Abstract
It has been shown that the direct seeding can enhance the performance of a free-electron laser (FEL) in terms of its spectral, temporal, and coherence properties and reduces fluctuations in FEL output energy and arrival-time jitter. The properties of the used seed photon pulse are of high importance. In this paper, we describe the influence of the ${M}^{2}$ onto the achievable power contrast between the direct seeded and the unseeded FEL radiation. The results of these studies are compared with the data from the high harmonic generation direct seeding experiment ``sFLASH'' in Hamburg, Germany. A method to measure ${M}^{2}$ from a single transverse intensity distribution of the high harmonics beam at waist is discussed.
Highlights
In the direct seeding process the quality of the external photon pulse (‘‘seed’’) plays an important role since the coupling and energy transfer between the electromagnetic field of the seed pulse and the electrons depends on the wave front of the photon pulse [1]
We describe the influence of the M2 onto the achievable power contrast between the direct seeded and the unseeded free-electron laser (FEL) radiation
We present a simulations study on the M2 impact on the output power of an FEL directly seeded using higher harmonics generated in gas (HHG)
Summary
In the direct seeding process the quality of the external photon pulse (‘‘seed’’) plays an important role since the coupling and energy transfer between the electromagnetic field of the seed pulse and the electrons depends on the wave front of the photon pulse [1]. A study on the quality of the photon pulse generated by an FEL emitting self-amplified spontaneous emission (SASE) using modal decomposition can be found in [8]. There are several other methods to measure the M2 value, e.g., the focus scan technique [9] or modal decomposition using computer generated holograms [10]. The M2 measurement technique used in his paper only needs a single transverse intensity profile, e.g., a CCD-camera image. This technique becomes a viable alternative in the cases when due to space limitations other methods cannot be used and/or knowledge on the modal content of the laser radiation is required
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