We present the crystalrad simulation code, combining all the features of the crystal simulation code for simulations of charged particles trajectories in a bent crystal and the radcharm++ code for calculation of the radiation spectrum. The crystalrad code is based on Monte Carlo simulations of trajectories in the planar and axial electric field either in a straight, bent, or periodically bent crystal taking into account multiple and single Coulomb scattering on nuclei and electrons, nuclear scattering and ionization energy losses. The trajectories simulated are used for calculation of radiation spectra by the Baier-Katkov method. We compare our simulations with experimental data taken at MAMI (MAinzer MIkrotron) as well as give an example for a possible future study with sub-GeV electrons interacting with Si bent crystals.

We consider electromagnetic radiation of a charged particle bunch moving uniformly along a corrugated planar metallic surface. It is assumed that the wavelengths under consideration are much larger than the period and the depth of corrugation. Using the method of the equivalent boundary conditions we obtain the Fourier transform of the Hertz vector. It is demonstrated that an ultrarelativistic bunch excites the surface waves, whereas volume radiation is absent. Fourier transforms of the surface wave components and spectral density of energy losses are obtained and analyzed.

We propose a scheme to modulate a low-emittance electron beam at THz frequency using a low-power optical THz source (seed). In this scheme, a matching section introduces tunable angular dispersion and focusing to the electrons. It helps to slice the electrons into “energy layers” and largely cancels out the impact of uncorrelated energy spread on electron modulation. Analytical and numerical studies have been carried out to demonstrate the proposed scheme. It is found that a strong electron density modulation (bunching) over the frequency range from 1 to 6 THz can be achieved when the peak power of THz seed is no more than 1 kW. The bunched beam can be used to generate superradiant undulator radiation with the peak power of 10s MW.

The semiclassical Baïer-Katkov (BK) formula is an approximation to the spectrum of the radiation by an ultrarelativistic electron in a nonuniform field, taking into account the recoil and spin effects. The question why it involves only the initial electron trajectory is addressed. When the field is an electromagnetic plane wave, the exact helicity amplitudes in the null-plane frame are derived using Volkov solutions of the Dirac equation and a modified form of the BK formula is obtained. In passing, the application to Compton scattering is outlined and interesting spin properties are noted. For a general field, two conditions of validity of the BK formula are proposed: (1) the field is approximately uniform in the transverse directions, (2) the relative velocity between the field and the electron is close to $c$. The applications to bremsstrahlung, coherent bremsstrahlung and channeling radiation are discussed.

The impact of the magnetic field profile on the operations of plasma-based Electron Cyclotron Resonance Ion Sources (ECRIS) has been debated and investigated for a long time, and a summary is given by the collection of the so-called “magnetic scaling laws”, i.e., some rules-of-thumb providing the optimal configuration of the axial and radial confining magnetic fields. Anyway, the optimal configurations have been found just looking at the output beam currents and charge states. Only recently, more advanced experiments have shown that the optimal performances are related to some “islands" in the parameters space leading to a dense, energetic but stable plasma. A general, systematic characterization of plasma density and temperature as a function of the magnetic scaling, and for different energy domains (i.e., for cold, warm, and hot electrons) is not yet available. This paper presents a multidiagnostics characterization of a hydrogen plasma heated by microwaves at around 6.8 GHz via Electron Cyclotron Resonance, and confined in an axisymmetric simple mirror magnetic trap. Investigation of plasma response to solely the axial field profile gives the unique opportunity to decouple the effect of simple mirror field from the radial components that are, usually, provided by a hexapole. Optical emission spectroscopy, Langmuir probe, and an x-ray silicon drift detector were simultaneously used to evaluate the plasma density and temperature in different energy domains, thus exploring the electron energy distribution function from a few eV to hundreds of keV. Plasma parameters have been measured as a function of the magnetic field profile (in particular by varying the ratio $B_{\min }/B_{\mathrm{ECR}}$), of the microwave power and of the neutral pressure. Furthermore, for the same scaling, the relative abundances of $H$ atoms and $H_2$ have been estimated. Data show smooth trends of electron density and temperatures versus microwave power and neutral pressure, but strong and even nonlinear responses as a function of the magnetic field profile variation, allowing us to connect semiempirical scaling laws to changes in plasma parameters.

