In our previous study of bunch lengthening for the planned Hefei Advanced Light Facility (HALF) storage ring, a periodic transient beam loading effect was found in the presence of passive harmonic cavities for some cases, which can dramatically affect the bunch lengthening as well as the beam stability [T. L. He et al., Bunch lengthening of the HALF storage ring in the presence of passive harmonic cavities, in Proceedings of the IPAC2021, Campinas, SP (JACoW, Campinas, SP, Brazil, 2021), pp. 2082–2085, TUPAB265]. In this paper, we continue to use HALF as an example, via both tracking simulation and semianalytical calculation, to comprehensively study the characteristics of this periodic transient effect, analyze its dependence on some relevant factors and investigate the impact of fill pattern on its periodicity. It is found that there exits a threshold current for this periodic transient effect, which can be increased by the reduction of $R/Q$ of harmonic cavity (HC) and the increase of main cavity (MC) voltage, HC detuning, momentum compaction, and energy spread, as well as the inclusion of short-range wake. In addition, the periodicity of bunch centroid oscillation can be affected by the damping time and MC beam loading, and completely interrupted due to the nonuniform fill pattern, but this is not helpful for improvement of bunch lengthening.

Typically, in self-Amplified spontaneous emission free electron laser (SASE FEL) based short-pulse schemes, pulse duration is limited by FEL coherence time. For hard x-ray FELs, coherence time is in a few hundred attosecond range while for XUV and soft x-ray FELs it is in the femtosecond regime. In this paper the modification of so-called chirp-taper scheme is developed that allows to overcome the coherence time barrier. Numerical simulations for XUV and soft x-ray FEL user facility FLASH demonstrate that one can generate a few hundred attosecond long pulses in the wavelength range 2–10 nm with peak power reaching hundreds of megawatts. With several thousand pulses per second this can be a unique source for attosecond science.

For a precise determination of the rf properties of superconducting materials, a calorimetric measurement is carried out with the aid of a so-called quadrupole resonator (QPR). This procedure is affected by certain systematic measurement errors with various sources of uncertainties. In this paper, to reduce the impact of geometrical uncertainties on the measurement bias, the modified steepest descent method is used for the multiobjective shape optimization of a QPR in terms of an expectation measure. Thereby, variations of geometrical parameters are modeled by the polynomial chaos expansion technique. Then, the resulting Maxwell’s eigenvalue problem with random input data is solved using the polynomial chaos-based stochastic collocation method. Furthermore, to assess the contribution of the particular geometrical parameters, the variance-based sensitivity analysis is proposed. This allows for modifying the steepest descent algorithm, which results in reducing the computational load needed to find optimal solutions. Finally, optimization results in the form of an efficient approximation of the Pareto front for a three-dimensional model of the QPR are shown.

Short bunch proton beams are of great significance for the applications of white neutron beams and muon beams. The accelerator complex of the China Spallation Neutron Source (CSNS) was designed to support the applications mainly based on neutron scattering techniques where the proton pulse length is not very sensitive. Some theoretical and experimental studies have been performed to see if one can extract a short-bunch proton beam by bunch rotation from the rapid cycling synchrotron (RCS) at CSNS. The experimental results at RCS have evidently displayed the bunch lengthening and rotation process, which demonstrates the effectiveness of this method even with a very short available time for the rf gymnastic processes and a high-intensity beam. With a beam power of 50 kW and normal longitudinal emittance at the injection, the proton beam with a bunch length of about 53% with respect to the one in the normal operation mode was obtained and transported to the spallation target. With a reduced longitudinal emittance at injection and the beam power of 30 kW, the shortest extraction bunch length obtained is about 26% of the one in the normal operation mode. Different machine settings have also been tested to show the impact of the desynchronization between the rf and magnetic fields, the influence of the nonadiabatic rise time, and the adiabatic decay time of the rf voltage on the extraction bunch length. The experimental results are well consistent with the theoretical and simulated ones. It is interesting to observe that space charge has a beneficial effect on the bunch lengthening which will result in a shorter bunch at the extraction with the later bunch rotation. The controlled desynchronization method between the rf and magnetic fields in an RCS was also proven successful.

