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Description
Harmonic lasing is an advanced FEL concept to produce short wavelengths with a low-energy electron beam by developing lasing at an odd harmonic of the FEL fundamental wavelength. Harmonic lasing self-seeded (HLSS) FEL uses a two-stage high-gain FEL with the first stage being a harmonic lasing FEL to produce the coherent seed for the second stage which serves as a high-gain FEL amplifier. HLSS has been theoretically studied for X-ray wavelengths and experimentally demonstrated between 4.5 and 15 nm*. Due to the SASE nature of the first stage, the harmonic seed pulses from the first stage do not have good temporal overlap with the core of the electron bunch in the second stage, thus reducing the output pulse energy and efficiency. We study an advanced version of HLSS by using a regenerative amplifier FEL (RAFEL) cavity consisting of multilayer mirrors to recirculate the harmonic wavelength in the first stage. We show in simulations that using the regenerative amplifier cavity with multilayer mirrors stretches and delays the harmonic seed pulses in time and narrows the radiation spectral bandwidth. The RAFEL harmonic seeding significantly increases the second stage output power and longitudinal coherence.
Footnotes
- E. A. Schneidmiller and M. V. Yurkov, Harmonic lasing in x-ray free electron lasers, Phys. Rev. ST Accel. Beams 15, (2012).
** E. A. Schneidmiller et al., First operation of a harmonic lasing self-seeded FEL, Phys. Rev. Accel. Beams 20, (2017).
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