TEXT-U error field measured from MHD dynamics

A new method is described for determining error field strengths using the dynamics of a rotating m identical with 2, n identical with 1 resistive MHD tearing mode as observed on TEXT-U. A set of external saddle coils induces a magnetic field whose m identical with 2, n identical with 1 component is...

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Bibliographic Details
Published in:Nuclear fusion 1998-04, Vol.38 (4), p.585-595
Main Authors: Craven, W.A, Wootton, A.J
Format: Article
Language:English
Subjects:
Demodulation
Exact sciences and technology
Magnetic confinement and equilibrium
Magnetic field measurement
Magnetohydrodynamics
Measurement and error theory
Metrology
Metrology, measurements and laboratory procedures
Physics
Physics of gases, plasmas and electric discharges
Physics of plasmas and electric discharges
Plasma dynamics and flow
Plasma flow
magnetohydrodynamics
Plasma stability
Tokamak devices
Tokamaks
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Summary:A new method is described for determining error field strengths using the dynamics of a rotating m identical with 2, n identical with 1 resistive MHD tearing mode as observed on TEXT-U. A set of external saddle coils induces a magnetic field whose m identical with 2, n identical with 1 component is either in phase or out of phase spatially with the intrinsic m identical with 2, n identical with 1 error field. This experimentally applied field, combined with the pre-existing error field, decelerates the plasma rotation by coupling to the tearing mode and induces a locked mode. The time evolution of the average rotation frequency and the Fourier amplitudes of a Mirnov signal observing the tearing mode are predicted analytically. This analytic result compares favourably with experimental data, which are analyzed for the frequency and Fourier amplitudes using variable frequency complex demodulation. The error field is measured at the q identical with 2 surface to be 0.45 plus or minus 0.09 G by noting when in the evolution the applied field cancels the error field. This technique is unique because it measures the field error in the equilibrium of interest and is extremely sensitive (the measured error is 2x10 super(-5)B sub(0)); because it is a dynamic measurement, it could conceivably be deployed in real time to prevent locked mode disruptions.
ISSN:0029-5515
1741-4326
DOI:10.1088/0029-5515/38/4/309