Effects of beam wobbling and target rotation on reducing target temperature are quantitatively considered with simulations and calculations. These manipulations with the beam and target reduce sharpness in the beam-density distribution, making it quasiuniform on the target surface. A uniform beam density is essential in prolonged experiments on the synthesis of superheavy nuclei using intense heavy-ion beams and actinide targets. The heavy-ion beam energy partially absorbed by the target and target backing heats them and transfers warmth to the surrounding by different means. The target temperature was initially considered for a stationary target using notions of heat transfer due to the thermal conductivity, radiation emission, and heat removal to dilute gas surrounding the target. The effects of the beam width, the amplitude of the wobbler, and the rotating target velocity on the beam-density distribution across the target surface and, consequently, on its temperature are further estimated with the same notions.

Controlling trapped magnetic flux in superconducting radio frequency (rf) cavities is of crucial importance in modern accelerator projects. In order to study flux trapping efficiency and sensitivity of surface resistance, dedicated experiments have been carried out on different types of low-$\beta$ superconducting accelerating cavities. Even under almost full trapping conditions, we found that the measured magnetic sensitivities of these cavity geometries were significantly lower than the theoretical values predicted by commonly used models based on local material properties. This must be resolved by taking account of geometrical effects of flux trapping and flux oscillation under rf surface current in such cavity shape. In this paper, we propose a new approach to convolute the influence of geometries. We point out a puzzling contradiction between sample measurements and recent cavity experiments, which leads to two different hypotheses to simulate oscillating flux trapped in the cavity surface. A critical reconsideration of flux oscillation by the rf Lorentz force, compared with temperature mapping studies in elliptical cavities, favored the results of previous sample measurements, which suggested preferential flux trapping of normal component to the cavity inner surface. Based on this observation, we builded a new model to our experimental results and the discrepancy between old theory and data were resolved.

It is known that dispersive coupling can be applied to realize three-dimensional cooling in stochastic cooling and laser cooling. In electron cooling, the transverse cooling rate is usually smaller than the longitudinal one, especially for high-energy beams. We find that dispersion can also be introduced into electron cooling (dispersive electron cooling) to redistribute the cooling rate between the longitudinal and transverse planes. In this paper, we present an analytical model to explain and estimate the rate redistribution in dispersive electron cooling, in which both ion dispersion and electron dispersion are included. It demonstrates that a small horizontal dispersion of the ions can enhance the horizontal cooling at the expense of longitudinal cooling, and the electron dispersion is also of benefit to this effect. We also find that this redistribution effect will be more significant when there is a large difference between the horizontal and longitudinal cooling rates, which will be of great significance for high-energy electron cooling in the future.

Linacs for high-energy physics, as well as for industry and medicine, require accelerating structures which are compact, robust, and cost-effective. Small foot-print linacs require high-accelerating gradients. Currently, stable-operating gradients, exceeding $100\mathrm{MV}/\mathrm{m}$, have been demonstrated at SLAC National Accelerator Laboratory, CERN, and KEK at X-band frequencies. Recent experiments show that accelerating cavities made out of hard copper alloys achieve better high-gradient performance as compared with soft copper cavities. In the scope of a decade-long collaboration between SLAC, INFN-Frascati, and KEK on the development of innovative high-gradient structures, this particular study focuses on the technological developments directed to show the viability of novel welding techniques. Two novel X-band accelerating structures, made out of hard copper, were fabricated at INFN-Frascati by means of clamping and welding. One cavity was welded with the electron beam and the other one with the tungsten inert gas welding process. In the technological development of the construction methods of high-gradient accelerating structures, high-power testing is a critical step for the verification of their viability. Here, we present the outcome of this step—the results of the high-power rf tests of these two structures. These tests include the measurements of the breakdown rate probability used to characterize the behavior of vacuum rf breakdowns, one of the major factors limiting the operating accelerating gradients. The electron beam welded structure demonstrated accelerating gradients of $90\mathrm{MV}/\mathrm{m}$ at a breakdown rate of ${10}^{-3}/(\mathrm{pulse}\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. Nevertheless, it did not achieve its ultimate performance because of arcing in the mode launcher power coupler. On the other hand, the tungsten inert gas welded structure reached its ultimate performance and operated at about a $150\mathrm{MV}/\mathrm{m}$ gradient at a breakdown rate of ${10}^{-3}/(\mathrm{pulse}\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. The results of both experiments show that welding, a robust, and low-cost alternative to brazing or diffusion bonding, is viable for high-gradient operation. This approach enables the construction of multicell standing and traveling-wave accelerating structures.

