With the rapid development of laser plasma accelerator, multiple functional beamlines have been designed to transport such pulsed particle beam with large energy spread and divergence angle. Accordingly, the demand for optimal design and online feedback of laser beamlines has also been put forward. We have developed a graphics processing unit–accelerated multiparticle beam transport simulation algorithm based on the open source project “HPSim” to meet the above need. The algorithm implements a series of functions such as the simulation of solenoids, the simulation of exponentially decreasing energy spectrum beams, and cross-programming and cross-language calls. In terms of computational efficiency, the algorithm is approximately 20 times more efficient than multiparticle simulation program tracewin, meeting the simulation time-consumption requirement of a hundred milliseconds for virtual accelerator software. The algorithm’s monoenergetic and exponentially decreasing energy spectrum beam simulation capabilities are verified in the beamline design for CLAPA-I and CLAPA-II at Peking University. The algorithm can be applied not only to virtual accelerator for real-time beam transport simulations but also to fast and efficient beamline magnets optimization design.

Steady-state microbunching (SSMB) storage rings are an appealing option for high-power extreme ultraviolet (EUV) light sources. In this paper, we propose a generalized longitudinal strong focusing (GLSF) scheme to boost the average power of short-wavelength radiation from an SSMB source with a technical demand within the present reach. This scheme employs transverse-longitudinal coupling dynamics to produce steady-state ultrashort bunches and reduces the required modulation laser power significantly by exploiting the low vertical beam emittance in a planar ring. Linear beam dynamics are studied in detail, including bunch compression, modulation cancellation, and vertical-longitudinal decoupling. An instance of a linear lattice layout is given, and kW-level quasicontinuous-wave 13.5-nm EUV radiation can be achieved in a GLSF SSMB storage ring with 1-MW modulation laser power. We believe that the GLSF scheme can also be used in conventional storage rings for various purposes concerning beam manipulation.

This study investigated an unknown instability in the crab-crossing scheme, which is caused by the interaction between beam loading on crab cavities and the coherent beam-beam force at the interaction point (IP). We developed an analysis method by formulating the transfer functions of the beam loading on the crab cavities and the related rf control loops, considering the beam-beam force on the colliding beams. The analysis was first applied to simple cases with no control loops to demonstrate the intrinsic stability of crab cavities under the beam loading and the beam-beam force, which corresponds to the Robinson stability for accelerating cavities. Furthermore, analysis with the control loops was applied to analyze unexpected oscillation phenomena observed in the crab-crossing operation in KEKB. The results obtained from the analysis were consistent with the observed phenomena at the following points: First, a time-domain simulation based on this analysis reproduced the observed coherent oscillation. Second, the relative orbit displacement between the colliding beams estimated in the analysis of the measured oscillation amplitude was consistent with the saturation effect of the beam-beam force at the IP. Third, rf-related parameter dependencies of stability obtained in the analysis were consistent with those in the measurements. The mechanism investigated in this study explained its essential function in the instability observed during the KEKB crab-crossing operation.

This paper discusses the trends and tradeoffs between transverse ${\sigma }_x$ and longitudinal ${\sigma }_z$ bunch dimensions, rf injector gradient, bunch charge, and intrinsic electron mean transverse energy (MTE), where all can be chosen to be independent, and the resulting effects on emittance and transverse brightness. Using a practical example of a quarter wave normal conducting photoinjector, it is computationally found that regardless of MTE and bunch charge, there is a universal relation between the gradient $E$ and the aspect ratio of the bunch (${\sigma }_x/{\sigma }_z$) leading to the highest brightness. This computational result is understood using an analytical formalism consisting of Kim’s emittance formulation and a two-dimensional space charge model. The results, obtained computationally and interpreted in a robust physical framework, could therefore provide the basis for an express mapping approach for emittance forecasting when used with practical injector system design requirements and limitations.

Beamstrahlung is a dominant effect in the beam dynamics of the high luminosity next-generation lepton collider FCC-ee. We characterize the beamstrahlung radiation for the beam parameters at the four working energies, from the Z-pole to the $t\overline{t}$ threshold, and present the effect of this radiation in the machine-detector-interface region. We discuss the conceptual need for a photon dump due to the high power produced, which is in the order of hundreds of kilowatts. We also discuss the detector induced backgrounds due to the incoherent $e^{+}{e}^{-}$ pairs produced by the interaction of the beamstrahlung photons at the interaction point.

In recent $e^{+}{e}^{-}$ colliders, a collision scheme with a large crossing angle has been designed and adopted. In two colliding beams, mode coupling between beam-beam oscillation modes is discussed in this paper. The typical $\sigma$ and $\pi$ modes are extended to include longitudinal motion. The tune shifts of the $l=0$ and $l=\pm 1$ modes are presented explicitly. The behavior of beam-beam modes is analyzed for different Piwinski angles. The instability due to the vacuum pipe impedance is discussed in terms of mode coupling between these beam-beam modes. Beam-beam simulations based on the strong-strong model have shown that a vertical beam-beam instability is induced by crosstalk between the beam-beam collision and the impedance. It is shown that the mode coupling theory for colliding beams reproduces the results of the beam-beam simulation. The impact of hourglass effects on the instability is also studied by simulation.

