Low-recycling, flat temperature profile discharges with higher performance, NBI-heating, and liquid lithium walls in the Lithium Tokamak Experiment-β

Recently, the Lithium Tokamak Experiment-β has achieved key prerequisites for a fusion device operating in the low-recycling regime. Previously, LTX and LTX-β demonstrated low-recycling flat T_e profiles transiently in ohmic discharges with solid Li coatings. Now, LTX-β has sustained steady flat T_e profiles and extended these operating conditions to neutral beam (NBI) heating and liquid Li walls with values of I_p, T_e, T_i, pressure, and τ_E 50-200% higher than LTX.

Li retains hydrogen and suppresses edge neutral cooling, allowing increased T_edge ~ T_core. Gas puffing can also cool the edge, so prior flat T_e discharges had decaying n_e after fueling ended. We show that careful fueling can exploit low ion-electron collisional coupling and sustain the unique high T_e, low n_e edge for multiple τ_E with steady n_e ~10¹⁹ m^-3.

The same low collisionality results in a long fast ion slowing down time τ_fi; thus heavily fueled, high n_e discharges show the strongest NBI heating, though with a cold edge. Still, careful fueling during the I_p ramp enables NBI heating with T_edge > T_core/2. Interpretive TRANSP analysis frequently shows τ_E ~ 2x H-mode scalings [DP Boyle et al NF 63 (2023) 056020], and generally, no degradation of τ_E with P_NBI is observed. An imminent upgrade will re-aim the beam and enable pulses longer than τ_fi; NUBEAM predicts doubled NBI heating and fueling.

Improvements in Li deposition led to cleaner Li surfaces and good plasma performance with liquid coatings in LTX. After further improvements in LTX-β, the first observations of flat T_e profiles with liquid Li have now been made, and plasma performance improvements similar to with solid coatings achieved. New experiments with visibly clean liquid Li coatings have reproduced and expanded on some of the solid Li results of the past two years, including quantification of the recycling coefficient.