UK XFEL is a multi-stage project to pursue ‘next-generation’ XFEL capabilities, either through developing a new facility in the UK or by investing at existing machines. The project’s Science Case envisages a step-change increase in the number of simultaneous experiments, with transform-limited (‘laser-like’) x-rays across a wide range of pulse durations and photon energies (up to ~20 keV) being delivered together with an array of synchronised sources, at high repetition rate to approximately ten FELs (evenly spaced pulses at approximately 100 kHz per experiment, with flexibility). A subset of applications require increased pulse energy and higher photon energies at low repetition rate or in short bursts. The project is now in the final year of its three-year conceptual design and options analysis phase, in which it has produced a conceptual design to efficiently meet these requirements, as well as conducting an analysis of the costs, socio-economic factors, and sustainability of the different investment options. The conclusions of this study are expected to be of general interest to the community.

The Relativistic Ultrafast Electron Diffraction and Imaging (RUEDI) facility is an approved project to provide ultrafast capability to UK researchers. The current design involves two separate beamlines for diffraction and imaging but with shared infrastructure including laser pump sources. This presentation describes recent progress in the design of the diffraction line. The diffraction line has a 2.4 cell S-band RF gun to produce 4 MeV electron bunches. Bunch compression to the sub-10 fs range is carried out with a triple bend achromat design that also suppresses arrival time jitter*. Interchangeable sample chambers are planned to allow wide ranging experiments from both solid samples at room and cryogenic temperatures and liquid and gas targets. Post sample optics are provided to image the diffraction pattern on to a high-resolution single electron sensitive detector. Temporal diagnostics including an RF TDC and THz deflector are included along with a spectrometer at the end of this line to measure beam energy.

Field emission or dark current from RF electron guns is problematic for accelerators for several reasons, and can be more difficult to predict, measure and understand, compared to the main beam. The work presented here is a study of the dark current emitted from the CLARA 3GHz 2.5 cell electron gun with exchangeable photocathode, which is a type of gun used for single pass high quality low emittance beams for uses such as Free Electron Lasers. The main purpose of these studies was to identify sources of dark current emission within the gun, by examining the dark current distribution and patterns on scintillating screens. It is shown that the features of the patterns can be reproduced qualitatively, and to some extent quantitatively, by finite element simulations. Thus, certain surfaces within the gun and cathode apparatus which contribute most significantly to problematic dark current can be identified, and further development and preparation of the guns and their cathodes can be informed to minimise dark current.

Powerful ERL for experiments (PERLE) is a novel energy recovery linac (ERL) test facility [1], designed to validate choices for a 50 GeV ERL foreseen in the design of the Large Hadron Electron Collider and the Future Circular Collider and to host dedicated nuclear and particle physics experiments. Its main goal is to demonstrate the high current, continuous wave, multipass operation with superconducting cavities at 802 MHz. With very high beam power (10 MW), PERLE offers an opportunity for controllable study of every beam dynamic effect of interest in the next generation of ERLs and becomes a "stepping stone" between the present state-of-the-art 1 MW ERLs and the future 100 MW scale applications.

MEIS and XPS studies have been conducted for a range of oxygen plasma treated copper samples (and one treated with Ar plasma), as part of a study of the preparation of photocathode surfaces. This procedure was seen to remove hydrocarbon contamination, but left an oxide film whose thickness depended on the treatment conditions used. The film thickness was seen to increase with longer treatment time and higher RF power levels with behaviour characteristic of a diffusion limited process. Annealing led to a removal of the oxide layer by diffusion of oxygen into the bulk of the samples. This indicates that where the oxide layer is sufficiently thin, it should be possible to produce a surface with low enough oxygen content to have adequate conductivity and high enough surface escape probability to allow photoemission at UV wavelengths. Ar plasma treatment was seen to result in a very thin oxide film which could satisfy these requirements and thus might be preferable. Data from single crystal copper samples showed similar total oxygen content for all the surfaces analysed.

The 2020 update of the European Strategy for Particle Physics emphasised the importance of an intensified and well-coordinated programme of accelerator R&D, supporting the design and delivery of future particle accelerators in a timely, affordable and sustainable way. This report sets out a roadmap for European accelerator R&D for the next five to ten years, covering five topical areas identified in the Strategy update. The R&D objectives include: improvement of the performance and cost-performance of magnet and radio-frequency acceleration systems; investigations of the potential of laser / plasma acceleration and energy-recovery linac techniques; and development of new concepts for muon beams and muon colliders. The goal of the roadmap is to document the collective view of the field on the next steps for the R&D programme, and to provide the evidence base to support subsequent decisions on prioritisation, resourcing and implementation.

