Plasma equilibrium reconstructions in the lithium tokamak experiment

The lithium tokamak experiment (LTX) (R ₀=0.4m, a=0.26m) is designed to explore the low-recycling, lithium wall operating regime for magnetically confined plasmas. A set of shell quadrants internal to the vacuum vessel and conformal to the plasma last closed flux surface is designed to be coated with lithium to produce the lithium plasma-facing surface. The shell quadrants are highly conductive in order to maintain an even thermal distribution, but this conductivity also permits eddy currents to flow that can be larger in magnitude than the plasma current. Due to this effect, plasma start-up is greatly complicated as is the development of an applicable equilibrium solver code. A suitable code, LTX LRDFIT, has been developed and benchmarked and has now been used to compare plasma flux surface reconstructions for discharges before and after initial lithium wall conditioning. Dramatic improvements in plasma performance and shaping have been noted with the introduction of lithium.