Observation of large Larmor radius instability in laser plasma expanding into a 10 T external magnetic field

Abstract

Flute instability produced by laser plasma expanding in a 10 T external magnetic field was studied in experiments. The plasma was generated by a 0.3 J ns laser ablating an aluminum target. The external magnetic field of approximately 10 T was provided by a pair of Helmholtz coils aligned parallel to the target surface. Initially, the plasma plume expands freely. The external magnetic field confines the plasma plume and, finally, forms a plasma cavity with a sharp plasma-field interface. Flute instability was observed at the plasma-field interface, which presents a salient kinetic feature rather than classical fluid instability. In the initial linear phase, the growth rate of the perturbation has good agreement with Large Larmor radius instability, which is larger than ion gyrofrequency. In the later nonlinear growth phase, the flute instability shows an obvious "fishbone" structure of kinetic instability, and the initial short wavelength perturbation shifts continually to longer wavelength mode and, finally, close to the density scale length. Our experiment reveals a new region of parameter space that reproduces the flute instability in the space experiments of an active magnetospheric particle tracer experiment and a combined release and radiation effects satellite.