 g/s and 1 kg/s. The tunnel ventilation worked at its standard speed of about 1.5 m/s. Downwind from the release point, oxygen concentration sensors were located at regular height and distance in the tunnel and the propagating Helium cloud was recorded at these points by video cameras. The results of these tests can be summarized as follows:

For releases of mass flows with 100 g/s or 320 g/s, a turbulent zone with an extension of a few metres around the release point develops, in which the released cold helium is vigorously mixed with the surrounding air. In this zone, the Helium warms up rapidly and raises by buoyancy to the ceiling. In some distance from the release point, a ceiling flow of helium is driven by the ventilation and by its own momentum towards the next shaft. Outside the turbulent zone, temperature, and oxygen levels at the height of a walking person (h < 2.5 m) are close to normal and permit a safe evacuation of personnel.
A release of Helium with a mass flow of 1 kg/s corresponds to a gaseous Helium volume of 5.6 m³/s at standard pressure and room temperature. The thermalized Helium gas displaces air in most of the tunnel cross section so that oxygen is rarefied over several hundred metres.

The Helium release experiments have yielded precious information for the operation of the LHC and for the safe design of future accelerator facilities. Few publications of theoretical/ numerical descriptions of cryogenic releases exist [25] and in general, this field is actively researched.

As protection measures it has been decided that personnel are not permitted to access the LHC tunnel in operational phases, where leakages may provoke helium releases of more than 320 g/s. This is the case during the powering of the superconducting magnets, when also electrical safety demands the exclusion of personnel form the tunnel area.

In periods when the magnet current is zero, a general risk assessment of the cryogenic system has shown that accidentally provoked helium releases cannot have a mass flow exceeding 100 g/s. In these phases, personnel are permitted to perform standard verifications and small maintenance after a specific risk assessment. This assessment is targeted to identify the risk of an accidental manipulation or damage of a cryogenic control device, potentially leading to a Helium release. Technical or organisational mitigation measures are prescribed in the risk assessment to reduce this probability.

2.3.3.3 Mitigation of Oxygen Deficiency Hazard

The following mitigation measures can be applied to reduce the risk that personnel are exposed to the oxygen deficiency hazard:

Process changes: consider storing the bulk of cryogenic fluid outside of regularly occupied rooms and introduce only the immediately required quantity.
Forced ventilation: in unventilated or poorly ventilated rooms, install a forced ventilation with sufficient flow to remove enough of the evaporated cryogenic liquid to remain at C(O₂) > 18%
Oxygen deficiency detectors: they determine C(O₂) by an electrochemical reaction and raise an alarm when C(O₂) decreases below a set point. In case of ODH alarm, personnel must leave the room immediately. Due to their larger volume, installed ODH detectors are more precise, on the other hand, portable personal ODH monitors are light-weight and less expensive, but less precise and prone to raise false alarms.

If neither of the technical measures above can sufficiently reduce the risk of exposure to ODH, then the workers accessing the hazard areas must carry personal protective equipment in the form of personal oxygen generators. Adopted from underground mining, these devices supply O₂ from a chemical reaction for approximately 30 min. They enable a safe evacuation in large facilities, such as accelerator tunnels, where the emergency exit may be far.

2.4 Radiofrequency Technologies

Radiofrequency technology is fundamental to the acceleration of particles (Fig. 2.13). An alternative, emerging technology is plasma wake field acceleration, which will be briefly touched in Sect. 2.5.1.

2.4.1 Principle of RF Acceleration

A charged particle can be accelerated by static voltage across a gap, but this simple acceleration principle is limited by the breakdown voltage of the electrical field in vacuum to a maximal voltage of a few MV. The fundamental idea of radiofrequency acceleration is to apply an alternating electrical field to a gap. The electrical field has the accelerating polarity when the charged particle passes the gap, and the particle is shielded from the decelerating polarity, for example in drift tubes in a linear accelerator, or simply by having left the gap in a circular accelerator. Figure 2.14 illustrates this principle.

