We examine the effect of higher-harmonic passive cavities (HHCs) on the beam response to rf noise. Upon invoking certain assumptions to make the problem tractable, we employ Vlasov methods to show that when the dipole approximation applies the HHCs have a generally limited impact. Beam loading in the main cavity is included in the analysis. We illustrate our results and the limitations of our model in application to the Lawrence Berkeley National Laboratory ALS (Advanced Light Source) and the future ALS Upgrade (ALS-U) offering validation against macroparticle simulations.

We derive evolution equations for the first and second moments of an initially mismatched, coupled, and displaced arbitrary Gaussian phase-space distribution under the influence of decoherence due to amplitude-dependent tune shift. Moreover, we find expressions for the asymptotic values of the beam matrix and the emittance and use them to evaluate error tolerances for injection.

We calculate equilibrium emittances and damping times due to the emission of synchrotron radiation for coupled storage ring lattices by evaluating the projections of the commonly used synchrotron radiation integrals onto the normal modes of the coupled motion. Orbit distortion is included by calculating off-axis contributions to the radiation integrals. We provide explicit formulas for fast forward calculation, which have been implemented into the interactive lattice design code opa.

With the development of high-performance photon sources which have extremely low emittance, autotuning magnet girders have drawn more and more attention, especially for diffraction-limited storage rings and free-electron lasers. The biggest challenge is to simultaneously obtain high stability and high flexibility. In this paper, an autotuning magnet-girder prototype has been designed and developed. Topological optimization, multipoint supports, and locking systems have been applied for magnet-girder design to improve the stability. The modal analysis accords with the vibration test well. The natural frequency of the magnet-girder assembly is deduced as high as 45.6 Hz, which demonstrates good stability. Ball-cam movers have been chosen as adjustment mechanisms, and a closed-loop control scheme has been used to pursue high accuracy. The kinematic resolution is better than $1\mu \mathrm{m}$, and the accuracy is better than $1\mu \mathrm{m}$ within the adjusting range of $\pm 5\mathrm{mm}$. Besides, it can eliminate most of the calibration, which can save much manpower and time. The tests demonstrate that the magnet girder can be used for beam-based girder alignment with high stability and high accuracy.

We show that under certain conditions a longitudinal mismatch between an electron and a hadron beam in a coherent electron cooling (CeC) scheme creates a circular attractor in the longitudinal phase space of the cooled hadrons. Formation of an attractor completely stops the cooling and results in anticooling (“heating”) causing small synchrotron amplitudes to grow to the attractor’s radius, rather than being damped. We present a theory of this effect, compare the analytical results with simulations and derive tolerances to possible sources of the longitudinal mismatch. We further show that under certain conditions a “weak” attractor, affecting hadrons with large synchrotron amplitudes, can appear in the hadrons’ longitudinal phase space and explain that formation of such an attractor does not require the presence of any mismatches.

The heavy heat load on optical components from the normal planar undulator has become a serious problem in the synchrotron radiation light source, especially for generating the low-energy photon in a high-energy storage ring and/or a diffraction-limited storage ring. The asymmetric magnet pole undulators have different period lengths between the upper and lower magnet poles. Taking the advantage of the asymmetric magnet pole undulators in building various magnetic fields, this paper proposes a new class of undulators to obtain the specific magnetic fields which can scatter the electron velocity away from the undulator axis. It consists of four magnet arrays and has a similar structure compared to the DELTA/APPLE-X undulator; however, each couple of magnet arrays has a different period length with the period ratio of $n_1:n_2$. Taking the one-to-two and two-to-three period ratios as examples, a novel Figure 8 undulator and a novel Knot undulator are demonstrated. The polarization state can be flexibly switched from the linear polarization to the circular polarization by tuning the phase shifts of magnetic fields, with the on-axis heat load greatly suppressed. The heat-load suppressions of linear polarization at two predetermined orthogonal directions are almost the same and fully effective. Analysis and simulations of undulator designs with the corresponding radiation characteristics are reported.

We present an ultracompact plasma-based method to measure spatial and temporal concurrence of intense electron and laser beams nonintrusively at their interaction point. The electron beam couples with a laser-generated seed plasma in dependence of spatiotemporal overlap, which triggers additional plasma production and manifests as enhanced plasma afterglow. This optical observable is exploited to measure beam concurrence with $\sim 4\mu \mathrm{m}$ spatial and $\sim 26.7\mathrm{fs}$ temporal accuracy, supported by auxiliary diagnostics. The afterglow interaction fingerprint is highly sensitive and enables ultraversatile femtosecond-micrometer beam metrology.

