Performance of St707 getter material in a rep-rated high power microwave sealed-tube vircator under UHV conditions

Abstract

This study focuses on the use of St707 non-evaporable getter (NEG) material in a high power microwave (HPM) sealed-tube virtual cathode oscillator (vircator) operated at 500 Hz repetition rate. High-current pulsed operation releases gases trapped within the bulk materials and gas monolayers on material surfaces, leading to localized plasma production in the A-K gap. This can lead to gap closure, shorten the duration of microwave emission, and spoil vacuum. A single current pulse increases the chamber pressure to the low 10−6 Torr range from an initial background pressure in the low 10−9 Torr range, desorbing approximately 1014 particles. At 500 Hz operation, a sufficiently large pumping speed (∼2,500 L/s) is necessary to evacuate desorbed particles from the vircator volume during consecutive shots. Previous work has identified hydrogen (H, H 2 ) as the main outgassing species during vircator operation, with contributions from CH 4 , N 2 , CO, CO 2 , and Ar as the other primary gas constituents[1]. The St707 NEG pumps shows an affinity for pumping hydrogen, making it a suitable choice to adsorb vircator outgassing species, of which hydrogen is an order of magnitude greater than any other gas species. Previously, without the presence of NEG material, degradation of microwave output power from the vircator has been observed during subsequent shots at 1 Hz, 10 second burst mode operation. Subsequently, an increase in chamber pressure from 1.25×10−6 Torr on the first shot to 10−5 Torr on the tenth shot has been experimentally observed. This paper details the performance of the St707 NEG material (70% Zr, 24.6% V, 5.4% Fe) for maintaining UHV conditions during rep-rated vircator operation. Pumping characteristics of multiple St707 NEG pumps in the presence of rep-rated high-current pulses are presented. Diagnostic results obtained with a residual gas analyzer to observe individual gas constituents and two inverted magnetron cold cathode gauges for absolute pressure are utilized to analyze vircator and getter performance in detail.