We study the feasibility of storing cold neutron beams in a strong focusing synchrotron. We also propose an alternating-current (ac) dipole magnet to be used as a neutron acceleration device and a pulsed quadrupole as a neutron beam kicker for injection and extraction. The ac acceleration device can provide adiabatic capture, acceleration, or deceleration of neutron beams. It seems feasible to design a high-quality neutron synchrotron for many future neutron beam applications, such as neutron life time measurements, monoenergetic neutron scattering experiments, and pencil neutron beam applications. This synchrotron can also be used as an injector for a neutron accumulator ring.

The High-Luminosity LHC (HL-LHC) upgrade with planned operation from 2029 onward has a goal of achieving a tenfold increase in the integrated number of recorded collisions thanks to a doubling of the intensity per bunch ($2.2\times {10}^{11}$ protons) and a reduction of ${\beta }^{*}$ (the $\beta$ value in the two high luminosity detectors, namely ATLAS and CMS) to 15 cm. Such an increase in recorded collisions would significantly expedite new discoveries and exploration. Crab cavities are an important component of the HL-LHC upgrade and will contribute strongly to achieving an increase in the number of recorded collisions. However, noise injected through the crab cavity radio frequency (rf) system could cause significant transverse emittance growth and limit luminosity lifetime. We presented a theoretical formalism relating transverse emittance growth to rf noise in an earlier work. In this follow-up paper, we summarize measurements in the super-proton synchrotron (SPS) at CERN that validate the theory, we present estimates of the emittance growth rates using state-of-the-art rf and low-level rf (LLRF) technologies, and we set the rf noise specifications to achieve acceptable performance. A novel dedicated feedback system acting through the crab cavities to mitigate emittance growth will be required. In this work, we develop a theoretical formalism to evaluate the performance of such a feedback system in any collider, identify limiting components, present simulation results to validate these studies, and derive key design parameters for an HL-LHC implementation of such a feedback system.

A new high transmission efficiency, easily tunable, and cost-effective beam collection system is proposed for laser-accelerated proton beams with large divergence angles and wide energy spectra. In previous experiments conducted at the compact laser plasma accelerator platform of Peking University, a beamline was initially constructed with a collection system based on an electromagnetic quadrupoles (EMQs) triplet. However, due to the limited acceptance angle, the EMQs exhibited a monoenergetic transmission efficiency of only 10% to 20% for laser accelerated proton beams. Permanent magnet quadrupoles (PMQs), known for their high magnetic field gradients, compact size, and lower expenses, are well suited for integration with other readily adjustable transmission elements like EMQs and solenoids, effectively enhancing the beam collection capabilities of the system. In this paper, we show that by introducing a pair of centimeter-sized PMQs in front of the EMQs for beam prefocusing, the beam’s transverse size is quickly compressed, enabling transmission of highly divergent protons. Experimental results demonstrate that the prefocusing by PMQs increased the system’s transmission efficiency by a factor of 2.44 to 6.01 compared to the original setup, while also enhancing the energy selection based on stronger chromatic effect. This method can be extended to 100 MeV high-energy proton beamlines and is crucial for applications of laser plasma accelerators.

Initiating the amplification process in a short-wavelength free-electron laser (FEL) by external seed laser pulses results in radiation with a high degree of longitudinal and transverse coherence. The basic layout in seeded harmonic generation involves a periodic electron energy modulation by laser-electron interaction in a short undulator (the “modulator”), which is converted into a density modulation in a dispersive section immediately followed by a long FEL undulator (the “radiator”) tuned to a harmonic of the seed laser wavelength. With the advent of more complex seeding schemes, density-modulated beams may need to be transported in drift sections before entering the radiator. Long FEL undulators may also contain several drift spaces to accommodate focusing elements and diagnostics. Therefore, it is of general interest to study the evolution of density-modulated electron beams in drift sections under the influence of repulsive Coulomb forces. At FERMI, a seeded FEL user facility in Trieste, Italy, systematic studies of the impact of varying drift length on coherent harmonic emission were undertaken. In order to make the underlying physics transparent, the emphasis of this paper is on reproducing the experimental findings with analytical estimates and a simple one-dimensional numerical model. Furthermore, the Coulomb forces in a drift section may be employed to enhance the laser-induced energy modulation and yield an improved density modulation before entering the FEL radiator.

