Nonlinear Robust Safety Factor Profile Control in Tokamaks via Feedback Linearization and Nonlinear Damping Techniques

Abstract

Tokamaks are toroidal devices in which a plasma is confined by means of helical magnetic fields with the purpose of obtaining energy from nuclear fusion reactions. The safety factor, $q$, measures the pitch of the helical magnetic field lines in a tokamak. Active control of the $q$ profile (i.e., spatial shape) is needed due to its relationship with plasma performance, steady-state operation, and magneto-hydrodynamic stability. However, the responses of some plasma magnitudes, such as the electron temperature, are difficult to model and introduce a high level of uncertainty in the model used for $q$-profile control design. Control algorithms that are robust against such model uncertainties must be developed in order to ensure successful q-profile regulation. In this work, a nonlinear, robust $q$-profile controller is designed using feedback linearization and nonlinear damping techniques. The controller makes use of plasma current modulation, neutral beam injection, electron-cyclotron heating & current drive, and electron density modulation as actuation methods. A simulation study is carried out for a DIII-D scenario to test the controller's performance under the presence of electron temperature uncertainties.