Dynamical aspects of multipacting induced discharge in a rectangular waveguide

Multipacting induced discharge in a rectangular waveguide is studied experimentally at a RF frequency of 500 MHz. The waveguide has a cross-section of 457 mm by 102  mm. The maximum forward RF power is 600 and 300 kW in traveling and standing wave mode, respectively. A continuous multipacting band structure is observed. Electron pick-up probes of antenna-type are used to measure the multipacting current and its longitudinal as well as horizontal distributions. The electron energy spectrum is measured with the retarding field method. The end-point energy of the spectra taken in traveling wave mode is in the range of 100-1000 eV and agrees well with the impact energy calculated by the classical multipacting theory. However, a large fraction of electrons has energies lower than 100 eV. Electron stimulated gas desorption is found to play a critical role in the dynamics of multipacting induced breakdown. It is concluded that the ionization discharge of desorbed gases is the immediate cause for RF breakdown.