We investigated a new cathode design and beam transport with the EPURE axis-1 injector in order to increase the beam characteristics at 3.8 MeV, 80 ns FWHM from the 2.0 kA nominal current to 2.6 kA corresponding to an average current density of $82\mathrm{A}/{\mathrm{cm}}^2$. Such current increase is highly desirable for improving the x-ray dose and hence radiographic performances. To achieve this, a time-dependent model based on the particle-in-cell method was developed in order to simulate the injector. Using results from calculations based on this model, a 17.2 cm AK gap diode with a larger radius cathode (3.175 cm) was designed, manufactured and tested. Experimental and calculated currents and emittances are qualitatively compared. The study provides a detailed understanding of the beam dynamics inside this type of high current, high energy injector.

Fluidized tungsten powder has been proposed as a potential target technology for particle accelerator applications with very high power highly focused pulsed beams. This has motivated a series of experiments carried out at the HiRadMat facility at CERN to study the response of a tungsten powder sample to an impinging high energy proton beam pulse. The main observation was that of beam induced lifting of the powder sample which was recorded by high speed video. In this paper we consider three mechanisms to explain the observed powder lift including aerodynamic, thermal expansion and induced charge effects. Simulations of the aerodynamic effect revealed that this could not explain the magnitude of the observed eruptions especially during tests carried out with the powder in a vacuum. Thermal expansion of tungsten particles giving rise to the eruption seems implausible due to the propensity for the powder to absorb perturbations. We show that the observations can be explained by a Coulombic eruption of the tungsten particles. The high energy beam leaves a pattern of charge distributed in the poorly conducting powder sample, which creates an electric field that consequently results in a force acting on the individual charged particles. We calculate the charge deposited, the electric field and the resulting acceleration and show that this is a plausible mechanism for causing the observed eruptions. We believe the response of a granular conductive sample to an incident proton beam has not previously been explained.

We consider the performance of free-electron lasers (FELs) in the oscillator configuration, using a hollow electron beam distribution instead of the usual Gaussian. Using the three-dimensional, time-dependent FEL code genesis, we show that for FEL oscillators lasing can be achieved over a much broader range of cavity configurations with a hollow electron beam. This occurs because with the hollow electron beam higher-order optical modes and mode competition are suppressed. We also find a substantial increase in the saturated out-coupled power, with the optical mode still remaining the fundamental ${\mathrm{TEM}}_{00}$ mode. For a hollow electron beam, even with a finite mirror misalignment the transverse optical mode profile remains close to a ${\mathrm{TEM}}_{00}$ mode but with higher out-coupled power than with perfectly aligned mirrors. Thus, it is preferable to operate FEL oscillators with a hollow electron beam rather than a Gaussian.

The high order mode (HOM) power losses induced by a particle beam in accelerator components should be taken into account during their design and the choice of HOM couplers. In this work the power losses due to HOMs in the proposed 400.79 MHz radio frequency (rf) cavity designs were evaluated for various energy options (Z, W, H, and $\mathrm{t}\overline{\mathrm{t}}$) of the future circular electron-positron collider (FCC-ee). The constraints for the rf cavity design are dictated by the Z machine with the lowest beam energy, but the highest beam current. Still certain beam filling patterns (bunch and train spacings) are not feasible and should be avoided in beam operation. For other energy options the power loss are sufficiently low and can be extracted by conventional HOM couplers.

In the blowout regime of plasma wakefield acceleration (PWFA), which is the most relevant configuration for current and future applications and experiments, the plasma flow that is excited by the ultrarelativistic drive beam is highly nonlinear. Thus, fast and accurate simulation codes are indispensable tools in the study of this extremely important problem. We have developed a novel algorithm that deals with the propagation of axisymmetric bunches of otherwise arbitrary profile through a cold plasma of uniform density. In contrast to the existing PWFA simulation tools, our code plebs (plasma-electron beam simulations) uses a new computational scheme which ensures that the transverse and longitudinal directions are completely decoupled—a feature which significantly enhances the speed and robustness of the new method. Our numerical results are benchmarked against the QuickPIC code and excellent agreement is established between the two approaches. Moreover, our new technique provides a very convenient framework for studying issues such as beam loading and short-range wakefields within the plasma cavity.

This study presents a further step within the ongoing R&D activities for the redesign of the CERN’s Antiproton Decelerator Production Target (AD-Target). A first scaled target prototype, constituted of a sliced core made of ten Ta rods $-8\mathrm{mm}$ diameter, 16 mm length-embedded in a compressed expanded graphite (EG) matrix, inserted in a 44 mm diameter Ti-6Al-4V container, has been built and tested under proton beam impacts at the CERN’s HiRadMat facility, in the so called HRMT-42 experiment. This prototype has been designed following the lessons learned from previous numerical and experimental works (HRMT-27 experiment) aiming at answering the open questions left in these studies. Velocity data recorded on-line at the target periphery during the HRMT-42 experiment is presented, showing features of its dynamic response to proton beam impacts. Furthermore, x-ray and neutron tomographies of the target prototype after irradiation have been performed. These non-destructive techniques show the extensive plastic deformation of the Ta core, but suggest that the EG matrix can adapt to such deformation, which is a positive result. The neutron tomography successfully revealed the internal state of the tantalum core, showing the appearance of voids of several hundreds of micrometers, in particular in the downstream rods of the core. The possible origin of such voids is discussed while future microstructure analysis after the target opening will try to clarify their nature.

