Dalian Coherent Light Source Research Articles

Study on the UV FEL single-shot damage threshold of an Au thin film.

The damage threshold of an Au-coated flat mirror, one of the reflective optics installed on the FEL-2 beamline of the Dalian Coherent Light Source, China, upon far-UV free-electron laser irradiation is evaluated. The surface of the coating is characterized by profilometer and optical microscope. A theoretical approach of the phenomenon is also presented, by application of conventional single-pulse damage threshold calculations, a one-dimensional thermal diffusion model, as well as finite-element analysis with ANSYS.

Multi-functional gas cell in the vacuum ultraviolet free-electron laser beamline.

A long gas cell, filled with noble gas, is typically positioned between the undulator and the first mirror in the free-electron laser (FEL) beamline to attenuate the laser power as required by the end-stations. In addition to attenuation, the gas cell also serves important functions in various applications, such as spectrometer calibration, resolving power evaluation during beamline commissioning, and filtering of third harmonic in FEL operations. These functions of the gas cell have been successfully tested and implemented at the Dalian Coherent Light Source, a vacuum ultraviolet FEL facility located in Dalian, China. The resolving power of higher than 5000 has been obtained, and accurate calibration has been completed using the gas cell. During operation, the third harmonic of the FEL was attenuated by approximately one order of magnitude with almost the same power of the fundamental. This greatly improved the signal-to-noise ratio at the end-stations.

An experimental application of machine learning algorithms to optimize the FEL lasing via beam trajectory tuning at Dalian Coherent Light Source

The lasing optimization of Free-Electron Laser (FEL) facilities is a time-consuming and challenging task. Instead of operating manually by experienced operators, implementation of machine learning algorithms offers a rapid and adaptable approach for FEL lasing optimization. Recently, such an experiment has been conducted at the vacuum ultraviolet FEL facility - Dalian Coherent Light Source (DCLS). Four algorithms, namely the standard and the neural network-based genetic algorithms, the deep deterministic policy gradient and the soft actor critic reinforcement learning algorithms, have been employed to enhance the FEL intensity by optimizing the electron beam trajectory. These algorithms have shown notable efficacy in enhancing the FEL lasing, especially the reinforcement learning ones which achieved convergence within only approximately 400 iterations. This study demonstrates the validity of machine learning algorithms for FEL lasing optimization, providing a forward-looking perspective on the automatic operation of DCLS.

Design and study of cavity quadrupole moment and energy spread monitor

A nondestructive method to measure beam energy spread using the quadrupole modes within a microwave cavity is proposed. Compared with a button beam position monitor (BBPM) or a stripline beam position monitor (SBPM), the cavity monitor is a narrow band pickup and therefore has better signal-to-noise ratio (SNR) and resolution. In this study, a rectangular cavity monitor is designed. TM220 mode operating at 4.76 GHz in the cavity reflects the quadrupole moment of the beam. The cavity plans to be installed behind a bending magnet in Dalian Coherent Light Source (DCLS), an extreme ultraviolet FEL facility. In this position, the beam has a larger dispersion, which is beneficial to measure the energy spread. A quadrupole magnet, a fluorescent screen, and a SBPM with eight electrodes is installed near the cavity for calibration and comparison. The systematic framework and simulation results are also discussed in this paper.

Design, fabrication and cold-test of an X-band accelerating structure for linearizer

X-band high gradient accelerating technology is a challenging and important technology in advanced electron linear accelerator facilities. The X-band accelerating structure can provide harmonic compensation to eliminate non-linear energy spread and realize linear compression of bunch in linac. In this paper, a special X-band traveling-wave accelerating structure is designed for linearizer, with accelerating gradient of 20 MV/m under the input power below 5 MW according to the requirement of Dalian Coherent Light Source. The fabrication and cold-test of the structure are successfully completed and the transmission efficiency of power is about 0.38.

Scientific research at Dalian coherent light source

Scientific research at Dalian coherent light source

Exploring the vacuum ultraviolet photochemistry of astrochemically important triatomic molecules.

The recently constructed vacuum ultraviolet (VUV) free electron laser (FEL) at the Dalian Coherent Light Source (DCLS) is yielding a wealth of new and exquisitely detailed information about the photofragmentation dynamics of many small gas-phase molecules. This Review focuses particular attention on five triatomic molecules-H2O, H2S, CO2, OCS and CS2. Each shows excitation wavelength-dependent dissociation dynamics, yielding photofragments that populate a range of electronic and (in the case of diatomic fragments) vibrational and rotational quantum states, which can be characterized by different translational spectroscopy methods. The photodissociation of an isolated molecule from a well-defined initial quantum state provides a lens through which one can investigate how and why chemical reactions occur, and provides numerous opportunities for fruitful, synergistic collaborations with high-level ab initio quantum chemists. The chosen molecules, their photofragments and the subsequent chemical reaction networks to which they can contribute are all crucial in planetary atmospheres and in interstellar and circumstellar environments. The aims of this Review are 3-fold: to highlight new photochemical insights enabled by the VUV-FEL at the DCLS, notably the recently recognized central atom elimination process that is shown to contribute in all of these triatomic molecules; to highlight some of the potential implications of this rich photochemistry to our understanding of interstellar chemistry and molecular evolution within the universe; and to highlight other and future research directions in areas related to chemical reaction dynamics and astrochemistry that will be enabled by increased access to VUV-FEL sources.

