We propose a new method that generates coherent Cherenkov radiation by controlling the tilt of the electron bunch. In this scheme, the wavelength band of coherent radiation depends on the transverse size of the electron bunch. This study demonstrates a coherent Cherenkov radiation (CChR) pulse in the THz region corresponding to the beam size. To generate the CChR, we used a 4.8-MeV electron bunch from a photocathode radio-frequency (rf) gun and controlled the angle through an rf transverse deflecting cavity. Our new method increased the pulse intensity by a factor of 7.3 compared to the case without tilting and the THz pulse characteristics well agreed with the prediction.

Interest in “all-metal” surface periodic structures to control charged particle beam dynamics and properties, and to generate coherent radiation in the THz frequency range has recently risen exponentially. While such structures have the benefits of metal, they are capable of providing similar properties to a dispersive dielectric. All-metal structures allow good temperature and space charge management. Transportation of high energy charge particle beams in the vicinity of the structure without risking material degradation makes them very attractive for particle acceleration and terahertz radiation generation applications. Here, the results of theoretical studies (via numerical modeling) of the electron beam dynamics and wakefield excitation and evolution along the cylindrical two-dimensional periodic surface structure will be presented, and the properties of the wakefields generated will be discussed. The wakefield acceleration of the witness bunch in the potential reaching $1\mathrm{GV}/\mathrm{m}$ will be demonstrated. The challenges will be identified and discussed, and the concept of the cascade accelerator based on such all-metal wakefield accelerator (AMWA) structures will be presented.

The China Spallation Neutron Source (CSNS) accelerator consists of an 80 MeV $H^{-}$ Linac and a 1.6-GeV rapid cycling synchrotron (RCS). The painting scheme is one of the most important factors for the control of low beam loss at high beam power. In the painting injection of the CSNS/RCS, position scanning is used in both horizontal and vertical planes, and the anticorrelated painting is the design scheme. By using the anticorrelated painting, the beam power has successfully reached 50 kW. However, some problems have been found in the higher power beam commissioning, for instance, too large beam size after painting, nonuniform beam distribution, large transverse coupling effect, and so on, resulting in additional beam loss and making it difficult to satisfy the requirements of a high-power user mode. In order to solve these problems, flexibility in the CSNS design has been exploited to implement the correlated painting by using the rising current curve of the pulse power supply. The effectiveness of the new method has been verified in the simulation and beam commissioning, which can well control the full beam emittance and improve the beam distribution. By using the new method, the beam power on the target has successfully risen from 50 kW to the design value of 100 kW. Under the mechanical structure of the anticorrelated painting scheme where the position scanning is used in both horizontal and vertical planes, the correlated painting has been successfully achieved by using the rising current curve.

The ALS-U is the upgrade of the existing Lawrence Berkeley National Laboratory Advanced Light Source to a diffraction-limited soft x-ray light source. Here we present the lattice correction studies and commissioning simulations demonstrating that the proposed machine design can be expected to deliver the intended performance when realistic errors and perturbations are fully accounted for. Critical to this demonstration are the high-fidelity, realistic simulations of the beam-based alignment process (both in turn-by-turn mode during early commissioning and with stored beam) that are now made possible by the Toolkit for Simulated Commissioning. In addition to presenting a statistical performance analysis based on a large number of lattice error realizations, we also study the range of further improvements that can be obtained by fine-tuning the correction chain to individual error seeds, mimicking the approach one would follow once the machine is built.

The superconducting radiofrequency electron linear accelerator of the Shanghai HIgh repetition rate XFEL aNd Extreme light facility (SHINE) will use 600 TESLA-style cavities with 1.3 GHz fundamental power couplers for continuous-wave operation. The first batch of 26 1.3 GHz coupler prototypes has been fabricated from three domestic manufacturers for qualification. Several key manufacturing processes have been developed and qualified, including high residual resistivity ratio copper plating, vacuum brazing of ceramic windows, electron beam welding, and titanium nitride coating. In addition, uniform quality control and inspection processes have been established to ensure the power couplers meet the requirements for the intended use in the SHINE project. All the 1.3 GHz coupler prototypes have been power conditioned with 14-kW traveling wave and 7-kW standing wave rf in continuous-wave mode. Even higher power levels have been demonstrated with 20-kW traveling wave and 10-kW standing wave rf, which indicates their robustness.

The CompactLight design for a next-generation x-ray free-electron laser utilizes a C-band injector. This requires that the harmonic system used to linearize the beam’s phase space must operate at X-band rf or higher. We investigate the optimum frequency for the harmonic system in the range of frequencies from 12 to 48 GHz. We describe the reasoning behind selecting 36 GHz (Ka-band) as our working harmonic frequency. The full linearizer system design including the power source, pulse compressor, and linearizing structure, along with options, is considered and presented. These designs are compared in terms of rf and beam dynamics performance. Two potential MW-level rf sources are discussed; a multibeam klystron and a gyro-klystron, while a klystron-based upconverter with an X-band driver is briefly discussed as an alternative path if even higher peak powers are needed. To further increase peak power, novel options for pulse compressors at Ka-band are discussed. Traveling and standing wave solutions for the structure are presented.

