EPI-Lo measures energetic particles in the lower portion of the ISIS energy range. As discussed in Sect. 2, the two ISIS instruments complement one another in their energy ranges and their sky coverage in order to obtain the comprehensive set of observations needed to understand solar energetic particle sources, acceleration and transport close to the Sun.
EPI-Lo is a novel, light-weight, high-heritage, time-of-flight (TOF) based, mass spectrometer that measures energetic electron (25–1000 keV) and ion (∼0.04–7 MeV for protons and ∼0.02–2 MeV/nuc for heavier ions) spectra and resolves all major heavy ion species and 3He and 4He over much of this energy range in multiple directions. ISIS thus covers the critical energy range from suprathermal energies (∼20 keV/nuc) up to the lower portion of the EPI-Hi energy range with a single instrument. The EPI-Lo characteristics and projected performance are summarized in Table 4.
EPI-Lo (Figs. 15 and 20) consists of eight sensor wedges mounted above an electronics box. It has 80 separate entrances (10 on each of eight wedges) densely sampling over half of the sky. This configuration permits full angular distributions without articulation or duty cycle, and allows for measuring the first-arriving, field-aligned ions at the spacecraft for a broad range of vector magnetic (B) field directions.
Understanding coronal acceleration processes requires ion-mass resolution sufficient to measure separately the intensities of the isotopes 3He and 4He (Fig. 21), and the elements C, O, Ne, Mg, Si, and Fe, while simultaneously providing angular coverage with at least 45 % energy resolution.
EPI-Lo achieves this resolution, including separation of 3He from 4He, over most of its angular coverage from 200 keV to 4 MeV total particle energy. EPI-Lo also returns high-resolution energy spectra (64 log-spaced energy bins) at reduced time resolution (necessitated by downlink limitations) and measures the required ion composition and pitch angle distributions from 30 keV/nuc to 0.3–1.0 MeV/nuc every 5 or 30 s. In this way ion acceleration histories are completely characterized with no uncertainty owing to an insufficient energy range or composition misidentification. EPI-Lo rejects background by requiring coincidence between the start and stop pulses for the TOF measurement, along with an energy measurement between appropriate TOF time gates. This rejection is non-linear, and is very effective (resulting in inconsequential rates of false valid events) for background counting rates in individual detectors (singles rates) below ∼106 s−1; projected singles rates from combined background and foreground sources are ≤3×105 s−1. Incoming ion velocities are determined by measuring the TOF between two thin (100 nm Start, 65 nm Stop) carbon-polyimide-aluminum (and palladium, Start only) foils. An ion passing through each of the foils produces secondary electrons, which are deflected toward a microchannel plate (MCP) producing “Start” and “Stop” pulses. The ion entrance angles are determined from the position where the Start electrons strike the MCP and are unique for each entrance foil location. The ion energy deposited in the SSD, together with velocity from the TOF, determines species through an onboard table lookup.
EPI-Lo also detects electrons from ∼25 keV to 1000 keV. EPI-Lo uses solid-state detectors (SSDs) shielded by aluminum flashing of ∼2 μm thickness as also used in multiple current and upcoming missions, e.g., Cassini-MIMI (Krimigis et al. 2004), MESSENGER EPS (Andrews et al. 2007), New Horizons PEPSSI (McNutt et al. 2008), Juno JEDI (Mauk et al. 2013), Van Allen Probes RBSPICE (Mitchell et al. 2013), and MMS EIS (Mauk et al. 2014). The relatively thick aluminum flashing naturally suppresses light, which is a very important feature for this intrinsically single parameter measurement. While the primary electron measurement does not identify which entrance aperture an electron enters through, each EPI-Lo wedge contains independent electron SSDs, so the sector of the sky is identified over an angular coverage similar to that for the ions. For the small subset of the electrons that produce secondary electrons as they transit the entrance foils, an additional mode that requires a Start pulse along with the SSD pulse identifies the specific entrance aperture that the electron entered through. This mode also provides better background rejection.
MCP detectors and SSD’s have been optimized for use on this mission. We also produced a prototype wedge (Fig. 22), which has been used for extensive testing and development of the EPI-Lo concept and design. Finally, lead development engineers and instrument scientists on JEDI, RBSPICE, and EIS are all participating in the EPI-Lo effort, as well the corresponding leads that developed and continue to operate PEPSSI.
