Long pulse high performance plasma scenario development for the National Spherical Torus Experiment

The National Spherical Torus Experiment [Ono , Nucl. Fusion, 44, 452 (2004)] is targeting long pulse high performance, noninductive sustained operations at low aspect ratio, and the demonstration of nonsolenoidal startup and current rampup. The modeling of these plasmas provides a framework for experimental planning and identifies the tools to access these regimes. Simulations based on neutral beam injection (NBI)-heated plasmas are made to understand the impact of various modifications and identify the requirements for (1) high elongation and triangularity, (2) density control to optimize the current drive, (3) plasma rotation and/or feedback stabilization to operate above the no-wall beta limit, and (4) electron Bernstein waves (EBW) for off-axis heating/current drive (H/CD). Integrated scenarios are constructed to provide the transport evolution and H/CD source modeling, supported by rf and stability analyses. Important factors include the energy confinement, Z(eff), early heating/H mode, broadening of the NBI-driven current profile, and maintaining q(0) and q(min)> 1.0. Simulations show that noninductive sustained plasmas can be reached at I-P=800 kA, B-T=0.5 T, kappa approximate to 2.5, beta(N)<= 5, beta <= 15%, f(NI)=92%, and q(0)> 1.0 with NBI H/CD, density control, and similar global energy confinement to experiments. The noninductive sustained high beta plasmas can be reached at I-P=1.0 MA, B-T=0.35 T, kappa approximate to 2.5, beta(N)<= 9, beta <= 43%, f(NI)=100%, and q(0)> 1.5 with NBI H/CD and 3.0 MW of EBW H/CD, density control, and 25% higher global energy confinement than experiments. A scenario for nonsolenoidal plasma current rampup is developed using high harmonic fast wave H/CD in the early low I-P and low T-e phase, followed by NBI H/CD to continue the current ramp, reaching a maximum of 480 kA after 3.4 s. (c) 2006 American Institute of Physics.