Rapidly developing diagnostic, operational, and analysis capability is enabling the first detailed local physics studies to begin in high beta plasmas of the National Spherical Torus Experiment (NSTX). These studies are motivated in part by energy confinement times in neutral-beam-heated discharges that are favorable with respect to predictions from the ITER-89P scaling expression. Analysis of heat fluxes based on profile measurements with NBI suggest that the ion thermal transport may be exceptionally low, and that electron thermal transport is the dominant loss channel. This analysis motivates studies of possible sources of ion heating not presently accounted for by classical collisional processes. Gyrokinetic microstability studies indicate that long wavelength turbulence with k θ ρ i ~ 0.1-1 may be suppressed in these plasmas, while modes with k θ ρ I ~ 50 may be robust. High harmonic fast wave (HHFW) heating efficiently heats electrons on NSTX, and studies have begun using it to to assess transport in the electron channel. Regarding edge transport, H-mode transitions occur with either NBI or HHFW heating. The power required for L-to H-mode transitions far exceeds that expected from empirical ELM-free H mode scaling laws derived from moderate aspect ratio devices. Finally, initial fluctuation measurements made with two techniques are permitting the first characterizations of edge turbulence.
Due to their highly shaped plasma and possible poloidal asymmetry in impurity concentration, spherical Tokamaks will require tomographic reconstruction of local emissivities to assess impurity content and transport. To collect in an effective manner the data required for such reconstruction, we develop arrays of high throughput “mini-monochromators” using extreme ultraviolet multilayer mirrors as dispersive elements and filtered surface barrier diodes as detectors. We discuss monochromator optimization and show that by working at near normal incidence throughput and spectral resolution are simultaneously maximized. A system proposed for tomographic reconstruction of C V and C VI resonance emission at 33.7 and 40.5 Å respectively, achieves 0.9 Å spectral resolution, 2 cm spatial resolution, and 0.2 ms temporal resolution, together with good sensitivity and background rejection. Preliminary results obtained from CDX-U low aspect ratio tokamak are also presented.
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