Backbombardment-Free Thermionic Cathode Electron Gun for Ampere Average Currents

One key advantage of superconducting electron accelerators is their ability to operate at high duty cycle and therefore at very high average beam powers. In high-current operation, small transverse emittances are needed to keep the beam losses low, particularly the losses to cryogenically-cooled surfaces. To fulfill these requirements, a new electron source capable of extremely high current and brightness needs to be developed. An ampere-class source would be capable of driving MW-power-level, compact, and efficient accelerators, such as those needed for waste treatment by electron beam irradiation. For dense waste streams, high power and energy of several MeV are required for throughput and penetration. As brightness increases, these electron sources can also drive accelerators for extremely high power free- electron lasers and high power beam-driven RF sources. Niowave currently operates a normal conducting RF electron gun with a thermionic cathode at 20 mA average current. Electron emission from this 350 MHz gun is gated via a novel combination of both DC and second harmonic biasing of the cathode. Unlike many conventional implementations, the gating design does not intercept any part of the electron beam, and as such allows for a higher brightness beam at extremely high currents. This project adapts that technology for use with a high-voltage transformer from the power distribution industry, greatly increasing the energy efficiency of the beam acceleration. In turn, the optics of the design have been revamped to create a clean, bright beam compatible with very low-loss injection into a superconducting accelerator. These improvements of this design are required to scale the current up by a factor of 100, to 2 amperes. This kind of accelerator opens up a number of applications, including sterilization of wastewater sludge at throughputs not possible today. In Phase II, the electron source will be built and tested in stages up to 2 A current with a duty factor greater than 20% (>50 kW average power). Phase III would be funded by an R&D program to create a MW electron accelerator comprising the new electron source and a 1-5 MV superconducting cryomodule. At this point, the system would be marketed commercially to wastewater treatment facilities.