Extend transient CSR to all elements downstream of a bend

[ChristopherMayes]

Summary

Removes the long-standing restriction that transient (drift) CSR is applied only to the first and second beam-line elements after a bend. With this change the transient CSR wake is applied to every element downstream of a bend, so the CSR effect is no longer truncated and is independent of how the drift region is segmented.

Builds on PR #17 (fix-out-of-bound-access).

Motivation

The 1D CSR wake kernel csrwakeTrIGF_FieldQuant (src/Appl/Field.f90) already implements the full Saldin et al. model, including the post-bend drift transient (Cases C and D), which is valid for arbitrary distance past a bend. However, the driver in src/Contrl/AccSimulator.f90 only tracked two elements of history (bitypeold, bitypeold2) and had three hard-coded geometry cases (in-bend, 1st-after, 2nd-after). Any element beyond the second after a bend silently received no CSR.

Consequently, breaking a post-bend drift into several shorter drifts (a common way to increase resolution) truncates the CSR effect after the second fragment. The examples/CSR/zeuthen chicane demonstrates this: ZB.PIP03 is split into 0.1 / 0.1 / 4.8 m, and the 4.8 m fragment previously got no CSR.

Change

The fix is bookkeeping only — no changes to the physics kernel:

• Retain the most recent bend's geometry across elements: flagbendcsr (is there an upstream bend requesting transient CSR), bendlencsr (its arc length). r0 already persists as the last bend's radius since it is only assigned in the bend branch.
• Accumulate the distance from the bend entrance in sentr, incremented by each element length at the end of the beam-line-element loop and reset to 0 at each new bend.
• Apply CSR to all downstream elements: flagcsr is set for the bend itself and for every element while flagbendcsr == 1.
• Unify the geometry into a single downstream formula
  (zwkmin = range(5)/gamma + (z - zbleng + sentr), bendlen = bendlencsr),
  replacing the 1st/2nd-after special cases and the dead "Not available for csr!" branch.

Scope / model

• Single most-recent-bend model (unchanged): each drift region uses the geometry of the nearest upstream bend. A new bend replaces the retained geometry.
• Multi-bend superposition is intentionally out of scope. Summing contributions from several upstream bends is not compatible with the Saldin fixed-geometry case approach (the geometry changes between bends) and would require a fundamentally different algorithm. This PR does not attempt it.
• On/off control is the bend's existing switch: input_switch (dparam(4)) > 500 enables transient CSR, which now propagates to all downstream elements; > 200 enables CSR inside the bend. No input-format change.

Validation

zeuthen chicane (attached as imactz_csr_zeuthen.zip — see Notes), 100k particles, mpirun -n 8. Split-invariance test — CSR must not depend on how the post-bend drift is segmented:

final quantity	before (split, truncated)	after (split)	after (single 5 m drift)
fort.26 z, col 4	1.4688	2.2074	2.2074
fort.26 z, col 6	9.5005	10.0662	10.0663
fort.24 x, col 6	+0.04082	−0.02246	−0.02246

• After the change, the split and single-drift lattices agree to 6–7 significant figures → CSR is applied consistently regardless of segmentation.
• The previous (truncated) result differs substantially (the transverse moment even changes sign), confirming the old behavior was dropping a physically significant CSR contribution over the long drift.

Files Changed

File	Change
src/Contrl/AccSimulator.f90	Retain post-bend geometry (flagbendcsr, bendlencsr, sentr); apply transient CSR to all downstream elements; unify the wake-geometry computation

Notes

• The validation lattice (zeuthen chicane) is attached to this PR as impactz_csr_zeuthen.zip, not committed to the repository. A proper in-repo example will be added in a follow-up.
• The NaN values in the last three columns of fort.26 are a pre-existing extended-diagnostic artifact, unrelated to this change (identical before and after).

Attachments

impactz_csr_zeuthen.zip

Acknowledgment

The investigation, implementation, and validation for this change were carried out with the assistance of Claude (Anthropic), used via GitHub Copilot.