FEL Operation Research Articles

Status and future of the soft X-ray free-electron laser beamline at the SHINE

The Shanghai High repetition rate XFEL and Extreme light facility (SHINE) is under construction and aims at generating X-rays between 0.4 and 25 keV with three FEL beamlines at repetition rates of up to 1 MHz. The soft X-ray FEL beamline, FEL-II, will be ready for commissioning in 2025. It is designed to cover the photon energy from 0.4 to 3 keV, in which the baselines of the FEL operation modes are self-amplified spontaneous emission (SASE), self-seeding, echo-enabled harmonic generation (EEHG), and polarization control. Therefore, a high repetition-rate external seed laser, large period length modulator, soft X-ray monochromator, planar undulator, and elliptically polarized undulator have been adopted in the FEL-II beamline. Several potentials such as an ultra-short pulse mode and a multi-color mode are also foreseeable without significant equipment changes in the follow-up operation. A dual-period undulator design is suggested for the echo-enabled harmonic generation (EEHG) commissioning, and it has great potential to break through the unreachable energy of the fully coherent X-ray in the future. The FEL-II beamline will deliver SASE radiation and fully coherent radiation in all the wavelengths of interest.

Reconstruction and beam-transport study of the cERL dump line for high-power IR-FEL operation

Infrared (IR) SASE-FEL emission was successfully generated for macro pulses of about 1 μs with the maximum repetition rate of 5 Hz at the cERL. In the future, high-power FEL operation will be planned to increase electron bunches drastically with energy recovery. Therefore, the dump line was redesigned and reconstructed to increase the momentum acceptance and to improve the magnet system for avoiding serious beam loss in the high power FEL operation. Furthermore, the first beam-transport study was performed by transporting the beam directly from the injector to the beam dump through the reconstructed dump line. In this paper, we present the reconstructed dump line and the beam-transport study. The new dump line can be a model for dump lines of high-power ERL-FELs.

Two-Color Operation of a Soft X-ray FEL with Alternation of Undulator Tunes

FLASH is the first soft X-ray FEL user facility, routinely providing brilliant photon beams for users since 2005. The second undulator branch of this facility, FLASH2, is gap-tunable, which allows for testing and using advanced lasing concepts. In particular, we developed a two-color operation mode based on the alternatingly tuned undulator segments (every other segment is tuned to the second wavelength). This scheme is advantageous in comparison with a subsequent generation of two colors in two consecutive sections of the undulator line. First, source positions of the two FEL beams are close to each other which makes it easier to focus them on a sample. Second, the amplification is more efficient in this configuration since the segments with respectively “wrong” wavelength still act as bunchers. We studied operation of this scheme in the regime of small and large separation of tunes (up to a factor of two). We developed new methods for online intensity measurements of the two colors simultaneously that require a combination of two detectors. We also demonstrated our capabilities to measure spectral and temporal properties of two pulses with different wavelengths.

Algorithm Verification of Single-Shot Relativistic Emittance Proposed Measuring Method

A 6 MeV hybrid photo-cathode gun is driving a THz-FEL in Ariel University, as well as other applications. An electron bunch with small transverse emittance is extracted from a copper photo-cathode using a 1 ps UV laser pulse, and then accelerated to a kinetic energy of 6.5 MeV. The Hybrid term is due to the unique standing wave-traveling wave sections in a single RF cavity. Since low emittance is crucial for FEL operation, the characterization of the electron beam requires measuring the transverse emittance, which will be compared with the predicted design and the 3D simulation obtained values, in order to verify their correctness. In this paper, we confirm the use of the multi-slit technique to measure emittance in the Hybrid beam in a single shot and develop a simple and convenient algorithm to be used in the experimental measurements. The experimental analysis requires image processing of the measured data, combined with a custom LabVIEW and Matlab scripts to control the hardware, and analyze the obtained data. Prior to experimentally measuring emittance, we perform a simulated experiment, using a simulated beam from the General Particle Tracer (GPT) code to test these algorithms and scripts, and compare the emittance obtained using the algorithm with GPT’s estimated emittance. Once concluded, this method will allow for a simple, fast and accurate single shot emittance measurement for the Hybrid accelerator beam.

