Beam-bunch arrival time has been measured for the first time by operating superconducting cavities, normally part of the linac accelerator array, in a bunch-detecting mode. The very high $Q$ of the superconducting cavities provides high sensitivity and allows for phase-detecting low-current beams. In detecting mode, the resonator is operated at a very low field level comparable to the field induced by the bunched beam. Because of this, the rf field in the cavity is a superposition of a “pure” (or reference) rf and the beam-induced signal. A new method of circular phase rotation (CPR), allowing extraction of the beam phase information from the composite rf field was developed. Arrival time phase determination with CPR is better than 1° (at 48 MHz) for a beam current of 100 nA. The electronics design is described and experimental data are presented.

Electron effects in the High Current Experiment (HCX) are studied via computer simulation. An approximate expression for the secondary electron yield for a potassium ion striking stainless steel is derived and compared with experimental results. This approximate expression has a peak of roughly 55 electrons at normal incidence at an ion energy of 60 MeV. Using an empirical angular dependence, the secondary electron yield is combined with a numerical simulation of the HCX ion beam dynamics to obtain an estimate for the number of secondary electrons expected per ion-wall collision in the HCX. This estimate is that approximately 150–200 electrons per ion collision may result in the HCX.

A longitudinal quadrupole ($q$-pole) instability was limiting the maximum stable current in the DA$\Phi$NE $e^{-}$ ring at a level of $\sim 700–800\mathrm{mA}$. In order to investigate the phenomenon, the instability threshold has been measured as a function of various machine parameters as radio frequency voltage ($V_{\mathrm{rf}}$), momentum compaction (${\alpha }_c$), number of bunches, fill pattern, etc. An unexpected interaction with the longitudinal feedback system, built to control the dipole motion, has been found and a proper feedback tuning has allowed increasing the threshold. The maximum stable beam current has now exceeded 1.80 A and it is no longer limited by the quadrupole instability.

This paper presents a scheme for three-step laser-based stripping of an ${\mathrm{H}}^{-}$ beam for charge exchange injection into a high-intensity proton ring. First, ${\mathrm{H}}^{-}$ atoms are converted to ${\mathrm{H}}^0$ by Lorentz stripping in a strong magnetic field, then neutral hydrogen atoms are excited from the ground state to upper levels by a laser, and the remaining electron, now more weakly bound, is stripped in a strong magnetic field. The energy spread of the beam particles gives rise to a Doppler broadened absorption linewidth, which makes for an inefficient population of the upper state by a narrow-band laser. We propose to overcome this limitation with a “frequency sweeping” arrangement, which populates the upper state with almost 100% efficiency. We present estimates of peak laser power and describe a method to reduce the power by tailoring the dispersion function at the laser-particle beam interaction point. We present a scheme for reducing the average power requirements by using an optical ring resonator. Finally, we discuss an experimental setup to demonstrate this approach in a proof-of-principle experiment.

The narrow band chaotic output of the self-amplified spontaneous-emission free-electron laser exhibits intensity spikes. In the linear regime before saturation, we use an approach developed by Rice to determine probability distributions for the peak values of intensity in both the time and frequency domains. We also find the average number of spikes per unit time or frequency. In addition, we derive joint probabilities for the intensity in the output pulse to have values $I_1$ and $I_2$ at times $t_1$ and $t_2$, and for the spectral intensity to have values ${\tilde{I}}_1$ and ${\tilde{I}}_2$ at frequencies ${\omega }_1$ and ${\omega }_2$.

We present a statistical analysis of the temporal and spectral properties of self-amplified spontaneous emission from an energy-chirped electron beam passing through a long undulator. It is found that the coherence time is independent of the chirp, while the range of spectral coherence is linearly proportional to it. We consider the use of a monochromator to pick out a small temporal slice of the radiation output. For the filtered radiation pulse, we determine the pulse duration, the number of modes, and the energy fluctuation. We apply our analysis to schemes proposed to generate short x-ray pulses at the Linac Coherent Light Source.

The upgraded Accelerator Test Facility (ATF) ${\mathrm{CO}}_2$ laser located at Brookhaven National Laboratory offers a unique opportunity to investigate laser wakefield acceleration (LWFA) with a $10.6\mathrm{\text{−}}\mu \mathrm{m}$ laser, a wavelength where little experimental work exists. While long laser wavelengths have certain advantages over short wavelengths, our modeling analysis has uncovered another important effect. The upgraded ATF ${\mathrm{CO}}_2$ laser will have a pulse length as short as 2 ps. At a nominal plasma density of $\sim {10}^{16}{\mathrm{cm}}^{-3}$, this pulse length would normally be considered too long for resonant LWFA, but too short for self-modulated LWFA. However, our model simulations indicate that a well-formed wakefield is nevertheless generated with electric field gradients of $E_z\gtrsim 2\mathrm{GV}/\mathrm{m}$ assuming 2.5 TW laser peak power. The model indicates pulse steepening is occurring due to various nonlinear effects. It is possible that this intermediate laser pulse length mode of operation may permit the creation of well-formed, regular-shaped wakefields, which would be needed for staging the LWFA process. Discussed in this paper are the model, its predictions for an LWFA experiment at the ATF, and the pulse steepening effect.

