On the Transport Physics of the Density Limit*

Proximity to the density limit is characterized by degraded particle confinement, rise in edge turbulence and collapse of zonal flows. We present extensive theoretical work (Hajjar et al. 2018, Phys. Plasmas 25, 062306) which shows that the key parameter governing the flow collapse transition is electron adiabaticity. We show that zonal flow production drops precipitously in the hydrodynamic electron regime, as the non-diffusive vorticity flux (i.e. residual stress) declines in proportion to adiabaticity. A simple physical argument establishes that the familiar cartoon of zonal flow production by eddy tilting fails in the hydrodynamic regime, as there causality and Reynolds stress tend to decouple. We show that reduced zonal flow production is the mechanism for particle confinement degradation in L-mode, and present a feedback loop mechanism for the cooling front. Several associated experimental studies are suggested. We also discuss the density limit in H-mode and its relation to the H to L back transition. A key element here is the invasion of the edge by turbulence, which spreads from the SOL.