When a charged-particle bunch in a storage ring is kicked to a large transverse offset, the time series describing the dynamics of the bunch centroid is determined both by the lattice focusing and by the Fourier transform of the 1D density profile of the bunch projected along the angle of the kick. In the presence of nonlinear focusing, we show that this fact can be exploited to enable 2D phase space reconstruction of the bunch (computational tomography) based only on turn-by-turn beam position monitor data. We demonstrate various tomography methods based on this principle, including machine learning methods, and discuss their advantages and disadvantages, and measure of reliability. We also mention a possible extension to 4D phase space computational tomography.

The first generation of S-band injectors for free-electron lasers has illustrated high reliability and performance by driving the latest generation of high brightness machines. However, the ultimate electron beam brightness of these devices is limited by the electric field that they can achieve on the cathode. With the aim of a higher beam brightness at SwissFEL, this paper presents the design of a C-band traveling-wave (TW) radio-frequency photogun that aims to offer the possibility to move to a cathode gradient up to $200\mathrm{MV}/\mathrm{m}$. This increased gradient comes from the ability to operate with rf pulse lengths approximately an order of magnitude shorter than the current state-of-the-art room-temperature S-band standing-wave rf photoguns. With this high cathode gradient, this novel gun is able to produce an electron bunch whose 5D beam brightness is 5 times greater than the current SwissFEL photogun. Furthermore, the low power dissipation within the photogun, resulting from the short rf pulse length and TW philosophy, opens up the opportunity for rf pulse repetition rates up to 1 kHz.

The predominant source of nonlinearity in most existing accelerators are sextupoles, which introduce nonlinear resonances. When the horizontal tune of an accelerator is near such a resonance line ($n{\nu }_x$), stable fixed points (SFPs) may appear in the horizontal phase space to form a second closed orbit different from the “zero” closed orbit. The stable islands in phase space surrounding the SFPs are also referred as transverse resonance island “buckets” (TRIBs). However, the TRIBs are not always present near a resonance line in a storage ring. Necessary conditions for TRIBs formation will be discussed in this paper. A 6-GeV lattice with the horizontal tune near a third-order resonance at $3{\nu }_x=50$ is then designed for the Cornell Electron Storage Ring (CESR). After loading this lattice into CESR, the TRIBs are observed while adjusting the horizontal tune near the third-order resonance, consistent with particle tracking simulations. TRIBs rotation in the horizontal phase space are also experimentally demonstrated with adjustment of a sextupole knob. These results are in good agreement with the theoretical calculation.

Crab cavities are key components used by most colliders to enhance luminosity. The local crab crossing scheme is adopted in the EicC (Electron-Ion Collider in China) project. Ideally, the beams would achieve head-on collisions at the interaction point subjected to transverse deflecting forces that linearly depend on the longitudinal position. However, in practical machine design and operation, unavoidable but realistic factors can ruin this perfect situation, including synchrobetatron resonance excited by the finite bunch length effect, imperfections in optics design, and errors in the crab cavity system. These may have significant effects on the colliding beam dynamics. This paper reviews the baseline design of the EicC project from the perspective of checking the rationality, exploring the adjustability, and investigating the tolerance of the design parameters. The self-consistent strong-strong simulation technique is employed. The simulation results suggest that increasing the proton crab cavity frequency up to 400 MHz has no impact on the luminosity lifetime since the proton beam-beam parameter is small. The option of providing more transverse space for forward detectors by doubling the crossing angle is feasible at this newly selected crab frequency. The optics imperfections are acceptable or manageable for both proton and electron beams. The cavity voltage errors need to be carefully controlled in both rings to ensure a better beam lifetime. In addition, the proton emittance growth caused by crab cavity noise is severe, and the tolerance is well beyond state-of-art levels. Electron beams, on the other hand, are not overly affected thanks to synchrotron radiation.

