The collisionality dependence of intermittency level in drift-wave turbulence in the stellarator TJ-K

Abstract

The link between plasma collisionality and the intermittency of edge drift-wave turbulence is investigated experimentally in the stellarator TJ-K and compared to results obtained from slab Hasegawa-Wakatani (HW) simulations. The kurtosis of experimental time series of density and potential fluctuations was analysed at different frequency scales. Using this method on data from a range of discharges, it was found that the intermittency level increases with increasing collisionality for density fluctuations, whereas potential fluctuations are generally self-similar. In addition, a high-order structure function analysis of density time-series data also shows a trend towards higher intermittency levels as collisionality is increased. HW simulations are found to produce a qualitatively similar trend in the intermittency level compared to experimental analyses. The transition to intermittent fluctuations at higher collisionalities can be understood in the framework of the HW model as the decoupling of the density field from the self-similar potential field when moving from the adiabatic to the hydrodynamic limit, where the density behaves like a passive scalar, resulting in the intermittency levels observed in the vorticity.