A scheme of generating a uniform ellipsoidal laser pulse for high-brightness photoinjectors is discussed. The scheme is based on the chromatic aberration of a dispersive lens. Fourier optics simulation reveals the interplay of group velocity delay and dispersion in the scheme, as well as diffractions. Particle tracking simulation shows that the beam generated by such a laser pulse approaches the performance of that by an ideal ellipsoidal laser pulse and represents a significant improvement from the traditionally proposed cylindrical beam geometry. The scheme is tested in an 800-nm, optical proof-of-principle experiment at lower peak power with excellent agreement between the measurement and simulation.

Strong beam-beam effects at the interaction point of a high-energy $e^{+}{e}^{-}$ linear collider such as the Compact Linear Collider (CLIC) lead to an emittance growth for the outgoing beams, as well as to the production of beamstrahlung photons and $e^{+}{e}^{-}$ coherent pairs. In this paper, we present a conceptual design of a 150 m long post-collision extraction line for the CLIC machine at 3 TeV, which separates the various components of the outgoing beam using a vertical magnetic chicane, before transporting them to their respective dump. In addition, detailed studies are performed in order to compute the power losses along the CLIC post-collision line. For the vacuum window at the exit of the post-collision line, we propose a thick (1.5 cm) layer of carbon-carbon composite, with a thin (0.2 mm) aluminum leak-tight foil. The stress levels in this exit window are estimated. Finally, we discuss the use of diagnostics along the post-collision line for monitoring and improving the quality of the $e^{+}{e}^{-}$ collisions and, in turn, the luminosity of the CLIC machine.

A superconducting energy-recovery linac (ERL) is under construction at Brookhaven National Laboratory (BNL) to serve as a test bed for an application to upgrades of the Relativistic Heavy Ion Collider (RHIC). The damping of higher-order modes in the superconducting five-cell cavity is of paramount importance and represents the topic of this paper. Achieving the damping by the exclusive use of two ferrite absorbers and the adoption of a space-saving step instead of the conventional taper are part of the exploratory study. Absorber properties which are portable to simulation programs for the ERL cavity have been obtained by measuring the absorber as a ferrite-loaded pill-box cavity. Measured and simulated results for the lowest dipole modes in the prototype copper cavity with one absorber are discussed. First room-temperature measurements of the fully assembled niobium cavity string are presented which confirm the effective damping of higher-order modes by the ferrite absorbers, and which give credibility to the simulated $R$ over $Q$’s in the ERL.

A few years ago, a novel multiturn extraction scheme was proposed, based on particle trapping inside stable resonances. Numerical simulations and experimental tests have confirmed the feasibility of such a scheme for low order resonances. While the third-order resonance is generically unstable and those higher than fourth order are generically stable, the fourth-order resonance can be either stable or unstable depending on the specifics of the system under consideration. By means of the normal form, a general approach to control the stability of the fourth-order resonance has been derived. This approach is based on the control of the amplitude detuning and the general form for a lattice with an arbitrary number of sextupole and octupole families is derived in this paper. Numerical simulations have confirmed the analytical results and have shown that, when crossing the unstable fourth-order resonance, the region around the center of the phase space is depleted and particles are trapped in only the four stable islands. A four-turn extraction could be designed using this technique.

Appreciable suppression of field emission (FE) from metallic surfaces has been achieved by the use of improved surface cleaning techniques. In order to understand the effects of surface preparation on field emission, systematic measurements were performed on five single crystal and three large grain samples of high purity ($\mathrm{RRR}>300$) niobium by means of atomic force microscope, x-ray diffraction, scanning electron microscope (SEM), and dc field emission scanning microscope. The samples were treated with buffered chemical polishing (BCP), half of those for $30\mu \mathrm{m}$ and others for $100\mu \mathrm{m}$ removal of surface layer, followed by a final high pressure water rinsing. These samples provided the emission at minimum surface fields of $150\mathrm{MV}/\mathrm{m}$ and those with longer BCP treatment showed the onset of field emission at slightly higher fields. A low temperature ($\sim 150°\mathrm{C}$) heat treatment in a high vacuum (${10}^{-6}\mathrm{mbar}$) chamber for 14 hours, on a selected large grain Nb sample, gives the evidence for the grain boundary assisted FE at very high fields of 250 and $300\mathrm{MV}/\mathrm{m}$. Intrinsic field emission measurements on the present Nb surfaces revealed anisotropic values of work function for different orientations. Finally, an interesting correlation between sizes of all investigated emitters derived from SEM images with respect to their respective onset fields has been found, which might facilitate the quality control of superconducting radio-frequency cavities for linear accelerators.

We study emittance growth for transport of uniform and Gaussian beams of particles in rf cavities and solenoids and show analytically its dependence on initial beam parameters. Analytical results are confirmed with simulation studies over a broad range of different initial beams.

