Why Can’t We Understand Basic SOL Interchange 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 the SOL. The development of a unique 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. (The only robust, consistent correlation is that of floating potential and Te fluctuations.) There is no trace of a primary linear instability or dominant mode. The PDFs of the transport fluxes all have mode 0; net transport arises solely from skew. Changes in azimuthal flow profile, independent of shear, have a strong effect on both turbulence and transport. These all want explanation. The most obvious experimental issue is the sheath boundary condition, which is nonlocal, with many manifestations. Although most turbulence Fourier spectra are featureless monotonic decreasing, many show complex features indicative of unknown secondary instabilities. The problems will be exacerbated in the more complex geometries of real tokamaks.