Leg D — foil scenarios

The foil leg tracks the beam to the stripper foil (landmarks.foil_element, or the first Foil of the deck) for its nominal state and a set of scenarios: the thickness list (foil.thickness_ug_cm2, 0 = the foil is missing), transverse offsets (foil.offsets_mm), a thinned foil (foil.thinned_fraction), and, in the full preset, tracked runs with a snapshot of the particles at the foil.

What is measured

Quantity	Model	Source
foil_sigma_x_mm, foil_sigma_y_mm	envelope Σ at the foil, or the rms of the snapshot	results.h5
foil_w_cm2_peak	peak power density of the snapshot at the beam's current × duty (× hits_per_particle)	analysis.loss_power.plane_power_density
strip_eff, h0_frac, missed_frac	the tracked H⁻ → H⁰ → p populations and the particles outside the foil extent	the beam's unstripped_table
strip_eff_analytic, h0_frac_analytic	the two-step model at the foil energy and scenario thickness	Foil.stripping_fractions

The stripping model

strip_model="two_step" converts a negative-ion beam H⁻ → H⁰ → p with the cross sections σ(−1→0) and σ(0→+1) calibrated so that the stripped fraction reproduces the two PIP-II design anchors — 99.956 % at 600 µg/cm² and 99.1 % at 380 µg/cm² for an 800 MeV H⁻ beam on carbon — scaled 1/β² to other energies and by √(Z/6) to other materials (a convention, not data). The calibration lands within a few per cent of the measured cross sections (Gulley et al., PRA 53, 3201 (1996)); the report labels the anchors as calibration points, not measurements. One uniform draw per particle, taken after the scattering and loss draws, assigns the charge state, so the historical kicks of a seeded foil are unchanged; unconverted ions are recorded on Beam.unstripped_table (HDF5 group unstripped/) and stay alive — they are not a loss, so transmission and the loss-power accounting stay honest. The model is inert for a positive species.

Deck line

; HELIX_FOIL STRIP C 600 gaussian strip_model=two_step dedx_model=bethe dx=0.5 dy=0 extent_mm=2.5,4 seed=7

The optional key=value tail (strip_model, dedx_model — mip or the β-dependent bethe mean loss —, dx, dy, extent_mm as one value or x,y half-sizes, seed) is written only when non-default, so existing decks round-trip unchanged; particles outside the extent miss the foil (no kick, no loss, recorded as missed). See Foil.

Outputs

legs/foil/foil.csv and the figures foil_spot.png and foil_efficiency_vs_thickness.png (the calibrated curve with the tracked points on it).