Abstract
Rapidly developing diagnostic, operational, and analysis capability is enabling
the first detailed local physics studies to begin in high β plasmas of the National
Spherical Torus Experiment (NSTX). These studies are motivated in part by the
observation of energy confinement times in neutral-beam-heated discharges
that are favourable with respect to predictions from the ITER-89P scaling
expression. For plasmas heated with neutral beam injection (NBI), analysis
based on profile measurements suggests that electron heat conduction is the
dominant thermal loss channel. Cases where early analysis indicates that ion
thermal conduction may be exceptionally low is motivating studies of possible
sources of ion heating not presently accounted for by classical collisional
processes. Gyrokinetic microstability studies indicate that long wavelength
turbulence with kθρi ∼ 0.1–1 may be stable or suppressed by E × B shear in
these plasmas, while modes with kθρi ∼ 50 may be robust. High harmonic
fast wave (HHFW) heating efficiently heats electrons on NSTX, and studies
have begun using it 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.