Nonlinear Burn Control of ITER’s Two-temperature Plasmas Using Optimal and Adaptive Allocation of Actuators with Uncertain Dynamics
V. Graber, E. Schuster
28th IAEA Fusion Energy Conference
Nice, France, October 12-17, 2020 -> May 10-15, 2021 (Remote)
Abstract
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ITER will be the first tokamak to sustain a fusion-producing, or burning,
plasma. If the plasma temperature were to inadvertently rise in this
burning regime, the positive correlation between temperature and the
fusion reaction rate would establish a destabi- lizing positive feedback
loop. Careful regulation of the plasma’s temperature and density, or
burn control, is required to prevent these potentially reactor-damaging
thermal excur- sions, neutralize disturbances and improve performance.
In this work, a Lyapunov-based burn controller is designed using a full
zero-dimensional nonlinear model. An adaptive estimator manages
destabilizing uncertainties in the plasma confinement properties and
the particle recycling conditions (caused by plasma-wall interactions).
The controller reg- ulates the plasma density with requests for deuterium
and tritium particle injections. In ITER-like plasmas, the fusion-born
alpha particles will primarily heat the plasma elec- trons, resulting
in different electron and ion temperatures in the core. By considering
separate response models for the electron and ion energies, the proposed
controller can independently regulate the electron and ion temperatures
by requesting that different amounts of auxiliary power be delivered
to the electrons and ions. These two commands for a specific control
effort (electron and ion heating) are sent to an optimal control allocation
module that maps them to the heating actuators available to ITER: an
electron cyclotron heating system (20 MW), an ion cyclotron heating
system (20 MW), and two neutral beam injectors (16.5 MW each). The control
allocator finds the optimal mapping by solving a convex quadratic program
that includes actuator saturation and rate limits. An adaptive control
allocation module is also developed as part of this work to handle
uncertainties in the mapping between the commanded control efforts and
the allocated actuators. This includes actuator efficiencies and the
fractions of neutral beam heating that are deposited into the plasma
electrons and ions. Furthermore, the adaptive allo- cator considers
actuator dynamics that contain uncertainties. The adaptive allocation
algorithm is formulated as a more computationally efficient dynamic
update-law instead of an optimization problem. Simulation studies assess
the performance of the proposed adaptive burn controller augmented with
each of the optimal control allocation modules.