Initial physics results from the National Spherical Torus Experiment

The mission of the National Spherical Torus Experiment (NSTX) is to extend the understanding of toroidal physics to low aspect ratio (R/a similar or e qual to1.25) in low collisionality regimes. NSTX is designed to operate wit h up to 6 MW of high harmonic fast wave (HHFW) heating and current drive, 5 MW of neutral beam injection (NBI) and co-axial helicity injection (CHI) f or noninductive startup. Initial experiments focused on establishing condit ions that will allow NSTX to achieve its aims of simultaneous high beta (t) and high-bootstrap current fraction, and to develop methods for noninducti ve operation, which will be necessary for Spherical Torus power plants. Ohm ic discharges with plasma currents up to 1 MA and with a range of shapes an d configurations were produced. Density limits in deuterium and helium reac hed 80% and 120% of the Greenwald limit, respectively. Significant electron heating was observed with up to 2.3 MW of HHFW. Up to 270 kA of toroidal c urrent for up to 200 ms was produced noninductively using CHI. Initial NBI experiments were carried out with up to two beam sources (3.2 MW). Plasmas with stored energies of up to 140 kJ and beta (t)=21% were produced. (C) 20 01 American Institute of Physics.