RESISTIVITY PROFILE EFFECTS IN NUMERICAL MAGNETOHYDRODYNAMIC SIMULATIONS OF THE REVERSED-FIELD PINCH

The influence of the resistivity profile on reversed-field pinch (RFP) dynamics is investigated numerically using a three-dimensional resist ive magnetohydrodynamic code. This investigation is motivated by exper imental observations on the EXTRAP-T1 RFP (Nordlund P et al 1994 Int. Conf. Plasma Physics and Controlled Nuclear Fusion Research IAEA-CN-60 /A6/C-P-6). Two cases with profiles mainly differing in the edge regio n, i.e. in the region outside the reversal surface, are simulated. It is found that increasing the resistivity in this region results in a f actor of two increase in magnetic fluctuation energy and an equal amou nt in the fluctuation-induced electric field. In spite of this, the pa rallel current decreases in the edge region, resulting in a factor two reduction of the field reversal ratio. The dynamics become more irreg ular and the characteristic timescale is reduced. The final state is c haracterized by a higher loop voltage, slightly lower values of the to tal (fluctuating plus mean part) magnetic energy and the magnetic heli city, but almost unchanged Taylor relaxation ratio. The results indica te that the edge region can be important for RFP confinement since coo ling of the plasma in this region can lead to an increased fluctuation level and degraded performance.