Electrical and Computer Engineering ETDs

Ehsan Vadiee

Publication Date

9-3-2013

Abstract

In this thesis the measurements and analyses of high power electromagnetic wave interaction with a laser-induced plasma (LIP) have been investigated. The laser-induced breakdown processes in air generated by focusing a 600 mJ, 7 ns pulselength, 1064 nm wavelength Nd:YAG laser onto a 30 μm spot size at an atmospheric pressure of 631 Torr are examined. The breakdown threshold electric field is measured and compared with classical and quantum theoretical ionization models at this short wavelength. A universal scaling analysis of these results allows one to predict some aspects of high power microwave (HPM) breakdown based on measured laser breakdown observations. The 1064 nm laser-induced effective field intensities for air breakdown data calculated based on the collisional cascade and multiphoton breakdown theories are used to compare with the experimental results. Experimental diagnostics of the laser deposition are interpreted using the compressible spatially adaptive radiation magnetohydrodynamic particle-in-cell code MAGIC invoking the ponderomotive approximation to describe the laser—plasma interaction. The 2-D MATLAB simulations of the temporal and spatial evolution of the LIP are also in agreement with the theoretical results. An X-band relativistic backward wave oscillator (RBWO) at the Pulsed Power, Beams, and Microwaves Laboratory was constructed at the University of New Mexico (UNM) and is used as the HPM source. The RBWO produces a microwave pulse of maximum power 400 MW, frequency of 10.1 GHz, and energy of 6.8 Joules. Special care was given to synchronize the RBWO and the pulsed laser system in order to achieve a high degree of spatial and temporal overlap. Different detectors are used to observe the overlap- a photodiode (PD), microphone, microwave detector, and others. Shadowgraphy with a nanosecond time resolution will be used to detect changes in the shockwave fronts when the HPM signal overlaps the laser pulse in time and space. Detailed experimental setup and experimental results are presented. 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 the 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 presented.

Document Type

Thesis

Language

English

Degree Name

Electrical Engineering

Level of Degree

Masters

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Prasad, Sarita

Second Committee Member

Christodoulou, Christos

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