The optical timing system of the FERMI facility underwent a significant upgrade to accommodate requests for additional pulsed links for remote lasers or diagnostic stations. Following the successful completion of compliance tests, the long-term performance of the extended system has been recently evaluated through out-of-loop measurements. In the setup each of the two pulsed subsystems, synchronized to the common optical master oscillator, feeds a stabilized fiber optic link. The relative stability between the outputs has been monitored at a remote location. The results achieved and the challenges encountered during the measurements will be discussed.

Peking University plans to conduct experimental research on a THz FEL (Terahertz Free Electron Laser) amplifier using a DC-SRF (Superconducting Radio Frequency) electron gun. The DC-SRF electron gun, which is capable of generating high-quality electron beams with high repetition rates and low emittance, is suitable for use in large scientific facilities such as FELs and ERLs. The experimental setup of the THz FEL amplifier mainly includes a 1.3GHz DC-SRF electron gun, a 2.856GHz RF (Radio Frequency) deflection cavity, a 2.4 GHz cavity-based Beam Arrival Monitor (BAM), a 1.3 GHz 2×9 Cell superconducting accelerator module, as well as photocathode drive laser systems and THz seed light systems. The two laser systems have repetition rates of 81.25 MHz and 100 MHz, respectively. Since the operating frequencies of the components on the THz FEL amplifier device are not identical and some frequencies do not have a multiple relationship, clock generation schemes based on PLL (Phase-Locked Loop) or mixers cannot fully meet the experimental requirements. Therefore, we have employed DDS (Direct Digital Synthesis) to generate the key frequencies. Additionally, to ensure the normal operation of the BAM, signal detection and processing of the BAM signals have been implemented based on the KC705 and FMC150 platforms.

Beam quality from photoinjectors is crucial for lasing in Free Electron Laser (FEL) facilities. While phase space measurement are usually limited to 2D with conventional methods, the recently-developed transverse deflecting cavities (TDCs) with variable polarization provide the capability to measure multi-dimensional phase space information. Such information could guide the improvement of beamline setup for optimal lasing performance. We therefore propose an S-band parallel-coupled TDC, in which two chains that deflect beam horizontally and vertically are independently fed by waveguides and variable polarization can be obtained by adjusting their relative amplitude and phase. This design offers several advantages, including tunability, single-frequency operation, compactness, and high shunt impedance. In this manuscript, physical and mechanical design of this TDC as well as the planned proof-of-principle experiment will be presented in detail.

In recent years, with the development of powerful THz source technologies, THz structures are widely utilized for electron beam manipulation, such as acceleration, deflection, compaction and diagnostics. Taking the bunch length measurement as an example, combining with high field strength and high resonant frequency, the THz structure based deflector could reach femtosecond or even sub-femtosecond resolution. In this paper, a 0.1THz Fabry-Perot resonator based structure will be introduced, which could provide time-dependent deflection for short electron beam to resolve the bunch length with high resolution. By adjusting the relative orientation of the beam direction and the E-field direction of the incident THz source, this structure is also potential for beam acceleration.

This tutorial addresses the realm of electrical, hybrid and specifically optical schemes for achieving a facility-wide synchronisation on the femtosecond level at free-electron lasers (FELs). After a brief introduction to the fundamental principles behind FEL operation and the significance of synchronisation for fully utilising their capabilities. Subsequently, it discusses various methods employed to achieve femtosecond-precision synchronisation, including low-noise timing references, different active stabilisation techniques, and advanced feedback algorithms. In addition, the tutorial provides an overview of the numerous challenges encountered in femtosecond optical synchronisation and solutions developed to overcome them. It discusses technological developments, such as ultra-stable optical lasers or timing diagnostics both for optical pulses and electron beams. Moreover, practical considerations for implementing such systems in FEL facilities are addressed, including stability requirements, scalability, and integration with experimental setups. Results from recent studies highlighting successful synchronisation implementations at prominent FEL facilities are presented.

