Integrated Control of Individual Scalars to Regulate Profiles and Improve MHD Stability in Tokamaks
A. Pajares, E. Schuster, A. Welander, J. Barr, N. Eidietis, K.Thome, D. Humphreys,
28th IAEA Fusion Energy Conference
Nice, France, October 12-17, 2020 -> May 10-15, 2021 (Remote)
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Abstract
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Simulations using the Control-Oriented Transport SIMulator (COTSIM)
and DIII-D experiments have been carried out to demonstrate the
performance of a new algorithm for simultaneous control of
individual-scalar magnitudes. The individual scalars considered in this
work include kinetic variables, such as the thermal stored energy (W)
or bulk toroidal rotation (Ωφ), and magnetic variables, such as the
safety factor profile (q) at different spatial locations (e.g., q95, q0).
In addition, the individual-scalars controller has been integrated with
other existing algorithms, such as the Off-Normal Fault Response (ONFR)
system and NTM suppression algorithms, in order to increase the level
of integration of the current DIII-D PCS architecture. Initial
simulations suggested that the plasma performance, which is related to
MHD stability, may be improved under integrated feedback. These simulation
results were later confirmed by DIII-D experimental results in steady-state
high-qmin scenarios. The confinement deterioration caused by NTMs in
these scenarios, which can be observed by reductions in W, can be
significantly ameliorated by employing individual-scalar feedback-control
techniques in conjunction with pre-emptive and/or catch-and-subdue NTM
suppression techniques. Also, feedback control is neces- sary to achieve
the desired q95 and q0 evolutions. Due to NTM development and lack of
off-axis NBI power, successful q0 regulation during experiments could
only be carried out for short periods of time. These results also suggest
that, when regulating several aspects of the plasma dynamics in a tokamak,
an individual-scalar control approach for the regulation of average
or point-wise plasma properties may be more suitable than a whole-profile
control approach due to the under-actuated nature of these devices.
Actuator availability is critical to achieve successful regulation of
the different scalars. Future work will focus on developing the individual-scalar
control scheme (e.g., including additional scalars such as li, βp or qmin),
its integration level with other algorithms (e.g., proximity control),
and the development of actuator management schemes.
