Emerging high-frequency accelerator technology in the terahertz regime is promising for the development of compact high-brightness accelerators and high resolution–power beam diagnostics. One resounding challenge when scaling to higher frequencies and to smaller structures is the proportional scaling of tolerances which can hinder the overall performance of the structure. Consequently, characterizing these structures is essential for nominal operation. Here, we present a novel and simple self-calibration technique to characterize the dispersion relation of integrated hollow THz waveguides. The developed model is verified in simulation by extracting dispersion characteristics of a standard waveguide a priori known by theory. The extracted phase velocity is in good agreement with the true value. In experiments, the method demonstrates its ability to measure dispersion characteristics of nonstandard waveguides embedded with their couplers with an accuracy due to systematic errors of $\approx 0.5\%$. Equipped with dielectric lining, the metallic waveguides act as slow wave structures, and the dispersion curves are compared without and with dielectric. A phase synchronous mode, suitable for transverse deflection, is found at 275 GHz.

Ion trapping occurs when a negatively charged beam ionizes residual gas inside an accelerator vacuum chamber, and the resulting ions become trapped in the beam potential. In addition to the well-understood coherent instability, trapped ions can cause incoherent effects, such as emittance growth and tune spread. Typically, simulation of ion effects is done using a weak-strong model, in which the ions are modeled using macroparticles, but the beam is assumed to be a fixed Gaussian distribution, with only centroid motion allowed. This type of model necessarily neglects intra-bunch effects. Recently, a residual gas ion modeling capability has been incorporated into the particle tracking code elegant. Both the beam and ions are modeled using macroparticles, so incoherent effects can be studied. The code models ion generation, movement of ions between bunches, and beam-ion interactions. It has also been parallelized and can be used in combination with other elegant elements. The code has been used to study ion instability in the present APS storage ring. Once several important effects are included (multiple ionization, transverse impedance, and charge variation between bunches), the simulations show good agreement with the measured data.

Beginning with a critical examination of the Lorentz-Abraham (LA) classical equation of motion for an extended charge and the closely related Lorentz-Abraham-Dirac (LAD) equation of motion for a mass-renormalized point-charge, the Landau-Lifshitz (LL) approximate solution to the LAD equation of motion is determined for an electron subject to a counterpropagating linearly or circularly polarized plane-wave pulse with an arbitrarily shaped envelope. A convenient three-vector formulation of the LL equation is used to derive closed-form expressions for the velocities and associated powers of the electron directly in terms of the time in the laboratory frame. The three-vector formulation also reveals definitive criteria for the LL solution to be an accurate approximation to the LAD equation of motion and for the LL solution to reduce to the solution of the Lorentz force equation of motion that ignores radiation reaction. Semiclassical analyses are used to obtain simple conditions for determining the regimes where the quantum effects of either Compton electron scattering by the incident photons or electron recoil produced by the emitted photons is significant. It is proven that the LL approximation becomes an inaccurate solution to the LAD equation of motion only for large enough electron velocities and plane-wave intensities that quantum recoil effects on the electron can greatly alter the classical solution. Comparisons are made with previously published analytical and numerical solutions to the LL equation of motion for the velocity of an electron in a counterpropagating plane wave.

The European Synchrotron Radiation Facility (ESRF) operates a 6 GeV fourth generation light source, the ESRF-Extremely Brilliant Source (EBS). This storage ring is the first to implement the hybrid multibend achromat lattice (HMBA) that provides a reduction of the horizontal emittance of approximately a factor 30 with respect to the former double bend achromat structure. By combining several innovative concepts, the HMBA lattice allows the achievement of ultralow emittance while maintaining excellent lifetime and dynamic aperture performance. In this paper, beam commissioning experience and results of the new ESRF storage ring will be presented with an emphasis on the specific challenges relating to the HMBA lattice. The issues, methods, and concepts developed during the commissioning period and first months of operation are discussed.

The neutron time-of-flight (n_TOF) facility at the European Laboratory for Particle Physics (CERN) is a pulsed white-spectrum neutron spallation source producing neutrons for two experimental areas: the Experimental Area 1 (EAR1), located 185 m horizontally from the target, and the Experimental Area 2 (EAR2), located 20 m above the target. The target, based on pure lead, is impacted by a high-intensity $20\text{−}\mathrm{GeV}/c$ pulsed proton beam. The facility was conceived to study neutron-nucleus interactions for neutron kinetic energies between a few meV to several GeV, with applications of interest for nuclear astrophysics, nuclear technology, and medical research. After the second-generation target reached the end of its lifetime, the facility underwent a major upgrade during CERN’s Long Shutdown 2 (LS2, 2019–2021), which included the installation of the new third-generation neutron target. The first- and second-generation targets were based on water-cooled massive lead blocks and were designed focusing on EAR1, since EAR2 was built later. The new target is cooled by nitrogen gas to avoid erosion-corrosion and contamination of cooling water with radioactive lead spallation products. Moreover, the new design is optimized also for the vertical flight path and EAR2. This paper presents an overview of the target design focused on both physics and thermomechanical performance, and includes a description of the nitrogen cooling circuit and radiation protection studies.

