Abstract
Observations of improved radio frequency (RF) heating efficiency in high-confinement (H-) mode
plasmas on the National Spherical Tokamak Experiment (NSTX) are investigated by whole-device
linear simulation. We present the first full-wave simulation to couple kinetic physics of the well
confined core plasma to the poorly confined scrape-off plasma. The new simulation is used to
scan the launched fast-wave spectrum and examine the steady-state electric wave field structure
for experimental scenarios corresponding to both reduced, and improved RF heating efficiency. We
find that launching toroidal wave-numbers that required for fast-wave propagation excites large
amplitude (kVm−1 ) coaxial standing modes in the wave electric field between the confined plasma
density pedestal and conducting vessel wall. Qualitative comparison with measurements of the
stored plasma energy suggest these modes are a probable cause of degraded heating efficiency. Also,
the H-mode density pedestal and fast-wave cutoff within the confined plasma allow for the excitation
of whispering gallery type eigenmodes localised to the plasma edge.