  Toroidal flow alone is generally thought to have important influence on
tokamak edge pedestal stability, even though theory analysis often predicts
merely a weak stabilizing effect of toroidal flow on the edge localized modes
(ELMs) in experimental parameter regimes. For the first time, we find from
two-fluid MHD calculations that such a stabilization, however, can be
significantly enhanced by increasing the edge plasma density. Our finding
resolves a long-standing mystery whether or how toroidal rotation can indeed
have effective influence on ELMs, and explains why the ELM mitigation and
suppression by toroidal rotation are more favorably achieved in higher
collisionality regime in recent experiments. The finding suggests a new control
scheme on modulating toroidal flow stabilization of ELMs with plasma density,
along with a new additional constraint on the optimal level of plasma density
for the desired edge plasma conditions.
