Low loop voltage startup and equilibrium control using multivariable poloidal field coils in the Hitachi Tokamak HT-2

Tokamak operation techniques to control the poloidal magnetic field using multivariable poloidal field coils (MPFCs) were applied to the Hitachi tokamak HT-2. Two problems encountered in operating a tokamak with MPFCs were identified: low-voltage startup and equilibrium control without interference. The key to their solution was accurate control of the poloidal magnetic field. To obtain multipole fields, a singular value decomposition was applied to a response matrix from the coil current to the magnetic flux value at the plasma surface region. The multipole fields are orthogonal bases of the poloidal field, and the interference was cleared using these modes. A control technique using the multipole fields was applied to control the null point position of the poloidal magnetic field during breakdown, which made it possible to get breakdown with a low loop voltage. During the flattop phase, good controllability without interference was obtained using the concept of a multipole magnetic field.