Laboratory experiments on Alfven waves caused by rapidly expanding plasmas and their relationship to space phenomena

There are many situations which naturally occur in space (coronal mass ejections, supernovas) or are man-made (upper atmospheric detonations) in which a dense plasma expands into a background magnetized plasma that can support Alfven waves. The Large Plasma Device (LAPD) at UCLA is a machine in which Alfven wave propagation in homogeneous and inhomogeneous plasmas has been studied. A new class of experiments which involve the expansion of a dense (initially n(laser-produced)/n(background) much greater than 1) laser-produced plasma into an ambient highly magnetized plasma capable of supporting Alfven waves will be presented. The 150 MW laser is pulsed at the same 1 Hz repetition rate as the plasma in a highly reproducible experiment. The laser beam impacts a solid target such that the initial plasma burst is directed across the ambient magnetic field. The interaction results in the production of intense shear and compressional Alfven waves, as well as large density perturbations. The waves propagate away from the target and are observed to become plasma column resonances. The magnetic fields of the waves are measured with a 3-axis inductive probe. Spatial patterns of the magnetic fields associated with the waves and density perturbations are acquired at over 10,000 spatial locations and as a function of time. Measurements are used to estimate the coupling efficiency of the laser energy and kinetic energy of the dense plasma into wave energy. The shear wave generation mechanism is due to field-aligned return currents, which replace fast electrons escaping the initial blast.