Free Electron Laser Research Articles

Current Status of TFCP Cryogenic System for DALS Pre-research Project

Abstract The Dalian Advanced Light Source (DALS), proposed by the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, is a novel Free Electron Laser (FEL) project that utilizes superconducting radio frequency (SRF) cavity technology. Before the DALS was approved by the central Government, a pre-research project named Accelerator Module Test Facility (AMTF) has been funded by the Dalian local government since 2019. In support of the AMTF project, a test facility cryo-plant (TFCP) with a cooling capacity of 370 W@ 2 K was designed in 2020. This paper presents the current status of the TFCP system, including the construction progress, the refrigerator commissioning results, the distribution valve box and the 2 K system.

Process design and calculation of DALS test facility cryogenic system using Ecosimpro

Abstract Dalian advanced light source (DALS) is a new Free Electron Laser (FEL) project based on SRF technology. The DALS test facility is used to test the key components of DALS, which requires a 370 W @ 2 K cryogenic system and is expected to be completed by end of this year. Dynamic simulation is often used in the cryogenic system process design, process calculation and control logic optimization. This paper introduces the establishment of cryogenic distribution system model, key components such as thermal shield, SRF cavity and cryomodule are modelled based on Ecosimpro platform. The safety relief model is also built for the dynamic relief process study. The dynamic cooldown process and safety relief process are simulated, the dynamic simulation software can be used to obtain more accurate process parameters in dynamic processes such as cooldown, safety relief, which is favourable for the design of cryogenic system and selection of components.

High accuracy simulations of extreme polarization purity of an X-ray beam for Brewster crystal diffraction

Abstract Recent advancements in powerful pump optical lasers and X-ray free-electron lasers have reignited interest in vacuum birefringence, a phenomenon predicted by quantum electrodynamics (QED). Detecting this subtle effect requires X-ray photons with exceptionally high linear polarization. To meet this need, several polarimeters employing channel-cut crystal monochromators, designed to deflect only σ-polarization while suppressing π-polarization, have been proposed. In this paper, we utilize the upgraded SHADOW code to perform high-accuracy simulations of the linear polarization of X-ray beams by Brewster deflection. Our results highlight a promising beamline configuration capable of achieving linear polarization purity on the order of 10−13. We propose using this setup for single-shot experiments to verify vacuum birefringence, offering a novel avenue for probing linear polarization purity in future experimental investigations.

Femtosecond laser-induced optical breakdown and cavitation dynamics in water imaged with an X-ray free-electron laser

Femtosecond laser-induced optical breakdown and cavitation dynamics in water imaged with an X-ray free-electron laser

RF design of a 1.3 GHz 3-cell superconducting cavity for high-current beam acceleration

Superconducting radio-frequency (SRF) cavities are widely used in particle accelerator facilities. The SRF-based energy recovery linac (ERL) equipped with special SRF cavities offers feasibility for application in high-power free electron lasers (FELs). This study aims to design and optimize a 3-cell cavity that serves as the accelerating structure for a 10 mA class injector of high-brightness ERL-FEL. We apply the middle-cell shape of the mature TESLA cavity, and the end-groups are optimized for high-current beam operation. The cavity is designed with two fundamental power coupler (FPC) ports, which enable high-power input by two FPCs. In addition, the beam pipe is enlarged to damp potentially strong higher-order modes (HOMs) induced by the high current beams. A multi-objective genetic algorithm is utilized to optimize the cavity geometry. The damping of HOMs, multipacting, and mechanical issues are also investigated to verify the rationality of the cavity design.

Development of cryogenically cooled permanent magnet undulators for PETRA IV

Abstract Cryogenically cooled permanent magnet undulators (CPMUs) are an improvement of the in-vacuum undulators (IVUs) which are widely used at various synchrotron radiation facilities and free-electron lasers in order to improve the photon flux towards shorter wavelengths. Especially beamlines with the need for high energy photons will benefit from shorter period insertion devices (IDs). For the planned ultra-low emittance photon source PETRA IV at Deutsches Elektronen-Synchrotron (DESY), we are designing CPMUs with a hybrid magnet structure based on PrFeB magnet material. First prototyping was done on magnet keepers and the pole-tuning mechanism. In addition, options for force-compensation are assessed. Crucial components are the link rods connecting the in-vacuum girders with load bearing out-of-vacuum girders. Simulations on the impact of their arrangement on the bending of the magnet structure and their thermal behaviour are presented.