The coupled-bunch instability for arbitrary multibunch configurations is investigated in its generality. The theoretical framework adopted is based on the general eigenvalue analysis based on the known formulas of the complex frequency shifts for the uniform multibunch configuration case. Closed formulas are derived for special cases. For the configuration consisting of a uniform filling pattern with a gap of missing bunches, the theoretical results are found to be in good agreement with measurements of the transverse coupled-bunch instability driven by the resistive wall impedance performed at the NSLS-II storage ring.

Recent studies have shown a novel coherent head-tail instability induced by beam-beam interaction with a large Piwinski angle. The instability has become an important issue during the designs of CEPC and FCC-ee. Simulations have further revealed that the longitudinal impedance has a strong impact on the beam stability, squeezing the horizontal stable tune area seriously. The cross-wake force has been introduced to represent beam-beam interaction. A mode coupling theory based on the localized wake (impedance) force has been developed to explain the instability. However, the theory did not consider the effects of longitudinal impedance. In this paper, we develop a new transverse mode coupling analysis method that could be used to study the beam-beam instability with and without longitudinal impedance. The result shows that the distortion of longitudinal phase space trajectory and the incoherent synchrotron tune shift induced by longitudinal impedance would reduce the stable tune area.

Laser-triggered ionization injection is a promising way of generating controllable high-quality electrons in plasma-based acceleration. We show that ionization injection of electrons into a fully nonlinear plasma wave wake using a laser pulse comprising of one or more Laguerre-Gaussian modes with combinations of spin and orbital angular momentum can generate exotic three-dimensional spatial distributions of high-quality relativistic electrons. The phase dependent residual momenta and initial positions of the ionized electrons are encoded into their final phase space distributions, leading to complex spatiotemporal structures. The structures are formed as a result of the transverse (betatron) and longitudinal (phase slippage and energy gain) dynamics of the electrons in the wake immediately after the electrons are injected. Theoretical analysis and three-dimensional simulations verify this mapping process leads to the generation of these complex topological beams. These beams may trigger novel beam-plasma interactions as well as produce coherent radiation with orbital angular momentum when sent through a resonant undulator.

We present design and realization of an ultrabroadband optical spectrometer capable of measuring the spectral intensity of multioctave-spanning light sources on a single-pulse basis with a dynamic range of up to eight orders of magnitude. The instrument is optimized for the characterization of the temporal structure of femtosecond long electron bunches by analyzing the emitted coherent transition radiation spectra. The spectrometer operates within the spectral range of 250 nm to $11.35\mu \mathrm{m}$, corresponding to 5.5 optical octaves. This is achieved by dividing the signal beam into three spectral groups, each analyzed by a dedicated spectrometer and detector unit. The complete instrument was characterized with regard to wavelength, relative spectral sensitivity, and absolute photometric sensitivity, always accounting for the light polarization and comparing different calibration methods. Finally, the capability of the spectrometer is demonstrated with a coherent transition radiation measurement of a laser wakefield accelerated electron bunch, enabling to determine temporal pulse structures at unprecedented resolution.

The electron gun of the REXEBIS charge breeder at the REX/HIE-ISOLDE facility at CERN has been upgraded from a standard magneto-immersed type to a gun using a nonadiabatic magnetic element. The results from the cathode emission and electron beam propagation tests are presented, as well as the charge breeding efficiency for the new design. Complete mass-scans of the extracted beam have been performed from which the level of cathode-originating contaminations could be established, as well as partial pressures of the most abundant residual gases in the ion trapping region. Furthermore, optimal breeding times for a broad range of elements and charge states, either introduced as a gas or externally injected as singly charged ions into the trapping region, are given for different electron currents. From these values, effective electron current densities have been derived. Finally, the axial ion energy distributions of various elements and charge states were also measured, and the derived ion temperatures were correlated with the ion and electron beam overlap factors.

