High performance plasmas in the National Spherical Torus Experiment (NSTX)

Abstract

Summary form only given. The National Spherical Torus Experiment has produced toroidal plasmas at low aspect ratio (A=R/a=0.86 m/0.68 m/spl sim/1.3) with plasma currents of 1 MA. 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 5 MW has been injected. Also, 4 MW of High Harmonic Fast Wave (HHFW) heating power has been applied, with 6 MW 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 /spl beta//sub 1/=22% (=2/spl mu//sub 0/p/B/sup 2/=a measure of efficiency), as calculated using the EFIT equilibrium reconstruction code, have been achieved, with pulse lengths in excess of 0.5s. /spl beta/ 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 >100 ms. 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 2 MW of heating power, as observed using the multi point Thomson scattering diagnostic. A challenge for spherical tori 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 260 kA of toroidal current. New diagnostics are rapidly coming on line, and results from these measurements will be presented.