In this paper, a self-consistent transverse beam dynamics framework is demonstrated that incorporates acceleration into the transverse beam dynamics studies for a proton linac machine. Two focusing schemes are developed and discussed: the FODO-like scheme and the minimum aperture scheme. The FODO-like scheme is a simple scheme, requiring only one quadrupole per cavity. The scheme is analytically solved to minimize the beam size at the cavity entrance/exit and ensures a constant beam size along the lattice, with respect to adiabatic damping due to longitudinally accelerating rf cavities. The minimum aperture scheme describes the regime that matches the beam ellipse to the acceptance ellipse of a cavity, allowing for the smallest possible aperture, for a given cavity length. A simple approximation of an rf cavity map is determined to allow changes in particle energy along a lattice, and acceleration is assumed only in the longitudinal direction.

The development of long, tunable structures is critical to increasing energy gain in laser-driven dielectric accelerators (DLAs). Here we combine pulse-front-tilt illumination with slab-geometry structures assembled by precisely aligning off-the-shelf 4 mm long transmission gratings to achieve up to 200 keV energy modulation for 6 MeV injected electrons. The effective interaction length is longer than 1 mm, limited by the dephasing of the accelerated particles in the structure. The piezo-based independent mounting system for the gratings allows tuning of the gap and field distribution inside the structure.

The low-energy muon (LEM) beamline at the Paul Scherrer Institute currently stands as the world’s only facility providing a continuous beam of low-energy muons with keV energies for conducting muon spin rotation experiments on a nanometer depth scale in heterostructures and near a sample’s surface. As such, optimizing the beam quality to reach its full potential is of paramount importance. One of the ongoing efforts is dedicated to improving the already applied technique of single muon tagging through the detection of secondary electrons emerging from an ultrathin carbon foil. In this work, we present the results from installing a thinner foil with a nominal thickness of $0.5\mathrm{\mu }\mathrm{g}{\mathrm{cm}}^{-2}$ and compare its performance to that of the previously installed foil with a nominal thickness of $2.0\mathrm{\mu }\mathrm{g}{\mathrm{cm}}^{-2}$. Our findings indicate improved beam quality, characterized by smaller beam spots, reduced energy loss and straggling of the muons, and enhanced tagging efficiency. Additionally, we introduce a method utilizing blue laser irradiation for cleaning the carbon foil, further improving and maintaining its characteristics.

The European Synchrotron Radiation Facility Extremely Brilliant Source (ESRF-EBS) is the first fourth generation 6 GeV storage ring (SR) light source making use of the hybrid multibend achromat lattice, reaching a natural horizontal emittance of 140 pm rad. Further, reducing the horizontal emittance would provide a more brilliant and a higher quality photon source for the EBS users. One way of achieving this is to operate the SR off-energy. The first approach reduces the electron beam energy by $-1\%$, which gives a 121 pm rad natural horizontal emittance. To fulfill operation requirements, the full ring had to be rematched, including both the quadrupoles and the sextupoles in the linear optics correction. The off-energy settings are then tested in the SR in terms of lifetime, injection efficiency, and operability.

Cherenkov optical beam loss monitors (oBLMs) based on optical fibers are a revolutionary achievement among the monitoring systems in particle accelerators. Due to the nature of the Cherenkov phenomenon and the intrinsic characteristics of optical fibers, the photon production and fiber light transmission need to be studied to deduce the total amount of Cherenkov photons detected by the photosensors attached at the end of the system. Simulations of the Cherenkov oBLM detector system have been implemented using the fluka software with postprocessing considering the light attenuation and capture factors in the optical fiber. In addition, the study includes the longitudinal beam loss localization of the controlled beam losses produced along the CERN Linear Electron Accelerator for Research (CLEAR) test facility line. In this paper, the Cherenkov photon detection by an oBLM installed in the CLEAR facility is simulated, analyzed, and compared with experimental data.

