Minimum Safety Factor Control in Tokamaks via Optimal Allocation of Spatially Moving Electron Cyclotron Current Drive
S.-T. Paruchuri, A. Pajares and E. Schuster
Proceedings of the 2021 IEEE Conference on Decision and Control (CDC)
Austin, Texas, USA, December 13-15, 2021
Abstract
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Tokamaks are torus-shaped devices designed to
confine a plasma (ionized gas at around 100 million degrees
where fusion reactions can take place) using helical magnetic
fields. Such magnetic confinement enables light ions, such as
isotopes of hydrogen, to stay confined long enough to undergo
a fusion reaction. The pitch of the helical magnetic field
in a tokamak is characterized by the safety factor q. The
safety factor is closely related to the magnetohydrodynamic
stability of the plasma. For instance, instabilities that can
degrade or even terminate plasma confinement can occur at
spatial locations with rational values of the safety factor q.
Thus, actively increasing the minimum magnitude of the safety
factor can reduce the occurrence of low-order (low rational
q values) instabilities. Non-inductive sources of current like
neutral beam injection (NBI) and electron cyclotron current
drive (ECCD) are used to control the q-profile. ECCD generates
electromagnetic waves to drive current and/or heat the plasma.
Mirrors are used to control the spatial region of incidence of
the generated electromagnetic waves. In this work, the ECCD
mirror’s position is treated as a controllable input, and its
effects are included in the response model used for control
design. A controller based on feedback linearization is proposed
to simultaneously allocate the NBI and ECCD powers and
the ECCD position to track a target minimum safety factor.
The effectiveness of the controller is assessed for a DIII-D
tokamak scenario in nonlinear one-dimensional simulations
using COTSIM (Control-Oriented Transport SIMulator).