Abstract Details

icc06_cdxu.pdf2006-02-21 10:12:58Richard Majeski

Low recycling discharges with full lithium wall coatings in CDX-U

Author: Richard Majeski
Submitted: 2005-12-21 16:06:33

Co-authors: T. Gray, R. Kaita, D. Mansfield, J. Spaleta, J. Timberlake, L. Zakharov, PPPL, V. Soukhanovskii, LLNL, R. Doerner, UCSD, R. Maingi, ORNL

Contact Info:
Princeton Plasma Physics Lab
P.O. Box 451
Princeton, NJ   08543

Abstract Text:
The Current Drive eXperiment – Upgrade (CDX-U) has now been operated with the vacuum vessel wall and limiter surfaces nearly completely coated with lithium. The CDX-U was a compact (R=34 cm, a=22 cm, B(toroidal) = 2 kG, Ip =100 kA, Te(0)~100 eV, ne(0)~ 5 x 10^19 m-3) spherical tokamak at the Princeton Plasma Physics Laboratory. Lithium wall coatings were applied with two techniques – electron beam-induced evaporation of a toroidal lithium-filled limiter, and vapor deposition from a resistively heated lithium-filled oven. The use of both systems produced 1000 Å thick lithium layers during the 5 minute period between discharges, as determined by a quartz crystal deposition monitor. Except for approximately 600 cm^2 of liquid lithium surface in the limiter, most of the lithium coating the limiters and walls was a solid, at ~20°C. New magnetic diagnostics were added in this last operating phase of CDX-U. A new version of the Equilibrium and Stability Code (ESC) has also been implemented. ESC is an equilibrium reconstruction code, which explicitly includes the effects of transient eddy currents in the vacuum vessel, without assuming axisymmetry in the vessel construction. Operation of CDX-U with full lithium wall coatings produced discharges with global recycling coefficients estimated at ~20%, which is the lowest recycling coefficient ever obtained for a magnetically confined plasma. Furthermore, low recycling discharges exhibited global energy confinement times which exceed previous CDX-U results by a factor of 5 or more. Confinement times for these discharges also exceed the ITER98pby1 confinement scaling, which includes data from the START device, by up to a factor of 4. This is the largest confinement enhancement ever observed in an ohmically driven tokamak. The mechanism for the observed improvement in confinement is by no means understood. However, it is tempting to suggest that the small region of good core confinement previously observed in CDX-U [T. Munsat, Phys. Plasmas 9 (2002)480] has greatly expanded, due to the expected mitigation of plasma-wall interactions in a very low recycling device. One of the possible benefits of a low recycling wall is that small (e.g. ICC-level), low recycling experiments may deliver confinement approaching that of much larger, high recycling devices (e.g. PoP-level), by greatly reducing the extent of the plasma “edge?. CDX-U has now been disassembled, and lithium experiments will be continued in the Lithium Tokamak eXperiment (LTX). The status of LTX will also be briefly summarized. First plasma is expected in summer 2006.

Characterization: A3,A4


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