How fluctuation intensity flux drives SOL expansion

Abstract

Predictions of heat load widths ${\lambda _q}$ based on particle orbits alone are very pessimistic. This paper shows that pedestal peeling-ballooning (P-B) magnetohydrodynamic (MHD) turbulence broadens the stable scrape-off layer (SOL) by the transport, or spreading, of fluctuation energy from the pedestal. ${\lambda _q}$ is seen to increase with ${\Gamma _\varepsilon }$ , the fluctuation energy density flux. We elucidate the fundamental physics of the spreading process. ${\Gamma _\varepsilon }$ increases with pressure fluctuation correlation length. P-B turbulence is seen to be especially effective at spreading, on account of its large effective mixing length. Spreading is shown to be a multiscale process, which is enhanced by the synergy of large and small-scale modes. Pressure fluctuation skewness correlates well with the spreading flux–with the zero crossing of skewness and ${\Gamma _\varepsilon }$ spatially coincident–suggesting the role of coherent fluctuation structures and the presence of intermittency in ${\lambda _q}$ broadening. ${\lambda _q}\sim B_{\text{p}}^{ - 1}$ scaling persists for the broadened SOL. We show that the spreading flux increases for increasing pedestal pressure gradient $\nabla {P_0}$ and for decreasing pedestal collisionality $\upsilon _{{\text{ped}}}^*$ . This trend is due to the dominance of peeling modes for large $\nabla {P_0}$ and low $\upsilon _{{\text{ped}}}^*$ . Ultimately, we see that a state of weak MHD turbulence, as for small ELMs, is very attractive for heat load management. Our findings have transformative implications for future fusion reactor designs and call for experimental investigations to validate the observed trends.

Keywords

• turbulence spreading
• SOL width broadening
• BOUT++ simulation
• small ELMs