Preliminary measurements of the edge magnetic field pitch from 2-D Doppler backscattering in MAST and NSTX-U
Authors: R. G. L. Vann, K. J. Brunner, R. Ellis,
G. Taylor, D. A. Thomas
Abstract:
The Synthetic Aperture Microwave Imaging (SAMI) system is a novel
diagnostic consisting of an array of 8 independently phased
antennas. At any one time, SAMI operates at one of the 16
frequencies in the range 10-34.5 GHz. The imaging beam is steered
in software post-shot to create a picture of the entire emission
surface. In SAMI's active probing mode of operation, the plasma
edge is illuminated with a monochromatic source and SAMI
reconstructs an image of the Doppler back-scattered (DBS) signal.
By assuming that density fluctuations are extended along magnetic
field lines, and knowing that the strongest back-scattered signals
are directed perpendicular to the density fluctuations, SAMI's 2-D
DBS imaging capability can be used to measure the pitch of the
edge magnetic field. In this paper, we present preliminary pitch
angle measurements obtained by SAMI on the Mega Amp Spherical
Tokamak (MAST) at Culham Centre for Fusion Energy and on the
National Spherical Torus Experiment Upgrade at Princeton Plasma
Physics Laboratory. The results demonstrate encouraging agreement
between SAMI and other independent measurements.
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