Overview of Physics Results from the National Spherical Torus Experiment
S.A. Sabbagh, ..., E. Schuster, et al. (Collaboration Paper)
Nuclear Fusion 53 (2013) 104007 (23pp)
Abstract
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Research on the National Spherical Torus Experiment, NSTX, targets physics understanding needed for extrapolation to a steady-state ST
Fusion Nuclear Science Facility, pilot plant, or DEMO. The unique ST operational space is leveraged to test physics theories for next-step
tokamak operation, including ITER. Present research also examines implications for the coming device upgrade, NSTX-U. An energy confinement
time, \tau_E, scaling unified for varied wall conditions exhibits a strong improvement of B_T\tau_E with decreased electron collisionality,
accentuated by lithium (Li) wall conditioning. This result is consistent with nonlinear microtearing simulations that match the experimental
electron diffusivity quantitatively and predict reduced electron heat transport at lower collisionality. Beam-emission spectroscopy
measurements in the steep gradient region of the pedestal indicate the poloidal correlation length of turbulence of about ten ion gyroradii
increases at higher electron density gradient and lower Ti gradient, consistent with turbulence caused by trapped electron instabilities.
Density fluctuations in the pedestal top region indicate ion-scale microturbulence compatible with ion temperature gradient and/or kinetic
ballooning mode instabilities. Plasma characteristics change nearly continuously with increasing Li evaporation and edge localized modes
(ELMs) stabilize due to edge density gradient alteration. Global mode stability studies show stabilizing resonant kinetic effects are
enhanced at lower collisionality, but in stark contrast have almost no dependence on collisionality when the plasma is off-resonance.
Combined resistive wall mode radial and poloidal field sensor feedback was used to control n=1 perturbations and improve stability. The
disruption probability due to unstable resistive wall modes (RWMs) was surprisingly reduced at very high beta_N/li > 10 consistent with
low frequency magnetohydrodynamic spectroscopy measurements of mode stability. Greater instability seen at intermediate beta_N is consistent
with decreased kinetic RWM stabilization. A model-based RWM state-space controller produced long-pulse discharges exceeding beta_N = 6.4 and
beta_N/li = 13. Precursor analysis shows 96.3% of disruptions can be predicted with 10ms warning and a false positive rate of only 2.8%.
Disruption halo currents rotate toroidally and can have significant toroidal asymmetry. Global kinks cause measured fast ion redistribution,
with full-orbit calculations showing redistribution from the core outward and towards V///V = 1 where destabilizing compressional Alfven
eigenmode resonances are expected. Applied 3D fields altered global Alfven eigenmode characteristics. High-harmonic fast-wave (HHFW) power
couples to field lines across the entire width of the scrape-off layer, showing the importance of the inclusion of this phenomenon in
designing future RF systems. The snowflake divertor configuration enhanced by radiative detachment showed large reductions in both
steady-state and ELM heat fluxes (ELMing peak values down from 19 MW m-2 to less than 1.5 MW m-2). Toroidal asymmetry of heat deposition was
observed during ELMs or by 3D fields. The heating power required for accessing H-mode decreased by 30% as the triangularity was decreased
by moving the X-point to larger radius, consistent with calculations of the dependence of E x B shear in the edge region on ion heat flux
and X-point radius. Co-axial helicity injection reduced the inductive start-up flux, with plasmas ramped to 1 MA requiring 35% less
inductive flux. Non-inductive current fraction (NICF) up to 65% is reached experimentally with neutral beam injection at plasma current
Ip = 0.7 MA and between 70-100% with HHFW application at Ip = 0.3 MA. NSTX-U scenario development calculations project 100% NICF for a
large range of 0.6 < Ip(MA) < 1.35.