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epr2014_abstract_eau_v4.pdf2014-05-30 10:44:45Ezekial Unterberg

Effects of Changes in the 3D Equilibria during RMP Application in Interpretation of Soft X-ray Images on DIII-D H-mode Discharges

Author: Ezekial A Unterberg
Requested Type: Consider for Invited
Submitted: 2014-05-30 10:43:00

Co-authors: M.W. Shafer, J.M. Canik, A. Wingen, N.M. Ferraro, T.E. Evans, J.H. Harris

Contact Info:
Oak Ridge National Laboratory
PO Box 85608
San Diego, CA   92186
USA

Abstract Text:
Externally applied non-axisymmetric magnetic fields on diverted, H-mode DIII-D discharges cause helical perturbations internal to the separatrix. These three-dimensional (3D) perturbations are measured using tangential imaging of soft X-ray (SXR) emission (>~ 300 eV) in the X-point region of the discharge. This work shows the first correlation between changes in the non-linear, 3D equilibrium parameters and the measured SXR structure. Recent work [1] has shown that very small external 3D magnetic perturbations in a tokamak equilibria can change the stability properties of otherwise stable equilibria. For example, modifying the field line shear causes localized changes in the ideal infinite-n ballooning stability and can be used as a proxy for kinetic ballooning mode (KBM) stability and micro-stability both of which change local transport properties. Similar phenomena has recently been observed in the TJ-K stellarator [2], where a direct correlation between localized turbulence and 3D equilibria properties was observed.
The work presented focuses on comparing the structure observed using the edge, SXR-emissivity, imaging system (SXRIS) [3] with 3D equilibria using the VMEC code on DIII-D during resonant magnetic perturbations (RMP) discharges. Modifications to the local field line shear are very similar to the structure seen in the SXR images. The changes are localized poloidally to the X-point region of a lower single null discharge and radial near the m/n=11/3 surface in both the VMEC equilibria and the SXRIS. The correlation between both potentially implies 3D localized KBM instabilities due to the RMPs are causing local transport modifications. Details for the models used for relating equilibrium structure with emissivity changes (i.e., synthetic diagnostic models) will be presented.
[1] T.M. Bird and C.C. Hegna, Nucl. Fusion 53 (2013) 013004.
[2] G. Birkenmeier et al., Phys. Rev. Letter 107 (2011) 025001.
[3] M.W. Shafer et al., Nucl. Fusion 52 (2012) 12200

Characterization: 3.0

Comments:

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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