Machine learning has emerged as a powerful solution to the modern challenges in accelerator physics. However, the limited availability of beam time, the computational cost of simulations, and the high dimensionality of optimization problems pose significant challenges in generating the required data for training state-of-the-art machine learning models. In this work, we introduce cheetah, a pytorch-based high-speed differentiable linear beam dynamics code. cheetah enables the fast collection of large datasets by reducing computation times by multiple orders of magnitude and facilitates efficient gradient-based optimization for accelerator tuning and system identification. This positions cheetah as a user-friendly, readily extensible tool that integrates seamlessly with widely adopted machine learning tools. We showcase the utility of cheetah through five examples, including reinforcement learning training, gradient-based beamline tuning, gradient-based system identification, physics-informed Bayesian optimization priors, and modular neural network surrogate modeling of space charge effects. The use of such a high-speed differentiable simulation code will simplify the development of machine learning-based methods for particle accelerators and fast-track their integration into everyday operations of accelerator facilities.

Plasma wakefield acceleration provides ultrahigh acceleration gradients of tens of GeV/m, providing a novel path toward efficient, compact, TeV-scale linear colliders, and high brightness free electron lasers. Critical to the success of these applications is demonstrating simultaneously high gradient acceleration, high energy transfer efficiency, and preservation of emittance, charge, and energy spread. Experiments at the FACET-II National User Facility at SLAC National Accelerator Laboratory aim to achieve all of these milestones in a single-stage plasma wakefield accelerator, providing a 10 GeV energy gain in a $<1\mathrm{m}$ plasma with high energy transfer efficiency. Such a demonstration depends critically on diagnostics able to measure emittance with mm mrad accuracy, energy spectra to determine both percent level energy spread, and broadband energy gain and loss, incoming longitudinal phase space, and matching dynamics. This paper discusses the experimental setup at FACET-II, including the incoming beam parameters from the FACET-II linac, plasma sources, and diagnostics developed to meet this challenge. Initial progress on the generation of beam ionized wakes in meter-scale hydrogen gas is discussed as well as commissioning of the plasma sources and diagnostics.

The measurement of phase space has always been an important topic in the field of accelerator physics, playing an indispensable role in understanding the beam dynamics. The phase space distribution of the injected beam is crucial for optimizing the multiturn accumulation injection process in synchrotrons. However, directly and accurately measuring the phase space distribution is a challenging task. In this study, we propose an innovative tomographic algorithm based on the measurement data of the accumulated beam profile obtained from the wall current monitor (WCM) in a synchrotron, to reconstruct the longitudinal phase space of the injected beam. Simulations were conducted for various initial distribution scenarios, and the results showed that this algorithm can achieve a difference of about 4% in the rms momentum spread between the initial and reconstructed phase space distribution of the injected beam. This algorithm has been applied to the China Spallation Neutron Source and successfully measured the momentum spread of the injected beam. Machine studies considering the phase error of the injected beam showed a high consistency between the reconstructed beam profiles and the measurement results from the WCM on Rapid Cycling Synchrotron. The research results demonstrate that this algorithm can be an effective approach for measuring the momentum distribution of the injected beam in a synchrotron. Furthermore, this method also has the potential to be extended to reconstruct the transverse phase space of the injected beam.

The novel experimental system “S-POD” (Simulator of Particle Orbit Dynamics) is employed to explore the stability of short hadron bunches in high-intensity linacs. In a previous study with the S-POD [M. Goto et al., Phys. Rev. Accel. Beams 25, 054201 (2022)], a static potential was used to focus the bunch in the longitudinal direction. We here make a step forward to include the possibility of pure synchrotron resonance, introducing periodic modulation to the longitudinal potential well. The modulation period was taken a half of the transverse alternating-gradient focusing period, which reflects the most typical lattice condition of a drift-tube linac. Detailed stability maps are constructed to reveal dangerous parameter regions where serious beam loss may occur due to resonance. We reconfirm the existence of various betatron and synchrobetatron resonance stop bands whose widths and locations change in tune space depending on the bunch intensity. It turns out that the periodicity of the longitudinal focusing potential brings about no pronounced effect on the resonance feature; the result is very similar to what we obtained in the previous study with a static longitudinal potential. As long as the lattice periodicity mentioned above is maintained, no serious noncoupling synchrotron resonance appears even with a high synchrotron phase advance above 90° per unit alternating-gradient cell. Severe envelope instability may, however, be excited in the longitudinal direction if the axial focusing force includes error components that affect the original lattice periodicity. The experimental observations can be explained with the stop-band diagram free from the concept of incoherent tune spread.

