Overview of physics results from the conclusive operation of the National Spherical Torus Experiment

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. An energy confinement time, τ E , scaling unified for varied wall conditions exhibits a strong improvement of B T τ 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 T i 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 β N /l i > 10 consistent with low frequency magnetohydrodynamic spectroscopy measurements of mode stability. Greater instability seen at intermediate β N is consistent with decreased kinetic RWM stabilization. A model-based RWM state-space controller produced long-pulse discharges exceeding β N = 6.4 and β N /l i = 13. Precursor analysis shows 96.3% of disruptions can be predicted with 10 ms warning and a false positive rate of only 2.8%. Disruption halo currents rotate toroidally and can have significant toroidal asymmetry. 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 × B shear in the edge region on ion heat flux and X-point radius. Co-axial helicity injection reduced the induct...

Benchmarking kinetic calculations of resistive wall mode stability

“…In each case, the focus was on the calculation of rotational resonances of thermal particles, so collisions were ignored and energetic particles were not included. Though obviously important, it is beyond the scope of this work to fully compare stability calculations to present experimental results, though MISK calculations of RWM stability have been compared to both National Spherical Torus Experiment 47 (NSTX) [19][20][21][22]24,34,48,49 and DIII-D (Refs. 9 and 25) experimental results, while MARS-K calculations have been performed for RWM stability in DIII-D (Refs.…”

Resistive wall mode active control physics design for KSTAR

“…This secondary mode could be controlled by using dual component sensor feedback as successfully demonstrated in NSTX. 16,18 …”

“…14 The addition of multiple plasma modes and a new model-based state space control technique 18 can improve the control capability in KSTAR by incorporating the effect of the full 3D wall eddy currents in real-time in future KSTAR experiments. …”

“…Energy dissipation depending on plasma rotation and kinetic effects can stabilize the RWM, 7,[9][10][11][12][13][14][15][16][17][18] however, this passive mode stabilization generally does not ensure full stabilization in all discharges, and active control of the RWM 16,18-34 is required for low disruptivity at high normalized beta. The growing n ¼ 1 RWM, which is a significant concern in advanced tokamak fusion plasmas, can be either non-rotating or slowly spinning (rotation frequency $Oð1=s w Þ.…”

An overview of recent physics results from NSTX

“…A cryo-pump is being considered for density control. A disruption prediction, avoidance and mitigation (PAM) system will be deployed using a large number of measurements and models for prediction and avoidance [5][6][7] and a massive gas injection (MGI) system at different poloidal locations to optimize gas penetration for reducing potential wall damage due to disruptive forces. Lithium evaporation and boronization will be used as the primary wall conditioning techniques, and off-line research centred on liquid lithium on metal substrates will support a planned phasing in of high-Z PFCs.…”