Accelerator-based Light Sources Research Articles

Conditional guided generative diffusion for particle accelerator beam diagnostics

Advanced accelerator-based light sources such as free electron lasers (FEL) accelerate highly relativistic electron beams to generate incredibly short (10s of femtoseconds) coherent flashes of light for dynamic imaging, whose brightness exceeds that of traditional synchrotron-based light sources by orders of magnitude. FEL operation requires precise control of the shape and energy of the extremely short electron bunches whose characteristics directly translate into the properties of the produced light. Control of short intense beams is difficult due to beam characteristics drifting with time and complex collective effects such as space charge and coherent synchrotron radiation. Detailed diagnostics of beam properties are therefore essential for precise beam control. Such measurements typically rely on a destructive approach based on a combination of a transverse deflecting resonant cavity followed by a dipole magnet in order to measure a beam’s 2D time vs energy longitudinal phase-space distribution. In this paper, we develop a non-invasive virtual diagnostic of an electron beam’s longitudinal phase space at megapixel resolution (1024 × 1024) based on a generative conditional diffusion model. We demonstrate the model’s generative ability on experimental data from the European X-ray FEL.

A dedicated application of evolutionary algorithms: synchrotron X-ray radiation optimization based on an in-vacuum undulator

Following the rapid growth in accelerator-based light sources research since the mid of 20th century, miscellaneous third generation synchrotron radiation (SR) facilities such as SSRL, APS, ESRF, PETRA-III, and SPring-8 have come into existence. These SR source facilities provide 1020–1025 photons/s/mrad2/mm2/0.1%BW peak brightness within the photon energy range of 10–105 eV. Since different measurement techniques are utilized at X-ray beamlines of SR facilities, many kinds of insertion devices (i.e., undulators and wigglers) and optical components (e.g., high-resolution monochromators, double-crystal monochromators, lenses, mirrors, etc.) are employed for each experimental setup as a matter of course. Under the circumstances, optimization of a synchrotron beamline is a big concern for many scientists to ensure required radiation characteristics (i.e., photon flux, spot size, photon energy, etc.) for dedicated user experiments. In this respect, an in-vacuum hybrid undulator driven by a 6 GeV synchrotron electron beam is optimized using evolutionary algorithms (EA). Finally, it is shown that EA results are well consistent with both the literature and the analytical calculations, resulting in a promising design estimation for beamline scientists.

Design and optimization of a 100 keV DC/RF ultracold electron source

An ultracold electron source based on near-threshold photoionization of a laser-cooled and trapped atomic gas is presented in this work. Initial DC acceleration to ∼10 keV and subsequent acceleration of the created bunches to 100 keV by RF fields makes the design suitable to serve as injector for accelerator-based light sources, single-shot ultrafast protein crystallography, applications in dielectric laser acceleration schemes, and potentially as an injector for free electron lasers operating in the quantum regime. This paper presents the design and properties of the developed DC/RF structure. It is shown that operation at a repetition frequency of 1 kHz is achievable and detailed particle tracking simulations are presented showing the possibility of achieving a brightness that can exceed conventional RF photosources.

Construction and tests of phosphor view screen station for monitoring transverse profile of electron beam at PCELL

Development of an accelerator-based light source for generating coherent terahertz (THz) and mid-infrared (MIR) free-electron laser (FEL) is ongoing at the PBP-CMU Electron Linac Laboratory (PCELL). For producing high quality radiation, suitable electron beam property is necessary. In particle accelerator technology, transverse beam distribution and profile can be measured by using several techniques. The beam monitoring using view screen station is one of popular and effective methods. At PCELL, we use stations consisting of phosphor screen equipped with CCD camera to monitor, measure and record the beam image along the accelerator system and beamlines. The phosphor screen was chosen due to its high spatial resolution. The phosphor film is uniformly coated on a thin aluminium plate. Its green emission when electrons hitting the screen is matched well with the respond of the CCD camera. The screen can be moved in and out from the vacuum chamber using the air pressurized linear actuator. This paper presents about design, construction and tests of the screen stations at PCELL. The resolution of measurements for beam transverse profile and emittance is also be discussed.

