Generation of a well-aligned bunch train is of vital importance for preserving the beam quality in a plasma wake-field accelerator. The simultaneous measurements of the relative misalignments in both planes, of the individual femtosecond electron bunches with THz repetition rate in a train, are reported and analyzed with simulations. The new method proposed in this paper demonstrates that the measurements can be done quickly by applying a single quadrupole scan together with a radio-frequency deflector (RFD), while the RFD strength and the RFD phase for each bunch do not need to be known.

We present the dimensioning of a photon-photon collider based on Compton gamma sources for the observation of Breit-Wheeler pair production and QED $\gamma \gamma$ events. Two symmetric electron beams, generated by photocathodes and accelerated in linacs, produce two gamma ray beams through Compton back scattering with two J-class lasers. Tuning the system energy above the Breit-Wheeler cross section threshold, a flux of electron-positron pairs is generated out of light-light interaction. The process is analyzed by start-to-end simulations. Realistic numbers of the secondary particle yield, referring to existing state-of-the-art set-ups and a discussion of the feasibility of the experiment taking into account the background signal are presented.

Based on Skobeltsyn Institute of Nuclear Physics (SINP) Moscow State University (MSU) experience in developing continuous-wave (cw) normal conducting electron linacs, we propose a design for such accelerators with beam energy of up to 10 MeV and average beam power of up to several hundred kW. An example of such design is the 1 MeV industrial cw linac with maximum beam power of 25 kW achievable with 50 kW klystron, which was recently commissioned at SINP MSU.

A full test of the cavity package concept under realistic operating condition was a necessary step before the assembly of the European XFEL (EXFEL) 3.9 GHz superconducting system and its installation in the accelerator. One cavity, equipped with magnetic shielding, power coupler and frequency tuner has been tested in a specially designed single cavity cryostat in one of the test benches of the DESY Accelerator Module Test Facility (AMTF). The cavity was operated at high pulsed power up to an accelerating field of $24\mathrm{MV}/\mathrm{m}$, above the quench accelerating field of $21\mathrm{MV}/\mathrm{m}$ achieved during the continuous wave (CW) vertical qualification test and with a large margin with respect to the EXFEL maximum operating specification of $15\mathrm{MV}/\mathrm{m}$ for the 3.9 GHz system. All subsystems under test—coupler, tuner, waveguide tuners, low level radio-frequency (LLRF) system—were qualified to their design performances.

Spallation sources create a large amount of neutrons with energies up to the GeV range. To shield for these neutrons, steel and concrete are important materials. By adding different aggregates to normal concrete, one can improve the shielding effect of the concrete. Some of these aggregates can influence the rate of hydration (reaction) of the cement or even completely inhibit the hydration. It is thus good practice to start the investigation of new shielding concretes by assessing the rate of cement hydration in the presence of new aggregates. This is preferably made with isothermal (heat conduction) calorimetry. In this paper we describe such tests made with a large number of different potential aggregates for a shielding concrete. We found a full range of influence on the hydration, from no influence to severely disturbed. In some cases smaller particles gave more disturbance.

In many modern heavy-ion accelerator facilities, gas strippers are used to increase the projectile charge state for improving the acceleration efficiency of ion beams to higher energies. For this application, the knowledge on the behavior of charge state distributions of heavy-ions after passing through dilute gases is of special interest. Charge state distributions of uranium (${}^{238}\mathrm{U}$), bismuth (${}^{209}\mathrm{Bi}$), titanium (${}^{50}\mathrm{Ti}$), and argon (${}^{40}\mathrm{Ar}$) ion beams with energies of $0.74\mathrm{MeV}/\mathrm{u}$ and $1.4\mathrm{MeV}/\mathrm{u}$ after passing through hydrogen (${\mathrm{H}}_2$), helium (He), carbon dioxide (${\mathrm{CO}}_2$), nitrogen (${\mathrm{N}}_2$), oxygen (${\mathrm{O}}_2$), neon (Ne), and argon (Ar) gases were measured. Gas stripper target thicknesses up to $100\mu \mathrm{g}/{\mathrm{cm}}^2$ were applied. The observed behavior of the charge state distributions, including their width and mean charge state, are discussed. The measurements show the highest equilibrium charge state at $1.4\mathrm{MeV}/\mathrm{u}$ for ${}^{238}\mathrm{U}$ on ${\mathrm{H}}_2$ gas of $29.2\pm 1.2$. Narrow charge state distributions are observed for ${}^{238}\mathrm{U}$ and ${}^{209}\mathrm{Bi}$ on ${\mathrm{H}}_2$ and He gas, which are highly beneficial, e.g., for the production of beams of high intensities in accelerators.

