Journal Article

Controlling beam trajectory and transport in a tapered helical undulator

Musumeci, P., Fisher, A., Jin, J.

2025

Overview

In this paper we present a detailed discussion of the helical undulator system developed for high extraction efficiency experiments in the tapered‐enhanced stimulated superradiant amplification regime. The design is based on permanent magnet technology and comprises two Halbach arrays orthogonally oriented and shifted by 90° with respect to each other. When used in low energy beamlines for THz generation, the electron beam trajectory and transport are particularly sensitive to the undulator off‐axis fields so that it becomes important to complement on‐axis field measurements with analysis and tuning of the higher‐order field components. Here we describe how the pulsed wire measurement technique can be effectively used to retrieve on‐ and off‐axis magnetic field characteristics of the undulator. A simple two‐dipole model is developed to guide the final adjustments to the permanent magnet positions along the array to tune the quadrupole and sextupole components of the field. In this work the transport of low energy electrons through helical magnetic undulators is discussed with application for THz generation in waveguide free‐electron lasers. To optimize beam transmission, the off‐axis fields sampled by low energy beams are tuned with a 3D pulsed‐wire measurement technique that reveals localized errors in the quadrupole and sextupole field moments.

• Publisher: International Union of Crystallography,John Wiley & Sons, Inc,International Union of Crystallography (IUCr)
• Subject: Arrays / Dipoles / Electron beams / free-electron lasers / magnetic measurements / Permanent magnets / pulsed wire / Quadrupoles / Research Papers / undulator tuning