In this work we investigate superconducting properties of niobium samples via application of the muon spin rotation/relaxation ($\mu \mathrm{SR}$) technique. We employ for the first time the $\mu \mathrm{SR}$ technique to study samples that are cut out from large and small grain 1.5 GHz radio frequency (rf) single cell niobium cavities. The rf test of these cavities was accompanied by full temperature mapping to characterize the rf losses in each of the samples. Results of the $\mu \mathrm{SR}$ measurements show that standard cavity surface treatments like mild baking and buffered chemical polishing performed on the studied samples affect their surface pinning strength. We find an interesting correlation between high field rf losses and field dependence of the sample magnetic volume fraction measured via $\mu \mathrm{SR}$. The $\mu \mathrm{SR}$ line width observed in zero-field-$\mu \mathrm{SR}$ measurements matches the behavior of Nb samples doped with minute amounts of Ta or $N$ impurities. A lower and an upper bound for the upper critical field $H_{c2}$ of these cutouts is found.

A new method of measuring of the rms longitudinal Twiss parameters of a beam in linear accelerators is presented. It is based on using sum signals from beam position monitors sensitive to the longitudinal charge distribution in the bunch. The applicability of the method is demonstrated on the superconducting section of the Oak Ridge Spallation Neutron Source linear accelerator. The results are compared to a direct measurement of the bunch longitudinal profiles using an interceptive bunch shape monitor in the linac warm section of the same accelerator. Limitations of the method are discussed. The method is fast and simple, and can be used to obtain the initial parameters for the longitudinal matching in linear accelerators where interceptive diagnostics are not desirable.

We report the first high-resolution measurement of superradiance, using coherent synchrotron radiation in the terahertz region from the Canadian Light Source synchrotron and a Michelson interferometer with a nominal frequency resolution of $0.00096{\mathrm{cm}}^{-1}$. Superradiance arises when a high degree of phase coherence exists between the radiation fields of the individual electron bunches, and manifests itself as a series of narrow spectral peaks at harmonics of the bunch frequency. We observe an enhancement factor of 16 at the spectral peaks, limited by the interferometer resolution. The spectral distribution and relative amplitudes of the superradiant peaks are modified by altering the pattern of bunches along the bunch train.

Beam loading compensation is a key for acceleration of a high intensity proton beam in the main ring (MR) of the Japan Proton Accelerator Research Complex (J-PARC). Magnetic alloy loaded rf cavities with a Q value of 22 are used to achieve high accelerating voltages without a tuning bias loop. The cavity is driven by a single harmonic ($h=9$) rf signal while the cavity frequency response also covers the neighbor harmonics ($h=8,10$). Therefore the wake voltage induced by the high intensity beam consists of the three harmonics, $h=8,9,10$. The beam loading of neighbor harmonics is the source of periodic transient effects and a possible source of coupled bunch instabilities. In the article, we analyze the wake voltage induced by the high intensity beam. We employ the rf feedforward method to compensate the beam loading of these three harmonics ($h=8,9,10$). The full-digital multiharmonic feedforward system was developed for the MR. We describe the system architecture and the commissioning methodology of the feedforward patterns. The commissioning of the feedforward system has been performed by using high intensity beams with $1.0\times {10}^{14}$ proteins per pulse. The impedance seen by the beam is successfully reduced and the longitudinal oscillations due to the beam loading are reduced. By the beam loading compensation, stable high power beam operation is achieved. We also report the reduction of the momentum loss during the debunching process for the slow extraction by the feedforward.

During the commissioning of the CERN Proton Synchrotron multiturn extraction, tests with different beam intensities were performed in order to probe the behavior of resonance crossing in the presence of possible space charge effects. The initial beam intensity before transverse splitting was varied and the properties of the five beamlets obtained by crossing the fourth-order horizontal resonance were studied. A clear dependence of the beamlets’ parameters on the total beam intensity was found, which is the first direct observation of intensity-dependent effects for such a peculiar beam type. The experimental results are presented and discussed in detail in this paper.

