A KINETIC MODEL OF MULTIPACTION FOR SRF CAVITIES FOR ACCELERATOR DRIVEN SUB-CRITICAL SYSTEM (ADSS)*

This work simulates multipaction in a 700 MHz elliptical SRF cavity. The cavity design is optimized using SUPERFISH. Then the electromagnetic field is re-computed with FEMLAB, a package that uses the finite element method, to obtain a more accurate field-mapping, and to make the field values available for computation of multipaction. In the multipacting subroutine, electrons are assumed to be released into the system from various points with different initial parameters. The electron trajectories are tracked until they hit the cavity surface. Leap-frog scheme is used to solve the Lorentz force equation for primary electrons, as it is easy to use and is accurate up to the second order. The position, velocity, phase and kinetic energy of primary electrons at each time step are calculated and stored. An interpolation function is used to calculate secondary emission yield (SEY) at different impact energies. With the emission of secondary electrons, their trajectories too are tracked along with primary electrons, in order to identify parameters responsible for multipaction. By repeating this process for large number of electrons, the multipacting trajectories and field levels are identified