A free electron laser-based 1+1′ Resonance-Enhanced Multiphoton Ionization scheme for rotationally resolved detection of OH radicals with correct relative intensities

The key role played by the OH radical as a reactive intermediate motivates advanced methods for state-resolved OH detection. In this work, we take advantage of the wavelength- and bandwidth-tunable vacuum ultraviolet (VUV) pulses produced at the Dalian Coherent Light Source to modify a previously reported 1+1′ UV + VUV Resonance-Enhanced Multiphoton Ionization scheme [J. M. Beames, F. Liu, M. I. Lester, C. Murray, J. Chem. Phys. 134, 241,102 (2011); J. M. Beames, F. Liu, M. I. Lester, Mol. Phys. 112, 897 (2014)], in which OH in its ground X2Π electronic state is first excited to the A2Σ+ state at around 281 nm, and subsequently ionized by 118 nm VUV radiation via the autoionizing (A3Π,3d) Rydberg state. By tuning the VUV-free electron laser so that its bandwidth covers the entire A3Π(v+ = 0, 3d) ← A2Σ+(v″ = 1) band, we obtain enhanced sensitivity and accurate relative intensities for quantitative determination of quantum state distributions. The relative line intensities observed in the experiment agree with the simulated absorption intensities to within an error of <1% of the integrated band intensity. The 1+1′ scheme is also compared to a convenient one-color 2+1 scheme [M. Collard, P. Kerwin, A. Hodgson, Chem. Phys. Lett.179, 422–428 (1991)], which suffers due to rapid predissociation of the D 2Σ− state used as resonant intermediate.

The role of the three body photodissociation channel of water in the evolution of dioxygen in astrophysical applications.

A recent experiment at the Dalian Coherent Light Source (DCLS) has provided measurements of the partial cross sections for the photodissociation of water vapor over an unprecedented range of wavelengths in the vacuum ultraviolet (VUV) region. It was found that the three body dissociation channel, H + H + O(3P/1D), becomes prominent at wavelengths shorter than the Lyman α-line at 121.6 nm. The present work explores the kinetic consequences of this discovery for several astrophysically motivated examples. The irradiation of a dilute low-temperature gas by unscreened solar radiation, similar to early stage photochemical processing in a comet coma, shows significant increase in the production of O2-molecules at shorter times, <1 day, that might physically correspond to the photochemical reaction zone of the coma. Several examples of planetary atmospheres show increased O-atom production at high altitudes but relatively little modification of the equilibrium O2 concentrations predicted by conventional models.

Quantum state-to-state vacuum ultraviolet photodissociation dynamics of small molecules

The present review focused on selected, recent experimental progress of photodissociation dynamics of small molecules covering the vacuum ultraviolet (VUV) range from 6 eV to 20 eV. These advancements come about due to the available laser based VUV light sources, along with the developments of advanced experimental techniques, including the velocity-map imaging (VMI), H-atom Rydberg tagging time-of-flight (HRTOF) techniques, as well as the two-color tunable VUV-VUV laser pump-probe detection method. The applications of these experimental techniques have allowed VUV photodissociation studies of many diatomic and triatomic molecules to quantum state-to-state in detail. To highlight the recent accomplishments, we have summarized the results on several important molecular species, including H2 (D2, HD), CO, N2, NO, O2, H2O (D2O, HOD), CO2, and N2O. The detailed VUV photodissociation studies of these molecules are of astrochemical and atmospheric relevance. Since molecular photodissociation initiated by VUV excitation is complex and is often governed by multiple electronic potential energy surfaces, the unraveling of the complex dissociation dynamics requires state-to-state cross section measurements. The newly constructed Dalian Coherent Light Source (DCLS), which is capable of generating coherent VUV radiation with unprecedented brightness in the range of 50–150 nm, promises to propel the photodissociation experiment to the next level.

