Transport-driven Scrape-Off-Layer flows and the boundary conditions imposed at the magnetic separatrix in a tokamak plasma

TLDR

The flows are driven by a ballooning‑like poloidal transport asymmetry that re‑symmetrizes SOL pressure variations, and experiments show that topology‑dependent flow boundary conditions also influence the L–H power threshold via central rotation. Plasma profiles and flows in the low‑ and high‑field side SOL are highly sensitive to magnetic separatrix topology, with near‑sonic parallel flows and secondary toroidal‑rotation/Pfirsch–Schlüter currents whose direction and magnitude depend on X‑point location; these transport‑driven flows impose topology‑dependent boundary conditions that couple across the separatrix, affecting toroidal rotation and producing positive or negative central co‑rotation increments in L‑mode discharges, and requiring higher input power when B×∇B points away from the X‑point.

Abstract

Plasma profiles and flows in the low- and high-field side scrape-off-layer (SOL) regions in Alcator C-Mod are found to be remarkably sensitive to magnetic separatrix topologies (upper-, lower- and double-null) and to impose topology-dependent flow boundary conditions on the confined plasma. Near-sonic plasma flows along magnetic field lines are observed in the high-field SOL, with magnitude and direction clearly dependent on X-point location. The principal drive mechanism for the flows is a strong ballooning-like poloidal transport asymmetry: parallel flows arise so as to re-symmetrize the resulting poloidal pressure variation in the SOL. Secondary flows involving a combination of toroidal rotation and Pfirsch–Schlüter ion currents are also evident. As a result of the transport-driven parallel flows, the SOL exhibits a net co-current (counter-current) volume-averaged toroidal momentum when B × ∇B is towards (away from) the X-point. Depending on the discharge conditions, flow momentum can couple across the separatrix and affect the toroidal rotation of the confined plasma. This mechanism accounts for a positive (negative) increment in central plasma co-rotation seen in L-mode discharges when B × ∇B is towards (away from) the X-point. Experiments in ion-cyclotron range-of-frequency-heated discharges suggest that topology-dependent flow boundary conditions may also play a role in the sensitivity of the L–H power threshold to X-point location: in a set of otherwise similar discharges, the L–H transition is seen to be coincident with central rotation achieving roughly the same value, independent of magnetic topology. For discharges with B × ∇B pointing away from the X-point (i.e. with the SOL flow boundary condition impeding co-current rotation), the same characteristic rotation can only be achieved with higher input power.