We describe a novel technique, based on image compression and machine learning, for transverse phase space tomography in two degrees of freedom in an accelerator beamline. The technique has been used in the CLARA accelerator test facility at Daresbury Laboratory: results from the machine learning method are compared with those from a conventional tomography algorithm (algebraic reconstruction) and applied to the same data. The use of machine learning allows reconstruction of the 4D phase space distribution of the beam to be carried out much more rapidly than using conventional tomography algorithms and also enables the use of image compression to reduce significantly the size of the data sets involved in the analysis. Results from the machine learning technique are at least as good as those from the algebraic reconstruction tomography in characterizing the beam behavior, in terms of the variation of the beam size in response to variation of the quadrupole strengths.

Energy-recovery linacs (ERLs) have been emphasised by the recent (2020) update of the European Strategy for Particle Physics as one of the most promising technologies for the accelerator base of future high-energy physics. The current paper has been written as a base document to support and specify details of the recently published European roadmap for the development of energy-recovery linacs. The paper summarises the previous achievements on ERLs and the status of the field and its basic technology items. The main possible future contributions and applications of ERLs to particle and nuclear physics as well as industrial developments are presented. The paper includes a vision for the further future, beyond 2030, as well as a comparative data base for the main existing and forthcoming ERL facilities. A series of continuous innovations, such as on intense electron sources or high-quality superconducting cavity technology, will massively contribute to the development of accelerator physics at large. Industrial applications are potentially revolutionary and may carry the development of ERLs much further, establishing another shining example of the impact of particle physics on society and its technical foundation with a special view on sustaining nature.

The minimum achievable particle beam emittance in an electron accelerator depends strongly on the intrinsic emittance of the photocathode electron source. This is measurable as the mean longitudinal and transverse energy spreads in the photoemitted electron beam (MLE and MTE respectively); consequently, MLE and MTE are notable figures of merit for photocathodes used as electron sources in particle accelerators. The overall energy spread is defined by the sum of the MTE and the MLE, and the minimization of MTE is crucial to reduce emittance and thus generate a high-brightness electron beam. Reducing the electron beam emittance in an accelerator that drives a Free-Electron Laser (FEL) delivers a significant reduction in the saturation length for an x-ray FEL, thus reducing the machine's construction footprint and operating costs while increasing the x-ray beam brightness. The ability to measure the transverse energy distribution curve of photoelectrons emitted from a photocathode is a key enabler in photocathode research and development that has prompted the Accelerator Science and Technology Centre (ASTeC) at the STFC Daresbury Laboratory to develop the Transverse Energy Spread Spectrometer to make these crucial measurements. We present details of the design for the upgraded TESS instrument with measured data for copper (100), (110), and (111) single-crystal photocathodes illuminated at UV wavelengths around 266 nm.

Although accelerator technology has matured sufficiently, state-of-the-art x-ray linacs for radiotherapy and cargo-scanning capture merely 30%-50% of the electrons from a thermionic cathode, requiring a higher cathode current and leaving uncaptured electrons to cause problems such as back bombardment on the cathode leading to a shortening of cathode life. Any solution to increase capture should be effective, simple, reliable, compact, and low cost in order to be adopted by the industry. To address this, we present the design of a 6 MeV high capture efficiency S-band electron linac that captures 90% of the initial dc beam. This linac does not require any extra parts that would increase the cost as the high efficiency is achieved via a low-field amplitude in the first bunching cell to decrease the number of back-streaming electrons, to velocity bunch the electron beam, and recapture back-streaming electrons. Under the low field amplitude, any electrons launched at decelerating phases travel backward with low speeds, thus most of them can catch the next rf cycle, and get reaccelerated/recaptured. As the electron speed is low, the cell length is also shorter than existing linacs. Such a short field is achieved by the use of asymmetric cells with differential coupling to the side-coupled cells. Our novel design has implications for all commercial high current thermionic gun linacs for increasing beam current and increasing cathode lifetime.

