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.

We present studies on enhanced SASE at the soft x-ray beamline Athos at SwissFEL. The generation of a series of current spikes with 790 nm spacing is injected into the Athos undulator beamline. Due to strong space-charge forces, the optimum performance occurs with a strong positive taper of the undulator field. There is a clear modal structure in the observed spectra, indicating the presence of a train of attosecond FEL pulses with no intrinsic phase-locking. The space-charge dynamics causes an elongation of the current spike and a redshift in the bunching spectra, which determines the optimum taper gradient.

High-gradient normal-conducting radio-frequency (rf) cavities are crucial components of the rectilinear cooling channel in a muon collider, providing the accelerating field necessary to sustain the rapid six-dimensional ionization cooling essential to achieve high luminosity. To reach this goal, a muon collider requires approximately ${10}^{12}$ muons per bunch. At such high beam intensities, beam loading induced by bunch-driven wakefields can significantly reduce the cavity accelerating gradient and compromise beam stability. This paper focuses on the conceptual rf design and beam-induced-effect analysis of copper cavities with so-called beam windows developed to meet the beam dynamics requirements of the rectilinear muon cooling channel. We first present the design and optimization of the cavities, evaluating their rf performance and power requirements. Thereafter, we analyze cavity wakefields and beam loading effects, proposing a new numerical method for their compensation, which takes the transverse distribution of the beam into account.

NbTiN thin films are promising candidates for superconducting multilayer structures aimed at surpassing the intrinsic field limits of bulk Nb in superconducting radio-frequency (SRF) cavities. In this work, ${\mathrm{Nb}}_{0.75}{\mathrm{Ti}}_{0.25}\mathrm{N}$ films grown by plasma-enhanced atomic layer deposition were systematically investigated with respect to their crystalline structure, surface morphology, and field emission behavior in their as-deposited state, as well as after annealing in vacuum at $900°\mathrm{C}$ and $1000°\mathrm{C}$, respectively. X-ray diffraction reveals that annealing induces significant grain growth, a pronounced (111)-texture, and a strong reduction of residual stress, with the $1000°\mathrm{C}$ samples exhibiting the most uniform and relaxed microstructure. Optical profilometry shows a clear decrease in macroscopic roughness by approximately 20%–25% after annealing, while atomic force microscopy measurements confirm that the nanoscale roughness remains low and largely unaffected by the heat treatments. Field emission scanning microscopy demonstrates that annealing substantially increases the onset field for electron field emission from values below $60\mathrm{MV}/\mathrm{m}$ in the as-deposited state to $180–220\mathrm{MV}/\mathrm{m}$ for annealed films, correlating with improved crystallinity, reduced stress, and enhanced film-substrate adhesion. Complementary ultraviolet photoelectronspectroscopy measurements yield a work function of ${\mathrm{\Phi }}_{\mathrm{NbTiN}}=(4.32\pm 0.07)\mathrm{eV}$, supporting the interpretation of the field emission scanning microscope data. The combined results show that high-temperature annealing, particularly at $1000°\mathrm{C}$, significantly enhances the structural and functional performance of NbTiN films, underscoring their potential for application in superconducting multilayer coatings for next-generation SRF cavities.

Superconducting magnets enable energy-frontier accelerators by generating strong magnetic fields to steer and focus the particles. Although high-temperature superconductors such as ${\text{REBa}}_2{\text{Cu}}_3{\mathrm{O}}_x$ (rebco, RE = rare earth) hold a strong potential for generating a higher magnetic field than Nb-Ti and ${\text{Nb}}_3\text{Sn}$, the associated magnet and conductor technology for accelerator applications is still in its infancy. The U.S. Magnet Development Program is developing rebco magnet technology in collaboration with industry. Here we report an experiment of making a dipole magnet called C3 using commercial high-temperature superconducting corc wires. The magnet, following a canted $\cos \theta$ design, generated a dipole field of 5.99 T at 4.2 K in its clear aperture of 65 mm at 6.795 kA when a resistive voltage of $105\mathrm{\mu }\mathrm{V}$ appeared across one of the coils in the magnet. The stored energy was 53 kJ at the peak field. The magnet showed no degradation in the current-carrying capability at 4.2 K after the thermal cycle. We report on the detailed design, fabrication, and performance of the C3 magnet that can be of interest to potential users of this emerging technology. We also discuss issues and research needs to inform future rebco magnet development. The experiment represented another step to addressing if the high-temperature superconducting accelerator magnet technology can increase the discovery capability of future particle accelerators.

