High performance plasmas on the National Spherical Torus Experiment

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, where R is the major radius and a is the minor radius of the torus) with plasma currents of 1.4 MA. The rapid development of the machine has led to very exciting physics results during the first full year of physics operation. Pulse lengths in excess of 0.5 s have been obtained with inductive current drive. Up to 4 MW of high harmonic fast wave (HHFW) heating power has been applied with 6 MW planned. Using only 2 MW of HHFW heating power clear evidence of electron heating is seen with HHFW, as observed by the multi point Thomson scattering diagnostic. A noninductive current drive concept known as coaxial helicity injection (CHI) has driven 260 kA of toroidal current. A neutral beam heating power of 5 MW has been injected. Plasmas with /spl beta//sub 1/ (= 2/spl mu//sub 0/ < p > /B/sup 2/ = a measure of magnetic confinement efficiency) of 22% have been achieved, as calculated using the EFIT equilibrium reconstruction code. /spl beta/ limiting phenomena have been observed, and the maximum /spl beta//sub 1/ scales with I/sub p//B/sub 1./ High frequency (> MHz) magnetic fluctuations have been observed. H-mode plasmas are observed with confinement times of > 100 s. Beam heated plasmas show energy confinement times in excess of those predicted by empirical scaling expressions. Ion temperatures in excess of 2.0 keV have been measured, and the power balance suggests that the power loss from the ions to the electrons may exceed the calculated classical input power to the ions.