In this paper we discuss a compact, laser-plasma-based scheme for the generation of positron beams suitable to be implemented in an all-optical setup. A laser-plasma-accelerated electron beam hits a solid target producing electron-positron pairs via bremsstrahlung. The back of the target serves as a plasma mirror to in-couple a laser pulse into a plasma stage located right after the mirror where the laser drives a plasma wave (or wakefield). By properly choosing the delay between the laser and the electron beam the positrons produced in the target can be trapped in the wakefield, where they are focused and accelerated during the transport, resulting in a collimated beam. This approach minimizes the ballistic propagation time and enhances the trapping efficiency. The system can be used as an injector of positron beams and has potential applications in the development of a future, compact, plasma-based electron-positron linear collider.

Beam-driven wakefield accelerators are foreseen to enable compact accelerator-based light sources and play a critical role in future linear-collider concepts. This class of wakefield acceleration has been extensively studied over the last four decades with a focus on demonstrating its ability to support high-accelerating gradient and, most recently, enhanced transformer ratios. Yet, the associated detrimental transverse wakefields have not been examined in as many details due to the limited diagnostics available. In this paper, we introduce a beam-based single-shot transverse-wakefield measurement technique. The approach employs a witness “hollow” electron beam to probe the wakefields generated by a drive bunch. We show how the transverse distortions of the hollow probe provide a direct measurement of the wakefield distribution within the area circumscribed by the probe. The ability to directly measure a full structure of the transverse wakefield could help to develop mitigation schemes and ultimately suppress the adverse beam-break-up instabilities. We discuss a practical implementation of the method and demonstrate its performance with the help of start-to-end simulations.

Niobium superconducting cavity performances are limited by the quench magnetic field of niobium. Maximum achievable accelerating field is estimated by simulation during the conception phase. The full geometry of the cavity is implemented but no roughness is taken into consideration. However, several experiences showed that the roughness can increase locally the magnetic field, leading to an effective peak magnetic field on the surface higher than expected by simulation. It can be at the origin of premature quench. The present article presents simulations of grains and defects with different sizes and its effect on the local magnetic peak field. The values implemented in the model are taken from actual surface measurements made by confocal microscopy on representative Nb samples prepared with usual surface treatments. Cut-out of fabrication defects and some thin films have been also explored. As expected, the mean roughness brought by usual chemical polishing as well as electropolishing surface treatment does not increase significantly the local field. But calculation based on actual values sometimes encountered in the surface measurement, or on specific defects, can reach very high field enhancement and can explain premature quench on some cavities within an otherwise good performing production batch.

Current experiments investigating radiation reaction employ high energy electron beams together with tightly focused laser pulses in order to reach the quantum regime, as expressed through the quantum nonlinearity parameter $\chi$. Such experiments are often complicated by the large number of latent variables, including the precise structure of the electron bunch. Here we examine a correlation between the electron spatial and energy distributions, called an energy chirp, investigate its significance to the laser-electron beam interaction and show that the resulting effect cannot be trivially ignored when analyzing current experiments. In particular, we show that the energy chirp has a large effect on the second central moment of the electron energy, but a lesser impact on the first electron energy moment or the photon critical energy. These results show the importance of improved characterization and control over electron bunch parameters on a shot-to-shot basis in such experiments.

Tsinghua University is currently developing a very-high-frequency electron gun that operates in continuous-wave (cw) mode at 216.667 MHz. This gun will serve as the beam source for the Shanghai high repetition rate XFEL and extreme light facility. The cavity profile has been optimized to minimize input power, peak surface electric field, and peak wall power density. The profile optimization also takes into account the suppression of multipacting effect. Additionally, thermal analysis has been performed to guide the design of water cooling channels in coordination with gun mechanical design. The fabrication of the gun has been completed, and the frequency and quality factor measured in cold test are in good agreement with simulation expectations. During high power conditioning, 75 kW cw radio frequency power was successfully fed into the gun, corresponding to a cathode gradient of $27\mathrm{MV}/\mathrm{m}$ and a gun voltage of 780 keV. This operating gradient is currently the highest in the world for room-temperature VHF guns. Under this condition, the maximum dark current collected by the Faraday cup at the gun exit was 376 nA when the strength of the gun solenoid was scanned. To measure and optimize the beam quality, a test beamline was constructed. After preliminary optimization, the 95% projected transverse emittance was 0.161 mm mrad for 10 pC bunches with a bunch length of 0.49 mm rms, 0.429 mm mrad for 50 pC bunches with a bunch length of 1.15 mm rms, and 0.853 mm mrad for 100 pC bunches with a bunch length of 1.44 mm rms.

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.