Harmonic Generation Free-electron Laser Research Articles

Tunable frequency comb from echo-enabled harmonic generation free electron laser

We propose a novel scheme for the generation of a tunable soft x-ray frequency comb in a seeded free-electron laser (FEL) via the chirped pulse beating technology. Our research shows that harnessing the chirped pulse laser to imprint a submillimeter-scale energy modulation in the phase space of the relativistic electron beam together with the process of echo-enabled effect, multichromatic output soft x-ray radiation with a series of longitudinal modes can be amplified in an echo-enabled harmonic generation (EEHG) free electron laser, which finally forms a comb-liked shape in the frequency spectrum with the mode spacing frequency at terahertz (THz) band. Based on the theoretical and numerical analyses of the modulation process, the electron beam maintains a nanometer-scale microstructures due to the echo effect, while the beam obtains an energy modulation at submillimeter-scale from the chirped pulse beating laser. This leads to a series of longitudinal modes in the downstream radiator. Three-dimension time-dependent simulation results show that a frequency comb with a central wavelength of 8.9 nm can be generated directly with the help of chirped beating laser. The frequency spacing between the frequency lines on the radiation spectrum can be tuned easily by changing the parameters of chirped beating laser.

Characterization of soft x-ray echo-enabled harmonic generation free-electron laser pulses in the presence of incoherent electron beam energy modulations

Echo-enabled harmonic generation free-electron lasers (EEHG FELs) are promising candidates to produce fully coherent soft x-ray pulses by virtue of efficient high harmonic frequency up-conversion from UV lasers. The ultimate spectral limit of EEHG, however, remains unclear, because of the broadening and distortions induced in the output spectrum by residual broadband energy modulations in the electron beam. We present a mathematical description of the impact of incoherent (broadband) energy modulations on the bunching spectrum produced by the microbunching instability through both the accelerator and the EEHG line. The model is in agreement with a systematic experimental characterization of the FERMI EEHG FEL in the photon energy range $130-210$ eV. We find that amplification of electron beam energy distortions primarily in the EEHG dispersive sections explains an observed reduction of the FEL spectral brightness that is proportional to the EEHG harmonic number. Local maxima of the FEL spectral brightness and of the spectral stability are found for a suitable balance of the dispersive sections' strength and the first seed laser pulse energy. Such characterization provides a benchmark for user experiments and future EEHG implementations designed to reach shorter wavelengths.

Coherent soft X-ray pulses from an echo-enabled harmonic generation free-electron laser

X-ray free-electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths. Echo-enabled harmonic generation in a free-electron laser enables 45th harmonic pulses from a 264 nm wavelength seed, yielding 5.9 nm wavelength coherent output.

Generation of coherent two-color pulses at two adjacent harmonics in a seeded free-electron laser

The growing requirements of pump-probe techniques and nonlinear optics experiments greatly promote the studies of two-color free-electron lasers (FELs). We propose a new method to generate coherent two-color pulses in a high-gain harmonic generation (HGHG) FEL. In this scheme, an initial tilted electron beam is sent though the modulator and dispersive section of an HGHG FEL to generate the bunching at harmonics of the seed laser. Then a transverse gradient undulator (TGU) is adopted as the radiator and in such radiator, only two separated fractions of the tilted beam will resonate at two adjacent harmonics of the seed laser and are enabled to emit the coherent two-color pulses simultaneously. The time separation between the two pulses are on the order of hundreds of femtoseconds, and can be precisely controlled by varying the tilted amplitude of the electron beam and/or the transverse gradient of the TGU radiator. Numerical simulations confirm the validity and feasibility of this scheme in the EUV waveband.

