The Touschek effect is one of the dominant loss mechanisms for lepton storage rings of low emittance (i.e. high bunch density) and low or moderate beam energy. So, in the past years, for all lepton circular machines, either colliders or modern synchrotron light sources, either due to high bunch density or to beam energy, more and more efforts have been made for a comprehensive evaluation of this effect. This paper describes the simulation tool developed to evaluate the actions relevant to minimize the Touschek effect, such as position and shape of scrapers and detector shieldings, interaction region optics design with a particular attention to sextupoles for their relevant nonlinear effects. Relevant results obtained through the years at the low energy ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ collider DAФNE are presented. A reduction of the luminosity normalized to the background of more than 1 order of magnitude is obtained with an increase in luminosity from ${10}^{31}$ to ${10}^{32}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ and a small decrease in lifetime. The corresponding data Monte Carlo simulation comparison for each action taken is presented, showing an overall good agreement.

An original method to determine the charged particle energy is developed. This method uses the dependency of waveguide mode frequency on the Lorentz factor of particles. It is central to this technique that the particle bunch generates Cherenkov radiation in a waveguide, and the mode frequencies depend essentially on the Lorentz factor. Here, we consider the case when radiation is excited in a circular waveguide with a dielectric layer. It is shown that structures with relatively thick layers are not convenient for the particle energy measurement because the dependence of the first mode frequency on the Lorentz factor is weak. In contrast, a structure with a thin layer is favorable for such a purpose because this dependency is more essential. Analytical and numerical investigations are performed. It is shown that the first mode amplitude is sufficient for measurements in the case of a pico-Coulomb bunch.

The spatiotemporal response of crystals in x-ray Bragg diffraction resulting from excitation by an ultrashort, laterally confined x-ray pulse is studied theoretically. The theory presents an extension of the analysis in symmetric reflection geometry [R. R. Lindberg and Y. V. Shvyd’ko, Phys. Rev. ST Accel. Beams 15, 050706 (2012)] to the generic case, which includes Bragg diffraction both in reflection (Bragg) and transmission (Laue) asymmetric scattering geometries. The spatiotemporal response is presented as a product of a crystal-intrinsic plane-wave spatiotemporal response function and an envelope function defined by the crystal-independent transverse profile of the incident beam and the scattering geometry. The diffracted wave fields exhibit amplitude modulation perpendicular to the propagation direction due to both angular dispersion and the dispersion due to Bragg’s law. The characteristic measure of the spatiotemporal response is expressed in terms of a few parameters: the extinction length, crystal thickness, Bragg angle, asymmetry angle, and the speed of light. Applications to self-seeding of hard x-ray free-electron lasers are discussed, with particular emphasis on the relative advantages of using either the Bragg or Laue scattering geometries. Intensity front inclination in asymmetric diffraction can be used to make snapshots of ultrafast processes with femtosecond resolution.

High brightness electron sources for linac based free-electron lasers (FELs) are being developed at the Photo Injector Test facility at DESY, Zeuthen site (PITZ). Production of electron bunches with extremely small transverse emittance is the focus of the PITZ scientific program. The photoinjector optimization in 2008–2009 for a bunch charge of 1, 0.5, 0.25, and 0.1 nC resulted in measured emittance values which are beyond the requirements of the European XFEL [S. Rimjaem et al., Nucl. Instrum. Methods Phys. Res., Sect. A 671, 62 (2012)]. Several essential modifications were commissioned in 2010–2011 at PITZ, resulting in further improvement of the photoinjector performance. Significant improvement of the rf gun phase stability is a major contribution in the reduction of the measured transverse emittance. The old TESLA prototype booster was replaced by a new cut disk structure cavity. This allows acceleration of the electron beam to higher energies and supports much higher flexibility for stable booster operation as well as for longer rf pulses which is of vital importance especially for the emittance optimization of low charge bunches. The transverse phase space of the electron beam was optimized at PITZ for bunch charges in the range between 0.02 and 2 nC, where the quality of the beam measurements was preserved by utilizing long pulse train operation. The experimental optimization yielded worldwide unprecedented low normalized emittance beams in the whole charge range studied.

