A compact, in-vacuum switchable beam dump is proposed as a first-of-a-kind engineering solution to improve operational flexibility in high-energy accelerator facilities. The design integrates beam-pass and beam-dump modes into a single wall-embedded unit, eliminating the need for external shielding and movable components outside the vacuum system. The concept is specifically developed for the superconducting fragment separator (Super-FRS) at Facility for Antiproton and Ion Research, making use of an existing opening in a modular radiation-shielding wall between experimental areas. It employs synchronous rotation to switch between an unobstructed vacuum beamline and a fully shielded beam-dump configuration. This architecture enables personnel access to downstream experimental areas while beam delivery continues in the upstream section. Two absorber configurations were investigated: an all-iron design and a mixed configuration incorporating the high-density tungsten alloy Densimet. Iron provides cost-effective bulk shielding and strong attenuation of secondary gamma radiation, whereas Densimet enables compact energy deposition with superior stopping power. The required thickness of additional downstream boron carbide shielding to adequately suppress slow neutrons differs between the two options. FLUKA Monte Carlo simulations confirm that both configurations satisfy the stringent personnel safety limit of $<3\mathrm{\mu }\mathrm{Sv}/\mathrm{h}$ in all relevant scenarios. Complementary vacuum simulations demonstrate that the beamline pressure remains below $5\times {10}^{-6}\mathrm{mbar}$ during operation. The results indicate that the proposed switchable beam dump offers a viable path to enhance facility availability and throughput at Super-FRS. The compact, modular concept is scalable and may be adapted to other high-energy accelerator facilities facing similar space and access constraints.

Coherent terahertz (THz) sources based on storage rings offer a compelling pathway to high-average-power radiation, leveraging the high revolution frequency of the electron beam. However, current achievable repetition rates are severely constrained by the radiation damping required to dissipate phase-space perturbations introduced during generation. To address this challenge, we propose a novel scheme incorporating an active demodulation mechanism. This method utilizes a phase-reversed laser interaction to actively compensate for the laser-induced energy modulation immediately following the radiation emission. By rapidly restoring the beam quality rather than waiting for natural radiation damping, the recovery time is significantly shortened. Simulations based on HLS-II, combined with an analytical damping estimate, suggest the feasibility of coherent 10 THz generation at repetition rates on the order of 100 kHz, corresponding to an average power of 6.2 mW.

In SuperKEKB, mitigation of sudden beam losses (SBLs) is essential to improve the luminosity. SBL refers to the rapid loss of a substantial portion of the stored beam current within a few revolutions. During SuperKEKB operation from 2019 to 2024, over 200 SBL events were observed, in which a beam abort with 5%–40% beam loss occurred within 2–3 turns, making high beam current operation difficult. The frequency of SBLs was very high, especially in the low-energy ring (LER). The occurrence of SBLs was highly concentrated during beam-current upgrade phases, presenting a severe operational bottleneck. While SBLs were rare during stable operations at lower currents, the frequency escalated to more than five events per day when operating at higher beam currents. These frequent events triggered intense beam losses, resulting in 11 quenches of the final-focus superconducting quadrupole magnets (QCS) within a period of less than three weeks in December 2024. Furthermore, SBL events could damage the collimator heads and the Belle II detector, causing devastating impacts on operation. SBLs occurred under both noncollisional and collisional conditions, but only under circumstances involving a small beam size at the interaction point and a high beam current. Most SBL events were accompanied by vacuum pressure bursts, primarily in the wiggler section of the LER, and an increase in beam size was also observed during these events. To investigate this phenomenon, SBL was artificially reproduced during beam operation and subsequently discharging the clearing electrodes. Inspecting the wiggler beam pipe, we found black stains, and removal of these stains led to a significant reduction in the frequency of SBLs. The black stains we discovered were identified as degraded vacuum sealant through observations using synchrotron radiation. This paper reports the results of an artificial reproduction test of the SBL phenomenon (knocking on the beam pipe, discharge inside the beam pipe), and an investigation of these black stains, including their composition and formation mechanism.

