The fate of the edge shear layer and the density limit

We elucidate the physics of edge shear layer collapse and consequential emergence of the density limit phenomenology. Neoclassical screening is one of the key factors fixing the strength of the zonal flow shear. Predator - prey dynamics with neoclassical screening shows that for weak enough turbulence drive, the zonal flow growth by negative turbulent viscosity fails to overcome the zonal flow damping. This threshold condition is linked to critical value of dimensionless parameter ρ_s/(ρ_scL_n)^1/2 which underpins the Greenwald density limit n_g∼I_p, where I_p is plasma current. Here, ρ_s is ion sound radius, ρ_sc is zonal flow screening length and L_n is density scale length. Smaller ρ_sc i.e., higher I_p expands the regime of zonal flow persistence. The limiting local edge density n_c for shear layer collapse scales favorably with I_p and integrated particle source S as n_c∼S^1/3I_p in the viscous damping regime and as n_c∼S^2/3I²_p in the charge exchange friction dominated regime. While the critical edge density clearly depends on S, it is also known that edge shear strength depends on the input power. Hence, the critical density might also have a power scaling. This (work in progress) will be discussed in detail in the meeting.