Reject unsupported periodic collision models#184
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This was referenced Jul 13, 2026
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Closing: the wp5 flux-pumping stack is retired, and this guard protects a driver ( |
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) ## Summary Extracts the two salvageable pieces of the retired wp5 flux-pumping stack onto the forced-periodicity lineage. The rest of that stack (#177–#181, #184) is closed as superseded. ## Krook argument `z0` (was #183) `interpolate_plasma_backs` interpolated the **derived** `z0` alongside the primitives. `z0` carries a `1/|k_parallel|` singularity at the resonance, so a four-point Lagrange stencil spanning `k_parallel = 0` returns values that violate its own defining formula. The fix interpolates primitives only and recomputes z0 = -(omega_E - omega - i nu) / (|k_parallel| sqrt(2) v_T) on the target grid — the same invariant the periodized-background path already follows. **Scope:** `z0` is consumed only by `Krook_kernel_plasma_prefacs.f90`. The Fokker-Planck path recomputes `x1/x2/A1/A2/I^{kl}` after interpolation and never reads `z0`, so **FP results are bit-unchanged**. This repairs the Krook hat path, whose nml still marks it `(unstable)` — plausibly part of why. ## Collision-frequency scale (was #182) New positive `&KIM_CONFIG` key `collision_frequency_scale` multiplying the calculated electron and ion collision frequencies before `z0` and the FP susceptibility inputs are assembled. Default `1.0` leaves every collision formula, profile, unit, and output unchanged. Non-positive values are rejected at startup; the value is displayed when it differs from 1.0 and stored in HDF5 provenance. Same shape as the existing `ion_flr_scale_factor` knob. No benchmark-acceptance claim — it is a sensitivity parameter. ## Tests `test_collision_scale_z0` (new) asserts the `z0` invariant on a stencil straddling `k_parallel = 0` and away from it, and that the scale reaches both `nu` and `z0` (including the imaginary part of `z0` tracking it). Developed red-first: against the pre-fix code the test fails with a **36% relative `z0` error at k_par = 2e-4** (`got (-0.0983, 0.0129)` vs `want (-0.4543, 0.0599)`), growing without bound toward the resonance. ## Validation - `make` — clean build - `ctest -E 'test_rhs_balance|test_periodic_convergence|test_periodic_vs_global'` — **32/32 passed** Known red gates excluded as usual: the pre-existing `test_rhs_balance` baseline failure, the periodization-deformation gate, and the periodic-vs-global 5% gate — none touched by this change. Supersedes #182, #183.
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The periodic-Fourier driver currently assembles the restored Krook kernels regardless of collision_model. Reject every model except Krook until a continuum Fokker-Planck kernel is implemented, so an input cannot silently claim different collision physics.
Invariant: the supported Krook periodic assembly, Fourier convention, CGS units, grid, dense solve, and boundary behavior are unchanged.
Verification
Failing before, using the parent binary with collision_model = FokkerPlanck:
Passing after, using the same input:
Relation to Markus's Fokker-Planck periodic branch
This guard closes a real semantic hole in the independent stack: a Krook kernel must not run under a Fokker-Planck label. Keep the guard until #187 verifies the FP kernel and #190 accepts the FP benchmark. #191 must replace the blanket rejection with explicit Krook/FP dispatch, not remove collision-model validation.