This paper describes the measurement of the transverse, two-dimensional profile of the high current proton beam in the linac at the Tsinghua University, by tomography mapping using a rotatable multi-wire scanner. The experiment is conducted at a beam energy of 3 MeV, a peak current of up to 32 mA, a repetition frequency of 20 Hz, and a beam pulse width of $50\mu \mathrm{s}$. The transverse density distribution is reconstructed utilizing the algebraic reconstruction technique (ART), by using the beam profiles measured at different rotation angles. The beam profile, measured at a fixed rotation angle, is obtained by 19 parallel carbon wires mounted on one board of the wire scanner monitor (WSM) moving along a straight line with a step increment of 0.1 mm. The results gathered from the computer tomography (CT) method could be verified by an independent profile measurement downstream of a multiple-slit aperture. This paper presents first measurement results of the high current proton beam during normal operation of the linac at the Tsinghua University. A dynamic range of $10^2$ for the two-dimensional distribution was achieved.

We performed a systematic investigation of the dc magnetic properties of superconducting niobium thin films deposited by high power impulse magnetron sputtering (HiPIMS) as a function of the main deposition parameters: the temperature, T, of the heated substrate and the applied dc bias voltage, V, during the sputtering process. The measured dc magnetization curves between 0 and 1000 mT were used to calculate the relative volume of each sample into which the applied magnetic field had penetrated, ${(\mathrm{\Delta }\mathrm{V}/\mathrm{V})}_M$. The sample deposited at $700°\mathrm{C}$ with $-80\mathrm{V}$ biased substrate exhibited the least penetration by the magnetic field. ${(\mathrm{\Delta }\mathrm{V}/\mathrm{V})}_M$ appeared to be highly dependent on the bias voltage at both room temperature and $500°\mathrm{C}$; however, a broad range of bias voltages showed comparatively similar results at increased temperatures of $700°\mathrm{C}$. Samples deposited at $700°\mathrm{C}$ exhibit smaller upper critical fields, ${\mathrm{H}}_{C2}$, than samples deposited at room temperature and $500°\mathrm{C}$, with the lower temperatures exhibiting a greater dependency on the applied bias. The films deposited at $700°\mathrm{C}$ also display a more stable magnetization curve suggesting that an enhanced flux pinning was achieved when compared to lower temperatures. Consequently, films with stable pinning were found to have the most repeatable dc magnetic behavior. Our results are particularly relevant to the superconducting radio-frequency accelerator scientific community where thin films have been suggested as a technology which may ultimately surpass the performance of bulk niobium. They are also relevant to the fundamental area of superconducting thin films and any applied area where thin films produced by HiPIMS are used, such as superconducting electronics.

We report on the first measurements of coherent microwave impulses from high-energy particle-induced electromagnetic showers generated via the Askaryan effect in a dielectric-loaded waveguide. Bunches of 12.16 GeV electrons with total bunch energy of $\sim {10}^3–{10}^4\mathrm{GeV}$ were preshowered in tungsten, and then measured with WR-51 rectangular (12.6 mm by 6.3 mm) waveguide elements loaded with solid alumina (${\mathrm{Al}}_2{\mathrm{O}}_3$) bars. In the 5–8 GHz $T{E}_{10}$ single-mode band determined by the presence of the dielectric in the waveguide, we observed band-limited microwave impulses with amplitude proportional to bunch energy. Signals in different waveguide elements measuring the same shower were used to estimate relative time differences with 2.3 ps precision. These measurements establish a basis for using arrays of alumina-loaded waveguide elements, with exceptional radiation hardness, as very high precision timing planes for high-energy physics detectors.

High-energy charged particle radiography has been used for diagnostics of high-energy density matter, and electrons can serve as a promising radiographic probe that acts as a complement to commonly used proton probes. Here we report on an electron radiography experiment using 45 MeV electrons from an S-band photoinjector, where scattered electrons, after interacting with a sample, are collected and imaged by a quadrupole imaging system. We achieve a spatial resolution of a few microns ($\sim 4\mu \mathrm{m}$) and a thickness resolution of a few percent for a silicon target of $300–600\mu \mathrm{m}$ in thickness. With additional dark-field images captured by selecting electrons with large scattering angles, we show that complementary information for determining external details such as outlines, boundaries and defects can be obtained.

