Recently, gradients on the order of $1\mathrm{GV}/\mathrm{m}$ have been obtained with single-cycle ($\sim 1\mathrm{ps}$) THz pulses produced by the conversion of high peak power laser radiation, in nonlinear crystals ($\sim 1\mathrm{mJ}$, 1 ps, up to 3% conversion efficiency). Such high intensity radiation can be utilized for charged particle acceleration. For efficient acceleration, a large number of accelerating cells have to be stacked together with delay lines in order to provide proper timing between accelerating fields and moving electrons. Additionally, THz pulses in individual cells need to be focused transversely to maximize the accelerating gradient. This focusing is currently done by the tapering of the waveguide. In this paper, we propose instead to use dielectric lenses for the robust focusing of THz pulses in accelerating cells. Each lens is made from the same material as the delay line in a monolithic unit. We have fabricated prototype units from quartz and silicon. Such an approach simplifies the fabrication and alignment of the multicell accelerating structure. We present a design in which a 100 microJoule THz pulse produces a 600 keV energy gain in a 5-mm long 10-cell accelerating structure for an ultrarelativistic electron. This approach can be extended to nonrelativistic particles.

Dielectric laser acceleration (DLA) achieves remarkable gradients from the optical near fields of a grating structure. Tilting the dielectric grating with respect to the electron beam leads to deflection forces, and the DLA structure can be utilized as a microchip undulator. We investigate the beam dynamics in such structures analytically and by numerical simulations. A crucial challenge is to keep the beam focused, especially in the direction of the narrow channel. An alternating phase focusing scheme is optimized for this purpose and matched lattice functions are obtained. We distinguish synchronous operation with phase jumps in the grating and asynchronous operation with a strictly periodic grating and well-designed synchronicity mismatch. Especially, the asynchronous DLA undulator is a promising approach, since a simple, commercially available grating suffices for the focusing lattice design. We pave the way toward experiments of radiation generation in these structures and provide estimates of the emitted radiation wavelength and power. The analytical models are validated by numerical simulations in the dedicated DLA simulation tool dlatrack6d and astra, where the underlying laser fields are computed by cst studio.

The recent achievements in the performance of superconducting rf (SRF) photoinjectors have opened a new era in the development of reliable high-brightness cw electron sources. While the SRF guns become one of the most promising technologies, the compatibility of the SRF environment with the complex photocathodes remains at the forefront of the modern accelerator science. The SRF cavities operate at cryogenic temperatures providing the ultrahigh vacuum environment highly beneficial for the photocathode performance. However, the necessity to keep the photocathodes at room temperature while being surrounded by the cavity walls that are kept at cryogenic temperatures creates an additional complexity for the SRF gun design and operation. The complex and volatile chemical compounds used for photocathodes have a high chance of contaminating the surfaces of the SRF cavity. When deposited, such compounds could create centers for cold electron emission which degrade the performance of the SRF guns. Such a circumstance would require the development of the in-situ processing techniques for restoring the SRF cavity performance. This paper presents the results of the successful implementation and application of the He conditioning method for cavity restoration using the existing SRF photoinjector at Brookhaven National Laboratory (BNL). The method has proven to be extremely effective and resulted in a dramatic improvement in the BNL gun performance.

The transverse emittance of a charged particle beam is an important figure of merit for many accelerator applications, such as ultrafast electron diffraction, free electron lasers, and the operation of new compact accelerator concepts in general. One of the easiest to implement methods to determine the transverse emittance is the phase advance scan method using a focusing element and a screen. This method has been shown to work well in the thermal regime. In the space charge dominated laminar flow regime, however, the scheme becomes difficult to apply because of the lack of a closed description of the beam envelope including space charge effects. Furthermore, certain mathematical, as well as beamline design criteria must be met in order to ensure accurate results. In this work, we show that it is possible to analyze phase advance scan data using a fully connected neural network (FCNN), even in setups, which do not meet these criteria. In a simulation study, we evaluate the performance of the FCNN by comparing it to a traditional fit routine based on the beam envelope equation. Subsequently, we use a pretrained FCNN to evaluate measured phase advance scan data, which ultimately yields much better agreement with numerical simulations. To tackle the confirmation bias problem, we employ additional mask-based emittance measurement techniques.

