Here we report on experiments, in which broadband space-level electron and proton flux was produced with LPAs having peak laser powers in the P ~ 150 TW to PW range. NASA’s AE8/AP8 and AE9/AP9 models16 were used to calculate the typical electron and proton flux at different orbits in the van-Allen belts. As a showcase, the electron spectral flux at the important GPS satellite orbit in the outer van-Allen belt is given in Fig. 1(a).

Figure 1: Electron flux in the inner van Allen belt according to the NASA AE9 model at various orbits. In (a), the flux on GPS orbit is given via contour plots as a function of orbital time, and in (b) the maximum spectral flux on various orbits from LEO to HEO is plotted, demonstrating the broadband energy range up to E ~ 10 MeV. The GPS electron spectrum at maximum flux from (a) is shown in (b) with the orange plot, and is used as a showcase for the experiments. The flux axis is logarithmic, indicating that the shape can often be approximated by exponential distribution functions. Full size image

To reproduce this broadband van-Allen belt level electron spectral flux, a university lab scale Ti:Sapphire laser17 at a power level of P ~ 150 TW was used. The laser pulses were focused to spot sizes in the range of a few μm2 on thin metal foil targets, corresponding to interaction intensities of I ≈ 1018–1020 W cm−2. Such laser-overdense plasma interaction is one of the most effective and reliable methods to convert laser energy into broadband electron flux, and also into protons via the TNSA mechanism18. In this scenario, it is well known that the resulting energy E of the accelerated electrons can be approximated by an exponential distribution N = N 0 exp(−E/k B T), where N is the number of electrons, k B is the Boltzmann constant and T the electron temperature. Established scalings by Wilks19, Beg20 and Kluge21, refined by particle-in-cell-simulations, were used to predict the effective temperature T eff = k B T as a function of laser intensity on target. By adjusting the laser intensity to values of I ≈ few 1019 W cm−2, the exponential electron flux was tuned to match the electron spectrum as present in the van Allen belt, e.g. T eff ≈ 0.6 MeV on the GPS orbit. Figure 2 illustrates the experimental setup. The laser–foil interaction produces broadband particle radiation, which was monitored with state-of-the-art diagnostics and was used to irradiate various commercial and radiation-hardened optocouplers as devices under test (DUT), see Methods section.

Figure 2: Experimental setup with 150 TW Ti:sapphire laser. The laser-solid-interaction produces broadband, broad-angle particle radiation with electrons in the 1–10 MeV range; protons are eliminated by a thin protection foil directly in front of the DUTs (not shown here, see methods). The electrons irradiate optocouplers (located 5 cm away from the target foil) and are then detected on an image plate stack to retrieve the spatially resolved temperatures and divergence. The high-resolution shadow of the optocouplers on a front image plate is demonstrated on the right hand side; single optocoupler pins and detailed internal structure of the devices are clearly resolved. A central hole allowed on-axis electrons to enter a permanent magnet spectrometer where simultaneous measurement was obtained on an additional image plate, and a Lanex scintillating screen. Full size image

Figure 3 demonstrates the successful reproduction of GPS-level electron flux at a laser intensity I ≈ 4.5 × 1019 W cm−2, producing electron flux with T eff ≈ 0.65 MeV. The agreement is especially good at the medium energy range, which is particularly important, whereas the number of high energy electrons is low due to the exponential decrease, and the numerous low energy electrons on the other hand would be absorbed by the spacecraft shielding.

Figure 3: Calculated (black (AE8) and blue (AE9) lines), PIC-simulated (red) and experimentally obtained (green) electron flux for GPS orbit. By fine-tuning the laser-plasma-interaction at an intensity I ≈ 4.5 × 1019 W cm−2 at an incidence angle of 45°, the experimentally obtained electron spectral flux was tuned to match the space-borne van-Allen belt spectral flux. Full size image

A second set of campaigns at the VULCAN PW-laser aimed at the production of broadband space protons. At these much higher laser pulse powers, protons with energies up to E ≈ 20 MeV were generated and used to irradiate a further set of optocouplers, as shown in Fig. 4. Again, the measured spectra retrieved from radiochromic film stacks have exponential slope and are particularly useful to reproduce certain similar space spectral flux on various orbits ranging from LEO to the proton-rich inner van-Allen belt at 5 k km altitude and beyond.

Figure 4: Results of proton irradiation at VULCAN. (a) compares the retrieved proton flux (solid black line, left y-axis) with the broadband flux at LEO and at higher altitudes of 5 k and 10 k km predicted by the AP9 model (dashed lines, right y-axis), and (b) shows the raw radiography images generated by the proton flux on the proton-sensitive radiochromic film (RCF) stack. Full size image

The irradiation of the optocouplers by reproduction of either the outer van-Allen belt electron flux or the inner van Allen belt proton flux has led to significant degradation of optocoupler performance, making use of state-of-the-art testing procedures adapted from the European Space Agency. This is shown in Fig. 5, where the current transfer ratio (CTR) of Vishay SFH6345 optocouplers after applying broadband electron and proton fluences of up to ≈7 × 109 e-/cm2, and ≈5.3 × 1010 p-/cm2, respectively, shows degradation of up to 4% when compared to unirradiated reference optocouplers.