Terahertz-slicing - an all-optical synchronization for 4th generation light sources.

A conceptually new approach to synchronizing accelerator-based light sources and external laser systems is presented. The concept is based on utilizing a sufficiently intense accelerator-based single-cycle terahertz pulse to slice a thereby intrinsically synchronized femtosecond-level part of a longer picosecond laser pulse in an electro-optic crystal. A precise synchronization of the order of 10 fs is demonstrated, allowing for real-time lock-in amplifier signal demodulation. We demonstrate successful operation of the concept with three benchmark experiments using a 4th generation accelerator-based terahertz light source, i.e. (i) far-field terahertz time-domain spectroscopy, (ii) terahertz high harmonic generation spectroscopy, and (iii) terahertz scattering-type scanning near-field optical microscopy.

Development and magnetic field measurement of a 0.5-m-long superconducting undulator at IHEP.

Undulators are important devices for accelerator-based light sources whose magnetic field quality determines the photon beam performance. Superconducting-type undulators have been developing rapidly in recent years around the world. The insertion device group at the Institute of High Energy Physics (IHEP) in China started an R&D project to develop prototypes of superconducting undulators (SCUs). A half-meter-long planar SCU has been produced recently. The SCU was designed based on the simulation program OPERA-3D giving a period length of 15 mm and a gap of 7 mm. This prototype was manufactured and fabricated precisely, and was then tested by vertically submerging in liquid helium in a Dewar. After quench training several times, the maximum current in the main coils reached 480 A. The magnetic field was measured by Hall probes mounted on a sledge in the middle of the undulator gap. The peak magnetic field reached 1 T. The measurement results indicate that correction coils with suitable current can not only optimize the magnetic first and second integrals but also reduce the phase error, which is expected bydesign.

A Compact Accelerator-Based Light Source for High-Power, Full-Bandwidth Tunable Coherent THz Generation

Terahertz (THz) radiation sources are increasingly significant for many scientific frontiers, while the generation of THz radiation with high-power at wide-tunable frequencies is still a limitation for most existing methods. In this paper, a compact accelerator-based light source is proposed to produce coherent THz radiation with high pulse energy and tunable frequency from 0.1 THz to 60 THz. By using a frequency beating laser-modulated electron beam and undulator taper, intense coherent THz radiation can be generated through undulators. Theoretical analysis and numerical simulations demonstrate that the proposed technique can generate narrow-bandwidth THz radiation with a pulse energy up to 6.3 millijoule (mJ) and the three-dimensional effects of beam has limited influence on its performance. The proposed technique will open up new opportunities for THz spectroscopic and time-resolved experiments.

Development of a low-emittance high-current continuous electron source

The Advanced Continuous-wave Electron injector aims to provide accelerator-based light sources with a high repetition rate and high average current electron beam. The injector consists of a DC thermionic gun generating a continuous electron beam, after which a radio-frequency (rf) cavity operating in dual mode at 1.5 and 3 GHz deflects the beam onto a knife-edge, creating a pulsed beam. A final rf cavity, also operating at 1.5 and 3 GHz, compresses the electron bunches for injection into a booster linac. The DC thermionic gun is fully operational and this paper presents the first results, demonstrating a continuous beam with a transverse emittance of 49±2 nm rad at 9.6 mA.