The Advanced Photoinjector Experiment (APEX) at the Lawrence Berkeley National Laboratory is dedicated to the development of a high-brightness high-repetition rate (MHz-class) electron injector for x-ray free-electron laser (FEL) and other applications where high repetition rates and high brightness are simultaneously required. The injector is based on a new concept rf gun utilizing a normal-conducting (NC) cavity resonating in the VHF band at 186 MHz, and operating in continuous wave (cw) mode in conjunction with high quantum efficiency photocathodes capable of delivering the required charge at MHz repetition rates with available laser technology. The APEX activities are staged in three phases. In phase 0, the NC cw gun is built and tested to demonstrate the major milestones to validate the gun design and performance. Also, starting in phase 0 and continuing in phase I, different photocathodes are tested at the gun energy and at full repetition rate for validating candidate materials to operate in a high-repetition rate FEL. In phase II, a room-temperature pulsed linac is added for accelerating the beam at several tens of MeV to reduce space charge effects and allow the measurement of the brightness of the beam from the gun when integrated in an injector scheme. The installation of the phase 0 beam line and the commissioning of the VHF gun are completed, phase I components are under fabrication, and initial design and specification of components and layout for phase II are under way. This paper presents the phase 0 commissioning results with emphasis on the experimental milestones that have successfully demonstrated the APEX gun capability of operating at the required performance.

We have developed high-efficiency normal-conducting rf accelerating structures by combining $H$-mode resonator cavities and a transverse beam focusing by permanent-magnet quadrupoles (PMQ), for beam velocities in the range of a few percent of the speed of light. The shunt impedance of interdigital $H$-mode (IH-PMQ) structures is 10–20 times higher than that of a conventional drift-tube linac, while the transverse size is 4–5 times smaller. Results of the combined 3D modeling—electromagnetic computations, multiparticle beam-dynamics simulations with high currents, and thermal-stress analysis—for an IH-PMQ accelerator tank are presented. The accelerating-field profile in the tank is tuned to provide the best propagation of a 50-mA deuteron beam using coupled iterations of electromagnetic and beam-dynamics modeling. Measurements of a cold model of the IH-PMQ tank show a good agreement with the calculations. Examples of cross-bar $H$-mode structures with PMQ focusing for higher beam velocities are also presented. $H$-PMQ accelerating structures following a short radio-frequency quadrupole accelerator can be used both in the front end of ion linacs or in stand-alone applications.

Laser-based techniques have been widely used for cleaning metal photocathodes to increase quantum efficiency (QE). However, the impact of laser cleaning on cathode uniformity and thereby on electron beam quality are less understood. We are evaluating whether this technique can be applied to revive photocathodes used for high-brightness electron sources in advanced x-ray free-electron laser (FEL) facilities, such as the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. The laser-based cleaning was applied to two separate areas of the current LCLS photocathode on July 4 and July 26, 2011, respectively. The QE was increased by 8–10 times upon the laser cleaning. Since the cleaning, routine operation has exhibited a slow evolution of the QE improvement and comparatively rapid improvement of transverse emittance, with a factor of 3 QE enhancement over five months, and a significant emittance improvement over the initial 2–3 weeks following the cleaning. Currently, the QE of the LCLS photocathode is holding constant at about $1.2\times {10}^{-4}$, with a normalized injector emittance of about $0.3\mu \mathrm{m}$ for a 150-pC bunch charge. With the proper procedures, the laser-cleaning technique appears to be a viable tool to revive the LCLS photocathode. We present observations and analyses for the QE and emittance evolution in time following the laser-based cleaning of the LCLS photocathode, and comparison to the previous studies, the measured thermal emittance versus the QE and comparison to the theoretical model.

The LHC is currently operating with a proton energy of 4 TeV and ${\beta }^{*}$ functions at the ATLAS and CMS interaction points of 0.6 m. This is close to the design value at 7 TeV (${\beta }^{*}=0.55\mathrm{m}$) and represented a challenge for various aspects of the machine operation. In particular, a huge effort was put into the optics commissioning and an unprecedented peak $\beta$ beating of around 7% was achieved in a high energy hadron collider.

Quench is a common phenomenon in a superconducting cavity and often limits the accelerating gradient of the cavity. Accurate location of the quench site, typically located at a material or geometrical defect, is the key to improve the cavity accelerating gradient. Here, the second sound propagation in liquid helium II is used to detect the quench location on the cavity. The technique is relatively convenient and complements the traditional temperature mapping which measures the “prequench” temperature rise on the cavity using an array of sensors. The speed of the second sound in liquid helium II is roughly $1.7\mathrm{cm}/\mathrm{ms}$ at 2 K which is sufficiently fast to provide a millimeter-size position resolution. However, the dynamics of the quench at the cavity surface are also found to significantly affect the achievable resolution with real cavities. Here we use a dynamic quench model, based on ANSYS, to calculate the quench area and the temperature distribution on the cavity. The detection error caused by the thermal conduction in the niobium was calculated.