Recent advances in laser-plasma accelerators, including the generation of GeV-scale electron bunches, enable applications such as driving a compact free-electron laser (FEL). Significant reduction in size of the FEL is facilitated by the expected ultrahigh peak beam currents (10–100 kA) generated in laser-plasma accelerators. At low electron energies such peak currents are expected to cause space-charge effects such as bunch expansion and induced energy variations along the bunch, potentially hindering the FEL process. In this paper we discuss a self-consistent approach to modeling space-charge effects for the regime of laser-plasma-accelerated ultracompact electron bunches at low or moderate energies. Analytical treatments are considered as well as point-to-point particle simulations, including the beam transport from the laser-plasma accelerator through focusing devices and the undulator. In contradiction to non-self-consistent analyses (i.e., neglecting bunch evolution), which predict a linearly growing energy chirp, we have found the energy chirp reaches a maximum and decreases thereafter. The impact of the space-charge induced chirp on FEL performance is discussed and possible solutions are presented.

The interaction of a circularly polarized laser pulse with a mixed solid target containing two species of ions is studied by particle in cell simulations and analytical model. After the interaction tends to be stable, it is demonstrated that the acceleration is more efficient for the heavier ions than that in plasmas containing a single kind of heavy ion and the acceleration efficiency is higher when its proportion is lower. To obtain monoenergetic heavy-ion beams, a sandwich target with a thin mixed ion layer between two light ion layers and a microstructured target are proposed. The influences of parameters of the laser pulse and target on ion acceleration are discussed in detail. It is found that, when the target is thick enough, a cold target is more appropriate for heavy-ion acceleration than a warm target, and the velocity of the reflected heavy ions is proportional to the laser amplitude.

The so-called spin response formalism, which is linear response theory applied to spin dynamics in storage rings, can calculate the resonance strengths for spin flippers in storage rings of arbitrary structure, including rings with Siberian Snakes and spin rotators. We calculate so-called spin response functions for a model of the RHIC lattice, for various scenarios of spin rotator settings.

We demonstrated a method with practical value for the measurement and global compensation of a complex coupling driving term $C$ of linear difference resonance, using a turn-by-turn beam position monitor (BPM) at the SPring-8 storage ring. The method is based on the perturbation theory with the single-resonance approximation. The accurate complex $C$ was obtained from coefficients of the eigenmode expansion of the coupled betatron oscillation observed near a target linear difference resonance. The global compensation for the measured $C$ was successfully carried out by determining optimal strengths of the two independent correcting skew quadrupoles for generating a counterterm $-C$ without using empirical methods. Meanwhile, we then confirmed that the determined optimal strengths gave a minimum vertical beam size by scanning the strength of the correcting skew quadrupole field. To demonstrate the validity of our method, $C$ was also measured while varying the strengths of the correcting magnets around the optimal values to generate deliberate skew quadrupole error fields. We confirmed that the measured values of $C$ agreed with those coming from the deliberate error fields.

Coherent synchrotron radiation has been studied effectively using a one-dimensional model for the charge distribution in the realm of small angle approximations and high energies. Here we use Jefimenko’s form of Maxwell’s equations, without such approximations, to calculate the exact wakefields due to this effect in multiple bends and drifts. It has been shown before that the influence of a drift can propagate well into a subsequent bend. We show, for reasonable parameters, that the influence of a previous bend can also propagate well into a subsequent bend, and that this is especially important at the beginning of a bend. Shielding by conducting parallel plates is simulated using the image charge method. We extend the formalism to situations with compressing and decompressing distributions, and conclude that simpler approximations to bunch compression usually overestimate the effect. Additionally, an exact formula for the coherent power radiated by a Gaussian bunch is derived by considering the coherent synchrotron radiation spectrum, and is used to check the accuracy of wakefield calculations.

Knowledge of the three-dimensional structure of a charged particle beam bunch is essential for understanding its evolution and for initializing computer simulations, especially when space charge is involved. This paper presents a novel experimental method for time-sliced mapping of the transverse phase space of a space-charge dominated beam based on tomographic principles. The combination of a high precision tomographic diagnostic with fast imaging screens and a gated camera are used to produce phase-space maps of two beams: one with a parabolic current profile and another with a short perturbation atop a rectangular pulse. The correlations between longitudinal and transverse phase spaces are apparent and their impact on the dynamics is discussed.

Recently, Stratakis and collaborators performed the tomographic phase space mapping of intense particle beams in the energy range of a few keV using a pair of solenoids [D. Stratakis et al., Phys. Plasmas 14, 120703 (2007)]. In this paper, we report the tomographic phase space mapping for the relativistic electron beam from a photocathode radio-frequency gun using a single solenoid at an energy of a few MeV. The experiment is conducted at a very low charge level aiming at mapping the phase space distributions of thermal emittance dominated beam. The phase space distribution is reconstructed with the algebraic reconstruction technique using the 14 beam profiles measured under various solenoid currents. Results from the computerized tomography (CT) technique are in good agreement with that from the solenoid scan method. The reconstructed phase space from the CT technique is further analyzed and shows a stochastic distribution which is a strong confirmation for the measurement of thermal emittance. Details on measurement principles and experimental implementation of the CT technique and solenoid scan method are presented.

