The National Spherical Torus Experiment (NSTX) research program and building the scientific basis for high beta, long pulse operating scenarios

摘要

Research on the National Spherical Torus Experiment is focusing on establishing long-pulse, high beta, high confinement operation. The effort is aimed at forming an extrapolable basis based on studies of high beta stability, transport. and turbulence, wave-particle interactions, and boundary physics. Capabilities include neutral beam (up to 7 MW) and high harmonic fast wave (HHFW, up to 6 MW) heating, toroidal fields up to 6 kG, and currents up to 1.5 MA. Plasmas with average toroidal beta of up to 35% have been obtained. Theoretical assessments of stability indicate that NSTX plasmas routinely exceed the no-wall beta limit by reaping the benefits of a nearby conducting wall combined with plasma rotation.. With NBI, both L mode and H mode confinement exceeds scaling law predictions, and efforts are beginning to assess the characteristics of the turbulence responsible for the transport. Analysis reveals exceptionally low ion thermal and particle transport in L mode and H modes exhibit substantial pressure profile broadening. Up to 6 MW of high harmonic fast waves (relative to the ion cyclotron-frequency) have been used to effectively heat electrons, and their acceleration of fast beam ions has been observed, and indications of current drive have been measured. Research on plasma startup without the aid of the solenoid is a high priority on NSTX. Toroidal currents up to 400 kA have been driven with coaxial helicity injection, and studies to assess flux closure and coupling to other current drive techniques will be discussed. Finally, preliminary results from heat flux scaling studies suggest that NSTX should be able to pulse lengths long compared to a current relaxation time with acceptable heat loads in the divertor.