Pedestal and ELM Stability Codes
Pedestal codes predict the height of the edge transport barrier and the stability limit that triggers edge-localized modes.
The edge transport barrier
In high-confinement operation a steep pressure gradient forms just inside the separatrix, the pedestal, whose height strongly sets core performance through profile stiffness. Pedestal and ELM codes predict how high this barrier can rise before edge instabilities, edge-localized modes (ELMs), relax it. The pedestal is where core and edge physics meet, and its prediction is a recognized hard problem.
The governing idea is that the pedestal is limited by coupled peeling-ballooning MHD stability and by the bootstrap-current and transport physics that build the gradient.
Peeling-ballooning stability
The pedestal grows until it hits the peeling-ballooning stability boundary, where current-driven peeling modes and pressure-driven ballooning modes become unstable. Dedicated MHD stability codes map this boundary in the space of pedestal height and edge current, giving the maximum stable pedestal.
Coupled models
Predictive pedestal models combine the peeling-ballooning limit with a model for the pedestal width, often tied to a transport constraint, to predict both the height and width self-consistently. This is a widely used engineering-physics approach, checked against experiment through validation studies.
Design relevance
For the Hyperion breeder, the pedestal boundary condition feeds core transport and thus predicted fusion power, so pedestal modeling is a load-bearing input to the whole-device simulation. The negative-triangularity, -0.30, shape has distinctive edge-stability behavior, making dedicated pedestal analysis part of the honest design case before construction.
- Predicts edge-barrier height and width
- Limited by peeling-ballooning stability
- Couples MHD limit with a width model
- Sets the core boundary condition