Dissertation Defense by Mr. Benjamin Oliker on Examination of Ionization in Cesium Diode Pumped Alkali Lasers with an Ion Chamber Diagnostic

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OSA Capitol Hill Visit

Posted: March 21, 2022

Examination of Ionization in Cesium Diode Pumped Alkali Lasers with an Ion Chamber Diagnostic

Date / Time: 
Monday, 28 MAR 2022, 9:30 am

PAIS, Room 3205

Wolfgang Rudolph (Chair / UNM)
Timothy Madden (AFRL),
Greg Pitz (AFRL)
Paul Schwoebel (UNM)
Payman Zarkesh-Ha (UNM)


Diode pumped alkali lasers (DPALs) are leading candidates for future high-power applications, with many potential utilities for the military, aerospace, communications, and scientific diagnostics. A critical step in their development is measurement and understanding of unwanted ionization processes that occur inside the laser, which decrease efficiency, reduce the usable alkali population, and increase heat load. In this dissertation, direct measurement of the ionization rate of a cesium DPAL gain medium are made for the first time, via application of an ion chamber diagnostic. Results will demonstrate that the rate of ionization is slow compared to pump absorption, with a maximum of less than one ionization event for every one million pump photons absorbed (<1 ppm). The slow ionization rate will be shown to be accurately predicted to within an order-of-magnitude via computer simulation based on known ionization processes. Measurements of fluorescence rates indicate that a collisional neutral-particle process, which populates highly excited Rydberg energy states, such as secondary energy pooling, warrants further investigation.

Ion chambers will be shown to be effective diagnostics for DPAL ionization rate measurement. Plasma production rates in the experiment were orders-of-magnitude above typical conditions, so additional research was done to ensure the accuracy of the diagnostic in this high plasma density regime. Standard ion chamber operation is at plasma densities sufficiently low that space-charge effects are negligible. However, at high density, it will be shown that the following processes occur: (1) space-charge limited ion drift, (2) Debye shielding preventing the electric field from penetrating a bulk plasma region, and (3) ambipolar diffusion across the bulk with possibly elevated electron temperature. Although a variety of processes occur only in a high-density ion chamber, none of these processes prevent accurate measurement of ionization rate under DPAL conditions.