Optimization and Design of Cutting-Edge-Technology Tools for Natural Sciences Research in the Twenty-First Century: X-Ray Free Electron Lasers

Regarding the chronologic evolution of accelerator-based light sources, optimization and design issues of cutting-edge-technology insertion devices (i.e., undulators, wigglers, etc.) have been a big competitive concern since the beginning of the twenty-first century. Although these sources are strongly restricted by technological limitations in terms of radiation wavelength, leading X-ray free-electron laser (FEL) facilities are spending considerable efforts to survive in this scientific competition by modifying/updating their insertion devices. In other words, even though they all generate almost identical free-electron laser pulses, advertising a “superconducting undulator based light source” in comparison with a “conventional undulator based light source” may sound more attractive for some scientists. However, in contrast to superconducting undulators, conventional undulators do not require cryogenic cooling systems resulting in a cost-effective operation as a matter of course. Under the circumstances, it is shown that generation of hard X-ray FEL pulses is feasible via unique PPM undulators driven by an electron linear accelerator (linac) within the energy range of 5–10 GeV. The results reveal good consistency with the operating X-ray FEL sources worldwide.

Utilization of RADFET sensors for x-ray undulators: experiential considerations on magnetic field profile before commissioning

Concerning accelerator-based light sources, electrons are accelerated in conformity with specific time structures (i.e. continuous wave, ‘cw’, or bunched), which arise out of accelerating frequencies of employed radiofrequency cavities. Thus, the concept of transverse emittance of electron bunches comes into prominence, simply defining the area of an ellipse in phase space where the electrons are occupied. In nonideal cases (i.e. in real life), when some fraction of these electrons sneak out of a bunch, the concept of ‘energy spread’ automatically arises. Since the energy spread resulting from off-axis relativistic electrons in an insertion device induces considerable radiation damage on magnetic structures of x-ray undulators, radiation-sensitive field effect transistors (RADFETs) are utilized for quantification of radiation intensity. Hence, it is crucial to bring out their influence on magnetic field profiles before commisioning in tunnels. In this respect, two different types of RADFET sensors were investigated over the magnetic poles of an undulator with 40 mm period length (shortened as an U40 undulator). The effects of the sensors on magnetic field were measured using transverse and longitudinal scans through 6.5 m-long magnetic measurement benches (MMBs). The results of two RADFET standard systems are presented, wherein one contains the magnetic material nickel. Finally, novel magnetic measurement techniques for x-ray undulators via unique MMBs are thoroughly explained as well.

A hard X-ray self-amplified spontaneous emission free-electron laser optimizationusing evolutionary algorithms for dedicated user applications

Accelerator-based fourth-generation light sources are utilized in a wide range of interdisciplinary applications such as nanotechnology, materials science, biosciences, and medicine. A hard X-ray free-electron laser (FEL), as a state-of-the-art light source, was optimized using evolutionary algorithms for dedicated user applications such as X-ray Raman scattering (XRS), resonant inelastic X-ray scattering (RIXS), and X-ray emission spectroscopies (XES). Optimal parameter sets were obtained for an in-vacuum planar undulator driven by an 8 GeV electron beam. Performance parameters of self-amplified spontaneous emission (SASE) operation (i.e. optimized SASE performance parameters through evolutionary algorithms) were found to be consistent with operating X-ray FEL facilities around the world. It is shown that FEL characteristics for specific user experiments can be optimized by finding several evolutionary algorithm solutions within the range of 5 keV to 10 keV.

Tailoring of an electron-bunch current distribution via space-to-time mapping of a transversely shaped, photoemission-laser pulse

Temporally-shaped electron bunches at ultrafast time scales are foreseen to support an array of applications including the development of small-footprint accelerator-based coherent light sources or as probes for, e.g., ultrafast electron-diffraction. We demonstrate a method where a transversely-segmented electron bunch produced via photoemission from a transversely-patterned laser distribution is transformed into an electron bunch with modulated temporal distribution. In essence, the presented transformation enables the mapping of the transverse laser distribution on a photocathode surface to the temporal coordinate and provides a proof-of-principle experiment of the method proposed in W. S. Graves, et al. as a path toward the realization of compact coherent X-ray sources, albeit at a larger timescale. The presented experiment is validated against numerical simulations and the versatility of the concept, e.g. to tune the current-distribution parameters, is showcased. Although our work focuses on the generation of electron bunches arranged as a temporal comb it is applicable to other temporal shapes.