Highly efficient superconducting radio-frequency cavities exhibit low heat loss and are used as components in accelerators and superconducting devices due to their high $Q$-values. The precise location of magnetic flux trapping in cavities is necessary to identify its effects on the performance of superconducting cavities. In this study, we report a new combined mapping system to measure the temperature and magnetic field on the equator. The proposed system comprehensively maps local magnetic field changes as magnetic flux trapping due to quenching. Our experimental results show that magnetic flux trapping due to quenching increases the local surface resistance of superconducting cavities at 2 K. Thus, the proposed system elucidates the relationship between local flux trapping due to quenching and surface resistance in superconducting cavities and highlights the effect of quenching on the surface resistance. This system can aid in the development of superconducting cavities with higher $Q$ values.

For the accelerator community and the vacuum scientists, the understanding of the beam interactions with a vacuum chamber is fundamental to provide solutions to mitigate pressure rises induced by electron, photon, and ion molecular desorption. Moreover, beam instabilities induced by ion and electron clouds must be investigated in order to find solutions to reduce them. This study presents in situ measurements of pressure evolutions and electrical currents performed during the LHC RUN II (2018). The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. These charged particles are accelerated away from the beam and reach the vacuum chamber wall, inducing, among other phenomena, stimulated desorption and secondary electron emission. Moreover, protons emit synchrotron radiations that also induce photodesorption and photoelectron production. Experimental measurements of the electrical signals recorded on copper electrodes were compared to calculations considering both the secondary electron yield of copper and the electron energy distribution. All measurements performed with the Vacuum Pilot Sector in the LHC ring show the importance of taking into account a large variety of phenomena in order to understand the pressure evolution in the LHC. Results show that the multipacting threshold, corresponding to an increase in the electron cloud density, strongly depends on the number of protons per bunch. Finally, the ion current was measured with a biased electrode lower than $-500\mathrm{V}$. It was much higher than expected, pointing its origin not only from simple beam-gas ionization but also from the ionization of the residual gas by the electron cloud.

A space charge algorithm is constructed that is a hybrid between envelope and multiparticle models. The transverse dynamics is simplified by tracking the transverse envelope of each macroparticle. The equations of motion are derived from the Hamiltonian-fluid formulation of the Vlasov Poisson system. A novel electrostatic self-field solver is derived that solves directly for the self-consistent on-axis potential and linear defocusing force, including longitudinal and beam pipe boundary conditions. The implementation of the algorithm, hyper3d, is presented. It is tested against the particle-in-cell code warp, using various configurations of a periodic focusing structure with rf cavities. The required number of macroparticles is reduced substantially compared to standard particle-tracking codes. This model is well adapted to cases where the transverse dynamics is linear and where the details of the longitudinal dynamics are the principal point of interest; an example is where a dc beam is converted to a bunched beam.

The main obstacle to the practical implementation of the dielectric wakefield acceleration (DWA) concept is the development of the beam breakup instability due to transverse dipole wakefields generated when the beam propagates off axis in the accelerating structure. One of the methods to suppress this instability is to elliptically shape the beam and accelerate it in a planar structure. Here, we report a detailed experimental investigation of the transverse dynamics of elliptical beams in planar dielectric structure with parameters mimicking future DWA modules. Both dipole and quadrupole wakefields’ effects on beam stability and projected emittance were studied. This study has highlighted the importance of counteracting quadrupole wakefields in future DWA implementations.