Commissioning and Long-Term Results of a Fully-Automated Pulse-Based Optical Timing Distribution System at Dalian Coherent Light Source

Commissioning and Long-Term Results of a Fully-Automated Pulse-Based Optical Timing Distribution System at Dalian Coherent Light Source

Study of the crosstalk evaluation for cavity BPM

In order to pursue high-precision beam position measurements for free-electron laser facilities, a cavity beam position monitor (CBPM) is employed to measure the transverse position that can meet the requirement of position resolution at a sub-micrometer or even nanometer scale. However, for the pill-box cavity BPM, crosstalk between the cavities will have an effect on the accurate measurement of beam position. To reduce the effect of crosstalk on CBPM performance and ease the measurement of the isolation between the cavities, the cavities with a slight difference in resonant frequency were designed and applied in the Dalian coherent light source and Shanghai soft X-ray free-electron laser facilities. Furthermore, two methods, the principal component analysis method and the method of harmonic analysis, are proposed in this paper to evaluate the crosstalk. The results demonstrate that the two methods are feasible in evaluating the crosstalk between the cavities.

Photodissociation dynamics of H2O at 111.5 nm by a vacuum ultraviolet free electron laser.

Photodissociation dynamics of H2O via the F̃ state at 111.5 nm were investigated using the high resolution H-atom Rydberg "tagging" time-of-flight (TOF) technique, in combination with the tunable vacuum ultraviolet free electron laser at the Dalian Coherent Light Source. The product translational energy distributions and angular distributions in both parallel and perpendicular directions were derived from the recorded TOF spectra. Based on these distributions, the quantum state distributions and angular anisotropy parameters of OH (X) and OH (A) products have been determined. For the OH (A) + H channel, highly rotationally excited OH (A) products have been observed. These products are ascribed to a fast direct dissociation on the B̃1A1 state surface after multi-step internal conversions from the initial excited F̃ state to the B̃ state. While for the OH (X) + H channel, very highly rotationally excited OH (X) products with moderate vibrational excitation are revealed and attributed to the dissociation via a nonadiabatic pathway through the well-known two conical intersections between the B̃-state and the X̃-state surfaces.

Generation of two-color ultra-short radiation pulses from two electron bunches and a chirped seeded free-electron laser

In this paper we describe a new method for the realization of two-color femtosecond radiation pulses in a seeded free-electron laser (FEL). The two-color pulses are obtained from two electron bunches and a chirped seeding laser. Compared to the previous methods based on seeded FELs, our method has the advantages of producing two-color FEL pulses with more flexible tunability both in the pulse durations and separations. Numerical simulations for the Dalian Coherent Light Source confirm that femtosecond XUV pulses with variable pulse durations and time delay can be directly generated from a chirped seed laser at 250nm by using this technique. We also show the possibility of performing a proof-of-principle experiment of this technique based on the Shanghai Deep-Ultraviolet FEL facility.

On-line spectral diagnostic system for Dalian Coherent Light Source

The Dalian Coherent Light Source (DCLS) is a Free electron laser (FEL) user facility currently under construction in the northeast of China. It is designed to work on high gain high harmonic principle with the capability of wavelength continuously tunable in the EUV regime of 50–150nm. The light source has unique features such as the turntable radiation frequency, wide spectral range, high brightness and peak power, very short pulse time structure, etc. A key diagnostic task in DCLS is the on-line source spectral characteristic recording during the source development, and for the definition of the experimental conditions. For this purpose, an online grazing incidence spectrometer with a toroidal mirror and a variable-line-spacing plane grating is designed and presented in this paper to monitor each single FEL pulse. A circular stage is chosen to fit the focal curve and to realize the wavelength scanning. This scanning mechanics is simpler and stable. Resolving power (λ/Δλ) of this spectrometer is better than 12,000 in the whole wavelength range.

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.

褶皱管束流能量稳定器

Stringent tolerance budget is restricted on the linear accelerator (linac) parameters, such as the mean beam energy stability, peak current variation, electron bunch arrival time jitter, and the transverse beam position offset, in order to generate stable output pulses in modern linac driven free electron laser facility. In this paper, a corrugated pipe is used as a beam energy stabilizer. Analytical and numerical methods for designing such kind of stabilizer are described, and the simulation results show that the beam energy jitter can be reduced more than 10% after this structure is adopted for the case of Dalian Coherent Light Source (DCLS) linac.

FEL polarization control studies on Dalian coherent light source

The polarization switch of a free-electron laser (FEL) is of great importance to the user scientific community. In this paper, we investigate the generation of controllable polarization FEL from two well-known approaches for Dalian coherent light source, i.e., crossed planar undulator and elliptical permanent undulator. In order to perform a fair comparative study, a one-dimensional time-dependent FEL code has been developed, in which the imperfection effects of an elliptical permanent undulator are taken into account. Comprehensive simulation results indicate that the residual beam energy chirp and the intrinsic FEL gain may contribute to the degradation of the polarization performance for the crossed planar undulator. The elliptical permanent undulator is not very sensitive to the undulator errors and beam imperfections. Meanwhile, with proper configurations of the main planar undulators and additional elliptical permanent undulator section, circular polarized FEL with pulse energy exceeding 100 μJ could be achieved at Dalian coherent light source.