Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

calized field errors that can cause wall locking of tearing modes, as shown in RFX-mod and mst.
RFX-mod experiments have also established that locking of tearing mode phases to the wall can
be mitigated by appropriate feedback using the full-coverage coil set.
research requirements
Present experiments suggest that it is possible to operate a RFP without a very conducting wall.
however, key questions remain, and the optimal control system has not been identified. Will
coils located inside the vessel be able to mitigate phase locking to the wall, optimize the plasma
shape and help for power handling (as in iteR)? Will it be necessary to include special flux loops
to control m=0 modes (m is poloidal mode number)? Will ad hoc saddle coils be able to compensate
for the residual error fields due to the unavoidable asymmetries of the magnetic boundary?
certain types of coils may be optimal for different instabilities. nonlinear coupling between m=1
and m=0 poloidal mode makes control of both harmonics important. separate coils optimized for
m=0 control may be needed. The toroidal field coil set in RFX-mod could in principle provide significant
m=0 mode control, but the precision of the currently installed power supply units is limited.
in both RFX-mode and extrap t2R, the issue of sideband aliasing (unwanted perturbations
in other modes) has been observed and improvements in control have been obtained by reducing
it either by increasing the number of actuators (t2R) or by removing it from the measurements in
real time (RFX-mod). it is vital to understand the required minimum number of control coils, and
how advanced control algorithms can surmount issues such as sideband aliasing.
The development of accurate models is essential for understanding the behavior of the present
control systems and for predicting the performance and control requirements of future devices.
several approaches are possible. an existing model of the electromagnetic boundary that includes
simplified transfer functions for the feedback-controlled current power supply units in a multiinput,
multi-output paradigm reveals the necessity to better understand and model the mutual
couplings of the feedback coil hardware. in the near term, merging the electromagnetic code cariddi
with the maRs code can advance this issue. The development of a full simulator of an RWm
control system for existing devices will also allow testing different kinds of model-based control
approaches, where standard Pid controllers will be replaced by more complex state-space controllers,
and which may also take into account the 3-d structure of the magnetic boundary. a parallel
approach to RWm control modeling is based on the development of a more complete physics model
of the plasma in a simplified geometry, taking into account plasma pressure, compressibility,
inertial, dissipation and longitudinal plasma rotation. in addition to RWm modes, optimization
of the magnetic boundary for the control of tearing modes is important, for example, identifying
the ideal non-axisymmetric deformation of the last closed magnetic surface.
SELF-COnSiStEnt REaCtOR SCEnaRiOS
in addition to the integration of current sustainment and improved confinement, a self-consistent
scenario must also integrate RWm control and plasma boundary control to achieve high-performance
RFP discharges and establish the knowledge base for a burning plasma experiment. The
RFX-mod and extrap t2R facilities have capability to begin addressing the optimization of RWm
control, but not for issues such as requirements on the proximity of feedback coils to the plasma
surface. mst can apply current profile control and possibly enhance plasma pressure, but cannot
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