Overview of recent physics results from the National Spherical Torus Experiment (NSTX)

Abstract

Abstract
\nThe National Spherical Torus Experiment (NSTX) has made considerable progress in advancing the scientific
\nunderstanding of high performance long-pulse plasmas needed for future spherical torus (ST) devices and ITER.
\nPlasma durations up to 1.6 s (five current redistribution times) have been achieved at plasma currents of 0.7MA
\nwith non-inductive current fractions above 65% while simultaneously achieving βT and βN values of 17% and 5.7
\n(%mTMA−1), respectively. A newly available motional Stark effect diagnostic has enabled validation of currentdrive
\nsources and improved the understanding of NSTX ‘hybrid’-like scenarios. In MHD research, ex-vessel radial
\nfield coils have been utilized to infer and correct intrinsic EFs, provide rotation control and actively stabilize the
\nn = 1 resistive wall mode at ITER-relevant low plasma rotation values. In transport and turbulence research,
\nthe low aspect ratio and a wide range of achievable β in the NSTX provide unique data for confinement scaling
\nstudies, and a new microwave scattering diagnostic is being used to investigate turbulent density fluctuations with
\nwavenumbers extending from ion to electron gyro-scales. In energetic particle research, cyclic neutron rate drops
\nhave been associated with the destabilization of multiple large toroidal Alfven eigenmodes (TAEs) analogous to
\nthe ‘sea-of-TAE’ modes predicted for ITER, and three-wave coupling processes have been observed for the first
\ntime. In boundary physics research, advanced shape control has enabled studies of the role of magnetic balance in
\nH-mode access and edge localized mode stability. Peak divertor heat flux has been reduced by a factor of 5 using an
\nH-mode-compatible radiative divertor, and lithium conditioning has demonstrated particle pumping and results in
\nimproved thermal confinement. Finally, non-solenoidal plasma start-up experiments have achieved plasma currents
\nof 160 kA on closed magnetic flux surfaces utilizing coaxial helicity injection.