Steerer¶

A thin steering corrector — a small dipole magnet that nudges the beam centroid back to the design orbit. Used in pairs (one horizontal, one vertical) at every BPM location for closed-orbit correction.

TL;DR¶

	TraceWin	HELIX
Keyword	`THIN_STEERING bx_l by_l aperture [elec]` or `STEERER ...`	`Steerer(name, bx_l, by_l, elec=False)`
`bx_l`	integrated B_x · L (T·m) — kicks in y	same (elec=1: ∫E_x·dl in volts — kicks in x)
`by_l`	integrated B_y · L (T·m) — kicks in x	same (elec=1: ∫E_y·dl in volts — kicks in y)
`elec`	0 = magnetic (default), 1 = electric	honoured (2026-07): electric kick via the electric rigidity Eρ = βc·Bρ

The thin kick — magnetic (crossed planes): Δx' = +sign(q)·by_l / Bρ, Δy' = +sign(q)·bx_l / Bρ; electric (same-plane, volts): Δx' = +sign(q)·bx_l / (βc·Bρ), Δy' = +sign(q)·by_l / (βc·Bρ) — in radians. (The TraceWin manual writes the magnetic x'-kick with the opposite sign; a corrector's polarity is a knob-sign convention absorbed by orbit correction.)

Tutorial¶

A steerer is a one-shot kick to redirect the beam centroid. Closed- orbit correction loops:

Read BPM positions (centroid x, y at each BPM).
Solve M·k = -x_BPM for steerer strengths k (M is the response matrix from each steerer to each BPM).
Apply the steerers; verify the orbit is corrected.

HELIX ships an orbit-correction driver (linac_gen.errors.correction.run_correction_from_lattice) that walks the lattice for ADJUST_STEERER / ADJUST_STEERER_BX / ADJUST_STEERER_BY cards, resolves each diag_n to the N-th DIAG_POSITION (BPM) marker, and applies the corresponding one-to-one or SVD correction. See Errors → Orbit correction for the full workflow.

6×6 transfer matrix¶

A steerer is a thin element with affine kicks — the linear 6×6 part is the identity, but the kick adds a constant to x' and y':

\[ M_{\text{steerer}} = I_{6\times 6}, \qquad \begin{pmatrix}\Delta x' \\ \Delta y'\end{pmatrix}_{\text{kick}} = \begin{pmatrix} +\operatorname{sign}(q)\,b_{y,L} / B\rho \\ +\operatorname{sign}(q)\,b_{x,L} / B\rho \end{pmatrix} \]

where bx_l = ∫B_x dl (T·m) gives the y kick, and by_l = ∫B_y dl gives the x kick — both with a plus sign. Positive by_l deflects positive charges in the +x direction; the sign flips automatically for negative species (H⁻).

Block	Effect
(x, x', y, y') 4×4 linear	identity (no focusing)
(Δφ, ΔW) 2×2 linear	identity (no longitudinal effect)
Affine offset on x', y'	constant kicks scaled by 1/p

Because the kicks are independent of (x, y), envelope tracking sees a steerer as the identity matrix — the kicks shift the centroid but leave σ unchanged.

Example¶

from linac_gen.elements.steerer import Steerer

st_x = Steerer(name="ST_H1", bx_l=0.0, by_l=1.0e-3)  # 1 mT·m → kick in x

API reference¶

linac_gen.elements.steerer.Steerer(
    name: str,
    bx_l: float = 0.0,            # T·m, kicks y
    by_l: float = 0.0,            # T·m, kicks x
)

Parameter	Default	Units	Notes
`name`	(required)	—	identifier
`bx_l`	0.0	T·m (elec: V)	integrated B_x; deflects in y (Δy' = +sign(q)·bx_l/Bρ) — elec: ∫E_x·dl, deflects in x
`by_l`	0.0	T·m (elec: V)	integrated B_y; deflects in x (Δx' = +sign(q)·by_l/Bρ) — elec: ∫E_y·dl, deflects in y
`elec`	False	—	electric flavour: same-plane kick / (βc·Bρ)

That is the entire constructor — a Steerer has no aperture and neither error-injection mixin. The STEERER card's aperture field is parsed and then dropped; elec round-trips through the writer. Note for orbit correction: the vmax T·m clip applies to whatever the knob's units are — volts for an electric steerer.

Source¶

linac_gen/elements/steerer.py:1