Saturation mechanisms for multipactors

Abstract

Summary form only given. The single-particle theory of electron multipactor is now well understood. The multipactor parameter space is spanned by the normalized magnetic and electric field parameters. In particular, for given normalized magnetic field /spl Omega/ multipactor can errupt within a certain range of electric field amplitudes /spl epsi//sub 1/</spl epsi/</spl epsi//sub 2/; to each value /spl epsi/ corresponds a different locked-phase /spl phi/. Once a multipactor has been launched, it is very important to know the saturation level; the latter determines the cavity power losses when the multipactor is a parasitic side effect, or the saturated current when a multipactor is deliberately triggered for the production of short period high current pulsed beams. The collective space charge effects must be included to address multipactor saturation. One proposed approach employs a single charge sheet and yields saturation when the secondary emission coefficient /spl delta/ equals one. Setting /spl delta/=1 fixes the impact parameters and therefore the value of the driving electric field /spl epsi/; the external RF field adjusts to that value through the nonlinear effect of the induced multipactor current on the cavity LC circuit. A limitation of the method is that one must start near the appropriate /spl epsi/ since the range of the voltage self-adjustment is usually small. A most general approach introduced here is based on multilayered space charge cloud involving electrons emitted at slightly different times. There is a continuous distribution of impact energies and interaction phases between the leading and the trailing edge of the bunch.