A 17 GHz single cell, standing wave, copper accelerator structure with an axisymmetric elliptical central cell sidewall was tested for internal and downstream dark current as a function of gradient up to $93\mathrm{MV}/\mathrm{m}$. The elliptical sidewall was predicted to suppress the internal dark current and the lower order multipactor modes as compared with a previously tested structure having a straight sidewall. During the conditioning phase of the elliptical sidewall structure, strong internal dark current generated by an $N=1$ multipactor mode was observed at gradients in the 10 to $20\mathrm{MV}/\mathrm{m}$ range. After conditioning with $2.2\times {10}^5$ pulses to $93\mathrm{MV}/\mathrm{m}$, the $N=1$ mode was completely suppressed and no multipactor resonances were observed. The internal dark current was reduced to a comparatively low level, much smaller than in the previously studied, straight sidewall structure, in good agreement with simulations. The energy spectrum of the electrons colliding with the sidewall was measured using an isolated side dark current monitor and a bias voltage. As the conditioning progressed, the electron energy spectrum showed an increase in the concentration of lower energy electrons, also in good agreement with simulations. Studies of internal dark current may help to understand the rf conditioning and ultimate performance of high gradient accelerator structures.

We report on the application of machine learning (ML) methods to extract longitudinal phase information such as parameters of the synchrotron damping oscillation. Parameters of the synchrotron damping oscillation are important for the evaluation of machine status and bunch stability. It is of concern to extract these parameters with high-speed and high-precision. The previous methods, such as multiparameter nonlinear fitting and table look-up, are slower and easily fall into local optimal solutions. Our approach based on ML-image processing consists of training a virtual diagnostic to predict parameters using the beam position monitor (BPM) electrical signal data as inputs. We find that when the noise of data is large, our ML-model can still get better results than other methods, an important step toward on-line multiparameter extraction from multidimensional raw data.

Conditioning of a metal structure in a high-voltage system is the progressive development of resistance to vacuum arcing over the operational life of the system. This is, for instance, seen during the initial operation of radio frequency (rf) cavities in particle accelerators. It is a relevant topic for any technology where breakdown limits performance and where conditioning continues for a significant duration of system run time. Projected future linear accelerators require structures with accelerating gradients of up to $100\mathrm{MV}/\mathrm{m}$. Currently, this performance level is achievable only after a multimonth conditioning period. In this work, a pulsed dc system applying voltage pulses over parallel disk electrodes was used to study the conditioning process, with the objective of obtaining insight into its underlying mechanics and, ultimately, to find ways to shorten the conditioning process. Two kinds of copper electrodes were tested: as-prepared machine-turned electrodes (“hard” copper) and electrodes that additionally had been subjected to high-temperature treatments (“soft” copper). The conditioning behavior of the soft electrodes was found to be similar to that of comparably treated accelerating structures, indicating a similar conditioning process. The hard electrodes reached the same ultimate performance as the soft electrodes much faster, with a difference of more than an order of magnitude in the number of applied voltage pulses. Two distinctly different distributions of breakdown locations were observed on the two types of electrodes. Considered together, our results support the crystal structure dislocation theory of breakdown and suggest that the conditioning of copper in high field systems such as rf accelerating structures is dominated by material hardening.

The generation of femtosecond soft x-ray free-electron laser (XFEL) pulses with an energy of hundreds of microjoules has been demonstrated at the Linac Coherent Light Source (LCLS) based on fresh-slice multistage amplifications. In this paper, we present a comprehensive simulation analysis and methods to improve fresh-slice multistage XFELs. A time-dependent transverse kick along the bunch is generated by two passive corrugated structures. The resulting oscillating orbit is controlled by downstream kickers, enabling different slices of the bunch to lase. The simulations reproduce the LCLS experimental results in terms of the pulse energy, bandwidth, and statistics of spectral spikes. The simulations reveal time-domain pulse profile properties, such as the pulse duration and structure, that were unavailable experimentally. We discuss possible issues connected with the accelerator setup and propose a simpler and more robust variation of the scheme to generate XFEL pulses shorter than 3 fs.

High-intensity, fully coherent x-ray pulse with a tunable polarization over a wide spectral range is of great importance to many experiments. In this paper, we propose a tapered crossed-polarized undulator configuration for an x-ray free electron laser oscillator (XFELO) to produce arbitrarily polarized x-ray pulses in the hard x-ray region. A numerical example utilizing the parameters of the Shanghai High-Repetition-Rate XFEL and Extreme Light Facility (SHINE) is presented to demonstrate the generation of polarization controllable, fully coherent hard x-ray pulses with 99.9% polarization degree and 20 kHz polarization switching rate. The analysis presents that this scheme holds the possibility to be used in an energy tunable XFELO built from multiple Bragg mirrors.

