Talk will cover state-of-the-art photocathodes for bright-beam and spin-polarized-beam generation.

Neutron scattering is an indispensable technique in material science research for providing solutions to important engineering challenges, including the ever-growing demand for more efficient batteries and fuel-cells. There are, however, limitations in the access and availability to the necessary neutron beams and this is worsening as nuclear research reactors continue to shut down. As a result, there appears to be market demand for an affordable, medium-flux, compact, accelerator-driven neutron source optimised for deployment in an industrial setting. In this paper, we present an overview of the beam specification and the high-level design choices for an electron linear accelerator that is optimised to drive such a facility.

RadiaSoft is developing machine learning methods to improve the operation and control of industrial accelerators. Because industrial systems typically suffer from a lack of instrumentation and a noisier environment, advancements in control methods are critical for optimizing their performance. In particular, our recent work has focused on the development of pulse-to-pulse feedback algorithms for use in dose optimization for FLASH radiotherapy. The PHASER (pluridirectional high-energy agile scanning electronic radiotherapy) system is of particular interest due to the need to synchronize 16 different accelerators all with their own noise characteristics. This presentation will provide an overview of the challenges associated with RF tuning for a PHASER-like system, a description of the model used to evaluate different control schema, and our initial results using conventional methods and machine learning methods.

In electron linear accelerators, the improvement of the acceleration gradient of the acceleration structure has been a continuous research topic for scientists, which can reduce the construction cost of the entire accelerator by increasing the accelerating gradient. For the CEPC and HEPS projects at IHEP, S-band 3 meters long and C-band 1.8 meters long accelerating structure has been devel-oped. The operating frequencies are 2860 MHz, 2998.8 MHz and 5720MHz respectively. CEPC linac is 30 GeV with S & C-band structures in the TDR phase. The high-power test gradient of S-band accelerating structure reach the 33MV/m. The C-band structures also designed and waiting for high power test. HEPS is 500 MeV linac in-jector and already conditioning for one year. The maxi-mum gradient achieved with the beam during commis-sioning was approximately 26 MV/m with a beam current of 7 nC. During actual operation, it has been functioning at around 20 MV/m. The electron beam has remained stable up to the present time.

Wakefield structures are broadly employed in free electron laser (FEL) facilities for beam manipulation. Compared with cylindrical geometries, planar structures are typically preferred due to their increased flexibility, allowing for tunable wakefield strength through gap adjustment. However, these planar configurations can induce time-dependent quadrupole wakefields, which require careful compensation in various applications. To address this issue, we propose a novel structure design incorporating four identical corrugated elements which are independently controllable. By adjusting the gaps between orthogonal pairs, the quadrupole wakefield can be either fully compensated to avoid emittance growth or significantly amplified to enhance beam mismatch for slice lasing control. This manuscript presents both the physical and mechanical design of the proposed structure, as well as the planned proof-of-principle experiment.

This work describes a C-band RF system for the SAPS (Southern Advanced Photon Source of China) test bench linear accelerator.SAPS' RF testing system comprises of a photocathode electron gun and a 2-metre-long equal gradient acceleration device.The klystron power source delivers energy to the photocathode electron gun and the travelling wave acceleration structure,respectively.Test the photocathode electron gun first,followed by the travelling wave acceleration structure.We investigated a short-pulse C-band spherical pulse compressor.The photocathode electron gun's preliminary high-power testing is now complete.

FERMI is the seeded free electron laser (FEL) user facility at Elettra laboratory in Trieste, operating in the VUV - soft X-ray spectral range. In order to extend the FEL radiation to shorter wavelengths, an energy increase from 1.5 GeV to 2.0 GeV is required in the linear accelerator (linac). This result is achievable by replacing the present old sections with the newly designed accelerating sections that can work at high gradient with lower transverse wakefields. A new high-gradient (HG) module was build and installed at the FERMI linac. We report here the recent experience on the conditioning and the results on the e-beam energy gain in operation.

Corrugated structures have recently been utilized for the time-resolved diagnostics of electron bunches and free-electron-laser (FEL) pulses across several FEL facilities: SwissFEL at PSI and European XFEL at DESY. This approach is simple and cost-effective, based on the self-streaking of electrons with a transverse wakefield enhanced in such structures. In this work, we optimize the design of a corrugated streaker for the wide range of beam parameters of the CERN Linear Electron Accelerator for Research (CLEAR) at CERN. We report on the fabrication of corrugated plates with various corrugation parameters and their initial installation for in-air measurement at CLEAR. Variable polarization streaking can be achieved either by mechanically rotating the plates or by utilizing two pairs of corrugated streakers. Additionally, we emphasize that when streaking in the vertical (or horizontal) direction with one structure, the undesired quadrupole wakefield can be compensated by the second orthogonally oriented streaker. This allows for a significant improvement in the resolution of the method.

