Calculations to Support In Situ Neutron Yield Calibrations at the Joint European Torus

The fusion power output of fusion plasmas is measured using the neutron yield detectors due to its linear relation to the fusion yield. Absolutely calibrated neutron yield detectors are thus a crucial part of the plasma diagnostics system and the absolute accuracy of their calibration must be ensured.

The transition of the Joint European Torus’s (JET’s) first wall material from carbon (C) wall to ITER-like (Be/W/C) first wall was a significant change in the structure of the machine and recalibration of the main neutron yield detectors was needed to maintain the required measurement uncertainty of less than ±10%. The neutron yield detectors were thus recalibrated through two in situ calibrations to deuterium-deuterium neutrons in 2013 and deuterium-tritium neutrons in 2017 using ²⁵²Cf spontaneous fission source and a compact neutron generator, respectively.

We describe the extensive neutronics calculations performed in support of these latest calibration experiments. These analyses were performed using Monte Carlo simulations to better understand the calibration procedure, optimize the experiments, ensure personnel safety, and quantify the effects of the uncharacteristic circumstances during calibration experiments. This paper focuses on assessments of the effects of the uncharacteristic circumstances, e.g., the presence of the remote handling system in the machine due to its use in neutron source delivery, difference in the neutron emission spectrum, and differences in the neutron source shape. Lessons learned, findings, and relevance for calibrations of future large tokamaks are discussed.