A compact X-band correction cavity chain (CC) has been successfully designed, fabricated and high power tested for the Compact Linear Collider (CLIC) project. The CC consists of eight spherical cavities and four polarizers, and each spherical cavity has a $Q_0$ of 45 000 and a diameter of 7 cm. The compact CC can correct a decaying pulse shape generated by a storage cavity without need of low efficiency pulse modulation or a solution based on long delay lines. A flattop output with small ripples can be obtained if the CC and the storage cavity are matched. Experiments of the CC with a barrel open cavity (BOC) were conducted on the XBOX2 test facility at CERN, and the high power results match the theoretical calculations. The appliance of the CC only reduces the pulse compression efficiency by 6% relatively if the transmission attenuation is ignored.

Interaction of the beam with the fundamental impedance of the accelerating cavities can limit the performance of high-current accelerators. It can result in a significant variation of bunch-by-bunch parameters (bunch length, synchronous phase, etc.) if filling patterns contain gaps that are not negligible compared with the cavity filling time. In the present work, this limitation is analyzed using the steady-state time-domain approach for the high-current option (Z) of the future circular electron-positron collider and for the future circular hadron-hadron collider. Mitigation of transient beam loading by direct rf feedback is addressed with evaluation of additional required generator power.

Charge lifetime of strained superlattice Gallium Arsenide photocathodes in dc guns is limited by ion back bombardment. In this work, we propose and present simulation results for an offset anode scheme to increase charge lifetime in dc guns. In this scheme, the axial symmetry of the cathode-anode configuration is broken by offsetting the anode transversely, while keeping the laser spot on the electrostatic center. This eliminates the bombardment of high energy ions on the cathode and enables maximum usage of the available cathode area. Depending on the size of the available cathode area, this method can increase the charge lifetime by an order of magnitude compared to the current best alternative method. An anode assembly, capable of in-vacuum movement, is required for this method, which has been designed and fabricated at Brookhaven National Laboratory.

We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between $0.64\times {10}^{10}$ and $9.6\times {10}^{10}$. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population provide information on cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on fitted ring-averaged secondary-yield properties of the vacuum chamber predicts tune shifts in good agreement with the measurements.

Precise manipulation of high brightness electron beams requires detailed knowledge of the particle phase space shape and evolution. As ultrafast electron pulses become brighter, new operational regimes become accessible with emittance values in the picometer range, with enormous impact on potential scientific applications. Here we present a new characterization method for such beams and demonstrate experimentally its ability to reconstruct the 4D transverse beam matrix of strongly correlated electron beams with subnanometer emittance and submicrometer spot size, produced with the HiRES beamline at Lawrence Berkeley National Laboratory. Our work extends the reach of ultrafast electron accelerator diagnostics into picometer-range emittance values, opening the way to complex nanometer-scale electron beam manipulation techniques.

We analyze the wave field of a charged particle bunch moving in a circular waveguide consisting of two parts: the first one has a corrugated wall and the other one has a smooth wall. It is assumed that the period and the depth of the corrugation are much less than the waveguide radius and wavelengths under consideration. The influence of corrugation on the electromagnetic field is analyzed using the method of equivalent boundary conditions (EBC). The cases of the bunch moving in both directions inside of such a waveguide are considered. We obtain analytical expressions for the main part of the field having discrete spectrum. Figures of a typical mode distribution of radiation are presented. We also perform numerical simulations in software package CST Particle Studio. Comparison of analytical and numerical results confirms the validity of the EBC method.

The angular drift of a laser beam is of particular concern in applications such as seeded free-electron lasers. A systematical study of the obliquely incident laser and electron beam interaction in an undulator is presented in this paper. Theoretical analysis and numerical simulations demonstrate that the interaction would imprint energy and angular modulations on the electron beam simultaneously. Compared with the normally incident pattern, the obliquely incident laser-electron interaction leads to reductions in the bunching factors of HGHG and EEHG. In the meanwhile, proactive applications of this multidimensional modulation technique may bring vitality to the field of laser-electron manipulation.