In recent decades, high-gain free-electron lasers (FELs) based on linear accelerators has been successfully developed around the world. Advanced beam diagnostics and feedback technology is one of the key factors to further improve the performance of facilities. Both beam position deviation and trajectory tilt can weaken the interaction between the electron beam and photon beam in the undulator, and seriously affect the FEL radiation performance. Existing measurement methods are not sufficient to achieve accurate measurement of beam trajectory tilt between compact undulator segments. Based on the working principle of beam-cavity, this paper proposes a method based on the measurement results of a single cavity BPM signal, which can simultaneously achieve real-time and in situ measurement of beam position and trajectory tilt, thus opening up a new way to improve the efficiency of FEL radiation. The feasibility of the scheme has been verified with the CBPM system in the Shanghai Soft X-ray FEL facility (SXFEL). The preliminary beam experiment results show that the resolution of beam trajectory tilt is better than $13.3\mathrm{\mu }\mathrm{rad}$ with a bunch charge of 100 pC.

Surface finish plays an essential role in the performance of superconducting radio frequency cavities. Several surface treatments have been developed to reduce surface resistance at a moderate accelerating gradient. We investigated the effects of sequential electropolishing on samples vacuum heat-treated at 300 and $600°\mathrm{C}$ and N-doped Nb samples using atomic force microscopy. The N-doping process precipitates niobium nitrides within grains and, most notably, continuously and deeply along some grain boundaries. Upon electropolishing, the nitrides are preferentially removed leaving behind a topographically imperfect surface marked by relatively deep holes and grooves with low radius of curvature edges. The progression of magnetic field enhancement and superheating field suppression factors upon electropolishing were investigated using atomic force micrographs. While minor changes in magnetic field enhancement and superheating field suppression factors are observed for the 300 and $600°\mathrm{C}$ heat-treated Nb, substantial improvements are observed for N-doped Nb. In this system, the most severe topographic defects are the grain boundary grooves which substantially suppress the superheating field. We find that the severity of topographic defects is related to the N-doping process.

A 10 GHz ECR ion source (PK-GANESA) with a new magnetic field topology was developed in a GANIL-Pantechnik collaboration. The performance of this source is analyzed through simulations of electron trajectories over a time of $20\mathrm{\mu }\mathrm{s}$ to ensure both rf-heating and magnetic confinement, using the code trapcad. The electron energy distribution functions obtained from the simulations are characterized with respect to radio frequency, heating power, and simulation time. The results are compared with a more traditional 10 GHz ECR source (NANOGANIII) presently used at GANIL. Our study demonstrates an improvement of electron confinement (a factor 10 increase) with increasing rf heating power which should in principle lead to the production of highly charged ions, at variance with the measured production of high-charge-state ions, significantly lower than for the NANOGANIII source. A tentative explanation of the difference between both sources is discussed.

The transverse injection painting scheme is widely adopted in many high-intensity hadron synchrotrons to alleviate space charge effects. As a common issue in such schemes, however, the utilization of the injection bump magnets during multiturn painting process can produce edge focusing effects, causing $\beta$-function beat and even beam loss. For instance, in the recently-built Rapid Cycling Synchrotron (RCS) of China Spallation Neutron Source (CSNS), the edge focusing effect of the dc injection bump magnets breaks the 4-fold symmetry of the RCS. In the upcoming upgrade project of the CSNS, the beam power is planned to increase to 500 kW. For such high-intensity circular accelerators operated with tunes above half-integer, the combined effect of the edge focusing and space charge becomes a key contribution to the beam loss, which should be subject to a strict control. To address this issue, the edge focusing effect of the injection bumps is analyzed via the model of coherent half-integer resonance, from which the stopband harmonics correction and $\beta$-beat correction method are proposed to minimize the emittance growth. To this end, 16 programmable trim quadrupoles are newly installed in the CSNS/RCS for the superperiodicity restoration. The experimental results show that the $\beta$-function is effectively compensated, the beam loss and the emittance growth are significantly reduced.

