Simulation of the neutron rate in ASDEX Upgrade H-mode discharges

The neutron rate is a direct indicator of the fusion performance in tokamaks. However, the accuracy of the measurement is limited by the wide range, covering several orders of magnitude, and by the delicate absolute calibration procedure. Moreover, a mere neutron counter does not distinguish between thermonuclear, beam–plasma and beam–beam fusion reactions. In this work we aim to use Monte Carlo simulations of the NBI deposition to improve our physics understanding of the correlation of the neutron yield with global plasma parameters. The modelling shows the beam–plasma reactions to be the main contribution to the neutron rate for PNBI > 5 MW. The comparison between experimental data and measurements allows the identification of systematic calibration factors for different calibration phases over several years of acquisition. The quality of the kinetic profile measurements is shown to play an important role in reducing the scatter of the simulated neutron rate around the measured value. The sensitivity to Zeff and to possible fast ion diffusion is investigated. Using an existing formula for the density dependence of Zeff the agreement between theoretical prediction and experimental measurements is considerably improved. Finally we derive a scaling law for the measured neutron rate in ASDEX Upgrade H-mode discharges depending on global parameters, in excellent agreement with a simple physics derivation for beam–plasma neutrons.