Multipactor in a Waveguide Iris

Microwave discharge in vacuum in a waveguide iris is analyzed under the approximation of a uniform electric field over the finite width of the iris. The analysis shows that the random electron drift due to the initial velocity of the secondary electrons leads to a significantly increased multipactor threshold as compared to the result predicted by the (infinitely wide) parallel plate configuration. This increase is determined by the value of the iris height-to-length ratio. For each fixed value of this ratio, the multipactor susceptibility charts can be generated in the traditional engineering units. An increase in this ratio results in a similar shrinkage of each of the multipactor resonance zones. Within each particular zone, the effect is more pronounced for the lower RF frequencies due to the longer transit time between the walls of the iris