Magnetic diffusion and current profiles during current reversal

Abstract

Computer studies are performed on the temporal changes of magnetic flux surfaces and current density profiles in a tokamak (of 25 cm minor radius) undergoing current reversal. The flux on the plasma boundary is forced to vary in time so as to model a total current reversal from positive to negative in about 5 ms. A two-dimensional computer code with radial and azimuthal spatial variations has been written, as well as a simpler one-dimensional code with only radial variation. The two-dimensional code shows that the flux variation takes place in way showing the formation of magnetic islands. Both codes show that the current penetration in the plasma is much slower than the reversal time. This slow resistive penetration occurs even with an enhanced resistivity factor which increases towards the boundary and with a temperature profile which decays in time towards zero current. After maintaining a constant flat negative current for certain period, results are also obtained on the profiles during reversing back rom negative to positive current and maintaining the constant positive current.