Overview Of Physics Results From NSTX
Authors: R. Raman, et. al.
Abstract:
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 lithium
coatings for wall conditioning, correction of non-axisymmetric field errors and control of n = 1 resistive wall
modes to produce high-performance neutral-beam heated discharges extending to 1.7 s in duration with noninductive
current fractions up to 0.7. The resistive wall mode control coils have been used to trigger repetitive
ELMs with high reliability, and they 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 on stability. High Harmonic Fast Wave (HHFW) heating has produced plasmas with central electron
temperatures exceeding 6 keV. The HHFW heating was used to show that there was a 20 - 40% higher power
threshold for the L - H transition for helium than for deuterium plasmas. A new diagnostic showed a depletion of
the fast-ion density 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
Alfven eigenmodes) can trigger TAE and EPM bursts, suggesting that fast ions are redistributed by highfrequency
AEs. The momentum pinch velocity determined by a perturbative technique decreased as the
collisionality was reduced, although the pinch to diffusion ratio, Vpinch / χφ, remained approximately constant.
The mechanisms of deuterium retention by graphite and lithium-coated graphite plasma facing components
(PFCs) have been investigated. To reduce divertor heat flux, a novel divertor configuration, 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. The resulting plasmas have characteristics needed to meet the objectives of the noninductive
start-up and ramp-up program of NSTX.
__________________________________________________
Submitted to: Nuclear Fusion 2011
__________________________________________________
Download PPPL-4683 (pdf 266 KB 42 pp)
__________________________________________________