As an advanced accelerator concept, laser wakefield acceleration, with its attractive acceleration gradient, has invoked great interest worldwide. The electron beams from laser wakefield acceleration were assumed to have the advantage of “jitterfree” or relatively small jitter, since the emergence of quasimonoenergetic bunch structures were considered to result from the trapping of electrons into the first bucket of the plasma wave. However, the emission times of the electron bunches from laser wakefield acceleration have not been real time monitored in experiments up to now. To examine this, we introduced the electro-optic spatial decoding method. The relative emission times of the electrons were observed to have variations with different plasma densities when using a laser with moderate intensity. The study shows that timing issues should not be ignored in some occasions of laser wakefield acceleration. The method used in this paper could be a candidate serving as a single-shot nondestructive electron bunch timing monitor for laser wakefield experiment and potential ultrafast pump-probe studies.

A fast-switching, high-repetition-rate magnet and power supply have been developed for and operated at TRIUMF, to deliver a proton beam to the new ultracold neutron (UCN) facility. The facility possesses unique operational requirements: a time-averaged beam current of $40\mu \mathrm{A}$ with the ability to switch the beam on or off for several minutes. These requirements are in conflict with the typical operation mode of the TRIUMF cyclotron which delivers nearly continuous beam to multiple users. To enable the creation of the UCN facility, a beam-sharing arrangement with another facility was made. The beam sharing is accomplished by the fast-switching (kicker) magnet which is ramped in $50\mu \mathrm{s}$ to a current of 193 A, held there for approximately 1 ms, then ramped down in the same short period of time. This achieves a 12 mrad deflection which is sufficient to switch the proton beam between the two facilities. The kicker magnet relies on a high-current, low-inductance coil connected to a fast-switching power supply that is based on insulated-gate bipolar transistors (IGBTs). The design and performance of the kicker magnet system and initial beam delivery results are reported.

We report the study of various beam dynamics issues arising from the scheme of using two-frequency crab cavities to generate short pulse x-rays for timed-resolved experiments in a storage ring while simultaneously supplying high brightness beams to other users. These issues include the prediction of the short pulse performance, the choice of system parameters, the impact of the crab cavities on injection efficiency and beam lifetime of the camshaft bunch, and the impact of crab cavities on the regular beam. The collective effect requirements and the separation of short pulses and regular beams are also discussed. With this scheme applied to the SPEAR3 ring, the short pulse flux would be equivalent to a 0.5 nC bunch with a duration of 3 ps (full width half maximum, fwhm), or a 2 nC bunch with a duration of 7 ps (fwhm).

In a multipass energy recovery linac (ERL), each cavity must regain all energy expended from beam acceleration during beam deceleration, and the beam should achieve specific energy targets during each loop that returns it to the linac. For full energy recovery, and for every returning beam to meet loop energy requirements, we must specify and maintain the phase and voltage of cavity fields in addition to selecting adequate flight times. These parameters are found with a full scale numerical optimization program. If we impose symmetry in time and energy during acceleration and deceleration, fewer parameters are needed, simplifying the optimization. As an example, we present symmetric models of the Cornell BNL ERL Test Accelerator (CBETA) with solutions that satisfy the optimization targets of loop energy and zero cavity loading. An identical cavity design and nearly uniform linac layout make CBETA a potential candidate for symmetric operation.

The space charge limit (SCL) of emission from photocathodes sets an upper limit on the performance of both high- and low-field electron guns. Generally, one is forced to strike a compromise between the space charge limit and the cathode’s intrinsic emittance [I. Bazarov et al., Phys. Rev. Lett. 102, 104801 (2009)]. However, it is possible to nearly eliminate the SCL due to the image charge by engineering the topography of the cathode’s surface. A cathode with a surface plasma frequency below the frequency spectrum of the accelerating electrons will greatly reduce the polarization of the cathode and its image-charge field, thereby mitigating the cathode’s space charge limit for photoelectric, thermionic, and field emission. In the work presented here, a theory for the image-charge field of a disk of charge being accelerated from the cathode surface is developed which includes the frequency-dependent effects of the cathode’s dielectric function. This theory verifies the concept of SCL mitigation with a wire array metamaterial to design a metacathode with greatly reduced image-charge fields. The details of the holistic metacathode design spanning 6 orders of magnitude in scale are described.

