Multipactor in High Power Radio-Frequency Systems for Nuclear Fusion

TL;DR

This paper reviews multipactor discharges in RF systems used in magnetic fusion experiments, highlighting their effects, challenges, and potential beneficial uses in vacuum conditioning.

Contribution

It provides a comprehensive overview of multipactor phenomena in high-power RF systems for nuclear fusion, including practical implications and research insights.

Findings

Multipactor discharges can damage RF components and limit power transmission.

Discharges are influenced by magnetic fields, frequency, and system geometry.

In some cases, multipactor is used intentionally for vacuum conditioning.

Abstract

Magnetic confinement fusion researches are an approach to demonstrate the feasibility of nuclear fusion power for energy production. In these experiments, mega-watt range power Radio-Frequency waves, from tens of MHz to hundreds of GHz, are injected into magnetically confined plasmas in order to increase its temperature and to extend its duration. These RF systems are subjected to the magnetic field environment of the experiments, ranging from few tenth of Tesla to Tesla, and various orientations depending of their locations. As these RF systems made of copper, silver or ceramics are located in vacuum environments, they are subject to multipactor discharges. These discharges are generally considered detrimental since they can lead to detuned RF systems, limit the RF power transmission in the plasma and eventually damage RF sources or components. In some case, especially in the MHz range of frequency, multipactor discharges can be desired for vacuum conditioning. This paper presents the various RF systems used in magnetic fusion experiments and review the work performed in the fusion research community on multipactor discharges for high power coaxial (MHz) and rectangular waveguides (GHz) transmission lines, with their practical implications on power delivery into the plasma.