First evaluation of the runaway electron measurement capability of the radial gamma-ray spectrometers diagnostic at ITER

Tokamak disruptions and the resulting multi-megampere beams of relativistic runaway electrons (REs) present severe machine-protection risks for ITER. While shattered pellet injection serves as the primary line of defense for mitigation, a robust diagnostic system is mandatory to evaluate the RE current and energy. This paper provides the first assessment of the RE measurement capabilities of the radial gamma-ray spectrometers (RGRS) in ITER 15 MA deuterium–tritium (DT) scenarios. Under steady-state conditions, the combined three-detector RGRS system is sensitive to RE currents as low as IRE,min ≈ 60 kA for neon densities of ≈1020m−3, which is within the requirement to make measurements every 10 ms and for currents down to 5% of the plasma current, i.e., 750 kA for a 15 MA full-power DT scenario, as well as within the desired safety limit of 150 kA. However, when accounting for the highly dynamic, millisecond-scale growth of the RE beam in realistic DT disruption scenarios, the practical detection threshold shifts into the 500 kA–4 MA range, with most scenarios having a threshold between 1 and 2 MA. We identify the post-disruption background due to the residual activation of the first wall as the primary modeling uncertainty, which, if negligible, could lower IRE,min detected by the RGRS system by up to a factor of ≈10, as expected for early DT experiments, when tritium will be injected in ITER for the first time. We also show that a single, tangential detector with a design similar to those of the RGRS could further lower the sensitivity to IRE,min by a factor of ≈60 using the current assumption about the background.