Despite being an old detector concept, position-sensitive parallel-plate avalanche counters (PPACs) remain widely used today for heavy-ion position and timing measurements. In modern rare isotope beam facilities and large-acceptance fragment separators, PPACs are used to characterize beam properties (diagnostics), facilitate beam delivery (tuning), and provide event-by-event beam tracking for particle identification (PID). Most popular particle localization methods in PPAC detectors are based on strip electrodes electrically connected to resistive-chain circuits or delay lines. More exotic systems include optical readouts based on recording electroluminescence light with high-granularity photodetector arrays or high-resolution resistive anode electrodes. PPACs with conventional resistive-chain or delay-line readouts achieve typical position and time resolutions of below 1 mm ($\sigma$) and around 150 ps ($\sigma$), respectively. In addition, delay-line systems are capable of counting rates above several hundred kHz for beam areas of a few millimeters square, and around 1 MHz for larger beam sizes. Resistive-chain readouts have limited rates of a few tens of kHz. A review of the operation principles and performance of PPAC detectors is presented in this paper. We will discuss decades-long experience building and operating PPACs developed at the National Superconducting Cyclotron Laboratory, and then refined at the Facility for Rare Isotope Beams (FRIB), mostly focusing on the delay-line readout technique (DLPPAC). We will also discuss problems that arise due to electric discharges at high rates and briefly describe ongoing developments at FRIB.

We explain how the accelerating field amplitude and phase vary with modulator voltage in pulsed radio frequency high-power amplifiers based on klystron tubes. Changes in modulator voltage give rise to correlated changes of amplitude and phase, affecting the properties of the accelerated beam, in particular energy, arrival time, and bunch duration. We show, both theoretically and experimentally, that there exists a postcrest acceleration phase (the magic angle) where the changes of beam energy due to phase and amplitude shifts caused by modulator-voltage variations cancel out. When accelerating at the magic angle, the klystron modulator voltage jitter no longer contributes to energy and arrival-time jitter in the accelerator. Off-crest operation at the magic angle can be implemented for bunch compression schemes in accelerators with arc-type bunch compressors, which have positive momentum compaction. The experimental results, obtained at the MAX IV laboratory, show the benefit of operating close to the magic angle in arc-type bunch compressors. In a direct measurement of normalized electron-energy jitter, the energy jitter was reduced by a factor of 1.8 down to $8.2\times {10}^{-5}$ when operating at the magic angle.

The scheme of multiturn painting beam injection in a dual harmonic rf system is widely employed in high-intensity proton or heavy ion synchrotrons to alleviate space charge effects. With a momentum offset of the injected beams, a large bunching factor can be achieved at the end of multiturn painting injection process. However, the momentum offset can increase the instantaneous beam peak current during the first $1/4$ synchrotron period, causing a reduced bunching factor during the beam injection process. To address this issue, a matched phase sweep method is developed. By matching the synchrotron oscillation frequency and beam injection period, an optimal curve for phase sweeping can be directly obtained. The effectiveness of the method has been checked via simulations and machine study, which are in good agreement. The matched phase sweep method is employed for the increase of the beam power in the upcoming upgrade project of the China Spallation Neutron Source and can also be generally applied for the operation in the presence of dual harmonic rf system in high-intensity hadron synchrotrons.

Coherent terahertz (THz) radiation sources based on a storage ring hold a significant future for generating high average power terahertz radiation, as the electron beam possesses a high revolution frequency. We propose a novel scheme to generate a high repetition rate coherent THz radiation leveraging the energy modulation self-amplification technology. In this scheme, utilizing the interaction of external intensity-modulated laser pulses with the electron beam can lead to a customized manipulation of the electron beam, such as a periodic longitudinal density modulation with a submillimeter scale for coherent THz radiation. Positively, it can propel the repetition rate when the peak power requirement of an external laser system is decreased. Thus, we employ a self-modulation method to enhance the initial weak energy modulation induced by the laser directly. In the meanwhile, the use of self-modulation can bypass the limitation that the bandwidth of a long modulator should be smaller than that of the external wideband laser source, which is necessary to shape the intensity envelope flexibly. The one-dimensional analytical description of electron longitudinal dynamic during the self-modulation process is given, which confirms the effective enhancement of the modulation at THz band. Three-dimensional time-dependent simulation results based on the parameters of Hefei light source-II show that the coherent THz radiation pulses with a repetition rate of 10 kHz can be generated.

