Matching to small beta functions is required to preserve emittance in plasma accelerators. The plasma wake provides strong focusing fields, which typically require beta functions on the mm-scale, comparable to those found in the final focusing of a linear collider. Such beams can be time consuming to experimentally produce and diagnose. We present a simple, fast, and noninvasive method to measure Twiss parameters in a linac using two beam position monitors only, relying on the similarity of the beam phase space and the jitter phase space. By benchmarking against conventional quadrupole scans, the viability of this technique was experimentally demonstrated at the FLASHForward plasma-accelerator facility.

Tuning of the final-focus system of a linear $e^{-}{e}^{+}$ collider is a challenging problem due to the strong nonlinearities of the system and small transverse beam sizes at the interaction point. To have reliable performance of a collider, nominal luminosity must be reached quickly under a set of various imperfections. Measuring luminosity at high energies is a nontrivial task, and, to get a precise measurement, many collisions must be sampled. Therefore, it is desirable to use other beam-beam signals for faster tuning. This paper studies the use of beamstrahlung and incoherent pairs in an extensive simulation study including static imperfections. Tuning results of the nominal lattice are compared to an updated lattice with a smaller vertical beam size at the interaction point. Finally, recovery time after machine downtime and misalignment of components due to ground motion is also studied.

Cryogenic bypass diodes are part of the baseline powering layout for the circuits of the new ${\mathrm{Nb}}_3\mathrm{Sn}$ based final focus magnets of the high luminosity Large Hadron Collider. They will protect the magnets against excessive transient voltages during a nonuniform quenching process. The diodes are located inside an extension to the magnet cryostat, operated in superfluid helium and exposed to ionizing radiation. Therefore, the radiation tolerance of different types of diodes has been tested at cryogenic temperatures in CERN’s CHARM irradiation test facility during its 2018 run. The forward bias characteristics, the turn-on voltage and the reverse blocking voltage of each diode were measured weekly at 4.2 K and 77 K, as a function of the accumulated radiation dose. The diodes were submitted to a total dose close to 12 kGy and a 1 MeV neutron equivalent fluence of $2.2\times {10}^{14}{\mathrm{cm}}^{-2}$. After the end of the irradiation program the annealing behavior of the diodes was tested by increasing the temperature slowly to 293 K. This paper describes the experimental setup, the measurement procedure and the analysis of the measurements performed during the irradiation program as well as the results of the annealing study.

This paper consists of two main parts regarding coherent synchrotron radiation (CSR). The first part extends the CSR theory of two particle interaction from a system of one bending magnet to two bending magnets, where the wake can leak from the first to the second. The new theory agrees well with the established simulation code Bmad. The second part of the paper presents the CSR simulation results on CBETA, the Cornell BNL Energy-Recovery-Linac (ERL) Test Accelerator, in which the magnets are so close to each other and the new extended theory becomes important. CBETA is the first multiturn ERL with superconducting radio frequency (SRF) accelerating cavities and a Fixed Field Alternating gradient (FFA) beamline. Simulations show that CSR causes phase space dilution that becomes more significant as the bunch charge and the number of recirculation passes increase. Potential ways to mitigate the CSR effects, including adding vacuum chamber shielding and increasing bunch length, are being investigated.

In this paper, we investigate the stability of the particle trajectories in fixed field alternating gradient accelerators (FFAs) in the presence of field errors. The emphasis is on the scaling radial sector FFA type: A collaboration work is ongoing in view of better understanding the properties of the 150 MeV scaling FFA at Kyoto University Institute for Integrated Radiation and Nuclear Science in Japan and progress toward high-intensity operation. Analysis of certain types of field imperfections revealed some interesting features that required the development of an analytical model based on the scalloping angle of the orbits. This helped explain some of the experimental results as well as generalize the concept of a scaling FFA to a nonscaling one for which the tune variations obey a well-defined law. Based on this, a compensation scheme of tune variations in imperfect scaling FFAs is presented. This is the cornerstone of a novel concept of a fixed tune FFA in which the scaling is not achieved at every azimuthal position of the ring but rather in an average sense.

Development of superconducting undulators (SCUs) continues at the Advanced Photon Source (APS). Several planar SCUs were designed and built, and two of them are currently in operation at the APS. In January 2018 a new helical SCU (HSCU) was installed on the storage ring at the APS. It has a 1.2 m-long magnet with bifilar winding that generates on its axis a single harmonic of about 6 keV x-rays. The magnet is housed in the newly designed compact cryostat equipped with four cryocoolers. The HSCU was extensively tested prior to its installation on the APS storage ring, and it demonstrated cryogenic and magnetic performance in accordance with the design specifications. The HSCU was rapidly and successfully commissioned, and in February 2018 it was transferred to user operations. Its performance as an x-ray radiation source has been studied and spectral characteristics confirmed. The HSCU is currently used to support unique scientific program at the APS.

