The dynamo effect in sustained reversed-field pinch discharges

Abstract

The paper presents a combination of theoretical and experimental results which show that the sustainment phase of a driven Reversed-Field Pinch (RFP) configuration is not fundamentally limited in time by toroidal field diffusion. Driven ZT-40M discharges where the external poloidal voltage circuit does not directly maintain toroidal field reversal are examined. Flux measurements on slowly rising current discharges clearly show that a toroidal flux regeneration mechanism (referred to as the 'dynamo-effect') must be present. Numerical simulations demonstrate that this effect is operative in the plasma on a short timescale ≤0.1 ms. This result is shown to be independent of the form of the resistivity profile. Thus, the possibility that the experimental results can be explained without a dynamo mechanism, if the temperature in the vicinity of the toroidal field reversal point is sufficiently high, is eliminated. Implications of these facts with regard to theoretical stability and transport studies of the RFP are discussed.