High performance plasmas in the national spherical torus experiment (NSTX)

The National Spherical Toms Experiment has produced toroidal plasmas at low aspect ratio (A = R/a = 0.86m/0.68m ∼ 1.3) with plasma currents of 1MA. The spherical torus confinement concept is promising because it holds the potential of confining high temperature plasmas with less applied magnetic field. Neutral beam heating power of 5MW has been injected. Also, 4MW of High Harmonic Fast Wave (HHFW) heating power has been applied, with 6MW planned. The rapid development of the machine has led to very exciting physics results during the first full year of physics operation. In particular, plasmas with β_t = 22% (=2μ₀p/B² = a measure of efficiency), as calculated using the EFIT equilibrium reconstruction code, have been achieved, with pulse lengths in excess of 0.5s. β limiting phenomena have been observed and the MHD modes that lead to this limit will be discussed. High frequency (>MHz) magnetic fluctuations have also been observed. Following a wall conditioning regime, which included bakeout at 1500C, helium glow discharge cleaning, and boronization, H-mode plasmas are observed with confinement times of > 100ms. H-modes on NSTX are limited by impurity accumulation, indicating further conditioning could improve plasma performance. Beam heated plasmas show energy confinement times in excess of those predicted by empirical scaling expressions. Clear evidence of electron heating is seen with HHFW using only 2MW of heating power, as observed using the multi point Thomson scattering diagnostic. A challenge for spherical tort is developing a non-inductive startup scheme, since the nearly spherical geometry makes the inclusion of a transformer difficult. Experiments using HHFW for profile control have successfully maintained a modified current profile. A non-inductive current drive concept known as Coaxial Helicity Injection (CHI) has driven 260kA of toroidal current. New diagnostics are rapidly coming on line, and results from these measurements will be presented.