(TH/P8-36) Toroidal Rotation in Tokamak Plasmas

J.D. Callen1), A.J. Cole1), C.C. Hegna1)
 
1) University of Wisconsin, Madison, WI, United States of America

Abstract.  Determining the magnitude and evolution of toroidal rotation in ITER plasmas is an important issue - for E×B flow shear control of anomalous transport, prevention of locked modes, ELM control via RMPs etc. Many effects influence toroidal rotation in tokamak plasmas. Momentum sources together with radial transport due to axisymmetric neoclassical, paleoclassical, and anomalous processes are usually considered. In addition, the toroidal rotation can be affected by magnetic field errors, which this paper concentrates on. Small, non-axisymmetric field errors in tokamaks arise from coil irregularities, active control coils and collective plasma magnetic distortions (e.g., NTMs, RWMs). Nonresonant field errors cause magnetic pumping (TTMP), ripple-trapping and radial drifts of bananas; they thus lead to radial non-ambipolar particle fluxes and neoclassical toroidal viscosity (NTV) flow damping over the entire plasma. Resonant field errors (FEs) cause localized J×B electromagnetic torques near rational surfaces in toroidally rotating plasmas. Toroidal flow inhibits penetration of resonant field errors into the plasma by producing a shielding effect on rational surfaces. Sufficiently large resonant FEs can lock plasma rotation at rational surfaces to the wall leading to magnetic islands and reduced plasma confinement or disruptions. Many of these processes can also produce momentum pinch and intrinsic rotation effects. A comprehensive picture of all the effects of field errors along with the usual radial plasma transport effects on plasma toroidal rotation has been developed using a fluid moment approach. The toroidal rotation equation results from setting to zero the net radial plasma current induced by the sum of all the non-ambipolar components of the particle fluxes in the plasma from these effects. In general, the NTV due to nonresonant field errors generates a global torque that attempts to rotate the plasma at an “intrinsic” rate that is in the “counter” (to the plasma current) direction and depends on the ion temperature gradient. Inclusion of NTV flow damping effects in a resonant field error penetration model for ohmic tokamaks predicts locking thresholds in better agreement with experimental results from a wide variety of tokamaks, especially with regard to the scaling of the threshold with electron density.

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