On the role of neutrals in achieving flat temperature profiles and exciting tearing mode activity in LTX-β

Flat T_e(r) - profiles are obtained earlier in LTX-β, with lithium coated first wall, once the external fueling is stopped. Due to strong hydrogen retention and low recycling of lithium coatings, the flat T_e profiles are obtained at reduced density. The goal of the present work is to explore the possibility of achieving this unique operation regime at higher and sustained densities with suitably tailored fueling, using the high field side gas puffing (HFS-GP) and/or the supersonic gas injection (SGI). LTX-β is equipped with toroidal and poloidal arrays of Mirnov probes, with one of the toroidal Mirnov probes being simultaneously acquired at a fast digitization rate (3 MHz). It is observed that beyond a critical fueling limit by either HFS-GP or SGI, the T_e profile shifts from flat to peaked and a tearing mode is also destabilized. The critical fueling limit is experimentally manifested through a line averaged density threshold (<n_e>_th) of 1 x 10¹⁹ m^-3 for the tearing mode to get destabilized. Below <n_e>_th flat T_e profiles are achieved while maintaining <n_e>. Mode analysis by singular value decomposition confirms the mode structure to be m/n = 2/1. Our hypothesis is that the destabilization of the tearing mode and peaking of T_e profiles beyond <n_e>_th is due to the edge cooling by the cold neutrals beyond a critical fueling flux. This is supported by neutral density estimation at the edge by DEGAS 2. Linear tearing stability analysis will be reported as a function of edge neutral flux to further investigate the hypothesis.