The Effect of Plasma Shaping on Performance in the National Spherical Torus Experiment (NSTX)
Authors: D.A. Gates, R. Maingi, J. Menard, S. Kaye, S.A. Sabbagh, G. Taylor, J.R. Wilson, et al.
Date of PPPL Report: November 2005
Invited Paper: the Forty-Seventh Annual Meeting of the APS Division of Plasma Physics, 24 – 28 October 2005, Denver, Colorado.
NSTX has explored the effects of shaping on plasma performance as determined by many diverse topics including: the stability of global 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 βt∼ 40%. Precise plasma shape control has been achieved on NSTX using real-time equilibrium reconstruction. NSTX has simultaneously achieved elongation k∼ 2.8 and triangularity δ ∼ 0.8. Ideal MHD theory predicts increased stability at high values of shaping factor S ≡ q95Ip/(αΒt), which has been observed at large values of the S ∼ 37[MA/(m ⋅ Tesla)] on NSTX. ELM behavior 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 Ip. The achievement of strong shaping, has enabled operation with 1s pulses with Ip = 1MA, and for 1.6s for Ip = 700kA. Analysis of the non-inductive current fraction as well as empirical analysis of the achievable plasma pulse length as elongation is varied will be presented. Data is presented showing a reduction in peak divertor heat load due to increasing in flux expansion.