Peeling-Ballooning Modes and the Edge Pedestal
The coupled current- and pressure-driven instability that limits the edge transport barrier and triggers ELMs.
The edge pedestal
In high-confinement (H-mode) operation an edge transport barrier forms, producing a steep pressure pedestal at the plasma boundary. The pedestal height strongly influences overall performance, but its gradient and edge current are limited by an MHD instability that couples two drives: the pressure-gradient (ballooning) drive and the edge-current (peeling) drive.
The coupled drive
- Ballooning drive grows with the edge pressure gradient
- Peeling drive grows with the edge current (largely bootstrap current from the steep gradient)
- Together they set a boundary in gradient-current space that the pedestal cannot cross
When the pedestal pushes against this peeling-ballooning boundary, the mode goes unstable and relaxes the edge in a burst, an edge-localized mode (ELM).
ELMs: benefit and hazard
ELMs periodically expel a fraction of the pedestal energy and particles. They usefully flush impurities but deposit large, transient heat loads on divertor surfaces. Large ELMs are a serious concern for plasma-facing components, so scenarios aim to mitigate or suppress them (pellet pacing, resonant magnetic perturbations, or naturally ELM-free regimes).
How it is modeled
The EPED-type approach predicts the pedestal height by combining peeling-ballooning stability (from MHD stability codes) with a model for the pedestal width. The stability boundary comes from the energy principle applied to edge modes with both pressure and current drive.
Shaping and Kronos
Plasma shape strongly affects the peeling-ballooning limit. Negative triangularity, used by the Hyperion breeder (-0.30), changes edge curvature and can reduce or avoid ELMs while still providing good confinement, which is one motivation for that shaping choice. Edge stability is evaluated as part of the design study.