In this paper we discuss the generation of a new class of high brightness relativistic electron beams, characterized by ultralow charge (0.1–1 pC) and ultralow normalized emittance ($<50\mathrm{nm}$). These beams are created in rf photoinjectors when the laser is focused on the cathode to very small transverse sizes ($<30\mu \mathrm{m}$ rms). In this regime, the charge density at the cathode approaches the limit set by the extraction electric field. By shaping the laser pulse to have a cigarlike aspect ratio (the longitudinal dimension much larger than the transverse dimension) and a parabolic temporal profile, the resulting space charge dominated dynamics creates a uniformly filled ellipsoidal distribution and the emittance can be nearly preserved to its thermal value. We also present a new method, based on a variation of the pepper-pot technique, for single shot measurements of the ultralow emittances for this new class of beams.

It is widely believed that a drive laser with uniform temporal and spatial laser profiles is required to generate the lowest emittance beam at the photoinjector. However, for a given 3 ps smooth-Gaussian laser temporal profile, our recent simulations indicate that a truncated-Gaussian laser spatial profile produces an electron beam with smaller emittance. The simulation results are qualitatively confirmed by later analytical calculation, and also confirmed by measurements: emittance reduction of $\sim 25\%$ was observed at the linac coherent light source (LCLS) injector with a truncated-Gaussian laser spatial profile at the nominal operating bunch charge of 150 pC. There was a significant secondary benefit—laser transmission through the iris for the truncated-Gaussian profile was about twice that compared to the nearly uniform distribution, which significantly loosens the laser power and quantum efficiency requirements for drive laser system and photocathode. Since February 9, 2012, the drive laser with the truncated-Gaussian spatial distribution has been used for LCLS routine user operations and the corresponding free electron laser power is at least the same as the one when using the nearly uniform spatial profile.

We analyze the electromagnetic field of a charged particle moving uniformly in a circular waveguide and crossing the boundary between a dielectric and a vacuum. Our study focuses on the case when Cherenkov radiation is generated in the dielectric. Analytical and numerical investigation of the waveguide modes is performed. We show that a large radiation can be excited in the vacuum area. The mode amplitudes in the vacuum can be greater than those in the dielectric. The field from a Gaussian bunch is also studied. We note that the effect under consideration can be used to generate a large quasimonochromatic or multimode radiation.

The acceleration of charged particles, producing collimated monoenergetic beams, over short distances holds the promise to offer new tools in medicine and diagnostics. Here, we consider a possible mechanism for accelerating protons to high energies by using a phase modulated circularly polarized electromagnetic wave propagating along a constant magnetic field. It is observed that a plane wave with dimensionless amplitude of 0.1 is capable to accelerate a 1 keV proton to 386 MeV under optimum conditions. Finally, we discuss possible limitations of the acceleration scheme.

In the experiment the efficiency of the 50 GeV proton beam extraction from accelerator by means of a bent crystal as a function of crystal orientation was measured. This allowed one to make a comparative analysis of efficiencies of high-energy protons deflection by different crystal atomic planes with different values of the electrostatic field. The results of simulation of high-energy protons deflection by means of crystal atomic planes and crystal atomic strings are also presented in the article. In the case of planar channeling the simulation shows a good agreement with experimental data. In the case of proton motion in the regime of stochastic scattering by bent atomic strings the simulation shows that angles of particle deflection are much greater than the critical channeling angle.

We have performed the beam-dynamics design of an ${\mathrm{H}}^{-}$ radio frequency quadrupole (RFQ) for the beam-current upgrade of the Japan Proton Accelerator Research Complex linac (RFQ III). LINACSrfqDES was set up to support the conventional design method, i.e., design with CURLI-type shaper followed by constant bore radius and with constant intervane voltage, supplemented by keeping the equipartitioned condition in the gentle buncher section. For the particle simulation, LINACSrfqSIM was used. The obtained transmission, transverse and longitudinal emittance are 98.5%, $0.21\pi \mathrm{mm}\mathrm{mrad}$, and $0.11\pi \mathrm{MeV}\mathrm{deg}$ for the input beam current of 60 mA and normalized rms emittance of $0.20\pi \mathrm{mm}\mathrm{mrad}$. This design satisfies the requirements of RFQ III.

This paper investigates the focusing properties of linear magnetic undulators, i.e., devices characterized by weak defocusing properties in the horizontal (wiggling) plane and strongly focusing in the vertical plane. The problem of identifying the conditions that ensure the existence of the electron beam eigenstates in the undulator lattice for a given working point of electron beam energy $E_b$ and resonant wavelength ${\lambda }_r$ is studied. For any given undulator lattice, a bandlike structure is identified defining regions in the $(E_b,{\lambda }_r)$ plane where no periodic matching condition can be found, i.e., it is not possible to transport the electron beam so that optical functions are periodic at lattice boundaries. Some specific cases are discussed for the SPARC FEL undulator.