Radio-frequency (rf) photoemission electron guns operating at higher accelerating gradients offer a pathway to produce brighter electron bunches. Brighter beams are expected to revolutionize many areas of science: they could form the backbone of next-generation compact x-ray free-electron lasers or provide coherent ultrafast quantum electron probes. We report on the experimental demonstration of an rf photoemission electron source supporting an accelerating field close to $400\mathrm{MV}/\mathrm{m}$ at the photocathode surface without major rf breakdown or significant dark current over a 3-week experimental run. This unprecedented gradient was achieved by operating the gun in a new regime with short rf pulses, $\sim 9\mathrm{ns}$ $X$-band (11.7 GHz). The demonstrated paradigm provides a viable path to form relativistic electron beams with unprecedented brightness.

A pulsed electron lens produces a betatron tune shift along a hadron bunch as a function of the longitudinal coordinates, which is a longitudinal detuning. An example of transverse detuning is the tune shifts due to octupole magnets. This paper considers a pulsed electron lens as a measure to mitigate the head-tail instabilities. Using a detailed analytical description within a Vlasov formalism, the coherent properties of the longitudinal and transverse detuning are presented. The analytical predictions are compared with the results of the particle tracking simulations. A pulsed electron lens is demonstrated to be a source of tune spread with two components: a static one, leading to Landau damping and a dynamic one, leading to an effective impedance modification, which we demonstrate analytically and in our particle tracking simulations. The effective impedance modification can be important for beam stability due to devices causing longitudinal detuning, especially for nonzero head-tail modes. The Vlasov formalism is extended to include the combination of longitudinal and transverse detuning. As a possible application at the SIS100 heavy-ion synchrotron [Facility for Antiproton and Ion Research (FAIR) at GSI Darmstadt, Germany], a combination of a pulsed electron lens with octupole magnets is considered.

Photon sources based on inverse Compton scattering, namely, the interaction between relativistic electrons and laser photons, are emerging as quasimonochromatic energy-tunable sources either as compact alternatives to synchrotron facilities for the production of low-energy (10–100 keV) x rays or to reach the 1–100 MeV photon energy range, which is inaccessible at synchrotrons. Different interaction layouts are possible for electron and laser beams, and several applications are being studied, ranging from fundamental research in nuclear physics to advanced x-ray imaging in the biomedical field, depending on the radiation energy range, intensity, and bandwidth. Regardless of the specific application, a reliable tool for the simulation of the radiation produced is essential for the design, the commissioning, and, subsequently, the study and optimization of this kind of source. Different computational tools have been developed for this task, based on both a purely analytical treatment and Monte Carlo simulation codes. Each of these tools has strengths and weaknesses. Here, we present a novel Monte Carlo code based on geant4 for the simulation of inverse Compton scattering in the linear regime. The code produces results in agreement with cain, one of the most used Monte Carlo tools, for a wide range of interaction conditions at a computational time reduced by 2 orders of magnitude. Furthermore, the developed tool can be easily embedded in a geant4 user application for the tracking of photons generated through inverse Compton scattering in a given experimental setup.

The Synchrotron Radiation Interferometry (SRI) has become a widely used technique to measure the small transverse size of the electron beam in the storage ring. In a typical SRI system for the routine storage ring operation, synchrotron radiation from a dipole magnet is used to illuminate a double slit with a small slit opening and a relatively large slit separation to form a large number of interference fringes on the observation plane. However, a different type of SRI is needed for intrabeam scattering (IBS) research to measure the beam size at ultralow currents and in a wide dynamic range. Such a system requires a double slit with a large slit opening to increase the light input while having the capability of accurately measuring the beam size with a range of visibility. By examining the impact of the nonuniform wave amplitude of synchrotron radiation and that of the varying visibility (due to a changing beam size) on the beam size measurement, we propose a new physics model for this type of SRI. This new model is validated using simulation, showing significantly improved results when compared with the conventional model. Based on this new model, we have developed and tested an SRI system dedicated to the IBS study on the Duke storage ring. This system has been used successfully to measure electron beams with about $10\mu \mathrm{A}$ of current and with a higher current but a variable size. This new physics model can also improve the measurement accuracy and consistency of the conventional SRIs, especially at low visibility.

