Conceptual design of the electron-optics of the magnetic electron recoil gamma-ray spectrometer.

In nuclear fusion reactors, deuterium and tritium are fused together. Most reactions produce a 4He ion and a 14 MeV neutron, while about 2 × 10-5 of them produce a 5He ion and a gamma photon. One of the most important measurements from a fusion reactor is the total fusion power, which can be measured using the 14 MeV neutrons or the gamma-rays. Using both provides two independent measurements, which is valuable for researchers and regulators. Facilities such as JET use scintillators such as LaBr3 to measure gamma-ray emission, but such detectors could struggle with background on high-power reactors such as SPARC and ITER. We propose that a magnetic electron recoil gamma-ray spectrometer (MERGS) can make gamma-ray measurements in these high-background environments. MERGS will consist of a foil that converts photons to electrons via Compton scattering, a magnetic electron-optic system that focuses and disperses the electron beam, and a detector array that counts the electrons and uses their positions to infer their energy. A conceptual design framework of MERGS is presented, along with three numerically optimized electron-optic geometries for three different resolution requirements. Simulations are presented showing that each presented design can meet its requirements. This shows that MERGS can measure the gamma-ray emission spectrum more effectively than scintillators for a variety of price points, enabling improved measurements of fusion power as well as the study of fast ion physics via other nuclear reactions.