Overview of Physics Results from NSTX
R. Raman, (E. Schuster), et al. (Collaboration Paper)
IAEA Fusion Energy Conference
Daejon, Korea, 11-16 October 2010
Abstract
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During the last two experimental campaigns, the low aspect-ratio NSTX
has explored physics issues critical to both toroidal confinement
physics and ITER. Experiments have made extensive use of both lithium
coatings for wall conditioning, n = 1 resistive wall mode control, and
non-axisymmetric field correction to reliably produce high-performance
neutral-beam heated discharges with non-inductive current fractions up
to 0.70, extending to 1.7 s in duration. The resistive wall mode
control coils have been used to trigger repetitive ELMs with high
reliability and have also contributed to an improved understanding of
both neoclassical tearing mode and resistive wall mode stabilization
physics, including the interplay between rotation and kinetic effects
in stability. High Harmonic Fast Wave (HHFW) heating produced plasmas
with central electron temperatures exceeding 6 keV. The HHFW was used
to show that there was little difference in power threshold for the
L–H transition for D and He plasmas, which suggests that operation in
helium may be the best approach to developing H-mode scenarios in the
early non-nuclear phase of ITER operation. A new fast ion diagnostic
showed a depletion of the fast ion profile over a broad spatial region
as a result of toroidicity-induced Alfvén eigenmodes (TAE) and
energetic particle modes (EPM) bursts. In addition, it was observed
that other modes (e.g. Global Alfvén eigenmodes) can trigger TAE and
EPM bursts, suggesting redistribution of fast ions by high-frequency
AEs. The momentum pinch velocity determined by a perturbative technique
decreased as the collisionality was reduced. The processes governing
deuterium retention by graphite and lithium-coated graphite plasma
facing components (PFCs) have been investigated. To reduce divertor
heat flux, a novel divertor configuration, called the “snowflake”
divertor was tested in NSTX and many beneficial aspects were found.
A reduction in the required central solenoid flux has been realized
in NSTX when discharges initiated by coaxial helicity injection were
ramped in current using induction and have produced the type of plasma
needed to meet the objectives of the non-inductive start-up and ramp-up
program of NSTX.