Simulation of spurious oscillations in high power klystrons

Summary form only given, as follows. Spurious oscillations can seriously limit a klystron´s performance from reaching its design specifications. These are modes with frequencies different from the drive frequency, and have been found to be localized in various regions of the tube. If left unsuppressed, such oscillations can be driven to large amplitudes by the beam, causing the main output signal to suffer from amplitude and phase instabilities that may lead to pulse shortening or reduction in power generation efficiency. The oscillation condition is determined by two factors: the circuit losses (to wall dissipation and external coupling through apertures), and the beam loading due to the mode. We will describe efficient methods to evaluate the external and beam-loaded Q´s of the circuit, using the electromagnetic and particle-in-cell (PIC) modules of the MAFIA code. The simulations constitute a numerical procedure by which a suspicious mode can be tested for possibility of oscillation. Results are presented for spurious modes observed during the testing of the first 150 MW S-band klystron, which was designed and built at the Stanford Linear Accelerator Center (SLAC) as a part of an international collaboration with Deutsches Elektronen Synchrotron (DESY).