The concept of a compact linear accelerator for industrial application suggested in reference* is based on the use of SRF cavities. The design of a thermionic electron source which can either be directly connected to a superconducting cavity or be part of a normal conducted injector cavity is described. The direct connection option is applied in a prototype 1½ cell 650 MHz SRF cavity capable of delivering a 12.5 mA average beam current with a beam power of 20 kW which is currently being developed at Fermilab. As an external option we present the development of a CW normal conducting 1.3 GHz RF injector which consists of a gridded RF gun integrated with the first cell of a copper booster cavity. The electron source concept is presented including the cathode-grid assembly and the gun resonator design. For the first case we considered thermal insulation of the cathode from the cavity, the cavity thermal load caused by the gun, including the static heat load, black body radiation, backward electron heating, etc. For both projects we present the results of beam dynamics optimization, RF, thermomechanical, and engineering designs.

Coherent synchrotron radiation has a significant impact on electron storage rings and bunch compressors, inducing energy spread and emittance growth in a bunch. Calculating the effects are computationally expensive, severally limiting the use of simulations. Here, we explore utilizing neural networks (NNs) to model the 3D wakefields of electrons in circular orbit in the steady state condition. NN models were trained on both Gaussian and more general bunch distributions, which evaluate much faster than physics-based simulations. Here, we explore how well the models generalize, by testing their ability to 1) extrapolate to Gaussians with smaller/larger widths 2) predict on distributions never encountered before (out of distribution generalization) using smoothed uniform cubes. We see the models are able to generalize, which makes them potentially useful in the design and optimization of accelerator apparatuses by enabling rapid searches through parameter space.

Space-borne accelerator technologies suffer from significant electron beam loss during beam acceleration and excessive energy spread of the output beam. LANL is proposing a deployable and compact solution using electrostatic potential depression (EPD) to achieve higher bunching, lower beam loss, and smaller energy spread. This buncher system involves the use of three EPD sections, each electrically insulated from the bunching cavities and with a separate high voltage power supply, whose leads will have to reach through vacuum and the insulator to bias the specific section of the buncher. This presents considerable challenges due to the triple junction problem and the presence of parasitic radio frequency fields leaking through the insulating material.

Gun5, the new generation of high-gradient normal conducting 1.3 GHz RF guns for linac driven free-electron lasers like FLASH and European XFEL is under development at the Photo Injector Test facility at DESY in Zeuthen (PITZ). Its improved cell geometry and cooling concept allow for RF pulse durations of up to 1 ms at 10 Hz repetition rate, at gradients of ~60 MV/m at the cathode. Gun5 is also equipped with an RF probe for measurements of the RF field inside the gun. The first gun of this type, Gun5.1, is in operation at PITZ since April 2022. Gun5.2 will be commissioned at the FALCO conditioning facility at DESY in Hamburg, starting in June 2024. This gun is equipped with a balanced (symmetric) RF waveguide feed to the coaxial power coupler to prevent a coupler kick and thus improve the beam quality delivered by the electron source. Further guns are currently in the manufacturing process. In parallel, studies towards a more reliable cathode spring design are ongoing, in order to overcome observed issues during the high duty cycle operation of Gun5.1. This article will give an overview on all those developments.

The FLASH 2020+ project at DESY includes, among other modernizations, an upgrade of the electron beam energy. Two accelerator modules were replaced and the RF distribution of the other modules was optimized. The limiting factors such as cavity quenching and field emissions are identified and measured at acceleration modules. At a later stage, based on those measurements, a high-power distribution adjustment scheme was proposed and the optimal operating point was demonstrated to achieve the design energy of 1.35 GeV with the nominal RF pulse length at FEL lasing conditions. After proper optimization and tuning of the low-level RF parameters, the linac successfully operated at maximum energy and delivered SASE-FEL radiation in the wavelength range below 3.2 nm. The measurement results as well as the achieved cavity gradients with energy gains are presented.

It has been found in benchmark tests that some Single Spoke Resonator Type-2 (SSR2) cavities have early field emission onset as well as strong multipacting barriers. A longstanding hypothesis is that field-emitted electrons in the high electric field accelerating gap can migrate and ignite multipacting bands in the low electric field regions of the cavity periphery. In this study, we use simulation techniques to examine multipacting behavior in SSR2 cavities from electrons seeded in common field emitter locations. Additionally, we investigated seed locations for areas in SSR2 cavities which may have poor coverage during high pressure water rinsing and compared the multipacting behavior.