With the rapid development of high-gradient laser plasma acceleration, implementing it in practical applications has become a priority. However, to go from “acceleration” to “accelerator,” a beam line system is required to accurately control the beam parameters according to different irradiation requirements. The laser-accelerated proton beam is characterized by a micron-scale original source size and a small emittance as low as 0.004 mm mrad [T. E. Cowan et al., Phys. Rev. Lett. 92, 204801 (2004)]. However, due to the broad energy spread and large divergence, its initial ultralow emittance will increase rapidly in the subsequent transmission process. This indicates that designing a beamline for laser-driven protons is challenging and differs significantly from that of a conventional accelerator. As a fundamental parameter for beam line design, we have theoretically derived the emittance growth law for laser-driven protons in both drift space and in a focusing element. The results demonstrate that the beam emittance deteriorates sharply with the energy spread and the square of the divergence angle. These theoretical calculations have been verified both in experiments and simulations. This work is helpful for designing subsequent beam lines that pursue high transmission efficiency and achromatic ability.

In electron storage rings, an accurate description of particle dynamics near third-integer resonance is crucial for various applications. The conventional approach is to extrapolate far-resonance dynamics to near resonance, but the difficulty arises because the nonlinear detuning parameter diverges at this critical point. Here we derive, via a suitable application of the canonical perturbation theory, a revised detuning parameter that is well behaved near resonance. The resultant theory accurately describes the morphology of resonance islands for a wide range of parameter space and facilitates its optimization.

Undulator tapering controls the resonance properties of the free-electron laser (FEL) amplification process. Wakefield energy losses in an undulator’s vacuum chamber are one of the factors that determine the undulator’s linear taper. While another contribution to energy losses, namely the losses due to spontaneous radiation, can be calculated analytically, estimating wakefield energy losses requires detailed knowledge of the chamber geometry and the electron beam current profile. We introduce a method for the automatic estimation of wakefield energy losses, which leverages noninvasive THz diagnostics, a current profile reconstruction algorithm enhanced with machine learning, and a recently developed analytical wakefield function for the European XFEL’s undulator beamline. The correctness of this method was validated by directly measuring wakefield-induced electron beam energy losses in the undulator section. This, in turn, enables the prediction of the optimal linear taper in the undulator.

The Super Proton Synchrotron (SPS) is the last stage in the injector chain for CERN’s Large Hadron Collider, and it also provides proton and ion beams for several fixed-target experiments. The SPS has been in operation since 1976, and it has been upgraded over the years. For the SPS to operate safely, its internal beam dump must be able to repeatedly absorb the energy of the circulating beams without sustaining damage that would affect its function. The latest upgrades of the SPS led to the requirement for its beam dump to absorb proton beams with a momentum spectrum from 14 to $450\mathrm{GeV}/c$ and an average beam power of up to $\sim 270\mathrm{kW}$. This paper presents the technical details of a new design of the SPS beam dump that was installed in one of the long straight sections of the SPS during the 2019–2020 shutdown of CERN’s accelerator complex within the framework of the Large Hadron Collider Injectors Upgrade Project. This new beam dump has been in the operation since May 2021, and it is foreseen that it will operate with a lifetime of 20 years. The key challenges in the design of the beam dump were linked to the high levels of thermal energy to be dissipated—to avoid overheating and damage to the beam dump itself—and high induced levels of radiation, which have implications for personnel access to monitor the beam dump and repair any problems occurring during operation. The design process, therefore, included extensive thermomechanical finite-element simulations of the beam-dump core and its cooling system’s response to normal operation and worst-case scenarios for beam dumping. To ensure high thermal conductivity between the beam-dump core and its water-cooling system, hot isostatic pressing techniques were used in its manufacturing process. A comprehensive set of instrumentation was installed in the beam dump to monitor it during operation and to cross-check the numerical models with operational feedback. The beam dump and its infrastructure design were also optimized to ensure it can be maintained, repaired, or replaced while minimizing the radiation doses received by personnel.

A beam gas curtain (BGC) monitor has been designed to obtain information about the relative position between the LHC proton beam and the hollow electron lens electron beam through a minimally invasive process. Its working principle relies on intersecting the path of both beams with a supersonic gas curtain, introduced transversely into the LHC beamline, to produce a fluorescence signal. As an intermediate project stage (phase II), a preliminary version of the BGC monitor has been installed into the LHC beamline. To ensure the successful integration of the monitor and subsequent operation under LHC ultrahigh vacuum conditions, a series of vacuum studies have been performed. These can be classified as follows: An off-line laboratory test campaign, to assess BGC behavior during pump down and gas injections; simulations and analytical calculations, to evaluate BGC behavior and estimate the impact of its installation and operation in the LHC. This document will briefly present the off-line tests campaign, followed by a more extensive description of the simulations performed.

