Initial studies of core and edge transport of NSTX plasmas

Abstract

-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.

The national spherical torus experiment (NSTX) research programme and progress towards high beta, long pulse operating scenarios

“…Heating efficiency, characterized by the value of the central electron temperature, seems to be highest for the slowest wave phase velocities. NSTX plasmas with predominantly electron heating exhibit a strong degradation in confinement, which is consistent with theoretical predictions of an increase in conduction due to ETG modes [40]. Attempts to measure the power deposition profile with modulated rf power have also showed a very stiff temperature profile response consistent with a marginally stable profile.…”

Microstability in a “MAST-like” high confinement mode spherical tokamak equilibrium

“…2 and 3), and the Princeton national spherical tokamak experiment (NSTX) (Ref. 4)]. Ion heat transport has been found to lie close to the level predicted by neoclassical theory.…”

“…Ion heat transport has been found to lie close to the level predicted by neoclassical theory. The NSTX experiment has reported dominant heat transport losses occurring in the electron channel, 4,5 and the study of microinstabilities could shed important light on the underlying reasons for this interesting observation.…”

Micro-tearing modes in the mega ampere spherical tokamak

“…When this occurs, the electrons are no longer confined to equilibrium flux surfaces and are able to move radially by moving along the perturbed field lines. This process can produce significant electron heat transport, which could explain the high ratios of T i /T e measured in a number of spherical tokamak (ST) plasmas (including those from START and NSTX [3,4]).…”