An x-ray Hanbury Brown-Twiss interferometry to diagnose a temporal profile of a femtosecond electron bunch (e-bunch) is presented. We show that intensity interference of spontaneous x-ray radiation from the e-bunch reflects the e-bunch profile. Based on this relationship, a temporal profile of the 8.1-GeV e-bunch at SPring-8 Angstrom Compact free-electron LAser (SACLA) that generates x-ray free-electron laser (XFEL) light is characterized through the intensity interference measurement. Combining this e-bunch profile with a numerical simulation, the XFEL pulse duration generated by the e-bunch is estimated to be less than 10 fs.

In this paper, we present a new kind of high power and high efficiency free-electron laser oscillator based on the application of the tapering enhanced stimulated superradiant amplification (TESSA) scheme. The main characteristic of the TESSA scheme is a high intensity seed pulse which provides high gradient beam deceleration and efficient energy extraction. In the oscillator configuration, the TESSA undulator is driven by a high repetition rate electron beam and embedded in an optical cavity. A beam-splitter is used for outcoupling a fraction of the amplified power and recirculate the remainder as the intense seed for the next electron beam pulse. The mirrors in the oscillator cavity refocus the seed at the undulator entrance and monochromatize the radiation. We discuss the optimization of the system for a technologically relevant example at $1\mu \mathrm{m}$ using a 1 MHz repetition rate electron linac starting with an externally injected igniter pulse.

The International Linear Collider (ILC) aims to employ field programmable gate array (FPGA)-based digital low level radio frequency (LLRF) control system to achieve its rf stability requirements. In this paper, we present a demonstration and evaluation of the minimal configuration of the digital LLRF control system for the ILC. The achieved stabilities are shown to fulfill the ILC requirements. We also developed signal monitoring capabilities using direct sampling technique and we successfully demonstrate the filling on resonance (FoR) method to reduce the required klystron power. Furthermore, we present the possible configurations of digital LLRF control system for the ILC in this paper. We performed the experiments at Superconducting rf Test Facility (STF) in High Energy Research Organization (KEK), Japan during the cavity conditioning in the autumn of 2016.

We propose and demonstrate a novel method to produce few-femtosecond electron beam with relatively low timing jitter. In this method a relativistic electron beam is compressed from about 150 fs (rms) to about 7 fs (rms, upper limit) with the wakefield at THz frequency produced by a leading drive beam in a dielectric tube. By imprinting the energy chirp in a passive way, we demonstrate through laser-driven THz streaking technique that no additional timing jitter with respect to an external laser is introduced in this bunch compression process, a prominent advantage over the conventional method using radio-frequency bunchers. We expect that this passive bunching technique may enable new opportunities in many ultrashort-beam based advanced applications such as ultrafast electron diffraction and plasma wakefield acceleration with externally injected beam.

The next generation of storage ring light sources will likely operate with high linear coupling, which could potentially prevent the use of off-axis injection as large horizontal oscillation of the injected beam is coupled to the vertical plane. We did experiments on the SPEAR3 storage ring to study how linear coupling impact the dynamic aperture and the off-axis injection efficiency. The results show that the dynamic aperture is significantly reduced and injection efficiency can drop to zero when operated on the coupling resonance. However, with large nonlinear detuning, the dynamic aperture and high injection efficiency can survive with the stored beam at full coupling because the injected beam is shifted away from the coupling resonance. Particle tracking simulation is also conducted to demonstrate the injected beam behavior in a nonlinear lattice with high coupling.