When a relativistic beam moves along a curved trajectory it loses energy on coherent radiation. The effect of this radiation on the beam dynamics is described by the so call coherent synchrotron radiation (CSR) longitudinal wake field. Among different models of the CSR wake, the simplest is the one-dimensional (1D) one where the beam is treated as a line-charge. The self-field of the beam on a curved trajectory also creates a transverse component of the force. Unfortunately, there is a confusion in the literature as to whether a 1D model can be worked out for the transverse force inside the beam. In this paper, we show how such a 1D model can be consistently derived for a general curvilinear beam orbit if one formulates the equations of motion for the beam particles in Hamiltonian form and uses a renormalized transverse force. Unfortunately, this scheme cannot be applied to the classical problem of relativistic beam passing through a single bending magnet, because the scalar potential in the initial conditions for the Hamiltonian variables cannot be defined in the 1D model. In this case, one should use the 3D transverse force, for which we derive analytical expressions. For the steady state, our calculations show an excellent agreement with computer simulations. We also calculate the transient effects for this force at the entrance to and the exit from the bend. Our results provide a new way to accommodate the transverse force into the existing and new simulation codes which is important for many applications of the high-current, small-emittance relativistic beams.

We demonstrate experimentally that hydrodynamic optical-field-ionized (HOFI) plasma channels can be generated at kHz-scale pulse repetition rates, in a static gas cell and for an extended period. Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated 1 ms apart are essentially the same. We demonstrate that HOFI channels can be generated at a mean repetition rate of 0.4 kHz for a period of 6.5 h without degradation of the channel properties, and we determine the fluctuations in the key optical parameters of the channels in this period. Our results suggest that HOFI and conditioned HOFI channels are well suited for future high-repetition rate, multi-GeV plasma accelerator stages.

A long-sought-after goal for photocathode electron sources has been to improve performance by temporally shaping the incident excitation laser pulse. The narrow bandwidth, short wavelength, and picosecond pulse duration make it challenging to employ conventional spectral pulse shaping techniques. We present a novel and efficient intensity-envelope shaping technique achieved during nonlinear upconversion through opposite-chirp sum-frequency mixing. We also present a numerical case study of the linac coherent light source-II photoinjector where transverse electron emittance is improved by at least 25%.

Off-line high-power tests of the fundamental power couplers prior to their on-line operations are of importance for ensuring their operating reliability and stability. To test the couplers using a limited power supply effectively, a movable standing wave (SW) resonant test system with an extraordinary power gain has been developed at Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). The system consists of a movable resonator having two movable shorts for the enhancement and movement of the SW field, and a tunable secondary coupler for feeding power into the resonator without reflection. The proof-of-principle structure of the system has been built and tested both at the low-power and at the high-power levels. The low-power test demonstrates that the moving range of the SW resonant field is over half a wavelength which ensures that the fundamental couplers can be tested by SW field at all reflected phases, and the system can provide a power gain ranging from 50 to 94, corresponding to 200–376 ($4\times 50–4\times 94$) of power gain in the case of traveling wave resonant ring system. Two types of multipacting inside the couplers were observed during the high-power tests and the mechanism of their influences on the power gain was analyzed. This movable and high-power-gain solution can be beneficial for the promotion of SW resonant test systems for fundamental couplers.

The low energy RHIC electron cooling (LEReC) system is the world’s first electron cooler utilizing radio frequency (rf) accelerated electron bunches, and a nonmagnetized electron beam. It is also the first electron cooler applied directly to colliding hadron beams. The unique approach to cooling makes beam dynamics in LEReC very different from the conventional electron coolers. Numerous LEReC parameters can affect the cooling rate. One of the most critical factors is the alignment of the electron and ion trajectories in the cooling section. In this work, we apply Bayesian optimization to check and if needed to optimize the trajectories’ alignment. Experimental results are presented and it is demonstrated that machine learning (ML) methods can be applied to perform the control tasks effectively in the RHIC controls system.

Active plasma lenses are compact devices developed as a promising beam-focusing alternative for charged particle beams, capable of short focal lengths for high-energy beams. We have previously shown that linear magnetic fields with gradients of around 0.3 kT/m can be achieved in argon-filled plasma lenses that preserve beam emittance [C.A. Lindstrøm et al., Phys. Rev. Lett. 121, 194801 (2018)]. Here we show that with argon in a $500\mu \mathrm{m}$ diameter capillary, the fields are still linear with a focusing gradient of 3.6 kT/m, which is an order of magnitude higher than the gradients of quadrupole magnets. The current pulses that generate the magnetic field are provided by compact Marx banks, and are highly repeatable. The demonstrated operation with simultaneously high-gradient, linear fields and good repeatability establish active plasma lenses as an ideal device for pulsed particle beam applications requiring very high focusing gradients that are uniform throughout the lens aperture.