Coherent terahertz Smith–Purcell free-electron laser with possible quantum effects

A quantum approach is proposed to describe the Smith–Purcell free-electron laser (SP–FEL) where the interacting electron is described quantum mechanically as a wave packet with a finite width. A quantum regime for the SP–FEL operation is newly defined when the electron wave packet width is larger or comparable to the interaction length. This quantum regime can be realized in SP–FELs in periodic nanostructures enabled by current nanofabrication advances. In this quantum regime, the gain linewidth is determined by the longitudinal size of the electron wave packet. The latter result can be used in measuring the spreading length of a free-electron quantum wave packet. It is found that as the width of the electron wave packet increases, the coherence of the emitted radiation is enhanced. Finally, whether in the classical or quantum regime, an analytical expression for the gain in a SP–FEL is derived. Then we propose an efficient scheme for a SP–FEL amplifier to generate terahertz radiation.

Self-consistent numerical approach to track particles in free electron laser interaction with electromagnetic field modes

In this paper we present a novel approach to FEL simulations based on the decomposition of the electromagnetic field in a finite number of radiation modes. The evolution of each mode amplitude is simply determined by energy conservation. The code is developed as an expansion of the General Particle Tracer framework and adds important capabilities to the suite of well-established numerical simulations already available to the FEL community. The approach is not based on the period average approximation and can handle long-wavelength waveguide FELs as it is possible to include the dispersion effects of the boundaries. Futhermore, it correctly simulates lower charge systems where both transverse and longitudinal space charge forces still play a significant role in the dynamics. For free-space FEL interactions, a source dependent expansion approximation can be used to limit the number of transverse modes required to model the field profile and speed up the calculation of the system evolution. Three examples are studied in detail including a single pass FEL amplifier, the high efficiency TESSA266 scenario and a THz waveguide FEL operating in the zero-slippage regime.

Distributed energy systems: Multi-objective optimization and evaluation under different operational strategies

In recent years, distributed energy systems (DES) have received an extensive attention, but they also face technical limitations in system architecture, operational strategies, and optimization methods. In this paper, for two typical DES, Natural Gas Cooling and Heating Triple Supply System (CCHP) and Hybrid Energy Distributed System (HDES), multi-objective optimization and evaluation have been carried out. In the optimization phase, for the two systems, a multi-objective mathematical programming (MOMP) model was developed to minimize the annual total cost and annual carbon emissions under three different operational strategies (heat load following (FTL), electrical load following (FEL), and load characteristic matching following (FCL)). Further, operational strategies were transformed into operational constraints, and models were solved using the augmented ε-constrain method. In the evaluation stage, the reference cost growth rate (CSR) and the reference CO2 emission reduction rate (CRR) evaluation indices were developed, and subsequently for two DES systems, comprehensive evaluations were conducted by incorporating with the weight distributed TOPSIS multi-objective decision-making method. Moreover, an office building in Nanjing was chosen as the baseline building, and the two DES systems were simulated. The results show that, under different operating strategies, CCHP has the advantage of lower cost, while the HDES has the advantage of the lower CO2 emission. Under the proposed FCL operation strategy, the maximum environmental benefits can be obtained. Compared to the FEL operation strategy, the CO2 emissions of CCHP and HDES have been reduced by approximately 4.2% and 2.9%, respectively. Compared to the FTL operating strategy, the CO2 emissions of CCHP and HDES have been reduced by approximately 15.2% and 13.7%, respectively.

Frequency modulated free electron laser.