A 300–600 MeV high-intensity scaling fixed-field alternating gradient (FFA) facility is proposed as a multipurpose proton driver for the China Spallation Neutron Source (CSNS) Phase-II upgrade. To identify a viable lattice configuration under strict engineering constraints, a statistical analysis methodology based on large-sample ray-tracing simulations comprising over 360,000 parameter configurations is applied. A two-stage scanning pipeline evaluates the topological limits of both FDF triplet and FD doublet structures under realistic multisource error budgets. Factoring in phase-space geometry, error tolerance, and facility integration, the findings indicate that a spiral FD doublet lattice with a super-periodicity of $N=16$ is preferred as the structural baseline; it avoids parameter bottlenecks and yields a large, resonance-sparse stable region. To characterize the coupled high-dimensional dynamics quantitatively, a machine learning feature attribution framework is applied. The framework identifies the spiral edge angle and the full-ring vertical tune as the primary drivers of beam survivability, and maps the underlying resonance stopbands from tracking data alone. Based on these results, a baseline configuration is established with a wide dynamic tune allowance that maintains long-term beam survivability within the physical and engineering boundaries of the facility.

The design and optimization of a broadband electron gun with a semi-immersed cathode for an Electron Beam Ion Source (EBIS) operating with a 5 T solenoidal magnetic field is presented. The work contributes to the development of high-performance, tunable EBIS for next-generation all-linac medical accelerators for cancer therapy, as well as other applications in accelerator physics. The system is primarily designed to meet the requirements for the production of fully stripped carbon ions. An acceptance criterion for the injection of the electron beam into a full magnetic field is derived and used to validate the design. The gun position is optimized for the transmission of an electron beam with a current of 2.0 A and a current density of $1000\mathrm{A}/{\mathrm{cm}}^2$ inside the solenoidal field. The operating range of the electron beam can be broadened from 0.2 to 3 A by using a nonadiabatic magnetic field on the beam path close to the gun, which transfers the transverse component of the electron energy into the longitudinal direction and therefore reduces the amplitude of the beam scalloping. The current density can be adjusted by the use of another coil surrounding the cathode.

The physics and ring design of the storage ring proton electric dipole moment (sr-pEDM) experiment, including the symmetries used to mitigate first-order and second-order systematic effects, are described in [Z. Omarov et al. Comprehensive symmetric-hybrid ring design for a proton edm experiment at below ${10}^{-29}\mathrm{e}·\mathrm{cm}$, Phys. Rev. D 105, 032001 (2022)]. The sr-pEDM experiment leverages a novel symmetric-hybrid, frozen-spin storage ring design using radial electric bending and alternate magnetic focusing to minimize systematic errors, enabling precise measurement of the vertical rotation of the polarization of polarized stored proton beams. In this work, we present a more systematic and detailed analysis of second-order effects, focusing in particular on the interplay between stray radial magnetic fields and electric quadrupole and sextupole fields. We demonstrate how the symmetries of the symmetric-hybrid ring design, including beam reversal and magnetic quadrupole polarity switching, can be used to suppress these effects to below the targeted sensitivity of the sr-pEDM experiment.

The x-ray free-electron laser (XFEL) facility SACLA uses a thermionic pulsed electron gun and a beam chopper to produce a low-emittance, short-bunch beam. Recently, a new type of fast pulse generator, which utilized wideband rf amplifiers and a new chopper chamber with high-impedance strip-line electrodes, was developed to reliably and accurately cut out a short bunch from the long-pulsed beam emitted from a single-crystal ${\text{CeB}}_6$ hot cathode. Using these apparatuses, we succeeded in forming nanosecond multibunch structure in the thermionic electron beam and generating a double-bunch XFEL pulse with a total pulse energy of 1 mJ at 10 keV. The results will provide new possibilities for XFEL beams: higher intensity, higher coherence, and higher pulse repetition rate.