A beam-monitor system for XFEL/SPring 8, “SACLA,” has been constructed. In order to maintain a stable self-amplified spontaneous emission (SASE) interaction, the straightness and overlap of the axes to within $3\mu \mathrm{m}$ between the electron beams and the radiated x rays for an undulator section of about 100 m length is necessary. This straightness means relative alignment to an experimental target sample. Furthermore, a temporal stability of 30 fs in order to maintain a constant peak beam current is also necessary to conduct stable SASE lasing. The monitor system was developed to satisfy these spatial and temporal stability and resolution criteria. The system comprises spatial monitors, such as cavity-type beam-position monitors and screen monitors, as well as temporal measurement instruments, such as current monitors, waveguide spectrometers, coherent synchrotron-radiation detectors, a streak camera, and an rf deflector. Commissioning of SACLA started from March 2011, and the monitors performed sufficient roles to tune the beams for lasing. The achieved overall performances of the system, including data acquisition, are: the beam position monitor has a spatial resolution of 600 nm in rms; the bunch-length monitors show ability to observe bunch lengths from 1 ns in an injector with velocity bunching to less than 30 fs after three-stage bunch compressors. The less than $3\mu \mathrm{m}$ spatial resolution of the screen monitor was also confirmed during practical beam operation. Owing to these fulfilled performances, such as the high spatial and temporal resolutions, stable lasing of SACLA has been achieved.

We describe a modification to the finite-difference time-domain algorithm for electromagnetics on a Cartesian grid which eliminates numerical dispersion error in vacuum for waves propagating along a grid axis. We provide details of the algorithm, which generalizes previous work by allowing 3D operation with a wide choice of aspect ratio, and give conditions to eliminate dispersive errors along one or more of the coordinate axes. We discuss the algorithm in the context of laser-plasma acceleration simulation, showing significant reduction—up to a factor of 280, at a plasma density of ${10}^{23}{\mathrm{m}}^{-3}$—of the dispersion error of a linear laser pulse in a plasma channel. We then compare the new algorithm with the standard electromagnetic update for laser-plasma accelerator stage simulations, demonstrating that by controlling numerical dispersion, the new algorithm allows more accurate simulation than is otherwise obtained. We also show that the algorithm can be used to overcome the critical but difficult challenge of consistent initialization of a relativistic particle beam and its fields in an accelerator simulation.

Using laser accelerated protons or ions for various applications—for example in particle therapy or short-pulse radiographic diagnostics—requires an effective method of focusing and energy selection. We derive an analytical scaling for the performance of a solenoid compared with a doublet/triplet as function of the energy, which is confirmed by TRACEWIN simulations. Generally speaking, the two approaches are equivalent in focusing capability, if parameters are such that the solenoid length approximately equals its diameter. The scaling also shows that this is usually not the case above a few MeV; consequently, a solenoid needs to be pulsed or superconducting, whereas the quadrupoles can remain conventional. It is also important that the transmission of the triplet is found only 25% lower than that of the equivalent solenoid. Both systems are equally suitable for energy selection based on their chromatic effect as is shown using an initial distribution following the RPA simulation model by Yan et al. [Phys. Rev. Lett. 103, 135001 (2009].

Stable operation of a laser-plasma accelerator near the threshold for electron self-injection in the blowout regime has been demonstrated with 25–60 TW, 30 fs laser pulses focused into a 3–4 millimeter length gas jet. Nearly Gaussian shape and high nanosecond contrast of the focused pulse appear to be critically important for controllable, tunable generation of 250–430 MeV electron bunches with a low-energy spread, $\sim 10\mathrm{pC}$ charge, a few-mrad divergence and pointing stability, and a vanishingly small low-energy background. The physical nature of the near-threshold behavior is examined using three-dimensional particle-in-cell simulations. Simulations indicate that properly locating the nonlinear focus of the laser pulse within the plasma suppresses continuous injection, thus reducing the low-energy tail of the electron beam.

Based on single particle tracking in the framework of classical Thomson scattering with incoherent superposition, we developed a relativistic, three-dimensional numerical model that calculates and quantifies the characteristics of emitted radiation when a relativistic electron beam interacts with an intense laser pulse. This model has been benchmarked against analytical expressions, based on the plane wave approximation to the laser field, derived by Esarey et al. [Phys. Rev. E 48, 3003 (1993)]. For laser pulses of sufficient duration, we find that the scattered radiation spectrum is broadened due to interferences arising from the pulsed nature of the laser. We find that by appropriately chirping the scattering laser pulse, spectral broadening can be minimized, and the peak on-axis brightness of the emitted radiation is increased by a factor of approximately 5.

The conceptual design for a nonscaling fixed field alternating gradient accelerator suitable for charged particle therapy (the use of protons and other light ions to treat some forms of cancer) is described.

