Speaker
Description
The generation of electron beams with high average and peak powers by an intense laser pulse is largely associated with the engineering of a plasma target, the parameters of
which determine both the efficiency of the particle acceleration process and the ability to rapidly resume upon the arrival of the next impulse. This work is aimed at creating a bright laser-plasma source of high-energy electrons and neutrons for the above applications based on the interaction of intense laser radiation with the plasma of a gas jet shaped by shock waves of additional laser pulses.
Experiments were conducted to accelerate electrons in hydrogen gas jet using a 1.5 TW femtosecond laser (50 fs, 75 mJ). In the absence of a shock wave, exponential spectra with energies up to 20–25 MeV were obtained. With the addition of a shock wave (5 mJ pulse, 4 ns lead time), a quasi-monoenergetic beam was formed with an energy of 30–60 MeV, a spectral width of 5 MeV, a divergence of 0.5°. The beam charge is determined by beam loading and varies from 20 pC for a 60 MeV beam to 60 pC at 30 MeV. Energy and charge control is achieved by varying the position of the shock wave. A neutron yield of 8x10^5 neutrons per shot was obtained on a lead secondary target.
3D PIC modeling (SMILEI) was performed, confirming the experiment. In experiments and PIC modeling, three stages of interaction were identified for a hydrogen jet: the formation of a plasma channel with self-focusing and self-modulation, the transition to a plasma bubble regime with increasing laser amplitude (a₀=2–3), and the self-injection of electrons accelerated to ~30 MeV with the formation of a quasi-monoenergetic spectrum. However, the high plasma density (nₑ=0.02–0.03n_cr) reduces the phase velocity of the wave to 0.98c, causing rapid dephasing, energy loss, and an increase in the transverse spread of electrons. This problem is solved by interrupting acceleration by modulating the density profile with a shock wave. This causes rephasing, allowing electron energies to increase to 60 MeV with a charge of 20 pC. By choosing a different position of the shock front that interrupts acceleration, it is possible to obtain beams with a charge up to 80 pC, but with an energy of 25–30 MeV. The decrease in energy is due to the beam loading effect.