Photonic time-stretch is a powerful method for recording electro-optic signals with terahertz bandwidth and high repetition rates. The method consist of modulating a chirped laser probe with the signal of interest. Then, the laser pulse is stretched it in time up to several nanoseconds, so that it can be read using an oscilloscope or ADC board. This technique has been shown to be efficient for monitoring the dynamics of Coherent Synchrotron Radiation (CSR) at SOLEIL, and to study electron bunch shape dynamics at KARA. However, the use of this technique has been strongly limited by the need of high bandwidth and costly oscilloscopes required for the readout. We present here a new design that allows a considerable reduction of the required oscilloscope bandwidth. A key point consists of using the 1550 nm wavelength for the probe. We will also present results obtained at SOLEIL, where THz pulses have been recorded, in single-shot and at MHz repetition rates, using an oscilloscope and ADC board with 1 to 3 GHz bandwidth.

The optical beam diagnostics at the BESSY II light source in Berlin have been improved significantly over the last few years. In particular, the streak-camera system has been extended in precision and sensitivity to allow two-dimensional imaging in time and space for equilibrated and non-equilibrated bunch patterns. In this paper, we prove experimentally and theoretically that we have reached a sub-ps RMS total time resolution using filtered synchrotron light. Detailed simulations, including the different physical time-dispersion mechanisms, show the influence of various band-pass and edge wavelength filters on the resolution. The limits for unfiltered near-visible synchrotron radiation (white-light) and the band-pass filter to achieve optimal time resolution are derived as well, providing a basis for more advanced beam-dynamics studies in the near future. (NIMA, 1062, May 2024, 169196)

Clarifying the longitudinal phase space distribution at the exit of a radio frequency quadrupole (RFQ) is crucial for precise beam tuning to minimize beam loss in a high-power superconducting linac. In this contribution, we introduce a method for direct measurement of the longitudinal emittance of a proton beam at the RFQ exit, which delivers an output energy of 1.51 MeV. Initially, we developed a bunch shape monitor (BSM) inspired by Feschenko’s design, achieving a resolution of 20 picoseconds. To conduct the direct measurement of longitudinal emittance, we integrate this BSM with a waist-to-waist beam transfer matrix, an energy-spread dipole, and a horizontal slit with a 0.2 mm width. The horizontal slit is positioned at the first waist at the dipole’s input, while the BSM wire is situated at the second waist, at the dipole’s output. This arrangement, enhanced by the waist-to-waist transfer matrix, improves the energy spread resolution to 0.01%. Using the BSM wire, we measure the energy spread and horizontal profile. Through adjusting the buncher voltage and synchronous phase, we use dipole and BSM to measure different longitudinal emittances and ascertain the effects of bunching and debunching conditions on the longitudinal phase space. Consequently, this comprehensive direct measurement setup for longitudinal emittance serves to elucidate the impact of RFQ and buncher on the longitudinal phase space distribution within a medium-energy beam transport (MEBT) system.

The shot-to-shot and non-invasive monitoring of the electron bunch length in a linac driven Free Electron Laser (FEL) relies on Bunch Compressor Monitors (BCMs). A BCM is designed to detect – and fully integrate in a given wavelength band - the radiation energy spectrum emitted at the threshold of the temporal coherence by the electron beam while crossing the last dipole of a magnetic chicane or a diffraction radiation screen placed just downstream of it. The BCM signal response is hence a direct - albeit non-absolute - function of the electron bunch length and of the beam charge as well. Due to its full non-invasiveness, a BCM is the ideal diagnostics to be integrated into the machine feedback to stabilize the bunch compression. Recently, we presented (*) a formal method for the absolute determination of the electron bunch length from the analysis of the signal readout of a BCM which is equipped with two independent detectors integrating the radiation energy pulse in two different wavelength bands. Theoretical highlights of the method as well as experimental results on the characterization in SwissFEL of electron beams with sub-fs bunch length will be presented in this contribution.

Accurate calibration of beam synchronous phase is essential for the optimal operation of accelerators. Traditional methods, such as the "phase scan method," not only consume significant machine runtime but are also susceptible to environmental disturbances. DESY has introduced a novel method based on "transient beam loading effects" for calibrating synchronous phase. However, this method requires the RF system to operate in an open-loop mode, limiting its applicability in proton linear accelerators. In this paper, leveraging the classical cavity differential equations, we propose a new method based on the steady-state "vector diagram of beam-induced voltage "for calibrating beam phase. This method enables on line calibration of beam phase and beam current under closed-loop operation of the radio-frequency cavities. We validated our approach on the CAFe (the Chinese ADS Front-end proton facility) at the Institute of Modern Physics, China, and the European Spallation Source. The measurement errors for beam current and phase using our method and the beam diagnostic system were 2% and within ±1 degree, respectively. Experimental results confirm the effectiveness of our method as a new solution for on line calibration of beam synchronous phase.