The development of models and the “Virtual Detector for Synchrotron Radiation” (vdsr) code that accurately describe the production of synchrotron radiation are described. These models and code are valid in the classical and linear (single-scattering) quantum regimes and are capable of describing radiation produced from laser-plasma accelerators (LPAs) through a variety of mechanisms including betatron radiation, undulator radiation, and Thomson/Compton scattering. Previous models of classical synchrotron radiation, such as those typically used for undulator radiation, are inadequate in describing the radiation spectra from electrons undergoing small numbers of oscillations. This is due to an improper treatment of a mathematical evaluation at the end points of an integration that leads to an unphysical plateau in the radiation spectrum at high frequencies, the magnitude of which increases as the number of oscillation periods decreases. This is important for betatron radiation from LPAs, in which the betatron strength parameter is large but the number of betatron periods is small. The code vdsr allows the radiation to be calculated in this regime by full integration over each electron trajectory, including end-point effects, and this code is used to calculate betatron radiation for cases of experimental interest. Radiation from Thomson scattering and Compton scattering is also studied with vdsr. For Thomson scattering, radiation reaction is included by using the Sokolov method for the calculation of the electron dynamics. For Compton scattering, quantum recoil effects are considered in vdsr by using Monte Carlo methods. The quantum calculation has been benchmarked with the classical calculation in a classical regime.

Two different tune measurement systems have been installed in the GSI Helmholtzzentrum für Schwerionenforschung heavy-ion synchrotron SIS-18. Tune spectra are obtained with high accuracy using these fast and sensitive systems. Besides the machine tune, the spectra contain information about the intensity dependent coherent tune shift and the incoherent space charge tune shift. The space charge tune shift is derived from a fit of the observed shifted positions of the synchrotron satellites to an analytic expression for the head-tail eigenmodes with space charge. Furthermore, the chromaticity is extracted from the measured head-tail mode structure. The results of the measurements provide experimental evidence of the importance of space charge effects and head-tail modes for the interpretation of transverse beam signals at high intensity.

The EUROnu project has studied three possible options for future, high intensity neutrino oscillation facilities in Europe. The first is a Super Beam, in which the neutrinos come from the decay of pions created by bombarding targets with a 4 MW proton beam from the CERN High Power Superconducting Proton Linac. The far detector for this facility is the 500 kt MEMPHYS water Cherenkov, located in the Fréjus tunnel. The second facility is the Neutrino Factory, in which the neutrinos come from the decay of and beams in a storage ring. The far detector in this case is a 100 kt magnetized iron neutrino detector at a baseline of 2000 km. The third option is a Beta Beam, in which the neutrinos come from the decay of beta emitting isotopes, in particular and , also stored in a ring. The far detector is also the MEMPHYS detector in the Fréjus tunnel. EUROnu has undertaken conceptual designs of these facilities and studied the performance of the detectors. Based on this, it has determined the physics reach of each facility, in particular for the measurement of violation in the lepton sector, and estimated the cost of construction. These have demonstrated that the best facility to build is the Neutrino Factory. However, if a powerful proton driver is constructed for another purpose or if the MEMPHYS detector is built for astroparticle physics, the Super Beam also becomes very attractive.

The field of hadron therapy is growing rapidly with several facilities currently being planned, under construction or in commissioning worldwide. In the “active scanning” irradiation technique, the target is irradiated using a narrow pencil beam that is scanned transversally over the target while the penetration depth is altered with the beam energy. Together, the target dose can thereby be conformed in all three dimensions to the shape of the tumor. For applications where a sharp lateral beam penumbra is required in order to spare critical organs from unwanted dose, beam size blowup due to scattering in on-line beam diagnostic monitors, air gaps and passive elements like the ripple filter must be minimized. This paper presents a model for transverse scattering of therapeutic hadron beams along arbitrary multislab geometries. The conventional scattering formulation, which is only applicable to a drift space, is extended to not only take beam optics into account, but also non-Gaussian transverse beam profiles which are typically obtained from the slow resonant extraction from a synchrotron. This work has been carried out during the design phase of the beam delivery system for MedAustron, an Austrian hadron therapy facility with first patient treatment planned for the end of 2015. Irradiation will be performed using active scanning with proton and carbon ion beams. As a direct application of the scattering model, design choices for the MedAustron proton gantry and treatment nozzles are evaluated with respect to the transverse beam profile at the focal point; in air and at the Bragg peak.

