Progress towards Steady State on NSTX
Authors: D.A. Gates, C. Kessel, J. Menard, G. Taylor, J.R. Wilson, and 94 co-authors.
Date of PPPL Report: January 2005
Date of Revised PPPL Report: March 2005
Published in: Nuclear Fusion 46:3 (2006)
Presented at the Fourth IAEA Technical Meeting on Steady-State Operation of Magnetic Fusion Devices and MHD of Advanced Scenarios, 1–5 February 2005 at the Institute for Plasma Research, Bhatt, India. Proceedings to be published on CD-ROM.
In order to reduce recirculating power fraction to acceptable levels, the spherical torus concept relies on the simultaneous achievement of high toroidal beta and high bootstrap fraction in steady state. In the last year, as a result of plasma control system improvements, the achievable plasma elongation on the National Spherical Torus Experiment (NSTX) has been raised from κ ~ 2.1 to κ ~ 2.6 — approximately a 25% increase. This increase in elongation has lead to a substantial increase in the toroidal beta for long pulse discharges. The increase in beta is associated with an increase in plasma current at nearly fixed poloidal beta, which enables higher toroidal beta with nearly constant bootstrap fraction. As a result, for the first time in a spherical torus, a discharge with a plasma current of 1 MA has been sustained for one second.
Data is presented from NSTX correlating the increase in performance with increased plasma shaping capability. In addition to improved shaping, H-modes induced during the current ramp phase of the plasma discharge have been used to reduce flux consumption and to delay the onset of MHD instabilities. A modeled integrated scenario, which has 100% noninductive current drive with very high toroidal beta, will also be discussed. The NSTX poloidal field coils are currently being modified to produce the plasma shape which is required for this scenario, which requires high triangularity (δ ~ 0.8) at elevated elongation (κ ~ 2.5). The other main requirement for steady state on NSTX is the ability to drive a fraction of the total plasma current with radio-frequency waves. The results of High Harmonic Fast Wave heating and current drive studies as well as electron Bernstein Wave emission studies will be presented.