In order to effectively improve the output quality of x-ray free electron laser (XFEL), we theoretically propose an XFEL scheme seeded by atomic inner-shell laser. Atomic inner-shell lasers based on neutral atoms and pumped by XFEL have been experimentally demonstrated [Rohringer et al., Nature (London) 481, 488 (2012), Yoneda et al., Nature (London) 524, 446 (2015)], which produced sub-femtosecond X-ray pulses with increased temporal coherence. It shows that, by using the inner-shell laser as a seed to modulate the electron bunch, very stable and almost fully-coherent short-wavelength XFEL pulses can be generated. The proposed scheme holds promising prospects in x-ray wavelengths, and even shorter.

rf photoinjectors produce incredibly bright electron beams enabling advanced photon science applications such as the current generation of free electron lasers and high energy x-rays and gamma-rays via laser-Compton scattering. A second generation 5.59 cell $X$-band rf gun has been developed, installed, conditioned, commissioned, tuned, and used to produce laser-Compton x-rays and multiple electron bunches. A charge per bunch from a few pC to 500 pC has been measured, consistent with a quantum efficiency of $5\times {10}^{-5}$ using a 263 nm 10 Hz photocathode drive laser. The rf gun has operated close to design performance at high gradient, and more reliably at lower gradient achieving a root mean square normalized emittance of 0.3 mm mrad at both 80 pC at $185\mathrm{MV}/\mathrm{m}$, and 40 pC at $165\mathrm{MV}/\mathrm{m}$. Thermal emittance is estimated at $0.55\mathrm{mm}\mathrm{mrad}/\mathrm{mm}$. Energy spread of 0.03% has been achieved. These results agree very well with modeling predictions for the operating conditions under which the measurements were made. Unusually disruptive breakdowns were observed with an applied magnetic field of $0.5T$ used for emittance compensation.

The development of high brightness electron sources can enable an increase in performance and reduction in size of extreme X-ray sources such as free electron lasers (FELs). A promising path to high brightness is through larger electric fields in radio-frequency (rf) photoinjectors. Recent experiments with 11.4 GHz copper accelerating cavities at cryogenic temperatures have demonstrated $500\mathrm{MV}/\mathrm{m}$ surface electric fields with low rf breakdown rates. However, when the surface electric fields are larger than $300\mathrm{MV}/\mathrm{m}$, the measured cavity quality factor, $Q_0$, decreases during the input rf pulse by up to 30%, recovering before the next rf pulse. In this paper, we present an experimental study of the rf losses, manifested as degradation of $Q_0$, in a copper cavity operated at cryogenic temperatures and high gradients. The experimental conditions range from temperatures of 10–77 K and rf pulse lengths of 100–800 ns, using surface electric fields up to $400\mathrm{MV}/\mathrm{m}$. We developed a model for the change in $Q_0$ using measured field emission currents and rf signals. We find that the $Q_0$ degradation is consistent with the rf power being absorbed by strong field emission currents accelerated inside the cavity.

The rapid cycling synchrotron (RCS) of the China Spallation Neutron Source (CSNS) employs painting injection to achieve a uniform beam distribution and to suppress the space charge effect. In the painting injection, a mismatch between the injection and the circulating beam orbit can cause nonuniform beam distribution and larger emittance, which may lead to beam loss. To match the injection beam orbit and the circulating beam orbit, it is necessary to identify the injection beam orbit relative to the circulating beam orbit at the injection point. However, the relative injection beam orbit is hard to measure directly. Theoretically, the relative injection beam orbit can be deduced from the turn-by-turn beam position of a single-turn injected beam, but in the RCS of CSNS, the intensity of a single-turn injected beam is too low to measure with a sufficient signal-noise ratio using a beam position monitor. In this paper, two effective methods based on multiturn injection and turn-by-turn beam position data are given to perform the match between the injection and the circulating beam orbit. Simulation results and practical application during the beam commissioning of the RCS of CSNS show the validity of the methods.

We consider the calculation of electromagnetic fields generated by a point charge passing through an anisotropic vacuum chamber of round or rectangular cross section with translational symmetry in the beam direction. The anisotropy causes a frequency shift and can have a significant effect on modal coupling. It must be accounted for in the design and analysis of accelerating structures. In this paper, field matching and finite-difference techniques for the anisotropic waveguides are developed. Additionally a method that mixes the field matching and the finite-differences is described. The numerical algorithms are implemented in a computer code and cross-checked on several examples. The impact of the anisotropy along different coordinate axes on the resonance frequency shifts is computed and analyzed.

A new scheme to produce very low emittance muon beams using a positron beam of about 45 GeV interacting on electrons on target is presented. One of the innovative topics to be investigated is the behaviour of the positron beam stored in a low emittance ring with a thin target, that is directly inserted in the ring chamber to produce muons. Muons can be immediately collected at the exit of the target and transported to two ${\mu }^{+}$ and ${\mu }^{-}$ accumulator rings and then accelerated and injected in muon collider rings. We focus in this paper on the simulation of the ${\mathrm{e}}^{+}$ beam interacting with the target, the effect of the target on the 6-D phase space and the optimization of the ${\mathrm{e}}^{+}$ ring design to maximize the energy acceptance. We will investigate the performance of this scheme, ring plus target system, comparing different multi-turn simulations. The source is considered for use in a multi-TeV collider in [F. Zimmermann, in Strategy for Future Extreme Beam Facilities, Accelerator Technology Institute of Electronic Systems, Vol. 44 (Warsaw University of Technology, 2017).].