High current storage rings, such as the Z-pole operating mode of the FCC-ee, require accelerating cavities that are optimized with respect to both the fundamental mode and the higher order modes. Furthermore, the cavity shape needs to be robust against geometric perturbations which could, for example, arise from manufacturing inaccuracies or harsh operating conditions at cryogenic temperatures. This leads to a constrained multiobjective shape optimization problem which is computationally expensive even for axisymmetric cavity shapes. In order to decrease the computation cost, a global sensitivity analysis is performed and its results are used to reduce the search space and redefine the objective functions. A massively parallel implementation of an evolutionary algorithm, combined with a fast axisymmetric Maxwell eigensolver and a frequency-tuning method is used to find an approximation of the Pareto front. The computed Pareto front approximation and a cavity shape with desired properties are shown. Further, the approach is generalized and applied to another type of cavity.

In state of the art photoemission electron sources, the transverse emittance of the electron beam is approaching its intrinsic value from the cathode. To reduce the intrinsic emittance, it is straightforward to tune the photon energy of the drive laser toward the photocathode emission threshold. This paper aims at constructing an improved model which takes into account an intermediate energy level to better explain the near threshold photoemission for semiconductors. A dynamic model is also proposed to evaluate the cathode degradation process due to surface oxidation. The models agree well with published results for different materials under various conditions, including electric field, temperature and vacuum condition. The dynamic model predicts that near threshold emission of an oxidized ${\mathrm{Cs}}_3\mathrm{Sb}$ cathode outperforms UV photoemission of typical metal cathodes in terms of both quantum efficiency and intrinsic emittance. With the oxidization layer, the robustness of semiconductor cathodes to gun vacuum condition may be comparable to a metal cathode. This, however, needs further testing.

We propose a method for numerical calculation of driving and detuning transverse beam coupling impedances of an asymmetric cavity. The method relies on eigenmode simulations and can be viewed as an alternative to time-domain wakefield simulations. A similar procedure is well established for symmetric cavities, and this paper extends it to the case of an asymmetric cavity. The method is benchmarked with time-domain wakefield simulations and its practical implementation is discussed.

NSLS-II is now one of the brightest synchrotron light sources worldwide, in user operations since 2014. Currently, 20 light-generating insertion devices (IDs) are installed: 3 damping wigglers and 17 undulators of various types. These devices introduce significant distortions into the magnet lattice of the storage ring. For each ID, the distortions have been locally corrected using quadrupole magnets located nearby the ID. The insertion devices have also a significant impact on the electron beam emittance. Precise measurements of the emittance were carried out at the Coherent Hard X-ray beamline for three different lattices: without IDs, with 3 pairs of damping wigglers only, and with all IDs. A minimum horizontal emittance of 0.76 nm was achieved. The radiation energy loss and relative energy spread were also measured using a streak-camera. The measured data show good agreement with analytical formulas and numerical simulations.

A new scheme with sequential resonant excitation for laser assisted charge exchange injection of ${\mathrm{H}}^{-}$ beams is proposed. In contrast with the one step excitation scheme, the proposed scheme requires significantly less laser power for high efficiency stripping. It also provides greater flexibility in the choice of laser wavelength for a given beam energy and extends the range of energies where laser stripping can be effectively applied. Calculations and experimental plans for 1.0 GeV and 1.3 GeV ${\mathrm{H}}^{-}$ beams at the Spallation Neutron Source are presented. Results indicate that the sequential excitation scheme allows the use of more convenient green laser with much smaller power in place of the UV laser previously used.

The resistive wall impedance of a vacuum chamber with elliptic cross section is of particular interest for circular particle accelerators as well as for undulators in free electron lasers. By using the electric field of a point charge and of a small dipole moving at arbitrary speed in an elliptical vacuum chamber, expressed in terms of Mathieu functions, in this paper we take into account the finite conductivity of the beam pipe walls by means of the surface impedance, and evaluate the longitudinal and transverse driving and detuning impedances for any beam velocity. We also extend the definition of the Yokoya form factors, valid in the thick wall regime, at any beam energy, and show that, in the ultra-relativistic limit, they coincide with the ones that are found in literature. The method is also extended to the multilayer vacuum chamber case. Under conditions generally satisfied with particle accelerator beam pipes, the classical transmission line theory can be used to modelling the impedance seen by a bunch in a vacuum chamber with several layers as an equivalent circuit with the same number of load impedances, giving, as result, a surface impedance that can be used in combination with the fields of the elliptic geometry to obtain the resistive wall impedance in an elliptical multilayer vacuum chamber. The results are also compared with a more time consuming 3D electromagnetic code and with solutions for known cases of circular and flat beam pipe.

