Recent Progress Toward an Advanced Spherical Torus Operating Point in NSTX
Authors: Stefan Gerhardt, et. al.
Abstract: Abstract. Progress in the development of integrated advanced ST plasma scenarios in NSTX [M.
Ono, et al., Nuclear Fusion 40, 557 (2000)] is reported. Recent high-performance plasmas in
NSTX following lithium coating of the plasma facing surfaces have achieved higher elongation
and lower internal inductance than previously. Analysis of the thermal confinement in these
lithiumized discharges shows a stronger plasma current and weaker toroidal field dependence
than in previous ST confinement scaling studies; the ITER-98(y,2) scaling expression describes
these scenarios reasonable well. Analysis during periods free of MHD activity has shown that the
reconstructed current profile can be understood as the sum of pressure driven, inductive, and
neutral beam driven currents, without requiring any anomalous fast ion transport. Non-inductive
fractions of 65-70%, and βP>2, have been achieved at lower plasma current. Some of these lowinductance
discharges have a significantly reduced no-wall βN limit, and often have βN at or near
the with-wall limit. Coupled m/n =1/1 + 2/1 kink/tearing modes can limit the sustained β values
when rapidly growing ideal modes are avoided. A βN controller has been commissioned and
utilized in sustaining high-performance plasmas. “Snowflake” divertors compatible with highperformance
plasmas have been developed. Scenarios with significantly larger aspect ratios have
also been developed, in support of next-step ST devices. Overall, these NSTX plasmas have
many characteristics required for next-step ST devices.
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Submitted to: Nuclear Fusion (December 2010)
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Download PPPL-4693 (pdf 5.1 MB 43 pp)
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