Liquid Metal Targets for High Power Electron Beams

The main focus of photonuclear physics has always been investigation of the internal spatial and spin structure of the nuclei and nucleons. While high energy photons (over 100 MeV) are used most often to probe the nuclear structure and look deep into the nucleus, intermediate energy (1-40 MeV) photonuclear reactions can also provide significant information via nuclear resonance fluorescence and spectroscopy, nuclear isomer production, nucleosynthesis, and other experiments. Efficient generators of high density photon flux are needed to support this important photonuclear physics research. One of the ways to produce a high flux photon environment is from a high power electron linac and a bremsstrahlung converter, which transforms the kinetic energy of the electron beam into photons. Conventional converters are made of solid, high Z metals with a high melting point, such as W or Ta. These solid converters cannot operate at high power (above 1 kW) without complicated cooling methods thus limiting photon flux density. A continuous-wave (CW) superconducting electron linac opens up the potential for machines with very high density photon flux, but requires a bremsstrahlung converter which can withstand the higher power of the electron beam (~ 100 kW). A liquid metal converter would remove concerns regarding possible converter melting. The liquid metal will simultaneously serve as converter and a coolant for the system. Niowave has resolved the first challenge by building reasonably priced superconducting electron accelerators to tackle Americas high-tech challenges in many fields. However, the lack of converters capable of withstanding high-power electron beam remains unsolved. We propose to develop a liquid metal target, capable of dissipating hundreds of kilowatts of electron beam power and providing very high photon flux density, over 1017 photons/cm2 per second for 40 MeV, 2.5 mA electron beam. In addition to the scientific uses for nuclear physics, this liquid metal converter would enable new commercial applications such as production of medical radioisotopes and bulk x-ray sterilization machines. We have completed preliminary testing of a prototype Pb-Bi eutectic (LBE) converter to a kilowatt of beam power. We propose to design of a much more complex high power liquid metal converter which can withstand up to 100 kW in a superconducting continuous wave (CW) linac such as being designed and built by Niowave.