PPPL-4126 is available in pdf format (1.1 MB).

Progress towards High Performance Plasmas in the National Spherical Torus Experiment (NSTX)

Authors: S.M. Kaye, M.G. Bell, R.E. Bell, S. Bernabei, J. Bialek, T. Biewer, W. Blanchard, et al.

Date of PPPL Report: November 2005

Published in: Nuclear Fusion 45:10 (2005) S168–S180
Presented at: the Twentieth IAEA Fusion Energy Conference, 1 – 6 November 2004, Tivoli Marinotel, Vilamoura, Portugal.

The major objective of the National Spherical Torus Experiment (NSTX) is to understand basic toroidal confinement physics at low aspect ratio and high βT in order to advance the ST concept. In order to do this, NSTX utilizes up to 7.5 MW of Neutral Beam Injection, up to 6 MW of High Harmonic Fast Waves, and it operates with plasma currents up to 1.5 MA and elongations of up to 2.6 at a toroidal field up to 0.45 T. New facility, diagnostic and modeling capabilities developed over the past two years have enabled the NSTX research team to make significant progress towards establishing this physics basis for future ST devices. Improvements in plasma control have led to more routine operation at high elongation and high βT (up to ~40%) lasting for many energy confinement times. βT can be limited by either internal or external modes. The installation of an active error field correction coil pair has expanded the operating regime at low density and has allowed for initial resonant error field amplification experiments. The determination of the confinement and transport properties of NSTX plasmas has benefited greatly from the implementation of higher spatial resolution kinetic diagnostics. The parametric variation of confinement is similar to that at conventional aspect ratio but with values enhanced relative to those determined from conventional aspect ratio scalings and with a ΒT dependence. The transport is highly dependent on details of both the flow and magnetic shear. Core turbulence was measured for the first time in an ST through correlation reflectometry. Non-inductive startup has been explored using PF-only and transient co-axial helicity injection techniques, resulting in up to 140 kA of toroidal current generated by the latter technique. Calculated bootstrap and beam-driven currents have sustained up to 60% of the flattop plasma current in NBI discharges. Studies of HHFW absorption have indicated parametric decay of the wave and associated edge thermal ion heating. Energetic particle modes, most notably TAE and fishbone-like modes result in fast particle losses, and these instabilities may affect fast ion confinement on devices such as ITER. Finally, a variety of techniques has been developed for fueling and power and particle control.