Stabilization of a high-order harmonic generation seeded extreme ultraviolet free electron laser by time-synchronization control with electro-optic sampling

Abstract

A fully coherent free electron laser (FEL) seeded with a higher-order harmonic (HH) pulse from high-order harmonic generation (HHG) is successfully operated for a sufficiently prolonged time in pilot user experiments by using a timing drift feedback. For HHG-seeded FELs, the seeding laser pulses have to be synchronized with electron bunches. Despite seeded FELs being non-chaotic light sources in principle, external laser-seeded FELs are often unstable in practice because of a timing jitter and a drift between the seeding laser pulses and the accelerated electron bunches. Accordingly, we constructed a relative arrival-timing monitor based on non-invasive electro-optic sampling (EOS). The EOS monitor made uninterrupted shot-to-shot monitoring possible even during the seeded FEL operation. The EOS system was then used for arrival-timing feedback with an adjustability of 100 fs for continual operation of the HHG-seeded FEL. Using the EOS-based beam drift controlling system, the HHG-seeded FEL was operated over half a day with an effective hit rate of 20%-30%. The output pulse energy was 20 µJ at the 61.2 nm wavelength. Towards seeded FELs in the water window region, we investigated our upgrade plan to seed high-power FELs with HH photon energy of 30-100 eV and lase at shorter wavelengths of up to 2 nm through high-gain harmonic generation (HGHG) at the energy-upgraded SPring-8 Compact SASE Source (SCSS) accelerator. We studied a benefit as well as the feasibility of the next HHG-seeded FEL machine with single-stage HGHG with tunability of a lasing wavelength.

Single-shot spatio-temporal measurement of relativistic electron beams by electro-optic sampling

“…However, highenergy charged particles have a relativistic effect called length contraction and their pulse widths (i.e., size in the propagation direction) shrink [1] . In particular, highly relativistic electron beams with pulse widths of a few tens of femtoseconds are utilized in high-order harmonic generation seeded free-electron lasers [2] . On the other hand, threedimensional beam sizes have a big impact on properties of generated X-ray pulses.…”

Longitudinal and transverse spatial beam profile measurement of relativistic electron bunch by electro-optic sampling

“…36) The measurement of the spatio-temporal, or transverse (radial) and longitudinal, profile of the electron beam is essential especially in a high-order harmonic generation seeded FEL. 37) This is because the electron bunch and the higherharmonic pulse need to be overlapped three-dimensionally while passing through the undulator. It was reported that the central location and the transverse spatial moment (the shape of the electron bunch) with the pulse width were successfully determined by EO sampling.…”