Abstract
Edge parameters play a critical role in high confinement mode ~H-mode! access, which is a key
component of discharge optimization in present day toroidal confinement experiments and the
design of next generation devices. Because the edge magnetic topology of a spherical torus ~ST!
differs from a conventional aspect ratio tokamak, H-modes in STs exhibit important differences
compared with tokamaks. The dependence of the National Spherical Torus Experiment ~NSTX! @C.
Neumeyer et al., Fusion Eng. Des. 54, 275 ~2001!# edge plasma on heating power, including the low
confinement mode ~L-mode! to H-mode ~L-H! transition requirements and the occurrence of
edge-localized modes ~ELMs!, and on divertor configuration is quantified. Comparisons between
good L-modes and H-modes show greater differences in the ion channel than the electron channel.
The threshold power for the H-mode transition in NSTX is generally above the predictions of a
recent International Tokamak Experimental Reactor ~ITER! @ITER Physics Basis Editors, Nucl.
Fusion 39, 2175 ~1999!# scaling. Correlations of transition and ELM phenomena with turbulent
fluctuations revealed by gas puff imaging and reflectometry are observed. In both single-null and
double-null divertor discharges, the density peaks off-axis, sometimes developing prominent ‘‘ears’’
which can be sustained for many energy confinement times, t E , in the absence of ELMs. A wide
variety of ELM behavior is observed, and ELM characteristics depend on configuration and fueling.
© 2003 American Institute of Physics. @DOI: 10.1063/1.1567288#
I. INTRODUCTION
A main goal of the National Spherical Torus Experiment
~NSTX!1 is to assess the attractiveness of the spherical torus
~ST! for pulse lengths longer than current diffusion times.
This is important since one possible advantage of operation
at low aspect ratio, A[R/a, would be a smaller size, and in
turn, a lower cost reactor concept. High confinement mode
~H-mode! studies are important to this goal since H-mode
profiles provide the stability needed for high b long pulse
operation.
Recent operation of NSTX has resulted in rapid progress
in increasing performance.2–4 This was accomplished
through reduction of intrinsic magnetic field errors, improvements
in machine