A phase space beam position and size monitor (ps-BPM) system was introduced previously to measure the electron source vertical position and angular motion along with the vertical source size and angular size at a single location in a synchrotron bend magnet beam line. The system was validated experimentally at the Canadian Light Source and demonstrated by a simulation that it is suitable for low-emittance light sources. In this work, the potential practicality of the ps-BPM system is explored with real-case studies. By combining the analysis in the time and frequency domain, information on the beam motion and size can be extracted and identified from both the source and the beam-line optics. Applications of the ps-BPM system at the Canadian Light Source are demonstrated by studying beam vibrations and machine insertion device field changes. Results of these studies provide guidelines for implementing a dedicated ps-BPM system at existing and planned synchrotron facilites.

We present a novel concept for the generation of gigahertz repetition rate high brightness electron bunches. A custom design 100 kV thermionic gun provides a continuous electron beam, with the current determined by the filament size and temperature. A 1 GHz rectangular rf cavity deflects the beam across a knife-edge, creating a pulsed beam. Adding a higher harmonic mode to this cavity results in a flattened magnetic field profile which increases the duty cycle to 30%. Finally, a compression cavity induces a negative longitudinal velocity-time chirp in a bunch, initiating ballistic compression. Adding a higher harmonic mode to this cavity increases the linearity of this chirp and thus decreases the final bunch length. Charged particle simulations show that with a 0.15 mm radius ${\mathrm{LaB}}_6$ filament held at 1760 K, this method can create 279 fs, 3.0 pC electron bunches with a radial rms core emittance of 0.089 mm mrad at a repetition rate of 1 GHz.

Planar nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] has emerged as a unique field emission source attractive for accelerator applications because of its capability to generate a high charge beam and handle moderate vacuum conditions. Most importantly, (N)UNCD sources are simple to produce: Conventional high aspect ratio isolated emitters are not required to be formed on the surface, and the actual emitter surface roughness is on the order of only 10–100 nm. A careful reliability assessment of (N)UNCD is required before it may find routine application in accelerator systems. In the present study using an L-band normal conducting single-cell rf gun, a (N)UNCD cathode has been conditioned to $\sim 42\mathrm{MV}/\mathrm{m}$ in a well-controlled manner. It reached a maximum output charge of 15 nC corresponding to an average current of 6 mA during an emission period of $2.5\mu \mathrm{s}$. Imaging of the emission current revealed a large number of isolated emitters (density over $100/{\mathrm{cm}}^2$) distributed on the cathode, which is consistent with previous tests in dc environments. The performance metrics, the emission imaging, and the systematic study of emission properties during rf conditioning in a wide gradient range assert (N)UNCD as an enabling electron source for rf injector designs serving industrial and scientific applications. These studies also improve the fundamental knowledge of the practical conditioning procedure via a better understanding of the emission mechanisms.

The powering of octupoles during third-integer resonant slow extraction has been studied and recently tested with the beam at the CERN Super Proton Synchrotron (SPS) in order to increase the extraction efficiency and reduce the induced radioactivity along the extraction straight. The octupoles distort the particle trajectories in phase space in such a way that the extracted separatrix is folded, which decreases the particle density impinging the wires of the extraction septum at the expense of increasing the extracted beam emittance. During experimental SPS machine studies a reduction of over 40% in the specific (per extracted proton) beam loss measured at the extraction septum was achieved. In this paper, the prerequisite studies needed to safely deploy the new extraction scheme in a limited time-frame are described, the experimental results are presented and an outlook given toward the next steps to bring slow extraction with octupoles into routine operation.

A challenge for the TW-class accelerators driving $Z$-pinch experiments, such as Sandia National Laboratories’ $Z$ machine, is to efficiently couple power from multiple storage banks into a single multi-MA transmission line. The physical processes that lead to current loss are identified in new large-scale, multidimensional simulations of the $Z$ machine. Kinetic models follow the range of physics occurring during a pulse, from vacuum pulse propagation to charged-particle emission and magnetically-insulated current flow to electrode plasma expansion. Simulations demonstrate that current is diverted from the load through a combination of standard transport (uninsulated charged-particle flows) and anomalous transport. Standard transport occurs in regions where the electrode current density is a few $10^4-{10}^5\mathrm{A}/{\mathrm{cm}}^2$ and current is diverted from the load via transport without magnetic insulation. In regions with electrode current density $>{10}^6\mathrm{A}/{\mathrm{cm}}^2$, electrode surface plasmas develop velocity-shear instabilities and a Hall-field-related transport which scales with electron density and may, therefore, lead to increased current loss.

Electron spin polarization occurs naturally in an electron storage ring due to spin-flip synchrotron radiation. The equilibrium polarization level that is achieved in Diamond Light Source was studied theoretically and experimentally. A novel approach that enables measurements during user operation is presented, compatible with beams of small vertical emittance in the range of pm rad. An array of four custom-made scintillation detectors has been installed around the beam pipe downstream from the collimators to capture a significant fraction of the lost particles and witness the depolarization process in the presence of a background loss rate. A new concept for detecting the depolarization resonance even when it coincides with higher-order betatron resonances was introduced. The momentum compaction factor was measured to be in very good agreement with the storage ring numerical model. In the end, we measured a good long-term stability of the beam energy during user operation time.