A new measurement system for wall thickness uniformity of a thin wall small aperture vacuum chamber

Abstract The vacuum external undulator is a critical component in synchrotron radiation light sources and free-electron lasers, featuring a small-aperture vacuum chamber that precisely guides the electron beam through the undulator. The uniformity of the wall thickness of the vacuum chamber significantly influences its beam quality. However, accurately measuring the thickness uniformity of long and narrow vacuum chambers poses significant challenges. A testing system utilizing a specific magnetic field has been developed to measure the wall thickness uniformity of vacuum chambers. The system synchronously measures the voltage between the permanent magnet and the iron ball, as well as the distance between the vacuum chamber’s outer surface and the permanent magnet. By calibrating the relationship between voltage and the iron ball’s height variation, the system enables rapid and accurate evaluation of wall thickness uniformity along the beam direction. Successful tests are conducted on an aluminum alloy vacuum chamber with a length of 4 meters and an inner diameter of 8 mm.

Commissioning results of the ATHOS-beamline at SwissFEL

Abstract The Soft X-ray Free Electron Laser ATHOS at SwissFEL came into operation at the end of 2019 together with a variable line space grating monochromator. The beamline alternatively distributes the FEL-beam to two end stations namely Maloja and Furka. A third end station, Diavolezza, is currently under construction and will be available in 2026. This article presents the current status of the beamline instrumentation and first commissioning results in the Maloja and Furka branches.

Commissioning progress of the first cryoplant for SHINE accelerator

Abstract This paper presents the commissioning progress of the first accelerator cryoplant for Shanghai high repetition rate x-ray free electron laser and extreme light facility (SHINE). To fulfil the cooling needs for the superconducting cavities in the SHINE accelerator, three cryogenic plants were contracted to Air Liquide Advanced Technologies (ALAT) in 2020. Each cryoplant could provide 4 kW cooling power at 2 K and mainly consists of a warm compressor station (WCS), a 4.5 K cold box, and a 2 K cold box. Following the success delivery and installation, the WCS had been commissioned and the final 100 hours test were achieved at the middle of 2023. Afterwards, the commissioning of the 4.5 K cold box progressed as expected and the first drop of liquid helium was obtained at the end of 2023. The 2 K cold box commissioning were enabled when the installation of the cryogenic transfer line in between had been finished. At the beginning of 2024, the cold compressor had been successfully tested to achieve the 2 K super-fluid inside the 2 K cold box. The promising commissioning progress of the first cryoplant demonstrates the capability to deliver cryogenic cooling to the SHINE accelerator that are being constructed.

Warm-up Time Estimation in case of Failure for the S3FEL Accelerator Cryogenic System

Abstract Shenzhen Superconducting Soft X-Ray Free Electron Laser (S3FEL) is under construction in Shenzhen, China by the Institute of Advanced Science Facilities (IASF). The cryogenic system is one of the most important systems in S3FEL, which provides a cryogenic environment for the superconducting accelerator. Once the cryogenic system shuts down in case of failure, the superconducting accelerator will stop running. Then, the pressure in the accelerator pipes will increase, which maybe caused safety valve jumping or cryogenic lines breaking. The warm up time from operation pressure to the specified relief pressure is helpful to select reasonable relief pressure and protective measures. In the present work, the calculations of the warm up time with different liquid helium levels under four specific operational faults and two initial conditions are conducted. In a case of liquid helium level with 70%, the warm up time of upstream and downstream were about 6.8 hours and 10.9 hours at an initial condition of 2 K superfluid helium, while they were about 4 hours and 6.1 hours at an initial condition of 4.5 K saturated liquid helium. The difference between upstream and downstream warm up time was mainly caused by the different lengths of cryogenic transfer line. Moreover, the variations of liquid helium level with time at a specified condition were also calculated. The results presented in this paper will provide useful information for the cryogenic system safety design and accelerator operation modes.

Generation of ultrafast user defined multi-micro-pulse burst structure from a femtosecond mode-locked fiber oscillator by using semiconductor optical amplifier for photoinjector.

We demonstrate and present detailed technical insights into the generation of a user-defined multi-micro-pulse burst structure from an in-house developed femtosecond mode-locked fiber oscillator using a standard semiconductor optical amplifier (SOA), which acts as an excellent candidate for an ultrafast fiber-based pulse-picker. An in-house developed 130MHz mode-locked fiber oscillator followed by a 500m long optical fiber as the pulse stretcher along with polarization control units was used to achieve a highly stable multi-micro-pulse structure at variable repetition rates by using and adapting a fiber coupled SOA. The timing system with a synchronous trigger setup has been explained in detail to achieve the multi-micro-pulse structure from the fiber laser system, which is being used to generate multi-micro-electron bunches in a photoinjector based free electron laser facility at IUAC named as Delhi Light Source. The technical insights from the detailed experimentation and results help bring out various advantages and challenges in the use of an SOA as an ultrafast pulse-picker for generating a multi-micro-pulse structure, which will be useful for widespread applications of the modern compact femtosecond lasers and in the field of photoinjector based systems for higher electron currents.

