Overview of Physics Results from the National Spherical Torus Experiment

S.A. Sabbagh, (E. Schuster), et al. (Collaboration Paper)

IAEA Fusion Energy Conference

San Diego, California, USA, October 8-13, 2012

Abstract

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. A 􏰃\tua_E scaling appropriate for varied wall conditions exhibits a strong improvement of B_T\tua_􏰃E with decreased electron collisionality produced by lithium (Li) wall conditioning. Nonlinear microtearing simulations match experimental electron diffusivity quantitatively and predict reduced electron heat transport at lower collisionality. Beam-emission spectroscopy measurements indicate the poloidal correlation length of pedestal turbulence ~10􏰄\rho_i increases at higher electron density gradient and lower Ti gradient. Plasma characteristics change nearly continuously with increasing Li evaporation and ELMs stabilize due to edge density gradient alteration. Global mode stability studies show stabilizing resonant kinetic effects are enhanced at lower collisionality. Combined radial and poloidal field sensor feedback controlled n = 1 perturbations and improved stability. The disruption probability due to unstable RWMs is reduced at high \beta_􏰅N/li > 11 consistent with low frequency MHD 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 􏰅N = 6.4 and 􏰅beta_􏰅N/li = 13. Precursor analysis shows 98% of disruptions can be predicted with 10ms warning and a false positive rate of only 6%. Disruption halo currents rotate toroidally and can have significant toroidal asymmetry. Global kinks cause measured fast ion redistribution. Full-orbit calculations show redistribution from the core outward and toward V||/V = 1 where destabilizing CAE resonances are expected. Applied 3D fields alter GAE characteristics. The snowflake divertor configuration enhanced by radiative detachment shows large reductions in both steady- state and ELM heat fluxes (steady-state peak values down from 7 MW/m2 to less than 1 MW/m2). Toroidal asymmetry of heat deposition is observed during ELMs or by 3D fields. Coaxial helicity injection has reduced the inductive startup flux, with plasmas ramped to 1MA requiring 35% less inductive flux. Non-inductive current fraction (NICF) up to 65% is reached experimentally with NBI at Ip = 0.7 MA and between 70 – 100% with high harmonic fast wave 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.