The horizontal and vertical betatron tunes vary during acceleration in TRIUMF 500 MeV cyclotron, leading the beam to cross the linear coupling resonance ${\nu }_r-{\nu }_z=1$ multiple times. This results in a measurable transfer of betatron amplitude between the horizontal and the vertical planes. In this paper, we report the observation of an unanticipated crossing of this resonance which cannot be understood from the tune diagram derived from the original magnetic field survey of the cyclotron. We present how we have resolved this conundrum and the technique we have developed to measure the vertical tune, which is particularly challenging in a cyclotron. To correct the deleterious effects of this particular crossing, rather than canceling the resonance driving term, a new strategy was adopted whereby the vertical tune is adjusted locally and pushed away from the resonance line. This is accomplished using the axial field of trim coils, while also maintaining the overall isochronism of the machine.

The Fermilab Accelerator Science and Technology facility is currently in operation with its linac based on TESLA-type superconducting rf cavities. Using a 3-MHz micropulse repetition rate with a long macropulse composed of up to 3000 micropulses, and with beam energies demonstrated at 300 MeV and projected to reach 800 MeV with two additional cryomodules, the feasibilities for a vacuum ultraviolet (VUV) and an extreme ultraviolet (EUV) free-electron laser oscillator (FELO) with the two energies are evaluated. We have used both the ginger code with an oscillator module and the minerva/opc code to assess FELO saturation prospects at 120 nm with a 5.0-cm-period undulator of 4.5-m length and the minerva/opc code to assess the FELO at 13.4 nm with adjusted parameters. The simulation results support saturation at both of these wavelengths which are much shorter than the demonstrated shortest wavelength record of 168.6 nm from a storage-ring-based FELO. This indicates superconducting rf linac-driven FELOs can be extended into this VUV-EUV wavelength regime previously only reached with single-pass FEL configurations. In addition, emittance-dilution effects due to wakefields in the cavities and the resulting submacropulse centroid slew effects on FELO performance are addressed using minerva/opc simulations for the first time.

The growing interest in the field of electron irradiation accelerators has led to various research opportunities, with a crucial emphasis on enhancing the irradiation homogeneity of accelerators to advance the industry of the irradiation process. In this study, we introduce a tunable and compact uniform spreading beamline that comprises a permanent magnet quadrupole doublet and an electron beam permanent magnet spreading system. This beamline could solve the problem of poor irradiation homogeneity caused by irregular beam spots in electron irradiation accelerators. Flexible and tunable magnetic yokes enable rapid changes to the magnetic field distribution based on different beam spot profiles or irradiation processing requirements, achieving uniform spreading of different beam spots and improving irradiation homogeneity. Additionally, we evaluated the performance of rectangular permanent magnet poles to minimize errors in future accelerator applications. The implementation of this beamline enhances irradiation quality and provides an experimental foundation for further investigating the interaction between electron beam irradiation rates and materials.

The Compact Linear Collider (CLIC) plans to start operation at a center-of-mass energy of 380 GeV. This paper reports on various optimizations of the CLIC 380-GeV Final Focus System (FFS). Analytical approximations have been used to guide the numerical optimizations. The first optimization consists of the reduction of the vertical beta function at the interaction point to $70\mathrm{\mu }\mathrm{m}$, which is at the limit of the hourglass effect. The second optimization focuses on minimizing the lengths of the final doublet quadrupoles to reduce the chromaticity. The third optimization searched for alternative optics configurations, resulting in the implementation of a novel dispersion profile along the FFS. This alternative optics boosts both total and peak luminosities by 7% and 9%, respectively.

