Abstract
We describe an application of laser-driven modulation in a dielectric micro-structure for the electron beam in a free-electron laser (FEL). The energy modulation is transferred into longitudinal bunching via compression in a magnetic chicane before entering the undulator section of the FEL. The bunched electron beam comprises a series of enhanced current spikes separated by the wavelength of the modulating laser. For beam parameters of SwissFEL at a total bunch charge of 30 pC, the individual spikes are expected to be as short as 140 as (FWHM) with peak currents exceeding 4 kA. The proposed modulation scheme requires the electron beam to be focused into the micrometer scale aperture of the dielectric structure, which imposes strict emittance and charge limitations, but, due to the small interaction region, the scheme is expected to require ten times less laser power as compared to laser modulation in a wiggler magnet, which is the conventional approach to create a pulse train in FELs.
Highlights
In this paper, we present and compare an alternative method to the conventional undulator modulation scheme
The remaining transverse modulation amplitude is 500 times smaller than the transverse momentum spread for the case of the 3 GeV electron beam in Athos at SwissFEL
We investigated a scheme which uses the fields in a dielectric micro-structure excited by a laser to modulate the ultra-relativistic electron beam of an free-electron laser (FEL) with the purpose of creating a pulse train for the ESASE scheme
Summary
We present and compare an alternative method to the conventional undulator modulation scheme. Athos is planned to deliver FEL radiation for wavelengths ranging from 0.65 nm to 5 nm from 2020 and user operation starting in 202110 Adjusting these chicanes to form overlap between each X-ray pulse with the subsequent slice of the electron bunch, the longitudinal coherence is transferred along the bunch, and the X-ray pulses become phase-locked[12]. Illuminating a dielectric double grating with a laser creates evanescent waves which travel inside the gap of the structure with a phase velocity defined by the periodicity of the structure and the wavelength of the illuminating laser. Due to the optical phase dependence of the interaction, net-acceleration of electrons can be achieved only for bunches significantly shorter than the wavelength of the driving laser. Since the SASE process strongly depends on the transverse slice emittance[21], we focus on longitudinal modulation only to avoid emittance growth after compression
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