The Fermilab Accelerator Science and Technology facility is currently in operation with its linac based on TESLA-type superconducting rf cavities. Using a 3-MHz micropulse repetition rate with a long macropulse composed of up to 3000 micropulses, and with beam energies demonstrated at 300 MeV and projected to reach 800 MeV with two additional cryomodules, the feasibilities for a vacuum ultraviolet (VUV) and an extreme ultraviolet (EUV) free-electron laser oscillator (FELO) with the two energies are evaluated. We have used both the ginger code with an oscillator module and the minerva/opc code to assess FELO saturation prospects at 120 nm with a 5.0-cm-period undulator of 4.5-m length and the minerva/opc code to assess the FELO at 13.4 nm with adjusted parameters. The simulation results support saturation at both of these wavelengths which are much shorter than the demonstrated shortest wavelength record of 168.6 nm from a storage-ring-based FELO. This indicates superconducting rf linac-driven FELOs can be extended into this VUV-EUV wavelength regime previously only reached with single-pass FEL configurations. In addition, emittance-dilution effects due to wakefields in the cavities and the resulting submacropulse centroid slew effects on FELO performance are addressed using minerva/opc simulations for the first time.

The growing interest in the field of electron irradiation accelerators has led to various research opportunities, with a crucial emphasis on enhancing the irradiation homogeneity of accelerators to advance the industry of the irradiation process. In this study, we introduce a tunable and compact uniform spreading beamline that comprises a permanent magnet quadrupole doublet and an electron beam permanent magnet spreading system. This beamline could solve the problem of poor irradiation homogeneity caused by irregular beam spots in electron irradiation accelerators. Flexible and tunable magnetic yokes enable rapid changes to the magnetic field distribution based on different beam spot profiles or irradiation processing requirements, achieving uniform spreading of different beam spots and improving irradiation homogeneity. Additionally, we evaluated the performance of rectangular permanent magnet poles to minimize errors in future accelerator applications. The implementation of this beamline enhances irradiation quality and provides an experimental foundation for further investigating the interaction between electron beam irradiation rates and materials.

The Compact Linear Collider (CLIC) plans to start operation at a center-of-mass energy of 380 GeV. This paper reports on various optimizations of the CLIC 380-GeV Final Focus System (FFS). Analytical approximations have been used to guide the numerical optimizations. The first optimization consists of the reduction of the vertical beta function at the interaction point to $70\mathrm{\mu }\mathrm{m}$, which is at the limit of the hourglass effect. The second optimization focuses on minimizing the lengths of the final doublet quadrupoles to reduce the chromaticity. The third optimization searched for alternative optics configurations, resulting in the implementation of a novel dispersion profile along the FFS. This alternative optics boosts both total and peak luminosities by 7% and 9%, respectively.

We propose to use tightly focused lasers to generate high quality electron beams in laser wakefield accelerators. In this scheme, the expansion of the laser beam after the focal position enlarges the size of wakefield bubble, which reduces the effective phase velocity of the wake and triggers injection of plasma electrons. This scheme injects a relatively long beam with high charge. The energy spread of the injected beam can be minimized if an optimal acceleration distance is chosen so that the beam chirp is suppressed. Particle-in-cell simulations indicate that electron beams with the charge on the order of nanocoulomb, the energy spread of $\sim 1\%$, and the normalized emittance of $\sim 0.1\mathrm{mm}\mathrm{mrad}$ can be generated in uniform plasma using $\sim 100\mathrm{TW}$ laser pulses. An empirical formula is also given for predicting the beam charge. This injection scheme, with a very simple setup, paves the way toward practical high-quality laser wakefield accelerators for table-top electron and radiation sources.

A single-shot method to reconstruct the transverse self-wakefields acting on a beam, based only on screen images, is introduced. By employing numerical optimization with certain approximations, a relatively high-dimensional parameter space is efficiently explored to determine the multipole components of the transverse-wakefield topology up to desired order. The reconstruction technique complements simulations, which are able to directly describe the wakefield composition based on experimental conditions. The technique is applied to representative simulation results as a benchmark, and also to experimental data on wakefield observations driven in dielectric-lined structures.

