Foil¶

A thin material foil that scatters and slows the beam. Used to model stripper foils (H⁻ → p⁺ charge exchange is the canonical PIP-II use) and any degrader / scattering foil in a beam line.

TL;DR (TraceWin users)¶

TraceWin has no foil card, so HELIX declares one with a comment-prefixed extension that stays invisible to TraceWin:

; HELIX_FOIL <name> <material> <thickness_ug_cm2>
; HELIX_FOIL STRIP C 600

Field	Meaning
`material`	`C`, `Be`, `Al`, `Cu`, `Mo`, or `W` (chemical symbol)
`thickness_ug_cm2`	areal density in μg/cm² (PIP-II strippers: 300–700)

The foil is a zero-length ThinKickElement: it applies its kick at one s-position and adds no length to the lattice.

Tutorial (newcomers)¶

A foil does two things to each macroparticle:

Multiple Coulomb scattering — many small-angle deflections add up to a random transverse kick on xp, yp. HELIX uses the Highland (1991) RMS-angle formula:

$$ \theta_0 = \frac{13.6\ \text{MeV}}{\beta c p}\,|z|\, \sqrt{x/X_0}\,\bigl[1 + 0.038\ln(x/X_0)\bigr] $$

where x/X₀ is the foil thickness in radiation lengths.

Energy loss — the mean ionisation loss (Bethe–Bloch, tabulated at minimum-ionising rates per material) is subtracted from dw, with Bohr/Vavilov straggling added as a Gaussian spread.

Both effects scale with charge² and with thickness; the scattering also grows for low-momentum (low-βcp) beams. They act only in multi-particle tracking — there is nothing to apply to an RMS envelope.

Example¶

from linac_gen.elements.foil import Foil

# A 600 μg/cm² carbon stripper foil.
foil = Foil(name="STRIP", material="C", thickness_ug_cm2=600.0,
            seed=1234)        # fixed seed → reproducible kicks

Pass a fixed seed in tests for reproducibility; leave it None (the default) in production so each run draws a fresh random stream.

API reference (developers)¶

linac_gen.elements.foil.Foil(
    name: str,
    material: str = "C",            # C, Be, Al, Cu, Mo, W
    thickness_ug_cm2: float = 600.0,
    aperture: float = 0.0,          # mm; 0 = unrestricted
    seed: int | None = None,        # RNG seed for the per-particle kicks
)

Parameter	Default	Units	Notes
`name`	(required)	—
`material`	`"C"`	—	one of C / Be / Al / Cu / Mo / W
`thickness_ug_cm2`	600.0	μg/cm²	areal density
`aperture`	0.0	mm	round aperture (0 = none)
`seed`	`None`	—	fix for reproducible scattering

The material database (radiation length X₀ and minimum-ionising dE/dx) is taken from PDG 2024.

Source¶

linac_gen/elements/foil.py:1