Measurement of the dispersion relation of plasma-loaded slow wave structure

Abstract

Summary form only given, as follows. Recent experiments with the plasma-loaded, 8.5 GHz, relativistic backward wave oscillator (BWO) showed an increase in the microwave interaction efficiency up to 40% and the possibility of operation at beam currents beyond the vacuum limit. Our goal is to analyze and optimize interaction between the electron beam and electromagnetic fields in plasma-loaded periodic slow wave structures filled with plasma. In this work, we measured electromagnetic dispersion characteristic of a plasma-loaded corrugated slow wave structure. A hydrogen flashover gun generated a plasma column which was guided by a magnetic field (2-15 kG) and filled a periodic slow wave structure. Since the structure is of finite length, resonances occur only for discrete values of the wavenumbers. Introduction of plasma into the slow wave structure was expected to cause upward frequency shifts of the resonances. The frequency upshifts associated with the TM/sub 01/ mode in the periodic slow wave structure were measured as a function of the background plasma density by a single port (S/sub 11/) method. In order to determine the background plasma density the same technique was used for a smooth wall cavity. We measured frequency shifts on the order of 0.1 GHz around a center frequency of 8.5 GHz for the plasma density of about 10/sup 11/ cm/sup -3/. The obtained resonance frequency upshifts in the plasma-loaded slow wave structure showed good agreement with theoretical calculations.