The mean volume reflection angle of a high-energy charged particle passing through a bent crystal is expressed as an integral involving the effective interplanar potential over a single crystal period. Implications for positively and negatively charged particles, and silicon crystal orientations (110) and (111) are discussed. A generic next-to-leading-order expansion in the ratio $E/R$ of the particle energy $E$ to the crystal bending radius $R$ is given. For positively charged particles, the dependence of the mean volume reflection angle on $E/R$ proves to be approximately linear, whereas for negatively charged particles the linear behavior is modified by an $E/R$-dependent logarithmic factor. Up-to-date experimental data are confronted with predictions based on commonly used atomic potentials.

Particle emittance quantifies the ability to focus and transport a beam, and is one of the most important parameters for a beamline. The emittance of a proton beam produced by ultraintense laser irradiation of a micron-thick flat solid target has been measured systematically for the first time, using three different methods at different positions along the transport beamline: pepper-pot method, quadrupole triplet scan technique, and single-shot emittance measurement. Emittance growth is shown both in experiments and in simulations using CST. An over 3-fold emittance growth was found for 5 MeV laser-driven protons with an energy spread of $\pm 2\%$ and divergence of $\pm 20\mathrm{mrad}$ after being transported 5.9 m in the experiment, due to the energy spread and angular dispersion of the protons.

Electron cloud ($e$-cloud) mitigation is an essential requirement for proton circular accelerators in order to guarantee beam stability at a high intensity and limit the heat load on cryogenic sections. Laser-engineered surface structuring is considered a credible process to reduce the secondary electron yield (SEY) of the surfaces facing the beam, thus suppressing the $e$-cloud phenomenon within the high luminosity upgrade of the LHC collider at CERN (HL-LHC). In this study, the SEY of Cu samples with different oxidation states, obtained either through laser treatment in air or in different gas atmospheres or via thermal annealing, has been measured at room and cryogenic temperatures and correlated with the surface composition measured by x-ray photoelectron spectroscopy. It was observed that samples treated in nitrogen display the lowest and more stable SEY values, correlated with the lower surface oxidation. In addition, the surface oxide layer of air-treated samples charges upon electron exposure at a low temperature, leading to fluctuations in the SEY.

We continue the study of single-particle dynamics in a more realistic cell, namely, the double-bend achromat with three families of quadrupoles. The cell is parametrized with the optical parameters including the phase advances and the horizontal beta functions at its entry and center. At zero chromaticity, we find that the landscape of the dynamic aperture in the tune plane can be captured by a simple formula constructed from the effective Hamiltonian of fourth order. Furthermore, the optimal dynamic aperture can be found by simply minimizing the third-order driving terms with equal weight in the effective Hamiltonian.

The transverse broadband impedances of major components in the BRing (booster ring) of HIAF (High Intensity Heavy-ion Accelerator Facility) are estimated using the analytical formulas or the wakefield solver in the CST Studio Suite. At low frequency, the transverse broadband impedance model of BRing is $Z_1^H(\omega )=-417.14i\mathrm{k}\mathrm{\Omega }/\mathrm{m}(\mathrm{horizontal})$ and $Z_1^V(\omega )=-530.19i\mathrm{k}\mathrm{\Omega }/\mathrm{m}(\mathrm{vertical})$, which are larger than the threshold impedance for the transverse mode-coupling instability. The ceramic rings in the vacuum chamber are the primary source of impedance. With a goal of mitigating the instability by reducing the impedance of ceramic rings, a high conductivity coating is discussed in detail. In addition, a prototype of ceramic rings-loaded thin-wall vacuum chamber is manufactured and the impedance measurements are performed. When ceramic rings are coated by $2\mu \mathrm{m}$-copper, the CST simulation and experiment results show that the transverse broadband impedance of ceramic rings-loaded thin-wall vacuum chamber can be reduced from $Z_1^H(\omega )=-291.69i\mathrm{k}\mathrm{\Omega }/\mathrm{m}$ and $Z_1^V(\omega )=-352.37i\mathrm{k}\mathrm{\Omega }/\mathrm{m}$ to $Z_1^H(\omega )=-46.16i\mathrm{k}\mathrm{\Omega }/\mathrm{m}$ and $Z_1^V(\omega )=-64.56i\mathrm{k}\mathrm{\Omega }/\mathrm{m}$. Furthermore, in this case the transverse broadband impedance model of BRing is reduced by more than 50% to $Z_1^H(\omega )=-171.61i\mathrm{k}\mathrm{\Omega }/\mathrm{m}$ and $Z_1^V(\omega )=-242.38i\mathrm{k}\mathrm{\Omega }/\mathrm{m}$.

Future hadron colliders aim at producing larger integrated luminosities by burning-off an increased fraction of the bunch particles. This burn-off removes particles unevenly in the bunch distribution generating an emittance growth. Analytical equations are derived describing the bunch distribution suffering luminosity burn-off and cooling from synchrotron radiation damping, far from the equilibrium emittance, and possibly including transverse collision offsets. This effect is evaluated for HL-LHC [G. Apollinari et al., Report No. CERN-2017-007-M, 2017], HE-LHC [A. Abada et al., Eur. Phys. J. Spec. Top. 228, 1109 (2019)], and FCC-hh [A. Abada et al., Eur. Phys. J. Spec. Top. 228, 755 (2019).] and included into a step-by-step simulation of the physics fill, together with intrabeam scattering, to evaluate impact on performance.