In diffraction-limited storage rings (DLSRs), due to the comparable energy loss from bending magnets to that from insertion devices (IDs), the independent adjustments of ID gaps can introduce random emittance variations, which need to be compensated. Hefei Advanced Light Facility (HALF) is a soft x-ray DLSR with both 20 long dispersion-free and 20 short dispersive straight sections. Its original emittance compensation scheme is based on the radiation damping effect, achieved by adjusting the gaps of two damping wigglers (DWs) located in long straight sections. This paper proposes a novel emittance compensation scheme for HALF based on the quantum excitation effect. In this new scheme, a short variable-gap wiggler is installed in a dispersive straight section, with the two DWs operating at their minimum gaps. The proposed scheme achieves emittance compensation comparable to the original scheme in both cases with and without intrabeam scattering, even though the wiggler is significantly shorter. It also yields significantly smaller energy spread variations and consequently, very small effective emittance variations at short dispersive straight sections, with the effective emittance considered basically compensated. Moreover, fixing the minimum gaps of the two DWs can provide additional beamlines for users as well as enhanced radiation damping, which not only compensates for the emittance growth caused by the short wiggler due to quantum excitation but also reduces the damping times to suppress intrabeam scattering. For DLSRs without dispersive straight sections, the proposed quantum-excitation-based emittance compensation scheme can be applied by converting a dispersion-free straight section into a dispersive one simply using a dipole chicane.

Laser wakefield acceleration (LWFA) is a promising approach for producing high-brightness electron beams in the GeV energy range, offering significant potential for compact next-generation accelerator facilities. In this work, we present a computational study of LWFA in a specially designed single-stage capillary gas cell target aimed at producing high-quality, GeV-class electron beams. The capillary cell includes a short ($\sim 2\mathrm{mm}$) injection region at the entrance filled with a helium (He) and nitrogen gas mixture. This is followed by a longer ($\sim 14\mathrm{mm}$) pure He section, which provides the required acceleration length and limits continuous ionization injection, thereby significantly reducing the energy spread of the accelerated beam. Hydrodynamic simulations are performed to optimize the capillary geometry and generate the required two-section gas-pressure profile. The resulting gas density distributions for various cases are then directly incorporated in particle-in-cell (PIC) simulations to study LWFA. In particular, our hydrodynamic simulations demonstrate how tailored density profiles in the acceleration section can be realized in a capillary gas cell, while the corresponding PIC simulations reveal how these profiles influence the acceleration process and the resulting beam quality. Using a 100 TW-class laser system with parameters relevant to the L2-DUHA laser at the ELI Beamlines Facility, the PIC results demonstrate electron acceleration to mean energies exceeding 1.0 GeV with high-quality beam properties. Self-injected He electrons are also observed, and their impact on the main beam quality is evaluated. The findings of this study provide valuable insights for upcoming LWFA experiments planned within the EuPRAXIA Project at the ELI Beamlines Facility.

We have constructed a 324-MHz interdigital $H$-mode drift tube linear accelerator (IH-DTL) with an alternating phase focusing method. This IH-DTL will be part of a muon linear accelerator (linac) being developed at the Japan Proton Accelerator Research Complex for a precise measurement of the muon anomalous magnetic moment and a search for the muon electric dipole moment. The IH-DTL is $1.45\mathrm{m}$ long and accelerates muons from $\beta =v/c=0.08$ to 0.28. Low-power tuning was performed using six slug tuners, which reduced the peak-to-peak on-axis field error to $1.4\%$. Following 94 h of high-power conditioning, the IH-DTL demonstrated stable operation at the nominal peak power of $390\mathrm{kW}$ and a duty factor of 0.1%, corresponding to the designed accelerating field ($E_0$) of $3.6\mathrm{MV}/\mathrm{m}$. This paper presents the construction, low-power rf tuning, and high-power performance of the 324-MHz IH-DTL.

