Operating with an edge transport barrier (ETB) is central to ITER's goal of attaining a fusion energy gain of ten. The evolution and stability of this ETB is governed through the interplay of MHD modes and microinstabilities. The ballooning formalism is a mathematical framework that can be utilised to understand the characteristics of these modes in the linear regime.
When applied to toroidal drift microinstabilities (e.g. ITG), the ballooning formalism predicts two distinct classes of global eigenmodes: the strongly growing Isolated Mode (IM) that exists under special conditions, and the relatively benign General Mode (GM) that is more generally accessible. Here we present findings from a new initial-value code, developed to study the dynamics of these linear branches in the presence of a time-evolving equilibrium toroidal flow-shear. The code has been further extended to incorporate the (quasi-linear) effect of intrinsic flow generated by these global structures on the modes themselves. The IM/GM dynamics could provide physical insights into understanding small-ELM regimes and intrinsic rotation - two unresolved physics issues that are of great significance to ITER.
Firstly, the IM is seen to form more rapidly than the GM. For our chosen fluid-ITG model, even though both structures are likely to form deep into the nonlinear regime, there is indication that close to marginal stability, these global modes might form much sooner to subsequently influence the nonlinear evolution. Secondly, in the presence of a critical flow-shear, a GM-IM-GM transition can take place to trigger a burst in the growth rate as the IM is accessed. These dynamics can occur on the right time-scale and form the basis of a new model for small-ELMs outlined in this work. Transient bursts are seen in the linear growth rate at high flow-shears, which may provide an alternative trigger for small-ELMs. Certain other seemingly robust features are reported, which could guide experimental efforts to test this theory. Finally, allowing for the feedback of the intrinsic flow on the mode structure, the IM seems to be a stable equilibrium when the external flow-shear is weak, whereas when strong equilibrium flow-shears dominate over the intrinsic flow, the GM solution is more likely. An approach to model the intrinsic flow profile from these global structures is suggested
