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

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.