EPI-Lo Expected Performance
The detectors in EPI-Lo are sensitive to charged particles and to light. The MCPs are sensitive to ultraviolet (UV) light, including both EUV and shorter far ultraviolet (FUV) wavelengths whereas the SSDs are sensitive to everything from X-rays to visible wavelengths. However, the mechanisms are different. MCPs respond to charged particles impacting their surface, and UV produces secondary electrons (photoelectrons) on surfaces (either the surface of the MCP itself, or the Start and Stop foils designed to produce secondary electrons as ions penetrate them, or other surfaces inside the sensor volume), which can then enter channels and be amplified similarly to signals from any other particle.
SSDs respond to particles as they deposit energy in the bulk material of the detector. They free bound electrons to create electron-hole pairs and so increase conductivity. In SSDs, light can also energize bound electrons sufficiently that they become free and increase conductivity. Energetic X-rays may deposit enough energy to create free electrons that then raise the energy of sufficient numbers of bound electrons so that the resulting current spike is above threshold. Lower energy photons will not do so, but enough of them arriving within an SSD event time constant (∼1 microsecond) can free sufficient numbers of electrons to register as events, or to raise the electronic noise of the detector unacceptably.
In both cases, light (both UV and visible) must be reduced to a level where these effects cannot degrade the particle measurements. Controlling the amount of light entering the sensor volume involves a variety of techniques: (1) eliminating stray-light leak paths into the instrument; (2) collimating the entrance apertures to eliminate exposure to trajectories that are not useful for particle analysis; (3) coating surfaces from which light may be reflected into the detectors with low-reflectance coatings; (4) using high-work-function surfaces in locations where UV photons may strike surfaces from which photoemission is undesirable; (5) designing the Start and Stop foils using materials and thicknesses that reflect and/or filter the UV and visible light, reducing the light that can directly enter the sensor volume to acceptable levels; (6) taking precautions against failure of this filtering approach as a consequence of small numbers of pinholes in the foils (either from dust impact or launch vibration damage); and, (7) designing the sensor timing constraints such that controlled rates of photoelectron production from UV photons do not impact particle measurements.
Briefly, the instrument design carefully considers: (1) stray light paths and avoids them; (3) coating surfaces as a second-order consideration relative to direct paths for light and UV; and (7) timing constraints fundamental to the instrument design: an approach successfully used on numerous past flight programs (the “hockey pucks”Footnote 1 on MESSENGER, New Horizons, Van Allen Probes, Juno, and MMS as well as the ENA imagers on Cassini and IMAGE).
All of these programs also rely on collimation and filtering by foils, and all were subject to pinhole degradation at various levels, so these considerations are not new; however, the SPP environment is sufficiently unique that careful consideration to these approaches is required for EPI-Lo.
The environment in which EPI-Lo will operate is one never before experienced. It has been extensively modeled, and there is considerable knowledge of the light environment. The SPP team has provided their best estimate of the worst-case photon environment as a function of solar elongation angle at perihelion. This, and the other expected SPP environments are specified in an internal Project-provided environmental design and test requirements document. Light at the ISIS location is dominated by Thomson-scattered photons, to first order a process that preserves the shape of the solar spectrum. The intensity of the scattered light is a fairly strong function of elongation angle, dominated primarily by the line-of-sight integrated electron density, which varies with elongation (Fig. 23). While dust-scattered light dominates at large elongations it is much weaker than the Thomson-scattered light at smaller elongations, so the foil filtering design required by the Thomson-scattered light at smaller elongations is more than adequate for the dust-scattered light.
We know the elongation angles of the various apertures on EPI-Lo. In considering these, we treat the EPI-Lo apertures in two distinct groups: those that contain sky elements within ∼12° elongation angle (the edge of the TPS is at about 8°), and those that do not. For those with elongation angles less than 12°, the rough order-of-magnitude (ROM) for the UV and light environment is approximately equivalent to having the disk of the Sun directly in the FOV of that aperture at 1 AU. This then means that the filtering requirements for those apertures are approximately the same as what would be required for EPI-Lo at 1 AU with the Sun directly in the aperture FOV.
