Neutron rate estimates in MAST based on gyro-orbit modelling of fast ions

Abstract

A discrepancy between predicted and measured neutron rates on MAST using TRANSP/NUBEAM has previously been observed and a correction factor of about 0.6 was needed to match the two: this correction factor could not be accounted for by the experimental uncertainties in the plasma kinetic profiles nor in the NBI energy and power [1]. Further causes of this discrepancy are here studied by means of TRANSP/NUBEAM and ASCOT/BBNBI simulations. Different equilibria, Toroidal Field ripples, uncertainties on the NBI divergence value and Gyro-Orbit effects were studied and simulations were performed with both transport codes. It was found that the first three effects accounted for only a 5 % variation in the fast ion density. On the other hand, full Gyro-Orbit simulations of the fast ions dynamics carried out in ASCOT/BBNBI resulted in an approximately 20 % reduction of the fast ion population compared to TRANSP/NUBEAM. A detailed analysis of the fast ion distributions showed how the drop occurred regardless of the energy at pitch values ≤ -0.4. The DRESS code was then used to calculate the neutron rate at the Neutron Camera detector's location showing that the discrepancy is considerably reduced when the full Gyro-Orbit fast ion distribution is used, with now the correction factor, used to match experimental and predicted neutron rates, being around 0.9.

Simulating the impact of charge exchange on beam ions in MAST-U

“…While TRANSP does use finite-Larmor-radius corrections to account for the gyroorbit for interactions with the background plasma and atoms, it is fundamentally a guiding-centre code and limited to following fast ions inside the confined plasma. As mentioned in section 1, the importance of following the full gyro-orbits of fast ions in MAST has been experimentally demonstrated [3,13]. Specifically, when compared to TRANSP, full gyroorbit following by ASCOT yielded predicted neutron emission rates that were quantitatively more consistent with the measurements [13].…”

“…The ASCOT CX model extends the existing capabilities for modelling fast-ion CX. Previous work on modelling the neutron rate in MAST suggests that it is necessary to follow the full gyro-orbits of fast ions to accurately reproduce experiments in the spherical tokamak geometry [3,13]. The full gyroorbit following capabilities of ASCOT combined with its nearoptimal scaling on supercomputers allows high-fidelity simulation of fast-ion populations in high-∇B geometries, such as the spherical MAST-U tokamak.…”

Effect of scintillator geometry on the energy resolution and efficiency of MAST neutron camera detectors

“…For the detector here studied, the relative reduction in efficiency for d = 5 cm is about 4 % compared to the efficiency for a = 1 (d = 0 cm) resulting in an average reduction in efficiency of approximately 2 %. This reduction in the efficiency is clearly insufficient to contribute to the discrepancy between measured and simulated neutron rates on MAST [9] which has been partially accounted for in a recent study [15] and linked to the modeling of the fast ion dynamics in the plasma. Although the reduction in the efficiency is negligible, due to their poor resolution the neutron camera upgrade is equipped with more traditional cylindrical liquid scintillator with a height of 1.5 cm and a diameter of 3 cm which have a resolution of about 8 % at the Compton edge for γ-rays of 1.275 MeV energy [16].…”

Verification and validation of the high-performance Lorentz-orbit code for use in stellarators and tokamaks (LOCUST)

“…These comparisons have been validated against experiment. In achieving this, credibility has been added to the conclusions of other parallel benchmarking activities, which may consider physics not included here [43].…”

“…The previous methodology was repeated for a spherical tokamak topology. Such devices tend to have steeper gradients [41], meaning any inaccuracies in fast-ion models will be exacerbated; in MAST for example, r Larmor can approach the order of the minor radius, and the impact on the validity of the guiding-centre approximation has been questioned [42,43]. MAST shot #29034 was selected to allow for comparison with measured fast-ion D-α (FIDA) emission [44].…”