Experimental Investigations of Shock Surface Perturbations in Laser Produced Plasmas(3/3)

Collisionless shocks are ubiquitous in various phenomena in astrophysical, space and evenlaboratory plasmas. It is widely believed that energetic particles in the universe or cosmic rays areaccelerated at collisionless shocks. At the very initial stage of the particle acceleration or theso-called injection process at collisionless shocks, some particles have to come back to the shockupstream, and thus, the particle reflection at the shock surface is essential in the injection process.The reflected particles can cause a number of instabilities and reformation of shock, resulting inshock surface perturbations. Although a number of analytic and numerical studies show the shocksurface perturbations and relevant particle acceleration and reflections, it is hard to observe this inspace and astrophysical plasmas. In this project we experimentally investigate the shock surfaceperturbations in laser-produced plasmas and observe the reflected or back streaming particles. Wefocus on the shock Mach number and the magnetic field orientation to the shock surface, which canbe controlled with the laser energy and target configurations with permanent magnets. We observethe shock images with optical diagnostics and local plasma quantities with Thomson scattering. Theenergy distribution functions of the reflected particles are measured with electron-ion spectrometers.We clarify the dependence of the shock surface perturbations on the shock Mach number and thefield orientation, and the relation between the shock surface perturbations and the particleacceleration.