A zero-field cooled superconducting shield is proposed to realize a high-field (3–4 T) septum magnet for the Future Circular Collider hadron-hadron (FCC-hh) ring. Three planned prototypes using different materials and technical solutions are presented, which will be used to evaluate the feasibility of this idea as a part of the FCC study. The numerical simulation methods are described to calculate the field patterns around such a shield. A specific excitation current configuration is presented that maintains a fairly homogeneous field outside of a rectangular shield in a wide range of field levels from 0 to 3 Tesla. It is shown that a massless septum configuration (with an opening in the shield) is also possible and gives satisfactory field quality with realistic superconducting material properties.

We present a design methodology for developing ultrasmall electron injectors and accelerators based on cascaded cavities excited by short multicycle THz pulses obtained from laser-driven THz generation schemes. Based on the developed concept for optimal coupling of the THz pulse, a THz electron injector and two accelerating stages are designed. The designed electron gun consists of a four cell cavity operating at 300 GHz and a door-knob waveguide to coaxial coupler. Moreover, special designs are proposed to mitigate the problem of thermal heat flow and induced mechanical stress to achieve a stable device. We demonstrated a gun based on cascaded cavities that is powered by only 1.1 mJ of THz energy in 300 cycles to accelerate electron bunches up to 250 keV. An additional two linac sections can be added with five and four cell cavities both operating at 300 GHz boosting the bunch energy up to 1.2 MeV using a 4-mJ THz pulse.

A high-acceleration-gradient linear accelerator (LINAC) for an x-ray free electron laser (XFEL) offers the advantages of a short accelerator length and low construction costs. In addition, the high pulse repetition rate of the LINAC, which can drive multiple x-ray beam lines, provides additional user opportunities for experiments involving XFEL. A $C$-band disk-loaded-type accelerating structure was developed to achieve a high acceleration gradient of $>50\mathrm{MV}/\mathrm{m}$ and an rf-pulse repetition rate of 120 pps, which is twice as high as that of the XFEL facility, SACLA (60 pps). The structure has a quasiconstant gradient and a traveling wave type with an accelerating mode of $\mathrm{TM}01-2\pi /3$. To reduce the surface electric fields, we employed a cross section with an ellipsoidal curvature around an iris aperture. The accelerating structure was manufactured for SACLA. High-power rf conditioning was conducted to investigate its performance. Owing to the conditioning, the acceleration gradient reached a value of more than $50.1\mathrm{MV}/\mathrm{m}$. The structure was operated without any serious issues at a repetition rate of 120 pps. The accelerating structures were installed in the dedicated accelerator for EUV-FEL at SACLA beam line-1. Finally, we obtained accelerated electron beams with the structures operated at an acceleration gradient of $41.4\mathrm{MV}/\mathrm{m}$.

In a free-electron laser, a transverse momentum offset (or “kick”) results in an oscillation of the centroid of the electron beam about the undulator axis. Studying the influence of this effect on the free-electron laser (FEL) interaction is important both from a tolerance point of view and for its potential diagnostic applications. In this paper, we present a self-consistent theoretical analysis of a high-gain FEL driven by such a “kicked” beam. In particular, we derive a solution to the three-dimensional, linearized initial value problem of the FEL through an orthogonal expansion technique and also describe a variational method for calculating the average FEL growth rate. Our results are benchmarked with genesis simulations and provide a robust theoretical background for a comparison with previous analytical results.

Chains of superconducting radio-frequency resonators are key components of modern particle accelerators such as the European XFEL, which is currently under construction in the north of Germany. In addition to the accelerating mode of the resonators, their beam excited higher order modes are of special interest, because they can harm the beam quality. In contrast to the accelerating mode, these modes are in general not confined within single resonators of the cavity string. For instance, eigenmodes can be localized between adjacent cavities or can be distributed along the entire chain of cavities. Therefore, the full chain has to be considered for a reasonable investigation of its resonant spectra. Accounting for such complex structures is computationally challenging and is therefore often avoided. In this article, the challenge is faced by using the so-called state-space concatenation approach, which is a combination of domain decomposition and model-order reduction. The technique allows for a reduction of the number of degrees of freedom by a factor of $\approx 1.471\times {10}^{-4}$. The method is employed to generate a compendium of eigenmodes in the chain of third harmonic cavities for the European XFEL. The results are discussed in detail and are compared with experimental measurements. The compendium serves as a reference for experiments (inter alia for diagnostics based on higher order modes) at the third harmonic cavity string of the European XFEL, it allows for qualitative understanding of resonant effects appearing in chains of cavities, and it is meant to be a proof of principle of the state-space concatenation approach to handle very long and complex radio-frequency structures. To the authors’ knowledge, it is the first time that a modal compendium of a structure with the given complexity is generated. The article presents geometrical details of the chain, defines quantities relevant to superconducting radio-frequency cavities, and describes the employed computational approach.

