Bow shock formation in a plasma flowing across randomized laser beams

High energy lasers interacting with flowing plasmas can produce a plasma response that leads to beam bending and, by momentum conservation, to slowing down of the plasma flow velocity [1]. For the incoming plasma flow, with a velocity slightly greater than sound speed, the plasma response to a ponderomotive force exerted by speckled laser beams is the strongest, such that slowing down of the flow leads to the formation of a shock. We present hydrodynamic simulations of plasma flow about speckled laser beams that demonstrate shock formation. Linearized theory of the plasma penetration length across a laser beam that is necessary to achieve subsonic flow velocity is confirmed in simulations. A cumulative effect of many speckles produces shock propagating first across the laser beam that emerges next from the speckled laser beam and freely propagates upstream with the velocity and jumps of the flow velocity and density that satisfy Rankine-Hugoniot relations. Perturbations of plasma parameters associated with the shocks are on the order of tens of the percent of their background values. We have repeated the simulations and the analysis for the randomized beams with the temporal incoherence using RPP and SSD corresponding to NIF parameters. For SSD we have also examined the influence of the inherent speckle motion on the ponderomotive force and the shock formation. We have established scaling laws accounting for the SSD parameters.  Results of the simulations and theoretical analysis are also discussed in the context of the planned NIF experiments.

[1] H.A. Rose, Phys. Plasmas 3, 1709 (1996).