Single-shot temporal characterization with the transverse deflecting structure PolariX and THz streaking at FLASH

Abstract At the free electron laser FLASH at DESY pulse length measurements can be performed with e.g. THz streaking or an analysis using the PolariX TDS. Since THz streaking examines the XUV pulse directly whereas the PolariX TDS focuses on the energy distribution of the (XUV pulse generating) electron bunch, both techniques are capable of analyzing the same XUV pulse simultaneously. We used a newly installed laser heater to shape the electron bunch and therefore influence the XUV pulse profile and compare the resulting pulse shapes measured by THz streaking and the PolariX TDS. We compare average pulse profiles as well as single-shot examples and discuss the challenges of both types of analysis.

Interferometric Solutions for Optics Stabilization at Superconducting Linac Driven X-ray Cavity Free Electron Laser Systems

Abstract The development of next generation X-ray laser sources with enhanced peak and average spectral brightness have been actively investigating concepts mimicking the laser regenerative amplifiers. These systems circulate X-ray photons through an undulator system using an optical cavity operating at X-ray wavelengths. The repeated interaction between the recirculating X-ray pulses and the successive fresh relativistic electron bunches leads to power build up and gain saturation, generating Fourier-limited, ultra-bright pulses. Compared to self-amplified spontaneous emission processes used in most currently operating free-electron laserss and the single-pass self-seeded methods, this regenerative amplification approach is another step forward in x-ray laser technology. However, as the regenerative systems are expected to be driving by a superconducting linac operating at MHz repetition rate such as the Linac Coherent Light Source (LCLS)-II HE linac currently under construction, the expected cavity round trip time dictates extended cavity lengths on the order of 100s of meters. Achieving and maintaining the optimal cavity optics alignment, thus the maximal interaction between the recirculating photons and the electron bunches, is one of the most critical challenges, considering in particular relative ground motion between optical benches separated by large distances across broad timescales. To address the stringent stability requirement, we propose the integration of a continuous wave laser guide system for the stabilization of the cavity, the accelerator, and the X-ray components. This paper outlines the concept and its potential implementation using the LCLS-II HE linac, at SLAC National Accelerator Laboratory.

Development of CoRDIA: An Imaging Detector for Next-Generation Synchrotron Rings and Free Electron Lasers

Abstract CoRDIA (Continuous Readout Digitizing Imager Array) is a hybrid pixel detector development targeted to 4th generation synchrotron sources and (continuous) high-rate Free Electron Lasers. Serving the latter it builds upon the concept of the AGIPD detector, employing a charge sensitive preamplifier with adaptive gain switching. The further signal path comprises of a Correlated Double Sampling stage and an 11 bit Analogue to Digital Converter (ADC), serving a sub array of 16 pixels. 128 ADCs connect to a multi-gigabit serial link to drive the images off chip. For this part CoRDIA adopts the ”GWT-CC ” implementation on the Timepix4 chip by Nikhef. A chip with 256 × 192 pixels will implement 24 of these blocks. Since the links conform to industry standards (IEEE 802.3ae), the subsequent data acquisition can be based on commercial components. Performance targets are a continuous frame rate of ≈150 kHz, single-photon sensitivity at <12 keV, and a dynamic range of a few thousand photons (@ 12 keV) with a silicon sensor. The energy range could be extended using active sensors or sensors from ”high-Z ” materials towards lower and higher photon energies.

Refurbishment of a cryogenic permanent magnet undulator for the Shanghai Synchrotron Radiation Facility

Abstract Shortening the period length is one direction of development in undulator technology, and it is also an effective approach for obtaining short-wavelength radiation and miniaturizing free electron laser facility. In 2017, the Shanghai Synchrotron Radiation Facility (SSRF) project team successfully developed a cryogenic permanent magnet undulator (CPMU) based on PrFeB magnets. This CPMU features a period length of 18mm and consists of 144 periods. With a magnetic gap of 6 mm, it achieves an effective peak magnetic field of 0.91 T. In September of the same year, successful radiation emission was achieved in the 13W straight section of the SSRF storage ring. However, due to operational issues, some components suffered damage, rendering it difficult to operate normally. In 2023, it was decided to refurbish the undulator by reprocessing the damaged components. Currently, it is undergoing integration and debugging, with plans for reinstallation in the storage ring during the summer of 2024. This paper reports the refurbishment project for CPMU, including magnetic field measurement and debugging results.