A particle moves in z-direction. If the electrical field was a constant across the gap g:

$$ {E}_z={E}_{z,0}=\raisebox{1ex}{${V}_0$}\!\left/ \!\raisebox{-1ex}{$g$}\right., $$

A particle with electrical charge e would gain the kinetic energy ΔE = e V₀. In reality, the electrical field oscillates with frequency ω:

$$ {E}_z(t)=\raisebox{1ex}{${V}_0$}\!\left/ \!\raisebox{-1ex}{$g$}\right.\cos \left(\omega t\right) $$

The field is at its maximum when the accelerated particle is in the centre of the gap. Then, the energy absorbed from the electrical field is:

$$ \varDelta E=e\ {V}_0T $$

where T is the transit-time factor. For the simple example of a sinusoidal field variation across a gap, T = 0.637. The energy gain of the charged particle per passage through the gap is determined by

The maximum Voltage V₀
The transit-time factor, which can be influenced by the geometry of the accelerating structure,

An accelerating gap is the simplest topology of an RF accelerating structure. Other topologies are drift-tube linacs (a succession of gaps, alternating with shielding drift-tubes) or accelerating cavities, in which standing or travelling electromagnetic waves impart kinetic energy on the particle.

2.4.2 Components of a RF Acceleration System

Figure 2.15 shows a block-diagram of an RF accelerating system. An amplifier converts DC input voltage into RF (high-frequency AC) output power. The power is transmitted by waveguides or coaxial cables to the accelerating cavity, where it is transferred to the beam particles.

2.4.2.1 RF Amplifier

An RF amplifier converts electrical power from a DC source into radiofrequency output power. In Fig. 2.15, the amplifier converts a DC input to radiofrequency output, modulated by the low-level RF signal at the amplifier’s control input. Frequency and phase of the RF output signal are determined by the low-level RF power, its amplitude and power are proportional to the current and voltage of the DC input. In the past, RF amplifiers were based on some type of vacuum tube. Today, technical solutions with solid-state amplifiers are possible.

In tetrode vacuum-tube RF amplifiers for CERN’s Super-Proton Synchrotron (SPS), packages of electrons were accelerated by anode voltages of 24 kV. In a Klystrons, a DC electron beam modulated by the low-level RF is accelerated up to 100 kV. When high-energy electrons collide with matter, they emit a Bremsstrahlung X-ray spectrum. In other words, vacuum tube-based RF amplifiers are sources of ionizing radiation

2.4.2.2 Waveguides

For operational reasons, the RF amplifiers are placed at a certain distance from the particle beam so that they can be monitored while the accelerator is working and serviced without having to enter a radiation area. The RF power from the amplifier is transported by waveguides or coaxial cables to the accelerating cavity in the accelerator tunnel. Above a frequency of a few 100 MHz and power exceeding some 10 kW, hollow waveguides are the most efficient means to transport RF energy. Electromagnetic waves can travel in hollow, metallic guides with rectangular cross section, when certain relations between the wavelength and the dimensions of the guide are met. These relations are derived in standard textbooks of electrodynamics, e.g [31]. If waveguides are incorrectly mounted, leakage radiation may escape. This radiation is non-ionising, electromagnetic radiation (NIR) and, depending on its power, may have adverse effects on persons in the vicinity.

2.4.2.3 RF Cavity

The shape of the metallic RF cavity is designed to maintain a standing or travelling radiofrequency electromagnetic field of the required frequency. The surface resistance of the metal constitutes a loss factor, and can be drastically reduced by making the cavity walls superconducting, for example by covering them with a thin layer of Nb.

2.4.3 Hazards from RF Systems

Radiofrequency systems may be the source of the following hazards which must be considered in a risk assessment:

Electrical hazards from the power source (Sect. 4.1)
Cryogenic hazards in superconducting RF systems (Sect. 2.3)
X-rays from bremsstrahlung from the electron beam in vacuum tube RF amplifiers or from spuriously emitted and accelerated surface electrons
Electromagnetic radiation leaking from radiofrequency systems, with frequencies too low to provoke ionisation: Non-Ionising Radiation.

2.4.3.1 Bremsstrahlung X-Rays

The X-ray bremsstrahlung spectrum from vacuum tube RF-amplifiers must be appropriately shielded. It has a terminal energy corresponding to the highest kinetic energy of the electrons in the vacuum tube, in some klystrons exceeding 100 kV. Exposure to ionizing radiation from RF amplifiers made the headlines in 2001 in Germany, when it was revealed that maintenance technicians of military RADAR devices in tanks and airplanes, fed by klystrons or magnetrons, showed a higher cancer incidence than the general population. This was traced back to their exposure to the partially unshielded radiation emitted by these devices during maintenance. Coincidentally, this occurred in both of the armed forces of former West- and East Germany. Vacuum tube RF amplifiers must be appropriately shielded to keep the emitted radiation below legal limits for non-designated areas. Klystrons bought from industry are delivered with shielding, and the radiation risk from them is low if the shielding is not compromised.