We report the experimental generation, acceleration, and characterization of a uniformly filled electron bunch obtained via space-charge-driven expansion (often referred to as “blow-out regime”) in an L-band (1.3-GHz) radiofrequency photoinjector. The beam is photoemitted from a cesium-telluride semiconductor photocathode using a short ($<200\mathrm{fs}$) ultraviolet laser pulse. The produced electron bunches are characterized with conventional diagnostics and the signatures of their ellipsoidal character are observed. We especially demonstrate the production of ellipsoidal bunches with charges up to $\sim 0.5\mathrm{nC}$ corresponding to a $\sim 20$-fold increase compared to previous experiments with metallic photocathodes.

We propose a simple method to significantly enhance the temporal coherence and spectral brightness of a self-amplified spontaneous emission (SASE) free-electron laser (FEL). In this purified SASE (pSASE) FEL, a few undulator sections (called slippage-boosted sections) resonant at a subharmonic of the FEL radiation are used in the middle stage of the exponential growth regime to amplify the radiation while simultaneously reducing the FEL bandwidth. In this slippage-boosted section, the average longitudinal velocity of electrons is reduced, which effectively increases the FEL slippage length that allows the radiation fields initially far apart to create a phase relation, leading to $n$ times increase in FEL cooperation length, where $n$ is the ratio of the resonant wavelength of the slippage-boosted section to that of the original FEL radiation. The purified radiation, as a seed with improved temporal coherence, is further amplified to saturation in the undulator sections tuned to the FEL wavelength. Using the linac coherent light source II (LCLS-II) parameters as an example, we show that with the proposed configuration the temporal coherence and spectral brightness of a SASE FEL can be significantly enhanced. This scheme may be applied to many SASE FEL light sources to enhance the FEL performance.

A detailed analysis is carried out of the various types of phase errors present in real undulator devices, and their statistical properties. The influence of phase errors on the radiation properties is also examined, distinguishing the effects on peak brightness and integrated flux, and including the effects of electron beam emittance and energy spread. The limitation of the usual expression for the reduction in intensity due to phase errors, based on the rms phase error, is explored, and a new parameter is introduced which correlates better with the reduction in integrated flux. The implications for operation of undulators in future lower emittance storage rings is also discussed.

Understanding coherent synchrotron radiation (CSR) effects in a bunch compressor requires an accurate model accounting for the realistic beam shape and parameters. We extend the well-known 1D CSR analytic model into two dimensions and develop a simple numerical model based on the Liénard-Wiechert formula for the CSR field of a coasting beam. This CSR numerical model includes the 2D spatial dependence of the field in the bending plane and is accurate for arbitrary beam energy. It also removes the singularity in the space charge field calculation present in a 1D model. Good agreement is obtained with 1D CSR analytic result for free electron laser (FEL) related beam parameters but it can also give a more accurate result for low-energy/large spot size beams and off-axis/transient fields. This 2D CSR model can be used for understanding the limitation of various 1D models and for benchmarking fully electromagnetic multidimensional particle-in-cell simulations for self-consistent CSR modeling.

The secondary emission yield (SEY) properties of colaminated Cu samples for LHC beam screens are correlated to the surface chemical composition determined by x-ray photoelectron spectroscopy. The surface of the as-received samples is characterized by the presence of significant quantities of contaminating adsorbates and by the maximum of the SEY curve (${\delta }_{\max }$) being as high as 2.1. After extended electron scrubbing at kinetic energy of 10 and 500 eV, the ${\delta }_{\max }$ value drops to the ultimate values of 1.35 and 1.1, respectively. In both cases the surface oxidized phases are significantly reduced, whereas only in the sample scrubbed at 500 eV the formation of a graphitic-like C layer is observed. We find that the electron scrubbing of technical Cu surfaces can be described as occurring in two steps: the first step consists in the electron-induced desorption of weakly bound contaminants that occurs indifferently at 10 and at 500 eV and corresponds to a partial decrease of ${\delta }_{\max }$; the second step, activated by more energetic electrons and becoming evident at high doses, increases the number of graphitic-like C-C bonds via the dissociation of adsorbates already contaminating the as-received surface or accumulating on this surface during irradiation. Our results demonstrate how the kinetic energy of impinging electrons is a crucial parameter when conditioning the surfaces of Cu and other metals by means of electron-induced chemical processing.