1 Introduction

In different irradiation scenarios, the interaction of intense laser pulses with matter results in the emission of various types of radiation, including electrons [1,2,3,4,5], ions [6,7,8,9,10], x-rays [11, 12], and positrons [13,14,15]. These high-energy particles may be employed to generate secondary radiation types, and specifically neutrons [16,17,18,19,20,21,22,23,24,25].

One method for laser generation of neutrons relies on primary beams of electrons generated by the direct laser acceleration (DLA) method [26]. In DLA, the electron beam is formed in a positively charged plasma channel that was partially vacated of electrons by the ponderomotive pressure exerted by the laser pulse [27]. Strong quasistatic azimuthal magnetic fields are generated through the driving of longitudinal electron currents by the intense laser pulse. These quasistatic fields mitigate the longitudinal dephasing between the electrons and the laser beam that would have otherwise taken place in vacuum [28], enabling the electrons to gain energy. Efficient DLA occurs in plasma targets whose density is nearing its critical value. Such densities may be obtained from solid foils pre-exploded by either precursor light native to the laser system [20, 26, 29, 30] or by a second laser pulse timed to precede the main pulse on nanosecond timescales [31,32,33].

For generating neutrons, a secondary metal “converter” target is placed downstream to the electron beam. Bremsstrahlung radiation, produced by the electron beam stopping in the target, induced photonuclear reactions in the sample resulting in neutron emission [20].

A high overall neutron dose can be obtained by repeated irradiation of a high number of targets. Tape-drive-based target systems have been used for laser acceleration of ions [34,35,36], electrons [37], and for generating X-ray [38], EUV [39], and THz [40] radiation. However, this technology poses stringent mechanical requirements that limit the choice of target material. The tape should be flexible enough to be spooled in a compact retainer and not too rigid to be suspended in a flat form and hold the mechanical stress without tearing. Typically, these targets are formed from copper, titanium, or polymer films with a thickness range of 5–25 \(\mu \)m.

Here, we present repeated generation of photoneutrons, using an automated target system that delivers ultrathin foils to the laser focus. An auxiliary prepulse turns the foils into a plasma plume target in which DLA of electrons is conducted by the main pulse. Neutrons are generated by these electrons in a high-Z converter target.

2 Experimental setup

We performed the experiments using the NePTUN 20 TW laser system at Tel Aviv University, which is described in details in Ref. [41]. We focused 25-fs long laser pulses, with energy of 300 mJ (on target), onto 800-nm-thick Au layers that were deposited over a 200-nm-thick SiN membranes [42]. We measured 70% of the laser energy within a circle of 5.7 \(\mu \)m diameter, resulting in a normalized laser intensity of \(a_0=3.9\). Controlled prepulses were generated from the residual energy of the front-end pump laser. These \(\lambda \)= 532 nm / E = 1.9 mJ / \(\tau \) = 43 ps pulses were optically synchronized with the main pulse. Before focusing, the prepulses were spatially filtered and collimated. 70% of their energy was contained within a circle of 8.5 \(\mu \)m diameter. A multiplane delay line was utilized to achieve a relative delay in the range of 0–90 ns between the pre- and main pulses [43]. Further details of the experimental setup are given in Ref. [44]. A detailed study of the plasma plume expansion is given in Ref. [45].

2.1 Target delivery system

We developed an automatic system that delivers up to 2000 thin-foil targets to the laser focus at an average rate of 0.1 Hz. A technical drawing and a photograph of the system are shown in Fig. 1a and b, respectively. The targets are mounted on 4 \(\times \) 26 \(\times \) 0.25 mm\(^3\) Si chips (photograph in Fig. 1c). Each chip supports 10 800-nm-thick free-standing Au membranes, which are photolithographed as 500 \(\mu \)m \(\times \) 500 \(\mu \)m squares. The microelectromechanical fabrication process is the same as the one described in Ref. [46]. The operation mechanism somehow resembles that of a rifle magazine. Hundred chips are mounted one behind the other, where they are pushed toward the laser irradiation plane by a linear actuator-backed piston. The first chip in the stack is pushed by another linear actuator sideways into a machined channel that is \(\sim \)10 \(\mu \)m wider than the chip, so that for each laser shot a new target is translated into the irradiation aperture. Once all ten targets on the chip are shot, the chip is translated out of the target holder and the actuator returns to its original position to allow a new chip to enter the channel.