We report the generation of up to 565 MW, 2.7 ns (FWHM) pulses at 11.7 GHz from a metamaterial structure in test at the Argonne Wakefield Accelerator. The highest power was generated by a train of eight 65 MeV electron bunches spaced at 1.3 GHz with a total charge of 355 nC. The metamaterial structure consists of 100 copper unit cells each consisting of a wagon-wheel plate and a spacer plate with a total structure length of 0.2 m. The 565 MW pulse generates a wakefield with a peak on-axis gradient of $135\mathrm{MV}/\mathrm{m}$ that could be used to accelerate a trailing main bunch. An estimated surface electric field of over $1\mathrm{GV}/\mathrm{m}$ is generated on the metamaterial plates at the peak power level but no evidence of breakdown was observed during testing. Tests with single electron bunches and with trains of bunches of up to 100 nC produced output power levels in excellent agreement with simulations. At higher total bunch charge, offsets of the bunches from the axis resulted in beam interception and reduced output power. Simulations indicate that a perfectly aligned bunch train would generate more than 1 GW of power from the structure.

We propose a method to simultaneously determine the magnetic centers of multiple quadrupoles in a transport line or a storage ring. The method finds the magnet centers by correcting the induced orbit shift due to a change of the quadrupole gradient strengths, with the goal of eliminating or minimizing such orbit shifts. The correction of the induced orbit shift is done by steering the orbit toward the quadrupole centers with correctors, using the response matrix of the induced orbit shift with respect to the correctors. The quadrupoles are selected with orbit corrector magnets and beam position monitors in between to ensure that orbit correction at the quadrupole locations can be achieved. The response matrix can be measured or calculated. Simulations with a section of the Linac Coherent Light Source II and the Stanford Positron Electron Asymmetric Ring (SPEAR3) storage ring are done to demonstrate the feasibility and performance of the method. It is also experimentally tested on SPEAR3. The method can be extended for beam-based alignment measurement of nonlinear magnets.

The SIS100 synchrotron as a part of the new Facility for Antiproton and Ion Research (FAIR) accelerator facility at GSI should be operated at the “space charge limit” for light- and heavy-ion beams. Beam losses due to space-charge-induced resonance crossing should not exceed a few percent during a full cycle. The recent advances in the performance of particle tracking tools with self-consistent solvers for the 3D space charge forces now allow us to reliably identify low-loss areas in tune space, considering the full SIS100 accumulation plateau of one second (160 000 turns) duration. A realistic magnet error model, extracted from precise bench measurements of the SIS100 main dipole and quadrupole magnets, is included in the simulations. Previously, such beam dynamics simulations required non-self-consistent space charge models. By comparing to the self-consistent simulations results, we are now able to demonstrate that the predictions from such faster space charge models can be used to identify low-loss regions with sufficient accuracy. The findings are applied by identifying a low-loss working point region in SIS100 for the design FAIR beam parameters. The bunch intensity at the space charge limit is determined. Several countermeasures to space charge are proposed to enlarge the low-loss area and to further increase the space charge limit.

The High-Luminosity target neutral beam absorber (TAXN) is the radiation absorber of the neutral particle debris generated at the beam collisions carried out in the ATLAS and CMS detectors at the Large Hadron Collider (LHC). The new absorber will protect the superconducting magnets from this radiation to prevent their quenching at a design luminosity of $7.5\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. This article describes the mechanical design of the TAXN assembly and the recombination chamber together with the thermal computations performed for the water-cooling and bakeout system prepared for the engineering design review held in January, 2021. It takes into account the internal integration of the zero degree calorimeter and beam rate of neutrals luminosity detector as well as their physics case overview. In addition, it provides the beam aperture breakdown and the heat deposition expected in the absorber.

The next generation of accelerator based scientific facilities, such as the megahertz repetition rate free electron laser and electron-ion collider, relies on photoelectron sources of high peak and high average brightness. Therefore, photocathodes with both low transverse momentum spread (TMS) and high quantum efficiency (QE) are required. The conventional positive correlation between QE and TMS leads to a tradeoff between peak and average brightness. In this study, a negative correlation between QE and TMS is experimentally observed for the first time on a semiconductor photocathode (${\mathrm{Cs}}_2\mathrm{Te}$). Two models are proposed to explain such an anomalous correlation. One model considers the junction between the semiconductor cathode and the metal substrate in the case of slightly excessive cesium deposition, and the other model considers different Cs-Te composites. Our studies inspire a new way to break the conventional correlation between QE and TMS, and open a new perspective to advance the beam brightness frontiers.

The $\mu \mathrm{E}4$-LEM beamline at Paul Scherrer Institute (PSI, Switzerland) is a special muon beamline combining the hybrid-type surface muon beamline $\mu \mathrm{E}4$ with the low-energy muon (LEM) facility and delivers ${\mu }^{+}$ with tunable energy up to 30 keV for low-energy muon spin rotation (LE-$\mu \mathrm{SR}$) experiments. We investigate a possible upgrade scenario for the surface muon beamline $\mu \mathrm{E}4$ by replacing the last set of quadrupole triplets with a special solenoid to obtain 1.4 times the original beam intensity on the LEM muon moderator target. In order to avoid the muon beam intensity loss at the LEM spectrometer due to the stray magnetic field of the solenoid, three kinds of solenoid models have been explored and the stray field of the solenoid at the LEM facility is finally reduced to the magnitude of the geomagnetic field. A more radical design, “Super-$\mu \mathrm{E}4$,” has also been investigated for further increasing the brightness of the low-energy muon beam, where we make use of the current $\mu \mathrm{E}4$ channel and all sets of quadrupole triplets are replaced by large aperture solenoids. Together with the new slanted muon target E, at least 2.9 times the original muon beam intensity can be expected in the Super-$\mu \mathrm{E}4$ beamline. Our work demonstrates the feasibility of upgrading surface muon beamlines by replacing quadrupole magnets with normal-conducting solenoids, resulting in higher muon rates and smaller beam spot sizes.

