Calculating ideal β-limit and testing feasibility of adding a second neutral beam with M3D-C1 in LTX-β*

Maximizing fusion gain necessitates tokamak operation at the highest possible beta (β) while avoiding both ideal and resistive MHD instabilities which reduce confinement and induce disruptions. β is the ratio of the plasma to the magnetic pressure and the theoretical β-limit (≈ 4l_i) calculated from ideal MHD stability can provide a useful guide for determining operation boundaries in the parameter space and could assist in planning any increase in auxiliary heating like neutral beams (NBI) in future. This limit can be increased by toroidal rotation and a nearby conducting wall. The ideal β-limit in LTX-β is calculated with the M3D-C1 code. A shell, constructed in four segments with stainless steel bonded to copper, serves as the first wall and is designed to accept lithium coatings over the high-Z substrate. This non-axisymmetric conducting shell is accounted for in the calculations. Interestingly, the maximum operational β in existing tokamaks is found to be limited not by ideal modes but by the onset of resistive MHD instabilities. Strong growth of resistive tearing modes has been observed in LTX-β due to edge cooling from the cold neutral inventory from edge fueling. These modes are accounted for while calculating β. LTX-β is equipped with a 20 kV, 35 A NBI and feasibility for adding a second NBI is explored. Adding extra NBI can push towards the β-limit, but momentum input can increase the β-limit itself through toroidal rotation. Results from M3D-C1 simulations will be presented.