One of the challenges for laser-driven proton beams for many potential applications is their stability and reproducibility. We investigate the stability of the laser driven proton beams through statistical analysis of the data obtained by employing a high repetition rate tape driven target system. The characterization of the target system shows the positioning of the target within $\sim 15\mu \mathrm{m}$ in the focal plane of an off-axis parabola, with less than a micron variation in surface flatness. By employing this stable target system, we study the stability of the proton beams driven by ultrashort and intense laser pulses. Protons with maximum energies of $\sim 6\pm 0.3\mathrm{MeV}$ were accelerated for a large number of laser shots taken at a rate of 0.2 Hz with a stability of less than 5% variations in cutoff energy. The development of high repetition rate target system may provide a platform to understand the dynamics of laser driven proton beams at the rate required for future applications.

Milling on two longitudinally split halves is one method to manufacture accelerating structures. This method is simple and allows one to avoid electromagnetic fields at bonding joints, making it attractive in manufacturing high-gradient accelerating structures. An X-band structure design with strong wakefield damping based on this manufacturing approach is studied in this work as an alternative design for the Compact Linear Collider (CLIC) main linac accelerating structures. The geometry of the structure is optimized to greatly reduce the surface fields, improve the efficiency, and suppress the wakefield. This structure features the baseline design of the CLIC main linac with additional advantages. This study may serve as a reference for designing other high frequency-band corrugated structures.

Superconducting linacs are capable of producing intense, stable, high-quality electron beams that have found widespread applications in science and industry. The 9-cell, 1.3-GHz superconducting standing-wave accelerating rf cavity originally developed for $e^{+}/e^{-}$ linear-collider applications [B. Aunes, et al. Phys. Rev. ST Accel. Beams 3, 092001 (2000)] has been broadly employed in various superconducting-linac designs. In this paper we discuss the transfer matrix of such a cavity and present its measurement performed at the Fermilab Accelerator Science and Technology (FAST) facility. The experimental results are found to be in agreement with analytical calculations and numerical simulations.

High-field superconducting rf cavities of the future circular collider $e^{+}{e}^{-}$ may require a kW-range superfluid helium refrigeration down to 1.6 K. Magnetic refrigeration operating below 4.2 K can be an alternative to the compression/expansion helium refrigeration. A significant difference between this application and previous magnetic refrigerator studies is its large cooling power, up to $10^3$ times larger than the other designs. Principles of magnetic refrigeration are described and various technical solutions are compared. A numerical model for the static magnetic refrigerator is presented, validated, and adapted to the needs of the positron-electron version of the future circular collider. A preliminary design of magnetic refrigerator suitable for low temperature, kW-range cooling is studied.

We present a setup based on the optical klystron concept, consisting of two undulator modules separated by a magnetic chicane, that addresses two issues in free-electron-laser (FEL) facilities. On the one hand, it allows increasing the intrinsic energy spread of the beam at the source, which is useful to counteract the harmful microbunching instability. This represents an alternative method to the more conventional laser heater with the main advantage that no laser system is required. On the other hand, the setup can be used to reconstruct the initial beam energy spread, whose typical values in FEL injectors around 1 keV are very difficult to measure with standard procedures.

Laplace’s equation is considered for scalar and vector potentials describing electric or magnetic fields in cylindrical coordinates, with invariance along the azimuthal coordinate. A series of special functions are found which, when expanded to lowest order in power series in radial and vertical coordinates, replicate harmonic polynomials in two variables. These functions are based on radial harmonics found by Edwin M. McMillan forty years ago. In addition to McMillan’s harmonics, a second family of radial harmonics is introduced to provide a symmetric description between electric and magnetic fields and to describe fields and potentials in terms of the same functions. Formulas are provided which relate any transverse fields specified by the coefficients in the power series expansion in radial or vertical planes in cylindrical coordinates with the set of new functions. This result is important for potential theory and for theoretical study, design and proper modeling of sector dipoles, combined function dipoles and any general sector element for accelerator physics. All results are presented in connection with these problems.

One of the factors which may limit the intensity in the Fermilab Recycler is a fast transverse instability. It develops within a hundred turns and, in certain conditions, may lead to a beam loss. The high rate of the instability suggests that its cause is electron cloud. We studied the phenomena by observing the dynamics of stable and unstable beams, simulating numerically the buildup of the electron cloud, and developed an analytical model of an electron cloud driven instability with the electrons trapped in combined function dipoles. We found that beam motion can be stabilized by a clearing bunch, which confirms the electron cloud nature of the instability. The clearing suggest electron cloud trapping in Recycler combined function magnets. Numerical simulations show that up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud buildup is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. In a Recycler combined function dipole this multiturn accumulation allows the electron cloud to reach final intensities orders of magnitude greater than in a pure dipole. The estimated resulting instability growth time of about 30 revolutions and the mode frequency of 0.4 MHz are consistent with experimental observations and agree with the simulation in the pei code. The created instability model allows investigating the beam stability for the future intensity upgrades.