Aligning linac accelerating structures using a copropagating octupolar mode

We propose a novel method to align accelerating structures such as those used in the Compact Linear Collider (CLIC) by exploiting a mode that copropagates with the normal accelerating mode. This mode has an octupolar dependence in the transverse direction and is caused by radial waveguides intended to damp higher-order modes. The nonlinear dependence of the octupolar mode makes it possible to determine the center of the structure from the nonlinear dependence of the transverse kick, observed on a downstream beam position monitor, while changing the transverse position of the beam with respect to the accelerating structures. We discuss the method, its tolerances and disentangling the individual misalignments of two adjacent accelerating structures that are powered from a single source.

Figure 1

Setup with a single accelerating structure with an octupole field. The beam travels from right to left and can be scanned transversely and parallel to the beam axis by moving the girder or by using two upstream steering magnets. The beam position shifts can be measured at a distance $L$ downstream from the octupole with a beam position monitor (BPM).

Figure 2

Relative error in fitted offset for different systematic errors in $KL$.

Figure 3

Aligning the beam in two accelerating structures both with octupole fields.