WEDGE-SHAPED JET PLASMA FOR RAMAN COMPRESSION OF LASER PULSES

Two processes of shortening an intense laser pulse are discussed in a transparent plasma: self-compression at wake wave excitation (Balakin et. al, 2013) and at stimulated Raman backscattering (Malkin et. al, 1999). Studying the possibility of amplification and compression of ultrashort (up to several field periods) laser pulses in a plasma based on the process of stimulated Raman backscattering is an important task aimed at creating next-generation superpower laser systems that generate ultrashort petawatt and exawatt laser pulses. However, there is a number of negative physical processes that may limit the effectiveness of Raman amplification. Most of these negative processes have been studied and ways are suggested to neutralize them. Among the most dangerous is the nonlinear frequency shift near the threshold for the overturning of the plasma wave (Balakin et. al, 2018). The use of a highly inhomogeneous jet plasma gives a significant density gradient along the jet. Accordingly, it is possible to compensate an excessively large frequency modulation of the pump due to the use of density inhomogeneity along the gas jet. In this case, Raman compression occurs without a significant loss of energy efficiency. Using a nozzle for a gas jet in the form of a long slit allows one create a long and uniform plasma in one of the directions having a wedge shape. The possibility of obtaining a high-energy output signal using wide-aperture laser pulses in a wedge-shaped plasma is predicted. Optimal parameters of the gas jet and laser pulses are proposed to ensure high efficiency and focusability, close to the ideal case. This research was supported by the Russian Science Foundation (Project 17-72-20111).

Vlasov–Maxwell simulations of backward Raman amplification of seed pulses in plasmas

We study the problem of the amplification of an ultra-short seed pulse via stimulated Raman backscattering (SRB) from a long pump pulse (assumed to have an envelope with a constant amplitude), in an underdense plasma. The SRB interaction couples the pump light wave to a daughter light seed wave propagating in the opposite direction, scattered off an electron plasma wave. In recent numerical simulations, it has been observed that besides stimulated Raman backward scattering (SRBS) and stimulated Raman forward scattering, other high-frequency kinetic instabilities can occur when modified distribution functions exist during the evolution of the system. In particular, we showed the prominent role played by kinetic electrostatic electron nonlinear (KEEN) waves (Afeyan et al., 2004). We continue this work by applying a relativistic Vlasov–Maxwell code to study stimulated KEEN wave scattering (SKEENS) and its role in the SRBS short pulse amplification processes. An analysis of the full spectrum of waves participating in the amplification processes is presented. The absence of spurious noise in grid-based Vlasov codes allows us to follow the evolution of the system with a kinetic (collisionless) description. This affords us a glimpse at the intricate phase-space structures such as trapped particle orbits, which coexist and interact nonlinearly in the electron distribution function.