The beam dump facility (BDF) is a project for a new facility at CERN dedicated to high intensity beam dump and fixed target experiments. Currently in its design phase, the first aim of the facility is to search for light dark matter and hidden sector models with the Search for Hidden Particles (SHiP) experiment. At the core of the facility sits a dense target/dump, whose function is to absorb safely the $400\mathrm{GeV}/\mathrm{c}$ Super Proton Synchrotron (SPS) beam and to maximize the production of charm and beauty mesons. An average power of 300 kW will be deposited on the target, which will be subjected to unprecedented conditions in terms of temperature, structural loads and irradiation. In order to provide a representative validation of the target design, a prototype target has been designed, manufactured, and tested under the SPS fixed-target proton beam during 2018, up to an average beam power of 50 kW, corresponding to 350 kJ per pulse. The present contribution details the target prototype design and experimental setup, as well as a first evaluation of the measurements performed during beam irradiation. The analysis of the collected data suggests that a representative reproduction of the operational conditions of the beam dump facility target was achieved during the prototype tests, which will be complemented by a postirradiation examination campaign during 2020.

A new project, high intensity heavy ion accelerator facility (HIAF), is currently under design and construction in China. The HIAF front end, composed of electron cyclotron resonance (ECR) ion sources, low energy beam transport (LEBT) and radio frequency quadrupole (RFQ), will produce and provide beams of ions with a mass up to uranium at a beam energy of $0.5\mathrm{MeV}/\mathrm{u}$. The typical beam intensity is designed up to 2 emA for the uranium beam with a charge state of $35+$. This paper presents an overall design of the front end for HIAF and discusses several key issues in the design. By modeling the beam extraction from the ECR source, we got a reliable starting beam condition to perform the design. Transverse coupling of the beam from the source was elaborated. To relieve the coupling we implanted two solenoids after the source. Space charge effect in the charge state selection of the ion source was evaluated. An overall space charge compensation degree of no less than 70% was predicted. A beam dynamics simulation was performed by using the initial particle distribution obtained from the extraction modeling. The simulation resulted in development of a beam collimation system in the LEBT to confine the transverse emittance. The RFQ design will follow the development of LEAF-RFQ at Institute of Modern Physics, which has successfully commissioned with several beams and demonstrated as an excellent design. Recent beam commissioning results of LEAF-RFQ will also be presented in this paper.

X-ray free electron lasers provide short, polarized, high-power pulses of x-ray radiation, where polarization properties are determined by the undulator magnetic field. We propose a cost-effective method to shape the polarization of FEL radiation on the sample both spatially, on a few-$\mu \mathrm{m}$ scale, and/or temporally on the 100-femtosecond scale. The method is based on coaxial superposition of two coherent radiation pulses with different frequencies emitted in two consequent undulators set to emit radiation with orthogonal polarization states. Its capabilities are demonstrated via numerical simulations for the SASE3 undulator line of the European X-ray Free-Electron Laser. Generation of x-ray free-electron laser pulses with shaped polarization may be interesting for studies of magnetism or ultrafast phenomena and will facilitate further developments of complex light engineering in the soft X-ray range.

The world’s first electron cooling based on the rf acceleration of electron bunches was experimentally demonstrated at the Low Energy RHIC Electron Cooler (LEReC) at Brookhaven National Laboratory. The critical step in obtaining cooling of the Au ions in the collider with this new approach was matching the electron and ion relativistic $\gamma$-factors with a relative error of less than $5\times {10}^{-4}$. Since the electron beam kinetic energy was just 1.6 MeV, it was required to set the absolute energy of electrons with an accuracy better than 0.8 keV. The method of setting electron energy in conventional coolers was unsuitable for LEReC and a new technique had to be developed. In this paper we describe our experience with measuring the electron beam energy at LEReC and precisely matching electron and ion $\gamma$-factors, which resulted in demonstration of the cooling.

Persistent currents in superconducting magnet introduce magnetic errors especially at low operating fields. In addition, their decay causes magnetic field variations and therefore drifts of the beam orbits, tunes, and chromaticities. To reduce field errors and suppress magnetic field variations, a new magnetic cycle was proposed for the low-energy beam operation at the Relativistic Heavy Ion Collider (RHIC). In the new magnetic cycle, the magnet current oscillates around the nominal operating current with diminishing amplitude a few times before it settles. The new magnetic cycle has been demonstrated experimentally to reduce field errors and the amplitude of magnetic field variations significantly and is essential for the ongoing RHIC Beam Energy Scan II (BES-II) program. This article will present beam-based experimental studies of the persistent current effects with the new magnetic cycle, and discuss its application in RHIC.

In the presence of longitudinal coherent electron cooling, the evolution of the line-density profile of a circulating ion bunch can be described by the 1D Fokker-Planck equation. We show that, in the absence of diffusion, the 1D equation can be solved analytically for a particular dependence of the cooling force on the synchrotron amplitude. For more general cases with arbitrary diffusion, we solved the 1D Fokker-Planck equation numerically, and the numerical solutions have been compared with results from macroparticle tracking.