Modern 3rd generation synchrotron light sources aim for 100% availability. No single beam interruption is acceptable and every beam disturbance should be investigated: what caused the interruption? Can it be avoided in the future? If it cannot be avoided, how can the recovery be accelerated? An automated event recording system has been implemented at the Swiss Light Source (SLS) in order to simplify beam distortion investigations with respect to a well-defined metrics. The system identifies beam disturbances and records automatically the type and duration of the event. Relevant information of the event, like control system archive data or shift summaries, is linked to the event and presented in Web pages. Tools for the automated evaluation of alarm logs are provided that generate summaries of a beam distortion. On the basis of this information each event will be assigned to a failure cause. The means to filter the events are provided. We will describe the concept and the implementation of the system at the SLS and our experiences with it. Finally, the SLS operation event logging system will be compared with failure analysis at other light sources.

We have conducted dielectric-breakdown tests on water subject to a single unipolar pulse. The peak voltages used for the tests range from 5.8 to 6.8 MV; the effective pulse widths range from 0.60 to $1.1\mu \mathrm{s}$; and the effective areas tested range from $1.8\times {10}^5$ to $3.6\times {10}^6{\mathrm{cm}}^2$. The tests were conducted on water-insulated coaxial capacitors. The two electrodes of each capacitor have outer and inner radii of 99 and 56 cm, respectively. Results of the tests are consistent with predictions of the water-dielectric-breakdown relation developed in [Phys. Rev. ST Accel. Beams 9, 070401 (2006)].

We have developed an initial-beam-loading compensation system by a new compensation method, where the system modulates the phase and amplitude of a low-level rf signal simultaneously, thereby optimizing a high-power rf waveform fed to an accelerating structure to compensate the beam energy spread. This compensation system is very compact and can easily be installed in and removed from a klystron system. This system was used in a beam test performed in the 125 MeV electron linac of the Laboratory for Electron Beam Research and Application in Nihon University. Experimental results demonstrate that this system effectively corrects the beam energy spread due to the initial-beam-loading effect. The new compensation method is expected to be effective in the compensation of energy spread in high-intensity and long-pulse beams in electron linacs.

A phenomenon of “spectral gaps” is observed in far-infrared free-electron lasers using partial waveguiding: the laser power falls down to zero at some particular wavelengths, whatever the beam adjustments are. We show that this effect is related to a periodic increase of the cavity losses, with a period depending on the dimensions of the waveguide. A code including diffraction effects exhibits good agreement with the measurements.

A cesium-free ${\mathrm{H}}^{-}$ ion source driven by a ${\mathrm{LaB}}_6$ filament is being operated for the beam commissioning of the accelerators at Japan Proton Accelerator Research Complex (J-PARC). The beam commissioning started in November 2006. As of June 2008, there have been 17 beam commissioning runs. The duration of each run was approximately 4–5 weeks. In these runs, the ion source has succeeded in providing beam to the J-PARC accelerators for 2720 hours. The interruption time due to the failure of the ion source is only approximately 50 hours. The availability of the ion source is calculated to be 98%. Since the fluctuation of the beam current is small, tuning the beam current once a day is sufficient to maintain the beam current within the error of a few percent. Recently, the ion source succeeded in producing the beam current of 36 mA with a flattop pulse width of $500\mu \mathrm{s}$ and a repetition rate of 25 Hz as required for the first stage of J-PARC.

Beam breakup instability in superconducting cavities is a serious problem. In this work, a four-cell LEP cavity installed in the KAERI linear accelerator is considered as an example. Dependence of the breakup instability threshold currents on the characteristics of a dipole mode was determined both analytically and numerically. An efficient technique to suppress breakup instability using rf beam focusing within a cavity is suggested. The technique involves applying TE-type monopole higher-order modes and is useful for multicell superconducting cavities with many trapped high-$Q$ dipole modes.

Usually, the emission of low-energy photons in electron-positron (or electron-ion) bunch collisions is calculated with the same approach as for synchrotron radiation (beamstrahlung). However, for soft photons ($E_{\gamma }<E_c$ where $E_c$ is a critical photon energy), when the coherence length of the radiation becomes comparable to the bunch length, the beamstrahlung approximation becomes invalid. In this paper, we present results of our calculation for this region based on approximation of classical currents. We consider several colliders with dense bunches. The number of low-energy photons $d{N}_{\gamma }$ emitted by $N_e$ electrons per bunch crossing in the energy interval $d{E}_{\gamma }$ is $d{N}_{\gamma }=\alpha g{N}_{e}d{E}_{\gamma }/E_{\gamma }$, where $\alpha$ is the fine-structure constant, and the function $g$, which depends on the bunch parameters, typically is of order unity for modern colliders. In particular, for the International Linear Collider, we find that $E_c=83\mathrm{keV}$ and $g=5.5$ at a vanishing beam axis displacement, and $g=0.88$, $E_c=0.24\mathrm{keV}$ for KEKB. We also calculate the specific dependence of $d{N}_{\gamma }$ on the impact parameter between the two beam axes. In principle, the latter aspect allows for online monitoring of the beam axis displacement.