To develop an isotope selective computed tomography imaging technique, a flat-laser Compton scattering γ-ray beam (F-LCS) by using a helical undulator installed in a storage ring has been proposed. An LCS beam with a broad energy spectrum and spatial distribution, keeping a small beam size, is preferable for multi-isotope imaging and qualitative evaluation. egs5 simulations assuming the BL1U beamline in UVSOR have been carried out and the result shows that the energy bandwidth of the LCS beam is widened from 2.7% to 22% (FWHM) with a beam size of 2 mm in diameter by varying the undulator $K$ value from 0 to 0.4. A proof of principle (POP) experiment has been carried out in the BL1U, where the APPLE-II undulator has been installed. The $K$ value dependency of the energy spectrum of the generated LCS beam was measured. As a result, a broader energy bandwidth of the LCS beam was observed as the undulator $K$ value increased. The measured energy spectra agreed with the egs5 results.

A harmonic kicker cavity was originally proposed as an ultrafast bunch exchange device between energy recovery linac (ERL) and the circulator cooling ring (CCR) of the Jefferson Laboratory Electron-Ion Collider (JLEIC). The cavity is a transverse deflecting cavity that delivers a sharp kick synthesized using a linear combination of harmonic modes, which can be developed as a normal conducting quarter wave resonator (QWR). We present the development of the QWR as a high-power device (up to 7.5 kW) ready for a beam test: rf design (that involves the rf power optimization, tuning mechanism of five harmonic modes, power coupling), multipacting study, mechanical design with a water cooling system for high power operation, fabrication, and the rf bench tests.

The optimization and advanced study of a laser-plasma electron injector are presented based on a truncated ionization injection scheme for high quality beam production. The smilei code is used with laser envelope approximation and a low number of particles per cell to reach computation time performances enabling the production of a large number of accelerator configurations. The developed and tested workflow is a possible approach for the production of a large dataset for laser-plasma accelerator optimization. A selection of functions of merit used to grade generated electron beams is discussed. Among the significant number of configurations, two specific working points are presented in detail. All data generated are left open to the scientific community for further study and optimization.

This paper presents a new approach to space charge dominated beamline design using an artificial intelligence (AI)-based optimization code named giotto. The code incorporates advanced algorithms for multiobjective genetic optimizations in particle accelerators, allowing efficient exploration of the parameter space and improved beam quality. The study demonstrates the application of giotto in the design of a high-brightness injector for an Energy Recovery Linac (ERL) called BriXSinO. The optimized injector features a low energy (4.5 MeV) and relatively high bunch charge (100 pC) operation. The results show promising beam parameters comparable to other ERL projects. Furthermore, the paper introduces innovative techniques, including bunches back-rotation the use of Lorentzian distributions in the fitness function. The approach successfully achieves dispersion closure in a space charge dominated dogleg. Overall, this work contributes to the advancement of accelerator science, offering a powerful methodology for beamline design and optimization. The new techniques and methodologies introduced have the potential to enhance the performance and stability of particle accelerators in various applications.

Reducing the mean transverse energy (MTE) of electrons emitted from photocathodes will improve the performance of linear accelerator applications like x-ray free electrons lasers (XFELs) and ultrafast electron diffraction (UED) and microscopy (UEM) experiments. Metallic photocathodes are popular for these applications due to their convenience of use. However, they typically have a very low quantum efficiency and require a large laser fluence in order to extract the desired charge density for applications like XFELs and single-shot UED/UEM experiments. Recent theoretical investigations have shown that nonlinear photoemission effects of multiphoton emission and electron heating increase the MTE dramatically at these large laser fluences. In this paper, we report on measurements of nonlinear near-threshold photoemission from a graphene-coated Cu(110) photocathode at laser pulse lengths of 130 fs, 1 ps, and 10 ps. We extrapolate our measured data to find the ideal irradiating photon energy that results in a minimum MTE from such metallic photocathodes for charge densities relevant to photoinjectors and specify quantum efficiency requirements to obtain the thermally limited MTE at these charge densities.

