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A microbunching instability (MBI) driven by beam collective effects can deteriorate electron beam quality in advanced x-ray free electron lasers. In this paper, we demonstrate a scheme to suppress microbunching instability by adjusting the compression ratio of two-stage bunch compressor in linear accelerator of the Shanghai soft X-ray Free-Electron Laser User Facility (SXFEL-UF). The calculation of the microbunching instability gain and the mechanism of MBI suppression scheme can be revealed by the theoretical model in this paper. Numerical simulations are presented to verify the analytical results. Furthermore, benefit by the high temporal resolution of transverse deflecting structure (TDS), beam experiments for direct analysis of MBI were carried out at SXFEL-UF. Theoretical calculations as well as simulation and consistent experimental results illustrate that the suppression of microbunching instability can be achieved by increasing the first stage compression ratio of the linac in the SXFEL-UF. The feasible theme provides a promising beam optimization method for future X-FEL facilities. [ABSTRACT FROM AUTHOR]

We developed a 2-mm-thick CdTe double-sided strip detector (CdTe-DSD) with a 250 μ m strip pitch, which has high spatial resolution with a uniform large imaging area of 10 cm 2 and high energy resolution with high detection efficiency in tens to hundreds keV. The detector can be employed in a wide variety of fields for quantitative observations of hard X-ray and soft gamma-ray with spectroscopic imaging, for example, space observation, nuclear medicine, and non-destructive elemental analysis. This detector is thicker than the 0.75-mm-thick one previously developed by a factor of ∼ 2.7, thus providing better detection efficiency for hard X-rays and soft gamma rays. The increased thickness could potentially enhance bias-induced polarization if we do not apply sufficient bias and if we do not operate at a low temperature, but the polarization is not evident in our detector when a high voltage of 500 V is applied to the CdTe diode and the temperature is maintained at − 20 °C during one-day experiments. The "Depth Of Interaction" (DOI) dependence due to the CdTe diode's poor carrier-transport property is also more significant, resulting in much DOI information while complicated detector responses such as charge sharings or low-energy tails that exacerbate the loss in the energy resolution. In this paper, we developed 2-mm-thick CdTe-DSDs, studied their response, and evaluated their energy resolution, spatial resolution, and uniformity. We also constructed a theoretical model to understand the detector response theoretically, resulting in reconstructing the DOI with an accuracy of 100 μ m while estimating the carrier-transport property. First, we evaluated the energy resolution using 22Na, 133Ba, and 57Co reconstructing the DOI effect from the energy correlation on both the anode and cathode sides. We obtained an energy resolution of 4.3 keV (FWHM) at 356 keV. Second, we formulated a theoretical detector-response model that reproduced the experimental data. Using the model, we determined the mobility-lifetime products of carriers μ τ e , h = (4 ± 1) × 1 0 − 3 , (1. 15 ± 0. 05) × 1 0 − 4 [cm 2 /V] with the space charge density of n s p = − 6. 0 × 1 0 10 [1/cm − 3 ] and then reconstructed the DOI with an accuracy of 100 μ m. Finally, we evaluated the spatial resolution and demonstrated to resolve patterns of 250- μ m -width slits with an X-ray test chart. We found no positional variation in the detector response. We realized the detector that has high energy resolution and high 3D spatial resolution with a uniform large imaging area. [ABSTRACT FROM AUTHOR]

A transmission diffraction grating is a useful tool for studying spectra in the soft X-ray and extreme ultraviolet range, created by the radiation of small–sized sources, such as laser produced plasma, vacuum spark, Z-pinch, X-pinch, etc. In almost all such experiments, a distance between the source and the grating is much larger than the size of the source, so the incident wave can be considered plane with sufficient accuracy, the distance from the recording detector to the grating is also much larger than the size of the grating and, therefore, Fraunhofer diffraction takes place here and the presence of a focusing element is not required. The interpretation of the results obtained using such a grating is difficult due to the fact that different diffraction orders overlap in the registered diffraction pattern. Therefore, there is a problem of reconstruction the true spectrum from an experimental diffraction pattern. In this paper, this problem is solved by approximating the function of the radiation intensity recorded on the diffraction pattern by a polynomial. In this case, due to the properties of the transmission diffraction grating, the desired function describing the spectrum being reconstructed is also a polynomial, which can be calculated according to the developed algorithm. Both the direct conversion of the spectrum into the diffraction pattern and the reverse conversion of the diffraction pattern into the spectrum do not change the radius of convergence, since multiplication is carried out by multipliers uniformly bounded from above and below. It follows from this that minor errors in the definition of the function describing the diffraction pattern cannot lead to significant errors in the reconstructed spectrum. [ABSTRACT FROM AUTHOR]

