Design and development status of the ITER Radial Gamma Ray Spectrometer

The ITER Radial Gamma Ray Spectrometer (RGRS) is an ITER diagnostic located in the Equatorial Port 01 undergoing its Preliminary Design Review and foreseen for Phase DT1 (2041). RGRS is expected to measure the density profile and energy distribution of α-particles through reactions with 10B, the current and maximum energy of runaway electrons through bremsstrahlung emissions, and fusion power via a radiative channel of the DT reaction. The diagnostic employs LaBr3 scintillators coupled with PMTs along 4 radial lines of sight, with LiH attenuators to reduce the background due to direct neutrons and a heavily hydrogenated and borated concrete-like mixture for neutron shielding. Performance assessments indicate RGRS can fulfil its functions regarding runaway electrons, while the feasibility of measuring α-particles is uncertain due to the intense gamma-ray background observed at JET. Further study is needed to confirm this measurement possibility. Additionally, while fusion power measurements appear possible, satisfying ITER requirements necessitates detailed knowledge of the gamma-ray-to-neutron branching-ratio of the DT reaction, which will be investigated through dedicated experiments, and possibly gamma-ray attenuators to reduce background.