The SPIRAL2 superconducting LINAC accelerates beams of different species, in a large energy range. During operation, the beam requested by the physics can change quite often and it is mandatory that beams that have been already tuned can be obtained again by simple application of the machine parameters already used. This reduces the accelerator retuning time and increases the machine availability for the physics experiences. Voltages and more particularly phases of all the cavities are among the crucial parameters for a quick retuning. Proper beam tuning is monitored via the Beam Position Monitors. This paper focuses on the phase issues, reminds the way the reference frequency distribution, the LLRF and the BPM works and are used in the tuning procedures, and summarizes the upgrade foreseen to improve the cavity phase setting reliability

The KEK injector linac injects high-charge electron and positron beams into the high-energy-ring and low-energy-ring of SuperKEKB respectively. The linac also injects electron beams to the two light source rings, PF ring and PF-AR. We operate simultaneous top-up injections into the four rings by using many pulsed magnets. We have been upgrading the linac to attain the higher-quality beam injections for the SuperKEKB rings. In the summer of 2023, large-aperture quadrupole pulsed magnets have been newly installed upstream of the linac and driven by new large-current pulse driver. The power of the new pulse driver is 600 A 400 V and is energy recovery type. We achieve high efficiency with simple pulse width control. I would like to introduce this high-power, high-efficiency pulse driver.

Transverse Deflecting Structures (TDS) are commonly used in Free Electron Laser (FEL) facilities for the measurement of longitudinal information of electron beam, including bunch length, temporal distribution, slice emittance, etc. Shenzhen Superconducting Soft-X-ray Free Electron Laser (S3FEL) is a high-repetition-rate FEL recently proposed for scientific research and applications. In S3FEL, TDSs that work at S-band (2997.222 MHz) and X-band (11988.889 MHz) will be utilized for the diagnosis and analysis of longitudinal phase space of electron bunches along the beamline. In this manuscript, we present the preliminary design of both S-band and X-band TDS systems of S3FEL, including system layout, deflecting structures, pulse compressors, RF distribution networks, etc. Additionally, we introduce a new parallel-coupled TDS cavity with variable polarization for multi-dimensional phase space diagnostics.

The performances and failure cases of the power couplers of the IFMIF/EVEDA RFQ and ESS DTLs have been analyzed with dedicated high-power test campaigns and multipacting simulation methods. The paper presents test and simulation methodology, results, and inputs for the next activities.

FLASH Therapy, a novel cancer treatment technique, aims to control the tumor-grown sparing the healthy tissue from radiation damage, increasing the therapeutic index. Translating FLASH therapy into clinical practice, especially for treating deep-seated tumors, necessitates achieving Very High Electron Energy (VHEE) levels within the 50-150 MeV range [2]. In the framework of the SAFEST project [3–7], Sapienza University, in collaboration with INFN, is actively developing a compact C-band linac demonstrator at the energy of 24 MeV (loaded) with a 100 mA peak current. This paper provides insights into the design strategy and electromagnetic characteristics, focusing on prototype testing and tuning conducted at the Sapienza Accelerator Laboratory. The progress of this innovative linac represents a step toward realizing an advanced FLASH VHEE source in cancer treatment.

We are developing a laser-driven ion accelerator aimed at downsizing heavy ion therapy devices. The ion beam produced by this accelerator exhibits low emittance(transverse emittance is approximately 10-3 π mm-mrad and longitudinal emittance is approximately 10-5 eV・s), with a very short pulse width (about picoseconds). As a result, the peak current reaches the kA level. However, explosive beam divergence is mitigated by co-moving electrons that neutralize the beam’s space charge in the high-density region immediately following acceleration. This study involved acceleration calculations and transport calculations of proton beams over 40 cm (up to just before the quadrupole magnet) using the Par-ticle-in-Cell (PIC) simulation code to assess the ion beam's space charge neutralization characteristics. This presentation will show the results of our simulations using the PIC code, which analyzed the degree of neutralization by co-moving electrons. The results suggest the potential for optimizing target thickness when utilizing of specific energy ions produced by laser-driven ion acceleration. The results suggest confirmation of the space charge neutralization phenomenon in the laser-accelerated ion beam.

Modern CW or pulsed Superconducting RF (SRF) accelerators require efficient RF sources controllable in phase and power with a reduced cost. Therefore, utilization of the high-power CW magnetrons as RF sources in SRF accelerator projects was proposed in a number of works. But typically, the CW magnetrons are designed as RF sources for industrial heating, and the lifetime of the tubes is not the first priority as it is required for high-energy accelerators. The high-power industrial CW magnetrons use the cathodes made of pure tungsten. The emission properties of the tungsten cathodes are not deteriorated much by electron and ion bombardments, but the latter causes sputtering of the cathode in the magnetron crossed fields. The sputtered cathode material covers the magnetron interior. That lead to sparks and discharges limiting magnetrons lifetime. We considered an analysis of magnetron failure modes vs. output power, developed a model of ionization of the residual gas in the magnetrons interaction space and simulated the spattering of the cathode in 100 kW CW magnetrons to estimate the life expectancy. Basing on results we proposed ways to increase the CW magnetrons longevity for SRF accelerators.

