Effects of lithium wall-coatings on impurity ions in the lithium tokamak experiment (LTX)

Summary form only given. By reducing neutral recycling and providing a low-Z plasma-facing surface, lithium wall coatings have led to major performance improvements in TFTR<;sup>1<;/sup>, CDX-U<;sup>2<;/sup>, and NSTX<;sup>3<;/sup>. The effects of lithium coatings on plasma performance are being studied in the Lithium Tokamak Experiment (LTX), a modest size (R=0.4 m, a=0.26 m) spherical tokamak. In LTX, lithium coatings can be applied to close-fitting stainless steel surfaces surrounding ~90% of the plasma. The surfaces can be heated to ~300°C, allowing plasma operations with partly or fully liquid lithium surfaces.A potential issue for lithium in fusion devices is the possibility of core impurity contamination due to lithium´s relatively high sputtering and evaporation rates. On the other hand, lithium coatings can also getter and bury more harmful, higher-Z impurities. Experiments on NSTX have shown very low levels of lithium contamination in the core plasma4. However, these experiments used partial lithium coatings on carbon surfaces and had high levels of carbon contamination, which caused a large friction force that pushed out lithium5. Here, we will present measurements of lithium and other impurities in a tokamak with all-metal walls fully coated in solid andlor liquid lithiwn. We present results of Doppler spectroscopy of intrinsic line emission in LTX, which measures the emissivity, velocity, and temperature profiles of the various impurity species. 15 toroidal views cover the plasma core with ria <; 0.7, while 6 pairs of poloidal views cover 0.2 <; ria <; 0.9. A novel highthroughput, high-resolution variable wavelength spectrometer has been installed6. It has similar capabilities to the existing Kaiser Holospec spectrometer (which measures Li III and C II emission), but can be adjusted between shots to measure any impurity lines in the visible range with high accuracy.