In accelerating cavities, electrons are emitted spuriously from the cavity surface at locations of high curvature, surface inhomogeneities or “dirt”. These electrons pick up a certain amount of the RF energy in the cavity and collide with its walls, emitting bremsstrahlung. The intensity of the emitted X-rays is the highest, when a cavity recently exposed to air is conditioned, a process where surface electrons are emitted purposefully to contribute to the removal of impurities from the surfaces. When RF cavities operate in the accelerator tunnel, these emissions do no present a risk for personnel, protected by the accelerator shielding and excluded by the access safety system (Sect. 5.3.1). During commissioning of new or modified cavities in test areas, proper shielding must be built around the test stands to protect the personnel from harmful ionizing radiation. Figure 2.16 shows a simulated contour map of ambient dose equivalent rate around a test bunker during the commissioning of RF accelerating structures with a field gradient of 25 MV/m in Beijing University [31].

In a two-step process, first the emission and spurious acceleration of surface electrons was simulated, and then the generation of bremsstrahlung X-rays from the stopping electrons. The light blue colour on the dose rate map outside of the bunker signifies a dose rate between 10 and 50μSv/h. This value would lead to a classification as a controlled radiation area under most European radiation protection regulations.

2.4.3.2 Electromagnetic Leakage Radiation

Non-ionising electromagnetic radiation leaking from RF structures and wave guides may have negative effects on human health. The subject of the next Sect. 2.4.4 are direct health effects of electromagnetic field exposure of the whole human body or of parts of it.

Indirect health effects occur when electromagnetic radiation alters or impedes the function of electric devices. A heart pacemaker or an insulin pump are electrical or electro-mechanical devices, equipped with electronic control circuitry. Any of these components may fail if they are coming under the influence of electromagnetic fields. The avoidance of such adverse effects is the subject of electromagnetic compatibility (EMC).

2.4.4 Health Effects of Electromagnetic Fields (EMF)

The field of non-ionising radiation covers a wide band of frequencies of electromagnetic radiation. The quantum energy of its photons is not high enough to provoke the ionisation of an atom in material struck by the radiation, therefore its name non-ionising radiation as compared to ionising radiation.

Nevertheless, the effect of exposure to non-ionising electromagnetic radiation can be as detrimental to health as that of ionising radiation.

The International Commission for Non-Ionizing Radiation Protection, ICNIRP, has updated its recommendations for electromagnetic radiation in the frequency range between 100 kHz and 300 GHZ in 2020 [30]. ICNIRP bases its recommendations for intensity limits of electromagnetic radiation only on substantiated effects. These are harmful effects of EMF on health which have been repetitively observed and documented in the scientific literature and which do not contradict current scientific understanding. The rigorous approach to rely on scientific evidence for adverse health effects of EMF ensures that ICNIRP recommendations do not follow unconfirmed claims.

2.4.4.1 Biophysical Effects of EMF

The biophysical effects of electromagnetic fields in the body can be classified in three groups:

The most important effects of EMF in the body are thermal effects. The electrical field components of EMF act on polar molecules (water!) and on charge carriers (ions and electrons) and confers kinetic energy to their translational, rotational, and vibrational degrees of freedom. This kinetic energy dissipates in the tissue, leading to a temperature rise. The adverse health effects of a continuous rise of body temperature have been demonstrated, and intensity limitations of non-ionising radiation exposure protect against these effects.
At frequencies around f ≈ 100 kHz, nerve stimulation which manifests itself in a “tingling” sensation has been described.
Very high intensities of high-frequency EMF, which may occur in the Fourier-decomposition of electromagnetic pulses, may lead to an alteration of the permeability of biological membranes. Such modifications in the body would lead to consequential adverse health effects. The effect has been demonstrated in in-vitro experiments, but at an intensity which is prevented by the limits on temperature effects.

As a summary, one can state that as of today, ICNIRP recognizes as the only substantiated adverse effect of non-ionising radiation on human health and safety the heating of exposed tissue. Spurious reports of electro sensitivity or the induction of cancer have not withstood epidemiological investigations.

