CORRECTING SMALL MAGNETIC-FIELD NON-AXISYMMETRIES

Coil placement errors break the axisymmetry of tokamak magnetic fields . The non-axisymmetries cause magnetic islands, which can enhance lock ed modes, decrease confinement and lead to disruptions. An algorithm i s presented for calculating the currents in small correction coils so that specified islands caused by known magnetic field errors will be e liminated. Surface of section plots indicate that the calculated corre ction coil currents will substantially eliminate islands. A slight mod ification of the algorithm allows one to calculate correction coil cur rents which give rise to isolated islands of prescribed size and toroi dal phase. Changing the toroidal phase would give rise to a rotating i sland, and rotating magnetic islands may be useful for plasma edge con trol.