Free-electron lasers are capable of producing high-quality radiation in a wide wavelength range and at very high power and brilliance. To reach improved performance especially regarding coherence, bandwidth and wavelength stability, different seeding techniques are often required. However, a successful implementation of seeding typically demands a very high-quality electron beam, in terms of energy distribution and emittance. Techniques like Echo-Enabled Harmonic Generation (EEHG) and High-Brightness SASE (HB-SASE) require, or are significantly enhanced, by a flat distribution of particles in the longitudinal phase space. In this paper we describe how the initially chirped longitudinal phase space in the proposed Soft X-ray Laser (SXL) at the MAX IV laboratory can be tailored by first overcompressing and then de-chirping the electron pulse. We also show that it is possible to retain the properties of the electron bunch and how such an enhanced distribution can drive HB-SASE "seeding", resulting in a significantly reduced bandwidth and a better wavelength stability. [ABSTRACT FROM AUTHOR]

Abstract We have been developing event-driven SOI Pixel Detectors, named "XRPIX" (X-ray soiPIXel) based on the silicon-on-insulator (SOI) pixel technology, for the future X-ray astronomical satellite with wide band coverage from 0.5 keV to 40 keV. XRPIX has event trigger output function at each pixel to acquire a good time resolution of a few μ s and has Correlated Double Sampling function to reduce electric noises. The good time resolution enables the XRPIX to reduce Non X-ray Background in the high energy band above 10 keV drastically by using anti-coincidence technique with active shield counters surrounding XRPIX. In order to increase the soft X-ray sensitivity, it is necessary to make the dead layer on the X-ray incident surface as thin as possible. Since XRPIX1b, which is a device at the initial stage of development, is a front-illuminated (FI) type of XRPIX, low energy X-ray photons are absorbed in the 8 μ m thick circuit layer, lowering the sensitivity in the soft X-ray band. Therefore, we developed a back-illuminated (BI) device XRPIX2b, and confirmed high detection efficiency down to 2.6 keV, below which the efficiency is affected by the readout noise. In order to further improve the detection efficiency in the soft X-ray band, we developed a back-illuminated device XRPIX3b with lower readout noise. In this work, we irradiated 2–5 keV X-ray beam collimated to 4 μ m ϕ to the sensor layer side of the XRPIX3b at 6 μ m pitch. In this paper, we reported the uniformity of the relative detection efficiency, gain and energy resolution in the subpixel level for the first time. We also confirmed that the variation in the relative detection efficiency at the subpixel level reported by Matsumura (2015) has improved. [ABSTRACT FROM AUTHOR]

Abstract The overlap between the electron beam and the radiation field is one of the most important characteristics during the free-electron laser (FEL) amplification processes, which will dramatically influence the quality of the FEL radiation. The beam-based alignment (BBA), first proposed in 1980s, has achieved great success in both theoretical and experimental studies on hard X-ray FELs. This paper gives a brief analysis on the undulator misalignment, which has been ignored in the original BBA algorithm. These misalignments, as revealed by the calculation and simulation, tend to have significant impacts on the soft X-ray FELs which are driven by the low energy accelerators. Furthermore, we demonstrate a method that can probably solve this dilemma using a searching algorithm — genetic algorithm. [ABSTRACT FROM AUTHOR]

In its startup configuration the SASE3 beamline of the European XFEL provides only soft X-ray radiation, linearly polarized in the horizontal plane. In order to enhance capabilities of this beamline an afterburner scheme is proposed. It will be used as a coherent radiator using the micro-bunched electron beam of the linear SASE3 system. Radiation with variable polarization, which covers the full SASE3 wavelength range can be generated. For the radiator a new type of undulator design called Apple-X will be used. In this paper the design is described and magnet parameters, which are compatible with the SASE3 afterburner are determined using RADIA simulations. The end structure of such a device is optimized for minimum 1st field integrals. [ABSTRACT FROM AUTHOR]

