The experiment described in this paper is the first study of the response of a static tungsten powder sample to an impinging high energy proton beam pulse. The experiment was carried out at the HiRadMat facility at CERN. Observations include high speed videos of a proton beam induced perturbation of the powder sample as well as data from a laser Doppler vibrometer measuring the oscillations of the powder container. A comparison with a previous analogous experiment which studied a proton beam interaction with mercury is made.

Here we describe some relationships between cavity shape and fields on and near its surface that can be used for better understanding of the surface field properties. The problem of accelerating cavity optimization lies in the search of the shape with minimal peak magnetic or electric field for a given acceleration rate. This problem became especially important due to widespread use of superconducting cavities where the maximal magnetic field appeared to have a hard limit. The peak magnetic field can be lowered if one can increase the peak electric field but the electric field is also limited because of field emission. The problem of minimal losses in a cavity is very close to the problem of minimal peak magnetic field, though it is not the same, it relates to the lowest average field for a given acceleration rate. The field configuration on the cavity surface is also responsible for the phenomenon of multipactor. Cavities with these properties—minimal peak fields, minimal losses, and absence of multipactor—are found within the set of elliptic cavities. Further improvement of these properties is possible if we step out of the limits of elliptic shapes.

For a proper design of the cryogenic layout of superconducting insertion devices it is necessary to take into account the heat load from the beam to the cold beam tube. In order to measure and possibly understand the beam heat load to a cold bore, a cold vacuum chamber for diagnostics (COLDDIAG) has been built. COLDDIAG is designed in a flexible way, to allow its installation in different light sources. In order to study the beam heat load and the influence of the cryosorbed gas layer, the instrumentation comprises temperature sensors, pressure gauges, and mass spectrometers as well as retarding field analyzers with which it is possible to measure the beam heat load, total pressure, and gas content as well as the flux of particles hitting the chamber walls. In this paper we describe the experimental equipment, the installation of COLDDIAG in the Diamond Light Source and selected examples of the measurements performed to show the capabilities of this unique instrument.

Nonlinearities from any periodic magnet in accelerators may strongly degrade the dynamics of the beams. This may be especially critical for magnets where the beam excursions are comparable to the good field region, as for example in wigglers, since the beam trajectory can be of the order of the pole width. A general method based on alternately shifting the magnetic axis of each pole to compensate this effect was proposed and applied to the $\mathrm{DA}\mathrm{\Phi }\mathrm{NE}$ wigglers, where an important integrated octupole was measured. This approach has been optimized by multipolar analyses and the effect on the beam dynamics verified by tracking studies. In this paper we report about the experimental validation of the magnetic model and the verification of the method by beam based measurements. The latter were performed after all the wigglers in the $\mathrm{DA}\mathrm{\Phi }\mathrm{NE}$ main rings had been modified according to the optimal configuration. These measurements were in agreement with the expectations and allowed experimentally proving the method.

In this paper, a genetic algorithm-based optimization is used to simultaneously minimize two competing objectives guiding the operation of the Jefferson Lab’s Continuous Electron Beam Accelerator Facility linacs: cavity heat load and radio frequency cavity trip rates. The results represent a significant improvement to the standard linac energy management tool and thereby could lead to a more efficient Continuous Electron Beam Accelerator Facility configuration. This study also serves as a proof of principle of how a genetic algorithm can be used for optimizing other linac-based machines.

Independently from conventional electron beam based procedures, the photon beam based diagnostic is an alternative way for electron trajectory alignment and commissioning of numerous undulator cells in high-gain short-wavelength free-electron lasers (FELs). In this paper, using the microbunched electron beam and the undulator fine-tuning technique, a novel method based on the spatial profile analysis of coherent harmonic radiation from individual or two consecutive undulator segments, the so-called coherent photon beam based diagnostic, is proposed for undulator alignment and commissioning in seeded FELs. The preliminary experimental results at the Shanghai deep ultraviolet FEL test facility are presented. It shows that the proposed method is effective and convenient in the electron beam trajectory control, the undulator magnetic gap verification, and the phase match between two undulator segments.