A self-amplified spontaneous emission free-electron laser (SASE FEL) is a device which is based on the creation of a very intense, relativistic electron beam which has very little temperature in all three phase planes. The beam in this system is described as having “high brightness,” and when it is bent repetitively in a magnetic undulator, undergoes a radiation-mediated microbunching instability. This instability can amplify the original radiation amplitude at a particular, resonant wavelength by many orders of magnitude. In order to obtain high brightness beams, it is necessary to compress them to obtain higher currents than available from the electron source. Compression is accomplished by the use of magnetic chicanes, which are quite similar to, if much longer than, a single period of the undulator. It should not be surprising that such chicanes also support a radiation-mediated microbunching interaction, which has recently been investigated, and has been termed coherent synchrotron radiation (CSR) instability. The purpose of this paper is to compare and contrast the characteristics of the closely related FEL and CSR microbunching instabilities. We show that a high-gain regime of the CSR instability exists which is formally similar to the FEL instability.

A novel superconducting (sc) structure is proposed, to accelerate a high intensity proton beam within the energy range of 5–20 MeV. The comparison with different sc structures from the point of view of both rf efficiency and technological aspects of manufacture looks promising. Results of M.A.F.I.A. simulations are given.

This paper makes use of the Vlasov-Maxwell equations to investigate collective excitations in an intense sheet beam, infinite in the $y$ and $z$ directions, propagating in the $z$ direction with directed kinetic energy $({\gamma }_b-1)m_b{c}^2$. The beam is confined in the $x$ direction by the smooth-focusing force ${\overrightarrow{F}}_{\mathrm{foc}}=-{\gamma }_b{m}_b{\omega }_{\beta \perp }^{2}x{\overrightarrow{e}}_x$, and perfectly conducting walls are located at $x=\pm x_w$. A self-consistent water bag equilibrium $f_b^0$ satisfying the steady-state $(\partial /\partial t=0)$ Vlasov-Maxwell equations is shown to be exactly solvable for the beam density $n_b^0(x)$ and electrostatic potential ${\phi }^0(x)$. A closed Schrödinger-like eigenvalue equation is derived, assuming small-amplitude perturbations $(\delta f_b,\delta \phi )$ about the self-consistent water bag equilibrium, and the eigenfrequency spectrum is shown to be purely real. The WKB approximation is employed to determine the eigenfrequency spectrum as a function of the normalized beam intensity $s_b={\widehat{\omega }}_{pb}^2/{\gamma }_b^2{\omega }_{\beta \perp }^2$, where ${\widehat{\omega }}_{pb}^2=4\pi {\widehat{n}}_b{e}_b^2/{\gamma }_b{m}_b$ and ${\widehat{n}}_b=n_b(x=0)$ is the on-axis number density of beam particles.

This paper presents the first experimental results of the transverse profile measurements using a vibrating wire scanner at the Yerevan synchrotron injector electron beam (6 nA after collimation). The advantage of this novel technique is that there is no need for secondary radiation or particle detectors. The local beam intensity is measured using the natural oscillation frequency of the wire, which is a function of the wire temperature.

Most studies of coherent synchrotron radiation (CSR) have considered only the radiation from independent dipole magnets. However, in the damping rings of future linear colliders, a large fraction of the radiation power will be emitted in damping wigglers. In this paper, the longitudinal wakefield and impedance due to CSR in a wiggler are derived in the limit of a large wiggler parameter $K$. After an appropriate scaling, the results can be expressed in terms of universal functions, which are independent of $K$. Analytical asymptotic results are obtained for the wakefield in the limit of large and small distances, and for the impedance in the limit of small and high frequencies.

A large-scale Vlasov simulation study of the microwave instability below transition energy in a beam confined between two barrier pulses is performed. Starting from a matched distribution function for the confined ion beam including the space charge impedance the stability threshold in the longitudinal impedance plane is obtained. A simple stability criterium is found to be in good agreement with the simulation results.

A nondestructive beam energy-spread monitor using multi-strip-line electrodes has been newly developed in order to measure and control the energy spread of a single-bunch electron beam at an $180°$ arc with a beam energy of 1.7 GeV in the KEKB injector linac. The detection principle is described based on a multipole-moment analysis of an electromagnetic field generated by a charged beam. A proof-of-principle experiment was performed to verify the analysis. The result shows that taking into account the second-order moment derived from the multipole-moment analysis, the analyzed transverse beamwidths are consistent with those measured by a screen monitor, and the energy spreads are estimated to be $0.150\%\pm 0.007\%$ and $0.264\%\pm 0.004\%$ for electron beams with charges of 0.9 and $8\mathrm{nC}/\mathrm{bunch}$ at the rf phase of the energy-spread minimum, respectively. In this report we describe in detail the experimental result using the beam energy-spread monitor along with a theoretical multipole-moment analysis.