In the pulsed-power accelerators that drive Z-pinch implosions, multi-MA currents rapidly heat the electrodes above the threshold for plasma formation. When these surface plasmas are transported into the transmission line gap, they shunt some portion of the driver current away from the load, referred to as “current loss.” In highly magnetized systems, current loss is well approximated by a Hall current proportional to the minimum plasma density in the gap and the electric field [N. Bennett, D. R. Welch, G. Laity, D. V. Rose, and M. E. Cuneo, Magnetized particle transport in multi-MA accelerators, Phys. Rev. Accel. Beams , 060401 (2021)]. This paper explores the mechanism that drives the initial plasma transport into the gap, which is difficult to isolate in large-scale power-flow simulations but is instrumental in seeding the current-loss mechanism. The stability and dynamics of an electron sheath emitted from a resistive cathode plasma in the presence of large magnetic fields are investigated using semianalytic theory and simulation. A dispersion analysis determines that the instability growth rate scales inversely to ion mass and magnetic field strength. This scaling is confirmed in kinetic models of an electron sheath and resistive plasma using the relativistic electromagnetic particle-in-cell code CHICAGO. The simulations are initiated with field magnitudes representative of the final feed of a Z-pinch driver and demonstrate that a resistive plasma instability grows on ns timescales, generating perturbed electric fields of sufficient strength to rapidly accelerate plasma ions into the gap. Although the resulting plasma density in the gap is 2 orders of magnitude below the surface density, it is sufficient to generate the published current losses.

In future storage rings, precise control of the electron-beam phase space will become increasingly important. A key challenge in these dynamics is maintaining the lattice’s isochronicity, which is crucial for applications such as optical stochastic cooling and steady-state microbunching. In particular, path-length deviations induced by intrabeam scattering (IBS) could limit the performance of steady-state microbunching light sources. In this paper, we present a more accurate formula for calculating the IBS-induced path-length deviation of an electron beam traversing the lattice, which can aid in lattice optimization. Additionally, we have developed a tool to simulate IBS-kick effects on beam dynamics. It can directly compute the IBS equilibrium emittance in storage rings and efficiently evaluate the associated path-length deviations for an electron beam traversing a lattice.

Inverse Compton scattering (ICS) is a promising method for generating coherent and tunable x-rays in a compact setup. In this paper, we present a theoretical framework describing the output of an ICS x-ray source for arbitrary interaction angles between pulsed electron and laser beams in the Thomson regime. This allows for analytic optimization of the x-ray beam properties by varying the parameters defining the geometry. In general, different x-ray applications require optimization of different x-ray beam properties, such as energy spread for x-ray spectroscopy and angular spread for x-ray scattering measurements. In this paper, we restrict ourselves to optimization of the x-ray brilliance, which is a comprehensive figure of merit for x-ray beam quality. The framework can be used, however, to optimize other x-ray properties. We investigate two specific ICS interaction geometries in particular: head-on scattering of a laser beam off an electron beam and scattering of a laser beam off an electron beam in a co-propagating geometry, interacting under a grazing angle. For head-on scattering, we show that a tightly focused, cylindrically symmetric laser pulse, which balances laser intensity and interaction time, optimizes the x-ray brilliance. For a co-propagating, grazing angle geometry, an elliptical focus of the laser pulse is required to mitigate the geometric reduction of the interaction time. We find that the latter geometry is especially useful for soft x-ray generation.

