Calculation of the non-inductive current profile in high-performance NSTX plasmas

Abstract

The constituents of the current profile have been computed for a wide range of high-performance plasmas in NSTX (Ono et al 2000 Nucl. Fusion 40 557); these include cases designed to maximize the non-inductive fraction, pulse length, toroidal-β or stored energy. In the absence of low-frequency MHD activity, good agreement is found between the reconstructed current profile and that predicted by summing the independently calculated inductive, pressure-driven and neutral beam currents, without the need to invoke any anomalous beam ion diffusion. Exceptions occur, for instance, when there are toroidal Alfvén eigenmode avalanches or coupled m/n = 1/1 + 2/1 kink-tearing modes. In these cases, the addition of a spatially and temporally dependent fast-ion diffusivity can reduce the core beam current drive, restoring agreement between the reconstructed profile and the summed constituents, as well as bringing better agreement between the simulated and measured neutron emission rate. An upper bound on the fast-ion diffusivity of ∼0.5–1 m2 s−1 is found in ‘MHD-free’ discharges, based on the neutron emission, the time rate of change in the neutron signal when a neutral beam is stepped and reconstructed on-axis current density.

Recent progress on spherical torus research

“…324 The fast ion transport due to the TAE avalanche and its effect on the NBI driven current has also been investigated. 325 In this study, an upper bound on the fast-ion diffusivity of $0.5-1 m 2 /s À1 is found. This level of fast ion diffusivity can significantly broaden the NBI driven current profile, which can be beneficial from the plasma stability point of view but can also lead to a loss of current drive efficiency.…”

“…113. 325 As can be seen in the figure, NSTX was able to scan a relatively wide range of A. In the inset, the j ¼ 3 plasma equilibrium achieve in NSTX is shown.…”

Detection of Disruptions in the High-β Spherical Torus NSTX

“…These false positives could be due to errors in the diagnostic data (Z eff in particular), or to neutron emission drops due to bursting Alfvenic modes [125,126] that are not captured in the simple slowing-down model. However, when the ratio of measured to predicted neutrons drops beneath 0.4, the analysis shows that a disruption almost always rapidly follows.…”

“…Hence, it is important to apply error field correction techniques that maintain the rotation [47,143], while monitoring the rotation for drops that may render the plasma more susceptible to these modes. Secondly, the core rotating n=1 modes that often occur in NSTX H-mode discharges [116,122,126,[144][145][146] have a tendency to first reduce the core rotation compared to the edge value, locking the plasma into a rigidly rotating system, followed by a rapid damping of the total plasma rotation. Hence, these modes can be detected by their impact on the rotation.…”

Investigation Of A Transient Energetic Charge Exchange Fux Enhancement ('spike-on-tail') Observed In Neutral-beam-heated H-mode Discharges In The National Spherical Torus Experiment

“…The TRANSP analysis code [53,54] uses measured temperature and density (including impurity) profiles, rotation velocity and radiated power to compute the beam ion deposition and density using a Monte-Carlo approach to determine the sources and losses of energy AFID has been applied, for example, to MHD-induced energetic ion redistribution and/or loss effects in simulations of the E||B NPA spectra [13] including horizontal and vertical scanning measurements, in transport [55] and in beam-driven plasma current [3,56]. In the following analysis, the TRANSP option to follow the fast ion gyro-orbit is used (as opposed to the guiding center).…”