A new self-consistent semianalytical method for calculating the stationary beam-induced voltage in the presence of arbitrary filling patterns and impedance sources in electron storage rings is presented. The theory was developed in space domain with resonator wake functions and in frequency domain with arbitrary impedance functions. The SIRIUS storage ring parameters were used to benchmark the results, demonstrating good agreement between the two approaches and with macroparticle tracking simulations. Additionally, a different approach to simulate the beam-loading compensation of active rf cavities was investigated in frequency domain, proving to be a more generic description than the methods generally used. The impact of broadband impedance on the longitudinal equilibrium was straightforwardly evaluated with the frequency-domain framework, without intermediate steps such as fitting broadband resonators or convolving short-range wakes with bunch distributions. Finally, a simple study of Touschek’s lifetime improvement with a passive higher harmonic cavity is presented.

The basic theory of free-electron lasers (FELs) indicates that the output wavelength of a seeded FEL operated in the high-gain harmonic generation (HGHG) configuration is determined by the wavelength of the seed laser and light is emitted when the undulators are tuned to an exact harmonics of the seed laser. In a realistic case, when taking into account the electron beam imperfections and the finite bandwidths of the seed and of the amplification process, the output wavelength is influenced by these factors and can deviate from the exact harmonic resonance. These effects are responsible for the small wavelength fluctuations of the FEL pulses but can also be exploited for an accurate FEL wavelength tuning. In this work, we show how the dispersive section, the curvature of the electron beam longitudinal phase-space and frequency pulling can influence the FEL wavelength and can be, in principle, used to control it. Furthermore, we show how one can reconstruct the electron beam longitudinal phase-space from the analysis of the FEL wavelength sensitivity to the seed laser delay with respect to the beam arrival time.

Spin-polarized lepton beams are an important aspect in the design of the future 100-km scale Circular Electron Positron Collider (CEPC). Precision beam energy calibration using resonant depolarization, as well as longitudinally polarized colliding beams, is being actively investigated. The attainable beam polarization level for various beam energies and application scenarios depends on the radiative depolarization in the collider rings. In this paper, the radiative depolarization effects are evaluated for a CEPC collider ring lattice with detailed machine imperfections and corrections. Simulations with the slim and Monte Carlo approaches using the bmad/ptc codes are compared with the theory of the effects of spin diffusion for ultrahigh beam energies and the validity of the theories, with special attention to that of so-called uncorrelated resonance crossing, is thereby addressed.

We explore the use of reduced physics models for efficient kinetic particle simulations of space charge limited (SCL) emission in inner magnetically insulated transmission lines (inner MITLs), with application to Sandia National Laboratories’ $Z$ machine. We propose a drift kinetic (guiding center) model of electron motion in place of a fully kinetic model and electrostatic-magnetostatic fields in place of electromagnetic fields. The validity of these approximations is suggested by the operational parameters of the $Z$ machine, namely, current pulse lengths of order 100 ns compared with Larmor periods typically smaller than ${10}^{-11}\mathrm{s}$, typical Larmor radii of a few (tens) of microns (magnetic fields of tens to hundreds of Tesla) compared with MITL dimensions of a few centimeters, and transient time of light waves along the inner MITL of order a fraction of a nanosecond. Guiding center orbits eliminate the fast electron gyromotion, which enables the use of tens to hundreds of times larger time steps in the numerical particle advance. Electrostatic-magnetostatic fields eliminate the Courant-Friedrichs-Lewy (CFL) numerical stability limit on the time step and allow the use of higher grid resolutions or, alternatively, larger time steps in the fields advance. Overall, potential computational cost savings of tens to hundreds of times exists. The applicability of the reduced physics models is examined on two problems. First, in the simulation of space charge limited emission of electrons from the cathode surface due to high electric fields in a radial inner MITL geometry with a short load. In particular, it is shown that a drift kinetic-based particle-in-cell (PIC) model with electrostatic-magnetostatic fields is able to accurately reproduce well-known physics of electron vortex formation, spatially and temporally. Second, deeper understanding is gained of the mechanism behind vortex formation in this MITL geometry by considering an exemplar problem of an electron block of charge. This simpler setup reveals that the main mechanism of vortex formation can be attributed to pure drift motion of the electrons, that is, the (fully kinetic) gyromotion of the electrons is inessential to the process. This exemplar problem also suggests a correlation of the spatial dimensions of vortices to the thickness of the electron layer, as observed in SCL simulations. It also confirms that the electromagnetic nature of the fields does not play an essential role. Finally, an improved hybrid fully kinetic and drift kinetic model for electron motion is proposed, as means of capturing finite Larmor radius (FLR) effects; the particular FLR physics that is missed by the drift kinetic model is the particle-wall interaction. By initializing SCL emitted electrons as fully kinetic and later transitioning them to drift kinetic, according to simple criteria, the accuracy of SCL simulations can be improved, while preserving the potential for computational efficiency.