For Time Of Flight (TOF) experiments in the Rare isotope Accelerator complex for ON-line experiments (RAON), specifically at the Korea Broad Acceptance Recoil spectrometer and Apparatus (KoBRA) and the Nuclear Data Production System (NDPS), the beam repetition rate must not be excessively high, and the bunch length needs to be suppressed. The pre-bunching and re-bunching systems will be operational to achieve these objectives. To measure the bunch shapes near the production targets of KoBRA and NDPS without disrupting the beam, we optimized and manufactured capacitive pick-up monitors for installation upstream of the production targets. Furthermore, an algorithm was developed to reconstruct the shape of non-relativistic ion beams using experimental results measured by capacitive pick-up type monitors. An experiment using bunched hydrogen ion beams was conducted to verify the pick-up monitors' capability to measure bunch shapes. This presentation discusses the design methodology, simulation results, and bench tests of the capacitive pick-up monitors for ion beams with non-relativistic speed and nanosecond-order bunch lengths. Additionally, experimental results using hydrogen ion beams are also presented.

This paper proposes a longitudinal phase space measurement and reconstruction technology of particle beam in a storage ring. The technology collects and analyzes the beam injection signals by a high-speed oscilloscope, so as to extract the phase and beam length information of the injected beam. The length of a single data collection covers several thousand circles, the measurement accuracy of the phase reaches 0.2ps, and the measurement accuracy of the beam length reaches 1ps. At the same time, we develop a single beam tracking software based on the mbtrack2 software package. The simulation software can record the phase space evolution of the beam after injection under different initial conditions. By matching with the simulation results, we can get various initial parameters of the experimental beam, including the initial phase, the initial beam length, the initial energy deviation, the initial energy dispersion, and the initial injection angle of the beam in the phase space. This technology enables us to understand the kinetic behavior of the particle beam deeply and to monitor and adjust the injection system in real-time. By obtaining the phase space distribution information of the particle beam in real-time, we can find and correct the deviation and instability in the injection system in time, so as to improve the injection efficiency and the quality of the particle beam.

The knowledge of the longitudinal bunch shape is of high interest to accelerator performance optimization and advanced beam application. Attracted by the ability to continuously monitor the beam in real time, there is always a demand for bunch-by-bunch and non-invasive diagnosis. However, such diagnosis is difficult to achieve for proton beam with high intensity and high repetition. Using the principle of electron beam deflection, electron beam probe has the potential of multi-function beam diagnosis. Here, we proposed the concept of real-time longitudinal bunch shape monitor with photocathode DC electron gun. Considering the realistic bunch distribution, we investigated the feasibility of this monitor using particle tracking simulation. The results and analysis of feasibility are reported in this paper.

The timing jitter of electron bunch will affect the temporal and power stability of FEL, as well as the resolution of pump-probe experiment at FELs. In order to improve the time stability of electron bunch by beam feedback, Shanghai high-repetition-rate XFEL and Extreme light facility (SHINE) will employ the Electro-optic Bunch Arrival Time Monitor (EOBAM) to accurately measure the electron bunch arrival time. This paper will introduce design of EOBAM, including the beam pick-up, the electro-optic front-end, the signal detection electronics and the high-level software. Then the latest research progress of the EOBAM for SHINE and the beam experiment for EOBAM prototype based on SXFEL will also be introduced. The experiment results show that the resolution of EOBAM based on the 18 GHz beam pick-up are 5.4 fs@200pC and 8.1fs@100pC.