We propose to use tightly focused lasers to generate high quality electron beams in laser wakefield accelerators. In this scheme, the expansion of the laser beam after the focal position enlarges the size of wakefield bubble, which reduces the effective phase velocity of the wake and triggers injection of plasma electrons. This scheme injects a relatively long beam with high charge. The energy spread of the injected beam can be minimized if an optimal acceleration distance is chosen so that the beam chirp is suppressed. Particle-in-cell simulations indicate that electron beams with the charge on the order of nanocoulomb, the energy spread of $\sim 1\%$, and the normalized emittance of $\sim 0.1\mathrm{mm}\mathrm{mrad}$ can be generated in uniform plasma using $\sim 100\mathrm{TW}$ laser pulses. An empirical formula is also given for predicting the beam charge. This injection scheme, with a very simple setup, paves the way toward practical high-quality laser wakefield accelerators for table-top electron and radiation sources.

A single-shot method to reconstruct the transverse self-wakefields acting on a beam, based only on screen images, is introduced. By employing numerical optimization with certain approximations, a relatively high-dimensional parameter space is efficiently explored to determine the multipole components of the transverse-wakefield topology up to desired order. The reconstruction technique complements simulations, which are able to directly describe the wakefield composition based on experimental conditions. The technique is applied to representative simulation results as a benchmark, and also to experimental data on wakefield observations driven in dielectric-lined structures.

To develop an isotope selective computed tomography imaging technique, a flat-laser Compton scattering γ-ray beam (F-LCS) by using a helical undulator installed in a storage ring has been proposed. An LCS beam with a broad energy spectrum and spatial distribution, keeping a small beam size, is preferable for multi-isotope imaging and qualitative evaluation. egs5 simulations assuming the BL1U beamline in UVSOR have been carried out and the result shows that the energy bandwidth of the LCS beam is widened from 2.7% to 22% (FWHM) with a beam size of 2 mm in diameter by varying the undulator $K$ value from 0 to 0.4. A proof of principle (POP) experiment has been carried out in the BL1U, where the APPLE-II undulator has been installed. The $K$ value dependency of the energy spectrum of the generated LCS beam was measured. As a result, a broader energy bandwidth of the LCS beam was observed as the undulator $K$ value increased. The measured energy spectra agreed with the egs5 results.

A harmonic kicker cavity was originally proposed as an ultrafast bunch exchange device between energy recovery linac (ERL) and the circulator cooling ring (CCR) of the Jefferson Laboratory Electron-Ion Collider (JLEIC). The cavity is a transverse deflecting cavity that delivers a sharp kick synthesized using a linear combination of harmonic modes, which can be developed as a normal conducting quarter wave resonator (QWR). We present the development of the QWR as a high-power device (up to 7.5 kW) ready for a beam test: rf design (that involves the rf power optimization, tuning mechanism of five harmonic modes, power coupling), multipacting study, mechanical design with a water cooling system for high power operation, fabrication, and the rf bench tests.

The optimization and advanced study of a laser-plasma electron injector are presented based on a truncated ionization injection scheme for high quality beam production. The smilei code is used with laser envelope approximation and a low number of particles per cell to reach computation time performances enabling the production of a large number of accelerator configurations. The developed and tested workflow is a possible approach for the production of a large dataset for laser-plasma accelerator optimization. A selection of functions of merit used to grade generated electron beams is discussed. Among the significant number of configurations, two specific working points are presented in detail. All data generated are left open to the scientific community for further study and optimization.

This paper presents a new approach to space charge dominated beamline design using an artificial intelligence (AI)-based optimization code named giotto. The code incorporates advanced algorithms for multiobjective genetic optimizations in particle accelerators, allowing efficient exploration of the parameter space and improved beam quality. The study demonstrates the application of giotto in the design of a high-brightness injector for an Energy Recovery Linac (ERL) called BriXSinO. The optimized injector features a low energy (4.5 MeV) and relatively high bunch charge (100 pC) operation. The results show promising beam parameters comparable to other ERL projects. Furthermore, the paper introduces innovative techniques, including bunches back-rotation the use of Lorentzian distributions in the fitness function. The approach successfully achieves dispersion closure in a space charge dominated dogleg. Overall, this work contributes to the advancement of accelerator science, offering a powerful methodology for beamline design and optimization. The new techniques and methodologies introduced have the potential to enhance the performance and stability of particle accelerators in various applications.