The Unexplained Complexities of Strong SOL Interchange-Type Turbulence

The Helimak is the least complex experimental realization possible of the cylindrical slab, the minimal model of the interchange instability, a “universal” instability of confined plasmas, especially in the SOL. The development of a unique magnetically baffled probe array has enabled simultaneous, local measurements of fluctuations in density, temperature, and true plasma potential as well as the turbulent transport fluxes of particles and electron thermal energy, revealing the remarkable complexity of this turbulence. Neither the three fields nor the transport are consistently correlated with each other; there is no trace of a primary linear instability or normal mode. The PDF of transport has a strong mode at 0; net transport arises solely from skew. Changes in azimuthal flow profile, even without increasing shear, has a strong effect on both turbulence and transport. These all want explanation. An important and robust conclusion from the observations is that floating potential fluctuations are highly correlated with temperature fluctuations, but not with plasma potential. Floating potential can be used only to infer properties of T_e turbulence. The Helimak is now at Shenzhen University.