Port gpout_dw / gpout_dw_matrix (psi-resolved torque response profiles and matrices) as the grid-logic diagnostic #423

Summary (revised after fortran-physics-reviewer consultation)

We need to port the ψ-resolved torque response machinery from gpec/gpout.f — gpout_dw (lines 2579–2650) and gpout_dw_matrix (2655–2891) — and use it as the standing grid-logic diagnostic.

The Julia PE currently outputs only the scalar boundary torque Energies/toroidal_torque = −2n·Im(⟨Φ_tot, Λ⁻¹Φ_tot⟩/4). A physics review against the Fortran confirms this scalar is a faithful port of gpout.f:1195–1201 (identical quadratic form, /4 normalization, total-flux argument, torque factor). But it is a boundary quadratic form of the inductance matrix — not the volume-resolved quantity:

• gpout_dw: ψ-resolved torque/energy density from the actual self-consistent EL solutions, dW(ψ) = Σ conj(u1)·u2 / 2 / (2μ₀) · 2n·i per step, splined; Re of the running ψ-integral = torque (N·m), Im = 2nδW.
• gpout_dw_matrix: the mpert×mpert response-matrix generalization per ψ step (dwks/dwk/gind/gres/gindp/gresp; normalized by ξ, Φ_boundary, Φ_x, and coordinate-independently).

Neither has a Julia equivalent (grep confirms). The ψ-integrated gpout_dw torque is the true apples-to-apples comparator for the grid-free KineticForces/fgar NTV torque — the comparison that in Fortran exposed ldp-grid inadequacy.

Measured data motivating the port (DIII-D kinetic-calculated, suppression on)

Per the review, the scalar-vs-fgar gap is not by itself a bug — the two are different constructions — so these rows are motivation, not defect evidence. Two genuine flags remain:

1. The scalar's grid instability at rot=1.0 (0.183 → 0.083 doubling the grid, while fgar holds to 0.5%) is a red flag on wt0/plasma_inductance convergence — 92% of the physical torque lives at ψ>0.9 where the ω_E sign flip sits within ~one grid cell and the Δ′ rational brackets clear knots. Worth its own investigation, independent of the port.
2. Until gpout_dw is ported, no volume-integral cross-check exists — the sharpest grid diagnostic from the Fortran workflow is missing from the Julia code.

Proposed work

1. Port gpout_dw (ψ-resolved torque/energy profile from solutions; the u1†u2 bilinear with the 2n/2μ₀ energy→torque factor) — output as a ψ profile (and its running integral) in gpec.h5.
2. Port gpout_dw_matrix (ψ × m × m torque/energy response matrix profiles).
3. Adopt ψ-integrated gpout_dw torque vs fgar torque as a tracked regression-harness quantity (a DIII-D kinetic-calculated case) — the designated referee for any kinetic grid logic.
4. Separately: investigate the scalar boundary torque's grid sensitivity (wt0 convergence near the pedestal ω_E crossing).

Full record: handoff/issue376/RESULTS.md §32 (experiment branch). Evidence h5s: ~/scratch_issue376/d3dkin/tx_*.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LzbLFQKyuRE5DYZmLokKmk