High-harmonic x-ray undulator radiation for nanoscale-wavelength free-electron lasers

We explore the undulator radiation (UR) from relativistic electrons in two-frequency undulators intended for use in x-ray free-electron lasers (FELs). We use an exact analytical account for the major sources of spectral line broadening of the spontaneous UR to describe the UR spectrum, intensity and emission line shape. We also show possible partial compensation of the off-axis divergence by ad hoc imposed weak transverse constant magnetic components. The harmonic radiation of the two-frequency undulator is studied. High-harmonics amplification in the two-frequency undulator is underlined for possible use in x-ray FELs and the generation of powerful x-ray radiation is demonstrated. The evolution of high harmonic power is studied with the help of the developed phenomenological self-amplified spontaneous emission FEL model. The performance of a high-gain harmonic generation FEL with a two-frequency undulator–prebuncher is explored. Harmonic multiplication cascades are introduced in FELs to obtain nanometer x-ray FEL radiation in reduced size FELs at ~30 m with seed from commercially available excimer laser and ~0.5 GeV electrons.

Parameter optimization and start-to-end simulation for the phase-merging enhanced harmonic generation free electron laser

Recently, a novel scheme called the phase-merging enhanced harmonic generation (PEHG) free-electron laser (FEL) has been developed to achieve higher harmonic bunching in a single stage set up. However, previous studies of the PEHG mechanism ignored the practical lattice configuration, leaving out the impact of the intrinsic beam divergence on the phase-merging effect. In this paper, a new method based on the beam transport matrix is proposed to comprehensively study the optimization conditions of the harmonic generation FELs. On the basis of this method, the new optimization conditions of the PEHG are obtained by taking into account both the intrinsic horizontal beam size and the intrinsic horizontal beam divergence. The parameter optimization and three dimensional start-to-end simulations are carried out using the typical beam parameters of the Shanghai Soft X-ray Free Electron Laser user facility. The simulation results are in agreement with the theoretical analysis and demonstrate the validity of the PEHG-FEL.

High gain harmonic generation free electron lasers enhanced by pseudoenergy bands

We propose a new scheme for high gain harmonic generation free electron lasers (HGHG FELs), which is seeded by a pair of intersecting laser beams to interact with an electron beam in a modulator undulator located in a dispersive section. The interference of the laser beams gives rise to a two-dimensional modulation in the energy-time phase space because of a strong correlation between the electron energy and the position in the direction of dispersion. This eventually forms pseudoenergy bands in the electron beam, which result in efficient harmonic generation in HGHG FELs in a similar manner to the well-known scheme using the echo effects. The advantage of the proposed scheme is that the beam quality is less deteriorated than in other existing schemes.

Phase-merging enhanced harmonic generation free-electron laser with a normal modulator.

A phase-merging enhanced harmonic generation free-electron laser (FEL) was proposed to increase the harmonic conversion efficiency of seeded FELs and promote the radiation wavelength towards the X-ray spectral region. However, this requires a specially designed transverse gradient undulator (TGU) as the modulator to couple the transverse and longitudinal phase space of the electron beam. In this paper, the generation of the phase-merging effect is explored using the natural field gradient of a normal planar undulator. In this method, a vertical dispersion on the electron beam is introduced and then the dispersed beam travels through a normal modulator in a vertical off-axis orbit where the vertical field gradient is selected properly in terms of the vertical dispersion strength and modulation amplitude. The phase-merging effect will be generated after passing through the dispersive chicane. Theoretical analysis and numerical simulations for a seeded soft X-ray FEL based on parameters of the Shanghai Soft X-ray FEL project are presented. Compared with a TGU modulator, using the natural gradient of a normal planar modulator has the distinct advantage that the gradient can be conveniently tuned in quite a large range by adjusting the beam orbit offset.

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.

Self-seeded FEL wavelength extension with high-gain harmonic generation

We study a self-seeded high-gain harmonic generation (HGHG) free-electron laser (FEL) scheme to extend the wavelength of a soft X-ray FEL. This scheme uses a regular self-seeding monochromator to generate a seed laser at the wavelength of 1.52 nm, followed by a HGHG configuration to produce coherent, narrow-bandwidth harmonic radiations at the GW level. The 2nd and 3rd harmonic radiation is investigated with start-to-end simulations. Detailed studies of the FEL performance and shot-to-shot fluctuations are presented.