Linac segment 1 (LS1) of the FRIB driver linac is composed of 15 cryomodules, consisting of 104 superconducting (SC) resonators and 39 SC solenoids. Four ion beam species (Ne, Ar, Kr, and Xe) were successfully accelerated up to $20.3\mathrm{MeV}/\mathrm{u}$ in LS1 and transported to the designated beam dumps located in folding segment 1 (FS1). 100% beam transmission was measured through all cryomodules and the warm section of LS1. High-power equivalent beams were delivered to the beam dump in two modes: pulsed and continuous wave (cw). In the pulsed mode, the peak intensity of the argon beam was $14.8\mathrm{p}\mu \mathrm{A}$ at 3% duty factor, which constitutes 30% of the FRIB design intensity for this particular ion beam. A cw argon beam was accelerated, demonstrating that the FRIB linac in its current configuration is the highest-energy cw superconducting hadron linac in the world. This paper presents a detailed study of beam dynamics in LS1 prior to and after charge stripping with a carbon foil.

A model is described in which electrical breakdown in high-voltage systems is caused by stochastic fluctuations of the mobile dislocation population in the cathode. In this model, the mobile dislocation density normally fluctuates, with a finite probability to undergo a critical transition due to the effects of the external field. It is suggested that once such a transition occurs, the mobile dislocation density will increase deterministically, leading to electrical breakdown. Model parametrization is achieved via microscopic analysis of oxygen-free high thermal conductivity Cu cathode samples from the CERN compact linear collider project, allowing the creation and depletion rates of mobile dislocations to be estimated as a function of the initial physical condition of the material and the applied electric field. We find analytical expressions for the mean breakdown time and quasistationary probability distribution of the mobile dislocation density, and verify these results by using a Gillespie algorithm. A least-squares algorithm is used to fit these results with available experimental data of the dependence of the breakdown rate on the applied strength of the electric field and on temperature. The effects of the variation of some of the assumptions of the physical model are considered, and a number of additional experiments to validate the model are proposed, which include examining the effects of the temperature and pulse length, as well as of a time-dependent electric field, on the breakdown rate. Finally, applications of the model are discussed, including the usage of the quasistatic probability distribution to predict breakdowns, and applying the predictions of the model to improve the conditioning process of the cathode material.

A theoretical investigation of radiation field produced by a charge moving through the dielectric concentrator for Cherenkov radiation is performed for the general case where a charge trajectory is shifted with respect to the target axis. The idea of dielectric target with specific profile of the outer surface was presented and investigated in our previous papers for the symmetric case. Here we show how nonsymmetric field components generated in the bulk of the target affect field distribution near the focus where strong concentration of the energy occurs. Possible applications of this target are discussed.

Cesium telluride (CsTe) photocathodes have been the primary choice for electron sources by worldwide accelerators, due to their high quantum yield, stable performance in complex operation environments and long lifetime. In this paper we compared the traditional sequential and the newly developed coevaporation growth of CsTe photocathodes by describing the chemical and structural evolution of each growth method, using in situ, real time x-ray characterization. From the codeposition method, we were able to achieve $\sim 2\mathrm{nm}$ surface roughness, high crystallinity and a quantum efficiency of 19% at 266 nm wavelength.

Monte Carlo shower simulations are essential for understanding and predicting the consequences of beam losses in high-energy proton and ion colliders. Shower simulations are routinely used at CERN for estimating the beam-induced energy deposition, radiation damage, and radioactivity in the Large Hadron Collider (LHC). Comparing these shower simulations against beam loss measurements is an important prerequisite for assessing the predictive ability of model calculations. This paper validates fluka simulation predictions of beam loss monitor (BLM) signals against BLM measurements from proton fills at 3.5 and 4 TeV and ${{}^{208}\mathrm{Pb}}^{82+}$ ion fills at $1.38A\mathrm{TeV}$. The paper addresses typical loss scenarios and loss mechanisms encountered in LHC operation, including proton collisions with dust particles liberated into the beams, halo impact on collimators in the betatron cleaning insertion, proton-proton collisions in the interaction points, and dispersive losses due to bound-free pair production in heavy ion collisions. Model predictions and measured signals generally match within a few tens of percent, although systematic differences were found to be as high as a factor of 3 for some regions and source terms.