Fig. 1
figure 1

Target positioning system. a Technical rendering of the target system, with a single chip positioned inside the channel. b Photograph of the experimental chamber, overlaid with an layout of the laser beam. c Photograph of a single chip supporting 10 500 \(\times \) 500 \(\mu \)m\(^2\) 800-nm-thick Au foil targets

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We characterized the system’s position stability along the laser axis using a displacement sensor (Philtec muDMS-RC32). Eighty targets were brought sequentially to the irradiation aperture at an average rate of 0.1 Hz. Figure 2a presents the measurement trace, which features a spread of 2.7 \(\mu m\) (RMS). Such variability is well-suited for conducting DLA of electrons, since in those experiment, the foils are turned into 100 \(\mu m\) scale plasma plumes that serve as the targets.

Fig. 2
figure 2

a Displacement measurements for 80 consecutive thin-foil targets which were positioned automatically at the laser’s focal point. The series if found to have a variability of 2.7 \(\mu m\) (RMS). b In situ microscope images of 153 target foils before and after each of them was shot in series. The timestamp of each image is indicated on top. The position of the first target in the series is indicated with a red square. c Zoomed-in images of a few example shots from (b)

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2.2 Direct laser acceleration of electrons

We used the target system to repeatedly produce MeV-level beams of electrons by DLA. The system brought a sequence of 153 thin-foil targets to the focus of the laser, at an average rate of 0.1 Hz. The double-pulse irradiation scheme was conducted with the experimental procedure described in Ref. [33]. The prepulse energy was set to be 1.9 mJ, and the prepulse to main-pulse delay was 45 ns. Figure 2b shows images of targets before and after they were shot, with a few example zoomed-in photographs in (c). The images were taken using a back-reflection telescope, consisting of the off-axis parabolic focusing mirror and a 15-cm focal length lens providing a magnification of about \(\times 1\). The red square on each image indicates the position of the first target in the series.

We measured the electron spectra generated in 27 laser consecutive shots, using a magnetic spectrometer described in Ref. [33]. The results are shown in Fig. 3 in solid blue, where the shaded region indicates one standard deviation. A fit to a Maxwell–Boltzmann distribution is shown with a dashed black line, from which the falloff of the spectrum is characterized with a fitted temperature value of 2.0 ± 0.13 MeV.

Fig. 3
figure 3

Average electron spectrum obtained from 27 consecutive laser shots is shown in solid, with one standard deviation indicated by the shaded area. A fit to a Maxwell–Boltzmann distribution is shown with a dotted line. A calculated Bremsstrahlung spectrum that those electron beams would generate in a thick \(^{238}\)U converter is shown by a dashed line. Overlaid is the photoneutron production cross section on \(^{238}\)U (right axis) calculated using Eq. 4

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3 Neutron generation

To demonstrate repeated photoneutron generation, we placed a 1-cm-thick sample of \(^{238}\)U 8-cm downstream to the targets. Upon stopping in the sample, the electrons produce Bremsstrahlung radiation, which induced photonuclear reactions leading to neutron emission [20]. A calculated Bremsstrahlung spectrum which corresponds to these measured electron beams (see derivation in the next section) is plotted in a dashed line in Fig. 3. The photoneutron production cross section for \(^{238}\)U, overlaid in the figure, partially overlaps with the produced Bremsstrahlung. We measured the integrated neutron flux using five BTI Bubble dosimeters [47] positioned 26-cm downstream to the \(^{238}\)U converter outside the vacuum vessel. The neutron flux was determined using the same procedure described in Refs. [20, 48]. A series of 153 laser shots that were taken over a span of 24 min resulted in a total of \(2.6 \times 10^7\) measured neutrons, corresponding to an average of \(1.7\times 10^{5}\) per shot.

4 Scalability

Using these measured yields and the results of past PW-level experiments, we validated an analytical estimation for how the photoneutron yields scale with the laser intensity. In these calculations, the temperature of the generated electron beams is assumed to scale as \(T_e\sim a_0\) [27]. We recently validated this assumption experimentally in the range of \(a_0\) = 2.6-\(-\)4.4 [44]. We also assumed that a constant fraction of the laser energy is converted into the electron beam. For a beam of electrons of a given temperature, we followed the method of Ref. [49] to estimate the generated number of neutrons.

The Bremsstrahlung cross section is calculated using the simple analytical model of Findlay et. al [50], given in the form of

$$\begin{aligned} \frac{\partial \sigma _{\gamma }}{\partial E_{\gamma }}= \alpha Z^2 \left( \frac{1}{E_\gamma } - \frac{\beta }{E_e} \right) \end{aligned}$$
(1)

where \(E_{\gamma }\) and \(E_e\) are the photon and electron energy, respectively, Z is the atomic number of the converter material, \(\alpha =0.011 b\), and \(\beta =0.83\) are constants with tabulated values in Ref. [51].

