Abstract
The major objective of the National Spherical Torus Experiment (NSTX) is to understand basic toroidal confinement
physics at low aspect ratio and high βT in order to advance the spherical torus (ST) concept. In order to do this,
NSTX utilizes up to 7.5MW of neutral beam injection, up to 6MW of high harmonic fast waves (HHFWs), and it
operates with plasma currents up to 1.5MA and elongations of up to 2.6 at a toroidal field up to 0.45 T. New facility,
and diagnostic and modelling capabilities developed over the past two years have enabled the NSTX research
team to make significant progress towards establishing this physics basis for future ST devices. Improvements
in plasma control have led to more routine operation at high elongation and high βT (up to ∼40%) lasting for
many energy confinement times. βT can be limited by either internal or external modes. The installation of an
active error field (EF) correction coil pair has expanded the operating regime at low density and has allowed for
initial resonant EF amplification experiments. The determination of the confinement and transport properties of
NSTX plasmas has benefitted greatly from the implementation of higher spatial resolution kinetic diagnostics.
The parametric variation of confinement is similar to that at conventional aspect ratio but with values enhanced
relative to those determined from conventional aspect ratio scalings and with a BT dependence. The transport
is highly dependent on details of both the flow and magnetic shear. Core turbulence was measured for the
first time in an ST through correlation reflectometry. Non-inductive start-up has been explored using PF-only
and transient co-axial helicity injection techniques, resulting in up to 140 kA of toroidal current generated by
the latter technique. Calculated bootstrap and beam-driven currents have sustained up to 60% of the flat-top
plasma current in NBI discharges. Studies of HHFW absorption have indicated parametric decay of the wave
and associated edge thermal ion heating. Energetic particle modes, most notably toroidal Alfv´en eigenmodes
and fishbone-like modes result in fast particle losses, and these instabilities may affect fast ion confinement on
devices such as ITER. Finally, a variety of techniques has been developed for fuelling and power and particle
control.