Recent progress towards an advanced spherical torus operating point in NSTX

Abstract

Progress in the development of integrated advanced ST plasma scenarios in NSTX (Ono et al 2000 Nucl. Fusion 40 557) is reported. Recent high-performance plasmas in NSTX following lithium coating of the plasma facing surfaces have achieved higher elongation and lower internal inductance than previously. Analysis of the thermal confinement in these lithiumized discharges shows a stronger plasma current and weaker toroidal field dependence than in previous ST confinement scaling studies; the ITER-98(y, 2) scaling expression describes these scenarios reasonably well. Analysis during periods free of MHD activity has shown that the reconstructed current profile can be understood as the sum of pressure driven, inductive and neutral beam driven currents, without requiring any anomalous fast-ion transport. Non-inductive fractions of 65–70%, and βP > 2, have been achieved at lower plasma current. Some of these low-inductance discharges have a significantly reduced no-wall βN limit, and often have βN at or near the with-wall limit. Coupled m/n = 1/1 + 2/1 kink/tearing modes can limit the sustained β values when rapidly growing ideal modes are avoided. A βN controller has been commissioned and utilized in sustaining high-performance plasmas. ‘Snowflake’ divertors compatible with high-performance plasmas have been developed. Scenarios with significantly larger aspect ratios have also been developed, in support of next-step ST devices. Overall, these NSTX plasmas have many characteristics required for next-step ST devices.

The dependence of H-mode energy confinement and transport on collisionality in NSTX

“…This is especially true using the gyroBohm assumption, with the normalized confinement scaling with collisionality as ν * e −1.50±0. 18 and ν * e −1.91±0.22 for the Bohm (α=2) and gyroBohm (α=3) assumptions respectively. Previous analysis of HeGDC+B discharges indicated this dataset to be more consistent with the gyroBohm than the Bohm assumption.…”

Detection of Disruptions in the High-β Spherical Torus NSTX

“…This change not only replaced a fraction of the graphite PFCs with a porous molybdenum surface, it resulted in a substantial increase in the fraction of discharges with lower-triangularity, so that the outer strikepoint would be in the vicinity of LLD. This is to be contrasted to previous campaigns, where higher-triangularity discharges were generally preferred due to the improved performance of discharges with strong shaping [120][121][122].…”

“…Such a model was used in Ref [127] to project operating points for NSTX-U, and is described here briefly. The plasma current, toroidal field, density, heating power, and geometry are used to evaluate the ITER-98pb(y,2) scaling expression [124] to predict the stored energy, volume average pressure € P th , the poloidal beta shown to predict the energy confinement in NSTX H-mode discharges [122] with reasonable accuracy when lithium conditioning [117,118] is used, as was the case in the majority of discharges used in this study. These data are then used to estimate the bootstrap current [128][129][130][131][132] and neutral beam current [133][134][135] from simple scaling expressions.…”

Simulation Of Microtearing Turbulence In NSTX

“…This scaling predicts improved confinement at smaller values of ν * envisioned for next generation STs [41,42]. It is important to clarify the cause of this scaling and whether it will hold at lower ν * [43].…”