Echo-enabled Harmonic Generation Scheme Research Articles

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

To align with the global trend of integrating synchrotron light source (SLS) and free electron laser (FEL) facilities on one site, in line with examples such as SPring-8 and SACLA in Japan and ELETTRA and FERMI in Italy, we actively explore FEL options leveraging the ultralow-emittance electron beam of the NSLS-II upgrade. These options show promising potential for synergy with storage ring (SR) operations, thereby significantly enhancing our facility’s capabilities. Echo-enabled harmonic generation (EEHG) is well-suited to SR-based FELs, and has already been demonstrated with the capability of generating extremely narrow bandwidth as well as high brightness, realized using diffraction-limited short pulses in transverse planes and Fourier transform-limited bandwidth in the soft X-ray spectrum. However, regarding a conventional EEHG scheme, the combination of the shortest seed laser wavelength (256 nm) and highest harmonic (200) sets the short wavelength limit to λ = 1.28 nm. To further extend the short wavelength limit down to the tender and hard X-ray region, a vital option is to shorten the seed laser wavelength. Thanks to recent advances in high harmonic generation (HHG), packing 109 photons at one harmonic within a few-femtosecond pulse could turn such a novel HHG source into an ideal seeding for EEHG. Thus, compared to the cascaded EEHG, the HHG seeding option could not only lower the cost, but also free the SR space for accommodating more user beamlines. Moreover, to mitigate the SASE background noise on the sample and detector, we combine the HHG seeding EEHG with the crab cavity short pulse scheme for maximum benefit.

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

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 h1 could be used as the input seed laser pulse to the second beamline with harmonic h2. Hence, the second radiator could potentially reach very high harmonics [h = h1(20)h2(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.

Toward fully coherent soft x-ray free-electron laser via echo-enabled harmonic generation in fourth generation synchrotron light sources.

Having previously reported on bunching via echo-enabled harmonic generation (EEHG) as an effective way to improve the longitudinal coherence in the NSLS-II storage ring [X. Yang et al., Sci. Rep. 12, 9437 (2022)], we demonstrate that this EEHG scheme can be easily adopted to any fourth generation diffraction-limited synchrotron light source with significant benefits. The advantage of the scheme is that it requires no change of the lattice and is fully compatible with other beamlines. Since the EEHG performance is mainly determined by the momentum compaction, beam emittances, and beta functions of a SR lattice, we have identified these crucial parameters and successfully built a generalized model, which can predict the performance of nearly any SLS. Regarding the fourth generation SLSs, momentum compactions are often significantly smaller; thus, to cover the x rays with a photon energy of up to 1keV, we utilize a specific design, including a 250nm seed-laser wavelength. Our model predicts that for most of the current and future fourth generation SLSs, the EEHG scheme can produce significant prebunching up to harmonic 200 and, thus, generate a few MW scale peak power at 1.25nm wavelength.

Coherent extreme ultraviolet free-electron laser with echo-enabled harmonic generation

The echo-enabled harmonic generation (EEHG) scheme holds promising prospects for efficiently generating intense coherent radiation at very high harmonics of a conventional ultraviolet seed laser. We report the lasing of the EEHG free-electron laser (FEL) at an extreme ultraviolet (EUV) wavelength with a seeded FEL facility, the Shanghai soft x-ray FEL. For the first time, we have benchmarked the basic theory of EEHG by measuring the bunching factor distributions over one octave down to the EUV region. Our results demonstrated the key advantages of the EEHG FEL, i.e., generation of very high harmonics with a small laser-induced energy spread and insensitivity to beam imperfections, and marks a great step towards fully coherent x rays with the EEHG scheme.

Generation of two-color polarization-adjustable radiation pulses for storage ring light source

To date, two-color pulses are widely used in pump–probe experiments. For a ring-based light source, the power of the spontaneous radiation fluctuates randomly in the longitudinal direction. It is difficult to produce two-color double pulses by optical methods. In this paper, we introduce a method based on the echo-enabled harmonic generation scheme that generates two-color pulses in a storage ring light source. By adopting crossed undulators and a phase shifter, the polarization of the two-color pulses can be easily switched. A numerical simulation based on a diffraction-limited storage ring, the Hefei Advanced Light Source, suggests that the time delay and spectral separation of the two pulses can be adjusted linearly by changing the pulse duration and chirp parameters of the seed laser. A circular polarization degree above 80% could be achieved.

Generating femtosecond coherent X-ray pulses in a diffraction-limited storage ring with the echo-enabled harmonic generation scheme

To study ultrafast processes at the sub-picosecond level, novel methods based on coherent harmonic generation technologies have been proposed to generate ultrashort radiation pulses in existing ring-based light sources. Using the High Energy Photon Source as an example, we numerically test the feasibility of implementing one coherent harmonic generation technology, i.e., the echo-enabled harmonic generation (EEHG) scheme, in a diffraction-limited storage ring (DLSR). Two different EEHG element layouts are considered, and the effect of the EEHG process on the electron beam quality is also analyzed. Studies suggest that soft X-ray pulses, with pulse lengths of a few femtoseconds and peak powers of up to 1 MW, can be generated by using the EEHG scheme, while causing little perturbation to the regular operation of a DLSR.