2.4.4.2 Dosimetric Quantities for Non-ionising Radiation

To describe the biophysical effects from EMF leading to adverse health effects and to define limits, one needs dosimetric quantities. ICNIRP defines quantities for energy absorption from EMF in its report [30]. The quantities which are suitable for limiting emissions from RF fields at particle accelerators can be described as follows:

SAR, Specific energy absorption rate, measured as a whole-body average in the unit W kg⁻¹, to quantify whole body energy absorption rate, leading to a rise of body core temperature
SAR_10g, Specific local energy absorption rate over a 10 g tissue cube (with side length of 2.15 cm) to quantify local energy absorption rate, leading to local heating
S_ab Specific Absorbed energy density, measured in the unit W m⁻², to quantify energy absorption rate to the skin from very high frequency EMF

2.4.5 Protection against NIR

2.4.5.1 Effect Levels and Basic Restrictions to Temperature Rise

When discussing body temperature rise, one distinguishes core temperature and local temperature. The core temperature is the temperature of the inner organs and amounts for a healthy adult to T_core = 37 °C. Core temperature may rise in immune reactions (fever) to help the body fend off infections, but in absence of fever, a long-term temperature rise of more than ΔT_core = 1 °C can lead to adverse health effects. Locally, body tissues shall not be heated by more than 5 °C to prevent effects, such as burns.

ICNIRP defines an effect level, the magnitude of an EMF, expressed in a suitable dosimetric quantity, from which on health effects are reported in the literature.

Modelling the effect of EMF on body tissue shows that a specific energy absorption rate SAR = 4 W kg⁻¹ over 30 minutes raises body core temperature by 1 °C. This level of SAR is therefore the effect level for whole-body exposure. As a comparison, an average human generates 1 W kg⁻¹ at rest, 2 W kg⁻¹ standing and 12 W kg⁻¹ when running. This heat loads are dealt with by the body’s temperature regulation system. Excess heat is transported to the skin, cooled by the surrounding air and, when this is not sufficient, by evaporation of sweat. The ICNIRP places a basic restriction on additional heat loads, which may overload the body’s regulation mechanism.

If only part of the body is exposed to EMF, then local effect levels for energy absorption rate apply. They are determined by the quantity SAR_10g. The effect levels are SAR_10g < 20 W kg⁻¹ for head and torso and SAR_10g < 40 W kg⁻¹ for the limbs. As tissue has an assumed density of ρ = 1 g cm⁻³_, the local power density must not exceed 20 mW cm⁻³ or 40 mW cm⁻³, respectively.

At frequencies above 6 GHz, the heating by EMF takes place mostly in the skin: at 6GHz, 86% of the power is absorbed within 8 mm of the body surface and leads to a rise of the local temperature. The suitable dosimetric quantity to describe energy absorption in this frequency range is specific absorbed energy density, S_ab. Here, effects over a small (1 cm²) and a larger (4 cm²) patch of skin are investigated, leading to two different effect levels of 400 W m⁻² and 200 W m⁻².

To derive basic restrictions to exposure by EMF for workers, ICNIRP divides the effect levels by a safety factor of 2 or 10 (for whole-body SAR). For the public, basic restriction levels are reduced by another factor of 5 (Table 2.5).

Table 2.5 Basic restrictions to the effects of electromagnetic radiation [30]. The table shows the body part to be protected, the frequency range, the maximal allowed temperature rise over a distinct volume, the applicable dosimetric quantity, the level, of the quantity at which health effects are observed with certainty and the derived exposure limits for workers and the public. The thermal effect is averaged over 6 min, with exception of the one on the whole body where the averaging interval is 30 min

Full size table

2.4.5.2 Reference Quantities for Non-ionising Radiation

Basic restrictions are defined within the body or parts of it and cannot be measured. For the assessment of non-ionising radiation at the workplace or in the public, reference quantities are defined which are accessible to measurement. Reference quantities are the incident electric field strength E_inc [V m⁻¹], incident magnetic field strength H_inc [A m⁻¹] and incident power density S_inc [W m⁻²]. The latter quantity is used for the determination of reference values for the skin.

ICNIRP has determined the field strengths which, under worst-case conditions, result in the limiting exposures to the whole body or specific tissues as expressed by the basic restrictions. The reference quantities depend on the frequency of the incident radiation and are shown for the occupational exposure of workers in Figs. 2.17 and 2.18.

2.5 Lasers at Accelerators

Lasers emit narrow beams of coherent, monochromatic optical radiation from the far ultraviolet (UV) (180 nm) to the far infrared (IR) (3000μm) range of wavelengths (Fig. 2.18). Lasers have wide-ranging applications in particle accelerator centres, both in the accelerator and in the adjacent laboratories and workshops.