In this paper, we present three-dimensional, time-dependent simulations of the University of Hawai‘i (UH) at Mānoa free-electron laser (FEL) oscillator. Using beam parameters from the UH facility, we study the pass-by-pass evolution of the radiation field, including its temporal, spectral, and transverse properties. At nominal bunch length, the radiation pulse develops temporal spiking near saturation, together with sideband formation and increased sensitivity to machine timing jitter. Our results show that modest cavity desynchronization can enhance the radiation energy by 63%. Large cavity desynchronization, on the other hand, can effectively suppress the spiking instabilities and improve robustness to timing fluctuations. Finally, we simulate a short-bunch operational mode with a bunch length comparable to the slippage length, which accelerates saturation and further amplifies the FEL power. Overall, these results provide a quantitative foundation for pulse control studies in the UH FEL oscillator and a critical benchmark for future experimental validation and machine optimization.

Laser manipulation plays a critical role in precisely tailoring relativistic electron beams through energy modulation, enabling the generation of coherent, intense, and ultrashort radiation in accelerator-based light sources such as synchrotron radiation facilities and free-electron lasers (FELs). However, laser-induced energy modulation inevitably degrades electron beam quality by increasing the energy spread, thereby limiting high-repetition-rate operation. Here, we investigate energy modulation and demodulation in a seeded FEL using two modulators separated by a tunable phase shifter. Analytical calculations and three-dimensional simulations show that a $\pi$ phase delay can nearly reverse the laser-electron beam interaction and substantially suppress the residual modulation. Diagnostics based on coherent undulator radiation and time-resolved measurements are established to characterize weak residual modulation, and a dedicated demodulation undulator is designed for controlled studies. Preliminary experiments performed at the Shanghai soft x-ray FEL facility using the existing seeding beamline demonstrate the suppression of laser-induced energy modulation. Together with the analytical and numerical studies, these results establish a practical framework for investigating the transition from energy modulation to demodulation in seeded FELs, with potential applications in high-repetition-rate, fully coherent x-ray sources with improved preservation of electron beam quality.

The maximum secondary electron yield, ${\delta }_{\max }$, is a key parameter governing electron cloud buildup in high-intensity proton accelerators, yet its direct in situ determination under operational conditions remains challenging. This paper presents a novel method to estimate ${\delta }_{\max }$ in the Large Hadron Collider (LHC) using electron cloud measurements performed in the Vacuum Pilot Sector. The approach exploits the characteristic evolution of electron cloud intensity during beam injection, in particular the transition from an initial build-up regime to a linear scaling of electron flux with the number of injected bunches. Electron cloud currents measured with dedicated electron pickups are compared with PyECLOUD simulations performed for varying ${\delta }_{\max }$ values. The threshold number of injected bunches required to reach the linear regime provides a robust observable from which ${\delta }_{\max }$ can be extracted with limited sensitivity to other surface-emission parameters. The method is applied to multiple LHC fills and surface materials. For conditioned copper liners, ${\delta }_{\max }$ values between approximately 1.16 and 1.30 are obtained, in agreement with independent analyses and literature values. Applying the technique over extended operation periods enables the reconstruction of surface conditioning curves as a function of cumulative electron dose. These curves show a rapid initial decrease in ${\delta }_{\max }$ before approaching an apparent plateau, which may lie below the detection limit of the method depending on the beam pipe material and location. The method is compatible with routine LHC operation and enables systematic monitoring of vacuum-surface conditioning.

This manuscript proposes a method to enable controlled high gradient particle acceleration when requiring self-modulation (SM) of the drive bunch. While electron bunch seeding of self-modulation has been realized at a plasma electron density [L. Verra et al., (AWAKE Collaboration), Controlled growth of the self-modulation of a relativistic proton bunch in plasma, Phys. Rev. Lett. 129, 024802 (2022)], it has not been demonstrated at higher plasma densities due to limitations of available seed bunch properties. As experimentally shown in this manuscript, truncating available seed bunches with a relativistic ionization front allows these limitations to be overcome. This seeding method is called truncated electron bunch seeding of self-modulation (teSSM), and experiments confirm that—when using teSSM—self-modulation becomes reproducible. Additionally, the seed wakefield amplitude is also increased, which is known to be advantageous because it shortens the length needed to reach self-modulation saturation. The presented results establish teSSM as a method for achieving controlled, high gradient particle acceleration with long drivers and available seed bunches.

