Hydrodynamic plasma waveguides initiated by optical field ionization have recently become a key component of multi-GeV laser wakefield accelerators. Here, we present the most complete and accurate experimental and simulation-based characterization to date, applicable to current multi-GeV experiments and future 100 GeV-scale laser plasma accelerators. Crucial to the simulations is the correct modeling of intense Bessel beam interaction with meter-scale gas targets, the results of which are used as initial conditions for hydrodynamic simulations. The simulations are in good agreement with our experiments measuring evolving plasma and neutral hydrogen density profiles using two-color short pulse interferometry, enabling realistic determination of the guided mode structure for application to laser-driven plasma accelerator design.

The fabrication process is presented for a typical transition section located between each cylindrical corrugated waveguide structure comprising the wakefield accelerator module. The transition section includes couplers for extracting the 180 GHz ${\mathrm{TM}}_{01}$ accelerating mode and separate couplers for extracting the 190 GHz ${\mathrm{HE}}_{11}$ dipole mode, both modes induced by the electron bunch traversing the cylindrical corrugated waveguide structure. Extraction of the high-power accelerating mode reduces the heat load due to the subterahertz wave power dissipation within the corrugated accelerating structure. Extraction of the low-power dipole mode serves the purpose of detecting the electron bunch transverse oscillations within the wakefield accelerator and identifying the onset of beam breakup instability. Comprehensive testing of the fully functional transition section with an electron beam was done at the Accelerator Test Facility in Brookhaven National Laboratory which verified the functionality of the transition section.

The wiggler-based amplifier of the microbunching instability in the electron beam is considered and compared to a longitudinal space charge amplifier consisting of the drifts and chicanes. The gain in the magnitude of a seed perturbation of the electron beam density is evaluated analytically as a function of the period of the perturbation, the electron beam parameters, the wiggler parameters, and the amplifier lattice parameters. The theory is compared with the results of the numerical calculations of the gain based on simulations of the electron beam transport through the amplifier lattice.

Coherent electron cooling is a novel method to cool dense hadron beams on timescales of a few hours. This method uses a copropagating beam of electrons to pick up the density fluctuations within the hadron beam in one straight section and then provides corrective energy kicks to the hadrons in a downstream straight, cooling the beam. Microbunched electron cooling is an extension of this idea, which induces a microbunching instability in the electron beam as it travels between the two straights, amplifying the signal. However, initial noise in the electron bunch will also be amplified, providing random kicks to the hadrons downstream which tend to increase their emittance. In this paper, we develop an analytic estimate of the effect of the electron noise and benchmark it against simulations. We also discuss how this effect has impacted the cooler design.

In this article, we summarize recent theoretical and experimental studies of the impedance and beam-induced heating of titanium-coated ceramic vacuum chambers used in the NSLS-II injection kickers. The impedance was calculated using the field matching theory assuming planar approximation and compared with the impedancewake2d code. For the coating thickness of a few microns, we demonstrated that the beam-induced power is dissipated in the titanium coating and that the longitudinally averaged two-dimensional power density is approximated by an analytical expression, thus allowing the use of a simplified model of the power density as input for the ansys code to simulate the temperature distribution with realistic nonuniform thickness of the Ti coating. For a few values of the NSLS-II beam current, we measured the beam-induced heating of two ceramic chambers using thermal sensors installed along the chamber and compared the measurement results with the simulations.

Intense highly charged ion beam production is essential for high-power heavy ion accelerators. A novel movable Vlasov launcher for superconducting high charge state electron cyclotron resonance ion source has been devised that can affect the microwave power effectiveness by a factor of about 4 in terms of highly charged ion beam production. This approach based on a dedicated microwave launching system instead of the traditional coupling scheme has led to new insight on microwave-plasma interaction. With this new understanding, the world record highly charged xenon ion beam currents have been enhanced by up to a factor of 2, which could directly and significantly enhance the performance of heavy ion accelerators and provide many new research opportunities in nuclear physics, atomic physics, and other disciplines.