A tuning method for hard x-ray free electron lasers is presented which combines electron-beam-based alignment (EBBA) with undulator radiation spectrum analysis. This method is conducive to achieving a dispersion-free orbit, finding the undulator field center, and determining the accurate value of the gap distance for the operating undulator $K$ for each undulator segment, and it proves to be highly reliable and robust for free electron laser lasing with variable-gap undulators. The effect of the natural focusing of the undulator on the EBBA is confirmed with measurements and a theoretical analysis. The tilt of an undulator segment subject to ground movement introduces an undulator field change which the electron beam interacts with and can be corrected by $K$ tuning to find a precise compensating gap distance. Lattice matching with a wire-scanner-based Twiss parameter measurement along the undulator line enables the EBBA reference orbit to coincide with the $e$-beam distribution.

In 2017, optics commissioning strategy for low-${\beta }^{*}$ operation of the CERN Large Hadron Collider (LHC) underwent a major revision. This was prompted by a need to extend the scope of beam-based commissioning at high energy, beyond the exclusively linear realm considered previously, and into the nonlinear regime. It also stemmed from a recognition that, due to operation with crossing angles in the experimental insertions, the linear and nonlinear optics quality were intrinsically linked through potentially significant feed-down at these locations. Following the usual linear optics commissioning therefore, corrections for (normal and skew) sextupole and (normal and skew) octupole errors in the high-luminosity insertions were implemented. For the first time, the LHC now operates at top energy with beam-based corrections for nonlinear dynamics, and for the effect of the crossing scheme on beta-beating and dispersion. The new commissioning procedure has improved the control of various linear and nonlinear characteristics of the LHC, yielding clear operational benefits.

High frequency high order modes can significantly affect the coupled-bunch stability in a circular accelerator. With a large enough shunt impedance they may drive a coupled-bunch transverse instability. We have developed an analytical model and implemented it in the nested head-tail Vlasov solver. The results show that the instability is characterized by the excitation of many azimuthal intrabunch modes, which have similar growth rates, that make the traditional remedies such as a flat resistive feedback and chromaticity inefficient in suppressing it. For the High Luminosity Large Hadron Collider this could result in a significant increase of the stabilizing Landau octupole current, up to $\sim 100$ A ($\sim 20\%$ of the maximum available current). In order to limit the increase below 10 A ($\sim 2\%$), the transverse shunt impedance has to be kept below $1\mathrm{M}\mathrm{\Omega }/\mathrm{m}$.

The beam-gas vertex (BGV) detector is an innovative instrument measuring noninvasively the transverse beam size in the Large Hadron Collider (LHC) using reconstructed tracks from beam-gas interactions. The BGV detector was installed in 2016 as part of the R&D for the High-Luminosity LHC project. It allows beam size measurements throughout the LHC acceleration cycle with high-intensity physics beams. A precision better than 2% with an integration time of less than 30 s is obtained on the average beam size measured, while the transverse size of individual proton bunches is measured with a resolution of 5% within 5 min. Particles emerging from beam-gas interactions in a specially developed gas volume along the beam direction are recorded by two tracking stations made of scintillating fibers. A scintillator trigger system selects, on-line, events with tracks originating from the interaction region. All the detector elements are located outside the beam vacuum pipe to simplify the design and minimize interference with the accelerated particle beam. The beam size measurement results presented here are based on the correlation between tracks originating from the same beam-gas interaction vertex.

We show a single-shot technique to measure topological and phase properties of radiation carrying orbital angular momentum. The single-shot method is effectively described as the one-dimensional case of a more general two-dimensional approach based on scanning interferometry (asymmetric lateral coherence). The validity of the method has been experimentally verified and the applicability to ultrarelativistic sources of hard x-rays has been discussed. The method is suitable to characterize phase and topological properties of x-ray sources by using simple apertures.

Microbunched electron cooling is a promising cooling technique that can find applications in future hadron and electron-ion colliders to counteract intrabeam scattering that limits the maximum achievable luminosity of the collider. To minimize the cooling time, one would use amplification cascades consisting of a drift section followed by a magnetic chicane. In this paper, we first derive and optimize the gain factor in an amplification section for a simplified one-dimensional model of the beam. We then deduce the cooling rate of a system with one and two amplification cascades. We also analyze the noise effects that counteract the cooling process through the energy diffusion in the hadron beam. Our analytical formulas are confirmed by numerical simulations for a set of model parameters.