An experimental program is described in this paper, aiming at detecting the formation of correlated coherent states (CCSs) in thin surface layers of crystals when bombarded by a very low energy proton or deuteron beam. CCSs are a generalization of “nonclassical” states of light, such as coherent and squeezed states, whose existence has been demonstrated long ago, giving rise to the remarkable development of quantum optics. In other fields, ranging from condensed matter physics to cosmology, such states have been intensively studied, but a clear signature of their existence is still lacking. This may be a clue to several unexplained phenomena, including the strong enhancement of nuclear fusion reaction rates in some crystal lattices, which have been reported on by several experiments and cannot be accounted for by electron screening only. Such an investigation is extremely relevant to nuclear astrophysics studies, few-body nucleon systems, and nucleon nucleon-interaction problems and, in particular, to energy-related nuclear fusion studies.

The proton-induced x-ray emission (PIXE) is the most commonly used ion beam analysis technique. It allows for nondestructive quantitative elemental characterization and is used in a variety of fields, particularly for the diagnosis of cultural heritage artwork. To provide mobile access to PIXE, a compact transportable radio frequency quadrupole (RFQ) linac has been designed and is being built at CERN. The PIXE RFQ has a length of only one meter and accelerates a proton beam to an energy of 2 MeV. This paper covers various rf design studies for the RFQ operating at 750 MHz. We show simulation results regarding cavity, equipment, and coupler geometry. Thermomechanical and beam dynamics studies are presented as well. The paper concludes with a conceptual study on dipole mode detuning by means of length adjustment.

The brightness and transverse coherence of undulator radiation in synchrotron light sources is investigated using the Wigner formulation in a full 4D treatment. New results are presented on the impact of energy spread on the brightness and coherence as a function of the emittance. In contrast to other work on this topic, it is found that as emittance decreases, the effect of energy spread increases, but as the diffraction limit is approached, the behavior of brightness and coherence diverge. The accuracy of the commonly used Gaussian approximation for brightness and coherence is also explored in detail, resulting in a new conclusion that the commonly used projected source sizes and divergences are not the appropriate quantities to use in the Gaussian approximation to obtain the most accurate results. New expressions are derived for the rms source size and divergence as a function of the energy spread for use in the Gaussian approximation.

In particle accelerators, pumping holes in a vacuum chamber can be a source of unwanted broadband coupling impedance, leading to beam instabilities. Analytical methods have been previously developed to estimate the impedance of holes in circularlike chambers, e.g., the beam screen of the Large Hadron Collider (LHC). More sophisticated chamber designs like that of the High Energy LHC and the Future Circular Collider (the hadron-hadron option) call for a different way to calculate the impedance. We propose using the decomposition of the wakefield into synchronous traveling waves and employing a numerical solver to find the impedance of each wave. This method is compared to the direct time domain wakefield calculation method, and its greater sensitivity to small impedances is shown.

Beam degradation is examined in a laser-plasma accelerator stage with a parabolic plasma channel when the laser pulse and/or the electron beam enters the channel off axis. Betatron oscillations in the beam become incoherent, resulting in a net increase of beam emittance through phase mixing. A quantitative model for transverse emittance growth due to misalignment in multistage accelerators, valid in the linear regime, is presented and compared with particle-in-cell simulations. The model is applied to a chain of laser-plasma accelerator stages, and tolerances are derived on the initial energy spread of the electron beam and misalignment in the multistage structure, with repercussions in high-energy physics applications of laser-plasma accelerators.

A scheme is discussed for momentum selection and momentum-spread reduction for muon-based experiments. The concept relies on placing a wedge absorber at a point along a beam transport system with nonzero dispersion. The technique has direct relevance to precision-science experiments such as the Fermilab Muon g-2 Experiment as it can enhance the muon beam intensity and therefore minimize the statistical uncertainty of the anomalous magnetic moment measurement. This paper presents a theoretical and numerical study about the orientation, material, geometrical parameters, and performance of this wedge. Results suggest a considerable increase in muon intensity for the Muon g-2 Experiment, when the optimal wedge is introduced along the beam path.

