Progress in laser-triggered high power microwave plasma interaction

Abstract

Experimental, theoretical, and simulation results of the propagation characteristics of a high power microwave pulse in an unbounded atmospheric-pressure laser-initiated plasma are described. Due to resonant and collisional absorption, the high power microwave pulse with relativistic parameter v = eEem/mωc has been demonstrated experimentally to permeate and be absorbed inside a non-uniform laser plasma despite having low frequency (ω<;<;ωp). When the overcritical laserinduced breakdown of air is irradiated with a high power microwave pulse, further ionization can be achieved due to the resonant excitation of a plasma surface wave by the evanescent component of the incident electromagnetic wave [1]. For low density plasmas, the electromagnetic waves lead to avalanche ionization from elastic electron-neutral collisions. In this regard, to enhance the breakdown region, a high-power Q-switched Nd:YAG laser at wavelength 1064 nm and energy per pulse of 600 mJ is focused onto a volume in air. An Xband relativistic backward wave oscillator (RBWO) at the Pulsed Power, Beams and Microwaves Laboratory at the University of New Mexico is used as the high power microwave source. The RBWO produces a microwave pulse with a TM01 radiation pattern that can be converted to a TE11 mode and has maximum power of 400 MW, and frequency about 10 GHz. A complete theoretical investigation using the impedance transformation method including scattering, partial reflections, and collisional absorption based on the transfer of energy from the wave to plasma and generation of fast electrons, is presented. A discussion on the effects of various unmagnetized plasma parameters on the transmitted, absorbed, and reflected power is calculated. Experimental diagnostic results of the laser deposition are interpreted using the compressible spatially adaptive radiation magnetohydrodynamic particle-in-cell code, Flash, invoking the ponderomotive approximation to describe the laser-plasma interaction. Features of the code, such as ray tracing and gas medium ionization, allows for greater confidence in data interpretation. 2-D simulations of the temporal and spatial evolution of the laser-induced plasma are in agreement with the theoretical results. Experimental results to-date will be provided as well.