Central safety factor control in DIII-D using neutral beam injection and electron cyclotron launchers in zero input-torque scenarios

The tokamak is a Torus-shaped machine whose final purpose is generating energy from nuclear fusion reactions. In order to achieve this goal, a reactant plasma is confined inside the tokamak by means of magnetic fields. For a tokamak to be commercially competitive, operation for long periods of time at high-performance operating points will be needed. Those high-performance scenarios are characterized by a steady-state, stable plasma operation in which the safety factor, a property of the plasma that measures the pitch of the magnetic field lines, plays a decisive role. In particular, control of the central safety factor, which is the value of the safety factor at the tokamak magnetic axis, is one of the crucial aspects to the success of tokamak devices due to its close relationship to magneto-hydrodynamic stability. Therefore, control algorithms for the central safety factor in tokamaks will be required. In the present work, a linear controller is proposed for the regulation of the central safety factor using neutral beam injection and electron cyclotron launchers. This controller is designed to guarantee a zero input torque delivered by the neutral beam injection system. The controller performance is tested via a simulation study in a DIII-D scenario.