A plasma channel undulator/wiggler may be created through the plasma wakefield excited by the beating of several Hermite-Gaussian laser modes propagating in a parabolic plasma channel. Control over both the betatron and undulator forces is conveniently achieved by tuning the amplitude ratios, colors, and order numbers of the modes. A special structure of the undulator/wiggler field without the focusing force near the propagation axis is generated inside the plasma wakefield by matching the strengths of the fundamental and first-order Hermite-Gaussian modes. The electron beam only experiences forced undulator oscillations in such a field, which significantly improves the quality of the emitted radiation. Since the value of the undulator strength parameter could be in a wide range, less or larger than unity, it is capable of generating narrow bandwidth x-ray, as well as the synchrotronlike high-energy $\mathrm{x}/\gamma$-ray, radiation by harmonics. Additionally, controlling the relative phases between the laser modes allows for polarization control of the plasma undulator. High-order harmonics produced from a circularly polarized plasma undulator clearly show the vortex nature and carry well-defined orbital angular momentum.

The SuperKEKB injector linac that includes the positron damping ring (DR) delivers low emittance electron and positron beams to the main rings. The beam injection phase from linac to the rings should be stabilized for long-term stable beam injection. The frequencies of the linac master oscillator (MO) and ring MO are 571.2 and 508.9 MHz, respectively. These two different frequency signals are monitored using the direct sampling technique with a common sampling rate. A phase drift of several degrees is observed between the linac and ring MOs. For stable beam injection, a phase drift compensation system is implemented at linac. The system performance was evaluated by the beam energy stability after the bunch compression system from DR to linac.

This paper addresses the generalization of the single-particle betatron resonance condition derived by Courant and Snyder more than a half century ago. A two-dimensional resonance condition including the effect of space-charge interaction was recently conjectured from one-dimensional Vlasov predictions made by Sacherer, Okamoto, and Yokoya [K. Ito et al., Phys. Rev. Accel. Beams 20, 064201 (2017)]. The condition is remarkably simple which only contains a few measurable quantities and indicates the possibility that twice as many resonance stop bands as expected from the conventional incoherent picture may exist at high beam density. Self-consistent multiparticle simulations are performed systematically to locate low-order stop bands in the tune diagram. The proposed betatron resonance formula is shown to explain the basic feature of numerical observations, which suggests that no serious incoherent resonance is activated inside the phase-space core of a dense beam matched to the external linear focusing potential. It is confirmed that the coherent tune-shift factor of any collective mode is less than unity and practically considered as a constant over the whole tune space in a typical high-intensity storage ring. The procedure for finding the optimum operating point of the ring is discussed on the basis of the coherent picture instead of the commonly used picture relying on the concept of incoherent tune spread. Despite years of theoretical efforts by many researchers, the coherent resonance concept is still not being employed for the construction of a stability tune diagram. We here provide a simple prescription to draw the diagram quickly. The present study also indicates the possibility of complete suppression of emittance exchange on particular difference resonances by choosing a proper emittance ratio.

An experimental study on the radiography of various metallic and biological plant samples with a minimum resolution of $\sim 75\mu \mathrm{m}$ using relativistic electron beams from the laser plasma accelerator is presented. Electron beams were generated from the interaction of Ti:sapphire laser pulses of 120 fs duration with underdense plasma of 4 mm length, created using Ar and He gas jets, with two different parameters, i.e., a maximum energy of $\sim 50\mathrm{MeV}$ (broad spectrum) and a quasimonoenergetic beam with a peak energy of $\sim 150\mathrm{MeV}$. geant4 Monte Carlo simulations were performed to reconstruct the radiographic images and were found to be consistent with the experimental results. Radiography of biological plant samples was also performed, and in this regard penetration depths of two types of electron beams in adipose tissue were estimated through a simulation. The effect of different electron beam parameters on the quality of the radiograph has been discussed.

The production of electron bunches with low transverse emittance approaches the thermal emittance of the photocathode as various aberrations are corrected. Recently, the coupled-transverse-dynamics aberration was theoretically identified as a significant source of emittance growth, and a corrector magnet was proposed for its elimination [Phys. Rev. Accel. Beams 21, 010101 (2018)]. This aberration arises when the beam acquires an asymmetric distribution that is then rotated with respect to the transverse reference axis, thus introducing a correlation in the vertical and horizontal planes. The asymmetry is introduced by a weak quadrupole field in the rf gun or emittance compensation solenoid and the rotation is caused by the solenoid. This manuscript presents an experimental study of the coupled-transverse-dynamics aberration in an rf photoinjector and demonstrates its elimination by a quadrupole corrector consisting of a normal and a skew quadrupole. The experimental results agree well with theoretical predictions and numerical simulations, showing that the corrected emittance could reach the thermal value for an electron bunch with a low charge and a short length.

