Dependence of Perpendicular Viscosity on Magnetic Fluctuations in a Stochastic Topology

Abstract

In a magnetically confined plasma with a stochastic magnetic field, the dependence of the perpendicular viscosity on the magnetic fluctuation amplitude is measured for the first time. With a controlled, ∼ tenfold variation in the fluctuation amplitude, the viscosity increases ∼100-fold, exhibiting the same fluctuation-amplitude-squared dependence as the predicted rate of stochastic field line diffusion. The absolute value of the viscosity is well predicted by a model based on momentum transport in a stochastic field, the first in-depth test of this model.

Intrinsic flow and tearing mode rotation in the RFP during improved confinement

“…The strong connection between tearing mode velocity and plasma flow has already been exploited in several devices to control plasma rotation and to do perturbative experiments that enable studies of momentum transport. 17,52 It would be interesting to see to what degree this could be used with active multimode feedback to shape the flow profile and open new avenues for controlling transport.…”

“…During standard RFP plasmas, momentum transport has been shown to be consistent with what is expected from the motion of plasma particles in the stochastic magnetic field produced by multiple overlapping tearing mode islands. 16,17,47 Momentum transport due to stochastic magnetic fields has also been shown to be an important process in stellarators. 48 In stochastic magnetic fields, the momentum diffusion coefficient is equal to the magnetic field line diffusivity multiplied by the sound speed.…”

“…In fact, the motion of resonant tearing modes has often been used to infer the motion of the plasma, assuming that the modes move with the plasma near the mode resonant surface. [13][14][15][16][17] Intrinsic flow drive mechanisms cause the plasma and modes to rotate spontaneously in the lab frame, and the Maxwell and Reynolds stresses produced by the modes also act back on the flow profile. 13,18 The relaxation of the current profile that is mediated by the modes is accompanied by forces that modify the flow profile as well.…”

“…13,19 Interaction of the modes with surrounding conducting walls and with magnetic field errors can also have a strong effect on the plasma rotation in the RFP. 17,[20][21][22] These effects are interesting scientifically but are also important to understand on a practical level. For example, the locking of the modes to the wall and the associated increase in plasma-wall interaction has often been an important limitation on RFP operation.…”

Kinematic viscosity estimates in reversed-field pinch fusion plasmas

“…Despite the interest in the RFP plasma viscosity, no unique consensus on either the form of the viscous term or the viscosity coefficient evaluation in RFP plasmas still exists [1]. In fact, the experimental estimates [11,12,13,14,15] carried on according to classical [16] or turbulent transport [17,18] theory display orders of magnitude differences. In addition, a more recent study [19] has shown the electrostatic turbulence to play a central role in fusion plasmas momentum transport.…”