Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

M. A. Miller, J. W. Hughes, T. Eich, G. R. Tynan, P. Manz, A. E. Hubbard, B. LaBombard, J. Dunsmore

Abstract

Experimental inferences of cross-field particle flux at the separatrix, $Γ_{\perp}^\mathrm{sep}$, show rapid growth near H-mode and L-mode density limits at high magnetic field on Alcator C-Mod. Increases in $Γ_{\perp}^\mathrm{sep}$ correlate well with proximity to high density operational boundaries as proposed by the separatrix operational space model. $Γ_{\perp}^\mathrm{sep}$ 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. $Γ_{\perp}^\mathrm{sep}$ 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.

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, , show rapid growth near H-mode and L-mode density limits at high magnetic field on Alcator C-Mod. Increases in correlate well with proximity to high density operational boundaries as proposed by the separatrix operational space model. 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. is well-organized by the characteristic wavenumber for resistive ballooning mode turbulence, , from interchange-drift-Alfvén fluid turbulence theory, with additional dependence on the cylindrical safety factor, , yielding an empirical limit to plasma operation of . This limit corresponds to the point where the perpendicular heat flux, , reaches the level of the parallel heat flux, , i.e. , beyond which point thermal equilibrium is not satisfied, resulting in a fold catastrophe.

Paper Structure

This paper contains 3 figures.

Figures (3)

  • Figure 1: (a) Separatrix operational space for subset of discharges at fixed $I_{P}$, $B_{t}$, and shape, all containing Ly$_{\alpha}$. The color variable shows $\mathrm{log}(\Gamma_{\perp}^\mathrm{sep})$. (b) $\Gamma_{\perp}^\mathrm{sep}$ plotted against $\alpha_{t}$ and (c) against $k_\mathrm{RBM}$. H-modes are shown as squares and L-modes are shown as circles.
  • Figure 2: $\Gamma_{\perp}^\mathrm{sep}$ plotted against $k_\mathrm{RBM}^{2}\hat{q}_\mathrm{cyl}$ for low $I_{P}$ (gray), mid $I_{P}$ (pink), and high $I_{P}$ (mid) discharges in H-mode (squares) or L-mode (circles). The vertical black dashed line represents $k_\mathrm{RBM}^{2}\hat{q}_\mathrm{cyl}=1$.
  • Figure 3: Fraction of heat flux in perpendicular (closed circles) and parallel directions (open circles) against (a) $1/k_\mathrm{RBM}^{2}\hat{q}_\mathrm{cyl}$ and (b) $1/\Lambda_{L}$ for H-modes (orange) and L-modes (blue). Vertical lines highlight unity on the abscissa.