Accelerator physics relies on numerical algorithms to solve optimization problems in online accelerator control and tasks such as experimental design and model calibration in simulations. The effectiveness of optimization algorithms in discovering ideal solutions for complex challenges with limited resources often determines the problem complexity these methods can address. The accelerator physics community has recognized the advantages of Bayesian optimization algorithms, which leverage statistical surrogate models of objective functions to effectively address complex optimization challenges, especially in the presence of noise during accelerator operation and in resource-intensive physics simulations. In this review article, we offer a conceptual overview of applying Bayesian optimization techniques toward solving optimization problems in accelerator physics. We begin by providing a straightforward explanation of the essential components that make up Bayesian optimization techniques. We then give an overview of current and previous work applying and modifying these techniques to solve accelerator physics challenges. Finally, we explore practical implementation strategies for Bayesian optimization algorithms to maximize their performance, enabling users to effectively address complex optimization challenges in real-time beam control and accelerator design.

Nonuniform transmission lines (NTLs) are widely used in pulsed power accelerators because they provide an efficient way to achieve impedance matching and pulse shaping. Since designing and constructing these accelerators typically demands substantial effort, finding the optimal impedance profile to maximize the power transmission efficiencies of the NTLs is important. In this paper, a convenient numerical method to determine the optimal impedance profile is proposed. First, the output of the NTL with arbitrary parameters is theoretically analyzed under arbitrary input conditions. It was found that only four factors affect the power transmission efficiency: the ratio of output impedance to input impedance, the ratio of input pulse width to the NTL’s one-way transit time, the normalized impedance profile, and the normalized input pulse. Based on these findings, a method designed to minimize the reflected component within the working frequency range is proposed. Using this method, an impedance profile demonstrating superior power transmission efficiency compared to the traditional exponential profile is identified. This work can provide a rapid and effective method to determine the impedance profile of the NTL, undoubtedly benefiting the design process of pulsed power accelerators.

Numerical optimizations on couplers of the traveling-wave (TW) accelerating structures usually require lots of calculation resources. This paper proposes a new technique for matching couplers to an accelerating structure in a more efficient and accurate way. It combines improved Kroll method with improved Kyhl method, thereby simplifying simulation process while achieving a high accuracy. This paper also presents the detailed design on couplers for a C-band constant-gradient (CG) accelerating structure based on this new technique. Such a new technique can be widely used for any TW accelerating structures working at different frequencies of S-band, C-band, and X-band including CG, constant-impedance (CI), and other structures with either electric couplers or magnetic couplers.

Achieving maximum electron beam brightness in photoinjectors requires detailed control of the 3D bunch shape and precise tuning of the beam focusing. Even in state-of-the-art designs, slice emittance growth due to nonlinear space charge forces and partial nonlaminarity often remains non-negligible. In this work, we introduce a new means to linearize the transverse slice phase space: a sacrificial portion of the bunch’s own charge distribution, formed into a wavebroken shock front by highly nonlinear space charge forces within the gun, whose downstream purpose is to dynamically linearize the desired bunch core. We show that linearization of an appropriately prepared bunch can be achieved via strongly nonlaminar focusing of the sacrificial shock front, while the inner core focuses laminarly. This leads to a natural spatial separation of the two distributions: a dense core surrounded by a diffuse halo of sacrificial charge that can be collimated. Multiobjective genetic algorithm optimizations of the ultracompact x-ray free electron laser injector employ this concept, and we interpret it with an analytic model that agrees well with the simulations. In simulation, we demonstrate a final bunch charge of 100 pC, peak current $\sim 30$ A, and a sacrificial charge of 150 pC (250 pC total emitted from cathode) with normalized emittance growth of $<20\mathrm{nm}\mathrm{rad}$ due to space charge. This implies a maximum achievable brightness approximately an order of magnitude greater than existing free electron laser injector designs.

A terahertz radiation enhancing scheme, in which a linearly polarized weakly relativistic laser pulse irradiates a target consisting of two parallel thin-solid layers with a certain gap, is proposed and studied by using two-dimensional particle-in-cell simulations. The radiation is known to be produced by laser-produced hot electrons via mechanisms such as coherent transition radiation at the target surfaces. Under optimized conditions, the energy conversion efficiency of terahertz radiation can be as high as 3.3%, which is nearly 1.5 times higher than that obtained with a single-layer target with the same drive laser. This is mainly due to the enhanced hot electron generation with moderate energy via multiple reflections of the laser pulse between the two target layers. The radiation has two peaks close to 30° from the target surface, which are more collimated than that with the single-layer target. The dependence of the terahertz radiation on a variety of target parameters is given, which can control the terahertz spectrum and radiation efficiency and thus provide guidance for experimental investigations. Moreover, both the coherent transition radiation and antenna radiation models are applied to explain the angular distributions of the terahertz emission found in the simulations.