We conducted a high-power test of a prototype cavity of a 324-MHz interdigital H-mode drift tube linac (IH-DTL) for the precise measurement of the muon anomalous magnetic moment ($g-2$) and electric dipole moment (EDM). This prototype cavity (short IH) was developed to verify the fabrication methodology for the IH-DTL cavity with a monolithic drift tube structure. The electromagnetic field distribution was measured and compared with the finite element method simulation results, and the fabrication accuracy of the monolithic drift tube was confirmed to satisfy the requirements. After 40 h of conditioning, the short IH was stably operated with an rf power of 88 kW, which corresponds to a 10% higher accelerating field than the design field ($E_0$) of $3.0\mathrm{MV}/\mathrm{m}$. In addition, the thermal characteristics and frequency response were measured, verifying that the experimental data were consistent with the three-dimensional model. In this paper, the design, fabrication, and low-power and high-power tests of this IH-DTL for muon acceleration are described.

In this article, we report on the latest investigations and achievements in proton beam shaping with our laser-driven ion beamline at GSI Helmholtzzentrum für Schwerionenforschung GmbH. This beamline was realized within the framework of the Laser Ion Generation, Handling, and Transport (LIGHT) collaboration to study the combination of laser-driven ion beams with conventional accelerator components. At its current state, the ions are accelerated by the high-power laser PHELIX via target normal sheath acceleration, and two pulsed high-magnetic solenoids are used for energy selection, transport, and transverse focusing. In between the two solenoids, there is a rf cavity that gives the LIGHT beamline the capability to longitudinally manipulate and temporally compress ion bunches to sub-nanosecond durations. To get optimal results, the rf cavity has to be synchronized with the PHELIX laser and therefore a reliable measurement of the temporal ion beam profile is necessary. In the past, these measurements showed unexpected correlations between the temporal beam profile and the phase as well as the electric field strength of the cavity. In this article, we present a numerical simulation of the beam transport through the LIGHT beamline which explains this behavior by a beam filamentation. We also report on our latest experimental campaigns, in which we combined transverse and longitudinal focusing for the first time. This led to proton bunches with a peak intensity of $(3.28\pm 0.24)\times {10}^8\mathrm{protons}/({\mathrm{ns}\mathrm{mm}}^2)$ at a central energy of ($7.72\pm 0.14$) MeV. The intensity refers to a circle with a diameter of ($1.38\pm 0.02$) mm that encloses 50% of the protons in the focal spot at the end of the beamline. The temporal bunch width at this position was ($742\pm 40$) ps (FWHM).

We report on experimental measurements of energy transfer efficiencies in a GeV-class laser wakefield accelerator. Both the transfer of energy from the laser to the plasma wakefield and from the plasma to the accelerated electron beam was diagnosed by simultaneous measurement of the deceleration of laser photons and the acceleration of electrons as a function of plasma length. The extraction efficiency, which we define as the ratio of the energy gained by the electron beam to the energy lost by the self-guided laser mode, was maximized at $19\pm 3\%$ by tuning the plasma density and length. The additional information provided by the octave-spanning laser spectrum measurement allows for independent optimization of the plasma efficiency terms, which is required for the key goal of improving the overall efficiency of laser wakefield accelerators.

Superconducting photoelectron injectors are promising for generating highly brilliant pulsed electron beams with high repetition rates and low emittances. Experiments such as ultrafast electron diffraction, experiments at the Terahertz scale, and energy recovery linac applications require such properties. However, optimizing the beam properties is challenging due to the high number of possible machine parameter combinations. This article shows the successful automated optimization of beam properties utilizing an already existing simulation model. To reduce the required computation time, we replace the costly simulation with a faster approximation with a neural network. For optimization, we propose a reinforcement learning approach leveraging the simple computation of the derivative of the approximation. We prove that our approach outperforms standard optimization methods for the required function evaluations given a defined minimum accuracy.

Photoinjectors are the main high brightness electron sources for x-ray free electron lasers (XFELs). Photoinjector emittance reduction is one of the key knobs for improving XFEL lasing, so precise emittance measurement is critical. It is well known that rms emittance is very sensitive to low intensity tails of particle distributions in the phase space, whose measurement depends on the signal to noise ratio and image processing procedures. Such sensitivities make the interpretations of beam transverse brightness challenging, leading to different emittance definitions to reduce the impact of tail particles. In this paper, transverse phase space is analyzed in action and phase coordinates for both analytical models and experiments, which gives a more intuitive way to calculate the beam core brightness.

In a recent paper, the adiabatic theory of Hamiltonian systems was successfully applied to study the crossing of the linear coupling resonance, $Q_x-Q_y=0$. A detailed explanation of the well-known phenomena that occur during the resonance-crossing process, such as emittance exchange and its dependence on the adiabaticity of the process, was obtained. In this paper, we consider the crossing of the resonance of nonlinear coupling $2Q_x-2Q_y=0$ using the same theoretical framework. We perform the analysis using a Hamiltonian model in which the nonlinear coupling resonance is excited and the corresponding dynamics is studied in detail, in particular looking at the phase-space topology and its evolution, in view of characterizing the emittance exchange phenomena. The theoretical results are then tested using a symplectic map. Thanks to this approach, scaling laws of general interest for applications are derived.