The target bandwidth of the Advanced Photon Source Upgrade (APS-U) fast orbit feedback (FOFB) is dc-1000 Hz where the synchrotron tune will be between 100 and 560 Hz. This overlap places coupled bunch mode 0 (CBM0) induced horizontal position offsets within the FOFB bandwidth range, affecting our ability to achieve APS-U goals for beam stability. In this article, we present a new feedback technique to damp CBM0 oscillations using the existent FOFB controller framework with no additional hardware and software requirements. Based on the theory of synchrotron oscillations, we developed a feedback algorithm that uses the beam horizontal position measurements at dispersive beam position monitors (BPM) as input and the low level rf (LLRF) phase as an actuator. We demonstrated via experiments in the present APS storage ring that this configuration can correct CBM0 oscillations, and also it can work simultaneously with orbit feedback used to damp ordinary betatron orbit. The simulation model for the proposed orbit to rf phase feedback system is implemented in matlab/simulink to guide us in the controller design. Simulation responses are in good agreement with measurements.

We present a setup for highly polarized proton beams using two parallel propagating laser pulses that have a carrier envelope phase difference of $\pi$. This mechanism is examined utilizing particle-in-cell simulations and compared to a single-pulse setup commonly used for magnetic vortex acceleration. We find that the use of the dual-pulse setup allows for peak energies of 124 MeV and good angular spread for two pulses with normalized laser vector potential $a_0=100$. Compared to a single pulse, we further observe higher polarization of the accelerated bunch.

A simple 2D model of the bubble formation in a plasma wakefield accelerator is developed and investigated. We show that in the case of a flat driver, the bubble may consist of two parts. The first part corresponds to a laminar flow where plasma electron streams do not cross and the second to the two-stream (turbulent) flow. The laminar flow turns out to be robust to the symmetry breaking. Building off of the developed model, we demonstrate that, in the case of the laminar flow and nonrelativistic plasma electrons, the transverse wakefield is absent inside the bubble even in the case of a transversely nonuniform plasma.

Ultrafast electron diffraction (UED) is a technique in which short-pulse electron beams can probe the femtosecond-scale evolution of atomic structure in matter driven far from equilibrium. As an accelerator physics challenge, UED imposes stringent constraints on the brightness of the probe electron beam. The low normalized emittance employed in UED, often at the 10-nm scale and below, is particularly sensitive to both applied field aberrations and space-charge effects. The role of aberrations is increasingly important in small probe systems that often feature multiple orders of magnitude variations in beam size during transport. In this work, we report the correction of normal quadrupole, skew quadrupole, and sextupole aberrations via dedicated corrector elements in an ultrafast electron microdiffraction beamline. To do this, we generate precise four-dimensional phase space maps of rf-compressed electron beams and find excellent agreement with aberration-free space-charge simulations. Finally, we discuss the role a probe-forming aperture can play in improving the brightness of bunches with appreciable space-charge effects.

