Field emission is one of the main factors that can limit the performance of superconducting radio frequency cavities. To reduce possible field emission in the Linac Coherent Light Source II (LCLS-II), we are developing plasma processing for 1.3 GHz nine-cell cavities. The ultimate goal of plasma processing will be to apply the technique in situ in the cryomodules in order to mitigate hydrocarbon-related field emission without disassembling them. Herein is presented the first systematic study of plasma processing applied to LCLS-II superconducting radio frequency cavities. Having developed a new method of plasma ignition for LCLS-II cavities, we applied plasma processing to 1.3 GHz cavities starting with a clean nitrogen doped cavity and proceeding with studying natural field emission and artificially contaminated cavities. All the cavities were cold tested before and after plasma cleaning in order to compare their performances. It was proved that this technique successfully removes carbon-based contamination from the cavity iris and that it is able to eliminate field emission in a naturally field emitting cavity. The effect of plasma processing on cavities exposed to vacuum failures was also investigated, showing positive results in some cases. This work shows how successful plasma processing is in removing hydrocarbon related contamination from the cavity surface without affecting the high Q-factors and quench fields characteristic of nitrogen doped cavities.

Relative narrow bandwidth-high energy radiation can be produced through Thomson scattering, where highly relativistic electrons collide with a laser pulse. The bandwidth of such a source is determined, among others factors, by the bandwidth of the laser pulse and the energy spread of the electrons. Here we investigate how the bandwidth of such a source can be minimized, with a particular emphasis on electron bunches with a correlated energy spread of several percent, that are typical for plasma based accelerator schemes. We show that by introducing a chirp on the laser pulse it is possible to compensate the broadening effect due to the energy spread of the electrons, and obtain the same bandwidth as a quasi-monochromatic plane wave laser pulse colliding with a monoenergetic electron bunch. Ultimately, the bandwidth of a Thomson source is limited by the acceptance angle and the initial transverse momentum of electrons (emittance).

Coherent synchrotron radiation (CSR) is a collective effect that mainly occurs when the trajectory of an electron beam is bent in a dipole magnet. It affects the electron beam by distorting the phase space along its slice distribution, which leads to emittance growth. Therefore, CSR should be suppressed to transport electron beams without further degradation of the emittance. In linear optics, CSR-induced emittance can be suppressed by controlling the Twiss parameters along the electron-beam transfer line. However, owing to some physical constraints, transfer-line optics may be governed by higher-order terms in the transfer map, and the use of a sextupole magnet to suppress these terms would be very challenging for low-energy-spread and low-emittance beams. Therefore, without using a sextupole magnet, we estimate the region of the Twiss parameters where the first-order terms are dominant along the transfer line by introducing chromatic amplitude. In this region, we can apply the suppression condition that is valid in a linear matrix system. This minimization of the emittance growth becomes even more important when the electron-beam transfer line is used for external injection into a plasma wakefield because mismatched beam conditions could induce an additional increase in the emittance during the acceleration. In this paper, we discuss a method of emittance-growth minimization driven by the CSR effect along the transfer line, which is particularly used for electron-beam injection into plasma wakefields. In addition, using the particle-in-cell simulation, we investigate the evolution of electron beam parameters during the acceleration through plasma wakefields in the presence of the CSR effect on the electron beam. We confirm that the beam emittance growth is minimized when the CSR effect is properly controlled. Otherwise, it is found that 11%–32% emittance growths by the CSR effect along the transfer line lead to additional 20%–40% increase of the maximum slice emittance.

Characterization and control of the transverse phase space of high-brightness electron beams is required at free-electron lasers or electron diffraction experiments for emittance measurement and beam optimization as well as at advanced acceleration experiments. Dielectric laser accelerators or plasma accelerators with external injection indeed require beam sizes at the micron level and below. We present a method using nano-fabricated metallic wires oriented at different angles to obtain projections of the transverse phase space by scanning the wires through the beam and detecting the amount of scattered particles. Performing this measurement at several locations along the waist allows assessing the transverse distribution at different phase advances. By applying a novel tomographic algorithm the transverse phase space density can be reconstructed. Measurements at the ACHIP chamber at SwissFEL confirm that the transverse phase space of micrometer-sized electron beams can be reliably characterized using this method.

