Vlasov-Maxwell simulation of simultaneous stimulated Raman forward and backward scattering

In stimulated Raman scattering (SRS), an electromagnetic pump wave decays into a scattered wave and a plasma wave. In long nearly uniform plasmas, the electrostatic fields may reach large amplitudes and thus trap and accelerate electrons to high energies. In laser fusion, even a small number of energetic electrons can cause severe preheating and thus prevent the efficient compression of the fuel capsule. In magnetic fusion, the beat wave process or SRS in the microwave region can be applied to current drive in tokamaks. In current drive, collisionless fast electrons are beneficial. Both in Raman forward (SRS-F) and backward scattering (SRS-B); the electron plasma waves travel forwards but their phase velocities can differ considerably. In most laser-plasma experiments, SRS-B dominates over SRS-F because of its larger gain. In well-underdense high-temperature plasmas, however, the Landau damping strongly limits SRS-B, which allows the growth of SRS-F. In some cases, solely SRS-F is excited. The parameters corresponding to reactor-grade laser-plasma experiments and to SRS current drive in fusion reactors lie in an intermediate region where both processes can occur simultaneously. The coupled mode theory predicts that in a homogeneous plasma slab, SRS-B and SRS-F, and correspondingly the respective plasma waves, are localized in spatially distinct regions: SRS-B in the front and SRS-F in the rear part of the plasma slab.