Generation of shear Alfven waves by expanding plasmas

Examples of one plasma expanding into another and the consequent radiation of wave energy are abundant in both nature and the laboratory. This work is an experimental study of the expansion of a dense laser-produced plasma (initially, n_lpp/n0 >> 1) into a magnetized background plasma (n₀ = 2 × 1012 cm −3) capable of supporting Alfven waves. The experiments are carried out on the upgraded Large Plasma Device (LAPD) at UCLA. It has been observed that the presence of a background plasma allows laser-plasma charge separation to occur that would otherwise be limited by large ambipolar fields. This charge separation results in the creation of current structures which radiate a variety of waves including shear Alfven waves. Shear waves propagate away from the target and are observed to become plasma column resonances. Conditions for increased current amplitude and wave coupling are investigated. Spatial patterns of the wave magnetic fields are measured and are used to estimate the coupling efficiency of the laser energy and kinetic energy of the dense plasma into shear wave energy. Additionally, the laser-plasma diamagnetism is investigated with and without a background plasma.