Link between tearing mode activity and flat temperature profiles via edge cooling by neutral inventory in LTX-β

Flat electron temperature profiles (T_e(r)) can be sustained while maintaining constant average density (n_e^ave) below a threshold (n_e^ave_th) of ~ 10¹⁹ m^-3 in LTX-β tokamak, which features lithium coated, low-recycling first wall. Above n_e^ave_th, T_e(r) shifts from flat to peaked and a tearing mode is destabilized. Due to low recycling, the achieved n_e^ave can be controlled precisely by external fueling and hence, a certain threshold of the edge neutral inventory is experimentally manifested through n_e^ave_th. Goal of the present work is to investigate the role of edge neutrals in determining T_e(r) and MHD stability in the unique low-recycling regime of LTX-β. Our hypothesis is that the peaking of T_e(r) beyond n_e^ave_th is due to the edge cooling by the cold neutrals beyond a critical fueling flux. At lower fueling flux, flat T_e(r) results in broader pressure profile and lower resistivity, which in turn stabilizes the tearing mode. This is supported by edge neutral density estimation by DEGAS 2 code. Linear tearing stability analysis with M3D-C1 predicts that plasmas with n_e^ave > 10¹⁹ are highly susceptible to a n = 1 tearing mode. q profile and T_e(r) scan confirms that broader pressure profile, influenced by the flatness of T_e(r) stabilizes the mode for n_e^ave < 10¹⁹ m^-3. This study shows for the first time that the neutral inventory at the edge could be a deciding factor for the achievability of the unique flat T_e(r) operation regime and the excitation of tearing activity.