The mid-Infrared region (2-5 um) is currently a frontier of laser science with short durations, where many molecular absorbing spectrums exist. The oscillator free electron lasers have advantages against solid-state laser systems, that include the fundamental generations of high-intensity mid-IR pulses with femto-seconds scale short duration, continuous variations of the central wavelength, and the high-repetitions of pulses due to RF accelerations of electron bunches. Especially, the coexistence of high-intensities and high-repetitions at GHz scales is important for the development of mid-IR frequency combs that may open up a new direction of molecule nonlinear reactions. In this presentation, we report on the importance of phase-locking between FEL pulses that grow up independently due to shot noises for the mid-IR frequency combs, and the states of development of a test phase-locking system, and introduce possible applications of the mid-IR frequency combs.

Producing bright electron beams is crucial for coherent light sources, where increasing the peak current is typically accomplished through bunch compression in magnetic chicanes. Alpha magnets, with their unique phase-space manipulation capabilities, have emerged as an attractive choice for compressing sub-10 MeV electron beams generated by radio frequency photoinjectors. This paper presents detailed numerical modeling of the beam dynamics of high-charge, bright bunches undergoing compression within an alpha magnet. The model incorporates space-charge effects and coherent synchrotron radiation, providing a comprehensive understanding of the complex interactions and behaviors of the electron beams during the compression process.

Machine learning (ML) tools have been growing in popularity for accelerator applications, but still struggle with time varying systems, for which they require lengthy brute-force re-training. LANL has developed generative machine learning (ML)-based tools, that utilize adaptive model independent feedback control theory together with hard physics constraints, to make the tools much more robust to distribution shift. These adaptive ML tools are able to extrapolate much further beyond the span of the training data and are thus much more robust for time-varying systems. This talk will give a broad overview of the challenges of various time-varying accelerator systems at various accelerator facilities (known as systems with distribution shift in the ML community) and will present adaptive ML tools for 6D phase space diagnostics of intense charged particle beams. The talk will give a general overview of diffusion-based generative models and also adaptive latent space tuning, which is the novel method we have developed for adaptive ML, and how we are strictly enforcing hard physics constraints in our ML tools, which traditional ML tools lack. We demonstrate our general methods for various accelerators: the 5-meter long ultra-fast electron diffraction (UED) HiRES compact accelerator at LBNL, the ~kilometer long plasma wakefield accelerator FACET-II at SLAC, and the LANL ion accelerator LANSCE.

The “FLASH” effect is currently a topic of considerable interest in radio-oncology. We present the design of a novel VHEE linac, to be built and installed at CHUV (Lausanne), capable of producing electron beams which deliver radiation at dose rates and time scales consistent with the FLASH effect. The design is based on X-band radio-frequency technology, developed at CERN for the CLIC study. The e-beam properties correspond to a CHUV specification and would allow large, deep seated, tumors to be treated. Construction of DEFT (DEEP Electron FLASH Therapy) will be assured by the company THERYQ in the context of a CHUV-CERN-THERYQ collaboration.

The Compact X-ray Light Source (CXLS) is a compact source of femtosecond pulses of x-rays that is now commissioning in the hard x-ray energy range 6-20 keV. It collides the electron beam from recently developed X-band distributed-coupling, room-temperature, standing-wave linacs and photoinjectors operating at 1 kHz repetition rates and 9300 MHz RF frequency with a Yb:YAG 1030 nm laser beam operating at high peak and average power at 1 kHz repetition rate with pulse energy up to 200 mJ. We present the performance of the CXLS accelerator, laser, and timing systems, and initial x-ray results.

The Laboratory for Electron Beam Research and Application (LEBRA) at Nihon University has been developing free electron laser (FEL), parametric X-ray radiation (PXR), and terahertz (THz) wave sources in collaboration with KEK and the National Institute of Advanced Industrial Science and Technology (AIST) using a 100 MeV electron linac. Each of these light sources is used for both internal and external collaborations. We are developing THz coherent edge radiation (CER), coherent transition radiation (CTR) and plane-wave coherent Cherenkov radiation (CCR) sources in the THz band for the FEL and PXR beamlines, respectively. In particular, we are developing THz wave sources using an artificial quartz hollow conical tube for the CCR source and a thin aluminum plate with a helical target surface for the THz-CTR optical vortex source. So far, we have performed parameter measurements, including beam profile and spectrum measurements, for the THz-CCR and the THz-CTR vortex beams. In this paper, we describe the development and characteristics of each THz wave source.

