Ion velocity distribution measurements in a magnetized laser plasma expansion

Abstract

Langmuir probes are used to study the expansion of a laser plasma into a large (17m long × 0.6m diameter) ambient magnetized cylindrical plasma. The expansion is either perpendicular or at 45° to the 300-600 G axial background field. One probe geometry allows data collection close to the ablation surface, inside a diamagnetic bubble formed during the laser plasma expansion. We measure expansion velocities of this diamagnetic cavity and the bulk laser plasma. Additionally, we detect fast ions along the axis of the ambient plasma column when the laser plasma expansion is directed at 45° to the background field. We obtain the fast ion velocity distribution by comparing the measured ion gyroradii to those predicted by Monte Carlo simulations of the ion trajectories in an external magnetic field. Experimental measurement of fast ion velocity distributions can help tune the experimental parameters that are required to drive collisionless shock waves through a laboratory plasma.