The Nonlinear State of Strong Interchange-Type Turbulence

The Helimak is an approximation to the infinite cylindrical slab with a size large compared with turbulence transverse scale lengths, but with open field lines of finite length. A pressure gradient in unfavorable magnetic curvature is unstable to interchange-type modes, leading to large amplitude nonlinear fluctuations similar to those in a tokamak SOL, except there are no field-line connections to favorable curvature. A novel magnetically-baffled probe cluster permits full characterization of the turbulence, including density, temperature, and plasma potential fluctuations as well as particle and thermal radial transport rates across the full plasma profile. Turbulence varies in a complex way with plasma parameters, but it can be most strongly modified by the application of bias to alter the transverse (poloidal, orthogonal to B and R) flow patterns.  Linear theory offers little guidance.  The local density, temperature, and potential fluctuations show modest time correlations at best, flow shear has little predictive value, and the local interchange growth rate is poorly associated with turbulent amplitudes, with strong turbulence persisting in regions of linear stability. Transport is stochastic, being only loosely time-correlated with its electrostatic driving terms.