Tunable High-Gradient Quadrupoles for a Laser-Plasma Acceleration-Based FEL
Résumé
The magnetic design and characterization of tunable high gradient permanent magnet based quadrupole, are presented. To achieve a high gradient field with a compact structure, permanent magnets are chosen rather than usual electro-magnets due to their small aperture. The quadrupole structure consists of two quadrupoles superimposed capable of generating a gradient of 210 T/m. The first quadrupole is composed of permanent magnets following a Halbach configuration shaped as a ring attaining a constant gradient of 160T/m, and the second of four permanent magnet cylinders surrounding the ring and capable of rotating around their axis to achieve a gradient tunability of ±50 T/m. Each tuning magnet is connected to a motor and is controlled independently, enabling the gradient to be tuned with a rather good magnetic center stability (±20 μm) and without any field asymmetry. Seven quadrupoles have been built with different magnetic lengths in order to fulfil the integrated gradient required. A set of QUAPEVA triplet are now in use, to focus a high divergent electron beam with large energy spread generated by a laser plasma acceleration source for a free electron laser application.