The infrared free-electron laser (FEL) offers a large tunability since the FEL gain remains high throughout the infrared spectral range, and the reflectivity of metal mirrors remains also close to unity. The main limitation comes from the diffraction of the optical beam due to the finite size of the vacuum chamber of the undulator. A solution is to use this chamber as a waveguide by adapting the radius of curvature of the cavity mirrors to this regime. Then, as has been shown before, a minimum appears in the spectrum that can be produced by the FEL. We discuss the physical mechanism of this particular regime and compare it to experiments using vacuum chambers of different transverse sizes. A good agreement is found with results of simulations and with a simple analytical formula.

Quadrupole fringe fields in the final focus system can be a source of aberrations in the interaction point transverse beam sizes. This paper investigates the fringe field impact on the transverse beam size in the ATF2, ILC, and CLIC lattices in the linear and non-linear regimes. The linear effects are studied by replacing the hard-edge quadrupolar field by the more realistic gradient fall-off. To address the nonlinear effects, the fringe fields are represented as high order kicks added to both sides of the hard-edge magnets. It will be shown that the linear fringe fields effects can be easily cured by tuning the quadrupole strengths. On the other hand, mitigation of the nonlinear fringe fields effects is more difficult and requires use of octupole magnets or, alternatively, increasing the value of interaction point horizontal beta function ${\beta }_x^{*}$.

The minimum obtainable transverse emittance (thermal emittance) of electron beams generated and trapped in plasma-based accelerators using laser ionization injection is examined. The initial transverse phase space distribution following ionization and passage through the laser is derived, and expressions for the normalized transverse beam emittance, both along and orthogonal to the laser polarization, are presented. Results are compared to particle-in-cell simulations. Ultralow emittance beams can be generated using laser ionization injection into plasma accelerators, and examples are presented showing normalized emittances on the order of tens of nm.

Field emission current from surfaces under vacuum and at high field strengths can be reduced by the injection of gas into the evacuated volume. In this paper, the effects of ${\mathrm{H}}_2$, He, ${\mathrm{N}}_2$, and Ar on this “dark” current emitted from a tungsten carbide point cathode for 2 cm gap distance is studied. Exposure to any of these gases at pressures on the order of ${10}^{-3}–{10}^{-2}\mathrm{Pa}$ was found to reduce the emission current by up to 90% with a time constant on the order of $\sim 1\mathrm{minute}$ as compared to the current at ${10}^{-6}\mathrm{Pa}$. The effect was strongly dependent on the gas nature, with Ar and ${\mathrm{N}}_2$ having larger effects at lower pressures than He and ${\mathrm{H}}_2$. The reduction was reversible, with the current increasing to near its original value with a time constant on the order of $\sim 1–10\mathrm{minutes}$ after pumping down. The effect of the gas remained in the absence of electric field, whatever the gas pressure. Mechanisms for these and related phenomena are discussed.

Performance of the x-ray free electron laser Linac Coherent Light Source (LCLS) and the Facility for Advanced Accelerator Experimental Tests (FACET) is determined by the properties of their extremely short electron bunches. Multi-GeV electron bunches in both LCLS and FACET are less than 100 fs long. Optimization of beam properties and understanding of free-electron laser operation require electron beam diagnostics with time resolution of about 10 fs. We designed, built and commissioned a set of high frequency $X$-band deflectors which can measure the beam longitudinal space charge distribution and slice energy spread to better than 10 fs resolution at full LCLS energy (14 GeV), and with 70 fs resolution at full FACET energy (20 GeV). Use of high frequency and high gradient in these devices allows them to reach unprecedented performance. We report on the physics motivation, design considerations, operational configuration, cold tests, and typical results of the $X$-band deflector systems currently in use at SLAC.