Past research at the Advanced Light Source (ALS) provided a proof-of-principle demonstration that deep learning methods could be effectively employed to compensate for the significant perturbations to the transverse electron beam size induced by user-controlled adjustments of the insertion devices. However, incorporating these methods into the ALS’ daily operations has faced notable challenges. The complexity of the system’s operational requirements and the significant upkeep demands has restricted their sustained application during user operation. Here, we introduce the development of a more robust neural network (NN)-based algorithm that utilizes a novel online fine-tuning approach and its systematic integration into the day-to-day machine operations. Our analysis emphasizes the process of NN model selection, demonstrates the superior performance of the NN-based method over traditional feedback methods, and examines the effectiveness and resilience of the new algorithm during user-operation scenarios.

We present a laser-assisted electron injection scheme for beam-driven plasma wakefield acceleration. The laser is collinear with the driver and triggers the injection of hot electrons into the plasma wake by interaction with a thin solid target. We present a baseline case using the AWAKE Run 2 parameters and then perform variations on key parameters to explore the scheme. It is found that the trapped witness electron charge may be tuned by altering laser parameters, with a strong dependence on the phase of the wake upon injection. Normalized emittance settles at the order of micrometres and varies with witness charge. The scheme is robust to misalignment, with a 1/10th plasma skin-depth offset ($20\mathrm{\mu }\mathrm{m}$ for the AWAKE case) having a negligible effect on the final beam. The final beam quality is better than similar existing schemes, and several avenues for further optimization are indicated. The constraints on the AWAKE experiment are very specific, but the general principles of this mechanism can be applied to future beam-driven plasma wakefield accelerator experiments.

Large energy acceptance arcs have been proposed for applications such as cancer therapy, muon accelerators, and recirculating linacs. The efficacy of hadron therapy can be improved by reducing the energy layer switching time, however this is currently limited by the small momentum acceptance of the beam delivery system ($<\pm 1\%$). A “closed-dispersion arc” with a large momentum acceptance has the potential to remove this bottleneck, however such a beamline has not yet been constructed. We have developed a design methodology for large momentum acceptance arcs with Fixed Field Accelerator optics, applying it to a demonstrator beam delivery system for protons at 0.5–3.0 MeV ($\pm 42\%$ momentum acceptance) as part of the Technology for Ultra-Rapid Beam Operation project at the University of Melbourne. Using realistic magnetic fields, a beamline has been designed with zero dispersion at either end. An algorithm has been devised for the construction of permanent magnet Halbach arrays for this beamline with multipole error below one part in ${10}^4$, using commercially available magnets. The sensitivity to errors has been investigated, finding that the delivered beam is robust in realistic conditions. This study demonstrates that a closed-dispersion arc with fixed fields can achieve a large momentum acceptance, and we outline future work required to develop these ideas into a complete proof-of-principle beam delivery system that can be scaled up for a medical facility.

Rotating gantries are commonly used in ion-therapy facilities to assist and support optimizing the dose distribution delivered to the patient. They are installed at the end of the beamlines and rotated mechanically in the treatment room. In synchrotron-based facilities, the gantries must be able to transport slowly extracted beams with essentially different emittance patterns in the two transverse planes. Such beams will be referred to as the asymmetric beams. A special device called rotator has been proposed as a possible solution. The worldwide first beamline with the rotator has been recently commissioned. The original rotator concept uses an “external” rotator that is a part (a module) of the beamline the gantry is connected to. In this paper, a novel gantry ion-optical concept integrating the rotator optics into the gantry optics is introduced. The first-order gantry transfer matrix satisfies the so-called sigma-matching ion-optical constraints, and—at the same time—it possesses the format of a rotator transfer matrix. The rotator-matching and the sigma-matching principles are combined in the gantry transfer matrix, which means that the sigma-matching gantry acts simultaneously as a rotator without the need for an extra rotator device. In addition, scattering in the gantry nozzle is used to balance the asymmetric beam emittances in the two transverse planes without an additional scattering foil. In this way, the presented ion-optical concept combines all three known matching techniques—the sigma matching, the rotator matching, and the scattering-foil matching—within the gantry beam transport system. Such a beam transport system provides the best matching result and full angular independence of the beam parameters at the gantry isocenter. It also makes it possible to optimize the beam parameters not only at the gantry isocenter but also at the beam monitors located in the gantry nozzle without increasing the number of gantry quadrupoles. There are two possible versions of such gantry optics: the point-to-point and the parallel-to-point optics. They both are presented in this paper. Theoretical calculations are supported by beam transport simulations performed with the winagile code. Feasibility of the newly proposed ion-optical concept is demonstrated on the MedAustron proton gantry. However, it can be applied to any rotating gantry at any ion-therapy facility. The presented design is the first rotatorlike gantry ion-optical concept worldwide.