The ultrafast and ultracold electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge. Its use is however still limited because of the required cold-atom laser-cooling techniques. Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module. This makes the technique more widely accessible and increases its applicability. We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches. The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition. The electron beams extracted from the GMOT have been characterized. Beam energies up to 10 keV were measured using a time-of-flight method. The normalized root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in $\epsilon =1.9\mathrm{nm}\mathrm{rad}$. The root-mean-squared transverse size of the ionization volume is $30\mu \mathrm{m}$ or larger, implying an electron source temperature in the few-10 K range, 2–3 orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.

A unit cell with one homogeneous bending magnet is commonly used in the design of multi-bend achromat (MBA) lattices for the new generation of diffraction limited storage rings. But significantly lower emittance at moderate focusing properties can be achieved by combining longitudinal gradient bends (LGB) and reverse bends (RB) in a periodic lattice unit cell. LGBs alone, however, are of rather limited gain. We investigate the emittance achievable for different unit cell classes as a function of the cell phase advance in a general framework, i.e., with a minimum of assumptions on the particular cell optics. Each case is illustrated with a practical example of a realistic lattice cell, eventually leading to the LGB/RB unit cell of the baseline lattice for the upgrade of the Swiss Light Source.

In a lepton storage ring of very high energy (e.g., in the $e^{+}{e}^{-}$ Higgs factory) synchrotron radiation from quadrupoles constrains transverse dynamic aperture even in the absence of any magnetic nonlinearities. This was observed in tracking for LEP and the Future Circular $e^{+}{e}^{-}$ Collider (FCC-ee). Here we describe a new mechanism of instability created by modulation of the particle energy at the double betatron frequency by synchrotron radiation in the quadrupoles. Energy modulation varies transverse focusing strength at the same frequency and creates a parametric resonance of the betatron oscillations with unusual properties. It occurs at arbitrary betatron frequency (the resonant detuning is always zero) and the magnitude of the parameter modulation of the betatron oscillation (strength of the resonance driving term) depends on the oscillation amplitude. Equilibrium between the radiation damping and the resonant excitation gives the boundary of the stable motion. Starting from 6d equations of motion we derive and solve the relevant differential equation describing the resonance, and show good agreement between analytical results and numerical simulation.

The design of machine protection systems for high-energy accelerators with high-intensity beams requires analyzing a large number of failures leading to beam loss. One of the most serious failures is an accidental impact of a large number of bunches at one location, for example, due to a deflection of the particle beams by the extraction kicker magnets with the wrong strength. The numerical assessment of such an event requires an iterative execution of an energy-deposition code and a hydrodynamic code, in case the hydrodynamic tunneling effect plays an important role in the beam-matter interactions. Such calculations have been performed for the CERN Large Hadron Collider (LHC), since the energy stored in the LHC beams exceeds previous accelerators by 2 orders of magnitude. This was done using the particle shower code fluka and the hydrodynamic code big2 [Tahir et al., Phys. Rev. ST Accel. Beams 15, (2012)]. Later, simulations for a number of cases for other accelerators at CERN and the Future Circular Collider (FCC) were performed [Tahir et al., Phys. Rev. Accel. Beams 19, (2016)]. These simulations showed that the penetration depth of the beam in copper or graphite could be an order of magnitude deeper when considering the hydrodynamic tunneling, compared to a static approximation. big2 is very efficient and used at GSI. It is always advantageous to cross-check the simulation results with different codes, because dedicated experiments are very complex or even impossible to carry out. For this purpose, it was decided to study the use of a commercial tool (Autodyn) for such calculations and to compare the results with previous work. This paper reports a benchmarking study against beam experiments performed at the HiRadMat (High-Radiation to Materials) facility using beams at 440 GeV from the Super Proton Synchrotron. Good agreement has been found between the simulation results and the experiments as well as previous simulations with fluka and big2 [Tahir et al., Phys. Rev. E 90, (2014)], particularly in terms of the penetration depth of the beam in copper. This makes the coupling of fluka and Autodyn an alternative solution to the simulation of the hydrodynamic tunneling, at least in the parameter range of this case study. Studies with other parameters are planned for FCC and other high-beam-power accelerators.