In the context of the Large Hadron Collider (LHC) injector upgrade, components with high contribution to the beam coupling impedance of the injector chain have to be identified and optimized to ensure the delivery of high-intensity proton beams to the LHC. The Super Proton Synchrotron (SPS) is the last accelerator in the LHC injector chain. In the existing design, the longitudinal beam coupling impedance of the SPS cavities limits the increase of the beam intensity in the SPS ring. Since the 200 MHz traveling wave cavities are one of the main contributors to the overall beam coupling budget of the SPS machine, different types of higher-order mode (HOM) couplers are used in the long 33-cell and 44-cell cavities for the damping of various HOMs. The location of the HOM couplers in the cavity, as well as their shape, affects the damping of HOMs. Finding a suitable arrangement of the HOM couplers requires solving a discrete optimization problem. The repetitive calculation of the beam coupling impedance by the conventional time-domain wakefield solvers in the optimization is hindered by the large size of the SPS cavities. The generalized coupled $S$-parameter method is a domain decomposition method for the calculation of the $S$ parameters and beam coupling impedance of large structures. In this paper, this method is employed to calculate the beam coupling impedance of the SPS cavities. Then, an optimization method is proposed to find an optimal arrangement of the HOM couplers in the cavities. The article presents the geometrical details of the SPS cavities, a short description of the generalized coupled $S$-parameter method, and a discrete optimization method applied to the SPS cavities.

We report a design work of a continuous-wave (cw) radio frequency quadrupole (RFQ) for the injector linac of the High Intensity heavy-ion Accelerator Facility project. The rf design and multiphysics analysis were conducted to enhance the long-term stability and reliability of the cavity. To minimize the peak modified Poynting vector and peak surface electric field associated with the rf breakdown rate, the cross section and the vanes segmentation of the RFQ cavity were optimized. Pi-mode stabilizing loops (PISLs) were adopted for the cavity to increase the mode separation between the quadrupole and the neighboring dipole modes, intending to reduce the emittance growth and beam loss caused by the field asymmetry of the quadrupole mode. Due to the lack of a universal criterion of acceptable mode separation, the field asymmetry of the quadrupole mode was directly used as the design criterion for the PISL. Tuners were designed to tune the cavity frequency and longitudinal field flatness. Thermal distributions on tuners with and without rf sealing were investigated, and we found that rf sealing was indispensable in decreasing the tuner’s temperature. A multiphysics analysis was carried out to maintain the cavity frequency while running with different rf power, in an attempt to avoid frequently adjusting the cooling water temperature when accelerating different ions. The analysis also verified that a large wall thickness could make the cavity possess a low stress and a minor deformation.

Slice energy spread is one of the key parameters in free electron laser optimizations, but its accurate measurement is not straightforward. Two recent studies from high energy ($\ge 100\mathrm{MeV}$) photoinjectors at SwissFEL and European x-ray free electron laser (XFEL) have reported much higher slice energy spread than expected at their XFEL working points (200–250 pC). In this paper, a new method for measuring slice energy spread at lower beam energy ($\sim 20\mathrm{MeV}$) is proposed and demonstrated at the PhotoInjector Test facility at DESY Zeuthen, and the results for 250 pC are much lower than those measured at high energy injectors.

High reliability and availability are primary goals for the operation of particle accelerators, especially for accelerator-driven subcritical systems (ADS). ADSs employ high-power beams for the transmutation of minor actinide; as a result, the amount and the radiotoxicity of the nuclear waste are considerably reduced. To this end, the Japan Atomic Energy Agency is designing a 30-MW continuous wave (cw) superconducting proton linear accelerator (linac) that supplies neutrons to an 800-MW subcritical reactor by a spallation process. The major challenge for an ADS linac is the strict control of the beam trip duration and its frequency to avoid thermal stress in the subcritical reactor structures. The maximum allowed beam trips for failures longer than a few seconds are estimated to be far below the rate achieved in current accelerators. Thus, we implemented a combination of hot standby and local compensation that enables a fast beam recovery. This work comprehensively investigated the tolerance of our linac lattice for the local compensations for failures in superconducting cavities and magnets. This scheme includes simultaneous compensation of multiple cavities in independent and same cryomodules that significantly enhance the reliability of the linac. The retuned schemes present acceptable beam performance to guarantee the integrity of the linac and the beam transport to the target; moreover, they satisfy the beam stability in the beam window. In addition, the readjusted elements are subjected to moderate stress to ensure a sustainable operation. This manuscript reports the beam dynamics results toward fulfilling the high reliability demanded by an ADS linac.