Assessing the two plasmon decay instability in ignition-scale hohlraums

In recent experiments Sean Regan, et. al. for the first time observed the 2ωpe instability from window plasma in hohlraum targets. This instability can also operate at peak power near the edge of the inner beams in the ablator plasma and near the edge of the outer beams in the liner plasma. Fortunately, only a small fraction of the laser energy was estimated to be at risk. A more quantitative assessment of the energy at risk at peak power and its sensitivity to variations in target design and to details of the instability threshold model will here be given. They also explore how strong collisionality restricts this instability in the Au wall plasma. They show that the instability threshold can be significantly reduced for laser beams with an angle of incidence of about 60 degrees due to the swelling of the laser field near its turning point. A simple model is given. It is also shown that for frequently cited plasma conditions, the SRS-scattered light wave can itself drive the 2ωpe instability. This effect is relevant for the nonlinear saturation of SRS and the resulting heated electron generation. Some estimates are given. Finally, several important issues concerning the high-energy electron distributions due to the 2ωpe instability and other laser plasma processes are discussed.