Bret B. Kreh

Date of Award


Document Type


Degree Name

Master of Science


Department of Engineering Physics

First Advisor

William Bailey, PhD


This work investigates the role that the beam-plasma instability may play in a thermionic converter. The traditional assumption of collisionally dominated relaxation is questioned, and the beam-plasma instability is proposed as a possible dominant relaxation mechanism. Theory is developed to describe the beam-plasma instability in the cold-plasma approximation, and the theory is tested with two common Particle-in- Cell (PIC) simulation codes. The theory is first confirmed using an unbounded plasma PIC simulation employing periodic boundary conditions, ES1. The theoretically predicted growth rates are on the order of the plasma frequencies, and ESl simulations verity these predictions within the order of 1%. For typical conditions encountered in thermionic converters, the resulting growth period is on the order of 7x10-11 seconds. The bounded plasma simulation PDP was used to evaluate the influence of finite geometry and the electrode boundaries. For this bounded plasma, a two-stream interaction was supported and resulted in nearly complete thermalization in approximately 5x10-10 seconds. Since the electron-electron cornsion rate of 109 Hz and the electron atom collision rate of 107 Hz are significantly slower than the rate of development of these instabilities, the instabilities appear to be an important relaxation mechanism.

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