We have designed a tapered dielectric-lined waveguide for the acceleration of sub-relativistic electron bunches with THz-frequency electromagnetic pulses. We consider an example design based on a commercial 100keV electron gun and a THz generation scheme driven by a mJ-level regenerative amplifier laser system. With a 12μJ THz pulse we simulated acceleration of a 100keV electron bunch to 162keV with very low energy spread. A second example design shows energy doubling from 100keV to 205keV using a 22.5μJ pulse. The former of these two designs has been assembled for experimental testing. We also discuss methods to improve the efficiency of the design process using 1D particle tracking to provide better estimates of the initial geometry before optimization.

Dust particulates are always present to some degree inside the vacuum space of particle accelerators, causing a variety of issues. At the LHC, beam loss events have been linked to the interaction of charged dust with the proton beams. In superconducting rf cavities, dust contamination leads to field emission, limiting the accelerating gradient and causing damage to external beamline components. Facilities such as the SLAC LCLS-II and TRIUMF electron linear accelerator see progressive onsets in field emission that cannot simply be explained by vacuum events. The environment of a particle accelerator provides an ideal opportunity for dust to gain charge, which is one of the main drivers of dust grain dynamics in vacuum. However, fundamental parameters such as the dust composition and charge to mass ratio of these grains are unique to each accelerator environment and remain largely unknown. We will present an analysis of dust samples taken from TRIUMF linear accelerators, detailing their size, composition and potential sources. Preliminary results from experimental studies on the charging, detachment and migration mechanisms acting on micron sized particulates will also be presented.

Recent developments in laser wakefield accelerators (LWFAs) lead us to consider employing this technology to accelerate electrons at the Advanced Photon Source (APS) facility. Previous experiments using LWFAs were performed at Argonne using the Terawatt Ultrafast High Field Facility. The injector complex serving the APS begins with an electron linac, producing beam energies on the order of 450 MeV. We consider that the infrastructure developed at the Linac Extension Area (LEA) could be usefully employed to develop a new LWFA injector for the APS linac. In the present work, we outline the proposed parameters of an LWFA using approximately a 100-TW-peak laser pulse focussed into a few-mm in extent pulsed gas jet. We are targeting electron beam energies in the range 300–500 MeV. Initially, we would use the LEA quads, diagnostics and electron spectrometer to demonstrate performance and characterize the LWFA beam, before moving the LWFA to inject into the Particle Accumulator Ring (PAR).

We propose further investigations on the longitudinal-space-charge-impedance mechanism for inducing microbunching of relativistic electron beams within the Advanced Photon Source S-band linac. The microbunched content is evaluated by observing the coherent enhancements of optical transition radiation (COTR) generated as the beam transits a metal-vacuum interface. The facility also uniquely includes both thermionic cathode and photocathode rf guns as electron sources for comparisons of effects. Previously, we addressed mitigation of the COTR’s deleterious effects in the 2-D visible-light beam images at 325 MeV*. By extending our wavelength coverage into the NIR, we will access the much stronger enhancements predicted (>100)** and elucidate their spectral content. We will use an existing optical transport line for visible to NIR COTR (0.4 to 3.0 microns) from the diagnostics cube in the tunnel to an enclosed, external optics table. The inexpensive addition of a NIR-sensitive photodiode and integrating circuit with an existing digital oscilloscope in the optical setup would provide immediate extension of the detectors’ wavelength coverage and would enable the testing of the current model predictions for the microbunching instability into the NIR.

Free-electron lasers (FELs) send an accelerated electron beam through a magnetic undulator to provide a source of continuously tunable, short (10s of fs), high-peak power (GW-scale) radiation. FELs have found many applications, particularly in the infrared, extreme ultraviolet (EUV) and X-ray regimes. However, current EUV and X-ray FELs are large (100s of m) and expensive facilities, limiting the accessibility of these sources. In this work, we present FEL simulations driven by a compact accelerator combining high-gradient short pulse two-beam wakefield accelerators [1] and short-period superconducting undulators [2]. An FEL demo based on a GeV-scale accelerator is discussed as a driver for a water-window ( 2.3-4.4 nm) FEL with a ≈ 50 m length. Such a proof-of-principle integrated facility would serve the dual purpose of supporting user-based research in the water-window regime, and providing a proving ground for these new technologies to later be applied to shorter wavelength FELs. Here, we present early design and simulation efforts with a focus on FEL-process modeling.

The delivery of high RF power---from hundreds of kW to MW---by klystrons, is linked with a high overall energy consumption. A research programme led by CERN in collaboration with the industry is being conducted to understand what limits klystron efficiency and how to develop high-efficiency klystrons. As a result of this program, two first prototypes of X-band (11.994 GHz) high-efficiency klystrons have been successfully designed and manufactured in collaboration with Canon Electron Tubes and Devices. The first results look promising, revealing a remarkable ~60% efficiency, and validating the proposed HE klystron technology. A comprehensive characterisation campaign has been conducted at CERN to verify and demonstrate these results. The methodology for the HEK tubes characterisation is based in two independent measurements: a RF power measurement, and a calorimetric methodology ---less subject to calibration inaccuracies. We describe the setups, principle of the calorimetry methodology, and we discuss the feasibility and precision of the results.

