Model-based Control of the Resistive Wall Mode in DIII-D:
A Comparison Study
Symposium on Fusion Technology
Rostock, Germany, September 15-19, 2008
| Abstract |
|
One of the major non-axisymmetric instabilities under study in the
DIII-D tokamak is the resistive wall mode (RWM), a form of plasma kink
instability whose growth rate is moderated by the influence of a
resistive wall. One of the approaches for RWM stabilization, referred
to as magnetic control, uses feedback control to produce magnetic
fields opposing the moving field that accompanies the growth of the
mode. These fields are generated by coils arranged around the tokamak.
One problem with RWM control methods used in present experiments is
that they predominantly use simple non-model- based
proportional-derivative (PD) controllers requiring substantial
derivative gain for stabilization, which implies a large response to
noise, leading to a requirement for high peak voltages and coil
currents, usually leading to actuation saturation. Motivated by this
limitation, current efforts in DIII-D include the development of
model-based RWM controllers. The General Atomics/Far-Tech DIII-D RWM
model represents the plasma surface as a toroidal current sheet and
represents the wall using an eigenmode approach [1]. Although the
plasma surface deformation cannot be directly measured in real time,
the magnitude and phase of the deformation can be diagnosed from
measurements by a set of 22 magnetic field sensors composed of poloidal
probes and saddle loops. An array of 12 control I-coils can then be
used to return the plasma to its original axisymmetric shape. Using an
estimator for the two orthogonal components of the assumed n=1 mode
pattern, the resultant plant can be constructed into a reduced form
from the original 12 input, 22 output. In particular, using a typical
quartet configuration for the I-coils and matched filter, the plant
can be simplified to a 3-input/2-output system. Optimal and robust
controllers have been designed exploiting the availability of the RWM
dynamic model. The controllers are tested through simulations, and
results are compared to present non-model-based PD controllers.
Advantages and disadvantages associated with the different control
approaches, including implications for experimental implementation and
use, are discussed.
[1] Y. In, et al, “Model-based dynamic resistive wall mode identification
and feedback control in the DIII-D tokamak,” Phys. Plasmas 13 (2006) 062512.