Fueling studies on the Lithium Tokamak Experiment

Abstract

Summary form only given. A series of experiments on the Lithium Tokamak Experiment (LTX) demonstrate that lithium wall coatings facilitate control of the neutral and plasma particle inventories. With fresh lithium coatings and careful gas injection programming, over 90% of the injected particle inventory can be absorbed in the lithium wall during a discharge. Furthermore, effective particle confinement times are shorter than the discharge duration. Thus the plasma density decays quickly in the absence of fueling, and the density can be precisely selected on a millisecond time-scale. A systematic study of the fueling efficiencies achieved with the three LTX fueling systems is presented. The fueling efficiency of the Supersonic Gas Injector is demonstrated to be strongly dependent on the distance between the nozzle outlet and plasma edge. The fueling efficiency of the gas puffer was improved by the addition of a guide tube to keep the gas flow collimated and directed towards the plasma edge. Contrary to results reported on other devices, a molecular cluster injection system yields fueling efficiencies that are at best equal to those achieved with supersonic gas injection. However, due to the collimated nature of the cluster injection beam, the fueling efficiency can be maintained with a large separation between the plasma edge and nozzle outlet. These results are consistent with the concept that the most efficient gas-based fueling source is one that produces highly directed neutrals that free-stream to the plasma edge without scattering. Furthermore, it is observed that the performance of LTX plasmas is strongly reduced by local saturation in lithium wall coatings due to repetitive fueling in one location. This suggests that the fueling sources most compatible with discharges run on lithium-coated walls are those that minimize the deposition of particles in the scrape-off layer, such as closely-coupled supersonic gas injection or molecular cluster injection.