Fast flow phenomena in a toroidal plasma

The bulk fluid velocity is measured spectroscopically with 10 μs time resolution in the Madison Symmetric Torus (MST) reversed-field pinch (RFP) [Fusion Technol. 19, 131 (1991)], a diagnostic capability used to study the fast flow dynamics associated with locked modes and the RFP dynamo. The phase velocity of the tearing modes and the fluid velocity accelerate between sawtooth events, reaching a maximum speed of about 20 km/s in a few ms. Both slow down at the sawtooth crash in ≈100 μs. This deceleration time scale is as calculated for the tearing modes from the action of electromagnetic torque on the magnetic islands, but is much faster than expected from the viscous torque on the bulk fluid. In the RFP, correlated fluctuations in the tearing modes and fluid velocity probably also generate current via the "RFP dynamo," 〈ũxB̃〉, where u is the bulk fluid velocity. Initial data indicate a possible increase in 〈ũ_φB̃ _r〉 during sawtooth events, coincident with toroidal flux generation.