Conventions for the EC reference labels and the ITER pedestal boundary
Two surrogates remain for M3: electron-cyclotron (EC) heating and current drive, learned from the GENRAY ray-tracing code, and the ITER pedestal, taken from a pretrained EPED neural network (EPEDNN). Both had been selected (OQ-21), but not how their outputs become TORAX inputs. The Primer explains sources and the pedestal; terms are in the Glossary.
We wrote one recommendation per open convention, and the owner accepted all of them as written.
EC labels. The current-drive calculation stays the one in the upstream ITER deck (an adjoint method). A second method with momentum conservation is run on a few cases as a check only. Electron heating keeps GENRAY’s two channels, wave-particle and collisional, separately. The current label is GENRAY’s flux-surface average of \(j \cdot B\) divided by \(B_0\). We read the GENRAY source to find which \(B_0\) this is: the vacuum toroidal field at the geometric centre stored in the equilibrium file, which is also TORAX’s definition. TORAX’s input is the same average without the division, so the coupling multiplies by TORAX’s own \(B_0\). Current is signed positive along the plasma current. Profiles go onto TORAX’s radial coordinate by a remap that keeps the volume integral of heating and the area integral of current exact in every cell. GENRAY’s radial coordinate ends slightly inside the last closed surface, and the remap rescales it with the equilibrium’s safety-factor profile. Labels are per megawatt injected. Accuracy targets are set after a first baseline.
ITER pedestal. EPEDNN returns a total pedestal pressure and a pedestal half-width in normalised poloidal flux, for three stability models and three solutions each. We use the model with both diamagnetic corrections and its first solution, which is also the upstream default. Ion and electron temperatures are equal at the pedestal top, so the temperature is the pressure divided by the electron density plus the total ion density. The pedestal density is the prescribed one, which is also an EPEDNN input. The pedestal top lies at one minus twice the half-width in poloidal flux; the upstream source documents the width as a half width. The equilibrium maps that point to TORAX’s toroidal-flux coordinate. Where the network’s inputs leave its training box, or an output is not finite, the prescribed pedestal is used instead. MAST-U keeps its prescribed pedestal.
The pedestal conversion is now code: pure JAX, compiled and differentiable, with the fallback selected per call. On the upstream ITER example it gives pedestal pressure 0.0772 MPa, T_ped 3.59 keV at n_e,ped 7e19 m^-3, top at psi_norm 0.935C-162. The checks are partly independent: the branch selection is compared with the network’s raw output vector, the temperature and coordinate conversions with hand calculations, and the fallback with inputs outside the box. They do not test the physics of the pedestal; the network is used as selected. The EC pipeline and pedestal checks pass in both modes (19 focused checks (13 EC pipeline, 6 pedestal contract) pass in full and Tier-A-only modesC-163).
The EC conventions are applied in the labelling run of the next entry. The pedestal conversion is not yet connected to TORAX. Remaining for OQ-21: the EC accuracy targets.
Technical details → EC label conventions, pedestal conversion. Decision → OQ-21.
Technical details → Surrogates › ec contract · Surrogates › pedestal contract
Decisions → OQ-21