In this article we demonstrate how a multiobjective genetic algorithm, like the nondominated sorting genetic algorithm II (NSGAII), and a selection tool, like the technique for order preference by similarity to ideal solution (TOPSIS), can be employed for beam matching and for optimizing the beam transport in the low and medium energy section of a modern hadron linac including space charge effect. Combining NSGAII with the particle tracking code TRAVEL v4.07, we determine the Pareto optimal front, and then apply TOPSIS for the final selection using the example of the 160 MeV ${\mathrm{H}}^{-}$ LINAC4 at CERN. We first determine the matching parameters yielding the optimum transport of the 45 keV ${\mathrm{H}}^{-}$ beam from the ion source to the radio-frequency quadrupole (RFQ) and then onwards at the entrance of the drift tube linac (DTL). Next, we optimize six parameters of the beam phase space to maximize the beam transmission from the exit of the RFQ to the DTL through the medium energy transport section. Finally, we benchmark our predictions with simulations based on the independent TRACE 3-D code and also against a beam experiment, in which the transverse emittance was obtained from a temporary slit-and-grid emittance diagnostic device behind the medium energy beam transport line.

We report on a broadband, picosecond two-stage optical parametrical chirped pulse amplification system, pumped at 515 nm by a frequency doubled Yb regenerative amplifier. The system, designed to be part of the new petawatt picosecond front end, delivers $30\mu \mathrm{J}$ pulses at a central wavelength of 870 nm with about 200 nm bandwidth at 100 Hz. The overall efficiency of the system (pump signal) is 8% while the second stage presents $\sim 17\%$ conversion efficiency.

The echo-enabled harmonic generation (EEHG) scheme holds promising prospects for efficiently generating intense coherent radiation at very high harmonics of a conventional ultraviolet seed laser. We report the lasing of the EEHG free-electron laser (FEL) at an extreme ultraviolet (EUV) wavelength with a seeded FEL facility, the Shanghai soft x-ray FEL. For the first time, we have benchmarked the basic theory of EEHG by measuring the bunching factor distributions over one octave down to the EUV region. Our results demonstrated the key advantages of the EEHG FEL, i.e., generation of very high harmonics with a small laser-induced energy spread and insensitivity to beam imperfections, and marks a great step towards fully coherent x rays with the EEHG scheme.

We consider the nondipolarity of axial channeling radiation generated in a tungsten single crystal at electron-beam energies of several GeV. The calculations are based on the realistic axial continuous potential. It could be shown that, compared to the dipole approximation, the nondipole approximation results in a considerable variation of the channeling radiation spectrum. In a simulation of positron production via conversion of $\gamma$ radiation into $e^{+}{e}^{-}$ pairs, the observed effect may play an important role.

The Xian gamma-ray light source (XGLS) is a proposed novel high brightness gamma-ray source based on inverse Compton scattering (ICS) between the high brightness electron and laser beams. The photoinjector of the accelerator is one of the main components that determines the electron beam quality and related gamma-ray parameters [Phys. Rev. Accel. Beams 21, 030701 (2018); 20, 080701 (2017); 8, 100702 (2005)]. In order to reduce the risk of this project, a photoinjector including a laser-driven photocathode rf gun, two S-band tubes, and a diagnostic section was constructed and installed from 2016, serving as a pilot plant for the development of the key technologies for the projects. The injector is designed to deliver high quality electron bunches of typically 500 pC charge and $\sim 120\mathrm{MeV}$ energy with low emittance and jitter at 10 Hz repetition. Initial system commissioning with an electron beam was finished in the end of 2017, with a goal of a stable 0.6–0.7 mm mrad emittance in a 500-pC bunch clearly demonstrated. In this paper, we report the experimental results and experience obtained in the commissioning, including rf gun, drive laser system, timing and synchronization system, and beam diagnostics.

