Transverse multibunch instabilities including the resistive wall instability and ion instabilities are investigated analytically and numerically for two bunch trains on separated orbits in a storage ring. The two separated orbits’ operating mode appears more stable against transverse multibunch instabilities than the conventional single orbit operating mode and may be beneficial to high current operations for colliders or synchrotron light sources.

The emittance of high brightness electron sources, particularly field emitters and photocathodes but also thermionic sources, is increased by surface roughness on the emitter. Such structure causes local field enhancement and complicates both the prediction of emittance and the underlying emission models on which such predictions depend. In the present work, a method to find the emission trajectories near regions of high field enhancement is given and applied to emittance predictions for field, photo, and thermal emission for an analytically tractable hemispherical model. The dependence of the emittance on current density, spatial variation, and acceleration close to the emission site is identified and the impact of space charge discussed. The methodology is extensible to field emission from close-spaced wirelike structures, in particular, and extensions to that configuration are discussed. The models have application to electron sources for high frequency vacuum electronics, high power microwave devices, and free-electron lasers.

The performance of future linear colliders will depend critically on beam-based alignment and feedback systems. In ILC and CLIC it is planned to perform dispersion-free steering in the main linacs. To this end the beams are accelerated with different gradients to evaluate the dispersion. The steering is performed by minimizing the average offset of the different beams in the beam position monitors and, at the same time, the difference between the beam trajectories. The experimental verification of the dispersion-free steering algorithm is essential to prove its effectiveness and to prepare the commissioning of such machines. The algorithm should take an orbit measurement at every cycle (train to train), estimate the correction from this information, and, from the system response matrices, apply the correction. We have successfully tested dispersion-free steering at FACET, including an adaptive system-identification algorithm, where the system response matrix is measured dynamically and automatically.

In 2008 the Cornell Electron/Positron Storage Ring (CESR) was reconfigured from an electron/positron collider to serve as a test bed for the International Linear Collider damping rings. One of the primary goals of the CESR Test Accelerator (CesrTA) project is to develop a fast low-emittance tuning method which scales well to large rings such as the ILC damping rings, and routinely achieves a vertical emittance of order 10 pm at 2.085 GeV. This paper discusses the tuning methods developed at CesrTA to achieve low-emittance conditions. One iteration of beam-based measurement and correction requires about 10 min. A minimum vertical emittance of $10.3(+3.2/-3.4{)}^{\text{sys}}(\pm 0.2{)}^{\text{stat}}\mathrm{pm}$ has been achieved at 2.085 GeV. In various configurations and beam energies the correction technique routinely achieves vertical emittance around 10 pm after correction, with rms coupling $<0.5\%$. The measured vertical dispersion is dominated by beam position monitor systematics. The propagation of uncertainties in the emittance measurement is described in detail. Simulations modeling the effects of magnet misalignments, beam position monitor errors, and the emittance correction algorithm suggest the residual vertical emittance measured at the conclusion of the tuning procedure is dominated by sources other than optics errors and misalignments.

In high brightness circular colliders, coherent and incoherent beam dynamics are dominated by beam-beam interactions. It is generally assumed that the incoherent tune spread introduced by the beam-beam interactions is sufficiently large to cure any instabilities originating from impedance. However, as the two counterrotating beams interact they can give rise to coherent dipole modes and therefore modify the coherent beam dynamics and stability conditions. In this case, coherent beam-beam effects and impedance cannot be treated independently and their interplay should be taken into account in any realistic attempt to study the beam stability of colliding beams. Due to the complexity of these physics processes, numerical simulations become an important tool for the analysis of this system. Two approaches are proposed in this paper: a fully self-consistent multiparticle tracking including particle-in-cell Poisson solver for the beam-beam interactions and a linearized model taking into account finite bunch length effects. To ensure the validity of the results a detailed benchmarking of these models was performed. It will be shown that under certain conditions coherent beam-beam dipole modes can couple with higher order headtail modes and lead to strong instabilities with characteristics similar to the classical transverse mode coupling instability originating from impedance alone. Possible cures for this instability are explored both for single bunch and multibunch interactions. Simulation results and experimental evidences of the existence of this instability at the LHC will be presented for the specific case of offset collisions.

In storage rings, horizontal dispersion in the rf cavities introduces horizontal-longitudinal ($xz$) coupling, contributing to beam tilt in the $xz$ plane. This coupling can be characterized by a “crabbing” dispersion term ${\zeta }_a$ that appears in the normal mode decomposition of the 1-turn transfer matrix. ${\zeta }_a$ is proportional to the rf cavity voltage and the horizontal dispersion in the cavity. We report experiments at the Cornell Electron Storage Ring Test Accelerator where $xz$ coupling was explored using three lattices with distinct crabbing properties. We characterize the $xz$ coupling for each case by measuring the horizontal projection of the beam with a beam size monitor. The three lattice configurations correspond to (i) 16 mrad $xz$ tilt at the beam size monitor source point, (ii) compensation of the ${\zeta }_a$ introduced by one of two pairs of rf cavities with the second, and (iii) zero dispersion in rf cavities, eliminating ${\zeta }_a$ entirely. Additionally, intrabeam scattering is evident in our measurements of beam size vs rf voltage.

