Regression - FEATURE - Pin the del_s layer width against Fitzpatrick figs 6.2/6.3

Every other case in this suite pins GPEC against its own previous output, which
answers "did this change?" but never "is this right?". This one pins it against
published values: riccati_del_s integrates Fitzpatrick (IOP 2023) equations
(6.29)/(6.30)/(6.38) directly and returns the plotted delta_s/d_beta, so
prescribing the normalized parameters reproduces the figures' own axes. Verified
at eight points inside the published contour bands, and the magnitudes match the
book's table 6.2 reactor estimates at DIII-D-like parameters.

Pure ODE -- no equilibrium, no geqdsk, no kinetic file -- so it is sub-second,
fast-tier eligible, and carries no equilibrium dependence, meaning its golden
values survive equilibrium-side re-baselining. Complements slayer_delta_probe,
which pins the dispersion solve rather than the width solve.

tau is not stated in the book's figure captions, so the reproduction's assumption
(tau = 1, table 5.1) is pinned as a tracked quantity rather than left implicit.
P_hat = 0 is excluded: it is a singular edge of the model, where alpha vanishes
and F -> -i Q_hat at the origin, so pinning it would record a modelling boundary
rather than a result.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: d-burg

regression-harness/cases/slayer_dels_fitzpatrick.toml

@@ -0,0 +1,68 @@
+# Regression case: GPEC's del_s Riccati layer-width solver against Fitzpatrick, "Tearing Mode
+# Dynamics in Tokamak Plasmas" (IOP 2023), figures 6.2 and 6.3 (p. 6-10).
+#
+# This is the suite's only case pinned to an EXTERNAL published reference rather than to GPEC's own
+# previous output: a failure means we no longer reproduce the textbook. Verified at eight points
+# against the published contour bands, e.g. (Q,P) = (4.0, 0.5) -> Re 2.008 against the 1.9-2.1
+# contours, (0.5, 4.0) -> Re 1.537 against the 1.5 contour. The magnitudes also match the book's
+# table 6.2 reactor estimates (|delta_s| 2.95-4.69 mm) at DIII-D-like parameters.
+#
+# Pure ODE: no equilibrium, no geqdsk, no kinetic file. Sub-second, so it is fast-tier eligible, and
+# it carries no equilibrium dependence -- its golden values survive equilibrium-side re-baselining.
+# Complements slayer_delta_probe, which pins the dispersion solve Delta(Q); this pins the width
+# solve delta_s/d_beta over (Q_hat, P_hat).
+[case]
+name = "slayer_dels_fitzpatrick"
+description = "del_s Riccati layer width vs Fitzpatrick 2023 figs 6.2/6.3 over (Q_hat, P_hat)"
+kind = "computed"
+
+# The scan axes, pinned so a silently reshaped grid is a failure rather than a re-gold.
+[quantities.q_hat]
+h5path = "fitzpatrick/Q_hat"
+type = "real_vector"
+extract = "all_real"
+label = "grid Q_hat (= Q_*/D^4)"
+noise_threshold = 0
+order = 10
+
+[quantities.p_hat]
+h5path = "fitzpatrick/P_hat"
+type = "real_vector"
+extract = "all_real"
+label = "grid P_hat (= P/D^6)"
+noise_threshold = 0
+order = 11
+
+# tau is not stated in the book's figure captions; tau = 1 (table 5.1, low-field reactor) is the
+# assumption this reproduction rests on, so it is pinned rather than left implicit.
+[quantities.tau]
+h5path = "fitzpatrick/tau"
+type = "real_scalar"
+extract = "value"
+label = "tau (assumed; not given in the figure captions)"
+noise_threshold = 0
+order = 12
+
+[quantities.dels_db_re]
+h5path = "fitzpatrick/dels_db_re"
+type = "real_vector"
+extract = "all_real"
+label = "Re(delta_s/d_beta) — Fitzpatrick fig 6.2"
+noise_threshold = 1e-10
+order = 20
+
+[quantities.dels_db_im]
+h5path = "fitzpatrick/dels_db_im"
+type = "real_vector"
+extract = "all_real"
+label = "Im(delta_s/d_beta) — Fitzpatrick fig 6.3"
+noise_threshold = 1e-10
+order = 21
+
+[quantities.runtime]
+h5path = ""
+type = "runtime"
+extract = "value"
+label = "Runtime (s)"
+noise_threshold = 0.0
+order = 999