Study of the output pulse stability of a cascaded high-gain harmonic generation free-electron laser

Cascading stages of high-gain harmonic generation (HGHG) have been demonstrated to be a promising candidate for producing fully coherent soft X-ray radiation directly from UV seed sources. However, the large shot-to-shot output pulse energy fluctuation may still be a serious problem for its user applications. In this paper, we study the effects of various electron beam parameters jitters on the output pulse energy fluctuations of a two-stage HGHG. Theoretical calculations and intensive simulations have been performed and the results demonstrate that the relative timing jitter between the electron bunch and the seed laser pulse is mainly responsible for the large output pulse energy fluctuation. Several methods that may be helpful to improve the FEL stability have also been discussed.

Phase-merging enhanced harmonic generation free-electron laser

Together with one of its variants, the recently proposed phase-merging enhanced harmonic generation (PEHG) free-electron laser (FEL) is systematically studied in this paper. Different from a standard high-gain harmonic generation scheme, a transverse gradient undulator is employed to introduce a phase-merging effect into the transversely dispersed electron beam in PEHG. The analytical theory of the phase-merging effect and the physical mechanism behind the phenomenon are presented. Using a representative set of beam parameters, intensive start-to-end simulations for soft x-ray FEL generation are given to illustrate the performance of PEHG. Moreover, some practical issues that may affect the performance of PEHG are also discussed.

Impact of Non-Gaussian Electron Energy Heating upon the Performance of a Seeded Free-Electron Laser

Laser-heater systems have been demonstrated to be an important component for the accelerators that drive high gain free electron laser (FEL) facilities. These heater systems suppress longitudinal microbunching instabilities by inducing a small and controllable slice energy spread to the electron beam. For transversely uniform heating, the energy spread augmentation is characterized by a non-Gaussian distribution. In this Letter, we demonstrate experimentally that in addition to suppression of the microbunching instability, the laser heater-induced energy distribution can be preserved to the FEL undulator entrance, significantly impacting the performance of high-gain harmonic generation (HGHG) FELs, especially at soft x-ray wavelengths. In particular, we show that the FEL intensity has several local maxima as a function of the induced heating caused by the non-Gaussian energy distribution together with a strong enhancement of the power at high harmonics relative to that expected for an electron beam with an equivalent Gaussian energy spread at an undulator entrance. These results suggest that a single stage HGHG FEL can produce scientifically interesting power levels at harmonic numbers m ≥ 25 and with current seed laser technology could reach output photon energies above 100 eV or greater.

Les collaborations CHG – neurologues libéraux : un mode de fonctionnement très divers. Une enquête du CNNHG

Coherent harmonic generation free-electron laser (CHG-FEL) acts as a harmonic amplifier of the seed laser. The spectral ratio, which is defined as the ratio of coherent radiation intensity and incoherent radiation intensity in infinitesimal bandwidth and solid angle aperture, can evaluate the performance of the CHG-FEL. In the experiment, we can only get the experimental integral ratio integrated over the actual bandwidth and solid angle aperture of the radiation measurement system. Because the coherent radiation and incoherent radiation are very different in the spatial and spectral structure, the experimental integral ratio is greatly influenced by the measurement system and cannot be directly used to evaluate the performance of the CHG-FEL. So, we must calculate the experimental spectral ratio according to the experimental integral ratio, the bandwidth and solid angle aperture of the measurement system and the parameters of the CHG-FEL. Our work provides a simple method for the calculation of experimental spectral ratio.

Simulation studies on laser pulse stability for Dalian coherent light source

Dalian Coherent Light Source will use a 300 MeV LINAC to produce fully coherent photon pulses in the wavelength range between 150–50 nm by high gain harmonic generation free electron laser (FEL) scheme. To generate stable FEL pulses, a stringent tolerance budget is required for the LINAC output parameters, such as the mean beam energy stability, electron bunch arrival time jitter, peak current variation and the transverse beam position offset. In order to provide guidance for the design of the Dalian Coherent Light Source, in this paper, the sensitivity of FEL pulse energy fluctuation to various error sources of the electron bunch was performed using intensive start-to-end FEL simulations.