A high-resolution cavity-based beam arrival/flight time measurement system has been established at SXFEL (Shanghai soft X-ray free-electron laser facility). During the development of the system, a correlation between the energy jitter and the beam flight time jitter was found which inspires and motivates us to further explore the possibility of using the beam arrival/flight time measurement system to measure the beam energy. On the one hand, beam energy is one of the most fundamental and critical parameters for accelerator facilities. Only precise handling of the beam energy enables precise control of the FEL (free-electron laser) radiation wavelength. On the other hand, there are only a few methods for beam energy measurement. The profile-based beam energy measurement method is commonly used but is interceptive. The BPM-based beam energy measurement scheme is non-interceptive but has a limited dynamic range and requires initial beam position calibration. This paper reports a promising application of a cavity-based high-resolution beam arrival/flight time measurement system: non-intercepting wide-range beam energy measurement. A brief introduction of the method principle and system implementation has been covered in this article. Moreover, a beam-based preliminary test has been performed and the test results show a linear correlation between the beam energy and the beam flight time under the current system configuration. The measured linear factor is 0.692 ps/MeV. In addition, the relative bunch energy measurement resolution is 0.0001. [ABSTRACT FROM AUTHOR]

The echo enabled harmonic generation (EEHG) scheme holds the ability for the generation of fully coherent soft x-ray free-electron laser (FEL) pulses directly from external UV seeding sources. In this paper, we study the feasibility of using a single stage EEHG to generate coherent radiation in the “water window” and beyond. Using the high-order operating modes of the EEHG scheme, intensive numerical simulations have been performed considering various three-dimensional effects. The simulation results demonstrated that coherent soft x-ray radiation at 150th harmonic (1.77 nm) of the seed can be produced by a single stage EEHG. The decreasing of the final bunching factor at the desired harmonic caused by intra beam scattering (IBS) effect has also been analyzed. [ABSTRACT FROM AUTHOR]

Beam-based alignment and feedback systems are essential for the operation of the Free Electron Lasers (FELs) and Linear colliders. They require a certain number of beam position monitors (BPMs) at selected locations and that their resolution can reach hundreds of nanometers or even the order of nanometers. Cavity BPMs adopting a resonant cavity structure having the advantage of high position resolution are widely used in the field of accelerators. A typical system includes a cavity BPM pickup, a radio frequency signal conditioning front-end and a digital signal processing electronic. How to systematically analyze the impact of the key parameters of each subsystem on the performance of the whole system, and combine the requirements of the system, operation mode of the facilities, the current level of equipment processing and the development of the electronics, so as to achieve the optimal and balanced allocation of the key technical indicators of each subsystem, is the primary issue to be considered when designing a CBPM system. In this paper, based on theoretical analysis, the relationship between the relative amplitude extraction uncertainty of the CBPM system and the key parameters of each subsystem is proposed, which could provide clear guidance for the design and optimization of a CBPM system. In addition, considering the engineering requirements of the Shanghai High repetition rate X-ray Free Electron Laser and Extreme Light facility (SHINE) for beam position monitor, the allocation of the key technical indicators of the CBPM system using the optimized design method is also introduced. Accordingly, the development of each subsystem was completed, and the beam test bench was also built at the Shanghai Soft X-ray FEL facility (SXFEL), based on the evaluation method of divided power, the beam experiment results show that the position measurement uncertainty of the CBPM system can reach 40 nm at the bunch charge of 100 pC, which is consistent with the theoretical analysis results and better than the requirements of the SHINE. [ABSTRACT FROM AUTHOR]