2.5.1 Application of Lasers at Accelerators

The intense light emitted by Lasers has specific applications at accelerators, for example:

Photo-cathodes illuminated by pulsed laser beams provide electron beams with a specified time structure, matched to the accelerating structures for linear accelerators [34].
Gamma factories: inverse Compton scattering of laser-generated photons on relativistic electron beams [44] or ion beams [41] provide photons with extremely high energies.
Laser wakefield acceleration (LWFA). An intense, short Laser pulse traversing a plasma cell-displaces the free electrons, creating an electrical wakefield (Fig. 2.19). Field gradients surpassing everything that is possible with RF cavities can be generated and electrons have been accelerated to 7.8 GeV over short distances [37]. The lasers used for LWFA have instantaneous powers in the Terawatt range.
Laser ion source. The sharply defined wavelength of a Laser permits isotope-selective ionisation of exotic atoms in radioisotope facilities, for example ISOLDE at CERN [36]. Several excitation steps are necessary to bring an electron to a loosely bound Rydberg state and finally to remove it from the atom. Tuneable lasers based on toxic dyes are mounted with frequency-doubling crystals and other optical elements on an optical bench (Fig. 2.20).

Unspecific, industrial applications of Lasers in particle accelerator centres are

Metrology, for example distance measurement, surveying, aligning of accelerator components, laser velocimetry, laser vibrometers.
Material processing, such as cutting, welding, drilling, photolithography, additive manufacturing (“3D printing”).
Communication, with laser diodes and optical fibres

2.5.2 Hazardous Effects of Lasers

The collimation of the laser light in narrow beams implies a high power density, expressed in the quantity irradiance E with the unit of surface power density [W m⁻²]. The time integral of irradiance is radiant exposure J, an energy surface density [J m⁻²]. The principal variable to determine the effects of laser light besides its wavelength is imparted energy. For pulsed lasers, radiant exposure is the relevant quantity because irradiance may assume misleadingly high values during the short pulse duration.

The relation between imparted energy and biological effects is strongly dependent on wavelength because different damage mechanisms are at play. The most critical organ for laser damage is the eye (Fig. 2.21). The eye has a physiological reaction to protect itself against intense light, the optical aversion reflex. It is effective for continuous-wave lasers of not too high irradiance. Powerful pulsed lasers may cause irreversible damage to the eye before the aversion reflex is even entering into action. The skin may also be affected when directly irradiated. The biophysical effects of laser radiation on the eye and skin are described in Table 2.6.

Table 2.6 Biophysical effects associated with exposure to laser light beyond the injury thresholds, adapted from [39]

Full size table

Injury thresholds of tissues after exposure to laser light were originally determined in animal experiments. Laboratory animals were exposed to laser beams with specific wavelength, duration, spot size and radiant exposure. After irradiation, damage to the critical parts of the eye (cornea, lens, or retina) was evaluated in an ophthalmological exam. The threshold value for damage is defined as the dose (radiant exposure) with a 50% probability for minimal injury. The animal data are supported by clinical data from human exposure, either of volunteers, accidental cases, or clinical patients. Today, lasers have many applications in medicine [43] and clinical data are available from ophthalmology and surgery. They are supported by biophysical models for the complex laser-tissue interactions [42]. Such models can be used to extrapolate injury thresholds to exposure conditions which are not experimentally investigated.

Other safety hazards of lasers are:

Fire hazard from powerful lasers, as their light output can ignite flammable material;
Chemical hazards from toxic and flammable dyes and solvents employed in the operation of tuneable lasers with adjustable wavelength;
Electrical hazards from power sources and control systems of lasers.

2.5.3 Protection Against Laser Exposure

2.5.3.1 Maximum Permissible Exposure Limits

In the previous section, injury thresholds for laser exposure were introduced, they are determined from animal experiments, clinical data, and modelling. For operational protection against laser exposure, Maximum Permissible Exposure (MPE) limits are derived by applying reduction factors (typically a factor of 10) to the injury thresholds. Depending on the application for pulsed or continuous-wave lasers, MPE are expressed either in the quantity irradiance E (W m⁻²) or radiant exposure J (J m⁻²), and they are tabulated as a function of wavelength, exposure duration and spot size in the reference publication of ICNIRP (International Commission for Non-Ionizing Radiation Protection) [38]. Figure 2.22 shows a graphic rendition of the MPE expressed in radiant exposure for different wavelengths and pulse times [45]. The plot shows MPE values varying over several orders of magnitude in short wavelength intervals, reflecting the complex interplay between the laser, and the tissues’ physical and physiological reactions.

In a practical situation, a laser specialist must determine the possible exposure scenarios, determine irradiance or radiant exposure for each scenario by measurement or calculation and compare these values with the applicable MPE. This process is complicated but must nevertheless be applied in laser applications in research and development as for example in Fig. 2.20.