The CERN Linear Accelerator for Research (CLEAR) at CERN is a user facility providing a 200 MeV electron beam for accelerator R&D and irradiation studies, including medical applications. In this paper we will outline the most recent improvements in CLEAR operation and beam control and delivery, and describe the upgrades under way, giving an update of their current status. These upgrades include a new front-end for the laser system which will enable an highly flexible time structure, better stability and higher repetition rates, and the implementation of a second beam line which will provide additional testing capability and whose optics has been designed to match user requirements. Finally, we will discuss the proposed future experimental programme of the facility, particularly in view of the novel capabilities provided by the upgrades.

Given the present availability of high-gradient accelerator technology for compact and cost-effective electron linacs in the 100-200 MeV energy range, the interest for Very High Energy Electron (VHEE) radiotherapy (RT) for cancer treatment recently reached an all-time high. Particular significance is assumed by the Ultra-High Dose Rate (UHDR) regime where the so-called FLASH biological effect takes place, in which cancer cells are damaged while healthy tissue is largely spared. VHEE beams from linacs are especially well adapted for FLASH RT, given their penetration depth and the high beam current needed to treat large deep-seated tumours. In recent years, several multidisciplinary user groups carried out a number of studies on VHEE and FLASH RT issues using the CERN Linear Accelerator for Research (CLEAR) user facility, in close collaboration with the local operation team. In this paper we give an overview of such activities and describe the main results of chemical and biological tests aimed at clarifying the damage mechanisms at the root of the FLASH effect and the relevant beam parameters needed to achieve it. We also describe the dedicated systems and methods developed and used in CLEAR for these activities, focusing on recent advances in the crucial aspects of uniform beam delivery and high dose rate real-time dosimetry.

For experiments requiring the longitudinal shaping of the beam at the exit of an electron linear accelerators, it is crucial to infer the initial beam profile at the entrance of the linear accelerator and key parameters. After passing through the dispersion section of beam bunch compressor, and the high-frequency system, the electron beam will undergo modulation on the longitudinal density. Based on the longitudinal dynamic process, this paper proposes to use automatic differentiation to provide the design of beam initial conditions and key parameters corresponding to a specific longitudinal profile of the beam at the exit of the linear accelerator. Finally, we implemented this method on a section of linear accelerator consisting of two L-band accelerating cavities, one S-band accelerating cavity, and a bunch compressor.

We have studied high-power terahertz-wave sources using a normal-conducting S-band linac at the Laboratory for Electron Beam Research and Application (LEBRA) at Nihon University [1, 2]. The developed coherent transition radiation (CTR) had a high energy of 1 mJ per macropulse [3]. However, the peak power of the CTR was approximately 100 kW and did not reach 1 MW, i.e., the level at which nonlinear optical phenomena are evident in the terahertz region. Therefore, we planned to generate high peak-power terahertz pulses by confining CTR micropulses in a ring-type resonator and superimposing them with CTR micropulses generated late within the resonator. By inserting a substrate with low absorption in the terahertz region into the resonator as an output coupler, it is possible to extract CTR pulses with high peak power while suppressing a cavity loss. In the presentation, we will report on this development plan based on the CTR pulse superimposition with the ring-type resonator at the LEBRA.

Conduction-cooled SRF niobium cavities are being developed for use in compact, continuous-wave electron linear accelerators for a variety of industrial applications. A 915 MHz two-cell cavity has been designed to achieve an energy gain of 3.5 MeV. The design of the cell shape aims at minimizing the peak surface magnetic field. Field flatness is achieved by adjusting the length of the outer end half-cells. The higher-order mode analysis shows that absorbers are not required for a moderate beam current of 5 mA. One of the beam tubes has two side-ports for insertion of coaxial fundamental power couplers. The mechanical design and analysis were done to maintain a stress near or less than 15.5 MPa for all anticipated loading conditions. This is half the measured yield strength and is to provide relief from creep when the cavity is evacuated and stored with outside atmospheric pressure.

RF long-term stability (drift) is as important as RF short-term stability for the stable operation of particle accelerators including PAL-XFEL. Increasing the performance of LLRF itself becomes an important factor in maintaining the long and short-term stability of the RF field. The reference tracking method applied to LLRF is effectively used as a method of reducing the drift of the RF phase. However, this drift improvement method was not applied to the RF amplitude. This time, the method of reference tracking was newly expanded to improve the RF amplitude drift. As a result of applying this new function to PAL-XFEL LLRF, it is showing some effect in improving the RF amplitude drift. We would like to share the progress so far.