It is shown that the output frequency of a free electron laser may be modulated to generate a series of modes that span a bandwidth of at least an order of magnitude greater than the normal FEL bandwidth. This new method of frequency modulated FEL operation has close analogies to frequency modulation in conventional cavity lasers. The FM-FEL is analysed and described in the linear regime by a summation over the exponentially amplified frequency modes. Simulations using a 3D, broad bandwidth, numerical code also demonstrate FM-FEL operation for parameters typical of FEL facilities currently under construction. Harmonic bunching methods are used to seed the FM-FEL modes to generate a temporally correlated frequency modulated output over a large bandwidth. This new, FM-FEL mode of operation scales well for X-ray generation, offering users a significantly new form of high-power, short wavelength FEL output.

Planar THz FELs Based on Intense Parallel Sheet Electron Beams and Intracavity Wave Scattering

A design for a planar FEL operating in the terahertz frequency range at the multimegawatt power level is being jointly developed at the ELMI accelerator by the Budker Institute of Nuclear Physics and the Institute of Applied Physics. The FEL oscillator is driven by parallel intense sheet electron beams of moderately relativistic energy, and transitions to the above frequency range via a two-stage cascade scheme. The first beam generates a powerful millimeter pump wave, which then travels through special waveguides to the parallel channel and is scattered by the second beam into a wave of terahertz radiation. Various possible arrangements of the FEL scheme are discussed, and results from their simulation are presented. The results from cold tests of the FEL’s electrodynamic system are reported.

Ultra-high brightness electron beams from very-high field cryogenic radiofrequency photocathode sources

Recent investigations of RF copper structures operated at cryogenic temperatures performed by a SLAC-UCLA collaboration have shown a dramatic increase in the maximum surface electric field, to 500 MV/m. We examine use of these fields to enable very high field cryogenic photoinjectors that can attain over an order of magnitude increase in peak electron beam brightness. We present beam dynamics studies relevant to X-ray FEL injectors, using start-to-end simulations that show the high brightness and low emittance of this source enables operation of a compact FEL reaching a photon energy of 80 keV. The preservation of beam brightness in compression, exploiting micro-bunching techniques is discussed. While the gain in brightness at high field is due to increase of the emission current density, further increases in brightness due to lowering of the intrinsic cathode emittance in cryogenic operation are also enabled. While the original proposal for this type of cryogenic, ultra-high field photoinjector has emphasized S-band designs, there are numerous potential advantages that may be conferred by operation in C-band. We examine issues related to experimental implementation in C-band, and expected performance of this type of device in a future hard X-ray FEL such as MaRIE.

RF injector design studies for the trailing witness bunch for a plasma-based user facility

The interest in plasma-based accelerators as drivers of user facilities is growing worldwide thanks to its compactness and reduced costs. In this context the EuPRAXIA collaboration is preparing a conceptual design report for a multi-GeV plasma-based accelerator with outstanding electron beam quality to pilot, among several applications, the operation of an X-ray FEL, the most demanding in terms of beam brightness. Intense beam dynamics studies have been performed to provide a reliable working point for the RF injector to generate a high-brightness trailing witness bunch suitable in external injection schemes, both in particle beam and laser driven plasma wakefield acceleration. A case of interest is the generation of a witness beam with 1 GeV energy, less than 1 mm mrad slice emittance and 30 pC in 10 fs FWHM bunch length, which turns into 3 kA peak current at the undulator entrance. The witness beam has been successfully compressed down to 10 fs in a conventional SPARC-like photo-injector and boosted up to 500 MeV in an advanced high-gradient X-band linac reaching the plasma entrance with 3 kA peak current and the following RMS values: 0.06% energy spread, 0.5 mm mrad transverse normalised emittance and a focal spot down to 1 μm. RF injector studies are here presented with the aim to satisfy the EuPRAXIA requests for the Design Study of a plasma-based user facility.