As part of the Large Hadron Collider high luminosity upgrade it is proposed to include crab cavities in the lattice in order to enhance the luminosity. For one proposed cavity design the dynamics of the cavity is considered in terms of its impact upon the dynamic aperture of the machine. Taylor maps of the cavity are created and used to perform this analysis with a full assessment of their validity. Furthermore from these Taylor maps, symplectic methods are developed further, guided by the knowledge gained in the study of the physics contained in them.

Maximizing the performance of modern linear accelerators working with high gradient electromagnetic fields depends to a large extent on ability to control breakdown rates near metal surfaces in the accelerating structures. Nanoscale voids, presumably forming in the surface layers of metals during the technological processing, can be responsible for the onset of the growth of a surface protrusion. We use finite element simulations to study the evolution of annealed copper, single crystal copper and stainless steel surfaces that contain a void under high electric fields. We use a fully coupled electrostatic-elastoplastic model in the steady state. Gradually increasing the value of an external electric field, we analyze the relationship of surface failure and depth of the void for the chosen materials with different elastoplastic properties. According to our results, the stainless steel and single crystal copper surfaces demonstrate the formation of well-defined protrusions, when the external electric field reaches a certain critical value. Among the three materials, annealed copper surface starts yielding at the lowest electric fields due to the lowest Young’s modulus and yield stress. However, it produces the smallest protrusions due to a significant strain hardening characteristic for this material.

A new theory is developed to evaluate longitudinal and transverse impedances of any size of ceramic break that is sandwiched between metal chambers. The theory has been numerically compared with the codes ABCI and CST studio. Excellent agreements are obtained with both codes, in particular with ABCI. The theory successfully reproduces resonance structures of the impedance due to trapped modes inside the ceramic break, which are enhanced by the difference of the dielectric constants between the ceramic and the surrounding space. Moreover, the theory can evaluate the impedance of the ceramic break with titanium nitride coating. We discuss several characteristics of the impedances, especially the difference between the impedances of the ceramic break covered with and without a conductive wall on its outer surface.

Neutrino beam with about 300 MeV in energy, high-flux and medium baseline is considered a rational choice for measuring CP violation before the more powerful Neutrino Factory is to be built. Following this concept, a unique neutrino beam facility based on muon-decayed neutrinos is proposed. The facility adopts a continuous-wave proton linac of 1.5 GeV and 10 mA as the proton driver, which can deliver an extremely high beam power of 15 MW. Instead of pion-decayed neutrinos, unprecedentedly intense muon-decayed neutrinos are used for better background discrimination. The schematic design for the facility is presented here, including the proton driver, the assembly of a mercury-jet target and capture superconducting solenoids, a $\mathrm{pion}/\mathrm{muon}$ beam transport line, a long muon decay channel of about 600 m and the detector concept. The physics prospects and the technical challenges are also discussed.

Charged particle radiography has become a promising new approach in the field of transmission radiography because of the invention of the magnetic imaging lens. The using of the imaging lens makes it possible for thick objects to get significantly improved transmission radiography. Currently, the conventional charged particle radiography only uses the information of the flux attenuation and the angular scattering of the transmitted particles to determine the properties of the sample. However, the energy loss of the incident particles introduced by ionizations throughout the object limits the spatial resolution of the image because of the chromatic blur. In this paper a new concept of imaging lens that uses the information of the energy loss is proposed. With a specially designed imaging lens, the information of the energy loss could result in apparent contrast in the final image. This design procedure of the energy loss imaging lens is presented, and a preliminary design is verified by numerical simulations. Experimental demonstration is also expected on a cyclotron at the Institute of Fluid Physics, CAEP.

A study into the maximum acceleration gradient, $E_z^{\max }$, in a collinear dielectric wakefield accelerator due to single bunch beam breakup (SBBU) is presented. The longitudinal wakefield is proportional to charge over radius squared ($E_z\sim Q/a^2$) which implies small $a$ is favorable for fixed $Q$. However, when the transverse wakefield ($E_{\perp }\sim Q/a^3$) is also considered then the SBBU instability severely limits the charge that can pass through a long structure as required for a large transfer of energy from the drive bunch to the witness bunch and hence limits the sustained acceleration that can be obtained.