Understanding the 3D collective long-term response of beams exposed to resonances is of theoretical interest and essential for advancing high intensity synchrotrons. This study of a hitherto unexplored beam dynamical regime is based on 2D and 3D self-consistent particle-in-cell simulations and on careful analysis using tune spectra and phase space. It shows that in Gaussian-like beams Landau damping suppresses all coherent parametric resonances, which are of higher than second order (the “envelope instability”). Our 3D results are obtained in an exemplary stopband, which includes the second order coherent parametric resonance and a fourth order structural resonance. They show that slow synchrotron oscillation plays a significant role. Moreover, for the early time evolution of emittance growth the interplay of incoherent and coherent resonance response matters, and differentiation between halo and different core regions is essential. In the long-term behavior we identify a progressive, self-consistent drift of particles toward and across the resonance, which results in effective compression of the initial tune spectrum. However, no visible imprint of the coherent features is left over, which only control the picture during the first one or two synchrotron periods. An intensity limit criterion and an asymptotic formula for long-term rms emittance growth are suggested. Comparison with the commonly used non-self-consistent “frozen space charge” model shows that in 3D this approximation yields a fast and useful orientation, but it is a conservative estimate of the tolerable intensity.

The achromatic telescopic squeezing (ATS) scheme has brought an essential conceptual foundation to the HL-LHC project, making possible a strong and clean (achromatic) reduction of ${\beta }^{*}$, an important parameter with respect to which several HL-LHC sub-systems were dimensioned (e.g., the 150 mm aperture of the new inner triplet quadrupoles) or justified (crab-cavities for mitigating the geometric luminosity loss factor at low ${\beta }^{*}$ and subsequent large crossing-angle). The basic mechanics of the scheme is shortly reminded, highlighting as well some of its by-products (Landau damping, long-range beam-beam mitigation with octupoles), and a recent improvement which made possible an early, but still partial, implementation of ATS optics in the LHC for the 2017 and 2018 LHC physics runs. The main focus of the paper will be on the experimental validation of the scheme, via the development of dedicated machine configurations and high-intensity beam tests which took place in Run 2. The paper will conclude on the configuration presently in mind to operate the LHC during its third exploitation period (Run 3), while trying to ensure a smooth enough transition toward the HL-LHC, in terms of optics, beam parameters, and dedicated beam manipulations (${\beta }^{*}$-leveling).

A dielectric-assist accelerating (DAA) cavity is a standing-wave accelerating cavity that attains a Q-value of over 100,000 at room temperature by using the reflection of the dielectric layer; the DAA cavity is expected to be used with a small rf power source and high-duty operation. Thus far, the maximum accelerating field of DAA cavities has been limited to a few MV/m by the multipactor discharges. Multipactor discharges are widely observed in the accelerator field, and their suppression has been studied to improve accelerator performance. By applying a diamondlike carbon (DLC) coating to reduce the secondary electron emission coefficient without sacrificing the Q-value of the cavity, we have demonstrated that a C-band rf pulse with a pulse length of $5.4[\mu \mathrm{s}]$ can be injected into a DAA cavity with a field of more than 10 [MV/m] while suppressing the multipactor discharges.

We present the results of numerical simulations and experimental studies about the effects of resonant and random excitations on proton losses, emittances, and beam distributions in the Large Hadron Collider (LHC). In addition to shedding light on complex nonlinear effects, these studies are applied to the design of hollow electron lenses for active beam halo control. In the High-Luminosity Large Hadron Collider (HL-LHC), a considerable amount of energy will be stored in the beam tails. To control and clean the beam halo, the installation of two hollow electron lenses, one per beam, is being considered. In standard electron-lens operation, a proton bunch sees the same electron current at every revolution. Pulsed electron beam operation (i.e., different currents for different turns) is also considered, because it can widen the range of achievable halo removal rates. For an axially symmetric electron beam, only protons in the halo are excited. If a residual field is present at the location of the beam core, these particles are exposed to time-dependent transverse kicks and to noise. We discuss the numerical simulations and the experiments conducted in 2016 and 2017 at injection energy in the LHC. The excitation patterns were generated by the transverse feedback and damping system, which acted as a flexible source of dipole kicks. Proton beam losses, emittances, and transverse distributions were recorded as a function of excitation patterns and strengths. The resonant excitations induced rich dynamical effects and nontrivial changes of the beam distributions, which, to our knowledge, have not previously been observed and studied in this detail. We conclude with a discussion of the tolerable and achievable residual fields and proposals for further studies.

