Predictive capability of MHD stability limits in high performance DIII-D discharges

Abstract

Results from an array of theoretical and computational tools developed to treat the instabilities of most interest for high performance tokamak discharges are described. The theory and experimental diagnostic capabilities have now been developed to the point where detailed predictions can be productively tested so that competing effects can be isolated and either eliminated or confirmed. The predictions using high quality discharge equilibrium reconstructions are tested against the observations for the principal limiting phenomena in DIII-D: L-mode negative central shear (NCS) disruptions, H-mode NCS edge instabilities, and tearing and resistive wall modes (RWMs) in long pulse discharges. In the case of predominantly ideal MHD instabilities, agreement between the code predictions and experimentally observed stability limits and thresholds can now be obtained to within several percent, and the predicted fluctuations and growth rates to within the estimated experimental errors. Edge instabilities can be explained by a new model for edge localized modes as predominantly ideal low to intermediate n modes. Accurate ideal calculations are critical to demonstrating RWM stabilization by plasma rotation and the ideal eigenfunctions provide a good representation of the RWM structure when the rotation slows. Ideal eigenfunctions can then be used to predict stabilization using active feedback. For non-ideal modes, the agreement is approaching levels similar to that for the ideal comparisons; ∆' calculations, for example, indicate that some discharges are linearly unstable to classical tearing modes, consistent with the observed growth of islands in those discharges.

Observation and analysis of a resistive mode with interchange parity in negative central shear plasmas in the DIII-D Tokamak

“…13͒ and DCON. 14 It has been shown that ideal MHD instabilities can be accurately reproduced 15 for DIII-D discharges with good quality internal equilibrium profile data and magnetic probe data, as is typically available for the discharges discussed here. Both codes show that the equilibrium is stable to all low n modes.…”

Observation of SOL current correlated with MHD activity in NBI heated DIII-D tokamak discharges

“…The nominal no-wall limit is ≈ 4 times the dimensionless internal inductance, l i , during an I p flat-top period, and ≈ 2.4 times l i for an I p ramp-up period for DIII-D discharges aimed at high performance [34]. In this article the purpose of comparing b N with a no-wall limit is simply to indicate that observed SOLCs may be relevant to an AT, not as a precise delineation of discharge regimes.…”

Anomalies in the applied magnetic fields in DIII-D and their implications for the understanding of stability experiments

“…DIII-D is a highly non-circular tokamak 11 that has carried out a wide variety of seminal studies of tokamak stability 12,13 . The interpretation of these experiments depends critically on understanding the anomalies in the applied fields and how the plasma interacts with them.…”