Abstract
The National Spherical Torus Experiment (NSTX) has made considerable progress in advancing the scientific understanding of high performance long-pulse plasmas needed for future spherical torus (ST) devices and ITER. Plasma durations up to 1.6 s (five current redistribution times) have been achieved at plasma currents of 0.7 MA with non-inductive current fractions above 65% while simultaneously achieving βT and βN values of 17% and 5.7 (%m T MA−1), respectively. A newly available motional Stark effect diagnostic has enabled validation of current-drive sources and improved the understanding of NSTX ‘hybrid’-like scenarios. In MHD research, ex-vessel radial field coils have been utilized to infer and correct intrinsic EFs, provide rotation control and actively stabilize the n = 1 resistive wall mode at ITER-relevant low plasma rotation values. In transport and turbulence research, the low aspect ratio and a wide range of achievable β in the NSTX provide unique data for confinement scaling studies, and a new microwave scattering diagnostic is being used to investigate turbulent density fluctuations with wavenumbers extending from ion to electron gyro-scales. In energetic particle research, cyclic neutron rate drops have been associated with the destabilization of multiple large toroidal Alfven eigenmodes (TAEs) analogous to the ‘sea-of-TAE’ modes predicted for ITER, and three-wave coupling processes have been observed for the first time. In boundary physics research, advanced shape control has enabled studies of the role of magnetic balance in H-mode access and edge localized mode stability. Peak divertor heat flux has been reduced by a factor of 5 using an H-mode-compatible radiative divertor, and lithium conditioning has demonstrated particle pumping and results in improved thermal confinement. Finally, non-solenoidal plasma start-up experiments have achieved plasma currents of 160 kA on closed magnetic flux surfaces utilizing coaxial helicity injection.
Highlights
Progress in plasma performance and understandingThe National Spherical Torus Experiment (NSTX) [1, 2] has made considerable progress in advancing the scientific understanding of high performance long-pulse plasmas needed for low aspect ratio spherical torus (ST) [3] concepts and for ITER
Plasma flat-top durations approaching five current redistribution times [9] and 50 energy confinement times have been achieved with the product of normalized beta and confinement enhancement, βNH89P, in the range needed for an spherical torus (ST)-based component test facility (CTF) [10]
The National Spherical Torus Experiment (NSTX) [1, 2] has made considerable progress in advancing the scientific understanding of high performance long-pulse plasmas needed for low aspect ratio ST [3] concepts and for ITER
Summary
The National Spherical Torus Experiment (NSTX) [1, 2] has made considerable progress in advancing the scientific understanding of high performance long-pulse plasmas needed for low aspect ratio ST [3] concepts and for ITER. Consistency between the reconstructed and the calculated total current density profiles can be obtained if MHD-induced diffusion of the NBI fast ions (χfast = 20– 50 m2 s−1) is assumed in the TRANSP calculation of the beamdriven current Such fast-ion redistribution can apparently convert a centrally peaked NBICD profile into a flat or even hollow profile. A comparatively lower level of AFID is invoked prior to the onset of MHD activity at t = 0.6 s in order to account for apparent fast-ion redistribution by TAE activity With this AFID model included, the reconstructed core current density profile is in much better agreement with the prediction [21], again consistent with MHD activity redistributing the NBI-driven current.
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