Multispectral x-ray pump-probe experiments call for synchronized two-color free-electron lasers (FEL). This mode often implies a laborious setup or an inefficient use of the undulator. We report on a simple and noninvasive approach tested at SwissFEL for a two-color x-ray delivering almost 60% of the pulse energy compared with a single color. In this new method, a ps UV pulse is overlapped to the photocathode drive laser increasing the beam emittance, which locally inhibits the FEL process. This scheme permits high-stability in energy and spectrum and the control of the two-color duration and intensity ratio. It enables shot-to-shot switching between one and two-color FEL and, since not associate to beam losses, it is compatible with high repetition-rate and high average-power FELs.

Beam breakup instability is a potential issue for all particle accelerators and is often the limiting factor for the maximum beam current that can be achieved. This is particularly relevant for energy recovery linacs (ERLs)with multiple passes where a relatively small amount of charge can result in a large beam current. Recent studies have shown that the choice of filling pattern and recirculation scheme for a multipass energy recovery linac can drastically affect the interactions between the beam and rf system. In this paper, we further explore this topic to study how filling patterns affect the beam breakup instability and how this can allow us to optimize the design in order to minimize this effect. We present a theoretical model of the beam-rf interaction as well as numerical modeling and show that the threshold current can vary by a factor of 5, and potentially, even more, depending on the machine design parameters. Therefore a judicious choice of filling pattern can greatly increase the onset of beam breakup, expanding the utility of future ERLs.

Since the introduction of the radio-frequency (rf) photoinjector electron source over thirty years ago, peak performance demands have dictated the use of high accelerating electric fields. With recent strong advances in obtainable field values, attendant increases in beam brightness are expected to be dramatic. In this article, we examine the implementation of very high gradient acceleration in a high frequency, cryogenic rf photoinjector. We discuss in detail the effects of introducing, through an optimized rf cavity shape, rich spatial harmonic content in the accelerating modes in this device. Higher spatial harmonics give useful, enhanced linear focusing effects, as well as potentially deleterious nonlinear transverse forces. They also serve to strongly increase the ratio of average accelerating field to peak surface field, thus aiding in managing power and dark current-related challenges. We investigate two scenarios which are aimed at unique exploitation of the capabilities of this source. First, we investigate the obtaining of extremely high six-dimensional brightness for advanced free-electron laser applications. We also examine the use of a magnetized photocathode in the device for producing unprecedented low asymmetric emittance, high-current electron beams that reach linear collider-compatible performance. As both of the scenarios demand an advanced, compact solenoid design, we describe a novel cryogenic solenoid system. With the high field rf and magnetostatic structures introduced, we analyze the collective beam dynamics in these systems through theory and multiparticle simulations, including a particular emphasis on granularity effects associated with microscopic Coulomb interactions.

Quasimonochromatic x rays are difficult to produce above 100 keV, but have a number of uses in x-ray and nuclear science, particularly in the analysis of transuranic species. Inverse Compton scattering (ICS) is capable of fulfilling this need, producing photon beams with properties and energies well beyond the limits of typical synchrotron radiation facilities. We present the design and predicted output of such an ICS source at CBETA, a multiturn energy-recovery linac with a top energy of 150 MeV, which we anticipate producing x rays with energies above 400 keV and a collimated flux greater than ${10}^8$ photons per second within a 0.5% bandwidth. At this energy, the anticipated flux exceeds that attainable from storage ring sources of synchrotron radiation, even though CBETA is a significantly smaller accelerator system. We also consider the consequences of extending the CBETA ICS source performance to higher electron energies, exploring achievable parameters and applications for MeV-scale photons. We foresee that future energy-recovery linacs may serve as ICS sources, capable of providing high energy photons unavailable at synchrotron radiation facilities or photon beams above approximately 300 keV which outperform sources at synchrotron radiation facilities in both flux and average brilliance.

Neutral uncharged molecules are affected by the electromagnetic field of a charged particle beam if they carry either an electric or a magnetic dipole moment. The residual gas in an accelerator beam pipe consists of such molecules. In this paper we study their dynamics. Under a few approximations, whose validity we explore and justify, we derive the equations of motion of neutral molecules and their invariants, determine the conditions for these neutral molecules to become trapped in the field of the beams as a function of beam-pipe temperature, and compute the resulting enhancement of molecule density in the vicinity of the beam. We demonstrate that large agglomerates of molecules, “flakes,” are much more likely to be pulled into the beam than single molecules, and suggest that this phenomenon might help explain some beam observations at the Large Hadron Collider.