First operation of a harmonic lasing self-seeded free electron laser

Harmonic lasing is a perspective mode of operation of X-ray FEL user facilities that allows to provide brilliant beams of higher energy photons for user experiments. Another useful application of harmonic lasing is so called Harmonic Lasing Self-Seeded Free Electron Laser (HLSS FEL) that allows to improve spectral brightness of these facilities. In the past, harmonic lasing has been demonstrated in the FEL oscillators in infrared and visible wavelength ranges, but not in high-gain FELs and not at short wavelengths. In this paper we report on the first evidence of the harmonic lasing and the first operation of the HLSS FEL at the soft X-ray FEL user facility FLASH in the wavelength range between 4.5 nm and 15 nm. Spectral brightness was improved in comparison with Self-Amplified Spontaneous emission (SASE) FEL by a factor of six in the exponential gain regime. A better performance of HLSS FEL with respect to SASE FEL in the post-saturation regime with a tapered undulator was observed as well. The first demonstration of harmonic lasing in a high-gain FEL and at short wavelengths paves the way for a variety of applications of this new operation mode in X-ray FELs.

Transverse Gradient Undulators and FEL operating with large energy spread

Undulators exhibiting a gradient of the field in the transverse direction have been proposed to mitigate the effects of the gain dilution in Free Electron Laser devices operating with large energy spread. The actual use of the device depends on the realization of a field distribution with quasi-vanishing quadrupolar terms in the tapering directions. We analyze the effect of a Transverse Gradient Undulator on the FEL operation and critically review the possibility of an appropriate field implementation.

Statistical properties of the radiation from SASE FEL operating in a post-saturation regime with and without undulator tapering

We describe statistical and coherence properties of the radiation from X-ray free-electron lasers (XFEL) operating in the post-saturation regime. We consider the practical case of the SASE3 FEL at the European XFEL. We perform comparison of the main characteristics of the X-ray FEL operating in the post-saturation regime with and without undulator tapering: efficiency, coherence time, and degree of transverse coherence.

Prospects for CW and LP operation of the European XFEL in hard X-ray regime

The European XFEL will operate nominally at 17.5GeV in SP (short pulse) mode with 0.65ms long bunch train and 10Hz repetition rate. A possible upgrade of the linac to CW (continuous wave) or LP (long pulse) modes with a corresponding reduction of electron beam energy is under discussion for many years. Recent successes in the dedicated R&D program allow to forecast a technical feasibility of such an upgrade in the foreseeable future. One of the challenges is to provide sub-Ångström FEL operation in CW and LP modes. In this paper we perform a preliminary analysis of a possible operation of the European XFEL in the hard X-ray regime in CW and LP modes with electron energies of 7GeV and 10GeV, respectively. We consider lasing in the baseline XFEL undulator as well as in a new undulator with a reduced period. We show that, with reasonable requirements on electron beam quality, lasing on the fundamental will be possible in the sub-Ångström regime. As an option for generating brilliant photon beams at short wavelengths we also consider harmonic lasing that has recently attracted a significant attention.

Statistical Theory of the SASE Fel Based on the Two-Particle Correlation Function Equation

The startup from noise problem in SASE FELs is usually treated in linear approximation. In this case amplification of initial density fluctuations may be calculated, and averaging over initial conditions may be fulfilled explicitly. In general nonlinear case the direct averaging is not applicable. During last years we developed the approach based on the BBGKY hierarchy for the n-particle distribution functions. The interaction of particles in FEL is retarded. Nevertheless, using special time-coordinate transformation, it is possible to eliminate the interaction lag and then to write down the BBGKY equations. Similar to plasma physics, the equations may be truncated after the second one (for the two-particle correlation function). Using this approach we consider several particular cases which illustrate some peculiar features of the SASE FEL operation

Accelerator Sources for THz Science: A Review

Free Electron Lasers have been around since 1977 providing not only a test bed for the physics of FELs and electron/photon interactions but as a workhorse of scientific research. More than 30 FELs are presently operating around the world spanning a wavelength range from the millimeter region to the hard x-ray using direct current and rf linear accelerators or storage rings as electron sources. The characteristics that have driven the development of these sources are the desire for high peak and average power, high micropulse energies, wavelength tunability, timing flexibility, and wavelengths that are unavailable from more conventional laser sources. Operation of FELs in the far infrared to terahertz regime poses special challenges which have been and are being addressed at a number of facilities around the world. This paper will review a number of former and existing FELs operating in this regime and discuss future efforts. Broadband collective radiation from relativistic electrons also plays a significant role in the production of FIR/THz radiation and several groups are taking advantage of this source for users. Applications for use of the radiation have evolved from simple imaging to complex pump probe tests of insulator/metal transitions and energy flow in organic molecules. We will discuss the technologies for generating the IR/FIR/THz radiation and cover some of the unique applications of such sources.