X-ray free-electron lasers (XFELs) are powerful tools to explore and study nature for achieving remarkable advances. Generally, seeded FELs are ideal sources for supplying fully coherent soft x-ray pulses. Benefiting from the high-frequency up-conversion efficiency, the cascading configuration with echo-enabled harmonic generation (EEHG) and high-gain harmonic generation (HGHG) holds promising prospects for generating fully coherent radiation at 1 nm wavelength. In this paper, we design and optimize EEHG-HGHG configuration using parameters of Shanghai High-Repetition-Rate XFEL and Extreme Light Facility. In addition, we systematically analyze the effect of relative timing jitter on the output FEL performance based on various start-to-end electron beams. The intensive numerical simulations show that the cascaded EEHG-HGHG scheme can achieve 1 nm FEL pulses with peak power up to 15 GW. Further sensitivity analysis indicates that the relative timing jitter between the electron beam and seed laser has a significant impact on the FEL performance. The RMS timing jitter of 3 fs can lead to the final output pulse energy fluctuations of 29%. [ABSTRACT FROM AUTHOR]

Besides the designed beam acceleration, the energy of electrons is changed by the longitudinal wakefields in a real free-electron laser (FEL) facility, which may degrade FEL performances from the theoretical expectation. In this paper, with the help of simulation codes, the wakefields induced beam energy loss in the sophisticated undulator section is calculated for Shanghai soft X-ray FEL, which is a two-stage seeded FEL test facility. While the 1st stage 44 nm FEL output is almost not affected by the wakefields, it is found that a beam energy loss about 0.8 MeV degrades the peak brightness of the 2nd stage 8.8 nm FEL by a factor of 1.6, which however can be compensated by a magnetic field fine tuning of each undulator segment. And the longitudinal coherence of the 8.8 nm FEL output illustrates a slight degradation, because of the beam energy curvatures induced by the wakefields. [ABSTRACT FROM AUTHOR]

The last decades have witnessed substantial efforts in the development of several detector technologies for X-ray fluorescence (XRF) applications. In spite of the increasing trend towards performing, cost-effective and reliable XRF systems, detectors for soft X-ray spectroscopy still remain a challenge, requiring further study, engineering and customization in order to yield effective and efficient systems. In this paper we report on the development, first characterization and tests of a novel multielement detector system based on low leakage current silicon drift detectors (SDD) coupled to ultra low noise custom CMOS preamplifiers for synchrotron-based low energy XRF. This new system exhibits the potential for improving the count rate by at least an order of magnitude resulting in ten-fold shorter dwell time at an energy resolution similar to that of single element silicon drift detectors. [ABSTRACT FROM AUTHOR]

Gd 2 O 2 S:Tb (P43) powder was widely used in soft X-ray detection due to its high light yield. Compared with P43, GAGG:Ce scintillator has high light transmittance, fast decay time and strong radiation resistance. Photonic crystals (PhCs) can be used to enhance the light output of scintillator. In this paper, two-dimensional (2D) photonic crystals that consist of square arrays of air holes etched into TiO 2 layer were fabricated on the exit surface of GAGG:Ce scintillator to enhance its light output. The luminescence spectra and the light output of GAGG:Ce scintillator with/without PhCs were measured at the BL08U1A beamline of Shanghai Synchrotron Radiation Facility. The results showed that the light output of GAGG:Ce scintillator with PhCs can be enhanced by a factor of 1.43 times compared with the reference GAGG:Ce scintillator, 2.35 times compared with YAG:Ce scintillator, 1.24 times compared with Gd 2 O 2 S:Tb scintillation ceramic and 0.76 times compared with P43 powder layer with the mass thickness of about 1.3mg/cm 2 with optical grease coupling. No significant change was observed in luminescence decay time between GAGG:Ce scintillator with and without PhCs. It makes GAGG:Ce scintillator with PhCs a promising material instead of P43 powder and YAG:Ce scintillator in soft X-ray detector. [ABSTRACT FROM AUTHOR]

Abstract: This paper is focused on the soft X-ray tomography system set-up at Tore Supra (DTOMOX) and the recent developments made to get precise information about plasma features from a raw or inverted data. The ultimate aim is not only to use this information in real time for visualisation but also for potential feedback, with a particular emphasis on the optimisation of the reconstruction technique and on simple analysis tools developed for an automatic treatment of these inverted data. Recent progress made on the optimisation of free parameters used for the tomographic inversion will be explained. Finally, future techniques using advanced imaging detector will be presented. In particular, preliminary investigation on the use on Tore Supra of Gas Electron Multiplier (GEM) detector for imaging purposes will be described also including some first observations in situ. [Copyright &y& Elsevier]