Delivery of high charge electron bunches into the main loop of an ERL (energy recovery linac) while preserving the emittance is challenging. This is because at the typical injection momentum, space charge forces still have a significant effect on the beam dynamics. In this work we consider the design of the merger for PERLE, an ERL test facility to be based at IJCLab in France. Previous simulations have shown that the baseline DC gun based injector can achieve the required emittance at the booster linac exit. The quality of the 500 pC bunches must then be preserved with space charge through the merger at total beam energy of 7 MeV keeping the emittance below 6 mm⋅mrad. The beam dynamics in the merger were simulated using the code OPAL and optimised using a genetic algorithm. Three possible merger schemes were investigated. The goal of the optimisation was to minimise the emittance growth while also achieving the required Twiss parameters to match onto the spreader at the main linac exit. A three dipole solution is then examined in more detail. PERLE AND THE PERLE INJECTOR PERLE is a proposed 500 MeV 3 turn ERL which is foreseen to be hosted at IJCLab in Orsay [1]. The injector for PERLE must be capable of delivering 500 pC bunches, with a RMS bunch length of 3 mm, an emittance of less than 6 mm⋅mrad, at a repetition rate of 40.1 MHz to give an average current of 20 mA. Table 1 shows the requirements on the beam at the exit of the main linac after the first pass. To achieve this low emittance with high average current a DC gun based injector will be used. This injector will consist of a 350 kV photocathode electron gun, a pair of solenoids for transverse beam size control and emittance compensation, a 801.58 MHz buncher cavity, a booster linac consisting of four single cell 801.58 MHz SRF cavities and a merger to transport the beam into the main ERL loop. The Twiss matching to the optics of the main ERL loop is also done in the merger. The layout of the injector with a possible merger example can be seen in Fig. 1. After the injection line is the main linac which consists of four five cell 801.58 MHz SRF cavities. ∗ Work at JLAB was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contracts DE-AC05-06OR23177 and DE-SC0012704. † B.R.Hounsell@Liverpool.ac.uk Table 1: PERLE Merger Specification

We present a study on the evolution of both transverse and longitudinal energy distributions due to oxygen-induced quantum efficiency degradation in the electrons emitted from a reflection-mode p-GaAs(Cs,O)-photocathode over an effective electron affinity range of chi*similar to -0.1 eV to chi*similar to +0.3 eV under illumination wavelengths lambda = 808, 635 and 532 nm. For lambda = 635 and 532 nm, we found that the mean transverse energy of electrons emitted as the photocathode effective electron affinity increased has a non-monotonic character. Our proposed fitting model links the measured response to changes in the relative number of thermalised and non-thermalised electrons in the total emitted photocurrent.

The Large Hadron electron Collider (LHeC) is designed to move the field of deep inelastic scattering (DIS) to the energy and intensity frontier of particle physics. Exploiting energy recovery technology, it collides a novel, intense electron beam with a proton or ion beam from the High Luminosity--Large Hadron Collider (HL-LHC). The accelerator and interaction region are designed for concurrent electron-proton and proton-proton operation. This report represents an update of the Conceptual Design Report (CDR) of the LHeC, published in 2012. It comprises new results on parton structure of the proton and heavier nuclei, QCD dynamics, electroweak and top-quark physics. It is shown how the LHeC will open a new chapter of nuclear particle physics in extending the accessible kinematic range in lepton-nucleus scattering by several orders of magnitude. Due to enhanced luminosity, large energy and the cleanliness of the hadronic final states, the LHeC has a strong Higgs physics programme and its own discovery potential for new physics. Building on the 2012 CDR, the report represents a detailed updated design of the energy recovery electron linac (ERL) including new lattice, magnet, superconducting radio frequency technology and further components. Challenges of energy recovery are described and the lower energy, high current, 3-turn ERL facility, PERLE at Orsay, is presented which uses the LHeC characteristics serving as a development facility for the design and operation of the LHeC. An updated detector design is presented corresponding to the acceptance, resolution and calibration goals which arise from the Higgs and parton density function physics programmes. The paper also presents novel results on the Future Circular Collider in electron-hadron mode, FCC-eh, which utilises the same ERL technology to further extend the reach of DIS to even higher centre-of-mass energies.