The X-ray free-electron laser facility SACLA generates X-ray SASE up to 20 keV in a compact 700 m long machine using a low-emittance thermal cathode electron gun, a high-field C-band normal-conducting 8 GeV linac and short-period in-vacuum undulators. The next upgrade plan for SACLA is to increase the repetition rate of the accelerator, which is currently 60 Hz, by one order of magnitude to 1 kHz maintaining the performance of the current SASE and electricity usage. Challenge is how to achieve high repetition operation without increasing the electric power consumption, which allows to reuse the same accelerator building, electrical plant, and cooling water system. To improve the power efficiency, we choose X-band as the radio frequency of the main accelerator instead of current C-band. A basic design and optimization of the accelerator are undergoing. As a testbed, we plan to introduce an X-band transverse deflector cavity to measure the temporal distribution of the electron beam downstream of the undulator. The development of equipment such as RF sources, pulse compressors, dummy loads, low-level RF control, which are common to the systems for high repetition, has also begun. We will report the design and the status of developments.

The UK is conducting a multi-stage project to analyse the case for major investment into XFELs, through either developing its own facility or by investing at existing machines. The project’s 2020 Science Case identified a clear need for ‘next-generation’ XFEL capabilities including near-transform limited x-ray pulses across a wide range of photon energies and pulse durations; evenly spaced high-repetition rate pulses; and a high-efficiency facility with a step-change in the simultaneous operation of multiple end stations. The project is developing a conceptual design to meet these requirements, significantly aided by collaboration with international XFELs. It is also guided by an extensive ongoing user engagement programme of Townhall meetings and other activities. Both the science requirements and the emerging conceptual design are expected to be of general interest to the community.

Use of a cavity-based X-ray free electron laser (CBXFEL) is potentially a way to dramatically improve the stability and coherence of existing XFELs. A proof-of-principle project is underway as a collaboration between Argonne National Laboratory (ANL), The Institute of Physical and Chemical Research in Japan (RIKEN), and SLAC National Accelerator Laboratory. The CBXFEL is expected to operate using 9.831 keV photons from LCLS, using synthetic diamonds as cavity Bragg mirrors. The LCLS copper linac will deliver two electron bunches 624 RF buckets apart, resulting in a total X-ray cavity length of 65500.87 mm. The final X-ray cavity design, and installation and production status will be presented.

BPMs have been used for decades since their easy-to-use absolute transverse position capability. Left signal minus right signal divided by the sum times the radius gives the beam position. The charge is “just” a relative measurement and has to be calibrated (or ironed) against a toroid signal. Even when the incoming charge variation is high (like 3% rms for the superconducting LCLS2), the relative variations are only 0.1%. This opens up quite some uses. Besides even small charge losses at beam restrictions like collimators or septum magnets it has been found that this signal is very useful in quantifying the charge loss during a wire scan since losses of around 2% are observed. By taking the difference of a few BPMs before and after the wire scanners signal-to-noise levels of up to 5000 are observed, making this method compatible to the typical scintillator plus photomultiplier setup. This is especially helpful where the first beam loss is hundreds of meters downstream since most of the scattered electron make it down the relatively wide bore of the superconducting cavities. An SVD method to analyze the data independent by human judgement is discussed.

A compact, high-voltage (HV) pulser in the nanosecond regime for transverse electromagnetic (TEM) kickers is presented. TEM kickers are electromagnetic deflectors used in particle accelerators to redirect bunches of particles out of their original trajectory into a new path, such as alternate beam paths, detectors, or other instrumentation devices. The circuit proposed in this design consists of two main portions: a gate driver and a HV switch. The gate driver consists of an isolated and high-speed gate driver, powered by an isolated DC/DC converter with dual output voltages. The HV switch portion was simulated in Ansys HFSS and is composed of a SiC MOSFET, LC resonance components, and specialized diodes. When switched, the MOSFET is used to pump a high voltage into the LC circuit and diode stack, and the ultrafast diode turnoff delivers the final HV pulse to the resistor load. Careful layout techniques were implemented for the MOSFET driver to reduce pulse to pulse instability. A 1 MHz repetition rate was the target of our design.

We designed, built and commissioned a beam diagnostic system based on a short S-band defector and a commercial klystron transmitter. A two feet long transverse-horizontally deflecting S-band rf structure (STCAV2) is installed the LCLS-II post-laser-heater diagnostic beamline at 100 MeV electron beam energy to measure the absolute electron bunch length and to allow time-resolved beam quality measurements such as vertical slide emittance and slice energy spread. The deflector is designed to produce 0.48 MeV peak kick at 300 kW of input power. The klystron transmitter, which uses a commercial solid-state modulator, is installed in the klystron gallery at the grade level. The low-level RF system is based on ATCA and developed in-house. We will report on the overall performance of the project, which was successfully completed, on May 31, 2024.

