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Recent Results from LTX

Author: Richard Majeski
Requested Type: Consider for Invited
Submitted: 2011-06-10 14:49:49

Co-authors: L. Berzak,E. Granstedt, C. M. Jacobson, R. Kaita, Tl Kozub, B. Leblanc, D. P. Lundberg, M. Lucia, J. Squire, L. Zakharov, T. Biewer, T. Gray, R. Maingi, K. Tritz, C. E. Thomas, V. Soukhanovskii

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

Abstract Text:
LTX is a low aspect ratio tokamak with R=0.4 m, a=0.26 m, and elongation=1.5. Design goals include a toroidal field of 3.2 kG, plasma current up to 400 kA, and a discharge duration of order 100 msec, although in 2010 the device operated at reduced parameters. LTX is the first tokamak designed to investigate modifications to equilibrium and transport when global recycling is reduced to 10 – 20%. LTX is fitted with a 1 cm thick heated (300 – 400 °C) copper liner. The plasma-facing surface of the liner is 1.5 mm stainless steel, explosively bonded to the copper, and is designed to be coated with lithium. The lower liner sections will retain several hundred cubic centimeters of liquid lithium, to form a lower liquid lithium limiter similar to that employed in CDX-U. [R. Majeski et al., Phys. Rev. Lett. 97 (2006) 075002] LTX is the first tokamak designed entirely to accommodate high temperature walls and a large in-vessel inventory of liquid lithium.
In 2010 LTX was operated with two new lithium evaporation systems. No other wall conditioning techniques such as boronization were employed, and there are no low-Z limiters, to prevent formation of sputtered low-Z (e.g. carbon) films, which react with lithium and increase recycling. Early discharges against the uncoated stainless steel liner wall in LTX were therefore impurity-dominated, with plasma currents in the 10 – 15 kA range, and short discharges of 4-6 msec duration. Operation with solid lithium liner coatings produced discharges with greatly increased plasma currents, up to 70 kA, and an increase in discharge duration to 20 msec. These parameters are similar to CDX-U discharges obtained with similar lithium wall coatings. Preliminary Thomson scattering data indicate core electron temperatures of 100 – 150 eV. Although good discharge parameters were obtained with room temperature, solid lithium wall coatings, operation with hot (300 °C) walls and presumably molten lithium films was not as effective. With hot walls, very rapid passivation of the lithium coatings was observed. Initial indications are that passivation was primarily due to segregation of oxygen and other impurities to the surface when the lithium is liquefied. These results are also relevant to NSTX operation with the LLD (liquid lithium divertor) system, and will be revisited in 2011.
In late 2010, the device was vented. For the 2011 run, both bakeout and active cooling of the vacuum vessel is being implemented, along with a novel set of lithium getter pumps. The intent is to significantly reduce the rate of passivation of the liquid lithium surface. A new liquid lithium filling system for the lower shell segments has been designed, and is currently being fabricated. These new systems will all be operable in summer 2011, and preliminary results will be presented.
Supported by US DOE contracts DE-AC02-09CH11466 and DE-AC52-07NA27344

Characterization: A6,D4

Comments:

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011