Effect of plasma shaping on performance in the national spherical torus experiment

Abstract

The National Spherical Torus Experiment (NSTX) has explored the effects of shaping on plasma performance as determined by many diverse topics including the stability of global magnetohydrodynamic (MHD) modes (e.g., ideal external kinks and resistive wall modes), edge localized modes (ELMs), bootstrap current drive, divertor flux expansion, and heat transport. Improved shaping capability has been crucial to achieving beta(t)similar to 40%. Precise plasma shape control has been achieved on NSTX using real-time equilibrium reconstruction. NSTX has simultaneously achieved elongation kappa similar to 2.8 and triangularity delta similar to 0.8. Ideal MHD theory predicts increased stability at high values of shaping factor S equivalent to q(95)I(p)/(aB(t)), which has been observed at large values of the S similar to 37[MA/(m center dot T)] on NSTX. The behavior of ELMs is observed to depend on plasma shape. A description of the ELM regimes attained as shape is varied will be presented. Increased shaping is predicted to increase the bootstrap fraction at fixed I-p. The achievement of strong shaping has enabled operation with 1 s pulses with I-p=1 MA, and for 1.6 s for I-p=700 kA. Analysis of the noninductive current fraction as well as empirical analysis of the achievable plasma pulse length as elongation is varied will be presented. Data are presented showing a reduction in peak divertor heat load due to increasing in flux expansion. (c) 2006 American Institute of Physics.