Ballooning instability

The ballooning instability is a pressure-driven instability characterised by wide poloidal spectra, generally at high toroidal mode numbers, $n>10$. Ballooning modes typically manifest in the outboard edge region of tokamaks operating in high confinement regimes, owing to large edge gradients. This outboard localisation is due to the curvature dependence of the more general interchange instability, where the inboard side of the tokamak typically possesses more favourable curvature.

Relationship with ELMs

Ballooning modes, coupled with the peeling instability, are a proposed mechanism for driving the ELM cycle, as part of the peeling-ballooning model of Edge Localised Modes (ELMs).