Session PM5: Mini-conference on the First Microns of the First Wall: Lithium Coatings and Surfaces

Lithium and Deuterium on NSTX Carbon Tiles

Evaporation of lithium onto the wall of NSTX has produced significant improvements in plasma conditions. The effect of this lithium on plasma-wall interactions is expected to depend mainly on its concentration near the surface of plasma-facing components. Here we report measurements of the concentration of lithium and deuterium (D) versus depth on 21 carbon tiles removed from NSTX after lithium deposition experiments in 2006. Measurements were done using nuclear reaction analysis. The lithium was observed to be within a few microns of the surface, which shows that diffusion of lithium into the carbon was less than a few microns. Lithium coverage was an order of magnitude lower on tiles shadowed from the lithium evaporation source than on unshadowed tiles at similar poloidal positions, whereas the D coverage was similar. This shows that the D coverage was not greatly changed by the lithium deposition. The D coverage was highest in the private flux region between inner and outer strike points of the high triangularity plasmas used. The presence of lithium at the strike points shows that it had not been removed by plasma erosion at these high flux locations.   

The lithium deposition on NSTX plasma facing components by LITER-1 evaporator in 2006

The deposition of lithium from the LITER-1 lithium evaporator to the in-vessel components of NSTX during the 2006 experimental campaign has been calculated using the recently written Cbebm code. Its numerical model represents a collisionless gas model which assumes re-evaporation of the Li molecules after their collision with the hot walls of evaporator. Theoretically, the model is valid up to lithium temperatures of 650 $^{\circ}$C. The code reproduces the real 3-D geometry of the evaporator canister and snout as well as the plasma facing components of NSTX, including the details of its carbon tiles. The simulation data can serve as a reference Li deposition distribution, which would be established in a perfectly clean vacuum vessel, and are compared with the lithium content in the surface layer of the sample tiles, obtained by Bill Wampler (SNL) using nuclear analysis.   

Surface analysis of lithium coatings in NSTX

Lithium coatings have been applied to NSTX plasma facing surfaces as part of a long term program to explore the potential for lithium to improve plasma and PFC performance. A LIThium EvaporatoR (LITER) directed a collimated stream of lithium vapor from an upper vacuum vessel port toward the graphite tiles of the lower center stack and divertor either before, or continuously between and during, discharges. Silicon witness coupons and other samples were retrieved from the vacuum vessel after the campaign and the surface morphology and elemental compostion examined with a XL30 FEG-SEM microscope equipped with an energy dispersed X-rays system sensitive to elements with atomic number greater than 4. The surfaces showed a complex morphology with nucleation sites apparent. Carbon and oxygen were the dominant impurities. Trace metals (Fe and Cr) were detected in the coating on the Si coupon, but not in a flake of bulk Li. We will present results of the surface analysis of samples exposed to Li evaporation in NSTX.   

Mass changes in NSTX surface layers with Li conditioning as measured with quartz microbalances

Three quartz crystal microbalances are deployed in plasma shadowed areas in NSTX at the upper and lower divertor and outboard midplane. Pulse-by-pulse material gain and loss (dynamic retention) is measured with a sensitivity of a fraction of an atomic monolayer. At the time of a plasma discharge a transient increase in mass of order 0.1 $\mu$g/cm$^2$ is seen. This decays in the interpulse period to a level either higher, lower or similar to that prior to the discharge. Following a days plasma operations a loss in mass is observed over several hours that parallels the deuterium outgassing. For the first discharge of the day, the relatively unsaturated hydrocarbon layer shows a step-up in mass independent of plasma conditions. In 2007 lithium was evaporated onto plasma facing components to control recycling. We will present data on the mass changes of surface layers with and without lithium coatings.   

In-situ elemental and chemical state characterization of lithiated surfaces under energetic particle bombardment.

Lithium has been considered a potentially viable plasma-facing surface enhancing the operational performance of fusion devices such as: TFTR and NSTX. Solid and liquid lithium has been studied extensively both in its erosion and hydrogen-retaining properties. However, questions still remain on the role of lithiated surfaces and multi-material interactions at the plasma edge. Lithiated surfaces include: liquid Li on metal substrates, Li alloys and Li coatings. The main processes studied here (e.g. erosion, H-retention) consist of spatial scales from a few monolayers at the vacuum/film interface to 100's nm deep. Techniques used include: low-energy ion scattering spectroscopy (LEISS), direct recoil spectroscopy, X-ray photoelectron spectroscopy and in-situ erosion diagnosis. LEISS diagnoses the first 2-3 monolayers. XPS gives chemical state data 10-nm into the lithiated surface. Three cases are presented in this paper: liquid Li, alloyed Li and Li coatings under D irradiation.   

Physical sputtering and chemical erosion studies on plain and lithiated graphite samples

PFC candidate materials must have characteristics allowing for high temperature resilience while limiting deuterium recycling and core contamination from erosion. Graphite is a good choice of material for its high temperature tolerance. However, to reduce deuterium recycling issues of the graphite surface, lithium has been used extensively as a coating on PFC surfaces, though many issues on physical and chemical sputtering still remain. The Ion-surface InterAction Experiment (IIAX) measures the absolute, angular-resolved and self-sputtering yields of many particle/target combinations. Baseline sputtering yield of an untreated ATJ graphite sample is very close to the predicted TRIM estimates with an average of 0.06 +/- 0.02 atoms / ion. Preliminary experiments show that Li was evaporated and deposited with thickness of 320 nm on a Si wafer. Li deposition on a ATJ graphite sample was verified using scanning electron microscopy. Chemical sputtering analysis on a ATJ graphite sample is done, and results confirm the operation of the RGA. Trail experiments on relative levels of Li to C collected during sputtering are analyzed using TOF-SIMS. A deposition rate of 10 nm/min is the most relevant to NSTX. Additional experiments using varying thicknesses and deposition rates of Li are described.   

Liquid lithium self-propulsion under applied heat loads

Recent experiments have lead to a resurgence in interest in liquid lithium plasma facing components (PFCs). Current plans on NSTX are to implement a Liquid Lithium Divertor (LLD) in the device. This system will utilize a porous metal foam with a thin layer of liquid lithium in contact with the divertor plasma. The liquid-solid system is examined and thermocapillary and thermoelectric magnetohydrodynamics (TEMHD) are deemed important effects. Thermocapillary forces were observed on the CDX-U device redistributing a point source heat load. In NSTX, these forces are expected to create a surface velocity on the order of 7cm/s for a 1mm layer. TEMHD may create additional forces on the liquid metal system. In the case of porous media, the capillary pumping may affect both TEMHD and thermocapillary induced flows. In order to accurately assess the power handling and particle pumping capabilities of liquid lithium PFCs, all these effects will need to be taken into account. The SLiDE apparatus has been designed in order to test thermocapillary and TEMHD flows with an incident heat flux in a laboratory scale environment. An overview of these effects in addition to results of liquid lithium imbibition experiments in a porous metal foam are shown.