Several approaches contribute to determining the required filtering properties of the foils. For visible light (which primarily concerns the SSDs), we have used flight experience from various instruments in Earth orbit. For example, the ISEE-1 Energetic Particle Detector (EPD) had 30 μg/cm2 of Al covering its SSD, and suffered from visible light contamination when the detector viewed the Sun directly. The IMP-8 EPD had 40 μg cm−2 of Al covering its SSD and did not respond to direct Sun in the visible, but it did respond to solar X-ray events. From this we conclude that 40 μg cm−2 of Al or the equivalent would be sufficient for EPI-Lo for elongations <12°. For these small elongation foils we are baselining a Start foil of 24 μg cm−2 of Al plus 18 μg cm−2 of palladium (Pd), which combined with the Stop foil containing 7.3 μg cm−2 of Al, yields a predicted noise level on the SSD of ∼0.04 keV μs−1, well below the electronic noise level of ∼7 keV μs−1.
For the larger elongation apertures we have baselined a total of 7.3 μg cm−2 of Al plus 18 μg cm−2 of Pd on the Start foil, and 7.3 μg cm−2 of Al on the Stop foil. The predicted noise level for this combination is 0.7 keV μs−1 in the SSD. We plan to test at this level, and increase the thickness of the aluminum layers if necessary. It should be noted that these are ROM estimates and not really directly comparable. The Thomson scattered light is diffuse, whereas sunlight at 1 AU is collimated. Appropriate descriptions in terms of photons at a particular wavelength are number of photons cm−2 s−1 for sunlight at 1 AU but number cm−2 s−1 sr−1 for the scattered light at 10 RS (∼perihelion for SPP). The way we employ our model is by using the EPI-Lo geometric factor for the apertures, which lie within a given elongation angle range we are modeling, and calculating the number of photons s−1 that hit the aperture foils, so the calculation is correspondingly conservative. We then run the filter model with a fraction of the full Sun at 1 AU determined such that we match that number of photons/s on the same aperture foils.
The foils employed as filters result in energy loss of ions entering the instrument before they reach the TOF section and subsequently the Stop foil and SSD. These energy losses and straggling affect the minimum energy and energy resolution at the lower end of the EPI-Lo energy range. However, based on TRIM and GEANT4 simulations, as well as experience with similar foils on previous instruments, the required foils will still permit EPI-Lo to provide its full, required measurements.
We calculated the transmission of visible light through Al and Pd. Aluminum is quite efficient at filtering visible light, but palladium is not, so we rely primarily on Al for filtering in the visible wavelength range with palladium included to filter short-wavelength EUV.
For UV transmission we have employed a filter model that we have maintained throughout the development of the Cassini INCA and IMAGE HENA, instruments and all of the “hockey pucks,” which incorporates the solar EUV spectrum from about 10 to 140 nm and uses Henke atomic scattering tables (http://henke.lbl.gov/optical_constants/asf.html). This model has been quite successful in modeling the expected responses of foil-based time-of-flight by energy (TOF × E) instruments to the UV environments of interplanetary space, Earth, Jupiter, and Saturn. Applying this model to the UV environment provided by the SPP team, the ISIS team determined that the predicted Start foil rates for an EPI-Lo quadrant from UV photo-electrons is ∼1×103 s−1. The predicted Stop rate is also ∼1×103 s−1. The predicted accidental TOF × E rate is ≪1.0 s−1 for the quadrant and, hence, not significant.
The foil materials are sufficiently thick that neither photoelectrons nor solar wind electrons (nor solar wind ions) will penetrate them. However, particles can enter through holes in the foils (either pre-existing, from launch, or produced by dust impacts in flight), so we also calculate and account for the susceptibility to backgrounds from photoelectrons and solar wind ions and electrons. The highest photoelectron intensities will be in the vicinity of the Sun-facing surface of the spacecraft TPS where the plasma sheath is predicted to have a thicknessFootnote 2 on the order of centimeters, therefore those electrons will not reach EPI-Lo (which is in the umbra, ∼3 meters from the TPS edge). EPI-Lo must be able to tolerate the photoelectron flux produced by the UV flux striking surfaces relatively close to the EPI-Lo apertures, and therefore umbral UV intensities. Those intensities are small (both in number flux and in energy) relative to solar wind electron intensities, therefore we consider only the latter.