The Novosibirsk Free Electron Laser (NovoFEL) is a powerful source of narrow-band terahertz and infrared radiation, operating at the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (INP SB RAS). It is based on an accelerator-recuperator system and is one of the main user facilities of the Siberian Synchrotron and Terahertz Radiation Center. In recent years, there has been active work to develop new diagnostics for measuring the parameters of the electron beam in the third stage of the NovoFEL. The laser generates pulses of radiation with picosecond durations in the mid-infrared range of 8-12 micrometers that is the challenge for the diagnostics. This paper describes the development of diagnostic systems for the spectral and temporal characteristics of laser radiation from the third stage of the NovoFEL facility. To record the radiation spectra, a diffraction monochromator was used in conjunction with a bolometric array as a detector. A nonlinear autocorrelator based on ZnGeP2 crystal was developed to measure the temporal profile of the radiation. The correct operation of the autocorrelator was demonstrated in experiments with YAG laser radiation acquired using a nonlinear β-BaB2O4 crystal. The paper presents results from measurements of the spectrum and autocorrelation function for laser radiation from the third stage of the NovoFEL. Self-consistency between the spectrum and autocorrelation functions is demonstrated.

Modern accelerator-based light sources rely on short bunches to generate intense photon pulses. To achieve this, the electron bunches from the accelerator need to be compressed longitudinally in a magnetic chicane. A valuable tool for the measurement of the signal in the bunch compressor is the use of broadband EM-detectors covering a spectral range from few 100 GHz up to THz frequencies. With this setup, bunch length variations caused by instabilities in the acceleration process can be measured that in turn also affects the secondary photon beam. In this paper, we demonstrate the pre-commissioning of broadband, room temperature Schottky THz detectors for the diagnosis of compressed short electron bunches at the TELBE facilities at the Helmholtz-Zentrum Dresden-Rossendorf, Germany. Qualitative bunch compression measurements have been carried out to diagnose the beam to optimize the machine setup and provide feedback to the beam-line scientists for optimum machine operation. These detectors are scheduled to be commissioned at free-electron facilities in near-future.

Bunch shape monitors based on the transverse modulation of low energy secondary emission elec-trons, will be used in the measurement of longitudinal beam density distribution in the upgrade of CSNS-II linac. A test bench for commissioning the 324MHz RF deflectors used in BSM has been built in the laborato-ry, which consists of a Kimball E-gun, a vacuum chamber for electron optics, an RF stimulator, a 324MHz RF power source, HV power supplies, a bending magnet and a set of MCP+Screen+camera+DAQ. This paper gives the design consideration, some results of the test bench and the continuing CST design of a λ/2 RF deflector.

Ultrafast micro-bunched electron beams have broad applications, including wakefield-based acceleration and coherent Terahertz sources, where precise diagnosis of individual bunch-to-bunch spacings is critical. However, high-precision direct measurements of these spacings remain challenging. This paper introduces a novel method capable of measuring these spacings with femtosecond temporal resolution using a THz-driven resonator. Simulations on a 3 MeV electron bunch train demonstrate a temporal resolution better than 10 fs for the bunch-to-bunch spacings. This method facilitates in-depth investigation of the longitudinal characteristics of the bunch train, promising significant advancement in narrow-band Terahertz sources and compact accelerators.

The jitter of the beam arrival time can significantly impact the synchronization between the seed laser and the electron beam, which will constrain the brightness and stability of the FEL. It is one of the important parameters for beam diagnostics. To align with the SHINE's (Shanghai HIgh repetition rate XFEL aNd Extreme light facility) requirements of a 1MHz repetition rate and a dynamic range from 10pC to 300pC, we developed a beam arrival time measurement system utilizing a cavity probe. This system is capable of achieving a specification of 20fs at a charge of 100pC. Our approach included designing measurement schemes based on intermediate and radio frequencies and establishing a comparative test platform at the SXFEL (Shanghai Soft X-ray Free-Electron Laser Facility). This article will detail the construction of two systems and compare their test results across various charges. It has also been confirmed that the system can accurately measure beam arrival times for charges less than 1pC. A major challenge identified was temperature drift, which significantly affects measurement accuracy and limits the system's application in beam feedback. To counter this, we implemented and evaluated constant temperature controls for the RF cables, demonstrating their effectiveness in enhancing measurement reliability.

