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. Further improvements of this design are required to scale the average current up by a factor of 100. The design goal for this SBIR project is operation with 2 amperes average current. This kind of accelerator opens up a number of applications, including sterilization of wastewater sludge at throughputs not possible today. Phase I will develop the design for the gun, including gating of a larger thermionic cathode that can operate at the ampere level. In Phase II, the electron source will be built and tested in stages up to 2 A average current, 100 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. Key Words – high-current electron sources, high-power electron beams, radio frequency linear accelerators