Nonlinear Burn Control in Tokamaks Using In-vessel Coils
A. Pajares, E. Schuster
Symposium on Fusion Technology
Prague, Czech Republic, September 5-9, 2016
Abstract
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Control of the plasma density and temperature to produce a certain
amount of fusion power, known as burn control, is one of the key issues
that need to be solved for the success of tokamak fusion reactors such
as ITER. In order to reach a high fusion power to auxiliary power ratio,
tokamaks must operate near temperature and density stability limits.
Therefore, active control to maintain a desired burn condition and avoid
instabilities is absolutely necessary. Previous work makes use of mainly
three different types of actuation: modulation of the auxiliary power,
modulation of the fueling rate, and controlled injection of impurities.
However, recent experiments showed the feasibility of modifying the
plasma energy by using the in-vessel coils as actuators. Inspired by
such experiments, a new burn control scheme is proposed in this work to
exploit the in-vessel-coil system in combination with auxiliary power
and fueling rate modulation. The in-vessel coils generate non-axisymmetric
magnetic fields that modify the confinement of the plasma, which
influences the plasma energy dynamics. By using the in-vessel coils,
energy losses can be enhanced when needed and thermal excursions can be
prevented. Moreover, actuation of the in-vessel coils may prevent the
injection of impurities and its associated drawbacks. A control-oriented
model has been developed to account for the influence of the in-vessel-coil
currents on the plasma burn. While much previous work uses linearization
techniques, a model-based nonlinear burn controller is proposed in this
work. This nonlinear control approach is applicable to a larger range of
operating conditions and is stable against a larger set of perturbations
when compared with linear control approaches. The effectiveness of the
controller is demonstrated via nonlinear simulation studies for different
plasma scenarios.