PPPL-4926
Upward-facing Lithium Flash Evaporator for NSTX-U
Authors: A.L. Roquemore, et. al.
Abstract:
NSTX plasma performance has been significantly
enhanced by lithium conditioning [1]. To date, the lower
divertor and passive plates have been conditioned by
downward facing lithium evaporators (LITER) as
appropriate for lower null plasmas. The higher power
operation expected from NSTX-U requires double null
plasma operation in order to distribute the heat flux between
the upper and lower divertors making it desirable to coat the
upper divertor region with Li as well. An upward aiming
LITER (U-LITER) is presently under development and will
be inserted into NSTX-U using a horizontal probe drive
located in a 6" upper midplane port. In the retracted position
the evaporator will be loaded with up to 300 mg of Li
granules utilizing one of the calibrated NSTX Li powder
droppers[2]. The evaporator will then be inserted into the
vessel in a location within the shadow of the RF limiters and
will remain in the vessel during the discharge. About 10
seconds before a discharge, it will be rapidly heated and the
lithium completely evaporated onto the upper divertor, thus
avoiding the complication of a shutter that prevents
evaporation during the shot when the diagnostic shutters are
open. The minimal time interval between the evaporation
and the start of the discharge will avoid the passivation of
the lithium by residual gases and enable the study of the
conditioning effects of un-passivated Li surfaces [3].
Two methods are being investigated to accomplish
the rapid (few second) heating of the lithium. A resistive
method relies on passing a large current through a Li filled
crucible. A second method requires using a 3 kW e-beam
gun to heat the Li. In this paper the evaporator systems will
be described and the pros and cons of each heating method
will be discussed.
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Submitted to: SOFE 2013 Proeedings (June 0213) 25th Symposium on Fusion Engineering, San Francisco, CA (June 10-14, 2013)
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Download PPPL-4926 (pdf 274 KB 5pp)
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