Experimental Demonstration of a Miniaturized S-Band High-Power High-Efficiency Multibeam Klystron

To meet the demands for miniaturization, lightweight, and fast transient response operation of S-band 3-MW multibeam klystrons (MBKs) in medical accelerator applications such as FLASH radiotherapy, a novel high-efficiency S-band 3-MW MBK is proposed in this article. The design employs a frequency-tuned gain cavity technique in the RF circuit to extend bandwidth, along with an ultracompact periodic reverse permanent magnet (PRPM) focusing system and locally trenched beam tunnels to enhance beam transmission. Theoretical analysis indicates that, at 2.998 GHz, the MBK achieves a pulsed output power of 3.7 MW with an RF conversion efficiency of 55.9%. The 3-dB power bandwidth under constant RF input drive exceeds 50 MHz, while the overall dimensions are as compact as0.2× 0.75m. A prototype of the proposed S-band MBK has been fabricated and experimentally verified. Hot-test results at 2.998 GHz show a DC beam transmission of 88% and a dynamic beam transmission of 82%. The measured pulsed output power reaches 3.7 MW, and the 3-dB bandwidth under constant RF drive is 45 MHz, which is about two times that of existing peers. In addition, the tube weight, including the focusing system, is about 45 kg. The experimental results agree well with theoretical predictions under comparable transmission conditions. These studies demonstrate the significant potential of the proposed compact, lightweight, and broadband S-band 3-MW MBK for future accelerator applications.