Current penetration in tokamak-type discharges

Abstract

This paper reports the results of a numerical-analytical study of current penetration in tokamak-type discharges. Results presented include: (1) A survey of microinstabilities proposed as relevant to tokamak start-up; (2) an examination of the energy transport equations with toroidally enhanced classical thermal conduction to argue resistive penetration and scaling properties; (3) tabular and graphical data from a 1-D MHD code applied to cylindrical geometry for a variety of assumed start-up conditions. The transport included is classical resistance and toroidal classical thermal conduction radially. The results show that current penetration properties of fast-rise, low-density devices, such as ST, Doublet II and ORMAK, differ from those of slow-rise, high-density devices such as the planned PLT and Doublet III experiments. In the latter case, the electrons and ions can remain collisionally well coupled throughout the rise of the driving field, and ion thermal conductivity can frustrate the tendency of Joule heating near the plasma boundary to cause skin formation. It is further concluded that most of the micro-instabilities proposed to explain “anomalous” current penetration will in practice never occur.