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ltx_epr2014r.pdf2014-08-08 11:54:16Richard Majeski

Enhanced confinement with liquid lithium PFCs in the Lithium Tokamak eXperiment (LTX)

Author: Richard Majeski
Requested Type: Consider for Invited
Submitted: 2014-05-29 01:50:39

Co-authors: R. Majeski, R. Bell, D. Boyle, A. Capece, A. Diallo, E. Granstedt, C. M. Jacobson, R. Kaita, B. Koel, T. Kozub, B. LeBlanc, M. Lucia, R. Maingi, E. Merino, J. Schmitt, G. Tchilingurian, PPPL, T. M. Biewer, J. M. Canik, T. K. Gray, ORNL, S. K

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Princeton Plasma Physics Lab
PO Box 451
Princeton, NJ   08543
USA

Abstract Text:
LTX is a low aspect ratio tokamak with a heated shell, which covers 85% of the plasma surface area (5 square meters). The shell can be heated to 300 - 400 C, and coated with lithium. The first coating approach used lithium evaporated from small heated crucibles, dispersed by a 5 mTorr helium backfill. The evaporation and pumpdown cycles required several hours, allowing the lithium coating to react with background gases. Subsequent heating of the coated shells to liquefy the lithium produced an impurity-laden limiting surface for discharges, and resulted in poor performance.[R. Majeski, et al., Phys. Plasmas 20 (2013) 056103] In 2014, a new approach to wall coatings was developed. The shells were preheated to 300 C, and previously applied lithium coatings were oxidized. An electron beam system is used to evaporate lithium from a pool at the bottom of the lower shell structure. The e-beam system produces a 50 – 100 nm coating of liquefied lithium within 5 minutes. The present system can coat half the shell inner surface. Compared to previous results with helium-dispersed coatings, discharges using the new approach have strongly reduced impurities, especially oxygen. Preliminary analysis of diamagnetic loop and other magnetic data indicates that confinement in LTX Ohmic discharges is now improved by 10x, compared to best previous results. Confinement times exceed H-mode scalings by 4-5x. This is the first experimental evidence that high performance tokamak discharges are compatible with large-area liquid lithium walls. In June, a second electron beam system will be installed to coat the remainder of the shell area. The LTX Thomson scattering system is undergoing realignment, and will be available in the same time frame. LTX is a collaborative effort between PPPL and ORNL, with additional participation by UCLA, LLNL, Johns Hopkins University, and the University of Illinois. This work is supported by USDoE contracts DE-AC02-09CH11466 and DE-AC05-00OR22725.

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Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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