How fluctuation intensity flux drives SOL expansion

Abstract

Predictions of heat load widths λq 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. λq is seen to increase with Γε , the fluctuation energy density flux. We elucidate the fundamental physics of the spreading process. Γε 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 Γε spatially coincident–suggesting the role of coherent fluctuation structures and the presence of intermittency in λq broadening. λq∼Bp−1 scaling persists for the broadened SOL. We show that the spreading flux increases for increasing pedestal pressure gradient ∇P0 and for decreasing pedestal collisionality υped∗ . This trend is due to the dominance of peeling modes for large ∇P0 and low υped∗ . 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.