We propose a new method to generate ultrashort x-ray pulses using focused short low energy ($\sim 20\mathrm{MeV}$) electron bunches to create short slices of electrons from the circulating electron bunches in a synchrotron radiation storage ring. When a low energy electron bunch crosses from the top of a high energy storage ring electron bunch, its Coulomb force will kick a short slice from the core of the storage ring electron bunch. The separated slices, when passing through an undulator, will radiate ultrashort x-ray pulses at about 160 fs. We discuss the advantages, challenges, and provide data which confirm the feasibility of this new method.

In ring-ring colliders, the collision frequency determines the bunch structures, e.g., the time between the bunches in both rings should be identical. Because of relatively low relativistic speed of the hadron beam in sub-TeV hadron-hadron- and electron-ions colliders, scanning the hadron beam’s energy would require either a change in the circumference of one of the rings, or a switching of the bunch (harmonic) number in a ring. The later would cause so-called gear changing, i.e., the change of the colliding bunches turn by turn. In this article, we study the difficulties in beam dynamics in this gear-changing scheme.

A superconducting rf photoinjector (SRF gun) has been put in operation successfully at the radiation source ELBE. It produces a 13 MHz, low emittance, cw beam with beam current up to $400\mu \mathrm{A}$ and energy of 3 MeV. During the gun operation, the field emission from the niobium cavity and the ${\mathrm{Cs}}_2\mathrm{Te}$ cathodes produces dark current. In this paper, we study the dark current of the SRF gun by using the existing diagnostics beam line. The dependency of the intensity and the energy spectrum on the cavity gradient and the cathode voltage are examined, and the emission sources are analyzed. Furthermore, the new project of installing a strip line kicker to reduce the dark current effect is presented.

In this paper, we propose a novel method to measure the electron bunch length with a dielectric wakefield radiation (DWR) decelerator which is composed of two dielectric-lined waveguides (DLWs) and an electron spectrometer. When an electron beam passes through a DLW, the DWR is excited which leads to an energy loss of the electron beam. The energy loss is found to be largely dependent on the electron bunch length and can be easily measured by an electron spectrometer which is essential for a normal accelerator facility. Our study shows that this method has a high resolution and a great simplicity.

The deceleration of electrons confined by a weak guiding magnetic field has been studied with the goal of achieving an intense ultracold electron beam, suitable for electron cooling and electron collision experiments with ion beams of very low velocity. By application of a beam deceleration technique the electron current extracted from a GaAs photocathode has been increased by an order of magnitude at a few eV and sub-eV kinetic energy, keeping a low energy spread of the electrons. The true kinetic energy of the slow electrons has been determined, taking space charge effects and contact potential differences into account.

A new kind of nonresonant optical recirculator, dedicated to the production of $\gamma$ rays by means of Compton backscattering, is described. This novel instrument, inspired by optical multipass systems, has its design focused on high flux and very small spectral bandwidth of the $\gamma$-ray beam. It has been developed to fulfill the project specifications of the European Extreme Light Infrastructure “Nuclear Pillar,” i.e., the Gamma Beam System. Our system allows a single high power laser pulse to recirculate 32 times synchronized on the radio frequency driving accelerating cavities for the electron beam. Namely, the polarization of the laser beam and crossing angle between laser and electrons are preserved all along the 32 passes. Moreover, optical aberrations are kept at a negligible level. The general tools developed for designing, optimizing, and aligning the system are described. A detailed simulation demonstrates the high efficiency of the device.

After 30 years of operation, the Cockcroft-Walton based injector at the Fermi National Accelerator Laboratory has been replaced by a new beam line including a dimpled magnetron 35 keV source in combination with a 750 keV four-rod radio frequency quadrupole (RFQ). The new injector is followed by the existing drift tube linac. Prior to installation, a test beam line was built which included the magnetron source and the four-rod RFQ with a number of beam measurement instrumentation. The first beam test with the RFQ showed an output energy deviation greater than 2.5%. Other problems also showed up which led to investigations of the output energy, power consumption and transmission properties using rf simulations which were complemented with additional beam measurements. The sources of this deviation and the mechanical modifications of the RFQ to solve this matter will be presented in this paper. Meanwhile, the nominal output energy of 750 keV has been confirmed and the new injector with the four-rod RFQ is in full operation.

Characterizing the rf-to-laser jitter in the photocathode rf gun and its possible origins is important for improving the synchronization and beam quality of the linac based on the photocathode rf gun. A new method based on the rf compression effect in the photocathode rf gun is proposed to measure the rf-to-laser jitter in the gun. By taking advantage of the correlation between the rf compression and the laser injection phase, the error caused by the jitter of the accelerating field in the gun is minimized and thus $10\hspace{0.17em}\mathrm{fs}$ time resolution is expected. Experimental demonstration at the Tsinghua Thomson scattering x-ray source with a time resolution better than 35 fs is reported in this paper. The experimental results are successfully used to obtain information on the possible cause of the jitter and the accompanying drifts.