Solar wind electron flux at perihelion is expected to rise to ∼2×1012 cm−2 s−1 (with a typical thermal energy of ∼100 eV; numbers provided by the SPP Project). Dividing by 2π sr, this amounts to about 3.2×1011 cm−2 s−1 sr−1. Solar wind electrons entering through pinholes in the EPI-Lo aperture foils at relatively low energy (≤100 eV) will behave like Start electrons inside the instrument; they will be focused onto the MCP areas that register Start events. These need to be limited to less than ∼1×106 s−1, preferably much less, but at this level the instrument will still function as designed; for example, the RBSPICE instruments on the Van Allen Probes mission routinely run with Start rates of 2 to 4×106 s−1 and still return well-behaved, calibrated TOF × E data.
The largest solid angles viewed by the Start foils are ∼0.2 sr, so those foils would see ∼6×1010 solar wind electrons cm−2 s−1. If we wish to limit the intensity to ∼106 s−1 for a quadrant, this requires that the total pinhole area through the 20 foils (of the quadrant, there being 10 apertures to the octant) be ∼2×10−5 cm2. The foil support grids are 180 lines/inch, or ∼70 lines/cm. This corresponds to a grid element width of 1/70 cm, or 143 μm. The grid wires are 12 μm in width, so a grid element has an area of ∼130×130 μm2, or about 1.7×10−4 cm2. This implies that with only one grid-element sized pinhole in any one of the 20 foils in a quadrant, we would already exceed our limit by about one order of magnitude (∼6×1010×1.7×10−4 cm2 ∼107 s−1).
In tests of high velocity dust impacting flight-like foils at the dust facility at the University of Colorado, only micron-sized holes were produced by the dust. Therefore, even though we do not expect a dust grain to be able to take out a whole grid element, during Phase B, we decided to take an even more conservative approach and use two foils in the collimators, separated by 0.5 cm minimum (effectively turning a single hole into an aperture-constrained “telescope”). Then the geometric factor for pinholes in both foils is Ω×1.7×10−4 cm2 where Ω is the solid angle defined by the two pinholes. This geometric factor is ∼6×10−8 cm−2 sr−1. Multiplying by 3.2×1011 cm−2 s−1 sr−1, we obtain a rate of ∼2×104 s−1. Even for one such pair of holes in every entrance of a quadrant, the total rate would amount to only ∼4.0×105 s−1, which more than meets our criterion.Footnote 3
It should be pointed out that the need for double foils for mitigation of solar wind electrons does not require both foils be of the same design. We only require the collimator foils to reduce the susceptibility to pinholes of the electron-blocking capacity. Therefore, our collimator foil is much simpler than the start foil, using a single layer (1000 Å of polyimide with 50 Å C flashing on the inward-facing side) sufficiently thick that pinholes on such a foil supported by a 180 line-per-inch grid become very unlikely.
The EPI-Lo electronics box contains all the electronics to run the instrument other than the energy and timing preamps, which are located in the sensor wedges. The box contains two octagonal boards mounted into metal frames. The boards stack one on top of the other, with an internal connector providing electrical interconnects between the boards. The functional breakup between the two boards minimizes the number of interconnects needed. See Fig. 24 for the block diagram.
The electronics are designed to handle solar energetic particle event intensities up to at least 106 particles cm−2 sr−1 s−1. This includes handling electron counts rates ≥70,000 counts per second and total ion count rates (SSD and MCP valid coincident event) of ≥70,000 counts per second. In both cases, these events can be either evenly distributed over the entire instrument or concentrated in one wedge. The term “handle” is used to mean that the incoming particles are processed in the instrument such that the particle types, directions, and rates can be determined. Note that ground software rate correction will be necessary when rates are sufficiently high (e.g. ≥106 starts per second or ≥40,000 total ion count rates per second). With the current mission and environment assumptions, we expect that rates will be sufficiently high only ∼5 % of the time to require such corrections.
