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.

Recent studies have demonstrated the effectiveness of feedback techniques in suppressing the bursting behavior caused by microbunching instabilities, leading to the stabilization of terahertz (THz) coherent synchrotron radiation (CSR) emission in storage rings. However, the use of feedback loops on the accelerator voltage without proper caution can lead to destabilization of the bunch position and result in beam losses, which limits the optimal feedback parameters. In this paper, we propose a solution to this limitation by introducing a second feedback loop on the phase of the accelerating voltage. The effectiveness of this strategy has been tested at the SOLEIL storage ring, representing a significant step forward in mitigating microbunching instability at high current charges.

A record accelerating rate was achieved earlier in standing wave (SW) SRF cavities when their shape was optimized for a lower peak surface magnetic field sacrificing the peak surface electric field. In view of new materials with higher limiting magnetic fields, expected for SRF cavities, in the first line of the ${\mathrm{Nb}}_3\mathrm{Sn}$, the approach to optimization of cavity shape should be revised. A method of equidistant optimization, offered earlier for traveling wave (TW) cavities is applied to SW cavities. It is shown here that without limitation by magnetic field, the maximal accelerating rate is defined not only by limitations of the electric field but to a significant degree by the cavity shape. For example, for a cavity with the aperture radius of $R_a=35\mathrm{mm}$, the minimal ratio of the peak surface electric field to the accelerating rate is about $E_{\mathrm{pk}}/E_{\mathrm{acc}}=1.54$. So, with the maximal surface field experimentally achieved $E_{\mathrm{pk}}\sim 125\mathrm{MV}/\mathrm{m}$, the maximal achievable accelerating rate is about $80\mathrm{MeV}/\mathrm{m}$ even if there are no restrictions by the magnetic field. Optimized cavity shapes with and without limitations by a magnetic field are presented. Another opportunity—optimization for a low magnetic field, is opening for the same material, ${\mathrm{Nb}}_3\mathrm{Sn}$, with the purpose of having a high quality factor and increased accelerating rate that can be used for industrial linacs with cryocooler-based cooling scheme.

Following the spectacular success of the third-generation light source over the past decades, a few new-generation light sources based on the multibend achromat (MBA) scheme have come into operation. Other such facilities are also under construction while existing ones are being upgraded. Likewise, the Swiss Light Source, which has been in operation for more than 20 years at the Paul Scherrer Institute, is to be upgraded with the present storage ring being replaced by an MBA low-emittance ring. A natural emittance of 158 pm at a beam energy of 2.7 GeV is achieved for a storage ring of comparatively short circumference, 288 m, providing a gross straight section of 83.6 m. The objectives of the upgrade were met under tight constraints by the application of two novel concepts: the introduction of both a longitudinal gradient and a reverse bend into the unit cell and pushing the superperiodicity to the number of arcs, effectively with “pseudosymmetry.” A detailed account of the MBA design and its expected performance is presented and the experience gained is highlighted with the aim to facilitate next-generation light source lattice design.

The LHC heavy-ion program with ${{}^{208}\mathrm{Pb}}^{82+}$ beams will benefit from a significant increase of the beam intensity when entering its high-luminosity era in Run 3 (2023). The stored energy is expected to surpass 20 MJ per beam. The LHC is equipped with a betatron collimation system, which intercepts the transverse beam halo and protects sensitive equipment such as superconducting magnets against beam losses. However, nuclear fragmentation and electromagnetic dissociation of ${{}^{208}\mathrm{Pb}}^{82+}$ ions in collimators generates a flux of secondary fragments, which are lost in downstream dispersion suppressor and arc cells. These secondary ions may pose a performance limitation in upcoming runs since they can induce magnet quenches. In order to mitigate this risk, an alternative collimation technique, relying on bent crystals as primary collimators, will be used in forthcoming heavy-ion runs. In this paper, we study the power deposition in superconducting magnets by means of tracking and fluka shower simulations, comparing the standard collimation system against the crystal-based one. In order to quantify the predictive ability of the simulation model, we present absolute benchmarks against beam loss monitor measurements from the 2018 ${{}^{208}\mathrm{Pb}}^{82+}$ run at $6.37Z\mathrm{TeV}$. The benchmarks cover several hundred meters of beamline, from the primary collimators to the first arc cells. Based on these studies, we provide a detailed analysis of ion fragmentation and leakage to the cold magnets and quantify the expected quench margin in future ${{}^{208}\mathrm{Pb}}^{82+}$ runs.

In the context of wave propagation in solids caused by particle-matter interactions, the composite structure of copper diamond is believed to have a significant impact on the material’s response. This limits the accuracy of isotropic homogeneous elastic and elastic-plastic models used in earlier studies modeling the material’s behavior under such conditions. This study aims to investigate the mesoscopic behavior of the copper diamond and discusses the advantages and limitations of modeling the internal composite structure of the material. The material response of CuCD was modeled in a 2D finite element simulation, considering internal wave propagation as a result of external impact and an internal thermal shock. Various homogeneous models were considered and compared with a mesoscopic model. The homogeneous models tested were found to be able to capture wave propagation effects in the material, and the inclusion of a hardening model allowed their performance to approach that of the mesoscale model considered, which is significantly more computationally demanding.

