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Fast motional Stark effect diagnostic measurements of hybrid-like magnetic flux-pumping and neoclassical tearing modes in the DIII-D tokamak

  • Author(s): King, Joshua Daniel
  • Advisor(s): Morse, Edward C
  • et al.
Abstract

MSE measurements reveal hybrid-like flux-pumping associated with 2/1 NTM-ELM coupling. Analysis of MSE signals using digital lock-in amplifiers shows the strength of the flux-pumping is more than twice that of typical hybrid discharges. This flux-pumping maintains the minimum safety factor above unity, thereby avoiding sawteeth. The strength of the flux-pumping and ELM-NTM coupling have a clear upward dependence on normalized beta and NTM-pedestal proximity. The size of the island does not appear to effect flux-pumping, except that the mode must be present, suggesting the island chain serves as a radial pivot surface around which poloidal flux is pumped from the core to the edge. This result implies that higher normalized fusion performance (lower q95 and higher beta) may be achieved in hybrid discharges that contain a partially suppressed 2/1 NTM. ELM-NTM coupling consists of an Alfv<é>nic timescale drop in the island width followed by a resistive recovery. The recovery phase is successfully modeled using the modified Rutherford equation. The depth of the drop in island width increases as the size of the ELM increases.

To aid in the design of a highly resolved MSE pedestal measurement, full spectral analysis was preformed on existing edge channels. This analysis has revealed that coherent core MHD oscillations cause interference with present dual PEM polarimeters. Avoiding this interference requires a dedicated pedestal polarimeter with second harmonic frequencies greater than those of MHD fluctuations.

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