Mitigation of the error field and eddy currents for the start-up of spherical tokamak using compensation and reverse coils

Abstract

The tokamak is the most popularly accepted device among the fusion community for future fusion reactors. It has an assembly of toroidal field (TF), central solenoid (CS), poloidal field (PF) coils, and compensation coils (CC) for sustaining plasma in the vacuum vessel (VV). Eddy currents are crucial for the steady-state operation of a tokamak. Major sources of eddy current in the tokamak are edge localized modes and transient currents in the CS and TF coils. Eddy currents strongly affect the tokamak startup scenarios. Eddy currents are produced due to flux changes, the vertical component of the magnetic field (VCMF) of the CS coil, and also by the fast rise of current in the TF coil in the initial phase. This study is aimed to estimate the eddy current density and magnitude of the VCMF by experimental and simulation tools. Eddy current density and VCMF are mitigated on the central cylinder and vacuum vessel, using a pair of six turn CC connected in series with CS and a reverse coil connected in series with TF coil. The current direction in the reverse coil is opposite to the direction of current in the Z-shaped connectors of the TF coil. Tokamak startup is optimized by decreasing the eddy current density, VCMF, and the resulting electromagnetic (EM) forces with the installation of CC and reverse coils.