Pedestal and ELM Stability Codes
Peeling-ballooning codes predict the height of the edge pedestal and the onset of edge-localized modes that set the plasma boundary condition.
The pedestal
In high-confinement operation the plasma forms a steep edge transport barrier, the pedestal, where pressure rises sharply over a narrow region. Because core transport is stiff, the pedestal height largely sets the whole profile. Predicting it is therefore central to predicting overall performance.
Peeling-ballooning limits
The pedestal is limited by coupled edge instabilities: ballooning modes driven by the steep pressure gradient, and peeling modes driven by the associated edge current. A peeling-ballooning stability code maps where an edge equilibrium becomes unstable in the space of pressure gradient and edge current, giving the maximum stable pedestal.
Edge-localized modes
When the pedestal reaches the peeling-ballooning boundary, it relaxes through an edge-localized mode (ELM) that expels a burst of energy and particles, then rebuilds. Large ELMs can damage plasma-facing components, so predicting their size and frequency, and finding regimes that avoid them, is a major modeling task.
The EPED-style approach
- Combine peeling-ballooning stability with a model for the pedestal width
- Find the pedestal height and width where both constraints are satisfied
- Deliver a predicted boundary condition for core transport codes
Coupling to the core
Pedestal predictions are handed to predictive transport codes as the edge boundary condition, closing the loop between edge stability and core performance. A design's projected output is only as trustworthy as its pedestal assumption, so this coupling is treated carefully and its uncertainty propagated forward.
Managing edge instabilities is also tied to divertor design, since ELMs deliver transient heat loads that the divertor must survive.