Transverse electron cooling of heavy molecular ions has been studied at the Test Storage Ring (TSR). Electron beams from a cold GaAs:(Cs,O) photocathode, with kinetic energies down to 31 eV, have been used for cooling of singly-charged ions of masses up to 41 u. We believe that these are the heaviest singly-charged ions for which successful electron cooling has been reported so far. Transverse ion-beam emittances $\ll 1\mu \mathrm{m}$ were reached after typically several seconds of cooling time. The measured transverse cooling rates agree with a simple binary-collision model, assuming a transverse electron temperature of approximately $1\mathrm{meV}/k_{\mathrm{B}}$. The results serve as benchmark for electron cooling at the new Cryogenic Storage Ring, which uses the same photocathode electron source and is targeting singly-charged ions of even higher mass.

Isochronous mass spectrometry based on heavy-ion storage rings is a powerful tool for direct mass measurements of very short-lived nuclei. Owing to the nature of in-flight separation of high-energy reaction products, many ion species with different mass-to-charge ratios ($m/q$) can be transmitted to and stored in the ring in one shot. However, high mass resolving power can be achieved only for a limited range of ion species with velocities well matching the isochronous condition of the ring. The knowledge of velocities of each stored ions is required to overcome this restriction. For this purpose, two time-of-flight (TOF) detectors were installed 18 m apart in one straight section of the cooler-storage ring CSRe in Lanzhou. The time sequences measured by the two TOF detectors for each stored ion were used for the precision determination of its velocity. A relative precision of the velocity is achieved to the level of ${10}^{-5}$ for individual ions. The betatron oscillations of the ion motion in the ring were clearly identified in the data and were taken into account in the analysis. The presented technique can be used for measurements of machine tunes and their dependence on particle momenta.

Generally, turn-to-turn fluctuations of synchrotron radiation power in a storage ring depend on the 6D phase-space distribution of the electron bunch. This effect is related to the interference of fields radiated by different electrons. Changes in the relative electron positions and velocities inside the bunch result in fluctuations in the total emitted energy per pass in a synchrotron radiation source. This effect has been previously described assuming constant and equal electron velocities before entering the synchrotron radiation source. In this paper, we present a generalized formula for the fluctuations with a non-negligible beam divergence. Further, we corroborate this formula in a dedicated experiment with undulator radiation in the Integrable Optics Test Accelerator (IOTA) storage ring at Fermilab. Lastly, possible applications in beam instrumentation are discussed.

At the electron linac for beams with high brilliance and low emittance (ELBE) center for high-power radiation sources, the second version of a superconducting radio-frequency (SRF) photoinjector has been put into operation and has been routinely applied for user operation at the ELBE electron accelerator. SRF guns are suitable for generating a continuous wave electron beam with high average currents and high beam brightness. The SRF gun at ELBE has the goal to generate short electron pulses with bunch charges of 200–300 pC at typical repetition rates of 100 kHz for the production of superradiant, coherent terahertz radiation. The SRF gun includes a 3.5-cell, 1.3-GHz niobium cavity and a superconducting solenoid. A support system with liquid nitrogen (${\mathrm{LN}}_2$) cooling allows the operation of normal-conducting, high quantum efficiency photocathodes. We present the design and performance of the SRF gun as well as beam measurement results of the operation with Mg photocathodes at an acceleration gradient of $8\mathrm{MV}/\mathrm{m}$ (4 MeV kinetic energy). In the last section, we discuss the SRF gun application for production of coherent terahertz radiation at the ELBE facility.

Stochastic cooling of electrons and positrons using their extreme ultraviolet radiation is considered. A few beneficial modifications of a stochastic cooling technique are described—a petaherz-scale bandwidth, ability to apply many uncorrelated corrections to particle offsets in one cooling system, cascade amplified stochastic cooling—all resulting in a simpler and more flexible cooling system producing fast cooling without the amplifier. This approach to cooling electron and positron beams easily outperforms radiation cooling in preparing low-emittance and low-energy-spread beams in moderate-energy storage rings.