Generation of tunable terahertz radiation based on the interaction of a frequency-chirped low power terahertz wave with a relativistic electron beam is proposed. The electron beam is first modulated temporally due to the ponderomotive force induced by the beat wave of two copropagating intense laser beams of slightly different frequencies. Then the electron beam interacts with a low-amplitude driver wave at terahertz frequency with a linear frequency chirping. A nonlinear transverse current density is driven that emits terahertz radiation at an up-shifted frequency with a relatively enhanced amplitude. The modulation process is analyzed theoretically by a complete set of fluid equations and considering the effects of both the electrons space-charge potential and the beat wave ponderomotive force. The initial density and energy of the electron beam are both optimized in order to improve the terahertz amplification process. The effect of the driver’s frequency chirping, as well as the effects of the initial electron beam density and energy on the emitted terahertz radiation are investigated numerically. It is shown that the frequency chirping of the driver wave plays a key role in terahertz amplification. It is shown that there is an optimum chirp value for which the terahertz radiation is amplified significantly compared with the case without chirping. Moreover, transverse effects of the laser beams and the driver wave on the emitted terahertz radiation are also investigated.

We discuss the choice of the magnetic lattice and parameter optimization of the fourth-generation light source SKIF (Russian acronym for Siberian Circular Photon Source) under construction in Novosibirsk. The study compares several basic lattice cells to procure one with low emittance and large dynamic aperture. The result is a developed lattice of SKIF with 72 pm natural emittance (at zero beam current and absent betatron coupling) at 3 GeV beam energy and 476 m circumference. Only two families of sextupoles provide horizontal and vertical dynamic apertures of 12 mm and 3.5 mm, respectively, and energy acceptance more than 5%. To check the potential of the found solution, we used slightly modified SKIF lattice to design 6 GeV light source and reached 33 pm natural emittance for 1075 m circumference with 40 straight sections. Again, only two sextupole families ensure sufficient dynamic aperture (7 mm horizontal and 4 mm vertical) and energy acceptance more than 5% for simple efficient injection and sufficient beam lifetime.

A numerical method to design nonlinear double- and multibend achromat (DBA and MBA) lattices with approximate invariants of motion is investigated. The search for such nonlinear lattices is motivated by Fermilab’s Integrable Optics Test Accelerator, whose design is based on an integrable Hamiltonian system with two invariants of motion. While it may not be possible to design an achromatic lattice for a dedicated synchrotron light source storage ring with one or more exact invariants of motion, it is possible to tune the sextupoles and octupoles in existing double- and multibend achromat lattices to produce approximate invariants. In our procedure, the lattice is tuned while minimizing the turn-by-turn fluctuations of the Courant-Snyder actions $J_x$ and $J_y$ at several distinct amplitudes, while simultaneously minimizing diffusion of the on-energy betatron tunes. The resulting lattices share some important features with integrable ones, such as a large dynamic aperture, trajectories confined to invariant tori, robustness to resonances and errors, and a large amplitude-dependent tune spread. Compared to the nominal National Synchrotron Light Source-II lattice, the single- and multibunch instability thresholds are increased and the bunch-by-bunch feedback gain can be reduced.

In dielectric laser-driven accelerators (DLA), careful tuning of drive-laser wavelength and structure periodicity is typically required in order to hit the resonant condition and match the phase velocity of the accelerating wave to the electron beam velocity. By aggressively detuning (up to 30 mrad) the angle of incidence of the drive laser on a double grating DLA structure, we show that it is possible to recover resonant phase matching and maximize the energy modulation of an externally injected 6 MeV beam in an 800 nm period structure driven using a 780 nm laser. These results show that it is possible to power DLA structures away from their design working point, and excite accelerating fields in the gap with phase profiles that change by a relatively large amount period-to-period. This flexibility is a key feature of DLAs and a critical element in the realization of phase modulation–based ponderomotive focusing to demonstrate MeV energy gain and large capture in a single DLA stage.