2.5.3.2 Laser Classification

For the application of the MPE limits stated by ICNIRP in standard situations, a different approach is chosen. Since one cannot expect that every Laser owner has the expertise to determine exposure values and compare them with the relevant MPE, one classifies Lasers based on their emission capabilities. Classes range from 1 for the least dangerous to 4 for the most dangerous Lasers. Classes may be subdivided to consider special situations. For each class, the maximal potential exposure risk is determined by measurement and calculation, and the Accessible Emission Limit (AEL) is determined for a few conservative exposure scenarios of the eye or the skin. The corresponding protective measures are applied to all lasers belonging to the same class. The classification of a laser is the duty of the manufacturer, who may refer to testing laboratories with the necessary expertise. Classification tests are designed to be rather “worst-case” and restrictive in order to ensure that a “low-class” (e.g. Class 1) laser does not present a hazard to the eye or skin even in reasonably foreseeable worst-case situations. The International Electrotechnical Commission (IEC) publishes the series of standards “Safety of Laser Products “, IEC 60825. The first part of the series [39] introduces the said classification (Table 2.7) and specifies the emission limits for each class, as a function of wavelength and pulse duration. Once classified, a label is affixed visibly on the laser to indicate the class that it belongs to. (Fig. 2.23).

Table 2.7 Laser Safety Classification, adapted from [39] and [40]. Class 1C, for medical and cosmetic applications, has been omitted

Full size table

2.5.3.3 Practical Laser Safety

Following the Hierarchy of Controls (Sect. 5.1.5), the most effective prevention measure against accidents with lasers is isolation so that access to the light path is impossible during operation. Class 1 also includes lasers with higher irradiance or radiant exposure than the AEL, but with a fully encapsulated light path and built in a way that they can only operate if the protection is intact.

If the light path of a laser from a class higher than 2 M cannot be fully enclosed, it must be installed in a laser room in which the entrance door is interlocked with the power source of the laser or an optical shutter, so that the laser beam stops once the door is opened. For classes 3B and 4, a key control for the laser room or for laser operation must be installed, permitting access to and operation of the laser only by trained personnel.

In laser set-ups where the direct beam runs in the open for purposes of adjustment, personal protective equipment (PPE) is the last resort. Hands and forearms can be protected from UV radiation by leather gloves and long sleeves, and the eyesight with goggles, attenuating the characteristic wavelength of the laser.

2.6 Beam-Intercepting Devices

As the name of this section indicates, it treats devices which interact directly with the particle beam by being placed in its path. Beam intercepting devices must be able to handle the energy transmitted to them, mostly in form of heat and ionising radiation. The principal occupational hazard of these devices is activation by interaction with particles (Sect. 3.1) and the resulting exposure of personnel to ionising radiation (Sect. 3.5.2), occurring during periods of maintenance and repair.

2.6.1 Collimators

In simple accelerator theory, an accelerated particle orbits in an accelerator or storage ring with no other interactions than with the electromagnetic guiding and accelerating fields. In reality, dissipative forces from collisions with rest-gas atoms and interactions with other particles make the particles deviate from their ideal orbit. Other sources of orbit errors are the pulsed electromagnetic fields in kickers and septa used to steer the beam from one accelerator into another one. Particles which do not experience the full strength of the “kick” deviate from the ideal orbit. The result of these dissipative and deviating interactions is a growth of the beam-size and an increased probability that beam particles collide with the accelerator components. These collisions are the source of the following adverse effects:

In accelerators with normal conducting magnets the collisions lead to secondary particle cascades with numerous nuclear interactions and lead to activation of the accelerator components (Sect. 3.3.2).
In accelerators with superconducting magnets the collisions of charged particles with the magnets lead to energy deposition outside the thermodynamic stability of the superconductor and lead to a quench (Sect. 2.2.2). The operational stability and availability of the accelerator make the avoidance of such beam loss mandatory.

The answer to these two points is to concentrate beam loss in a few locations of the accelerator, where it does not damage equipment or quench magnets, and where secondary particle cascade and the resulting activated material can be shielded [48]. Collimators, consisting of massive absorber blocks, present artificial restrictions in the accelerator beam line to absorb particles steering off the ideal orbit. Very high energy particles are scattered in a large solid angle by the collimator material, thus diluting their damaging effects.