This paper describes the construction, rf measurement and tuning, rf conditioning, and beam commissioning of a compact 325 MHz interdigital $H$-mode (IH) drift tube linac (DTL) with modified KONUS (Kombinierte Null Grad Struktur, combined zero-degree structure) beam dynamics at Tsinghua University. This IH-DTL accelerates a proton beam from 3 MeV to 7 MeV in about 1 m. With a high rf frequency of 325 MHz, the cavity is designed to be compact while retaining a high average gradient of $4\mathrm{MV}/\mathrm{m}$. The efficient shunt impedance $Z{T}^2$ is $101\mathrm{M}\mathrm{\Omega }/\mathrm{m}$. The beam test results show good agreement with the beam dynamics design. The developed compact IH-DTL can be applied to the injectors of medical proton machines.

The increasing demands for electron beams with high quality and low timing jitter for accelerator-based applications such as ultrafast electron microscopy (UEM) and ultrafast electron diffraction (UED) have stimulated active efforts to develop suitable injectors for these accelerators. One promising direction in this regard is using of a bimodal rf gun operating in fundamental mode and a phase-locked harmonically-related higher mode. Work reported in this paper is on analysis and design of a bimodal rf gun operating in S-band as the fundamental and C-band as the second-harmonic modes. The rf design for such an rf gun includes the new design like directional coupler and rectangular mode launcher, which presented good transmission, low reflective cross-talk, and allowed multiple iterations of the modification. Compared to the fundamental mode, the new design achieved reduction in rf emittance and energy spread for MeV UEM initial distribution, and reduction in slope of flight time as function of injection phase for MeV UED applications.

Radiation reaction force in a relativistic beam, also known as the CSR wakefield, is often computed using a 1D model of a line charge. While this model can serve as a useful tool for a quick calculation, in many cases its accuracy is not sufficient. In particular, this model misses the so-called compression effects associated with the change of the electromagnetic energy when the beam is compressed longitudinally or transversely. The existing 3D simulation codes that take this effect into account are often slow and are not easy to use. In this work, we propose a new approach to the calculations of radiation and space charge longitudinal forces in free space based on the use of the integrals for the retarded potentials. Our main result expresses the rate of change of particles’ energy through 2D (in a 2D model) or 3D integrals along the beam orbit.

We report on laser spectroscopic measurements on ${\mathrm{Li}}^{+}$ ions in the experimental storage ring ESR at the GSI Helmholtz Centre for Heavy Ion Research. Driving the $2s{{}^{3}S}_1(F=3/2)\leftrightarrow 2p{{}^{3}P}_2(F=5/2)\leftrightarrow 2s{{}^{3}S}_1(F=5/2)$ $\mathrm{\Lambda }$-transition in ${}^7{\mathrm{Li}}^{+}$ with two superimposed laser beams it was found that the use of circularly polarized light leads to a disappearance of the resonance structure in the fluorescence signal. This can be explained by optical pumping into a dark state of polarized ions. We present a detailed theoretical analysis of this process that supports the interpretation of optical pumping and demonstrates that the polarization induced by the laser light must then be at least partially maintained during the round trip of the ions in the storage ring. Such polarized ion beams in storage rings will provide opportunities for new experiments, especially on parity violation.

Accurate knowledge of the charged particle bunch longitudinal (time) profile is important in the context of wakefield accelerators, Compton Light sources, x-ray SASE FELs and THz radiation sources. However, it is still a challenge to obtain this information for subpicosecond long bunches and microbunched beams with the required femtosecond (fs) resolution and nondestructively. Apart from determining the profile in a nondestructive manner, the ideal bunch diagnostic would enable extraction of all required information in a single shot, have a sufficiently high repetition rate to monitor each bunch, small footprint, good cost efficiency and reliability. In this paper we present the design of a longitudinal bunch profile monitor that can determine the charged particle bunch profile with femtosecond resolution, nondestructively and in a single shot via the spectral analysis of coherent Smith-Purcell radiation (cSPr). It is based on the simultaneous deployment of three gratings with different periodicities, each with its own set of detectors. The number of the frequency sampling points is equal to the number of the optical channels and the discrimination against background radiation is based on the different polarization properties of cSPr and background radiation. The rationales for the choices made to optimize the monitor operation will be presented and the criteria for determining the number of frequency sampling points will be discussed. The possible future developments of the monitor are also presented and discussed.