A relatively simple method of a guess of multipactor appearance on a cavity cell equator was described earlier in Phys. Rev. Accel. Beams 19, 102002 (2016). For this guess, evaluation of electric field components in two definite points near the equator is sufficient, it gives the value of a so-called geometrical parameter $p$ of the cell. If this parameter is big enough, for niobium superconducting cavities, it was found as $p>0.28÷0.3$, conditions for multipactor exist. However, this method works, as described, only for the inner cells of a multicell cavity or for single-cell cavities. An explanation is in the breakdown of symmetry of the electric field near the equator when the end cell consists of asymmetric half-cells and has a regular inner cell from one side and a beam pipe from another side. Multipactor in such cells shifts from the equator point. This paper is an addendum to the earlier presented paper, here it is shown how to find a correct value of the geometrical parameter $p$ taking the asymmetry into account.

The amplitude dependent tune shifts from either space charge or beam-beam interactions are calculated analytically with the inclusion of synchrotron oscillations and multiple interactions around the ring. Simpler formulas are derived under limits of bunches longer than the transverse sizes, equal and unequal transverse sizes, etc. This is used to derive semianalytical forms for the density distribution of the tune shifts. The tune spread and the density distribution are needed to understand beam decoherence or Landau damping with either interaction. The tune footprints due to space charge in IOTA are simulated using pyorbit and found to be in good agreement with the theoretical predictions.

$C$-band technology is emerging as an exciting innovative approach to the creation of compact new accelerators. Besides the possibility to sustain higher gradients and higher repetition rate operation at normal conducting temperature, it also allows one to increase the machine performance in terms of beam brightness. We propose the design study of a normal conducting, high gradient $C$-band injector aiming at the production of high brightness electron beams—up to $2.6\times {10}^3\mathrm{TA}/{\mathrm{m}}^2$—at high repetition rate—up to 1 KHz—as desired to enhance the capabilities of modern radiation sources. This paper reports on beam dynamics studies that guided the injector design looking for a good compromise between the machine compactness and performances. For this purpose, a new standing wave $C$-band gun consisting of 2.6 cells has been designed to enhance the final beam brightness together with its own mode launcher to ensure the needed peak field at the cathode—up to $180\mathrm{MV}/\mathrm{m}$. Several working points in terms of peak current, transverse emittance and brightness are explored, showing the flexibility allowed by the proposed photoinjector. The integration of the gun mode launcher, that relies on a four-port configuration, in the injector design and its effect on the beam quality are also reported.

Polarization switching of synchrotron radiation has wide application prospects in various areas, requiring a high switching frequency between horizontally linear polarization (HLP) and vertically linear polarization (VLP) or between left-hand circular polarization (LCP) and right-hand circular polarization (RCP). To make it possible for realizing a fast polarization switching, in this paper we simplify and incorporate the previous schemes based on segmentally helical undulators connected by phase shifters. Adjacent undulators are set by the same or opposite polarization state to generate circularly or linearly polarized radiation. Theoretical analysis and simulations for radiation characteristics are reported in details. By combining the polarization adjustment of each helical undulator with the phase shifters, both the switching for HLP/VLP and LCP/RCP can be realized in the same undulator scheme. The on-axis heat load can always be effectively suppressed. Benefiting from the extremely low emittance and taking into account the space occupied by phase shifters, the proposed scheme shows a significant advantage in preserving the photon flux with a high polarization degree in a diffraction limited storage ring compared with a typical third generation storage ring. In addition, we also study the influence of beam deviation at each undulator segment, the segment number limited by the practical length of straight section and the issues related to user experiments.

Beam stability is conventionally described by means of the stability diagram, which is a threshold line in a complex plane of a particular intensity parameter, the coherent tune shift; the diagram separates stable and unstable states. The incoherent tune spread of the beam particles normally causes the transverse stability diagram to be an asymmetric bell-shaped curve above the real axis; for a given optical nonlinearity, the diagram is determined by the beam distribution function only. Recently, such a diagram was measured by means of an antidamper, see Antipov et al., Phys. Rev. Lett. 126, 164801 (2021). Such measurements open the door to a new method of beam diagnostics, finding the beam phase space density as a solution of an inverse stability problem. The main idea, methods of its implementation, possible obstacles and optimizations are discussed.

We measured the surface resistance of titanium, amorphous carbon, and nonevaporable getter (NEG) coatings on copper samples, representative of surface treatments of beam-facing components in the High-Luminosity Large Hadron Collider at CERN. Measurements were done at room temperature as well as at liquid nitrogen temperature (77 K) by means of a novel large dielectric resonator operating at 3.4 GHz. The impact of a 100 to 400 nm titanium layer or a 50-nm amorphous carbon layer was negligible both at room temperature and down to 77 K in comparison to copper, while a thick (1140 nm) NEG layer showed a measurable increase in the surface resistance, being more significant at low temperature. The dielectric resonator proved to be a useful tool to characterize the surface resistance of flat samples with different surface treatments and sizes up to 10 cm for accelerator applications. Furthermore, its construction and operation are much simpler compared to other test devices currently in use, and it can provide accurate experimental data for the evaluation of the beam coupling impedance.