This paper presents a first experimental demonstration of a new nondestructive method for aperture measurements based on ac dipoles. In high intensity particle colliders, such as the CERN Large Hadron Collider (LHC), aperture measurements are crucial for a safe operation while optimizing the optics in order to reduce the size of the colliding beams and hence increase the luminosity. In the LHC, this type of measurements became mandatory during beam commissioning and the current method used is based on the destructive blowup of bunches using a transverse damper. The new method presented in this paper uses the ac-dipole excitation to generate adiabatic forced oscillations of the beam in order to create losses to identify the smallest aperture in the machine without blowing up the beam emittance. A precise and tuneable control of the oscillation amplitude enables the beams to be reused for several aperture measurements, as well as for other subsequent commissioning activities. Measurements performed with the new method are presented and compared with the current LHC transverse damper method for two different beam energies and two different operational optics.

A miniaturized Penning ion source, capable of generating a milliampere ${\mathrm{H}}^{+}$ beam with low anode power supply and low gas consumption, was produced and tested for neutron tubes. For a better understanding and optimization of the Penning ion source, magnetic field, beam extraction system, and plasma simulations were conducted to research the optimization of the Penning ion source. The distance between the two cathodes is optimized based on the plasma simulations. Based on the optimization of the magnetic field, extraction system, and the distance of the cathodes, the extraction current of the Penning source had been increased from 1.21 to 1.72 mA, and the ${\mathrm{H}}^{+}$ ratio had been increased from 36% to 42.3%.

In SuperKEKB high-current beam operation, achieving the stability of the zero mode associated with the accelerating mode of the rf system is an important concern because of heavy beam loading. The issue is a more serious problem in the high-energy ring (HER) compared to the low-energy ring (LER) because of the high impedance of superconducting cavities operating in the HER. As a countermeasure, the rf system is equipped with a direct rf feedback system (DRFB) to effectively reduce the impedance of the cavities and a zero mode oscillation damper. The effect of these mitigations on improving the stability while coexisting with other control loops was quantitatively studied using two independent methods, namely, analysis with a characteristic equation and simulation in the time domain. For the analysis, we extended the transfer functions in the Pedersen model to include the DRFB to analyze the rf system with beam loading, while coexisting with the other control loops such as amplitude and phase controls. The consistency between both methods was confirmed by comparing the results obtained from them for different cases. Furthermore, simulation results were compared with measurements conducted during beam operation; they were consistent with each other. Having confirmed the validity of these methods, they were applied to extensively study the stability for future higher beam current operations in SuperKEKB. Particularly, several operation parameter sets, including possible system modifications, were identified for the HER and LER with considerably good margins to compensate for possible deficiencies in the performance of cavities or control functions at higher beam currents. The results can be used as guidelines for future beam operations with beam current increasing in a stepwise manner.

Dust grains interacting with the beam of particle accelerators are believed to be the cause of several detrimental effects such as beam losses, emittance growth, pressure bursts, and even quenches of superconducting magnets. Experimental observations suggest that these grains are positively charged in electron storage rings and negatively charged in the Large Hadron Collider (LHC). In this paper, the charging mechanisms for dust grains in the LHC are discussed and a possible explanation for the observed polarity is presented. Electron collection, secondary electron emission, and photoelectric emission are considered because of the presence of electron clouds and synchrotron radiation. It is found that the same mechanisms can explain both the positive grain polarity observed in electron storage rings and the negative polarity in the LHC. As a consequence of the charge acquired, the possibility of grains orbiting the beam is discussed. The orbital dynamics in a logarithmic potential is analyzed and critical parameters for describing such orbits are introduced. Finally, LHC beam losses attributed to beam-dust interactions with multiple loss peaks are presented. It is shown that they have an amplitude and a peak separation consistent with what can be expected for orbiting grains.

The Advanced Wakefield (AWAKE) Experiment is a proof-of-principle experiment demonstrating the acceleration of electron beams via proton-driven plasma wakefield acceleration. AWAKE Run 2 aims to build on the results of Run 1 by achieving higher energies with an improved beam quality. As part of the upgrade to Run 2, the existing proton and electron beamlines will be adapted and a second plasma cell and new 150-MeV electron beamline will be added. The specification for this new 150-MeV beamline will be challenging as it will be required to inject electron bunches with micron-level beam size and stability into the second plasma cell while being subject to tight spatial constraints. In this paper, we describe the techniques used (e.g., numerical optimizers and genetic algorithms) to produce the design of this electron line. We present a comparison of the methods used in this paper with other optimization algorithms commonly used within accelerator physics. Operational techniques are also studied including steering and alignment methods utilizing numerical optimizers and beam measurement techniques employing neural networks. We compare the performance of algorithms for online optimization and beam-based alignment in terms of their efficiency and effectiveness.