An ion entering the sensor through one of the collimator apertures will deposit energy in the SSD and produce secondary electrons in the Start and Stop foils, which are amplified by the MCP and collected in distinct positions on the anode. This collection of time-correlated signals is called an event for the purpose of the description of the electronics that follows. The Event board directly processes the sensor SSD and anode preamp output signals, and contains all necessary analog and digital circuitry to process and store event information on an event-by-event basis. The energy signals from the eight SSD preamplifier sets are processed in parallel peak-detect/discriminator circuit/ADC chains. The MCP anode signals are processed via constant-fraction discriminators (CFDs) and time-to-digital conversion (TDC) circuitry; these measured time differences are converted into event look direction and particle velocity in the field-programmable gate array (FPGA). FPGA-based event logic also determines which signals comprise valid ion and electron events and coordinates all event hardware processing timing. A soft-core processor (i.e. a processor implemented in VHDL) is also embedded in the FPGA to provide all command, control, telemetry, and data processing functions of the instrument. SRAM, MRAM, and PROM memory storage is provided on the board to support the processor.
EPI-Lo uses a pulse-width technique to determine the energy deposited in each detector for energies above ∼1 MeV (Paschalidis 2008). This method, used on the Juno JEDI, the RBSPICE, and MMS EIS, allows the energy dynamic range to be extended from ∼1.5 MeV to a total energy ∼15–20 MeV. In order to cover fully iron composition with no gaps between EPI-Lo and EPI-Hi, the maximum energy measured will be extended to ∼85 MeV for iron (1.5 MeV/nuc for Fe nuclei). Three separate approaches to this extension of the iron energy range have been identified, and the preferred approach will be finalized and tested in the engineering model prior to instrument CDR. Each sensor uses an existing, flight-qualified, application-specific integrated circuit (ASIC) containing preamplifier/shaper circuits to amplify the SSD and APD (analog peak detect) signals, shape the pulse, and generate timing triggers on the rise and fall of each pulse. These signals feed into the EPI-Lo FPGA where the coincidence logic and other digital processing begin. The EPI-Lo FPGA-based processor accumulates events into rates, and packetizes the telemetry products.
The Power board contains both the low and high voltage power supplies. The low voltage portion takes spacecraft primary power on a single 9 pin connector and generates 1.5 V (for the FPGA core), 3.3 V (primarily for digital interface logic, memories, and TDCs), and 5 V (primarily for analog functions). A 15 V output powers the high voltage power supply (HVPS). The HVPS generates the necessary high voltage outputs for the sensor MCP and electron optics, with a maximum voltage output of 3300 V. It can independently control high voltage to each of the four quadrants. A bias supply provides up to 250 V for the SSDs.
The four anode boards are located directly under the MCPs and each board covers two octants (sensor wedges). Each anode board (a “quadrant”) has 20 start pick-up anodes configured as a delay line, and two stop pick-up anodes tied together. One pre-amplifier reads out each of the two sides of the delay line, and one pre-amplifier reads out the combined stop anodes. Locating the pre-amplifiers directly on the anode board reduces system noise.
Each of the eight SSD assemblies has an energy board that contains the pre-amplifiers and shapers for each of the electron, ion, and anti-coincidence detectors. The energy board is mounted directly to the back of the SSD to minimize noise. The electron and ion SSDs have pulse height analysis electronics on the event board while the anti-coincidence detector is monitored by a comparator with a programmable threshold to give a simple yes/no particle event result. The energy board also supports test inputs, both external and from the event board, and filtering for the bias voltage.
EMI/EMC Design Considerations
Of principal concern for EMC design are the power supplies. These are controlled to a frequency window centered at n×50 kHz with n≥3 and 500 ppm wide overall operating conditions and time. The LVPS is synchronized to 200 kHz by a 400 kHz clock provided by the digital boards. EPI-Lo has a 40 MHz oscillator and EPI-Hi has a 58.8 MHz oscillator; both evenly divide to 400 kHz. To minimize interference, transformers and large inductors are placed as far from the box walls as possible, and stable currents are employed to minimize changes in magnetic emissions. EPI-Lo does have nickel grids, and concerns associated with those are mitigated with careful handling, use of non-magnetic tools, and testing.
