Design and modeling of multistage depressed collectors by using 3D conformal finite-difference time-domain particle-in-cell simulations

The feasibility of designing and modeling a multistage depressed collector (MDC) by using 3D conformal finite-difference time-domain (CFDTD) particle-in-cell (PIC) simulations as implemented in 3D PIC code, VORPAL has been studied. A simple circuit model employing feedback mechanisms has been implemented to provide stable time-dependent voltages for each stage of the depressed collector, and an arbitrary space-time dependent spent-beam distribution is given in our timedomain simulations. We demonstrate the design of a five-stage depressed collector recovering a triangular-like spent-beam distribution imported from a large signal simulation, achieving an energy recovery efficiency of 75%. The MDC after optimization can be integrated into the limited space of an S-band traveling wave tube for a compact microwave power module (MPM).