Development of an L-band continuous-wave buncher at Tsinghua University

Abstract

This paper presents the development of an L-band continuous-wave (CW) two-cell buncher at Tsinghua University for a very-high-frequency electron gun photoinjector. The dynamic simulations indicate the VHF injector generally employs 2-stage compression by the buncher and the first acceleration structure. The optimized voltage for the buncher is about 340 kV, with a shunt impedance of 7.93 MΩ and an RF power dissipation of 14.4 kW. Multipole components due to the waveguide coupler slot are reduced with a four-port design. Multiphysics simulations demonstrate that the maximum temperature rise is 35 °C and the rf heating induced frequency shift is -650 kHz. The resonant frequency, quality factor, and field balance are consistent with the design after tuning during the cold test. About 14 kW rf power has been successfully fed into the buncher during offline tests. After that, the buncher was mounted on the test beamline. The first phase of the beam commissioning has been completed. Unfortunately, the 1.3 GHz solid-state power source of the test beamline could only deliver 6.5 kW rf power, limiting the buncher to operate at 230 kV voltage. Nevertheless, the buncher operated stably during the beam commissioning, successfully compressing 50 pC bunches to 1.15 mm rms and 100 pC bunches to 1.44 mm rms, and maintaining good beam quality.