Neutral beam heating of flat-temperature profile plasmas in LTX-β

Flat temperature profiles are achieved in LTX-β via a lithium coated first wall. A 20 kV Neutral Beam Injector, installed in 2019, has recently been observed to heat the plasma. This auxiliary heating, essential for studying the energy confinement scaling in the low-recycling environment and the response of flat temperature profiles to auxiliary heating, was achieved only after a concerted modeling and experimental campaign. Utilizing thermocouple data from the calorimeter, the neutralization fraction was optimized, a misalignment of the source was detected and corrected, and a measure of the beam width as a function of perveance was made, consistent with CHERs data. Modeling suggests that the higher current discharges made available as of 2021 after an upgrade to the ohmic power supply have moved LTX-β into a regime where good coupling is now possible but remains challenging. Many factors influence beam coupling, including the beam energy, perveance, and tangency radius, as well as plasma density, current, temperature, and tearing mode activity. Although TRANSP modeling suggests beam heating increases with beam energy up to the max operating voltage of 20 kV, heating was not observed until the beam tangency radius was increased by intentionally misaligning the beam source relative to the neutralizer tank and operating at the mid-range energy of 13 kV. Exploration of the variables controlling beam heating in LTX-β are still underway and the present findings will be presented, alongside future plans to install an NPA and realign the beam during an upcoming vent.