Radioactivity after use. The dose rate levels around a collimator in a storage ring are in general so high – a few mSv/h in working distance – that the equipment cannot be maintained shortly after the stop of the accelerator beam.
Radioactive contamination. A collimator operates under high or ultrahigh vacuum conditions. During collimator maintenance the vacuum tank is opened and workers may be exposed to dust and debris from collimator materials which have become brittle by constant particle beam impact. Cooling water or hydraulic fluids in a collimator may be slightly activated, although by experience, this is rarely of concern.
In a rigorous interpretation of the legislation, the actuators or motors make the collimator a “machine” which falls under the corresponding directives (see 4.2.1). However, equipment for the use in fundamental research may be exempted from the prescriptions of the directive and the local prescriptions should be clarified with the licensing authorities (Fig. 2.24).

One strategy to mitigate high personal radiation dose from activated collimators is to keep a damaged collimator in an intermediate storage and wait until the dose rate is acceptable before attempting a repair. The acceptable level depends on the planned duration of the work (Sect. 3.5). Alternatively, one may decide beforehand that collimators are expendable and not even try to repair them. The design of a collimator contributes also to the optimization of radiation protection: a plug-and-play assembly allowing rapid exchange of components saves time and dose.

2.6.2 Targets

One can distinguish between different types of targets at an accelerator: production targets, experimental targets, and stripping foils. They have in common that they are placed in the path of an accelerated particle beam to provoke beam-matter collisions.

2.6.2.1 Production Targets

Production targets serve to produce unstable particles in collisions with the primary beam. Examples are spallation targets from heavy metals to produce neutrons, or light metal targets to produce antiprotons. Production targets are usually massive objects to optimise the yield of the desired particles.

The yield of a production target is proportional to the number of reactions within the target, which is in turn the product of the beam intensity, the length of the primary particle’s path within the target and the production cross section of interest. Besides the reaction of interest, the primary charged particle will have numerous other interactions within the target material, mediated by the electro-magnetic and the strong fundamental interactions. These processes are described with more detail in section 3.2. As a bottom line, most of the energy deposited during the passage of charged particles through matter serves to heat the target material [47], ionisation and energy leaving the target in form of radiation and new particles are often negligible. In spallation sources [5, 8], the neutron production targets cope with several MW of absorbed beam power. An aggravating factor is the pulsed time structure of the beam which leads to even higher instantaneous values of power during the beam impact. This places stringent requirements on the mechanical and thermal resistance of the targets.

2.6.2.2 Experimental Targets

Experimental targets are placed in the path of a particle beam, the properties of the reaction products of collisions (particle type, momentum, spin) are observed with detectors and provide insight to fundamental interaction mechanisms. As a rule, experimental targets are thin to let the reaction products escape without alteration of their properties in secondary collisions.

Experimental targets may consist of radioactive material: to produce superheavy isotopes one bombards targets enriched in unstable transuranium isotopes with beams of the heaviest possible stable or long-lived elements, lead, gold, or the Uranium-isotope ²³⁸U. The use of these radioactive targets is subject to authorisation procedures in national regulations. Their handling requires the application of strict administrative procedures. Most of the transuranium isotopes employed in production targets for superheavy isotopes are alpha emitters with a high radiotoxicity. They must be handled with utmost care to prevent internal exposure of personnel.

A special type of experimental target is the instrumented beam dump. It is a thick target, surrounded and followed by detectors to investigate the debris from collisions in the search of long-lived exotic particles.

2.6.2.3 Stripping Foils

Stripping foils are very thin foils (in the range of micrometres) placed in the path of a negatively charged beam to remove orbital electrons. They are used to strip heavy ions from all electrons, or to remove two electrons from the H- ion in an efficient injection process in a synchrotron.

2.6.3 Beam Dumps

Particle beams carry high energy and can damage accelerator equipment by different interaction mechanisms (Sect. 3.2). This energy must be deposited safely once the beam has interacted in an experiment or has become too diluted to provide meaningful collision rates with the opposing beam in a collider. The beam is directed onto a massive absorber where most of its energy is absorbed or dissipated. The high beam energy poses challenges to cooling and mechanical stability of the dumps.

One distinguishes between external and internal beam dumps. External beam dumps are constructed at the end of a transfer beam line. When the signal for dumping the beam is given, kicker magnets deflect it out of its normal orbit into the transfer beam line. The extraction kicker and transfer beam line are optimised for the transport of particles within a narrow energy band. The LHC is equipped with two external dumps and transfer lines optimised for the terminal energy of LHC, 7 TeV.