System-on-a-chip (SoC) bus systems are typically confined on-chip and rely on higher level components to communicate with the outside world. The idea behind the EtherBone (EB) protocol is to extend the reach of the SoC bus to remote field-programmable gate arrays or processors. The EtherBone core implementation connects a Wishbone (WB) Ver. 4 Bus via a Gigabit Ethernet based network link to remote peripheral devices. EB acts as a transparent interconnect module towards attached WB Bus devices. EB was developed in the scope of the WhiteRabbit Timing Project at CERN and GSI/FAIR. WhiteRabbit will make use of EB as a means to issue commands to its timing nodes and control connected accelerator hardware.

A reliable transverse emittance measurement for high-brightness electron beams is of utmost importance for the successful development of fourth generation light sources and for the beam transport in plasma-based accelerators. When the beam exhibits a significant energy spread, typical quadrupole scan emittance measurements may be affected depending on the beam properties and on the quadrupoles arrangement. The emittance degradation induced by chromatic effects in measurements involving magnetic lattices is evaluated analytically for different configurations. Analytical and numerical calculations compared with measurements have been used to evaluate the consequent error on the emittance value measured for single and double quadrupole schemes and for typical operating conditions at the SPARC facility.

The field emission characteristics of niobium electrodes were compared to those of stainless steel electrodes using a DC high voltage field emission test apparatus. A total of eight electrodes were evaluated: two 304 stainless steel electrodes polished to mirrorlike finish with diamond grit and six niobium electrodes (two single-crystal, two large-grain, and two fine-grain) that were chemically polished using a buffered-chemical acid solution. Upon the first application of high voltage, the best large-grain and single-crystal niobium electrodes performed better than the best stainless steel electrodes, exhibiting less field emission at comparable voltage and field strength. In all cases, field emission from electrodes (stainless steel and/or niobium) could be significantly reduced and sometimes completely eliminated, by introducing krypton gas into the vacuum chamber while the electrode was biased at high voltage. Of all the electrodes tested, a large-grain niobium electrode performed the best, exhibiting no measurable field emission ($<10\mathrm{pA}$) at 225 kV with 20 mm cathode/anode gap, corresponding to a field strength of $18.7\mathrm{MV}/\mathrm{m}$.

Harmonic lasing in a free electron laser with a planar undulator (under the condition that the fundamental frequency is suppressed) might be a cheap and efficient way of extension of wavelength ranges of existing and planned x-ray free electron laser (FEL) facilities. Contrary to nonlinear harmonic generation, harmonic lasing can provide much more intense, stable, and narrow-band FEL beam which is easier to handle due to the suppressed fundamental frequency. In this paper we perform a parametrization of the solution of the eigenvalue equation for lasing at odd harmonics, and present an explicit expression for FEL gain length, taking into account all essential effects. We propose and discuss methods for suppression of the fundamental harmonic. We also suggest a combined use of harmonic lasing and lasing at the retuned fundamental wavelength in order to reduce bandwidth and to increase brilliance of x-ray beam at saturation. Considering 3rd harmonic lasing as a practical example, we come to the conclusion that it is much more robust than usually thought, and can be widely used in the existing or planned x-ray FEL (XFEL) facilities. In particular, Linac Coherent Light Source (LCLS) after a minor modification can lase to saturation at the 3rd harmonic up to the photon energy of 25–30 keV providing multigigawatt power level and narrow bandwidth. As for the European XFEL, harmonic lasing would allow one to extend operating range (ultimately up to 100 keV), to reduce FEL bandwidth and to increase brilliance, to enable two-color operation for pump-probe experiments, and to provide more flexible operation at different electron energies. Similar improvements can be realized in other x-ray FEL facilities with gap-tunable undulators like FLASH II, SACLA, LCLS II, etc. Harmonic lasing can be an attractive option for compact x-ray FELs (driven by electron beams with a relatively low energy), allowing the use of the standard undulator technology instead of small-gap in-vacuum devices. Finally, in this paper we discover that in a part of the parameter space, corresponding to the operating range of soft x-ray beam lines of x-ray FEL facilities (like SASE3 beam line of the European XFEL), harmonics can grow faster than the fundamental wavelength. This feature can be used in some experiments, but might also be an unwanted phenomenon, and we discuss possible measures to diminish it.