Toward a fully coherent tender and hard X-ray free-electron laser via cascaded EEHG in fourth-generation synchrotron light sources

Abstract

Free-electron-laser-based beamlines utilize fully coherent laser pulses with extremely narrow bandwidth allowing direct use of X-rays without monochromators. This could be very beneficial for all users of current and future fourth-generation diffraction-limited synchrotron light sources (DL-SLSs) who need narrowband full-coherence high-brightness X-ray pulses. Based on our previous finding, i.e. that separating the two stages of echo-enabled harmonic generation (EEHG) with a few extra bending-magnet sections provides an effective way to increase the momentum compaction of chicane 1, one can simultaneously achieve adequate prebunching at extremely high harmonics as well as keep the energy modulation to the ideal minimum. This could open the door for cascaded EEHG, toward fully coherent tender and hard X-ray wavelengths. Built on our compact design of a twin-pulse seeding electron beam with an adjustable delay and timing jitter at the level of a few femtoseconds, a cascaded EEHG can be implemented, which includes two EEHG beamlines, where the radiation pulse generated by the first beamline with harmonic h 1 could be used as the input seed laser pulse to the second beamline with harmonic h 2. Hence, the second radiator could potentially reach very high harmonics [h = h 1(20)h 2(25–100)] from 500 to 2000, corresponding to tender and hard X-ray wavelengths. It is demonstrated that the cascaded EEHG scheme is compatible with almost any current or planned fourth-generation DL-SLS, with significant benefits for space-limited storage rings in particular. The main advantage is that this scheme requires almost no change of the storage-ring lattice and is fully compatible with other beamlines. Current proposals for rings with much longer straight sections would add self-amplified spontaneous emission as another viable option for storage-ring-based free-electron lasers.

High Harmonic Generation Seeding Echo-Enabled Harmonic Generation toward a Storage Ring-Based Tender and Hard X-ray-Free Electron Laser

“…Thus, finding a way to shorten the seed laser wavelength becomes the most feasible option. One way is to apply the cascaded EEHG scheme [13], as shown in Figure 1a, and the other is to find a seeding source with a short wavelength [10][11][12], as shown in Figure 1b. The drawbacks of the cascade scheme are not only significantly scaling up the occupancy of an SR, which is already crowded, as well as the overall cost, but also increasing the technical challenges and the operational complexities.…”

“…An EUV HHG source seeding EEHG scheme could enable a compact design of the SR-based FEL toward the tender and hard X-ray region, eliminating the need of building two cascaded EEHG beamlines [13]. Considering the intrinsic features of the EEHG-forming multiple energy stripes in the longitudinal phase space (LPS) via the first-stage pre-bunching then utilizing an HGHG type of harmonic bunching in the second stage [13]-it would be ideal if one can seed the second stage with an optimized EUV-HHG source at the shortest possible wavelength 28.5 nm, whereas the highest peak power, a few-femtosecond pulse duration and narrowest linewidth of 0.1-0.2 eV are simultaneously fulfilled. These HHG parameters, including 1•10 9 photons per pulse, are derived from an overall photon flux of 6•10 12 photons per second at a repetition rate of 5 kHz.…”

“…They can be achieved under optimally driven conditions, a capability provided by KMLabs Inc., founded by Dr. Henry C. Kapteyn [26]. In addition, to optimize the energy-stripe formation in the LPS for maximum prebunching under the peak power constraint of an EUV-HHG source, limited by 1•10 9 photons per pulse with a 10 fs pulse duration, it is preferrable to seed the first stage with a conventional Ti:sapphire laser at its third harmonic (256 nm) and vary the two-stage separation to optimize the momentum compaction of the first stage (R 1 ) [1][2][3], while simultaneously mitigating the smearing effect induced with incoherent synchrotron radiation (ISR) [13]. The optical transport lines for the 256 nm seed laser of stage 1 and the EUV-HHG source of stage 2 should be built in an ultra-high vacuum to avoid air turbulence, achieving the required µrad-level spatial pointing jitter, which is limited by the transverse walk-off between the seed laser and the electron bunch, not exceeding a small fraction of the beam size [13].…”

“…The total reflectivity of two EUV mirrors at the HHG seed wavelength equals to T = R 2 , around 30% [13]. In an overly optimistic case, one could directly send an HHG pulse into the modulator, with T being closer to 100%.…”

“…From harmonic 50 to harmonic 200, only about 30-40% decrease in the achievable bunching factor is predicted while using a reasonable laser power for seeding. Regarding the nominal seeding via a conventional solid-state laser system, e.g., the third harmonic of Ti:sapphire laser at the wavelength of 256 nm [1][2][3]13], the shortest wavelength that can be achieved is determined by the combination of the short seed laser wavelength of 256 nm and the highest harmonic of 200, which is difficult to achieve, around λ = 256 nm/200 = 1.28 nm [14].…”