Edge dW scan: unbracketed Newton in free_compute_wv_spline throws ConvergenceFailed

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Evaluación

Dificultad
3/5
Tiempo estimado
1-2 días
Aptitud para principiantes
74/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
julia

Línea de trabajo

Read free_compute_wv_spline in src/ForceFreeStates/Free.jl and follow the call path through _handle_edge_dW_scan!, scan_edge_dW!, and findmax_dW_edge! in EulerLagrange.jl. Check how q_spline behaves over [dW_edge_scan_start, psilim], then reproduce the n = 1 edge scan if possible. Done means the scan completes when an unbracketed Newton solve would fail and existing converged cases retain the same roots.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Summary

free_compute_wv_spline (src/ForceFreeStates/Free.jl) inverts q(ψ) at each edge-scan node using an unbracketed Roots.Newton() started from a ψ-linear guess. On one equilibrium in a parameter scan, Newton did not converge. The whole run then aborted with Roots.ConvergenceFailed("Algorithm failed to converge") during the n = 1 edge δW scan.

The same code is still on develop, at about line 222.

Where

qi   = qedge + (intr.qlim - qedge) * ((i - 1) / npsi)
psii = ctrl.dW_edge_scan_start + (intr.psilim - ctrl.dW_edge_scan_start) * ((i - 1) / npsi)
psi_array[i] = find_zero((psi -> profiles.q_spline(psi) - qi, psi -> profiles.q_deriv(psi)),
                         psii, Roots.Newton())

Call path: _handle_edge_dW_scan! → scan_edge_dW! → findmax_dW_edge! (EulerLagrange.jl:1140) → free_compute_wv_spline (Free.jl:232).

Context

  • Build: 728a7e7d5 (develop).
  • Settings: n = 1, no wall, psihigh = 0.998, mpsi = 256, grid_type = "core_edge_packed", dW_edge_scan_start = 0.97, and psilim set from rational_offset_fraction = 0.2.
  • The equilibrium is an H-mode with q95 ≈ 4, where q rises steeply over the last few percent in ψ. The g-file's FPOL is single precision, and GPEC already warns about that.
  • The failure came from one point of a 96-point scan. Its neighbors, with slightly lower and slightly higher pedestal-region current, ran normally, so the failure looks like Newton's sensitivity to its starting guess rather than a bad equilibrium.
  • I can't attach the equilibrium publicly, but I can share it privately.

Likely cause

The target q values are spaced evenly in q, while the starting guesses are spaced evenly in ψ. Where q(ψ) is strongly convex near the edge, the guess sits far from the root. A Newton step can then overshoot past psilim, into the spline's ExtendExtrap region, or oscillate.

Suggested fix

Over [dW_edge_scan_start, psilim], q is monotonic and the root is bracketed by construction. A bracketed solve would always converge:

psi_array[i] = find_zero(psi -> profiles.q_spline(psi) - qi,
                         (ctrl.dW_edge_scan_start, intr.psilim), Roots.Brent())

Alternatively, keep Newton and fall back to the bracketed solve when it fails. Wherever Newton converges today, either option returns the same root, so no existing result changes.

Lenguaje dominante
Julia
Estrellas
11
Forks
7
Merge medio
3 d 11 h
PR fusionados (30 d)
68

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  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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