Landau cavities used to lengthen the bunches in storage rings necessarily constitute a significant impedance. Because of the particular phase of the field required for bunch lengthening, they are often detuned quite considerably from resonance, more so than the main cavities. As a result, their impedance can excite the first coupled-bunch mode such that it becomes unstable. This phenomenon has previously been predicted [M. Venturini, Phys. Rev. Accel. Beams 21, 114404 (2018)] and characterized in simulations [T. He, Phys. Rev. Accel. Beams 25, 024401 (2022)] but experimental observation is yet to be documented. In this paper, the experimental observation of coupled-bunch modes-$\pm 1$ excited by the Landau and main cavities in a fourth-generation light-source storage ring is presented. Features of the instability such as amplitude and coherent frequency at saturation have been measured and its dependency on the main rf voltage has been explored. The impact of a parked main cavity has also been investigated.

A new regime in radiofrequency (rf) breakdown, named the breakdown insensitive acceleration regime (BIAR), was observed in an 11.7 GHz metamaterial structure for wakefield acceleration driven by rf pulses with a duration of a few nanoseconds. In the BIAR, rf breakdown occurs without interrupting potential beam acceleration, resulting in greater resilience to breakdown. We have investigated the possibility that BIAR can support higher gradients by characterizing the breakdown in a high-power test. The peak gradient reached $190\mathrm{MV}/\mathrm{m}$ when the structure was powered by 6 ns long rf pulses with 115 MW peak power. The short rf pulses were extracted from 65 MeV electron bunch trains with a total charge of up to 210 nC. This work has revealed the benefits of short-pulse acceleration by characterizing rf breakdown in the previously unexplored parameter space.

We present a novel concept of longitudinal bunch train compression that can manipulate a relativistic electron beam across hundreds of meters. This concept holds the potential to compress the electron beam produced by a conditional linear accelerator at a high ratio, elevating its power to a level comparable with large induction accelerators. The method employs the spiral motion of electrons in a uniform magnetic field to fold hundreds-of-meters-long trajectories into a compact setup. The interval between bunches can be fine-tuned by modulating their spiral movement. We explore this method with the particle dynamic simulation. Compared to setups of similar size, such as a chicane, our method can compress bunches at considerably larger scales. Consequently, it opens up new possibilities for generating high-power beams using compact devices at lower costs.

The development of cross-bar H-type cavities (CH-DTL, $H_{21}$ mode) is ongoing to improve the technology of this attractive type of drift tube linac (DTL). In comparison with the conventional DTL, the H-type cavities can reach a higher effective field gradient and are competitive in shunt impedance at an energy range up to 100 A MeV. H-mode DTL’s profit in shunt impedance additionally when applying the KONUS beam dynamics. They are in use at research laboratories as well as at hospitals. This paper describes a new concept to approach the zero mode in CH-type cavities, by extending the cavity diameter at the tank ends in combination with tilted drift tube stems. RFQs of the four-vane type are operated as well in the ${\mathrm{H}}_{210}$ mode and the strategy for voltage flattening can be partly applied there too if conventional vane undercuts are causing problems. Up to around 35 MeV, it is attractive to integrate one or more triplet lenses into each cavity, as one KONUS section is relatively short and would not exploit the full rf power of 3 MW klystrons which are available above 300 MHz. Such a cavity is denoted as a coupled CH-cavity CCH. Three possible arrangements of those internal triplet lenses are discussed and are compared to each other. The operating modes as well as higher modes are then compared with a lens-free CH cavity, where the lens position was filled by ordinary drift tubes. As a result, it is shown that the rf behavior and resonance frequency for the higher harmonics of the frequency band are very similar for all four investigated arrays. This means, that the tuning behavior of the CCH cavity can be simply deduced from the lens-free CH cavity by replacing an even number of ordinary drift tubes ($\mathrm{n}\beta \lambda$) with a lens-containing drift tube with adequate length and large outer diameter. This large drift tube itself oscillates like an Alvarez-type drift tube. rf simulations on a 30-gap CH cavity show that the reduction in shunt impedance is about 10% when installing a lens with length 2 beta lambda.