Phase stable coaxial cables are widely used for the transmission of reference signals, monitoring signals and control signals in accelerator Low-level RF, beam measurement and control systems, especially for high requirements of time/phase stability. The change in ambient temperature will change the electrical length of the coaxial cables leading to the transmission time and signal phase drift, this effect is termed as temperature coefficient of delay (TCD). The TCD curves at room temperature (15~40°C) of various types of coaxial cables commonly used in particle accelerators and other industries are measured. Some cables are tested for the first time. The cables with lowest coefficients are CommScope LDF2-50A, Zhongtian HCAAYZ-50-12 and Trigiant HCTAYZ-50-22, for different cable diameters. According to attenuation, mechanical and TCD parameters, these three cables are chosen in the HEPS phase reference line system and Linac LLRF system respectively.

Chinese Academy of Engineering Physics terahertz free electron laser facility (CTFEL) is a superconducting linac-based user facility. It provides laser pulses with frequencies from 0.1 THz to 4.2 THz. CTFEL works in pulsed mode with a repetition of 10 Hz where up to about 54000 bunches at a bunch spacing of 18.5 ns are accelerated in one macro-pulse. To satisfy the high-precision synchronization requirement from user experiments, the synchronization system based on coaxial lines is updated to a continuous laser carrier and Michelson interferometer-based system. The timing system is updated to the event system.

Monitoring the beam length and maintaining stability during operation is crucial for Free Electron Laser (FEL) user facilities. A monitor based on Coherent Synchrotron Radiation (CSR) is an ideal candidate, and it has been successfully developed at the Shanghai X-ray Free Electron Laser (SXFEL). This article presents the basic principles, system configuration, and experimental results. The results show that the monitor is capable of measuring the bunch length within a range of 0.6 ps to 1.4 ps, achieving a precision better than 10%.

Online beam monitoring is crucial for enhancing the efficiency and availability of high-power accelerator operations. While real-time monitoring of transverse beam parameters is commonly employed in modern accelerators, there is a scarcity of online measurement techniques for longitudinal beam characteristics. We are currently developing an online tool for measuring fundamental longitudinal beam parameters: synchronous phase and energy gain. This endeavor is founded upon a comprehensive understanding of beam-RF cavity interactions, facilitated by advanced hardware platforms, flexible software applications, and computationally intensive algorithms. Validation of our measurement methods has been conducted using beam and RF data acquired during the latest beam commissioning at the European Spallation Source (ESS). This validation encompassed both single-cell and multi-cell cavities, affirming the reliability and feasibility of our techniques. Furthermore, comprehensive comparative analyses were performed, aligning results from various measurement methodologies with theoretical calculations, enhancing our understanding of measurement accuracy. Our ongoing research aims to provide accelerators with robust and real-time monitoring tools for longitudinal dynamics aspects based on beam and RF cavity interaction, thereby ensuring optimal efficiency and performance in high power accelerator operation.

In order to achieve nondestructive measurement and feedback of beam energy and energy spread for high repetition frequency Linac, an eight-stripline beam energy and energy spread monitor have been designed to replace destructive monitor sunch as fluorescent screen. Different from the conventional evenly arranged stripline structure, an unevenly arranged stripline layout is proposed to improve the sensitivity. At the same time, impedance transition structures are added to the Feedthroughs and striplines connection parts to further enhance the system sensitivity and resolution. The electronics adopts the method of separating the analog front-end and digital acquisition part from each other, and has the function of bunch-by-bunch measurement and data storage with a high repetition frequency of 1 MHz. The processing of the monitor has now been completed. Experiments show that the position sensitivity with an inner diameter of 63 mm reaches 0.0538 $mm^{-1}$, which is close to the theoretical result of numerical calculation. Compared with monitors of traditional structure, the performance has been greatly optimized.

The non-uniformity, longitudinal oscillations, and space charge effects in a multi-bunch filled electron stor-age ring can lead to significant deviations in the meas-urement of longitudinal beam parameters. Selecting a single bunch for measurement can effectively improve the measurement accuracy of longitudinal beam parame-ter, under normal multi-bunch operation mode. This pa-per introduces an optical pulse selection system based on an RTP crystal Pockels cell, which is controlled by fast electronics and high-voltage electronics, to study the complex longitudinal beam dynamics. By adjusting the driving voltage frequency and trigger delay of the high-voltage driver, precise selection of single pulses in a multi-bunch filling mode can be achieved. Offline cali-bration experiments have verified the potential feasibility of selecting specific bunches or bunch trains within the multi-bunch operation, which is of great significance for diagnosing longitudinal characteristics and instabilities of the beam in electron storage rings.