If particles with many different energies must be absorbed in a synchrotron (for example, particles shortly after injection and at different terminal energies), an internal dump is the best choice. Internal dumps resemble collimators [46]. Particles can pass through an internal dump by a large beam space without interacting or even being deviated from their orbit (Fig. 2.25). Upstream of the dump, a kicker is mounted which can deflect the particles into the absorber block, where part of their energy is absorbed, and part dispersed as a secondary particle cascade. The combination of a kicker followed shortly by an internal dump has a large acceptance for particle energies. An internal dump is mounted in the SPS synchrotron at CERN, where protons are accelerated from 20 GeV to 450 GeV. The internal dump allows beam aborts at any energy.

The hazards of beam dumps are similar to those of collimators. Due to the high number of absorbed particles, the radioactivity of a dump is so high that personnel can only work for a limited time in their vicinity. Internal dumps are surrounded by massive radiation shielding to protect personnel who must pass the dump location or work in its vicinity, external dumps are located in blind tunnels where no other access is required. Because the repair of a dump is possible only after storage for radioactive decay for a few years, one must have spares in store to replace the failed dump.

Notes

1.
In special cases, a collider can be realized within a single storage ring when the colliding particles are antiparticles to each other: Examples are the SPS (proton-antiproton) and the LEP (electron-positron), both at CERN. CERN’s LHC looks like a single ring, but in reality, the magnet yoke houses two magnets with opposite polarity.
2.
In the Alps, this is possible on the summits of Aiguille du Midi (Chamonix, FR) and Klein Matterhorn (Zermatt, CH).

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Otto, T. (2021). Risks and Hazards of Particle Accelerator Technologies. In: Safety for Particle Accelerators. Particle Acceleration and Detection. Springer, Cham. https://doi.org/10.1007/978-3-030-57031-6_2

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Risks and Hazards of Particle Accelerator Technologies

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Keywords

2.1 Accelerators for Pedestrians

2.1.1 Why Particle Accelerators?

2.1.2 The Particle Accelerator Family

2.1.2.1 Linear Accelerator

2.1.2.2 Cyclotron

2.1.2.3 Synchrotron

2.1.2.4 Storage Ring

2.1.2.5 Free-Electron Laser

2.1.2.6 Collider

2.1.3 Particle Acceleration from Source to Target

2.2 Magnets

2.2.1 Normal Conducting Magnets

2.2.2 Superconducting Magnets

2.2.3 Safety Aspects of Magnets

2.2.3.1 Magnetic Field Hazard

2.2.3.2 Health Effects of Magnetic Fields

2.2.3.3 Magnetic Forces

2.2.3.4 Electrical Hazards of Magnets

2.3 Cryogenics

2.3.1 Production of Low Temperatures

2.3.2 Cryogenic Fluids

2.3.2.1 Overpressure

2.3.2.2 Cold Burns

2.3.3 Oxygen Deficiency Hazard from Cryogenic Fluids

2.3.3.1 Oxygen Deficiency from Nitrogen Gas

2.3.3.2 Oxygen Deficiency from Helium Gas

2.3.3.3 Mitigation of Oxygen Deficiency Hazard

2.4 Radiofrequency Technologies

2.4.1 Principle of RF Acceleration

2.4.2 Components of a RF Acceleration System

2.4.2.1 RF Amplifier

2.4.2.2 Waveguides

2.4.2.3 RF Cavity

2.4.3 Hazards from RF Systems

2.4.3.1 Bremsstrahlung X-Rays

2.4.3.2 Electromagnetic Leakage Radiation

2.4.4 Health Effects of Electromagnetic Fields (EMF)

2.4.4.1 Biophysical Effects of EMF

2.4.4.2 Dosimetric Quantities for Non-ionising Radiation

2.4.5 Protection against NIR

2.4.5.1 Effect Levels and Basic Restrictions to Temperature Rise

2.4.5.2 Reference Quantities for Non-ionising Radiation

2.5 Lasers at Accelerators

2.5.1 Application of Lasers at Accelerators

2.5.2 Hazardous Effects of Lasers

2.5.3 Protection Against Laser Exposure

2.5.3.1 Maximum Permissible Exposure Limits

2.5.3.2 Laser Classification

2.5.3.3 Practical Laser Safety

2.6 Beam-Intercepting Devices

2.6.1 Collimators

2.6.2 Targets

2.6.2.1 Production Targets

2.6.2.2 Experimental Targets

2.6.2.3 Stripping Foils

2.6.3 Beam Dumps

Notes

References

Accelerators

Magnets

Cryogenics

RF Technologies

Lasers

Beam-Intercepting Devices

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