Design of an X-ray undulator: optimization considerations for SASE operation

Accelerator-based fourth-generation light sources, namely, free-electron lasers (FELs), offer unique radiation characteristics, such as tunable, coherent, high-power, ultra-short pulses. They rely on novel technology with challenging parameters, from which the practicability is currently being proved by world-class facilities like the European XFEL, LCLS, FLASH, and SACLA. When contriving such superior light characteristics, “state-of-the-art” linear accelerator (linac) and undulator technologies come into prominence. In this respect, design and simulation studies for a planar X-ray undulator are considered to optimize the FEL generation process by self-amplified spontaneous emission (SASE). Three main performance parameters for SASE operation (i.e., 1D gain length, saturation power, and saturation length) are compared and discussed by means of numerical calculations and simulation results. It is shown that hard X-ray FEL pulses (down to sub-angstroms) are generable via in-vacuum hybrid undulators driven by an 8 GeV electron linac.

Accelerator-based terahertz transmission imaging at the PBP-CMU Electron Linac Laboratory in Thailand

Terahertz radiation from an accelerator-based light source at the PBP-CMU Electron Linac Laboratory is aimed for terahertz transmission imaging purpose. Femtosecond electron bunches are accelerated with a linear accelerator and are guided to hit an aluminium radiator to produce coherent transition radiation. The backward radiation is collimated and focused with an optical system to pass through the samples. The transmitted radiation is detected and analysed for terahertz image construction. Materials having different geometries and dielectric constants are examined to demonstrate the imaging ability. Radiation filtering is also incorporated in order to improve the resolution of the image. The results show high capability of using this light source for the future terahertz imaging studies and applications.

Accelerator-based X-ray sources: synchrotron radiation, X-ray free electron lasers and beyond.

The evolution of synchrotron radiation (SR) sources and related sciences is discussed to explain the 'generation' of the SR sources. Most of the contemporary SR sources belong to the third generation, where the storage rings are optimized for the use of undulator radiation. The undulator development allowed to reduction of the electron energy of the storage ring necessary for delivering 10 keV X-rays from the initial 6-8 GeV to the current 3 Gev. Now is the transitional period from the double-bend-achromat lattice-based storage ring to the multi-bend-achromat lattice to achieve much smaller electron beam emittance. Free electron lasers are the other important accelerator-based light sources which recently reached hard X-ray regime by using self-amplified spontaneous emission scheme. Future accelerator-based X-ray sources should be continuous wave X-ray free electron lasers and pulsed X-ray free electron lasers. Some pathways to reach the future case are discussed. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Performance of K -edge subtraction tomography as an application of parametric x-ray radiation

We developed a novel method for element detection utilizing an x-ray source based on parametric x-ray radiation (PXR), an accelerator-based light source at the Laboratory for Electron Beam Research and Application (LEBRA), Nihon University. The method is a type of $K$-edge subtraction (KES) imaging that uses the drastic change of x-ray absorption around the $K$-shell absorption edge of the target element. Using the properties of PXR, simultaneous KES imaging is possible, and can easily be applied to 3-dimensional (3D) computed tomography (CT). We demonstrated the feasibility of simultaneous KES-CT in our previous work. In this study, we investigate the quantitative performance of simultaneous KES imaging for a sample containing the element strontium (Sr) for which the $K$-edge energy is 16.1 keV. Results of a simultaneous KES-CT experiment employing the LEBRA-PXR source confirm that the imaging method can provide a 3D distribution of the element with a value proportional to the Sr concentration. Concerning sensitivity to element concentration, at least in the region of 0.5%-concentration, the sample was successfully distinguished from the region without Sr in the 3D tomographic image obtained using the element-imaging technique.