The Shanghai soft X-ray Free-Electron Laser facility (SXFEL) is a fourth-generation linac-based light source, capable of producing X-ray pulses with duration of tens of femtosecond. The photocathode laser and the seed laser for external seeding FEL therefore have tight requirements for relative arrival time to the machine and electron bunch. To reach required energy and wavelength to drive photocathode, as well as for external seeding FEL, further optical amplification and frequency conversion is needed. the femtosecond-stable pulsed optical reference, which are delivered via fiber length stabilizers. In this paper, we present the development status of the fiber length stabilizers for the laser-arrival time measurement.

The Novosibirsk Free Electron Laser (NovoFEL) facility consists of three free electron lasers (FELs) installed on different tracks of the Energy Recovery Linac (ERL). These FELs share a common acceleration system, which allows for the generation of high average electron currents, typically around 10 mА. This high current facilitates the production of significant average FEL powers, often exceeding 100 watts in the spectral range between THz and mid-infrared wavelengths. Precise measurement of electron beam parameters is crucial for monitoring the performance of the accelerator and fine-tuning its operating modes. The length of the electron bunch is particularly important, as it directly influences the efficiency of laser radiation generation. This study focuses on the dependence of the electron bunch length on the parameters of the radio frequency (RF) and bunching systems for the first and second FELs at NovoFEL. Measurements were conducted using a Cherenkov aerogel radiator in conjunction with a streak camera to accurately determine the electron beam properties. The measurement results, along with a plan for future experiments, are discussed in detail in this publication.

With the refurbishment completed, the optical beamline delivers all the required diagnostics. This paper reports on their status focusing in particular on the R&D beamline branch. The additional branch is equipped with programable mirror and lens position controllers allowing elaborate optical optimisation. This system is used for educational purposes and for improving the source point imaging system through the study of polarisation characteristics. Test systems for an ultra-fast diode and a THz detector are equipped with CMOS cameras and polarisation filters. Furthermore the R&D branch complements the existing diagnostics to measure bunch lengths and investigate non-linear beam dynamics.

This paper describes the optical beam diagnostics available at the BESSY II booster synchrotron. For the first time, diagnostics are established to investigate the distribution of the electron beam in all three dimension. A permanent installation of a source-point imaging system aided by a telescope optic depicts the transverse properties of the electron beam. Additionally, the bunch length is measured using a streak camera with a resolution in the picosecond range. Both systems can work in parallel and are able to observe the non-equilibrium beam dynamics over the entire booster ramp.

Laser-to-RF synchronization plays a crucial role in various scientific and technological domains. It is instrumental in generating high-quality electron beams, producing high-performance FEL pulses, ex-ploring ultrafast dynamical processes, and achieving precise measurements and transmission. Passively mode-locked femtosecond lasers are known for their exceptionally low noise characteristics, particularly in the high-offset frequency range, where jitter remains less than 5 fs from 1 kHz to 1 MHz. Meanwhile, RF master oscillators provide outstanding long-term sta-bility in the offset frequency range. This paper demon-strates that integrating the low-noise performance of passively mode-locked femtosecond laser with the superior stability of RF master oscillator enables the achievement of 10-fs-level synchronization. By im-plementing an RF-based phase-locked loop (PLL) scheme, we achieved an absolute timing jitter of 17.4 fs integrated from 10 Hz to 1 MHz.

We present the experimental results of the longitudinal phase space measurement using the well-known wakefield deflector driven by the dechirper. When the electron bunch travels through the dechirper, electrons at the head of bunch generate the strong transverse wakefield which forces the trailing electrons to be transversely streaked. In such a way, the temporal structure of bunch can be reconstructed by analyzing the distribution of transverse profile at the downstream of dechirper. In the soft X-ray line of the PAL-XFEL (Pohang Accelerator Laboratory, X-ray Free Electron Laser), the dechirper composed of 1.4-meter-long corrugated metallic walls streaks the bunch horizontally via the wakefield. By combining with the bending magnet having the vertical dispersion, the longitudinal phase space of electron beam can be interpreted as the spatial distribution at the screen monitor. We show the results of the longitudinal phase space measurement using the wakefield deflector.