Abstract
The undulator system in the European X-ray Free Electron Laser is mainly comprised of 5-m long undulator segments and 1.1 m long intersections in between. The longitudinal component of the electrons' velocity is reduced when traveling inside an undulator due to the wiggle motion. Therefore the optical phase is detuned. The detune effect is also from the undulator fringe field where electron longitudinal speed also deviates from the oscillation condition. The total detune effect is compensated by a magnetic device called phase shifter, which is correspondingly set for a specific undulator gap. In this paper we investigate the homogeneity of the fringe field from different undulators. Different phase matching criteria are studied. The field fitting technique for the phase matching in high accuracy is demonstrated in detail. The impact by air coil is also studied. Eventually the matching test by spontaneous radiation simulation is made. A test method for high sensitivity to matching error is proposed.
Highlights
High gain free electron lasers (FELs) using the principle of self-amplified spontaneous emission (SASE) are so far the only way to generate FEL radiation in the hard x-ray range [1,2,3,4,5]
For the FEL process the interruption of the undulator implies a problem: The longitudinal speed of the electrons in the intersection is different from that inside the undulator and the optical phase matching between laser field and electron motion is perturbed
K-parameters for different undulators at the same gap, which exceed the limits given by the Pierce parameter, ΔK K
Summary
High gain free electron lasers (FELs) using the principle of self-amplified spontaneous emission (SASE) are so far the only way to generate FEL radiation in the hard x-ray range [1,2,3,4,5]. Alternatives, such as harmonic generation, exist that generate more stable radiation [6] Aside from such differences both require long undulator systems with lengths from tens of meters up to about 200 meters depending on the radiation wavelength and electron beam parameters. For the soft x-ray FEL SASE3 the undulator system is comprised of 21 U68 type undulator segments with a total length of about 112 m Both U40 and U68 use the same standard 5 m long mechanical support structure. Undulators and phase shifters are well characterized before installation: Undulators were accurately tuned for small trajectory and phase errors in the operational gap range [8,9]. It has been demonstrated that the magnetic data are accurate enough [14]
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