High-Voltage Pulse Modulator for S-Band Klystron in IR-FEL RF System

A high-power S-band klystron based RF system has been developed at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, to meet the RF power requirements of the infrared free-electron laser (IR-FEL) facility. The modulator employs a line-type topology comprising a high-voltage pulse-forming network (PFN), charging power supply, a thyratron switch, a pulse transformer (PT), a programmable logic controller (PLC) based interlock system, and an S-band klystron. This article presents a comprehensive mathematical analysis of the deQing-based resonant charging system and compares it with conventional analysis. The results indicate that conventional analysis tends to overestimate pulse-to-pulse PFN voltage regulation. A mathematical expression for determining the inductance of a tunable PFN inductor is derived and validated by experimental measurements. Additionally, this article compares air-insulated and oil-immersed modulators, providing key design insights to aid in selecting the optimal configuration. The design details of other essential modulator components are also presented. During initial commissioning, the system achieved a peak RF power of 15 MW with a pulsewidth of $12~\mu $ s at a repetition rate of 5 Hz.