Equilibrium Reconstruction Improvement via Kalman-filter-based Vessel Current Estimation at DIII-D
Y. Ou, M.L. Walker, E. Schuster, J.R. Ferron
Symposium on Fusion Technology
Warsaw, Poland, September 11-15, 2006
Abstract
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The efficient and safe operation of large fusion devices relies on
accurate knowledge of many of the discharge parameters. Unfortunately,
the values of several discharge parameters, such as plasma shape and
current density distribution, are not directly measured. However,
these values can be reconstructed from magnetic field and flux
measurements. Equilibrium codes, such as EFIT, calculate the
distributions of flux and toroidal current density over the plasma and
surrounding vacuum region that best fit, in a least square sense, the
external magnetic measurements, and that simultaneously satisfy the
MHD equilibrium equation (Grad-Shafranov equation). Once the flux
distribution is known, it is possible to reconstruct the plasma
boundary for shape control purposes.
The most general approach to the fitting problem treats all toroidal
current sources as unknown values. Thus, in addition to the plasma
toroidal current, the currents in the external poloidal field (PF)
coils can be free parameters and, potentially, the induced currents in
the vacuum vessel and support structures can be treated this way as
well. There are direct measurements of the external PF coil currents,
but these measurements have uncertainties that can be properly
accounted for in the least squares fitting procedure by solving for
the external currents using the measurements as constraints. A similar
procedure could be followed for the vessel currents if they were
measurable. Unfortunately, this is not usually the case and vessel
currents are often neglected in the fitting procedure.
The important effect of vessel or structure currents has been
recognized in many plasma control applications. Kalman filtering theory
is used in this work to optimally estimate the current in the tokamak
vessel. With the ultimate goal of improving the equilibrium
reconstruction for the DIII-D tokamak, the real-time version of the
EFIT algorithm is modified to accept the estimated vessel currents.
Furthermore, it will be shown that the integration of Kalman filter
estimation into the equilibrium reconstruction algorithm provides a
new way to validate and refine the plasma dynamic model.