L3 for the Breeder: The ELM-Free Negative-Triangularity Shape
Negative triangularity offers an ELM-free edge, but only if the shape is held precisely; L3 makes that a continuous control guarantee.
Why negative triangularity
The breeder runs at negative triangularity, delta -0.30, because that edge shaping can suppress edge-localized modes (ELMs), the periodic edge instabilities that dump heat onto plasma-facing components. An ELM-free edge protects the divertor and vessel and improves the machine's duty. But the benefit is contingent: it holds only while the plasma sits in the negative-triangularity configuration with adequate stability margin. Lose the shape and the advantage goes with it.
Holding the regime
L3 turns ELM-free operation into a continuous control objective. The free-boundary PINN reports the actual triangularity and boundary; shape-control MPC drives it to the delta -0.30 target; the stability PINN reports the margin to the peeling-ballooning boundary that governs the edge. MPC constrains against that margin, so it will trade a little shape-tracking error to keep the edge comfortably inside the ELM-free window rather than riding its limit.
- Objective: hold delta -0.30 boundary with peeling-ballooning margin
- Sensors: Mirnov/flux graph for edge MHD, Thomson for the pedestal profile
- Risk watched: edge approaching the peeling-ballooning boundary
- Response: shape/profile adjustment before an ELM or mode onset
The anomaly ensemble watches the edge specifically: a pedestal building toward the stability boundary, or an edge magnetic signature growing, is a precursor that the ELM-free regime is about to break. Catching that sub-threshold lets MPC back off the pedestal gently, preserving the ELM-free character instead of taking an edge event.
This is a clear example of L3's role: the physics advantage of negative triangularity is real but conditional, and the twin's job is to keep the machine inside the condition, continuously and with margin, rather than assuming the favorable regime persists on its own.