Optical Science and Engineering ETDs

Publication Date

Summer 8-1-2022

Abstract

Results presented here examine the effect of changing gas pressure on the radio frequency (RF) emissions of an ultrashort pulse laser filament plasma and how those emissions vary longitudinally in the laser focal region. We use a WR284 rectangular waveguide with a 1.5 cm hole that allows the beam through. A 3.2 GHz microwave signal is emitted in the waveguide, and signals are received through a waveguide-to-coax antenna connected to an HP8470B Schottky diode. By enabling and disabling the 3.2 GHz signal, we measure both the self-emitted RF from a USPL filament and subsequently the degree of attenuation a filament has on the 3.2 GHz injected signal. After subtracting the self-emission RF from the attenuation signal, we obtain a corrected attenuation of the injected microwave signal by a USPL filament. We use this method at regular intervals along the laser propagation direction to obtain a longitudinal distribution of filament RF properties. This RF diagnostic is used to determine filament size resulting in a length from 5.84 cm to 11.9 cm depending on gas pressure. We also characterize the plasma decay by measuring the recovery time of the signal attenuation. A series of glass vacuum tubes is used with a 1.2 cm diameter section encompassing the focusing region so measurements can be obtained along the plasma while air pressure is varied from atmosphere to 0.01 Torr. These measurements are combined with an optical inspection of plasma fluorescence. Wide-field images allow us to optically measure the filament length to compare with results gathered during the microwave scattering measurements. Optical filament length measurements differ at all gas pressures tested with an average result 9.6 cm shorter than when measured electrically. Images taken with a magnifying lens assembly allow us to identify and measure individual filaments along the plasma resulting in an average filament diameter of 121 µm. Combining the filament length and diameter measurements allow us to estimate a single filament's volume. The signal attenuation results coupled with the filament volume measurements allow us to estimate filament conductivity for a wide range of gas pressures.

Degree Name

Optical Science and Engineering

Level of Degree

Masters

Department Name

Optical Science and Engineering

First Committee Member (Chair)

Jean-Claude Diels

Second Committee Member

Andreas Schmitt-Sody

Third Committee Member

Wolfgang Rudolph

Keywords

Ultrashort, USPL, filamentation, plasma, femtosecond

Sponsors

Air Force Office of Scientific Research

Document Type

Thesis

Language

English

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