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Prospects for Non-invasive Internal Magnetic Field Measurements via Polarimetry on the HIT-SI Spheromak

Author: Nathaniel K. Hicks
Requested Type: Poster Only
Submitted: 2009-12-06 01:10:32

Co-authors: R. J. Smith, T. R. Jarboe, J. S. Wrobel, HIT-SI Team

Contact Info:
University of Washington
Box 352400
Seattle, WA   98195-2
USA

Abstract Text:
The Helicity Injected Torus – Steady Inductive (HIT-SI) spheromak experimental program aims to investigate helicity injection current drive for magnetic confinement. The study of this concept would be facilitated by a non-perturbative diagnostic of the internal magnetic field in HIT-SI discharges, enabling measurement of achieved current, q- and λ-profiles. Motional Stark effect measurements utilizing a diagnostic neutral beam are not feasible on HIT-SI, so instead the technique of polarimetry will be pursued. In this established technique, a linearly polarized probe beam is introduced into the plasma, and its plane of polarization rotates due to the component of the magnetic field parallel to the beam. For a “vertical” measurement in a toroidal geometry, measurement of this angle α allows calculation of the line-integrated poloidal magnetic field component; however, to do so, the electron density must also be known. Conveniently, the same probe beam can be used for an interferometric measurement of the electron density. Local values of the parallel magnetic field component must be obtained via interpolation of line-averaged values between multiple viewing chords, and performing numerical inversion. The calculated B profile in turn provides data for the current and q-profiles, as well as the λ-profile. A preliminary design for such a diagnostic operating at millimeter wavelengths on HIT-SI is presented here, as are projected measurement capabilities in HIT-SI plasmas. The experimental data are foreseen to inform and validate computer modeling of such plasmas. For example, magnetic measurements of MHD mode activity will represent clear macroscopic conditions that should be reproducible in MHD codes under development at the PSI-Center.

This research is supported in part by an appointment to the U.S. Department of Energy Fusion Energy Postdoctoral Research Program administered by the Oak Ridge Institute for Science and Education.

Characterization: A1,E3

Comments:
Please place with the other HIT-SI posters.