In order to explore physics in the EUV to soft x-ray region, we have designed a machine which is capable of accelerating a $\sim 250\mathrm{pC}$ electron bunch to an energy of $\sim 1\mathrm{GeV}$. The front end of the CLARA (Compact Linear Accelerator for Research and Applications) system at Daresbury Labs will be used as an S-band injector of $\sim 180\mathrm{MeV}/\mathrm{c}$, sub-ps FWHM, $\sim 250\mathrm{pC}$ electron bunches into the XARA (X-Band Accelerator for Research and Application) system. A rf feasibility study has been carried out for a structure operating in the $2\pi /3$ mode at a frequency of 11.9942 GHz which is fed by a SLED klyston setup. The average cell of this structure has an iris radius of $⟨a⟩=3.2\mathrm{mm}$ and a shunt impedance of $⟨R_s⟩=106.55\mathrm{M}\mathrm{\Omega }/\mathrm{m}$. A high target gradient of $80\mathrm{MV}/\mathrm{m}$ for a single-bunch operation of the linac is necessary due to spatial constraints at Daresbury Labs. We have also implemented Gaussian detuning of the linac in order to future-proof the project for potential multibunch operation of the machine. After combining the rf study with an analysis of the uncoupled long-range wakefield and the short-range transverse wakefields, the optimal structure parameters are outlined as a compromise between the shunt impedance, electrical breakdown rate and wakefields in the structure. As novel designs will be tested using this free-electron laser (FEL) an increased beam charge may be useful. Therefore a beam dynamics study via the particle tracking code elegant has been performed to assess how the beam quality evolves while traveling through the XARA rf structures for different bunch charges and beam offsets. These simulations reveal how the bunch is disturbed for varying bunch charges and offsets and give an initial indication of how sensitive the beam parameters (beam centroid position, emittance, RMS beam size, etc.) are to the wakefields generated in XARA. An analytical formulation of the beam motion as it travels through the XARA linac has been utilized to calculate the emittance growth. This allows for comparison between analytical and numerical simulation of the beam dynamics to give confidence in the results. The beam dynamics study shows that for a bunch charge of $Q_b=250\mathrm{pC}$ and a beam offset of $C_x={\sigma }_z/2=0.253\mathrm{mm}$, the normalized emittance growth at the end of the XARA linac is $\mathrm{\Delta }{\epsilon }_{Nx}/{\epsilon }_{Nx}\sim 270\%$. This may be mitigated by the design of a magnetic lattice which would include kickers to recenter the beam on the electrical axis of the structure, reducing the effect of the wakefields on the bunch. Currently it is probable that a maximum bunch charge of $\sim 250\mathrm{pC}$ will be utilized for this machine; however a future design of a magnetic focusing lattice may allow higher charges to be viable and will reduce the emittance growth.

Mitigation of beam backgrounds via collimators is critical for the success of the Belle II experiment at the SuperKEKB electron-positron collider. We report on an improved simulation methodology, which includes a refined physical description of the collimators and beam pipe, our first implementation of collimator tip scattering, and in which the existing beam particle tracking software has been embedded into a new sequential tracking framework. These improvements resolve longstanding discrepancies between measured and predicted Belle II background levels, and significantly reduce the computing time required to optimize the collimation system in simulation. Finally, we report on collimator aperture scans, which confirm the accuracy of the simulation and suggest a new method for aligning the collimators.

Ultrashort electron bunches are becoming increasingly important for applications such as free-electron lasers, time-resolved electron diffraction/microscopy, and advanced accelerator concepts. We report femtosecond electron bunch generation with a unique rf gun and its precise measurement using a transverse deflecting cavity. A bunch length of $323\pm 13\mathrm{fs}$ with 120 pC ($\sim 7.5\times {10}^8\mathrm{electrons}$) was achieved at a distance of just 3 m from the cathode. A longitudinal phase space distribution was also achieved, and we successfully experimentally confirmed the linearly energy chirped electron bunch generation and self-compression by the velocity difference.

The main physics program of the CERN Super Proton Synchrotron (SPS) is dedicated to the fixed target physics experiments hosted in the North experimental Area (NA). Protons are delivered to the NA via third-integer resonant slow extraction over an almost 5 s flattop. In order to maximize the usable intensity delivered to the experiments, the flux of extracted particles should be kept as constant as possible. This is a very general requirement for fixed target experiments served by synchrotrons. Power supply ripples are a well-known issue in resonant slow extraction, affecting the quality of the spill. A long-standing effort is ongoing at CERN to characterize the SPS slow extraction frequency response to its main power supplies. In this paper, beam dynamics simulations are employed to understand and characterize the process, combined with dedicated beam based measurements.