The formation of the beam halo in charged particle accelerators is studied in the framework of a stochastic-hydrodynamic model for the collective motion of the particle beam. In such a stochastic-hydrodynamic theory the density and the phase of the charged beam obey a set of coupled nonlinear hydrodynamic equations with explicit time-reversal invariance. This leads to a linearized theory that describes the collective dynamics of the beam in terms of a classical Schrödinger equation. Taking into account space-charge effects, we derive a set of coupled nonlinear hydrodynamic equations. These equations define a collective dynamics of self-interacting systems much in the same spirit as in the Gross-Pitaevskii and Landau-Ginzburg theories of the collective dynamics for interacting quantum many-body systems. Self-consistent solutions of the dynamical equations lead to quasistationary beam configurations with enhanced transverse dispersion and transverse emittance growth. In the limit of a frozen space-charge core it is then possible to determine and study the properties of stationary, stable core-plus-halo beam distributions. In this scheme the possible reproduction of the halo after its elimination is a consequence of the stationarity of the transverse distribution which plays the role of an attractor for every other distribution.

In the design of the vacuum chamber of a storage ring, special care has to be taken to keep the impedance as low as possible, in order to not degrade the quality of the circulating beam. Since the start of the commissioning of the Italian synchrotron light source ELETTRA in 1993, several low gap insertion vacuum chambers with internal aperture of 14 mm have been installed into the storage ring. Monitoring of their individual impedances and effects has been carried out with measurements on the electron beam after each new installation. The subsequent evolutions of the total longitudinal and transverse impedances are presented in this paper. Furthermore, in contradiction with the foreseen single-bunch effect, noticeable excitations of particular narrow band longitudinal coupled multibunch instabilities as a function of the position of the blades of the vertical scraper have been observed. In order to achieve a deeper insight of the phenomenon, an exhaustive characterization of both the longitudinal and transverse impedances of the scraper has been carried out.

We present a particle core model study of the space charge effect on high intensity synchrotron beams, with specific emphasis on the Proton Storage Ring (PSR) at Los Alamos National Laboratory. Our particle core model formulation includes realistic lattice focusing and dispersion. We transport both matched and mismatched beams through real lattice structure and compare the results with those of an equivalent uniform-focusing approximation. The effects of lattice structure and finite momentum spread on the resonance behavior are specifically targeted. Stroboscopic maps of the mismatched envelope are constructed and show high-order resonances and stochastic effects that dominate at high mismatch or high intensity. We observe the evolution of the envelope phase-space structure during a high intensity PSR beam accumulation. Finally, we examine the envelope-particle parametric resonance condition and discuss the possibility for halo growth in synchrotron beams due to this mechanism.

The dispersion relation for the resistive hose instability in a charged particle beam with a flattop density profile is derived from the linearized Vlasov-Maxwell equations. Stability properties of the resistive hose instability where the perturbations are initiated at the beam entrance are investigated. In particular, the complex eigenfrequency $\Omega$ in the dispersion relation is expressed as a function of the real oscillation frequency $\omega$ of the excitation at the beam entrance. As expected, the growth rate $\mathrm{Im}\Omega ={\Omega }_i$ decreases rapidly as the conducting wall approaches the beam ($r_w/r_b\to 1$). The growth rate also decreases substantially as the frequency ratio $\omega /{\nu }_c$ increases, where ${\nu }_c$ is the electron collision frequency. Stability properties for perturbations propagating through the beam pulse from its head to tail are also investigated. In this case, the growth rate $\mathrm{Im}\omega$ is calculated in terms of the real oscillation frequency $\Omega$ of each beam segment. It is shown that the resonance frequency $\Omega ={\Omega }_r$ corresponding to the infinite growth rate detunes considerably from the betatron frequency ${\omega }_{\beta }$ of the beam particles. It is also found that the bandwidth corresponding to instability is narrow when the plasma electron collision time ($1/{\nu }_c$) is long compared with the magnetic decay time (${\tau }_d$).

Low-energy, background electrons are ubiquitous in high-energy particle accelerators. Under certain conditions, interactions between this electron cloud and the high-energy beam can give rise to numerous effects that can seriously degrade the accelerator performance. These effects range from vacuum degradation to collective beam instabilities and emittance blowup. Although electron-cloud effects were first observed two decades ago in a few proton storage rings, they have in recent years been widely observed and intensely studied in positron and proton rings. Electron-cloud diagnostics developed at the Advanced Photon Source enabled for the first time detailed, direct characterization of the electron-cloud properties in a positron and electron storage ring. From in situ measurements of the electron flux and energy distribution at the vacuum chamber wall, electron-cloud production mechanisms and details of the beam-cloud interaction can be inferred. A significant longitudinal variation of the electron cloud is also observed, due primarily to geometrical details of the vacuum chamber. Such experimental data can be used to provide realistic limits on key input parameters in modeling efforts, leading ultimately to greater confidence in predicting electron-cloud effects in future accelerators.