October 31, 2026

Which limit does the plasma edge reach first?

As the edge pressure gradient builds, the plasma moves through the stability diagram until it meets a boundary.

Negative Triangularity Tokamak Design

Which limit does the plasma edge reach first?

Change the triangularity and the edge current to see which instability stops it, and what that does to the pedestal.

Figure · Edge MHD stability · schematic

Scenario
Triangularity δ+0.40 −0.50+0.5
Edge current drive0.28 lowhigh
a Cross-section and mode structure
Pressure Mode perturbation δp
b Edge stability diagram
Stable Peeling Ballooning High-n ballooning
c Edge pressure profile

Pedestal building

building
Limiting instability
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2nd-stability access
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Gradient at limit, α
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Pedestal vs δ = +0.4
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Schematic, not computed from an equilibrium. (a) Poloidal cross-section with elongation κ = 1.6 and the chosen triangularity. Colour inside the plasma shows pressure; the navy and cyan band at the edge shows the pressure perturbation of the limiting mode, which grows during an ELM. (b) Edge stability in the plane of normalized pressure gradient α and edge current density j. As the gradient builds, bootstrap current grows with it, so the edge follows the dashed path. The solid curve is the finite-n peeling–ballooning boundary; the hatched region is unstable to high-n ballooning modes. (c) Edge pressure profile; the dashed curve is the pedestal at the peeling–ballooning limit for δ = +0.4.

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