Observations of optical synchrotron radiation emitted over millions of passes from a few electrons circulating in the particle accumulator ring at the Advanced Photon Source were performed with a digital CMOS camera and a synchroscan streak camera operating at 117.3 MHz. Discrete changes of integrated counts in the CMOS image region of interest are ascribed to single electron steps in a 10-s sensor integration time. Circulations of a single electron at 375 and at 425 MeV were demonstrated in the 12-bit digital CMOS camera images. The streak camera operating at the 12th harmonic of the fundamental revolution frequency at 9.77 MHz was used to measure the bunch length from 0.5-nC circulating charge down to tens of electrons or $<10\mathrm{aC}$. The measurements were performed with $\sim 8\text{−}\mathrm{ps}$ temporal resolution uniquely at such ultralow charges. These results also serve as a proof-of-principle longitudinal profile diagnostic for the predicted effects of optical stochastic cooling at low charge in commissioning experiments at the Fermilab Integrable Optics Test Accelerator ring.

As a part of SPIRAL1 upgrade, several experiments were carried out to understand the transport of $1+$ (monocharged) ion beam through the SP1 ECR charge breeder ion source and to investigate the physical processes involved in charge breeding. Ion beam transport simulations were performed to reproduce the trends of the experimental results (charge breeding efficiency versus $1+$ beam injection energy $\mathrm{\Delta }\mathrm{V}$) by transporting the $1+$ beam through a potential map that reflects the presence of the ECR (electron cyclotron resonance) plasma. The role of Coulomb collisions in the capture of $1+$ beam leads to a necessary detailed analysis for an accurate description of the charge breeding process. This was done by using a full six-dimensional (6D) phase space Monte Carlo charge breeding (MCBC) code. MCBC models Coulomb collisions of the injected $1+$ beam in an ECR plasma and atomic processes (ionization and charge exchange). A simplified background ECR plasma model is implemented in the code to reproduce the trends of three different experiments (interaction of ${\mathrm{Na}}^{1+}$ with He plasma, ${\mathrm{K}}^{1+}$ with He plasma and ${\mathrm{K}}^{1+}$ with ${\mathrm{O}}_2$ plasma). The model is able to reproduce the low charge state ($1+$ and $2+$) experimental trends by tuning each plasma parameter (plasma density, plasma ion temperature and electron temperature) independently. The estimated plasma parameters obtained from each case are presented and their contribution in charge breeding process are discussed.

The Compact Linear Collider (CLIC) could provide $e^{+}{e}^{-}$ collisions in two detectors simultaneously possibly at a bunch train frequency in the linac twice the baseline design value. In this paper, a novel dual beam delivery system (BDS) design is presented in order to serve two interaction regions (IR1 and IR2) including optics designs and the evaluation of luminosity performance with synchrotron radiation (SR) and solenoid effects for both energy stages of CLIC, 380 GeV and 3 TeV. IR2 features a larger crossing angle than the current baseline. The luminosity performance of the novel CLIC scheme was evaluated by comparing the different BDS designs with and without the detector solenoid field effects. It has to be highlighted that the impact of the detector solenoid on luminosity had not been evaluated for the current CLIC baseline, which amounts to a loss of about 4% that corresponds to the same value of the old baseline design. At 380 GeV the novel dual BDS design features same luminosities than the current baseline. However at 3 TeV the luminosity performance is reduced by 2% from the baseline design for the IR1 and by 33% for the IR2. The dual CLIC BDS design provides adequate luminosities to two detectors and proves to be a viable candidate for future linear collider projects.

Negative hydrogen ions are often used for injecting protons from linacs to storage rings via charge-exchange injection. In this process, the two electrons are stripped by a foil or laser to produce protons which can be merged with an existing beam without significantly affecting its dynamics, allowing high intensities of protons to be accumulated. However, this capability comes with the drawback that the outer electron of an ${\mathrm{H}}^{-}$ ion has a low binding energy and can easily be stripped away prior to injection. This paper addresses the following stripping mechanisms: interactions with residual gas in the beam pipe, blackbody radiation from accelerator components, and electromagnetic fields from accelerator optics (Lorentz-force stripping) and particles within the bunch itself (intrabeam stripping); with a discussion on how to avoid excessive activation from stripped ${\mathrm{H}}^0$ particles and protons. We also demonstrate that the proportion of stripped ${\mathrm{H}}^0$ colliding with a nearby beam pipe or machine-element walls presents only roughly 10% of those lost in stripping; the remaining stripped particles traverse to the end of a linac or local straight section, which may relax the limits for allowable stripping-based beam loss in ${\mathrm{H}}^{-}$ accelerators.