A thermionic cathode gridded electron gun used in injectors for different types of circular and linear particle accelerators and for energy recovery configurations was studied. Both theory and numerical simulation were used to explore the relationship between the bunch charge and bunch length. The electron gun is based on a Pierce-type geometry. It was initially designed using Vaughan synthesis followed by optimization using a 2D electron trajectory solver TRAK. After optimization, the grid in front of the cathode was inserted and the RF field was introduced through a coaxial waveguide structure. The complete gun was simulated using the PIC code MAGIC. High duty cycle operations at frequencies 1.5 GHz and 3.0 GHz, were investigated using different combinations of both the bias and the RF voltage applied between the cathode and the grid. The beam dynamics results from the PIC showed that a minimum bunch length of 106 ps could be achieved with a bunch charge of 33 pC when the driving RF frequency was 1.5 GHz. Operating at the higher RF frequency of 3GHz did not significantly reduce the bunch length. The normalized emittance of about 5.6 mm-mrad was demonstrated in PIC simulations.

The compact linear accelerator for research and applications (CLARA) is an ultrabright electron beam test facility being developed at STFC Daresbury Laboratory. The ultimate aim of CLARA is to test advanced free electron laser (FEL) schemes that can later be implemented on existing and future short-wavelength FELs. In addition, CLARA is a unique facility to provide a high-quality electron beam to test novel concepts and ideas in a wide range of disciplines and to function as a technology demonstrator for a future United Kingdom x-ray FEL facility. CLARA is being built in three phases; the first phase, or front end (FE), comprises an S-band rf photoinjector, a linac, and an S-bend merging with the existing versatile electron linear accelerator beam line; the second phase will complete the acceleration to full beam energy of 250 MeV and also incorporate a separate beam line for use of electrons at 250 MeV; and the third phase will include the FEL section. The CLARA FE was commissioned during 2018, and the facility was later made available for user experiments. Significant advancements have been made in developing high-level software and a simulation framework for start-to-end simulations. The high-level software has been successfully used for unmanned rf conditioning and for characterization of the electron beam. This paper describes the design of the CLARA FE, performance of technical systems, high-level software developments, preliminary results of measured beam parameters, and plans for improvements and upgrades.

The transverse phase space of a beam in an accelerator can be characterized using well-established methods based on observation of changes in the beam profile between screens at different locations along the beamline, or on observation of changes on a single screen for different strengths of upstream quadrupoles. Studies on CLARA FE (the Compact Linear Accelerator for Research and Applications Front End, at Daresbury Laboratory, UK) show that where the beam has a complicated (nonelliptical) distribution in transverse phase space, conventional analysis techniques aimed at characterizing the beam in terms of the emittances and Courant--Snyder parameters fail to provide a good description of the beam behavior. Phase space tomography, however, allows the construction of a detailed representation of the phase space distribution that provides a better model for the beam. We compare the results from three measurement and analysis techniques applied on CLARA FE: emittance and optics measurements from observations on three screens; emittance and optics measurements from quadrupole scans on a single screen; and phase space tomography. The results show the advantages of phase space tomography in providing a detailed model of the beam distribution in phase space. We present the first experimental results from four-dimensional phase space tomography, which gives an insight into beam properties (such as the transverse coupling) that are of importance for optimizing machine performance.

We compare three techniques for characterising the transverse phase space distribution of the beam in CLARA FE (the Compact Linear Accelerator for Research and Applications Front End, at Daresbury Laboratory, UK): emittance and optics measurements using screens at three separate beamline locations; quadrupole scans; and phase space tomography. We find that where the beam distribution has significant structure (as in the case of CLARA FE at the time the measurements presented here were made) tomography analysis is the most reliable way to obtain a meaningful characterisation of the transverse beam properties. We present the first experimental results from four-dimensional phase space tomography: our results show that this technique can provide an insight into beam properties that are of importance for optimising machine performance.

The injector for PERLE, a planned ERL test facility, must be capable of delivering 500 pC bunches at a repetition rate of 40.1 MHz to provide a beam with 20 mA average current with a projected rms emittance of less than 6 mm mrad. This must be achieved at two different operational voltages 350 kV and 220 kV for unpolarised and polarised operation respectively. The PERLE injector will be based on an upgrade of a DC photocathode electron gun operated previously at ALICE ERL at Daresbury. The upgrade will add a load lock system for photocathode interchange. This paper presents the results of a re-optimisation of the electrode system as ALICE operated with a bunch charge of around 80 pC while PERLE needs a bunch charge of 500 pC. This re-optimisation was done using the many-objective genetic algorithm NSGAIII to minimise both the slice emittance and transverse beam size for both required operational voltages.