P.W. Terry and Cavendish McKay
Department of Physics, University of Wisconsin-Madison
There is evidence that various enhanced confinement regimes in the MST reversed field pinch involve suppression of turbulence by flow shear.1 One of these is spontaneous and occurs after a transition with features that are similar to the L-H transition in tokamaks.
An important issue in the RFP that does not arise in tokamaks is how the measured flow shear, which is localized to a narrow edge layer of 1-2 cm, affects the global tearing modes that dominate core transport. The amplitude of these modes does appear to be reduced in MST. We perform a linear tearing mode stability analysis for a global m=1, n=6 tearing mode with a narrow, localized flow shear layer in the external tearing mode region. For sufficiently strong flow shear, delta-prime is reduced, provided the product of the wavenumber and the distance between the rational surface and the shear layer is not much greater than one. The growth rate and delta-prime are evaluated for MST parameters to determine if this effect can account for the observed reduction in amplitude.
We also examine the physics of the transition to the enhanced confinement
regime. Experimental evidence suggests that the transition is not
driven by the pressure gradient. The transition is also easier to
achieve (i.e., the discharge can have lower current) when the reversal
layer is further from the wall. We investigate both electrostatic
and magnetic Reynolds stresses as the flow shear drive, and an effective
flow viscosity related to distance from the wall as the toroidal flow damping
mechanism. Such a viscosity could arise from either a coupling of
magnetic modes with wall modes, or from the effect of neutral penetration.
The transition is of the second order variety, occurring when the Reynolds
stress of fluctuations, whose radial structure includes the effect of flow
shear, overcomes the flow viscosity. We will evaluate simple transition
thresholds and determine if their requirements are consistent with conditions
present in sawtooth triggered transitions in MST.
Work supported by US DOE
1. B. Chapman, et al., Phys. Plasmas, submitted.
2. P.H. Diamond, et al., Phys. Rev. Lett. 72, 2565 (1994).