The transverse phase space of a beam in an accelerator can be characterized using well-established methods based on observation of changes in the beam profile between screens at different locations along the beamline, or on observation of changes on a single screen for different strengths of upstream quadrupoles. Studies on CLARA FE (the Compact Linear Accelerator for Research and Applications Front End, at Daresbury Laboratory, UK) show that where the beam has a complicated (nonelliptical) distribution in transverse phase space, conventional analysis techniques aimed at characterizing the beam in terms of the emittances and Courant–Snyder parameters fail to provide a good description of the beam behavior. Phase space tomography, however, allows the construction of a detailed representation of the phase space distribution that provides a better model for the beam. We compare the results from three measurement and analysis techniques applied on CLARA FE: emittance and optics measurements from observations on three screens; emittance and optics measurements from quadrupole scans on a single screen; and phase space tomography. The results show the advantages of phase space tomography in providing a detailed model of the beam distribution in phase space. We present the first experimental results from four-dimensional phase space tomography, which gives an insight into beam properties (such as the transverse coupling) that are of importance for optimizing machine performance.

Paraxial optics of charged particles in solenoid magnets with axisymmetric field is considered. Analytic and symplectic transverse maps for charged particle motion through solenoids with 3D magnetic fields have been derived. The formula for transverse maps contains both linear and nonlinear effects caused by entrance and exit fringe fields of the solenoid. The explicit transfer map can be used for applications of accelerator physics that use solenoid magnets.

A hollow electron beam has been proposed as an active control tool to remove the beam halo from high-energy, high-current hadron or ion machines (such as the High-Luminosity Large Hadron Collider). To study the halo removal rate and assess the effect on the ion beam core, one of the two electron lenses in the Relativistic Heavy Ion Collider was changed from a Gaussian beam profile to a hollow profile. We describe the design and verification of the hollow electron beam parameters as well as the methods to minimize the hollow beam profile distortions, which can result in an ion beam emittance increase. The hollow beam alignment with the ion beam by using a backscattered electron detector has been demonstrated. Furthermore, experiments were carried out to explore the efficiency of the halo removal by scanning the current and inner radius of the hollow electron beam, which is pulsed either every turn or every nth turn. The effects of the hollow electron beam on the ion beam emittance and luminosity were also assessed experimentally by scanning the inner radius of the electron beam.

We present a method to measure the transverse size and position of an electron or proton beam, close to the injection point in plasma wakefields, where other diagnostics are not available. We show that transverse size measurements are in agreement with values expected from the beam optics with a $<10\%$ uncertainty. We confirm the deflection of the low-energy ($\sim 18\mathrm{MeV}$) electron beam trajectory by Earth’s magnetic field. This measurement can be used to correct for this effect and set proper electron bunch injection parameters. The advanced wakefield experiment at CERN (AWAKE) relies on these measurements for optimizing electron injection.

The longitudinal acceptance of the BESSY II storage ring has been measured. To our knowledge, such a measurement has never been performed in electron storage rings. The study is also motivated by the fact that for future diffraction limited light sources a couple of longitudinal injection schemes were proposed recently. In these injection schemes, the energy and/or timing of the injection beam is shifted, resulting in a large synchrotron oscillation amplitude of the injected beam. It is, therefore, of importance to reveal whether we can evaluate the longitudinal acceptance precisely when we design a storage ring with a longitudinal injection scheme. The experimental results showed a good agreement to the acceptance evaluated through the accelerator model, including the special feature arising from the synchrotron radiation. We also found that, as a by-product, the bunch length of the injected beam can be obtained from the acceptance measurement data. The result is in good agreement with the design value of the injector booster synchrotron.

The effect of Landau damping is often calculated assuming a Gaussian beam distribution in all transverse degrees of freedom, which agrees reasonably well with beam measurements. However, the stability of the beam is strongly dependent on the details of the distribution. The present study focuses on the slow evolution of the transverse bunch distribution, for a bunch excited by a coherent white noise source, damped by a transverse feedback system, and with detuning dependent on the transverse actions. The mechanism is modeled by the Fokker-Planck equation. It corresponds to a diffusion that is zero for particles with tune equal to the average tune of the bunch, and which is growing quadratically with the tune in the vicinity. The evolving distributions are then used to calculate the evolving stability diagrams, and thus the long-term evolution of the Landau damping. The relative effective octupole current is reduced faster for stronger noise, stronger linear detuning coefficients and weaker damper gain. The relevant parameters for this mechanism are scanned. With relevant parameter values for the LHC, this mechanism can cause a reduction of the effective octupole current by at most $10\%/\mathrm{h}$.