Heat load measurements of XFEL cryomodules using helium evaporation method

Abstract The European XFEL Free Electron Laser (EuXFEL) at DESY is at 2K operation since January 2017. User operation with a maximum beam energy of 17,5 GeV began in September 2017. Studies are ongoing for a possible upgrade to operate XFEL at higher duty factors and lower cavity gradients. For this purpose, dynamic heat loads should be measured precisely to evaluate the cryomodules performances and the needed cooling capacity of refrigeration plant. This paper describes a heat loads measurement method based on measurements of the amount of helium evaporated from LHe II bath during a certain time period. A distinctive feature of this method is its insensitivity to eventual leaks across the seats of JT-valves. Furthermore, possible errors in the LHeII level readings can be minimized using this method. This paper summarizes the experience gained so far with this method at the EuXFEL linac and the CryoModule Test Bench (CMTB). Issues that have arisen during the measurements are discussed and conclusions are drawn. Results of the heat load measurements in CW mode are presented for a single cryomodule at CMTB.

Novel passive resonant structures for precision assembly of synchrotron X-ray optics

Abstract Many optics at synchrotron and free electron laser facilities need to be cooled to dissipate the heat imparted by the intense photon beams. For indirectly cooled optics, distortion of the surface can occur when excessive or asymmetrical clamping forces are applied during assembly. To address this issue, it is necessary to monitor the effects of assembly and subsequent life cycle of the clamping forces applied to the crystal. We present experimental verification of a non-contact approach to the monitoring of clamping forces applied to the first crystal of a monochromator at Diamond Light Source using additively manufactured passive resonant structures. Laser vibrometery demonstrates the efficacy of this novel approach for assembly of indirectly cooled optics based on feedback from passive resonant structures. Fizeau interferometry of the optical surface reveals greater uniformity and repeatability when compared with a conventional approach to assembly. Furthermore, frequency shifts in the range of 20-60 Hz were observed in the passive structures after cryogenic cooling, indicative of a change in clamping force of up to approximately 25%.

The MID Instrument of European XFEL: Upgrades and Experimental Setups

Abstract This article provides examples of setups and upgrades currently under development at the Materials Imaging and Dynamics (MID) instrument of the European X-Ray Free-Electron Laser (EuXFEL): the first installations of the Multi-environmental multi-Detector Setup (MDS_2), its next design scenarios in the MID Instrument and the Multi-Purpose Chamber 2 project design status, with the first successful commissioning of its Breadboard Assembly.

Speckle analysis as a characterization tool for the focusing properties of diamond X-ray lenses

Abstract The availability of intense X-ray beams at megahertz repetition rate allows for new scattering and imaging experiments using the unique pulse train structure of the European X-ray Free-Electron Laser (EuXFEL). However, the resulting heat load can pose challenges for X-ray optics and potentially limit the efficiency. In this context, X-ray optics made of diamond emerge as a promising solution better suited for the extreme beam conditions at EuXFEL. In this article, we demonstrate a simple speckle analysis to determine the size of the focused beam in a caustic scan. The lenses are 1D planar diamond lenses and experiments were performed at the Materials Imaging and Dynamics (MID) station of EuXFEL. We find a minimum beam size of 630 nm and compare the speckle method with the more conventional method of wire scanning.

Multiple Detector Stage at the MID instrument of European XFEL

Abstract The Multiple Detector Stage is an ancillary detector setup for the Materials Imaging and Dynamics instrument at the European X-Ray Free-Electron Laser Facility. It is developed to improve the current capabilities concerning X-ray detection and make entirely new experiments possible. A unique feature of the MID instrument is the large flexibility in positioning of the AGIPD detector relative to the sample. This enables a large variety of instrument configurations ranging from small-angle to wide-angle X-ray scattering setups. A recurrent request from the users, which is currently not enabled, is the option of simultaneously recording both wide- and the small angle scattering by using two area detectors. The aim of developing MDS is to provide this missing capability at MID so that SAXS and WAXS experiments can be performed in parallel. The MDS will not be installed permanently at the instrument but only on request to provide as much flexibility as possible. In this article, the background and status of the MDS project is described in detail.