Echo-Enabled Harmonic Generation Studies for the FERMI Free-Electron Laser

Studying ultrafast processes on the nanoscale with element specificity requires a powerful femtosecond source of tunable extreme-ultraviolet (XUV) or x-ray radiation, such as a free-electron laser (FEL). Current efforts in FEL development are aimed at improving the wavelength tunability and multicolor operation, which will potentially lead to the development of new characterization techniques offering a higher chemical sensitivity and improved spatial resolution. One of the most promising approaches is the echo-enabled harmonic generation (EEHG), where two external seed lasers are used to precisely control the spectro-temporal properties of the FEL pulse. Here, we study the expected performance of EEHG at the FERMI FEL, using numerical simulations. We show that, by employing the existing FERMI layout with minor modifications, the EEHG scheme will be able to produce gigawatt peak-power pulses at wavelengths as short as 5 nm. We discuss some possible detrimental effects that may affect the performance of EEHG and compare the results to the existing double-stage FEL cascade, currently in operation at FERMI. Finally, our simulations show that, after substantial machine upgrades, EEHG has the potential to deliver coherent multicolor pulses reaching wavelengths as short as 3 nm, enabling x-ray pump–x-ray probe experiments in the water window.

Design study of an echo-enabled harmonic generation free electron laser for the 24th harmonics

In this paper, we present an echo-enabled harmonic generation (EEHG) scheme to generate a free electron laser (FEL). The EEHG FEL utilizes two modulators and two chicanes that can generate a high harmonic density modulation with a low initial beam energy modulation. Using the beam parameters of a 150-MeV electron accelerator, we show that the EEHG scheme enhances FEL performance and generates coherent radiation with a wavelength of 100 nm. In this scheme, the 150-MeV electron beam is energy-modulated by using two lasers with wavelengths of 800 and 2400 nm. The modulated electron beam is sent to two chicanes whose dispersive strengths are 8.2 and 1.0 mm to convert energy modulation to density modulation. Finally, the density-modulated electron beam is sent to a radiator that is tuned to a wavelength of 100 nm and generates radiation with a peak power of 0.3 GW. We performed simulations of the laser-beam interaction by using the GENESIS code and showed that intense coherent radiation with a wavelength of 100 nm could be achieved from the two seeding lasers under the designed EEHG scheme.

Feasibility study of generating ultra-high harmonic radiation with a single stage echo-enabled harmonic generation scheme

The echo enabled harmonic generation (EEHG) scheme holds the ability for the generation of fully coherent soft x-ray free-electron laser (FEL) pulses directly from external UV seeding sources. In this paper, we study the feasibility of using a single stage EEHG to generate coherent radiation in the “water window” and beyond. Using the high-order operating modes of the EEHG scheme, intensive numerical simulations have been performed considering various three-dimensional effects. The simulation results demonstrated that coherent soft x-ray radiation at 150th harmonic (1.77nm) of the seed can be produced by a single stage EEHG. The decreasing of the final bunching factor at the desired harmonic caused by intra beam scattering (IBS) effect has also been analyzed.

Designs and numerical calculations for echo-enabled harmonic generation at very high harmonics

The echo-enabled harmonic generation (EEHG) scheme for driving an FEL using two seeded energy modulations at much longer wavelengths than the output wavelength is a promising concept for future seeded FELs. There are many competing requirements in the design of an EEHG beamline which need careful optimization. Furthermore, revised simulation tools and methods are necessary because of both the high harmonic numbers simulated and the complicated nature of the phase space manipulations which are intrinsic to the scheme. This paper explores the constraints on performance and the required tolerances for reaching wavelengths well below 1/100th of that of the seed lasers, and describes some of the methodology for designing such a beamline. Numerical tools, developed both for the GENESIS and GINGER FEL codes, are presented and used here for more accurate study of the scheme beyond a time-averaged model. In particular, the impact of the local structure in peak current and bunching, which is an inherent part of the EEHG scheme, is evaluated.

A proposed method to characterize high harmonic microbunching in EEHG operation of SDUV-FEL

The echo-enabled harmonic generation (EEHG) scheme offers remarkable efficiency for generating high harmonic microbunching with a relatively small energy modulation. A proof of principle experiment of the EEHG scheme has been proposed at the Shanghai deep ultraviolet (SDUV) free electron laser (FEL) facility, where the 4th harmonic of the seed laser is amplified in the 9 m long radiator. To explore the advantages of the EEHG scheme, in this paper, a method of measuring the coherent high harmonic radiation of the radiator is proposed to investigate the electron beam microbunching corresponding to the 10th-20th harmonics of the seed laser. The principle of the proposed method, comparisons with existing methods and the simulation results are presented and discussed.

Operating the SDUV-FEL with the echo-enabled harmonic generation scheme

Using the recently proposed echo-enabled harmonic generation (EEHG) free-electron laser (FEL) scheme, it is shown that operating the Shanghai deep ultraviolet FEL (SDUV-FEL) with single-stage to higher harmonics is very promising, with higher frequency up-conversion efficiency, higher harmonic selectivity and lower power requirement of the seed laser. The considerations on a proof-of-principle experiment and expected performance in SDUV-FEL are given.

Echo-enabled harmonic generation free electron laser

In this paper, we systematically study the echo-enabled harmonic generation (EEHG) free electron laser (FEL). The EEHG FEL uses two modulators in combination with two dispersion sections that allow one to generate in the beam a high harmonic density modulation starting with a relatively small initial energy modulation of the beam. After presenting an analytical theory of the phenomenon, we address several practically important issues, such as the effect of incoherent synchrotron radiation in the dispersion sections, and the beam transverse size effect in the modulator. Using a representative realistic set of beam parameters, we show how the EEHG scheme enhances the FEL performance and allows one to generate a fully (both longitudinally and transversely) coherent radiation. As an example, we demonstrate that 5 nm coherent soft x rays with GW peak power can be generated directly from the 240 nm seeding laser using the proposed EEHG scheme.