Speaker
Description
Neutron diagnostics play a crucial role in fusion reactors by providing essential data for plasma control, machine protection, and nuclear safety. In ITER, neutron diagnostics are exposed to a wide spectrum of neutron energies, ranging from thermal to fast neutrons, and operate across a broad neutron flux range (10⁶ to 10¹⁴ n.cm⁻²s⁻¹). These detectors are critical for quantifying fusion plasma parameters, including neutron emissivity, D-T ion fueling ratio, total neutron yield, and fusion power.
Achieving high operational accuracy requires in-situ calibration using neutron generators with 2.45 MeV and 14.1 MeV neutron energies, as in DD and DT plasmas. The purpose of in-vessel calibration is to accurately determine detector calibration coefficients, particularly for the most sensitive detectors like the Divertor Neutron Flux Monitors (DNFM). This process involves linking experimental counts to physical plasma parameters while considering coefficients accounting for the plasma neutron source profile, machine integration, and detector response.
However, full in-situ calibration is challenging due to the size of the ITER tokamak, the limited yield of the neutron generator and the low sensitivity of systems designed for high-fusion power operation. Therefore, a comprehensive calibration strategy that incorporates high-detail neutronic simulations is necessary to minimize uncertainties and improve cross-calibration accuracy. This strategy includes sensitivity studies to address the effect of uncertainties in the geometrical factors of the tokamak environment and in the material nuclear properties. Then, by determining the corrective coefficients of the detector response, sensor readings are adjusted for geometric and background effects. These coefficients will be derived from a dedicated characterization experiment. But DNFM characterization is not straightforward, due to the detectors' location outside the Vacuum Vessel. Indeed, the moderated neutron spectrum is complex to reproduce in the laboratory, which prevent characterization in the same operational field. This requires a thorough understanding of the sensor response, from neutron interaction to signal formation, considering all relevant parameters, such as filling gas pressure and fissile deposit layer thickness, while also addressing operational constraints like magnetic fields, electromagnetic noise, and temperature.
This study focuses on different phases of the in-vessel calibration strategy, detailing the DNFM unit design, assessment of the impact of the tokamak environment and surrounding materials, characterization objectives, modeling scheme using GEANT4 Monte-Carlo code for fission fragment transport, and recommendations for experimental procedure.
Disclaimer: The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
