Modeling of high power klystrons using TESLA

Summary form only given. TESLA (Telegraphist´s Equations Solution for Linear beam Amplifiers) is a highly efficient code designed to simulate linear beam vacuum electronic devices with cavities, such as klystrons, extended interaction klystrons, twystrons, and coupled cavity amplifiers. The model includes a self-consistent, nonlinear solution of the three-dimensional electron equations of motion and the solution of time-dependent field equations. The model differs from the conventional Particle in Cell approach in that the field spectrum is assumed to consist of a carrier frequency and its harmonics with slowly varying envelopes. Also, fields in the external cavities are modeled with circuit like equations and couple to fields in the beam region through boundary conditions on the beam tunnel wall. TESLA has been verified by comparisons with the PIC code MAGIC and validated by comparison with measured performance of high power klystron. While the simulations of high gain, high power klystrons show good agreement with the results of measurement, they also point out difficulties in the operation of TESLA. The main numerical problem in the simulation of high gain klystrons is the artificially low decay rate of cavity fields when the resonant frequency of the cavity is far from the carrier frequency. To eliminate this problem we have reengineered the discrete time advance algorithm so that all eigenfrequencies of the discrete system match those of the corresponding continuous time equations. The modified version of TESLA has better performance than the old version because steady state solutions can now be reached using larger time steps.