Framework for study of neoclassical tearing modes in NSTX

A framework for the study of neoclassical tearing modes in NSTX has been developed. This work is motivated by the need to interpret the experimental data and to provide a predictive modeling prior to the experiments regarding the neoclassical tearing mode activities in NSTX and other tokamaks. The framework focuses on solving the generalized Rutherford equation [1,2] for the time evolution of the magnetic island width. Each term in the generalized Rutherford equation is allowed a degree of freedom, represented by a coefficient, to incorporate the measurement uncertainties in the parameters involved. The coefficients are fitted to match the simulated and measured time evolution of the magnetic island width. The input data for the model is provided by synthetic soft X-ray diagnostics, which provides the measured magnetic island width [3], and TRANSP, which self-consistently calculates both thermal and fast ion transport considering the effect of the measured magnetic island width utilizing the Kick model assumption [4]. The results show that adding a fast ion term [5] to the generalized Rutherford equation is necessary for the fitted coefficients to be within a realistic range in NSTX. It has been found that the stabilization effect of ion polarization current can be exceeded by the destabilization effect of fast ion driven uncompensated cross field current for otherwise marginally stable neoclassical tearing mode. The result of a database study involving NSTX discharges will be presented.

[1] Fredrickson et al., PoP 7 4112 (2000)

[2] Poli et al., NF 58 016007 (2018)

[3] Yang et al., PPCF 63 045003 (2021)

[4] Podestà et al., PPCF 56 055063 (2014)

[5] Cai, NF 56 126016 (2016)