Turbulent Nature of the Quasicontinuous Exhaust Regime for Fusion Plasmas

Abstract

We demonstrate a mechanism for reconciling high confinement with heat exhaust in fusion plasmas. Global fluid turbulence simulations of the Quasicontinuous Exhaust regime in the ASDEX Upgrade tokamak show that a quasi-coherent mode (QCM) causes the pedestal foot to oscillate across the separatrix and eject ballistic blobs into the scrape-off layer (SOL), reproducing not only mean profiles but also fluctuation spectra and mode structure seen in experiments. The QCM is a kinetic ballooning mode that develops an extended radial correlation length via electromagnetic self-organization of turbulence, thereby driving enhanced transport, with Maxwell stress and finite Larmor radius effects mediating the process. The blobs are launched when resistivity excites a secondary mode that originates from the X-point and interacts with QCM. The blob-dominated SOL temperature fall-off is then well decoupled from the pedestal-foot gradient set by the QCM.

Figures (4)

• Figure 1: Composite view of plasma density fluctuations. The color scale is saturated near the divertors. The top-right inset shows toroidal fluctuations; bottom-right inset shows the time series of blob generation.
• Figure 2: (a-c) $n$, $T_e$, and $T_i$ profiles compared to experimental estimates (black markers with error bars indicating uncertainties). Shaded bands show the fluctuation envelope spanning the $1\%$--$99\%$ quantiles. (d) Fluctuating $T_e$ profiles sampled every $0.01\, ms$ over $0.5\, ms$ on a logarithmic y-axis, compared to Thomson scattering. The blue and red straight lines denote linear regressions of the mean $T_e$ in the simulation. (e) Histogram of $\tilde{n}$. The dashed lines mark the $\pm 2.5\sigma$ thresholds.
• Figure 3: (a) Binormal Fourier spectra of $|\tilde{\phi}|$ at $\rho_{\mathrm{pol}} = 0.999$ in the plasma frame. The dashed and dash–dotted lines indicate the KBM and RXM dispersion in \ref{['eqn:disp']}, respectively. (b) Production of fluctuations $\hat{\sigma}_\phi\hat{\sigma}_n$ ($\hat{\sigma}$ denotes the standard deviation normalized against its value at OMP) at $\rho_{\mathrm{pol}} = 0.999$ as a function of the parallel connection length to OMP, comparing Ref. and C1. (c) Cross correlation between $\tilde{\phi}$ and $\tilde{n}$, plotted vs. connection length at different flux surfaces. $\lozenge$ and $\circ$ denote the heights $z=-0.5m$ (where QCM reaches half-value) and $z=-0.8m$ (near the X-point), respectively. $\bigstar$ marks the zero-cross (where interchange occurs).
• Figure 4: (a) Mean profiles of $E_r$, $\partial_rp_i/(en)$ and $\tilde{\phi}^2$, comparing the reference case and the test where the Maxwell stress is removed. (b) Cross coherence between $\tilde{\phi}$ and $\tilde{n}$, comparing the reference case and the test without FLR.