Aspect ratio dependence of fast ion effects on neoclassical tearing mode growth from comparison between DIII-D and NSTX

Fast ions can facilitate the growth of small islands by reducing the stabilizing ion polarization currents in the island sheath region [1]. This relatively new discovery is included in interpretation

of the previous study about aspect ratio dependence on neoclassical tearing mode growth at DIII-D and NSTX, which feature almost a factor of two difference in aspect ratio [2]. The results show that in both devices, the ratio of fast ion contribution to the ion polarization current contribution within the modified Rutherford equation is significant while the island is small, thus confirming the importance of including fast ion effect across the aspect ratio. Each term is calculated using the output from TRANSP which includes profiles of thermal and fast ion density and temperature self-consistent with the equilibria and measurements in both tokamaks [1]. Details of the calculation will be presented, along with the implication of results to ITER and fusion pilot plants.

[1] Yang et al., Plasma Phys. Control. Fusion 64 095005 (2022)

[2] La Haye et al., Phys. Plasmas 19 062506 (2012)