All electronic parts are selected for proven radiation tolerance: total ionizing dose (TID) >100 krad, no latch-up, and acceptable single-event upset (SEU) performance. Triple module redundancy (TMR) and error correction code (ECC) are used on vulnerable registers inside the FPGA. PROM-based boot code is used to ensure reliable memory loading and checking capabilities. Analog parts are selected with low dose effects in mind. Parts comply with Goddard Space Flight Center’s (GSFC’s) “EEE-INST-002” Level II requirements.
EPI-Lo uses a symmetric design to enable the wide field of view in a compact, low-mass package. Parts consist of the eight wedge assemblies and their closeouts, a top close out, the “spider” frame, which holds the wedges, and the event- and power-board slices, which comprise the components of the electronics box (Fig. 25). The common wedge design is shown in Fig. 26. Each contains an MCP assembly, an SSD assembly, and a collimator set. Alignment of all of the pieces enables the coverage and functionality of the detector overall. Preliminary design analysis has been done with a finite element model (FEM) using 89,077 nodes and 70,645 elements.
The preliminary structural analysis of the baseline design was performed using MSC Nastran, MAYA SATK®, and Femap for analysis. The model was simplified wherever possible to reduce solution time. Printed wiring assemblies (PWAs) were modeled as plate elements with uniform stiffness, thickness, and density, and the instrument model was oriented to the ISIS bracket configuration in relation to the spacecraft panels.
We performed a mechanical modal analysis to determine mechanical resonances. The analysis showed the first mode to be 304 Hz (for the Event PWA) and 553 Hz for the overall instrument. The analysis environmental input levels per spacecraft requirements were performed for all three orthogonal axes relative to the spacecraft panel. The 3-σ acceleration random response enveloped the static load requirement as desired for EPI-Lo displacements, stresses, and forces. The random-vibration PWA displacement response may be relatively high for electrical, electronic, and electromechanical (EEE) part solder/lead-wire fatigue resistance, and further analysis is planned after EEE parts placement is finalized. All margins of safety are positive for the current model configuration, and a detailed analysis will be carried out for the flight configuration to confirm that the flight design has positive margins and meets the minimum frequency requirement.
EPI-Lo software consists of instrument Common Software (reused from other projects), SPP Host Software, EPI-Lo Application Software, and EPI-Lo Boot Software. The Common software includes packet telemetry, command handling, macros (stored command sequences), memory management, monitoring and alarm generation, and status reporting. About 50 % of the application software and >90 % of the instrument boot code is reused.
EPI-Lo software identifies ion species and energies on-board by referencing look up tables stored in non-volatile memory that define carefully tailored regions (“boxes”) in the TOF vs. Energy parameter space. In the instrument’s ion-composition mode, each incoming ion event is associated with a 32-bit species and energy accumulation bin that will record the total number of events per integration interval within the defined box. The accumulation bins are arranged in packets and sent to the SSR. A selection of raw event data is also sent to the SSR, allowing more detailed compositional studies (but with decreased statistics due to downlink data volume constraints). We have used these techniques successfully many times on past missions (most recently, and with very similar software, on PEPSSI, RBSPICE, and JEDI).
In the spacecraft interface software, command and telemetry use the 115,200-baud Universal asynchronous receiver/transmitter (UART) protocol with 8 data bits plus odd parity. There is a redundant (side A and B) interface, for which the command arrival determines the active side; telemetry is sent only to that side. Dynamic side switching is supported. The system is interrupt driven: per-byte interrupt for command and telemetry, and side A and B 1PPS interrupts.
EPI-Lo measurements are intended to generate the information needed to derive differential intensity j in (cm2 sr s keV/nuc)−1. The goal of EPI-Lo characterization and calibration efforts is to develop the quantitative procedures for converting the count rates R (s−1) reported by EPI-Lo into estimates of j for the various defined ranges of energies, particle species, and arrival angles.