Achieving high-quality electron beams from laser wakefield accelerators critically relies on density tailoring to control electron dynamics during injection, acceleration, and extraction. We report on the experimental observation of electron beam acceleration and shaping, in transverse momentum and longitudinal phase space, controlled by plasma density tailoring in a gas cell. Electron beams with a FWHM charge of 40 pC at an energy of 190 MeV, 3.4% energy spread and an rms divergence of 0.46 mrad, corresponding to a transverse momentum spread of $0.2{\mathrm{m}}_{\mathrm{e}}\mathrm{c}$, have been measured. These beams have a peak spectral brightness of up to $8\mathrm{pC}/\mathrm{MeV}/\mathrm{mrad}$. Simulations using experimental parameters as input show that acceleration in the plasma plateau leads to chirped electron beams which then undergo transverse momentum spread reduction in a plasma down-ramp followed by dechirping in a 10 mm long plasma tail, leading to the measured peaked spectra. The comparison of experimental results with and without long plasma tail confirms this analysis.

The optimization of nonlinear dynamics for a storage ring lattice aims to achieve large dynamic aperture (DA) and momentum aperture (MA), which are critical for beam injection and lifetime. One of the main factors affecting MA is off-momentum DA. In our previous work, it was demonstrated that reducing on- and off-momentum resonance driving terms (RDTs) along the ring is very effective in enlarging on- and off-momentum DAs. In this paper, the previous effectiveness analysis is formulated into a practical nonlinear optimization method, in which the on- and off-momentum ring-averaged RDTs are minimized simultaneously while constraints on tune shifts and the working point are imposed. Off-momentum ring-averaged RDTs are evaluated over a range of momentum deviations to optimize MA more effectively. The dependence of the ring-averaged RDT and DA on tune is also analyzed, showing that reducing the ring-averaged RDT can help guide the working point away from structural resonance lines and control tune shifts with momentum to avoid dangerous resonance lines. Applied to the SOLEIL storage ring, this efficient ring-averaged RDT optimization enables a rapid iterative workflow of optimization, analysis, and refinement, providing physical insight into the lattice and leading to improved nonlinear solutions. One optimized solution achieves a DA comparable to that of the reference solution obtained from tracking-based optimization while improving the beam lifetime, as validated experimentally on the SOLEIL storage ring.

The Super Tau-Charm Facility (STCF) is a proposed high-luminosity electron-positron collider operating at beam energies of 1–3.5 GeV, with a nominal peak-luminosity target exceeding $0.5\times {10}^{35}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ at 2 GeV. At such low energies, the combined effects of beam-beam interactions in the crab-waist collision scheme, collective effects, and Touschek scattering impose stringent requirements on the dynamic aperture, momentum acceptance, and beam lifetime. This paper presents the design and nonlinear optimization of a one-fold lattice for the STCF collider rings. The design procedure consists of three main steps: lattice-independent optimization of the global machine parameters under luminosity, beam-beam, collective effects, and lifetime constraints; design of a compact interaction region (IR) with local chromatic correction and dedicated crab-waist sextupoles; and full-ring nonlinear optimization. The last step employs a two-stage strategy combining analysis-driven preoptimization with tracking-based refinement. Particular emphasis is placed on controlling momentum- and amplitude-dependent nonlinearities in the IR and preserving the nonlinear transparency of the crab-waist sextupole pair. At 2 GeV, the optimized lattice achieves a theoretical peak luminosity of $1.1\times {10}^{35}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ while maintaining sufficient dynamic aperture, momentum acceptance, and a Touschek lifetime of approximately 600 s. These results demonstrate the effectiveness of the proposed staged optimization strategy and provide a systematic framework for the design of high-luminosity, low-energy electron-positron colliders.