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)
optional tail	`[auto\|landau\|gaussian] [strip_model=two_step] [dedx_model=bethe] [dx=… dy=…] [extent_mm=x,y] [seed=N]` — written only when non-default (see Foil scenarios)

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 Highland's RMS-angle formula in the form of Lynch & Dahl (1991) that the Particle Data Group's review gives:

$$ \theta_0 = \frac{13.6\ \text{MeV}}{\beta c p}\,|z|\, \sqrt{x/X_0}\,\bigl[1 + 0.038\ln\bigl(x z^2/(X_0 \beta^2)\bigr)\bigr] $$

where x/X₀ is the foil thickness in radiation lengths, z the particle's charge number and β its speed. (Before 2026-10-24 the logarithm was of x/X₀ alone; with z²/β² in it θ0 is 2.3 % larger for 800 MeV protons and 20 % larger for 24 MeV ones.)

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

Without a seed (the default) the foil follows the run: a beam made by create_beam with a seed — the GUI and the CLI always give one, also for a beam read from a file — draws the foil's kicks from a stream seeded by that seed and the foil's name, so a run repeats exactly and two foils draw different numbers. A seed on the foil itself fixes the stream instead, whatever the beam's seed. Either way every new beam starts the stream afresh, so a run that reuses the lattice (as the GUI does) repeats too. A beam built by hand without a seed, through a foil without one, draws a fresh random stream each run. The bunches of a bunch train share their seed, so they all see the same foil kicks, as they share their macroparticles.

The rms energy loss is small, but it is uncorrelated with the beam: on a long, strongly chirped bunch — a phase–energy correlation of −0.9998, say — it widens the thin longitudinal ellipse a lot. The Landau tail then makes the post-foil longitudinal emittance converge slowly with the particle count: in such a case it grew by between 5 % and 190 % over 20 foil seeds with 5 000 particles, and by 40–67 % with 500 000. Use 50 000 particles or more when that number matters.

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`	—	`None`: follow the beam's seed (see above); an integer fixes the stream, restarted for every beam
`strip_model`	`"off"`	—	`"two_step"`: H⁻ → H⁰ → p with cross sections calibrated to the PIP-II anchors; unconverted ions on `Beam.unstripped_table` (not a loss)
`dedx_model`	`"mip"`	—	`"bethe"`: β-dependent Bethe mean loss instead of the MIP floor
`dx`, `dy`	0.0	mm	foil offset (the tracker/envelope wrap translates the beam into the foil frame)
`extent_mm`	`None`	mm	half-size (one value or `(x, y)`); particles outside miss the foil

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

Source¶

linac_gen/elements/foil.py:1