We propose a short period undulator based on the electromagnetic field pattern in a THz-driven split ring resonator structure. An analytical model is developed that allows us to assess the key performance parameters of the undulator and to estimate the emitted radiation spectrum. Different geometric configurations are compared in detail using numerical simulations. A 100 MeV electron bunch with 5 pC charge is shown to emit narrow band 83 eV photon pulses with a peak brightness of approximately ${10}^{19}\mathrm{photons}/(\mathrm{s}{\mathrm{mrad}}^2{\mathrm{mm}}^{2}0.1\%\mathrm{BW})$ when passing through the 100 mm long undulator with a 1 mm period.

A disturbance observer (DOB)-based control approach was verified using the low-level radio-frequency systems of superconducting (SC) cavities operated in a continuous wave (CW) mode in a compact energy recovery linac (cERL) at KEK. This approach compensates for both predictable and unpredictable disturbances in a radio frequency (rf) system such as beam loading, Lorentz force detuning, and microphonics. This paper generalizes the DOB method in pulsed SC rf systems in the cERL at KEK. The results show that the pulse-to-pulse stabilities of the rf system are improved as expected by using this control approach. Furthermore, this method is validated by successfully compensating long pulse beam-loading effects in cERL.

Prior to establishing operation of the world’s first multiturn superconducting energy recovery linac, the Cornell-BNL energy recovery test accelerator was configured for one-turn energy recovery. In this setup, direct measurement of the beam loading in each of the main linac cavities demonstrated high energy recovery efficiency. Specifically, a total one-turn power balance efficiency of 99.4%, with per cavity power balances ranging from 99.2–99.8% was measured. When accounting for small particle losses occurring in the path length adjustment sections of the return loop, the per cavity power balances correspond to per cavity single particle energy recovery efficiencies ranging from 99.8 to 100.5%. A maximum current of $70\mu \mathrm{A}$, limited by the an incomplete radiation shielding installation around the main beam stop at the time of these measurements, was energy recovered.

The Compact Linear Collider (CLIC) targets a nanometer beam size at the collision point. Realizing this beam size requires the generation and transport of ultralow emittance beams. Dynamic imperfections can deflect the colliding beams, leading to a collision with a relative offset. They can also degrade the emittance of each beam. Both of these effects can significantly impact the luminosity of CLIC. In this paper, we examine a newly considered dynamic imperfection: stray magnetic fields. Measurements of stray magnetic fields in the Large Hadron Collider tunnel are presented and used to develop a statistical model that can be used to realistically generate stray magnetic fields in simulations. The model is used in integrated simulations of CLIC at 380 GeV including mitigation systems for stray magnetic fields to evaluate their impact on luminosity.

A transverse feedback system can effectively mitigate the emittance growth caused by injection oscillations and machine noise in hadron beams. However, as its action on the beam depends on beam position measurements of finite accuracy, it introduces additional noise on its own. The machine noise is in general strongest at low frequencies. Hence, the feedback is less needed at high frequencies. In this paper, two theories for the reduction of the machine noise induced emittance growth rate, with a bunch-by-bunch feedback, have been extended to a multibunch feedback. The extended theories show quantitative agreement with sophisticated macroparticle simulations. The emittance growth caused by the beam position measurement noise is numerically found to be only weakly dependent on the feedback’s cutoff frequency, while it is strongly dependent on the single-bunch gain. The ultimate goal of this study is to find the optimal transverse feedback bandwidth and gain, determined by the minimization of the total emittance growth rate. The optimum depends on the ratio between the amplitudes of the beam position measurement error and the machine noise, the power spectrum of the machine noise, the response of the feedback filters, and the magnitude and details of the detuning. For the illustrative case of the Large Hadron Collier during collision in run 2, the optimum is found at the currently lowest possible cutoff frequency of 0.5 MHz, with a single-bunch damping time of approximately 270 turns. Using a chromaticity of 15 units, the minimal emittance growth rate at this cutoff frequency is 72% lower than with a bunch-by-bunch feedback. If the beam position measurement error can be reduced relative to the machine noise, the optimum will shift to larger single-bunch gains, or equivalently shorter single-bunch damping times.