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.

We present a method to actively outcouple radiation from an x-ray regenerative amplifier. The method uses an undulator within a cavity formed by the diamond mirror and is driven by a high repetition rate linac to build up a high power, narrow bandwidth seed within the cavity. Shaping the electron beam via laser heater modulation on a subsequent pass increases the bandwidth of the x-ray free-electron laser pulse beyond the bandwidth of the diamond mirrors, transmitting the radiation from the cavity. A fraction of the power from the shaped pulse is returned to the cavity, reducing the subsequent number of passes needed to rebuild the seed and increasing the duty factor. The cavity seeding may be used to produce short pulses that are significantly more stable than those produced via self-amplified spontaneous emission (SASE) or enhanced SASE.

In this paper, we present the effects of linear transverse-longitudinal coupling on beam size at the interaction point (IP) of a collider with a local crab crossing scheme, when time-dependent transverse deflection (crab kicks) and dispersive orbit intertwine near IP. The analytic propagation formula and the closed orbit form of the crab dispersion and momentum dispersion are derived. The nonzero momentum dispersion at crab cavities and the nonideal phase from crab cavities to IP are detailed with the derived propagation formula to predict the beam size distortion at IP with or without the beam-beam interaction. The linear results are compared with nonlinear simulation using the weak-strong beam-beam code.

The usage of optical transition radiation (OTR) for profile monitoring of relativistic electron beams is well known. This paper presents the case for beam diagnostic application of OTR for nonrelativistic ion beams. In addition to expected linearly polarized transition radiation in the plane of observation, a significant component of observed radiation is unpolarized. The unpolarized contribution increases as the target is tilted toward the grazing angles with its intensity, an order of magnitude higher than the polarized radiation. This unpolarized radiation is shown to have the characteristics of OTR and is qualitatively explained as the OTR generated on a rough target surface. This increase in observed radiation can be used advantageously for transverse profile measurements. Further systematic effects such as the dependence of light yield on beam current, comparison of the measured transverse profiles with secondary electron emission based grid, target heating, and prompt nature of observed radiation are reported.

We have developed a new chromatic compensation scheme for the electron storage ring with two low-beta interaction regions in the Electron-Ion Collider. The hybrid scheme consists of the modular chromatic matching of periodic systems and beamlines. The first-order chromatically matched solutions are linearly parameterized with the local linear chromaticities that control the higher order chromatic beatings. The parameterization enables an efficient optimization of dynamic aperture. As a result, we successfully achieve the 1% design criterion for the momentum aperture in the ring.

Understanding and controlling of nonlinear beam dynamics are essential to the performance of storage ring light sources. In order to measure and correct limitations due to nonlinear beam dynamics, an effective method is needed to excite the beam oscillation to large amplitudes. Autoresonance is a method that enables the control of the amplitude of a driven nonlinear oscillator by sweeping the frequency of the driver. In this study, autoresonant excitation of beam transverse oscillations in storage rings is studied. The threshold of the drive amplitude for autoresonance is theoretically obtained, for cases with or without damping. The results are in good agreement with simulations for a simple storage ring model, as well as for models of actual storage rings. Application of the theory to experimental data on the SPEAR3 storage ring also validates the results.

In an electron storage ring, the emittances are determined by the beam energy and the linear lattice; the effects of lattice nonlinearity are usually negligible. In this study, we show a case where the vertical emittance has a strong quadratic dependence on the vertical chromaticity, a form of lattice nonlinearity. In this case, a crab cavity is present in the lattice, which couples the longitudinal and vertical motions of the beam. A theoretic analysis is conducted to derive the emittance dependence on chromaticity, using the normal form approach to decouple the longitudinal and vertical motion. Particle tracking simulations are performed to verify the analytic results and a good agreement is found. The dependence of equilibrium emittances on nonlinear lattice properties as demonstrated in this paper may provide new possibilities for design options and machine control.

This article presents a reliability analysis method and a series of case studies of different architectures for interlock systems of large and complex research infrastructures. Interlock systems play a crucial role in the protection of different types of machines, including present and future particle accelerators and fusion experiments. These infrastructures require multibillion Euros investments and accidents could cause irreparable damage. Protection systems are needed to prevent damage from an unintended release of large amounts of stored energy or power. Interlock systems define the signal exchange between the sensors that detect non-nominal conditions and the actuators that bring the machine into a safe state through a protection shutdown. The design of machine protection systems in general and interlock systems, in particular, is caught between the desired machine safety and machine availability, and the requirements vary between different infrastructures. For some infrastructures, interlock systems must be designed to strictly avoid unintentional shutdowns, as these can have a significant impact on the lifetime of vital equipment or their primary operational purpose. For others, unintentional shutdowns due to a failure in the interlock system are acceptable as long as their number is small compared to protection shutdowns caused by failures of other equipment, in order to maximize their scientific output. The case studies presented in the article compare different interlock architectures based on the probability of specific failure scenarios occurring.

Measuring the electron source size is essential for determining the emittance of synchrotron radiation. Pinhole imaging is the most common technique which measures the source profile in all transverse directions. The grating interferometry technique measures the coherence function of the x-ray beam, which indirectly determines the source size. The newly developed phase-space beam position and size monitor (ps-BPM) system provide information on electron source properties: position, angle, size, and divergence. These three techniques were used to determine the source size at a bend magnet beamline at the Canadian Light Source. In this work, we compare experimental results from these measurements at the same time, which is used as a cross-calibration procedure for each method.

Tuning machine parameters of particle accelerators is a repetitive and time-consuming task that is challenging to automate. While many off-the-shelf optimization algorithms are available, in practice their use is limited because most methods do not account for safety-critical constraints in each iteration, such as loss signals or step-size limitations. One notable exception is safe Bayesian optimization, which is a data-driven tuning approach for global optimization with noisy feedback. We propose and evaluate a step-size limited variant of safe Bayesian optimization on two research facilities of the PSI: (a) the SwissFEL and (b) HIPA. We report promising experimental results on both machines, tuning up to 16 parameters subject to 224 constraints.