Experimental Evidence for Increased Particle Fluxes Due to a Change in Transport at the Separatrix near Density Limits on Alcator C-Mod

Abstract

Experimental inferences of cross-field particle flux at the separatrix, ${\Gamma }_{\perp }^{\mathrm{sep}}$, show rapid growth near H-mode and L-mode density limits at high magnetic field on Alcator C-Mod. Increases in ${\Gamma }_{\perp }^{\mathrm{sep}}$ correlate well with proximity to high density operational boundaries as proposed by the separatrix operational space model. ${\Gamma }_{\perp }^{\mathrm{sep}}$ grows as the L-mode density limit and the H-L-mode back transition boundaries are approached, consistent with expectations of plasma instability-driven turbulence suggested by theory, confirming the power dependence of density limits. ${\Gamma }_{\perp }^{\mathrm{sep}}$ is well-organized by the characteristic wavenumber for resistive ballooning mode turbulence, $k_{\mathrm{RBM}}$, from interchange-drift-Alfvén fluid turbulence theory, with additional dependence on the cylindrical safety factor, ${\hat{q}}_{\mathrm{cyl}}$, yielding an empirical limit to plasma operation of $k_{\mathrm{RBM}}^2{\hat{q}}_{\mathrm{cyl}}=1$. This limit corresponds to the point where the perpendicular heat flux, $Q_{\perp }$, reaches the level of the parallel heat flux, $Q_{\parallel }$, i.e. $Q_{\perp }\approx Q_{\parallel }$, beyond which point thermal equilibrium is not satisfied, resulting in a fold catastrophe.