In electron guns, generating high-intensity beams comes with the cost of increasing the number of secondary ions produced by the residual gas ionization which can be a limiting factor in the cathode lifetime. However, it is often overlooked that the longitudinal potential generated by the beam can be the main factor influencing secondary ions to drift back toward the source and limit the maximum beam intensity. Using 3D simulations, this paper shows how the flux of trapped ions depends on the beam parameters. The results provide a practical framework that can be used as a guide in the design of the electron gun focal point to mitigate the ion back bombardment.

Time-resolved diagnostics at free-electron laser (FEL) facilities, in particular electron beam longitudinal phase space (LPS) and FEL power profile measurements, provide information highly valuable for users, machine development studies, and beam setup. We investigate the slice energy resolution of passive streaker setups, in particular the effect of an energy chirp on the measured slice energy spread. Downstream of the hard x-ray SASE2 beamline at the European XFEL, these measurements are enabled by a single-plate nonmovable passive wakefield streaker, essentially a rectangular corrugated plate placed inside a vacuum chamber. We show measurements with a time resolution down to a few femtoseconds and an energy resolution down to a few MeVs.

Novel schemes for generating ultralow emittance electron beams have been developed in past years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Recent theoretical work has proposed a laser-based method capable of resolving emittances in the sub 0.1 mm mrad regime by modulating the electron phase space ponderomotively. Here we present the first experimental demonstration of this scheme using a laser wakefield accelerator. The observed emittance and source size are consistent with published values. We also show calculations demonstrating that tight bounds on the upper limit for emittance and source size can be derived from the “laser-grating” method even in the presence of low signal to noise and uncertainty in laser-grating parameters.

The simultaneous laser-driven acceleration and angular manipulation of the fast electron beam are experimentally demonstrated. The bunch of multi-MeV energy charged particles is generated during the propagation of the femtosecond laser pulse through the near-critical plasma slab accompanied by plasma channeling. Plasma is formed by the controlled breakdown of a thin-tape target by a powerful nanosecond prepulse. The electron beam pointing approach is based on the refraction of a laser pulse in the presence of a strong radial density gradient in the breakdown of the tape with a small displacement of the femtosecond laser beam relative to the breakdown symmetry axis. A shift of several micrometers makes it possible to achieve beam deflection by an angle up to 10° with acceptable beam charge and spectrum conservation. This opens up opportunities for in situ applications for scanning objects with an electron beam and the multistage electron beam energy gain in consecutive laser accelerators without bulk magnetic optics for particles. Experimental findings are supported by numerical particle-in-cell calculations of laser-plasma acceleration and hydrodynamic simulations.

The phenomenon of electric breakdown poses serious challenges to the design of devices that operate in high electric field environments. Experimental evidence points toward breakdown events that are accompanied by elevated temperatures and dark current spikes, which is attributed to high-asperity nanostructure formation that enhances the local electric field and triggers a runaway process. However, the exact mechanistic origin of such nanostructures under typical macroscopic operational conditions of electric field and magnetic-field-mediated heating remains poorly understood. In this work, we simulate the evolution of a copper surface under the combined action of the electric fields and elevated temperatures. Using a mesoscale curvature-driven surface evolution model, we show how a copper surface can undergo a type of dynamical instability that naturally leads to the formation of sharp asperities in realistic experimental conditions. Exploring the combined effect of fields and temperature rise, we identify the critical regimes that allow for the formation of breakdown precursors. The results show that thermoelastic stresses, while not essential, can significantly lower the critical electric field required for runaway surface instability, which is consistent with experimental observations that thermal effects can increase breakdown rates.