Particle accelerators are invaluable tools for research in the basic and applied sciences, such as materials science, chemistry, the biosciences, particle physics, nuclear physics and medicine. The design, commissioning, and operation of accelerator facilities is a nontrivial task, due to the large number of control parameters and the complex interplay of several conflicting design goals. The Argonne Wakefield Accelerator facility has some unique challenges resulting from its purpose to carry out advanced accelerator R&D. Individual experiments often have challenging beam requirements, and the physical configuration of the beam lines is often changed to accommodate the variety of supported experiments. The need for rapid deployment of different operational settings further complicates the optimization work that must be done for multiple constraints and challenging operational regimes. One example of this is an independently staged two-beam acceleration experiment which requires the construction of an additional beam line (this is now in progress). The high charge drive beam, well into the space charge regime, must be threaded through small aperture (17.6 mm) decelerating structures. In addition, the bunch length must be sufficiently short to maximize power generation in the decelerator. We propose to tackle this problem by means of multiobjective optimization algorithms which also facilitate a parallel deployment. In order to compute solutions in a meaningful time frame, a fast and scalable software framework is required. In this paper, we present a general-purpose framework for simulation-based multiobjective optimization methods that allows the automatic investigation of optimal sets of machine parameters. Using evolutionary algorithms as the optimizer and opal as the forward solver, validation experiments and results of multiobjective optimization problems in the domain of beam dynamics are presented. Optimized solutions for the new high charge drive beam line found by the framework were used to finish the design of a two beam acceleration experiment. The selected solution along with the associated beam parameters is presented.

In-vacuum undulators have been widely operated in many synchrotron radiation facilities across the world. They usually are required to be operated at a smaller magnet gap than those of other undulators. Thus, operating challenges including impedance effects on the stored electron beam are introduced by these devices. In this paper, we report the efforts in solving the problem of coupled-bunch instabilities caused by an in-vacuum undulator in the SPEAR3 storage ring. Using beam based measurements, cold rf measurements, and numerical simulations, the source of the beam instabilities is characterized as trapped modes in the vacuum chamber. Using numerical models, we explored several approaches to reduce the strength of the trapped modes and found that ferrite dampers were the most effective and simplest way for mode damping in our SPEAR3 in-vacuum undulator. The results of the first rf cold measurement on an in-vacuum undulator equipped with these ferrite dampers agree well with numerical simulations.

A 10-Hz repetition rate, Nd:YAG pulsed laser ($\lambda =1064\mathrm{nm}$, pulse energy of 0.69 J, pulse duration of 3 ns) irradiated a Xe double-stream gas-puff target source. The interaction gives rise to the formation of plasma and emission of soft x-ray and extreme ultraviolet radiation. The produced plasma was investigated and characterized by a silicon carbide (SiC) and a commercial silicon (Si) detector, applying different spectral filters. Some parameters such as the plasma stability and its evolution (time trace profile and pulse time duration) are presented and discussed, evidencing pros and cons of the employment of SiC detectors with respect to the traditional Si for laser-generated plasma diagnostic.

Operational experience of a 500 kV photoemission gun at the compact energy recovery linac (cERL) of the High Energy Accelerator Research Organization is presented. The gun, developed at the Japan Atomic Energy Agency, was found to have failures in two out of the ten-segment ceramic insulator just after installation at cERL. The gun had been operated at 390 kV with eight segments until April 2015 and provided a 0.9 mA beam. An additional two-segment insulator was installed on the top of the existing insulators to recover the high-voltage performance. The gun was then conditioned up to 539 kV and has been operated stably at 500 kV. No discharge caused by the gun itself was observed at 500 kV once the high threshold voltage for stable operation exceeded 500 kV. A dark current of a few picoamperes was generated at 500 kV from a photocathode puck with a semiconducting wafer, while no dark current was observed without a semiconducting wafer. Stable generation of a 500 keV beam with current greater than 0.8 mA was demonstrated for more than two hours.