Bunch Length Measurement Employing Cherenkov Radiation from a Thin Silica Aerogel

The temporal profile measurement for ultra-short electron bunches is one of the key issues for accelerator-based coherent light sources. A bunch length measurement system using Cherenkov radiation (CR) is under development at the Research Center for Electron Photon Science, Tohoku University. This system allows for the real-time diagnostics of electron bunches. The system comprises a thin silica aerogel as the Cherenkov radiator, a specially designed optical transport line, and a high-speed streak camera. The electron bunch length can be obtained by measuring the time spread of the CR from the electrons passing through the radiator medium using the streak camera. In this paper, we describe the novel bunch length measurement system using CR, discuss the expected time resolution of the system, and finally present the measurement results.

Coherent transition radiation from femtosecond electron bunches at the accelerator-based THz light source in Thailand

A THz radiation source based on electron linear accelerator (linac) has been built and commissioned at the Plasma and Beam Physics (PBP) Research Facility, Chiang Mai University (CMU) in Thailand since 2005. The accelerator system consists of a thermionic cathode RF electron gun, a magnetic bunch compressor in a form of an alpha-magnet, an S-band travelling-wave linac structure, beam steering and focusing magnets, beam and radiation diagnostic instruments, control system and other support components. Electron bunches with an average kinetic energy of about 8 MeV, a bunch charge of 100 pC, and a bunch length of 300 fs can be produced from the PBP-CMU electron linac system. Coherent THz radiation generated via transition radiation technique is used as a source of the THz spectroscopy and THz imaging applications. The measured power spectrum indicates that the radiation obtained from short electron bunches covers the frequency range of 0.15–1.80 THz or the wavenumber range of 5–60 cm−1. Experimental results reveal that a peak radiation power as high as 0.2 MW per micropulse and an average radiation power of 2.5 mW can be obtained. Generation and characterization of the coherent THz transition radiation as well as some examples of experiments to achieve electron bunch length and radiation power spectrum are reported and discussed in this paper.

Towards femtosecond-level intrinsic laser synchronization at fourth generation light sources.

In this Letter, the proof of principle for a scheme providing intrinsic femtosecond-level synchronization between an external laser system and fourth generation light sources is presented. The scheme is applicable at any accelerator-based light source that is based on the generation of coherent radiation from ultrashort electron bunches such as superradiant terahertz (THz) facilities or X-FELs. It makes use of a superradiant THz pulse generated by the accelerator as an intrinsically synchronized gate signal for electro-optical slicing. We demonstrate that the scheme enables a reduction of the timing instability by more than 2 orders of magnitude. This demonstration experiment thereby proves that intrinsically synchronized time-resolved experiments utilizing laser and accelerator-based radiation pulses on few tens of femtosecond (fs) to few fs timescales are feasible.

Probing ultra-fast processes with high dynamic range at 4th-generation light sources: Arrival time and intensity binning at unprecedented repetition rates.

Understanding dynamics on ultrafast timescales enables unique and new insights into important processes in the materials and life sciences. In this respect, the fundamental pump-probe approach based on ultra-short photon pulses aims at the creation of stroboscopic movies. Performing such experiments at one of the many recently established accelerator-based 4th-generation light sources such as free-electron lasers or superradiant THz sources allows an enormous widening of the accessible parameter space for the excitation and/or probing light pulses. Compared to table-top devices, critical issues of this type of experiment are fluctuations of the timing between the accelerator and external laser systems and intensity instabilities of the accelerator-based photon sources. Existing solutions have so far been only demonstrated at low repetition rates and/or achieved a limited dynamic range in comparison to table-top experiments, while the 4th generation of accelerator-based light sources is based on superconducting radio-frequency technology, which enables operation at MHz or even GHz repetition rates. In this article, we present the successful demonstration of ultra-fast accelerator-laser pump-probe experiments performed at an unprecedentedly high repetition rate in the few-hundred-kHz regime and with a currently achievable optimal time resolution of 13 fs (rms). Our scheme, based on the pulse-resolved detection of multiple beam parameters relevant for the experiment, allows us to achieve an excellent sensitivity in real-world ultra-fast experiments, as demonstrated for the example of THz-field-driven coherent spin precession.