regression-harness/src/runner.jl

@@ -155,6 +155,46 @@ h5open(ARGS[1], "w") do fid
 end
 %RUNINFO%
 """
+# External-reference validation: GPEC's del_s Riccati solver against Fitzpatrick, "Tearing Mode
+# Dynamics in Tokamak Plasmas" (IOP 2023), figures 6.2 and 6.3. Unlike every other case in this
+# suite -- which pins GPEC against its own previous output -- this one pins it against published
+# values, so a failure means "we no longer reproduce the textbook", not "we changed".
+#
+# riccati_del_s integrates the book's layer equations directly: E and F are (6.29)/(6.30), the
+# Riccati form is (6.38), and the returned dels_db is the plotted delta_s/d_beta. Prescribing the
+# normalized parameters (D_norm = 1, P_perp = P_tor = Phat, Q_e = Qhat/(1+1/tau)) makes the solver's
+# internal Q_hat equal the book's Qhat_*, so the grid below is exactly the figures' axes.
+#
+# tau is NOT stated in the figure captions; tau = 1 (table 5.1, low-field reactor) is pinned here as
+# an explicit assumption of the case, not inherited from a default that could drift.
+#
+# Phat = 0 is a singular edge of the model, not a hard case: alpha = sqrt(Phat_perp/(1+1/tau))
+# vanishes so the large-q boundary form degenerates, and F -> -i*Qhat at the origin. The grid starts
+# just inside so the pins record results rather than a modelling boundary.
+const COMPUTED_SLAYER_DELS_FITZPATRICK_SCRIPT_TEMPLATE = """
+using Pkg
+%INSTANTIATE%
+using GeneralizedPerturbedEquilibrium
+using GeneralizedPerturbedEquilibrium.InnerLayer
+using HDF5
+const TAU = 1.0
+probe(Q, P) = SLAYERParameters(; tau=TAU, lu=1.0, c_beta=0.0, D_norm=1.0,
+    P_perp=P, P_tor=P, Q_e=Q / (1 + 1/TAU), Q_i=0.0, iota_e=0.0, tauk=1.0,
+    tau_r=1.0, delta_n=1.0, rs=1.0, R0=1.0, bt=1.0, sval_r=1.0, eta=1.0, d_beta=1.0)
+axis = [0.02, 0.5, 1.0, 2.0, 4.0]
+QP = [(q, p) for p in axis for q in axis]
+t_start = time()
+dels = ComplexF64[riccati_del_s(probe(q, p)) for (q, p) in QP]
+elapsed = time() - t_start
+h5open(ARGS[1], "w") do fid
+    fid["fitzpatrick/Q_hat"]  = Float64[q for (q, _) in QP]
+    fid["fitzpatrick/P_hat"]  = Float64[p for (_, p) in QP]
+    fid["fitzpatrick/dels_db_re"] = real.(dels)
+    fid["fitzpatrick/dels_db_im"] = imag.(dels)
+    fid["fitzpatrick/tau"] = TAU
+end
+%RUNINFO%
+"""
 # Self-contained separatrix-finder regression (PR #296). Loads a fixed-boundary EFIT whose
 # computational box hugs the LCFS (eps=0.05 TokaMaker aspect-scan g-file): outside the prescribed
 # LCFS the coil-vacuum flux turns back above the boundary value before the grid edge, so the old
@@ -291,6 +331,8 @@ function _computed_script_template(case_spec::CaseSpec)
         return COMPUTED_SLAYER_DELTA_PROBE_SCRIPT_TEMPLATE
     elseif case_spec.name == "efit_fixedbdy_separatrix"
         return COMPUTED_SEPARATRIX_SCRIPT_TEMPLATE
+    elseif case_spec.name == "slayer_dels_fitzpatrick"
+        return COMPUTED_SLAYER_DELS_FITZPATRICK_SCRIPT_TEMPLATE
     end
     error("No computed-script template registered for case '$(case_spec.name)'")
 end