Cavity Beam Position Monitor (CBPM) adopting a resonant cavity structure and using the characteristic modes excited by the electron beam to measure the beam parameters, has the advantage of high position resolution and is widely used in Free-electron Laser (FEL) facilities. The wake field of different modes carries different bunch information, the amplitude and phase of the signals of different modes can be extracted through the signal processing method to obtain the characteristic parameters of the source bunch. In the application of bunch charge and position measurement, the accurate amplitude extraction method for cavity BPM signal is the primary issue to be considered when designing the data acquisition and processing system. In this paper, through theoretical analysis and numerical simulation, it is proved that the optimal algorithm of amplitude extraction for CBPM exists, and the dependence between the data processing window size and the decay time of the cavity BPM under the optimal design is given. In addition, the relationship between the optimized amplitude extraction uncertainty and the noise-to-signal ratio, sampling rate of data acquisition and processing system, and the decay time of the cavity BPM is also proposed, which can provide clear guidance for the design and optimization of the CBPM system. Accordingly, the CBPM system in Shanghai Soft X-ray FEL (SXFEL) facility is optimized, and the resolution improved from 273 nm to 177 nm at the bunch charge of 500 pC. [ABSTRACT FROM AUTHOR]

We present the assembly and preliminary results of a prototype detector, which was developed to evaluate the performance of the Topmetal-II − for application in Soft X-ray Polarimeter (SXP). SXP is used to measure the polarization of X-rays emitted from randomly occurring gamma-ray bursts (GRBs) and other specific dense celestial bodies. The Topmetal-II − is a silicon pixel sensor designed to directly track charged particles. The test was conducted on a gas mixture of Ne and DME at 0.8 atm pressure, and the Topmetal-II − sensor was used to image the photoelectron tracks. Typical real X-ray tracks obtained under four conditions are presented in this paper. The modulation factor derived from the tracks of 100% polarized 4.5 keV X-rays reached ∼ 29% at ∼ 53% signal efficiency, with a residual modulation of the order of ∼ 2% for unpolarized 5.9 keV X-rays. [ABSTRACT FROM AUTHOR]

The increasing demands for electron beams with short bunch length and high brightness for Soft X-ray Free Electron Laser (SXFEL) have stimulated active efforts to develop suitable photoinjectors for these accelerators. One promising direction in this regard is increasing the gradient of rf gun. The rf design, beam dynamics and cold-test results of a novel 3.6 cell C-band 150 MV/m rf photocathode gun are presented in this paper. The rf gun and its beam dynamics modeling have been done using a combination of codes including POSSION, CST, and ASTRA. The impact of first half-cell length, cell number, and solenoid on beam dynamic are analyzed and described. The gun has state-of-the-art rf features including: elliptical irises profile; a non-brazed cathode plate; and a rectangular mode launcher. The fabrication and cold-test of the rf gun prototype are successfully implemented, and the measured solenoid field agrees well with simulations. [ABSTRACT FROM AUTHOR]

All-sky surveys of X-ray transient objects in the soft X-ray band are essential for observing high-redshift events to probe unexplored physical conditions in the early universe and discovering mysterious electromagnetic counterparts of gravitational-wave sources. HiZ-GUNDAM is a future satellite mission that intends to perform a wide-field survey in the soft X-ray band (0.4–4 keV). The HiZ-GUNDAM X-ray detector has Si pixel sensors that require fine positioning accuracy (e.g. tens of micrometers), high detection efficiency, a large detection area (tens of cm 2) and, a relatively high frame rate of ∼ 10 frames per second that can be achieved by a back-illuminated Si CMOS image sensor. One of the most promising candidates is GSENSE6060BSI with 6144 × 6144 pixels fabricated by Gpixel Inc. In this paper, for the initial performance test, we used a smaller-sized CMOS sensor, GSENSE400BSI-TVISB, which is similar to GSENSE6060BSI in terms of pixel characteristics, such as pixel size, epilayer thickness, and resistivity, except for the number of pixels (2048 × 2048). We conducted a detailed study on the spectroscopic performance of GSENSE400BSI-TVISB in the soft X-ray band and its radiation tolerance for space applications. For the spectroscopic performance, low-energy X-ray lines such as Al-K α (1.5 keV), Mn-K α (5.9 keV), and Mn-K β (6.5 keV) were clearly detected. Furthermore, using the obtained X-ray events, we estimated > 50% detection efficiency in the 0.4–4 keV band, corresponding to a ∼ 10- μ m Si thickness of the detectable layer. We also performed radiation tolerance tests against gamma rays from 60Co, 100-MeV protons and 5.5-MeV alpha rays (5–30 krad, corresponding to 6–30 year operation in orbit). After the irradiation tests, we observed that GSENSE400BSI-TVISB has strong radiation tolerance for our purpose: the lower detectable energy of ∼ 0.4 keV can be obtained if the CMOS is operated with an exposure of 0.1 s at − 20 °C. [ABSTRACT FROM AUTHOR]

