Conditioning technique for high power RF vacuum transmission line components using multipactor plasma

Multipactor is a low power, electron multiplication based resonance breakdown phenomenon in vacuum often observed in radio frequency (RF) and microwave systems. A multipactor discharge is often undesirable as it can create a reactive component that detunes the resonant cavities, generates noise in communication system and induces gas desorption from the conductor surfaces. Multipactor breakdown on dielectric surface is also a major concern for failure of vacuum window in klystrons, cyclotrons and accelerators. Despite of these, the multipactor discharge is not absolutely undesirable. Its usefulness is being explored in electron gun technology, plasma display technology, ICRH antenna conditioning etc. Since multipactor is a pure electron resonance phenomenon, it can happen without any gas being present in the system. Nevertheless, the massive electron bombardment on conductor surfaces removes substantial amount of adsorbed gas species and increases the neutral pressure. In the presence of neutrals these resonant electrons ionize the gaseous atoms and forms the plasma known as multipactor plasma. Many of the high power RF components used in the vacuum transmission line interface sections of ICRH system are having restricted access to surface condition them especially the inner conductors of transmission line sections and dielectric material of vacuum window. By suitably choosing the operating frequency and a minimum pressure, it is possible to form a multipactor plasma in the above components to condition them. Such a conditioning technique is adopted for conditioning of vacuum window and vacuum transmission line of ICRH system. In this paper, an overview of multipactor plasma, a brief description of the test set up, testing conditions and conditioning results are presented.