Metal additive manufacturing technologies are rapidly becoming an integral part of the advanced technological portfolio for the most demanding industrial applications. These processes are capable of fabricating three-dimensional components with near-net shape quality by depositing the constituent materials in a layer-by-layer fashion. This fabrication approach provides numerous advantages over conventional manufacturing methods, including enhanced design flexibility, reduced production costs and lead times, rapid prototyping, and the possibility to repair damaged parts. In recent years, the growing demand for novel accelerator components with improved performance characteristics, integrating structures such as drift tubes and internal cooling channels, has prompted the exploration of additive manufacturing in the field of particle accelerators. Radio-frequency components, beam intercepting devices, and vacuum systems have been prototyped using various metallic materials and additive manufacturing technologies, demonstrating performance levels comparable to the conventionally manufactured counterparts in preliminary tests. However, the absence of established qualification protocols and the uncertain reliability of additively manufactured parts under the demanding conditions typical of accelerator applications pose significant challenges to the integration of additive manufacturing processes into the fabrication practices of these components. This paper provides a comprehensive review of documented applications of metal additive manufacturing in particle accelerators, highlighting benefits, challenges, and opportunities for future improvements. The main requirements and currently available test setups for the assessment of additively manufactured components in applications involving ultrahigh vacuum and intense electromagnetic fields are also discussed.

Bragg diffraction from crystals is widely used to select a very narrow spectral range of x-ray pulses. The diffracted signal can be used to calibrate the photon energy, a fact that can be exploited very effectively at x-ray free-electron lasers (XFELs). This work describes three crystal-based methods used to this goal at a major x-ray FEL facility, the European XFEL. These methods, which have a wide applicability, have been developed in relation to the hard x-ray self-seeding setup installed at the SASE2 undulator. They are fast and straightforward to implement since they are based on techniques for extracting and identifying crystal reflections from standard diagnostic raw data while still delivering few electronvolts resolution.

Active plasma lensing, a compact method for intensifying the focus of charged particle beams by providing a magnetic field gradient of kT/m, has emerged as a sought-after technology in laser plasma accelerator applications. However, the utilization of active plasma lenses faces significant hurdles when dealing with laser-driven proton pulses, characterized by their broad bandwidth and high divergence. To address this challenge, we developed a novel active plasma lens with a variable radius, specifically designed to optimize lens geometry in accordance with the beam envelope, and performed the first measurement of its focusing ability. The experimental findings reveal that, compared to conventional cylindrical active plasma lenses, our radius-varying lens exhibits a 2.0-fold improvement in single-energy transmission efficiency, while maintaining comparable achromatic ability. This breakthrough is anticipated to significantly contribute to the miniaturization of laser proton accelerators.

We present a novel design of a ferroelectric fast reactive tuner (FE-FRT) capable of modulating mega-VAR reactive power on a submicrosecond timescale. The high reactive power capability of our design extends the range of applications of reactive tuners to numerous applications. We present a detailed analytical model of the performance of a megawatt-class reactive power device and benchmark it against finite-element method eigenmode and frequency domain electromagnetic simulations. We introduce new features, including an annulus design for the ferroelectric capacitors and capacitive window coupling to the cavity. We consider thermal design issues and nonlinear effects in the ferroelectric. The model covers several configurations, allowing control of the frequency of superconducting and normal-conducting cavities in a variety of applications and frequencies. We calculate that the FE-FRT designed should be capable of handling around 0.45 MVAR of reactive power with around 3 kW of resistive losses, providing a frequency tuning range of 8 kHz in an example of 400 MHz cavity geometry.