Laser-based proton accelerators offer promising advantages for many sensitive applications due to their compact size. However, achieving uniform beams with a narrow energy spread remains a challenge. Here, a beamline simulation is performed using the tracewin and srim codes to optimize the spectral and spatial characteristics of a laser-driven proton beam. This beamline, comprised of four magnetic quadrupoles and equipped with a thin axial absorber rod in conjunction with an energy selection aperture for narrowing the energy spread, a carbon scatterer foil for beam spot smoothing, and an angular selection aperture for beam collimation, effectively transports a selected proton beam energy within the range of 50–250 MeV. In this work, laser-accelerated protons with a wide energy spread were transported through the proposed beamline, which was optimized for 109 MeV protons. The protons emerged well-collimated with a significantly narrowed energy spread of $94.8\pm 3.9\mathrm{MeV}$ (a reduction from the intended 109 MeV due to passage through the scatterer) and a homogenized, 11 mm diameter beam spot while maintaining a transmission efficiency above 2.2%.

The microbunching instability has been a long-standing issue for high-brightness free-electron lasers (FELs) and is a significant showstopper to achieve full longitudinal coherence in the x-ray regime. This paper reports the first experimental demonstration of microbunching instability mitigation through transverse Landau damping, based on linear optics control in a dispersive region. Analytical predictions for the microbunching content are supported by numerical calculations of the instability gain. The effect is confirmed through the experimental characterization of the spectral brightness of the FERMI FEL under different transverse optics configurations of the transfer line between the linear accelerator and the FEL.

Beams with cross-plane coupling or extreme asymmetries between the two transverse phase spaces are often encountered in particle accelerators. Flat beams with large transverse-emittance ratios are critical for future linear colliders. Similarly, magnetized beams with significant cross-plane coupling are expected to enhance the performance of electron cooling in hadron beams. Preparing these beams requires precise control and characterization of the four-dimensional transverse phase space. In this study, we employ generative phase-space reconstruction techniques to rapidly characterize magnetized and flat-beam phase-space distributions using a conventional quadrupole-scan method. The reconstruction technique is experimentally demonstrated on an electron beam produced at the Argonne Wakefield Accelerator and successfully benchmarked against conventional diagnostics techniques. Specifically, we show that predicted beam parameters from the reconstructed phase-space distributions (e.g., as magnetization and flat-beam emittances) are in excellent agreement with those measured from the conventional diagnostic methods.

We present a scattering slow extraction method employing a rotating foil in the Rapid Cycling Synchrotron (RCS) to produce a stable 1.6 GeV single-particle proton beam at CSNS. The foil scatters particles on the beam’s edge over multiple turns, with those at large scattering angles being separated and extracted through a Lambertson magnet. By adjusting the scatter foil’s rotational speed and position, we can precisely control the intensity and repetition rate of the extracted beam. This approach enables the efficient extraction of extremely low-intensity proton beams from CSNS-II/RCS. By setting single-particle collimators in the downstream beamline, a single-particle proton beam is achieved. Using g4pyorbit, we simulated the scattering slow extraction process in detail, confirming that a stable 1.6 GeV single-particle proton beam can be extracted, the beam energy can be adjusted from 0.8 to 1.6 GeV using the degrader, the time interval between adjacent single protons can be adjusted from hundreds of nanoseconds to tens of microseconds, to meet the needs of different experiments. Both the beam loss and the residual radiation doses are kept at low levels.