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eau_icc_2011.pdf2011-06-10 16:54:58Zeke Unterberg

Edge Soft X-ray Imaging and Inversion Techniques for Measurement of Magnetic Topology during 3-D Magnetic Perturbations

Author: Zeke A. Unterberg
Requested Type: Consider for Invited
Submitted: 2011-06-10 16:55:32

Co-authors: M.W. Shafer, D.J. Battaglia, J.M. Canik, T.E. Evans, J.H. Harris, S. Ohdachi, R. Maingi, and S. Meitner

Contact Info:
PO Box 85608
San Diego, CA   92186

Abstract Text:
A new tangential 2D soft x-ray imaging system (SXRIS) was recently installed on DIII-D to directly measure the edge island structure in the X-point region. The main motivation for the measurement of the island structure inside the plasma is to help validate the models of plasma response when applying resonant magnetic perturbations (RMPs) and the resulting effect on edge localized mode (ELM) stability. 2D SXR imaging can yield information on the 3D magnetic topology of the plasma under the assumption of particles being line-tied to the magnetic field. This new diagnostic has the spatial resolution to image islands with widths >2 cm near the X-point where the flux expansion is large and island sizes increase. Core MHD studies using a similar diagnostic technique (i.e., tangential viewing with visible cameras, pinhole optics and a scintillator) have been used on both tokamaks and stellarators, but in all these cases the interpretation of the images is complicated by the chordal integration and requires advanced inversion techniques. Such image inversion requires a regularization method because the numerical problem is ill-posed, and the diagnostic layout is inherently composed of limited-angle measurements.
Given the complexity of data analysis, the inversion methods have been exercised on synthetic modeling of DIII-D RMP discharges near the plasma edge as well as core MHD from a wide-field SXR imaging system installed on NSTX. The inversion methods are examined in the context of noise, spatial sensitivity, and symmetry assumptions. The synthetic DIII-D diagnostic calculation is based on estimates of the 3D SXR emissivity that have been calibrated against the NSTX SXR system. The synthetic SXRIS modeling using typical DIII-D RMP discharges indicates integration times ≥ 1 ms with accurate equilibrium reconstruction are needed for small island (≤ 3 cm) detection. The total signal-to-noise ratio at these parameters is estimated to be ≥ 100 and, therefore, gives enough sensitivity to extract the estimated 1-10% island emissivity perturbation from the total SXR emissivity. The inversion methods are also tested against data from core NSTX 2/1 islands. Such core modes are larger in both signal intensity and spatial extent and also have clear signatures on other diagnostics – thus making the analysis relatively simpler. The final results comparing measured and modeled island sizes will be presented.
This work was supported in part by the US Department of Energy under DE-AC02-00OR22725 and DE-FC02-04ER54698.

Characterization: A5,D1

Please see attached PDF for author and co-author affiliations, which I hope will be used when publishing the abstract. Thank you.

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011