One-Dimensional Particle-Field Simulation Code for Single-Extractor Klystrons Optimization

Output efficiency of a klystron is strongly determined by the beam bunching quality, and the size optimize of the extractor. In this work, an automated optimization framework for single-extractor S-band klystrons is developed using the original one-dimensional particle-field interaction code. The Particle Swarm Optimization algorithm maximizes output power by tuning cavity gap width, external quality factor, resonant frequency, and position, while energy-conservation and divergence-coefficient criteria ensure physically valid solutions. Simulations show that under small beam currents, optimized output efficiency (similar to 70%) consistent with 3D particle-in-cell (PIC) simulation results. For kA current operation, the framework provides reliable operational ranges, with PIC simulations yielding similar to 50% efficiency. The method efficiently guides cavity design, offering a robust and physically consistent tool for further multi-cavity optimization of klystrons.