The Thomson scattering system on the lithium tokamak experiment

The lithium tokamak experiment (LTX) is a spherical tokamak with R0 =0.4 m, a=0.26 m, BTF ∼3.4 kG, IP ∼400 kA, and pulse length ∼0.25 s. The goal of LTX is to investigate tokamak plasmas that are almost entirely surrounded by a lithium-coated plasma-facing shell conformal to the last closed magnetic flux surface. Based on previous experimental results and simulation, it is expected that the low-recycling liquid lithium surfaces will result in higher temperatures at the plasma edge, flatter overall temperature profiles, centrally peaked density profiles, and an increased confinement time. To test these predictions, the electron temperature and density profiles in LTX will be measured by a multipoint Thomson scattering system. Initially, TS measurements will be made at up to 12 simultaneous points between the plasma center and plasma edge. Later, high resolution edge measurements will be deployed to study the lithium edge physics in greater detail. Technical challenges to implementing the TS system included limited "line-of-sight" access to the plasma due to the plasma-facing shell and problems associated with the presence of liquid lithium.