We calculated the Bremsstrahlung photon emission of electrons stopping in a thick converter by integrating the emission of thin layers of thickness \(\Delta l\), using Eq. 1. The number of generated photons of energy \(E_{\gamma }\) in the \(i_{th}\) layer is obtained by integrating \(\frac{\partial \sigma _{\gamma }}{\partial E_{\gamma }}\) for electron energies exceeding \(E_{\gamma }\)

$$\begin{aligned} \frac{dN_{\gamma , i}}{dE_{\gamma }}(E_{\gamma }) = n_a \Delta l \intop _{E_e=E_{\gamma }}^{\infty } \frac{\partial \sigma _{\gamma }}{\partial E_{\gamma }} \frac{dN_{e,i}}{dE_e} dE_e, \end{aligned}$$
(2)

where \(n_a\) is the converter’s material atomic density. The number of photoneutrons generated in the i-th layer is then

$$\begin{aligned} N_{n,i}= n_a \intop _{E_{\gamma ,th}}^{\infty } \sigma _{n} \frac{dN_{\gamma ,i}}{dE_{\gamma }} l_{at} dE_{\gamma }, \end{aligned}$$
(3)

where \(\sigma _n\) is the giant dipole resonance (GDR) nuclear cross section, and \(E_{\gamma ,th}\) is the energy threshold for photonuclear reactions, which is about 8 MeV for most materials [52]. At the energy range around where the GDR peaks, the attenuation length of the high-energy photons \(l_{at}\) is rather constant [53], and so its measured value at E\(_{\gamma }\) = 10 MeV is used in the calculation. \(\sigma _n\) is obtained from parameterized empirical data [54]

$$\begin{aligned} \sigma _n = \sigma _0 \frac{ E_{\gamma }^2 \Gamma _R^2 }{ (E_{\gamma }^2-E_R)^2 +\Gamma _R^2E_{\gamma }^2 }, \end{aligned}$$
(4)

where \(\sigma _0\) = 2.5A mb, A is the mass number, \(E_R = 40.3/A^{0.2}\) MeV, and \(\Gamma _R\) = 0.3\(E_R\). The total number of neutrons is obtained numerically by summing the contribution of each layer \(N_n=\Sigma _i N_{n,i}\), where the distribution of \(\frac{dN_{e,i}}{dE_e}\) is updated after each layer according to the stopping power of the material for electrons [55]. We note that in this calculation, electro-neutron production which is suppressed relatively to the photoneutron channel [56] is neglected. Positron generation [57] and photoelectric absorption [57, 58] are also neglected here.

For this work, we chose three converter materials that are frequently used: \(^{238}\)U for its high neutron generation cross sections, \(^{65}\)Cu for the relatively short lifetimes of its decay products, and \(^{186}\)W, for its relatively high heat conductivity, a quality important for high current applications. The results of this calculation are presented in Fig. 4. The number of neutrons generated per shot (solid line) and the number of photons with energies exceeding the GDR threshold (dashed line) are plotted vs. the normalized laser intensity. Overlaid is the scaling of the electron temperature described above, used in this calculation. The measured neutron yields (per shot) of this work, as well as those of Ref. [20], are indicated on the figure.

Fig. 4
figure 4

Calculated yields for photoneutron (solid lines) and Bremsstrahlung photons (dashed line) as a function of laser strength parameter. The graph displays three different converter target materials: \(^{238}\)U, \(^{65}\)Cu, and \(^{186}\)W. Overlaid is the estimated electron beam temperature (right axis). Two experimental data points, from this work and from Ref. [20], are indicated with squares

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5 Conclusions

We presented a system capable of automatically delivering thin-foil targets to the focus of a high-intensity laser. Its positioning accuracy in the laser’s propagation direction of 2.7 \(\mu \)m (RMS) makes it suitable for irradiation scenarios of a longer Rayleigh range, or for cases when the foil is first turned into a plasma plume target of larger scales, such as for DLA of electrons. Two hundred Si chips supporting ten foil targets each may be loaded into the system.

We demonstrated using the system to generate photoneutrons by double-pulse irradiation of Au foils. A mJ-level prepulse was used to turn each foil into a 100-\(\mu \)m scale plume of plasma that served as a target for DLA of electrons by the main pulse. This demonstration ended after 153 shots when two chips were fed into the irradiation channel together, which is known as a “Type 3 malfunction” for the case of firearms [59]. In our future runs, this limitation will be resolved by manufacturing chips with a smaller thickness variability.

The average shot rate of 0.1 Hz was limited by electronic communication resets of the linear actuators with their controllers, as a result of the strong electromagnetic pulse generated by the return currents to the irradiated target holder [60, 61]. We previously shown how this problem may be mitigated using a programmable controlled electronic relay that disconnects the stages at the time of the shots [46].

We presented a method for evaluating how these yields scale with the laser intensity. For a current-day state-of-the-art systems at the intensity frontier, such as the 10 PW @1/min repetition rate system in ELI-NP [62], we project the production of \(10^{13}\) neutrons/hr.