Session KP01 - Poster Session III.
POSTER session, Tuesday afternoon, March 23
Exhibit Hall, GWCC
We present a transition scenario for the spontaneous Enhanced Confinement (EC) regime observed in the Madison Symmetric Torus (Chapman, et al., Phys. Rev. Lett.) 80 2137 (1998) which is based on the combination of global magnetic turbulence and localized flow shear. It appears that the key physics leading to ECs is the spontaneous generation of a turbulence-suppressing shear flow outside the reversal layer. The stabilizing influence of this flow shear has been investigated both analytically and numerically for arbitrary current profiles, and thresholds for stabilization are obtained. We propose that the emergence of the shear flow is related to the generation of magnetic Reynolds stresses during the excitement of high n, ømega_*-tearing modes in a sawtooth crash. This effect could be explained and quantified using a model based on the reduced MHD equations.(Strauss, Phys. Fluids B) 4 (1) 3 (1992) The increase in Re \langle \tildeB_r \tildeB_\theta \rangle is related to the induction of a large imaginary part in the relevant eigenfunction by the combined effect of diamagnetism and shear flow. After the crash the role played by the Reynolds stress in driving the edge flow is taken over by the steepened pressure gradient created by suppression of turbulence, and the enhanced confinement is maintained. Key elements of the theory are compared with experimental observations.