Toroidal stray field effects on the operational flexibility of poloidal field coils in superconducting tokamaks

The discreteness of the toroidal field coils in tokamaks generates a toroidal stray magnetic field outside a vacuum vessel, as well as toroidal field ripples inside it. It is shown that the stray field considerably restricts the operational flexibility of superconducting poloidal field (PF) coils due to the increase of magnetic fields at the positions of PF coils. It is also shown that outer shaping coils are more vulnerable to these effects than central solenoid coils. As an example of the investigation, calculation results for the superconducting KSTAR device are presented and analysed in detail. From the study, it is concluded that the toroidal stray field effects should be incorporated in the design and operation of poloidal field coils in superconducting tokamak devices. (author)