  High peak and average power free-electron lasers (FELs) in the terahertz
region (THz) require small diameter, low-emittance, and high voltage electron
beams. This paper presents two 1.5-2 MV, 100-200 A, thermionic cathode electron
source approaches for compact megawatt range peak power, multi-kilowatt average
power, high repetition rate, THz FELs. The preferred beam generation system
includes grading electrodes and is quite compact compared to the other standard
diode gun approaches. Both provide highly compressed beams at the waist having
low values of normalized rms emittance. In particular, the new injector
approach with grading electrodes, operating at a body voltage of 1.5-2 MV and
100 A, has a normalized rms emittance of roughly 10 mm-mrad at the beam waist.
Power supply switching considerations are considered in the paper as well as
considerations for very high-voltage multi-stage depressed collectors for
device efficiency enhancement. Based on these designs, we provide performance
estimates for FELs operating in the THz spectral region.