We show untenability of key points of the recent article of N. Biancacci, E. Metral, and M. Migliorati [Phys. Rev. Accel. Beams 23, 124402 (2020)], hereafter the article and the authors. The main Eqs. (23) are shown to be incorrect even as an approximation. The article claims the solution of this pair of equations to be in “excellent agreement” with the pyheadtail simulations for CERN PS, which is purportedly demonstrated by Fig. 6. Were it so, it would be either a signal of a mistake in the code or a consequence of a nonsmoothness of the simulations.

Significant magnetic cross talks were observed between neighboring magnets on the Extremely Brilliant Source (EBS) low emittance storage ring. The main sources for these cross talks are the short longitudinal distances between magnets, which reach 47 mm in some cases. It affects mainly the dipole bending angles and the quadrupole gradients. A 1% bending angle correction was applied to the (permanent) bending magnets before their installation in the storage ring to compensate for the cross talks, while gradient errors as high as 1.8% were observed on quadrupoles. Intensive 3D magnetic simulations gave the longitudinal distribution of the cross talk errors for various magnet pairs and current settings. The error distribution depends on the magnet pairs: cross talks may create errors at magnet edges or all along the magnets, with same or opposite polarity. The simulations were validated by magnetic measurements, the discrepancies between measured and simulated values being in the ${10}^{-4}$ range with respect to the nominal gradients. Cross talk induced gradient errors were added to all quadrupoles and neighboring magnets in the lattice model. Even if it complicated the commissioning, the cross talks have no impact on the final performances of the lattice as the design machine parameters were reached for the most part.

Based on continuously recorded beam positions and corrector excitations from, for example, a closed-orbit feedback system we describe an algorithm that continuously updates an estimate of the orbit response matrix. The speed of convergence can be increased by adding very small perturbations, so-called dither, to the corrector excitations. Estimates for the rate of convergence and the asymptotically achievable accuracies are provided.

We propose a new method to measure and correct linear optics and coupling for storage rings with beam position monitor (BPM) data. Two correctors in each transverse plane are used to modulate the closed orbit in an appropriate pattern to sample the linear optics. The orbit modulation data are fitted to the lattice model in a similar manner as the usual method of fitting the orbit response matrix. The closed orbit modulation measurements can be done much faster, and the number of sample points is not limited by the number of correctors. The method also has a number of advantages over methods that rely on turn-by-turn BPMs. A simulation is done to demonstrate the working principles of the method.

For the Linac Coherent Light Source II (LCLS-II) project at SLAC, a 1.3 GHz superconducting rf (SRF) linac is being constructed that will generate 4 GeV electron bunches at a high repetition rate to drive x-ray free electron lasers. The LCLS-II electron source, which comprises the first three meters of the electron injector, includes two normal-conducting, continuous-wave rf cavities: a one-cell, 185.7 MHz gun and a two-cell, 1.3 GHz buncher. It also includes a gun load-lock system that allows photocathodes to be changed under vacuum. The components in this beam-line section were designed and built by Lawrence Berkeley National Laboratory based on experience from their advanced photoinjector experiment program. In combination with the SLAC UV laser system, the electron source is designed to produce beam rates up to 1 MHz with average currents up to $30\mu \mathrm{A}$ initially. The source was installed in mid-2018, well in advance of the SRF linac, which is now nearing completion. The source was commissioned over a two-year period, and this paper presents results including electron beam and dark current characterization.

Submicron defects represent a well-known fundamental problem in manufacturing since they can significantly affect performance and lifetime of virtually any high-value component. Positron annihilation lifetime spectroscopy is arguably the only established method capable of detecting defects down to the subnanometer scale but, to date, it only works for surface studies, and with limited resolution. Here, we experimentally and numerically show that laser-driven systems can overcome these well-known limitations, by generating ultrashort positron beams with a kinetic energy tuneable from 500 keV up to 2 MeV and a number of positrons per shot in a 50 keV energy slice of the order of ${10}^3$. Numerical simulations of the expected performance of a typical mJ-scale kHz laser demonstrate the possibility of generating MeV-scale narrow-band and ultrashort positron beams with a flux exceeding ${10}^5\mathrm{positrons}/\mathrm{s}$, of interest for fast volumetric scanning of materials at high resolution.

Several configurations of the High Luminosity Large Hadron Collider, whose performance at collision is mainly limited due to the strong beam-beam long-range interactions, are studied in the presence of dc wire compensators. This analysis is based on analytical and numerical calculations where the main observables are the dynamic aperture (correlated to the beam lifetime) and the frequency map analysis. It is demonstrated that, with a proper optimization of the wire compensator parameters (distance from the beam and wire current) and without violating the machine protection restrictions, these long range beam-beam interactions can be very well mitigated, making these scenarios viable and complementary with respect to the present HL-LHC baseline. The impact in the integrated luminosity and the operational flexibility gained are also presented.