At the 1.5-GeV synchrotron light source DELTA operated by TU Dortmund University, intense and ultrashort radiation pulses in the vacuum ultraviolet and terahertz regimes are generated by the interaction of femtosecond laser pulses with electron bunches in an undulator. The resulting periodic modulation of the electron energy is converted into a density modulation in a dispersive section, giving rise to coherent radiation emission at harmonics of the seed wavelength in a second undulator. This process is known as coherent harmonic generation (CHG). This paper describes the experimental setup and concentrates on the spectral aspects of CHG radiation under variation of properties of the seed laser pulse and the dispersive section. CHG spectra at the second harmonic of the 800 nm seed wavelength were recorded using a grating spectrometer equipped with an image-intensified CCD camera. Numerical simulations to model the CHG spectra for different group-delay dispersion and third-order dispersion were carried out, and a convolutional neural network was implemented to predict these parameters from measured CHG spectra. The results show that the tunable laser parameters directly translate into the spectrotemporal properties of the coherently emitted pulses.

The High Energy Photon Source is the first fourth-generation light source in China, featuring a hybrid multibend achromat lattice. This innovative design incorporates high gradient quadrupoles with an aperture of 26 mm, a gradient of about $80\mathrm{T}/\mathrm{m}$, and high-order field harmonics of less than $4\times {10}^{-4}$. This paper details the optimized magnetic design, precise fabrication techniques, batch field measurements, and a method for field harmonic compensation utilizing a magic finger. A comparative analysis was conducted on nine types of the 480 pure quadrupole magnets that share the same cross section but differ in iron lengths. These variations include laminated or solid cores and closed or open yokes. The cross section, characterized by a narrow concave and convex pole contour, a tapered pole root, and saddle coils, is designed to mitigate magnetic saturation while enhancing the iron filling ratio. The impact of iron saturation on the transfer function and the evolution of current-dependent multipole fields has been systematically investigated, along with the effects of mechanical errors and inhomogeneities in the magnetic properties of the iron core on harmonics. We demonstrate a comprehensive engineering solution for high gradient small-aperture quadrupole magnets, which has been validated through mass production.

We report the experimental implementation of a data-driven chaos indicator (DDCI) [Y. Li et al., Data-driven chaos indicator for nonlinear dynamics and applications on storage ring lattice design, Nucl. Instrum. Methods Phys. Res. A 1024, 166060 (2022)] for online optimization of the National Synchrotron Light Source II storage ring. The DDCI quantifies the predictability of electron beam dynamics using turn-by-turn beam position monitor data. A surrogate model of the one-turn map is first trained, and its out-of-sample predictive uncertainty is then employed as a measurable indicator of chaos. By tuning sextupole magnets to mitigate nonlinear effects, a clear enlargement of the dynamic aperture is achieved, accompanied by a corresponding improvement in injection efficiency.

Undulator tapering is an important technique in free-electron lasers and synchrotron radiation sources, used to enhance radiation efficiency, broaden spectral bandwidth, and perform other related functions. However, conventional tapering methods, such as gap variation or electromagnetic control of individual magnets, involve complex mechanical structures or power supply systems, posing challenges in tuning the taper profile while maintaining good field integral performance in practice. This paper presents a beat frequency undulator that enables dynamic control of undulator tapering over a wide range. The concept employs two permanent magnet arrays with slightly different periods, whose magnetic field superposition generates a longitudinal beat frequency envelope, creating a tunable taper profile without gap adjustment or electromagnetic tuning. By longitudinally shifting one array, the taper strength and direction can be reconfigured in real time. This approach offers inherent advantages in mechanical simplicity and maintains a field integral comparable to that of a standard planar undulator, providing a robust and flexible solution for undulator tapering.

The inverse-Cherenkov dielectric laser accelerator (ICR-DLA) holds great promise as a compact, on-chip accelerator for a wide range of future applications. However, two significant challenges—bunch dispersion/deflection and phase slippage—have hindered its development in the subrelativistic regime. In this paper, we propose an approach that addresses both issues simultaneously by utilizing a single laser pulse to illuminate a staged dielectric prism. Our method leverages the phase slippage experienced by subrelativistic electrons during high-gradient acceleration, allowing these electrons to encounter alternating focusing and defocusing forces throughout the acceleration process. This technique enables stable, long-range bunch transport within a miniaturized acceleration channel. We show that cascading focusing and acceleration of subrelativistic electrons can be achieved in a multistage acceleration structure, laying a foundation to bridge the gap between subrelativistic and relativistic regimes, which is crucial for the realization of a practical on-chip particle accelerator.