X-ray Imaging Spectroscopy Mission (XRISM) is the next Japanese X-ray astronomical satellite to be launched in 2021 Japanese fiscal year. We are developing one of the XRISM instruments "Xtend", which is an X-ray CCD camera combined with an X-ray telescope, and achieves the wide field of view of 3 8 ′ × 3 8 ′ in 0.4–13 keV. In 2019, twelve flight-model (FM) candidate CCD chips were fabricated by Hamamatsu Photonics K.K. We conducted screening experiments to examine whether the FM candidates met requirements for the Xtend CCDs, and selected the four FM chips from them. We constructed a screening system, with which we can examine various CCD performances by illuminating characteristic X-ray lines in a ∼ 0. 5 –14 keV band or optical lights. With this system, all the twelve candidates were confirmed to satisfy the requirements. We then selected four chips with the best performance, in terms of e.g., their charge transfer inefficiencies, energy resolutions, soft X-ray sensitivities, and optical light leakages. In this paper, we report an overview of the screening system, and procedures and results of the screening process. [ABSTRACT FROM AUTHOR]

Recent advances in the studies of laser-plasma accelerators (LPAs) and optical undulators enable the possibility of designing a compact x-ray free-electron laser (FEL). However, the LPA beams usually have a percent-level energy spread that will degrade the FEL performance. To compensate for the energy spread effect, an optical undulator with a transverse gradient has been proposed, in which a dispersed LPA beam is injected into a horizontally polarized Gaussian laser with a horizontal offset relative to the laser center. Such a scheme, however, leads to an inherent orbit deflection and calls for accurate orbit correction. In this paper, we propose an alternative method that does not induce any orbit deflection and requires less total laser power. In this method, a dispersed LPA beam is sent into an optical undulator with a vertical gradient generated by the combination of a Hermite-Gaussian T E M 01 mode laser and a T E M 00 mode laser. Simulations suggest that this method is feasible to generate megawatt-level soft x-ray FEL by using an LPA beam with the energy of a few tens of MeV. [ABSTRACT FROM AUTHOR]

Lawrence Berkeley National Laboratory is designing a new electron synchrotron for scientific research using synchrotron radiation. An upgrade of the Advanced Light Source (ALS) would position the nation to retain its international competitiveness in the soft X-ray sciences. The ALS is a 1.9 GeV storage ring operating at 500 mA of beam current. It is optimized to produce intense beams of soft X-rays, which offer spectroscopic contrast, nanometer-scale resolution, and broad temporal sensitivity. Upgrading the ALS storage ring from a triple-bend-achromat to a multibend-achromat (MBA) lattice design will provide a soft X-ray source that is up to 100–1,000 times brighter than today's ALS and will provide a significantly higher fraction of coherent light in the soft X-ray region than is currently available at the ALS. The upgraded ALS will offer the highest coherent flux of any existing or planned storage-ring facility, worldwide, up to a photon energy of 3.5 keV. The addition of an accumulator ring that enables on-axis, swap-out, recovery, and exchange of bunch trains is required to achieve this coherence and brightness. Analysis of the radiation generated and shielding required by the addition of this new accelerator is presented in this paper. [ABSTRACT FROM AUTHOR]