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Liquid metal plasma-facing components (PFCs) have been proposed as a means of solving several problems facing the creation of economically viable fusion power reactors. To date, few demonstrations exist of this approach in a diverted tokamak and we here provide an overview of such work on the National Spherical Torus Experiment (NSTX). The Liquid Lithium Divertor (LLD) was installed and operated for the 2010 run campaign using evaporated coatings as the filling method. The LLD consisted of a copper-backed structure with a porous molybdenum front face. Nominal Li filling levels by the end of the run campaign exceeded the porosity void fraction by 150%. Despite a nominal liquid level exceeding the capillary structure and peak current densities into the PFCs exceeding 100 kA m-2, no macroscopic ejection events were observed. In addition, no substrate line emission was observed after achieving lithium-melt temperatures indicating the lithium wicks and provides a protective coating on the molybdenum porous layer. Impurity emission from the divertor suggests that the plasma is interacting with oxygen-contaminated lithium whether diverted on the LLD or not. A database of LLD discharges is analysed to consider whether there is a net effect on the discharges over the range of total deposited lithium in the machine. Examination of H-97L indicates that performance was constant throughout the run, consistent with the hypothesis that it is the quality of the surface layers of the lithium that impact performance. The accumulation of impurities suggests a fully flowing liquid lithium system to obtain a steady-state PFC on timescales relevant to NSTX. [ABSTRACT FROM AUTHOR]

Transient coaxial helicity injection (CHI) in the National Spherical Torus Experiment (NSTX) has generated toroidal current on closed-flux surfaces without the use of the central solenoid. When induction from the central solenoid was added, CHI initiated discharges in NSTX achieved 1MA of plasma current using 65% of the solenoid flux of standard induction-only discharges. In addition, the CHI-initiated discharges have lower density and a low normalized internal plasma inductance of 0.35, as desired for achieving advanced scenarios. Transient CHI will be used for non-inductive plasma start-up on the upgrade to NSTX (NSTX-U) that is now under construction. It will have numerous improvements that significantly enhance CHI start-up capability. The TSC code is now starting to be used for full discharge simulation, which includes solenoid-less start-up with CHI and subsequent non-inductive current ramp-up using neutral beams. These results suggest that the increased injector flux capability of NSTX-U should allow CHI start-up at more than the 400 kA level and that the new tangential neutral beam system on NSTX-U should be able to ramp the current to the 1MA levels. [ABSTRACT FROM AUTHOR]

Lithium wall coatings have been shown to reduce recycling, suppress edge-localized modes (ELMs), and improve energy confinement in the National Spherical Torus Experiment (NSTX). Here we document the effect of gradually increasing lithium wall coatings on the discharge characteristics, with the reference ELMy discharges obtained in boronized, i.e. non-lithiated conditions. We observed a continuous but not quite monotonic reduction in recycling and improvement in energy confinement, a gradual alteration of edge plasma profiles, and slowly increasing periods of ELM quiescence. The measured edge plasma profiles during the lithium-coating scan were simulated with the SOLPS code, which quantified the reduction in divertor recycling coefficient from∼98% to∼90%. The reduction in recycling and fuelling, coupled with a drop in the edge particle transport rate, reduced the average edge density profile gradient, and shifted it radially inwards from the separatrix location. In contrast, the edge electron temperature (Te) profile was unaffected in the H-mode pedestal steep gradient region within the last 5% of normalized poloidal flux, ψN; however, the Te gradient became steeper at the top of the H-mode pedestal for 0.8 < ψN < 0.94 with lithium coatings. The peak pressure gradients were comparable during ELMy and ELM-free phases, but were shifted away from the separatrix in the ELM-free discharges, which is stabilizing to the current-driven instabilities thought to be responsible for ELMs in NSTX. [ABSTRACT FROM AUTHOR]

The 2nd International Symposium on Lithium Applications for Fusion Devices (ISLA-2011)was held on 27-29 April 2011 at the Princeton Plasma Physics Laboratory (PPPL) with broad participation from the community working on aspects of lithium research for fusion energy development. This community is expanding rapidly in many areas including experiments in magnetic confinement devices and a variety of lithium test stands, theory and modeling and developing innovative approaches. Overall, 53 presentations were given representing 26 institutions from 10 countries. The latest experimental results from nine magnetic fusion devices were given in 24 presentations, from NSTX (PPPL, USA), LTX (PPPL, USA), FT-U (ENEA, Italy), T-11M (TRINITY, RF), T-10 (Kurchatov Institute, RF), TJ-II (CIEMAT, Spain), EAST (ASIPP, China), HT-7 (ASIPP, China), and RFX (Padova, Italy). Sessions were devoted to: I. Lithium in magnetic confinement experiments (facility overviews), II. Lithium in magnetic confinement experiments (topical issues), III. Special session on liquid lithium technology, IV. Lithium laboratory test stands, V. Lithium theory/modeling/comments, VI. Innovative lithium applications and VII. Panel discussion on lithium PFC viability in magnetic fusion reactors. There was notable participation from the fusion technology communities, including the IFE, IFMIF andTBMcommunities providing productive exchanges with the physics oriented magnetic confinement lithium research groups. It was agreed to continue future exchanges of ideas and data to help develop attractive liquid lithium solutions for very challenging magnetic fusion issues, such as development of a high heat flux steady-state divertor concept and acceptable plasma disruption mitigation techniques while improving plasma performance with lithium. The next workshop will be held at ENEA, Frascati, Italy in 2013. [ABSTRACT FROM AUTHOR]