Investigation of fast particle redistribution induced by sawtooth instability in NSTX-U

Abstract

The effects of sawtooth on fast ion transport have been studied in reproducible, 2 s long sawtoothing L-mode discharges during the 2016 experimental campaign on National Spherical Torus Experiment Upgrade (NSTX-U) (Menard et al 2012 Nucl. Fusion 52 083015). Analysis of the discharges demonstrated that standard sawtooth models (full/partial reconnection models) in the TRANSP code were not capable to fully reproduce the fast ion redistribution induced by sawtooth crashes. Some global parameters such as neutron rate can be recovered while detailed features, e.g. distribution functions, estimated using the models were different from the experimental observation. The standard sawtooth models in TRANSP do not take into account the different effect of sawtooth crashes depending on fast ion energy and orbit type and that may cause the disagreement between experiments and simulations. In this work, the newly developed kick model has been applied to replace the standard sawtooth models for the fast ion transport. TRANSP simulation results using the kick model, taking into account the characteristics of fast ion such as energy and pitch angle, can reproduce experimental neutron rates within 10% difference. The qualitative comparison of the measurements and synthetic diagnostics of fast ion D-alpha (FIDA) and solid state neutral particle analyser (SSNPA) using the TRANSP simulation results with kick model shows good agreements.