Plasma-based positron acceleration is still an open question, as the most efficient regimes for electron acceleration (quasilinear and blowout) are not directly applicable to positrons. Nevertheless, positron acceleration is a stepping stone on the path toward a plasma-based lepton collider. In this work, we propose a scheme for positron acceleration based on the spatial overlap of a driver (electron or laser) beam and a positron beam, also known as a fireball beam. Under appropriate conditions, these beams can self-consistently evolve toward a hollow driver and focused positron beam on-axis, driving plasma waves suitable for positron acceleration. This evolution seems to be a manifestation of the current filamentation instability. We discuss how the self-consistent dynamics affect the beam quality and perform a preliminary tolerance study.

The China Spallation Neutron Source upgrade project (CSNS-II) has completed the preliminary design report and is expected to start in the fall of 2023. It plans to add two types of superconducting cavities: the 324-MHz double spoke superconducting cavity and the 648-MHz elliptical superconducting cavity in the linac section of the accelerator to upgrade the beam power from 100 to 500 kW, requiring more than 40 new fundamental power couplers (FPCs). These couplers will operate at pulse mode with a peak power ranging from 100 to 380 kW. To master the entire R&D process of the FPC for superconducting cavities, meanwhile, provide the horizontal test condition for various superconducting cavities research, a pair of 324-MHz power couplers for the double spoke superconducting cavity are developed. This coupler is a coaxial structure with a disk-warm ceramic window, and its characteristic impedance is $50\mathrm{\Omega }$. Multiphysics design is performed to ensure that the coupler meets the actual requirements. Structures of the coupler are optimized to realize a clean assembly with the cavity in Class 10 clean room and avoid window failure due to the bombardment of electrons generated by possible cavity field emission. We have successfully fabricated two FPCs and high power conditioned them on the test bench at room temperature. The test results show that FPCs can transmit a peak radio frequency (rf) power of 2 MW with a duty factor of 1.625% in traveling wave mode and an rf power of 500 kW with a duty factor of 3% in standing wave mode with arbitrary reflection wave phase. It meets the requirement of spoke cavities for CSNS-II—200 kW maximum power with a duty factor of 3%. Multipacting (MP) zones found during the high-power conditioning agree reasonably well with those predicted by simulations.

The properties of the failure shots on an induction voltage adder are described. This induction voltage adder is composed of six identical prime pulsed power sources, six induction cavities, and a lossless vacuum insulation output transmission line which is terminated by a rod-pinch diode. The failure modes include too early and too late firing of the laser-triggered gas switch in the prime pulsed power sources and abnormal diode impedance declining. The amplitudes of the voltage pulses across the feed port and azimuthal transmission line of the induction cavity are increased by 38% and 48%, respectively, in the event that this cell is driven too early. On the contrary, a reverse voltage pulse will be produced at the induction cavity when it is fired too late. The peak value of the reverse voltage pulse is about 50% of that of the normal polarity pulse. The abnormal rod-pinch diode impedance declining taking place occasionally decreases the load voltage and x-ray dose remarkably. However, the average amplitude of the voltage pulses across the six induction cavities is almost the same as that of a normal shot. We have also analyzed the property of the voltage pulses across the induction cell under the circumstance that only one induction cavity is fired. Both the experimental and circuit simulation data suggest that in this case the voltage pulses on the induction cell are nearly identical even though the load impedance varies vastly.

We consider analytical expressions for beam impedance of round, rectangular, and $\mathrm{\Pi }$-shaped waveguides with anisotropic surface impedance of arbitrary nature. The formulas are given for relativistic and nonrelativistic cases and the impedance matrix of general form. The field matching technique for layered structures with layers of uniaxial anisotropy and anisotropic impedance boundary condition at the last layer is described. The analytical methods are applied to the real structure examples with corrugations, dielectrics, and anomalous skin effects.