Calibration for a particle instrument like EPI-Lo means determining the following:
Transfer function from counts into flux (engineering units to physical units)
Correct rates for effects such as dead times and hardware or software saturation
Response to visible and ultraviolet light
Response to high-energy (out of band) particle backgrounds
Calibration will be done primarily using the APL particle accelerator (e.g., McNutt et al. 2008), a versatile system capable of producing a broad range of ion species at energies from 20 to 170 keV. The system includes an electron-impact ionization source, extraction gap, Einzel Lens and Wien filter mounted in the insulated terminal structure along with all associated power supplies. The system produces beams of H, He, O, and noble gas ions with intensities over the range of 102–106 particles cm−2 s−1 at the target position (∼mm2 to cm2). These tests will be supplemented with a variety of radioactive sources (e.g., 241Am) as stimuli.
In addition, calibration runs will be made using the accelerator at Goddard Space Flight Center. A proton beam will be used to scan both angles to complete the characterization of the transfer function. The sensor response to electrons will be characterized from ∼100 keV to 1 MeV, and heavy ions (He, O, and Ar) will also be used to characterize the instrument response. Calibrations of the response to both foreground and background ions and electrons at very high energies (up to ∼20 MeV) require additional specialized facilities. These include the accelerator at Lawrence Berkeley National Laboratory (Berkeley Lab) and an accelerator at Idaho National Laboratory (INL).
Calibration will begin on the prototype unit to validate the instrument design and performance. This activity will also be used to establish testing procedures for the flight unit. Most sensor-level calibration for EPI-Lo occurs sequentially, with subsystems sequenced through fixed calibration setups. Sensor wedges are integrated with the electronics, followed by instrument-level testing and pre-qualification calibration, which establishes a calibration baseline. The integrated EPI-Lo undergoes EMI/EMC, vibration, and thermal vacuum testing. Final calibration is performed to ensure no changes have occurred during environmental testing. In addition to ground calibration and pre- and post-environmental qualification, in-flight calibration is used to finalize knowledge of the full instrument response. For instance, data will be used to monitor MCP gain, and efficiencies can be tracked separately for each wedge by using relative Start, Stop, and SSD rates. There are also on-board pulsers to calibrate the SSD and anode electronics. Such in-flight calibration times will also naturally allow cross-calibration between EPI-Lo and EPI-Hi.
All EPI-Lo electronics parts have been used in recent flight instruments, are currently available, and have a minimum tolerance of >100 krad. The electronics box thickness meets that dose level with a factor of two margin. EPI-Lo contains detectors sensitive to certain contaminants, e.g. hydrocarbons; however, all detectors reside within cavities behind cover foils protected pre-launch by GN2 purge. EPI-Lo is located out of direct impact by thruster plumes; high voltage can be turned down for thruster firings if analysis indicates temporary pressure increases from such events.
Data Collection Strategy and Products
EPI-Lo delivers telemetry in Consultative Committee for Space Data Systems (CCSDS) packets using the CCSDS file Delivery Protocol (CFDP). Most data are losslessly compressed (using the FAST algorithm) prior to packetization; compression factors of >1.6 have been achieved with similar instruments on recent deep-space missions (e.g., MESSENGER and New Horizons). Sensor-event collection, command-and-telemetry processing, and instrument control functions are implemented by the instrument processor, an approach successfully used for the previously mentioned instruments now in operation or development.
The telemetry formats are flexible and adjustable by ground command. EPI-Lo has low average data rates fitting the nominal allocation suggested in the original SPP Announcement of Opportunity (NASA Science Mission Directorate 2014) and high data rates to allow for higher time resolution and additional event data, sometimes called pulse height analysis (PHA) data. EPI-Lo has ion counting rates for up to 64 energy bins, at least 9 species, and 80 angular “pixels” (one for each aperture). Event data include high-resolution TOF and energy PHA values. Energy-spectra data are collected and averaged from the selected detectors. High-resolution electron rates are also collected.
Data products conform to time, angle, and energy-resolution requirements that address the science objectives. Rate data for ions are generated nominally at 30 and 5 s time-resolution (time resolution can also be adjusted by ground command between 1 s and several hours). High-resolution energy spectra are sent every few minutes so that fast developing features can be followed. The high level of flexibility enables maximal use of telemetry allocation and permits adjusting the operating parameters in response to evolving flight experience if needed.