Superconducting Radio Frequency (SRF) technology is a proven solution for generating high-power electron beams (EB), suitable for tasks like purifying wastewater from challenging impurities such as PFAS. This study elaborates on effectiveness of EB treatment and outlines design considerations for a 1.3 GHz SRF linac operating at 5 MeV with an average beam current of 10 mA. Nu-merical analyses for the accelerator system, ensuring that the beam reaches 5 MeV with the desired characteristics, lead to a compact beamline structure. This structure includes a 100 kV thermionic gridded electron gun, a 1.3 GHz 3-cell low beta buncher cavity, and three 2-cell 1.3 GHz accelerator cavities, along with necessary focusing solenoids, all fitting within 3 meter. Given the need for high beam current, achieving a high bunch repetition rate is important. We therefore will employ the RF gating to the grid of the electron gun. The results of the numerical studies will be presented at this conference.

Various types of radioisotopes (RIs) are used in the field of nuclear medicine for diagnosis, such as PET and SPECT. In recent years, RIs are applied to therapy of cancer and the Ac-225 has been confirmed to be effective in the treatment of advanced cancer. One of the promising RI production methods for medical application is the use of high-intensity beam in accelerators. In the case of an electron accelerator, a photonuclear reaction is used in the RI production process. We have started research and development of a 4K niobium-tin (Nb3Sn) superconducting RF (SRF) electron accelerator system for RI production, which can be operated with a compact conduction cooling system and does not require a large-scale cooling system. As a first step, we plan to develop a single-cell Nb3Sn superconducting cavity and a cryomodule, and to demonstrate its performance by beam acceleration experiments. In this presentation, we report the basic design of the SRF electron linac and R&D project of the 35 MeV SRF linac for the medical RI production.

The generalized longitudinal strong focusing (GLSF) scheme is a potential approach for a steady-state mi-crobunching (SSMB) storage ring, leveraging the ultra-low vertical emittance in the storage ring. It achieves active vertical-longitudinal coupling through an inser-tion unit, further compressing bunch length from the hundreds of nanometers scale in the main ring to the nanometers scale, thus emitting radiation. Due to the extremely short bunch length, coherent synchrotron radi-ation (CSR) effect may significantly impact beam dynam-ics. We developed a particle tracking program based on one-dimensional CSR model to preliminarily evaluate the influence of CSR effect in the GLSF scheme under current design parameters. Our work contributes to the future optimization of the GLSF scheme.

Accelerator-based light sources require high brightness electron bunches to improve performance in exploring structure of matter. Higher acceleration gradient is the key to generate high brightness electron bunches and is more feasible with higher frequency and shorter pulse length electromagnetic wave according to previous empirical formulas. A tapered rectangle waveguide structure driven by terahertz wave is designed as a compact electron gun. A nanotip is fabricated by focused ion beam (FIB) in the center to enhance the field and to emit electrons. The average emission charge per pulse is measured by Pico ammeter, and the peak value reaches 10fC. The max electron energy beyond 4keV is measured from the signal of channel electron multiplier behind a -4kV metal girds, revealing that maximum acceleration gradient is beyond 100MeV/m. These results indicate promising performance of compact terahertz electron gun in high brightness electron injection. Further research will be done in the future.

The design and tuning of a storage ring for a fourth-generation synchrotron light source is very demanding. Recently, some research groups have considered techniques based on quasi-invariants of motion to address this task. This contribution presents tools, based on a quasi-invariant of motion method, for the description and optimisation of the electron dynamics in a storage ring. An overview of this quasi-invariant formalism in the context of electron dynamics in storage rings for synchrotron light sources is presented. Quasi-invariant surface techniques to study and optimise the dynamics of a particular model are shown in detail. The relevance of the distorted chromatic index for cell tuning and for determining a working point of a machine is highlighted. These techniques are implemented to optimise the horizontal electron dynamics generated by a ring model based on a 7BA cell, with 20 cells, 81 pm rad emittance and approximately 490 m circumference, and the results are presented.

The high-repetition-rate infrared terahertz free-electron laser (IR-THz FEL) facility are progressing in the preliminary research stage, which can achieve the demand for a tunable, high-power-light source in the long wavelength spectrum and form a complementary structure of advantages with the Hefei Advanced Light Facility (HALF). In this paper, we present the design of a bunch compressor which can compress the bunch length to reach the peak current of 118 A. We also present an approach to optimize the RF parameters for the accelerating modules, which makes it feasible to generate a high-quality beam bunch that can reach the requirements for future FEL applications.