UC Berkeley

This thesis presents the development of a THz-driven electron gun and associated beam characterization assembly. The THz gun is a 2 cell electroformed copper structure and uses a copper tip field emission cathode. The cells operate in the pi mode at 110.081 GHz and are designed to be powered by a 110 GHz gyrotron oscillator. For 500 kW of input power, the gun is expected to produce bunches of electrons accelerated to a peak energy of 365 keV. Electromagnetic simulations predict the fields on the surface of the cathode will reach 3.9 GV/m for this input power, resulting in 51 fC bunches. The beam is accelerated over a distance of 1.6 mm in the two cells.

The gun structure and characterization assembly are designed to operate over a range of input powers beyond the nominal operating point of 500 kW. The electron beam characterization assembly consists of a focusing solenoid, a dipole-based energy spectrometer, an on-axis microchannel plate detector, and a Faraday cup. The setup is designed to measure the energy spread, beam size, and current to characterize the gun performance. Each element in the assembly was custom designed for this setup and the expected energy range of the beam. The expected performance is based on modeling of the electromagnetic performance of the cells and 3D particle simulations of the beam dynamics and transport.

Fabricated gun cells were cold tested and an extensive study of cell tuning was performed. Several copper tip structures were successfully tuned using mechanical compression. This is the first demonstration of the mechanical tuning of individual W-band accelerator cavities. The effects of plating and etching were also studied. Further electromagnetic and particle modeling was performed to characterize the expected performance of the fabricated and tuned structures. One set of tuned cells was incorporated into the full assembly and cold tested under vacuum. The cell resonances occur at their expected frequencies and the performance is preserved after cycling between air and vacuum.