We demonstrate a novel scheme for imposing or removing large energy chirp on relativistic electron bunches. This method relies on the transverse-to-longitudinal mixing accomplished with a set of transverse deflecting cavities. The working principles of the new concept are explained using the first-order matrix formalism in the approximation of the linear single-particle dynamics. The scheme demonstrates versatility regarding the average beam energy and, hence, positioning along the accelerator beam line. The performance of the scheme is numerically investigated with the elegant particle-tracking code. It is shown that the associated nonlinear effects causing emittance deterioration for the extreme quantities of the total energy spread can be effectively compensated by optimization of the Twiss parameters while relying on the eigenemittance analysis. The impact of the longitudinal space charge effects on the beam dynamics in the proposed scheme is also investigated.

The capability of accelerating electron bunches at high repetition rate is one of the key performance criteria for all high average power particle accelerator applications. High gradient laser-driven acceleration holds the potential for greatly reducing size and costs of future machines, but typically requires very high peak laser powers. On the other hand, MHz pulse trains of TW-class laser beams are much beyond the state of the art, so that laser recycling and recirculation is a necessary step to bridge that gap. In this experiment we demonstrate for the first time an inverse free electron laser accelerator (IFEL) operating within an active optical cavity showing the ability to laser-accelerate electron bunch trains in burst mode at $>20\mathrm{MHz}$ repetition rate. The experimental setup, synchronization challenges and acceleration results are presented. It is found that careful control of the dispersive properties of the cavity is required in order to sustain high accelerating gradients over many passes in the laser pulse train.

Movable collimators for the SuperKEKB main ring, which is a two-ring collider consisting of a 4 GeV positron and 7 GeV electron storage ring, were designed to fit an antechamber scheme in the beam pipes, to suppress background noise in a particle detector complex named Belle II, and to avoid quenches, derived from stray particles, in the superconducting final focusing magnets. We developed horizontal and vertical collimators having a pair of horizontally or vertically opposed movable jaws with radiofrequency shields. The collimators have a large structure that protrudes into the circulating beam and a large impedance. Therefore, we estimated the impedance using an electromagnetic field simulator, and the impedance of the newly developed collimators is lower compared with that of our conventional ones because a part of each movable jaw is placed inside the antechamber structure. Ten horizontal collimators and three vertical collimators were installed in total, and we installed them in the rings taking the phase advance to the final focusing magnets into consideration. The system generally functioned, as expected, at a stored beam current of up to approximately 1 A. In high-current operations at 500 mA or above, the jaws were occasionally damaged by hitting abnormal beams, and we estimated the temperature rise for the collimator’s materials using a Monte Carlo simulation code. In order to avoid damage to the jaw, we have started developing a collimator with low atomic number materials.

We explore the possible use of particle beam storage rings as quantum computers. More precisely, we consider creating an ion trap system, in which the same computational basis states can be defined as in a modern ion trap system, but in which the ions have a constant velocity and are rotating in a circular trap. The basic structures that we explore are classical and ultracold crystalline beams. What we propose is a novel method that uses the ion trap quantum computer concept, but puts the ions into a rotating frame of reference. The benefits of this approach are discussed.

Finding a truly local observable for perturbations of the linear beam dynamics is a nontrivial task, contrary to the nonlinear regime, where local resonance driving terms already exist. The phase beating between two locations depends on errors outside of this region. However, phase advances between four nearby locations can be arranged in a way to cancel the contributions from errors outside of this region up to first order. The resulting local observable contains valuable information about quadrupolar lattice imperfections. This report seeks to explore this local phase beating observable and to test its usefulness for gaining insight in the linear optics imperfections of a circular accelerator.

The “Montague resonance” is a familiar issue in high-intensity proton rings, which causes emittance exchange between the two transverse planes, having a significant influence on their high-intensity performance. In this paper, we discuss the characteristic effects of this resonance on the formation of the beam distribution during multiturn injection painting that are measured experimentally at the 3-GeV rapid cycling synchrotron in the Japan Proton Accelerator Research Complex. In addition, we discuss an optimization methodology for injection painting for the case involving emittance exchange.

In this paper we investigate the production of a muon beam by positrons on target. We describe the characteristics of the muon beam as produced by different target configurations. We present an optics for the muon accumulator ring, discussing how it fulfils the large energy acceptance requirement and the high order chromaticity correction at the target, as well as other parameters relevant to enhance the muon bunch quality. We discuss the muon beam dynamics through the ring for the configuration of the single-pass LEMMA scheme, and the optimization study performed with the goals of maximizing the muon bunch population and minimizing its emittance.

