The transition of a multipactor to a low-pressure gas discharge

Multipactor discharges can cause severe problems in high voltage rf systems like rf antennae or transmission lines of ion cyclotron resonance heating (ICRH) in nuclear fusion devices, where they may initiate gas breakdown. To study this eventual transition from a starting multipactor into an ordinary rf discharge detailed investigations were performed using a parallel plate geometry, 50 MHz operating frequency and up to one kilovolt rf amplitude. Measurements of electric data (rf amplitude, absorbed and reflected power, discharge current), electronic parameters (electron current density and energy distribution) and light emission were used for characterization. As a main result a new type of well defined discharge regime was identified, which seems important for the transition from multipactor into gas breakdown in rf devices.