Experimental Measurements of Ultra-Thin Bragg Crystals for LCLS Beam-Sharing Operation

The successful lasing and operation of the LCLS hard X-ray FEL has brought tremendous interest to the user community spanning a wide range of scientific disciplines including physics, chemistry, structural biology, and material science. It created demand on beam time that is often left unfulfilled. Here we report experimental measurements of ultra-thin silicon single-crystal membranes for potentially beam-sharing the LCLS beam. The samples included the (111), (220), and (400) orientations with thicknesses ranging from 5 to 20 μm. Both high-resolution rocking curves and topographic data were first obtained using synchrotron X-rays, demonstrating near ideal diffraction qualities. Subsequent tests using the full LCLS beam revealed lattice distortions from beam-induced membrane vibrations, which were shown to be effectively reduced by ambient air and smaller membrane dimensions. High diffraction quality thin-diamonds in the (111) orientation are also being pursued as a parallel effort. Both approaches are paving a way for a practical beam-sharing implementation at LCLS in the near future.

Coherent and spontaneous emission in the quantum free electron laser

We present an analysis of quantum free electron laser (QFEL) dynamics including the effects of spontaneous emission. The effects of spontaneous emission are undesirable for coherent short-wave generation using FELs and have been shown in previous studies to limit the capabilities of classical self amplified spontaneous emission (SASE)-FELs at short wavelengths ∼1 Å due to growth of electron beam energy spread. As one of the attractive features of the QFEL is its potential as a relatively compact coherent x-ray source, it is important to understand the role of spontaneous emission, but to date there has not been a model which is capable of consistently describing the dynamics of both coherent FEL emission and incoherent spontaneous emission. In this paper, we present such a model, and use it to show that the limitations imposed by spontaneous emission on coherent FEL operation are significantly different in the quantum regime to those in the classical regime. An example set of parameters constituting a QFEL using electron and laser parameters which satisfy the condition for neglect of spontaneous emission during coherent QFEL emission is presented.

Generation of doublet spectral lines at self-seeded X-ray FELs

Self-seeding schemes, consisting of two undulators with a monochromator in between, aim to reduce the bandwidth of SASE X-ray FELs. We recently proposed to use a new method of monochromatization exploiting a single crystal in Bragg transmission geometry for self-seeding in the hard X-ray range. The obvious and technically possible extension is to use such kind of monochromator setup with two (or more) crystals arranged in a series to spectrally filter the SASE radiation at two (or more) closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet (or multiplet) spectral lines. Exploitations of such mode of operation involve any situation where there is a large change in cross-section over a narrow wavelength range. In this paper we consider the simultaneous operation of the LCLS hard X-ray FEL at two closely spaced wavelengths. We present simulation results for the LCLS baseline, and we show that this method can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band, i.e. within 10 eV. An interesting aspect of the proposed scheme is a way of modulating the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating “mode-beat” component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. Thus, at saturation one obtains FEL-induced modulations of energy loss and energy spread in the electron bunch at optical frequency. These modulations can be converted into density modulation at the same optical frequency with the help of a weak chicane installed behind the baseline undulator. Powerful coherent radiation can then be generated with the help of an optical transition radiation (OTR) station, which have important applications. In this paper we briefly consider how the doublet structure of the XFEL generation spectra can be monitored by an optical spectrometer. Furthermore, the OTR coherent radiation pulse is naturally synchronized with the X-ray pulses, and can be used for timing the XFEL to high power conventional lasers with femtosecond accuracy for pump-probe applications.