A compact X-band backward traveling-wave accelerating structure

Very high-energy electrons (VHEEs) are potential candidates for FLASH radiotherapy for deep-seated tumors. We proposed a compact VHEE facility based on an X-band high-gradient high-power technique. In this study, we investigated and realized the first X-band backward traveling-wave (BTW) accelerating structure as the buncher for a VHEE facility. A method for calculating the parameters of single cell from the field distribution was introduced to simplify the design of the BTW structure. Time-domain circuit equations were applied to calculate the transient beam parameters of the buncher in the unsteady state. A prototype of the BTW structure with a thermionic cathode-diode electron gun was designed, fabricated, and tested at high power at the Tsinghua X-band high-power test stand. The structure successfully operated with 5-MW microwave pulses from the pulse compressor and outputted electron bunches with an energy of 8 MeV and a pulsed current of 108 mA.