Two-colour pump–probe experiments with a twin-pulse-seed extreme ultraviolet free-electron laser

Exploring the dynamics of matter driven to extreme non-equilibrium states by an intense ultrashort X-ray pulse is becoming reality, thanks to the advent of free-electron laser technology that allows development of different schemes for probing the response at variable time delay with a second pulse. Here we report the generation of two-colour extreme ultraviolet pulses of controlled wavelengths, intensity and timing by seeding of high-gain harmonic generation free-electron laser with multiple independent laser pulses. The potential of this new scheme is demonstrated by the time evolution of a titanium-grating diffraction pattern, tuning the two coherent pulses to the titanium M-resonance and varying their intensities. This reveals that an intense pulse induces abrupt pattern changes on a time scale shorter than hydrodynamic expansion and ablation. This result exemplifies the essential capabilities of the jitter-free multiple-colour free-electron laser pulse sequences to study evolving states of matter with element sensitivity.

Impact of radiator length in the emitted power for a high gain harmonic generation free-electron laser

We present a numerical study that characterizes the dependence on the radiator length of the output power produced by a free-electron laser (FEL) operated in the high gain harmonic generation (HGHG) configuration. Using the main parameters of the FERMI@Elettra FEL, numerical simulations of the FEL process have been performed for different lengths of the radiator. Our results show that in the case of HGHG the achievable output power has a dependence on the radiator length that is linear. The impact of the electron beam parameters on the achievable maximum power vs radiator length dependence is also studied. A normalization of the results to the FEL saturation power and to the FEL gain length shows that this dependence can be expressed by a universal linear equation that, in some conditions, is independent on the electron beam current and brightness. The reported results could be useful for the design of future FELs based on the HGHG scheme and could be used for a quick estimate of the best undulator length.

Oscillator seeding of a high gain harmonic generation free electron laser in a radiator-first configuration

A longitudinally and transversely coherent, high repetition rate x-ray source with widely tunable wavelength is desired for a variety of experimental applications. A free electron laser (FEL) powered by an electron beam from a superconducting linac can reach the desired peak and average x-ray power levels with transverse coherence. However, generating longitudinally coherent x-ray pulses is a significant challenge, especially at high repetition rate. This paper presents a one-dimensional theoretical and numerical investigation of a method to achieve longitudinal coherence and high repetition rate simultaneously. We propose a ``radiator-first'' configuration, wherein an FEL oscillator follows a high gain harmonic generation (HGHG) FEL. The oscillator generates seed power that is directed upstream to initiate the HGHG process in a following electron bunch. This configuration allows for the generation of radiation at short wavelength, which is highly sensitive to energy spread, to occur before the longer wavelength oscillator, whose performance is not seriously degraded by the beam heating in the upstream radiator. The dynamics and stability of this radiator-first scheme is explored analytically and numerically. A single-pass, 1D map is derived using a semianalytic model for FEL gain and saturation. Iteration of the map is shown to be in good agreement with simulations. A numerical example is presented for a soft x-ray FEL.

Design study for the cascaded HGHG experiment based on the SDUV-FEL

Cascading stages of high-gain harmonic generation (HGHG) free electron laser (FEL) is a promising way to produce fully coherent X-ray radiation. As a test facility for modern FEL R&D, the Shanghai deep ultraviolet FEL (SDUV-FEL) is now under upgrading for the cascading two stages of HGHG experiment. Since the energy of the electron beam is as low as about 185 MeV after upgrade, the total harmonic number of this two stages HGHG is only 2×2, and the wavelength of the final radiation is 196.5 nm which is the 4th harmonic of the 786 nm seed laser. With help of three-dimensional simulation codes, design studies on the FEL physics for the cascaded HGHG experiment are present based on the parameters of the upgraded SDUV-FEL facility. It is found from the simulation results that the part of the electron beam which has been used in the first stage can still generate powerful radiation in the radiator of the second stage, and this radiation will be difficult to be separated from the radiation generated by the fresh part of the electron beam. To overcome this problem, a novel method based on the energy spectrum of the electron beam is proposed in this paper to demonstrate the “fresh bunch” technique.

Quantum-coherent energy transfer: implications for biology and new energy technologies

The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. Sir William Henry Bragg Nobel Prize For Physics, 1915 There is a growing body of scientific evidence that light-initiated reactions in the molecular components of photosynthetic