Traditional spiral inflectors of the Belmont-Pabot type are commonly used for axial injection of external ion beams into cyclotrons. These inflectors are designed to control the trajectory of the central path, and do not actively focus the beam in the vertical and longitudinal directions. This can introduce effects such as a large vertical divergence and a debunching longitudinal spread, making it difficult to match the injection line emittance to the cyclotron acceptance. In an attempt to overcome this, some recent inflectors have started incorporating electrodes specially shaped to produce field gradients along the central path, thereby influencing the inflector optics. This method has shown some success, and at iThemba LABS an inflector was built exhibiting good vertical focusing. However, it performed poorly longitudinally, worse than traditional spiral inflectors. In this article a generalized field gradient spiral inflector design is presented, based on a mathematical description of all possible first-order field gradients along the central path. Such a design is numerically optimized to simultaneously focus longitudinally and vertically. Experimental studies of this design show a 60% improvement in overall current extracted from the cyclotron.

The Landau damping mechanism plays a crucial role in providing single-bunch stability in the LHC, high-luminosity LHC, and other existing as well as previous and future circular hadron accelerators. In this paper, the thresholds for the loss of Landau damping (LLD) in the longitudinal plane are derived analytically using the Lebedev matrix equation (1968) and the concept of the emerged van Kampen modes (1983). We have found that for the commonly used particle distribution functions from a binomial family, the LLD threshold vanishes in the presence of the constant inductive impedance $\mathrm{Im}Z/k$ above transition energy. Thus, the effect of the cutoff frequency or the resonant frequency of a broadband impedance on beam dynamics is studied in detail. The findings are confirmed by direct numerical solutions of the Lebedev equation as well as using the Oide-Yokoya method (1990). Moreover, the characteristics, which are important for beam operation, as the amplitude of residual oscillations and the damping time after a kick (or injection errors) are considered both above and below the threshold. Dependence of the threshold on particle distribution in the longitudinal phase space is also analyzed, including some special cases with a nonzero threshold for $\mathrm{Im}Z/k=\text{const}$. All main results are confirmed by macroparticle simulations and consistent with available beam measurements in the LHC.

Cooling of hadron beams is critically important in the next generation of hadron storage rings for delivery of unprecedented performance. One such application is the electron-ion collider presently under development in the US. The desire to develop electron coolers for operation at much higher energies than previously achieved necessitates the use of radio-frequency (rf) fields for acceleration as opposed to the conventional, electrostatic approach. While electron cooling is a mature technology at low energy utilizing a dc beam, rf acceleration requires the cooling beam to be bunched, thus extending the parameter space to an unexplored territory. It is important to experimentally demonstrate the feasibility of cooling with electron bunches and further investigate how the relative time structure of the two beams affects the cooling properties; thus, a set of four pulsed-beam cooling experiments was carried out by a collaboration of Jefferson Lab and Institute of Modern Physics (IMP). The experiments have successfully demonstrated cooling with a beam of electron bunches in both the longitudinal and transverse directions for the first time. We have measured the effect of the electron bunch length and longitudinal ion focusing strength on the temporal evolution of the longitudinal and transverse ion beam profile and demonstrate that if the synchronization can be accurately maintained, the dynamics are not adversely affected by the change in time structure.

Recent developments in superconducting radio-frequency (SRF) research have resulted in significant increases of cavity performance. Advances have been made in both reducing rf surface resistance and pushing the readily achievable accelerating gradient by using novel SRF cavity treatments including surface processing, custom heat treatments, and flux expulsion. These developments have been largely driven by specific tests on single-cell elliptical cavities, while TEM cavities have typically been designed and developed as specific to a project application and not as standard test articles that could be replicated across various labs. To address this, two purpose-built research cavities, one quarter-wave and one half-wave resonator, have been designed and built to allow characterization of TEM-mode cavities with standard and novel surface treatments. The cavities are intended as the TEM-mode equivalent to the 1.3 GHz single-cell cavity, which is the essential tool for high-frequency cavity research. Given their coaxial structure, the cavities allow testing at the fundamental mode and higher harmonics, giving unique insight into the role of rf frequency on fundamental loss mechanisms from intrinsic and extrinsic sources. In this paper, the cavities and testing infrastructure are described, and the first performance measurements of both cavities are presented. Temperature-dependent surface resistance data are analyzed to extract both the temperature-dependent and temperature-independent components and their dependence on the rf field and frequency. In particular, the temperature-dependent component was found to be at low fields $\propto {\omega }^{1.8(1)}$ at 4.2 K and $\propto {\omega }^{1.7(2)}$ at 2 K, agreeing fairly well with the theoretical model. The growth of the temperature-dependent surface resistance with increasing field amplitude matches both exponential and quadratic growth models fairly well in the examined range. The independent component is determined to be $\propto {\omega }^{0.7(1)}$, matching roughly with anomalous losses, while no clear field dependence was determined. In addition, first measurements of a $120°\mathrm{C}$ baking treatment and of the external magnetic field sensitivity are presented.