Experimental investigation of electron multipactor discharges at very high frequency

Abstract

Multipactor discharges are a resonant condition in which electrons impact a surface in phase with an alternating electric field. The discharge is sustained by electron multiplication from secondary emission. As motivation, multipactor discharges can adversely affect many different rf systems in vacuum, and this work provides an improved general understanding of multipactor and gives promising results for improved performance and reliability of these systems. The Coaxial Multipactor Experiment (CMX) creates and investigates multipactor discharges in both parallel plate and coaxial geometries at very high frequency (VHF). CMX provides the first detailed investigation of multipactor energy distribution functions for both coaxial and parallel plate geometries with the use of retarding potential analyzers (RPA). A 1-D particle tracking simulation supports these experimental distributions and yields the underlying physics behind the distribution shape. Experimental and simulation energy distributions have a low energy population of defocused electrons due to space charge effects and RPA emission, and a high energy population responsible for sustaining the discharge. Results show a higher energy distribution for the coaxial geometry as compared to the parallel plate geometry with the same electrode spacing, implying that coaxial geometries are more susceptible to multipactor. These results are supported by CMX susceptibility data, which are provided for both coaxial and parallel plate electrodes. Lastly, similar multipactor experiments were performed on Alcator C-Mod rf systems, allowing the discovery of multipactor-induced glow discharge in these systems.

(cont.) Results suggest the onset of this glow discharge causes the observed C-Mod neutral pressure limits. These results are further supported by CMX experiments, and a new, 50 pim sandblasted copper surface treatment has been shown to sufficiently lower 6 < 1 for multipactor prevention on CMX and raise the minimum pressure for glow discharge breakdown. This surface treatment shows no significant degradation of high voltage handling, and it is proposed for implementation on the multipactor-susceptible regions of C-Mod rf systems.