In this paper we report on recent two-dimensional (2D) electron beam size measurements with a nonconventional synchrotron radiation interferometric technique based on x-ray heterodyne near field speckles (HNFS). The method relies on Fourier analysis of the random speckle patterns generated by a water suspension of nanospheres to assess the full 2D transverse coherence of the incoming x rays. The horizontal and vertical electron beam sizes are then retrieved by means of statistical optics approaches. The manuscript thoroughly describes the HNFS technique, and shows experimental results obtained at the ALBA Synchrotron Light Source. By changing the machine coupling, beam sizes as small as $5\mu \mathrm{m}$ are measured, thus improving on past measurements reported in the literature and proving the HNFS diagnostics suitable for low-emittance particle beams.

Inverse Compton scattering sources are finding increasing use as intense sources of high-energy photons. When operated at high field strength, ponderomotive detuning of the scattered emission can lead to decreased source performance. Up to now, the calculations of spectra for such nonlinear Thomson scattering have been done assuming a perfectly aligned electron interacts with the incident laser beam and several authors have investigated whether pondermotive detuning may be mitigated or cured by suitable incident laser chirping prescriptions. In order to determine if these chirping prescriptions are suitable in real beams with nonzero emittance, it is necessary to include misaligned boundary conditions in the electron motion and calculate the resulting spectra from the exact motion. In this paper we provide the exact solution for the electron equations of motion in the case of a misaligned electron passing through a laser pulse of high field strength. This solution is then used to calculate the scattered radiation distribution and we determine the emittance limits for the simplest chirping prescription.

Cavity-based x-ray free-electron lasers (CBXFEL) will allow the use of optical cavity feedback to support generation of fully coherent x-rays of high brilliance and stability by electrons in undulators. CBXFEL optical cavities comprise Bragg-reflecting flat crystal mirrors, which ensure x-rays circulation on a closed orbit, and x-ray refractive lenses, which stabilize the orbit and refocus the x-rays back on the electrons in the undulator. Depending on the cavity design, there are tens of degrees of freedom of the optical elements, which can never be perfectly aligned. Here, we study signatures of misalignment of the optical components and of the undulator source with the purposes of understanding the effects of misalignment on x-ray beam dynamics, understanding misalignment tolerances, and developing cavity alignment procedures. Betatron oscillations of the x-ray beam trajectory (both symmetric and asymmetric) are one of the characteristic signatures of cavity misalignment. The oscillation period is in the general case a noninteger number of round-trip passes of x-rays in the cavity. This period (unlike the amplitude and offset of the oscillations) is independent of the type of misalignment and is defined by cavity parameters. The studies are performed on an example of a four-crystal rectangular cavity using analytical and numerical wave optics as well as ray-tracing techniques. Both confocal and generic stable cavity types are studied.

The Beam Energy Scan phase II (BES-II), performed in the Relativistic Heavy Ion Collider (RHIC) from 2019 to 2021, explored the phase transition between quark-gluon plasma and hadronic gas. BES-II exceeded the goal of a fourfold increase in the average luminosity over that achieved during Beam Energy Scan phase I (BES-I), at five gold beam energies: 9.8, 7.3, 5.75, 4.59, and $3.85\mathrm{GeV}/\mathrm{nucleon}$. This was accomplished by addressing several beam dynamics effects, including intrabeam scattering, beam-beam, space charge, beam instability, and field errors induced by superconducting magnet persistent currents. Some of these effects are especially detrimental at low energies. BES-II achievements are presented, and the measures taken to improve RHIC performance are described. These measures span the whole RHIC complex, including ion beam sources, injectors, beam lifetime improvements in RHIC, and operation with the world’s first bunched beam Low Energy RHIC electron Cooler (LEReC).

Numerous applications of Mössbauer spectroscopy are related to a unique resolution of absorption spectra of resonant radiation in crystals, when the nucleus absorbs a photon without a recoil. However, the narrow nuclear linewidth renders efficient driving of the nuclei challenging, restricting precision spectroscopy, nuclear inelastic scattering and nuclear quantum optics. Moreover, the need for dedicated x-ray optics restricts access to only few isotopes, impeding precision spectroscopy of a wider class of systems. Here, we put forward a novel Mössbauer source, which offers resonant photon flux for a large variety of Mössbauer isotopes with strongly suppressed electronic background. It is based on relativistic electrons moving through a crystal and emitting parametric Mössbauer radiation essentially unattenuated by electronic absorption. As a result, a collimated beam of resonant photons is formed, without the need for additional monochromatization. We envision the extension of high-precision Mössbauer spectroscopy to a wide range of isotopes at accelerator facilities, also using dumped electron beams.