Enhanced toroidal flow stabilization of edge localized modes with increased plasma density

Abstract

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.

Theory of coupled resistive drift and resistive drift ballooning instabilities in fusion plasma

“…It is observed that for given collisionality, all states of L-H transition lie under the resistive drift-Alfven regimes and are stable to ideal ballooning mode (see ref: Scott, 1997). The two-fluid MHD model is extensively used by plasma community (Wagner et al 1982;Gohil et al 1994;Connor and Wilson 2000;ASDEX team 1989;Righi et al 1999;Chankin 1997;Marashek et al 1997;Shikui Cheng et al, 2019) to describe fundamental theory of fusion plasma and rapid revival of interest with computational calculations is observed (Shen et al 2020;Ren et al 2020;Zhu and Reader 2013;Cheng et al 2017;Xi et al 2012;Meneghini et al 2015). The four field plasma model has been introduced to describe the coupling effects of microscopic and macroscopic dynamics for edge regime tokamak plasma.…”

A nonlinear simulation study of the effect of toroidal rotation on RMP control of ELMs

“…However, there does not yet exist a definitive conclusion or understanding regarding the stability of ELMs in the presence of various equilibrium flows. While simulation studies of Aiba et al [12,14] indicate that a sheared toroidal flow can destabilize a type-I ELM, the investigations of Cheng et al [15] report a flow induced stabilization of the ELMs. Sheared poloidal rotation can have either a stabilizing or destabilizing influence depending on the direction of the flow [12].…”

“…In an actual operational scenario additional physical factors such as plasma rotation can introduce significant modifications in the dynamics of ELMs. A few past studies [12][13][14][15] have shown that toroidal and or poloidal rotation can significantly influence the stability properties of ELMs. However, there does not yet exist a definitive conclusion or understanding regarding the stability of ELMs in the presence of various equilibrium flows.…”

Solving the Grad–Shafranov equation using spectral elements for tokamak equilibrium with toroidal rotation

“…Meanwhile, plasma flow and flow shear can also directly affect plasma stability and transport [8,9,10,11,12,13]. In particular, flow shear may have stabilizing effects on neoclassical tearing modes(NTMs) [14,15], tearing modes (TMs) [16,17,18,19] and edge localized modes(ELMs) [20,12,21,22]. It is found that sufficient toroidal flow opens up a stability window for resistive wall mode (RWM) [23,24,25,26,11].…”