Symplectic maps are routinely used to describe single-particle dynamics in circular accelerators. In the case of a linear accelerator map, the rotation number (the betatron frequency) can be easily calculated from the map itself. In the case of a symplectic nonlinear map, the rotation number is normally obtained numerically, by iterating the map for given initial conditions, or through a perturbation approach. Integrable maps, a subclass of symplectic maps, allow for an analytic evaluation of their rotation numbers. In this paper we propose an analytic expression to determine the rotation number for integrable symplectic maps of the plane, if an integral is explicitly known, and present several examples, relevant to accelerators. If the integral of motion is not explicitly known, one can obtain the rotation number numerically as outlined in Appendix B. These new results can be used to analyze the topology of the accelerator Hamiltonians as well as to serve as the starting point for a perturbation theory for maps.

Due to the tiny apertures of dielectric laser acceleration grating structures within the range of the optical wavelength, wakefields limit the bunch charge for relativistic electrons to a few femtocoulomb. In this paper, we present a wakefield upgrade of our six-dimensional tracking scheme DLAtrack6D in order to analyze these limitations. Simulations with CST Studio Suite provide the wake functions to calculate the kicks within each tracking step. Scaling laws and the dependency of the wake on geometrical changes are calculated. The tracking with wakefields is applied to beam and structure parameters following recently performed and planned experiments. We compare the results to analytical models and identify intensity limits due to the transverse beam breakup and strong head-tail instability. Furthermore, we reconstruct phase advance spectrograms and use them to analyze possible stabilization mechanisms.

Thermal emittance and quantum efficiency (QE) are key figures of merit of photocathodes, and their uniformity is critical to high-performance photoinjectors. Several QE mapping technologies have been successfully developed; however, there is still a dearth of information on thermal emittance maps. This is because of the extremely time-consuming procedure to gather measurements by scanning a small beam across the cathode with fine steps. To simplify the mapping procedure and to reduce the time required to take measurements, we propose a new method that requires only a single scan of the solenoid current to simultaneously obtain thermal emittance and QE distribution by using a pattern beam with multiple beamlets. In this paper, its feasibility has been confirmed by both beam dynamics simulation and theoretical analysis. The method has been successfully demonstrated in a proof-of-principle experiment using an L-band radio-frequency photoinjector with a cesium telluride cathode. In the experiment, seven beamlets were generated from a microlens array system and their corresponding thermal emittance and QE varied from 0.93 to $1.14\mu \mathrm{m}/\mathrm{mm}$ and from 4.6% to 8.7%, respectively. We also discuss the limitations and future improvements of the method in this paper.

This paper reports the observation of beam energy widening and distortion from Gaussian due to a nonvanishing horizontal chromaticity at a quasi-isochronous storage ring. The result originates from an average path-length dependence on the betatron oscillation amplitudes, which is intimately correlated to the transverse chromaticities. It is the first experimental validation of the impact of such a nonlinear transverse-longitudinal coupling effect on the equilibrium beam characteristics in a storage ring. The results could be important for quasi-isochronous rings, steady-state microbunching, nonscaling fixed-field alternate gradient accelerators, etc., where very small momentum compaction or large chromaticity is required.

The mechanism of steady-state microbunching (SSMB) has been proposed [D. F. Ratner and A. W. Chao, Phys. Rev. Lett. 105, 154801 (2010)] to generate high-power coherent radiation at a high repetition rate or in continuous-wave mode using electron storage rings. In this paper, the related single-particle dynamics are theoretically and numerically studied, and important results are presented. The investigated effects are longitudinal quantum radiation excitation, nonlinear momentum compaction, and linear and nonlinear coupling of the transverse and longitudinal motion. Although this analysis is oriented toward SSMB, some of the analyzed effects are also crucial in cases such as coherent harmonic generation, bunch slicing, bunch compression, free-electron laser beam transport lines, and quasi-isochronous rings, which involve precise longitudinal phase-space manipulations.

In circular accelerators, transverse beam emittances can be exchanged by means of a coupling resonance crossing. It was first demonstrated experimentally at CERN accelerators, before a theoretical study revealed its mechanism. The existing theory of emittance exchange seems, however, to be deficient. In this study, we establish a theory of emittance exchange using the matrix formalism. Furthermore, we have found that the mechanism of emittance exchange is analogous to the transition dynamics of two-state quantum systems, thereby allowing the Landau-Zener formula to be applied for the prediction of transverse emittance values after nonadiabatic crossing.