This article is an extended version of the talk is given at the IPAC19 (Melbourne, Australia, May 2019) on the occasion of acceptance of the ACFA/IPAC19 Nishikawa Tetsuji Prize for a recent, significant, original contribution to the accelerator field, with no age limit with citation “…for original work on electron lenses in synchrotron colliders, outstanding contribution to the construction and operation of high-energy, high-luminosity hadron colliders and for tireless leadership in the accelerator community.”

This contribution compiles the benefits of lattice symmetry for global closed orbit correction, which relies on the decomposition and inversion of the orbit response matrix (ORM). A symmetric arrangement of beam position monitors (BPMs) and correctors results in structured ORMs of circulant or block-circulant type. These structured matrices provide favorable properties in terms of robustness of the ORM inversion against modeling errors due to information compression and a transparent interpretation of analytical vector spaces of BPMs and correctors as well as reduced computational complexity for inversion in the case of larger ORMs. A nearest-circulant approximation is introduced for the extension of these benefits for near-symmetric lattices. A relation between harmonic orbit correction and singular value decomposition (SVD)-based orbit correction is presented. The practical advantages of symmetry exploitation are demonstrated with the help of simulations and experiments in the context of synchrotrons of Facility for Antiproton and Ion Research (FAIR).

In the main ring (MR) of the Japan Proton Accelerator Research Complex (J-PARC), the power supplies of magnets induce large ripples in the betatron tunes. The tune ripples degrade the quality of the beam spill on the slowly extracted beam. To improve the spill quality, the J-PARC MR employed a feedback system using a beam spill monitor; however, the existing system could not correct the beam spills suitably, since the tune ripples were too large. We developed a new correction system using the information of magnet currents as an input of the correction. The new system consists of current monitors and a corrector quadrupole, and it allows us to correct the ripples in real time. Using this system, we successfully corrected the tune ripples for the first time in the J-PARC MR. The correction system improved the spill quality from 3% (without any correction) to 30% (with the new correction), whereas the quality using the present feedback system is 17.4%.

At SwissFEL, a fully noninvasive characterization of the beam energy distribution can be performed by means of a synchrotron radiation monitor (SRM) imaging in the visible the transverse profile of the electron beam in a magnetic chicane. Under conditions of off-crest acceleration in the SwissFEL injector, according to a first order Taylor series expansion, the distribution of the relative energy of the electron beam can be linearly expressed as a function of the distribution of the electron longitudinal coordinates via the coefficient of the rf induced beam energy chirp. It is hence possible to express the distribution of the electron longitudinal coordinates at the entrance of the magnetic chicane of SwissFEL as a function of the distribution of the dispersed electron trajectories in the horizontal plane of the magnetic chicane. Machine parameters and instrument data at SwissFEL can be beam-synchronously acquired. A shot-by-shot correlation between the analysis results of the SRM camera images and the linear coefficient of the beam energy chirp resulting from the rf parameters of the injector can be hence established. Finally, taking into account the compression factor of the magnetic chicane, the electron bunch length at the exit of the magnetic chicane can be expressed as a function of the horizontal size of the electron beam imaged by the SRM. Bunch length measurements performed by means of the SRM in the first magnetic chicane of SwissFEL will be presented.

We report on the effect of laser spatiotemporal coupling in laser plasma accelerators. Pulse front tilt in the driving laser causes asymmetry in the wakefield, resulting in deflection of the electron beam from the laser axis. We explore the physical mechanisms and propose a quantitative model of electron steering, which is validated with particle-in-cell simulations. Even a small amount of pulse front tilt can result in beam steering in the final down ramp of the plasma profile, which may lead to unexpected beam-pointing errors or fluctuations. On the other hand, it can be used to govern the final beam direction, which has consequences for staging laser plasma accelerators in a high-energy physics collider as well as x-ray generation for biological imaging.