The twin-bunch technique has been a subject of intense investigation due to its application in diverse fields, such as two-color free-electron lasers and beam-driven wakefield acceleration. In this paper, we investigate the generation of twin bunches (tunable in both time and energy space) via a velocity bunching (VB) scheme in a traveling wave accelerator (TWA) by varying the TWA phase across a very large range for the first time. The results of both simulations and experiments are presented. The results show that when the phase injected into the TWA tube is set to $\ll -100\mathrm{degrees}$, VB occurs in a deep overcompression mode, where the twin bunches are continuously and widely tunable under a large TWA phase acceptance. The energy and time spaces can be adjusted in the ranges [0–3] MeV and [0–6] ps, respectively, by varying the phase of the injected bunch train from $-170\mathrm{degrees}$ to $-120\mathrm{degrees}$. The deep overcompression mode is a newly explored working mode for the VB scheme. Moreover, the emittance of each sub-bunch can be preserved when the strength of the long solenoid surrounding the TWA is optimized for emittance compensation, thus maintaining the high-quality beams from photoinjectors.

The Old Dominion University Compact Light Source (ODU CLS) design concept is presented—a compact Inverse Compton Light Source (ICLS) with flux and brilliance orders of magnitude beyond conventional laboratory-scale sources and greater than other compact ICLS designs. This concept utilizes the physics of inverse Compton scattering of an extremely low emittance electron beam by a laser pulse of rms length of approximately two-thirds of a picosecond ($2/3\mathrm{ps}$). The accelerator is composed of a superconducting radio frequency (SRF) reentrant gun followed by four double-spoke SRF cavities. After the linac are three quadrupole magnets to focus the electron beam to the interaction point (IP). The distance from cathode surface to IP is less than 6 m, with the cathode producing electron bunches with a bunch charge of 10 pC and a few picoseconds in length. The incident laser has 1 MW circulating power, a 1 micron wavelength, and a spot size of 3.2 microns at the IP. The repetition rate of this source is 100 MHz, in order to achieve a high flux despite the low bunch charge. The anticipated x-ray source parameters include an energy of 12 keV, with a total flux of $2.2\times {10}^{13}\mathrm{ph}/\mathrm{s}$, the flux into a 0.1% bandwidth of $3.3\times {10}^{10}\mathrm{ph}/(\mathrm{s}0.1\%\mathrm{BW})$, and the average brilliance of $3.4\times {10}^{14}\mathrm{ph}/(\mathrm{s}{\mathrm{mm}}^2{\mathrm{mrad}}^{2}0.1\%\mathrm{BW})$.

Errors from discretization and large data volume of field maps is a concern for beam dynamics simulations with respect to achievable accuracy and to the required amount of time. High-order singular value decomposition (HOSVD) has recently emerged as simple, effective, and adaptive tool to extract the essentials from multidimensional data. This paper is on the feasibility of compression and noise reduction of electromagnetic field map data with HOSVD. The method has been applied to an electric field map of a DTL cavity with 11 m in length comprising 55 rf-gaps. The original field map data of 220 MB was converted into practically noise-free data of just 20 KB. Noise was reduced by 95% as demonstrated using a cubic cavity for which the analytical field map is available.

We experimentally demonstrate a single-shot arrival time monitor for short picosecond infrared free-electron laser (IR FEL) pulses based on balanced optical cross-correlation with a synchronized fs table-top laser. Employing this timing tool at the Fritz Haber Institute IR FEL, we observe a shot-to-shot timing jitter of only 100 fs and minute-scale timing drifts of a few picoseconds, the latter being strictly correlated with the electron beam energy of the accelerator. We acquire sum-frequency cross-correlation data with micropulse resolution, providing full access to the IR FEL pulse shape evolution within the macropulse. These measurements provide unprecedented insights into the occurrence of limit-cycle oscillations of the FEL intensity as a consequence of subpulse formation. Our experimental results are complemented by four-dimensional simulations of the nonlinear pulse dynamics in a low-gain FEL oscillator based on Maxwell-Lorentz theory.

The beam-beam interaction is one of the most severe limitations on the performance of circular colliders, as it is an unavoidable strong nonlinear effect. As one aspires for greater luminosity in future colliders, one will simultaneously achieve stronger beam-beam interactions. We study the limitations caused by strong incoherent head-on beam-beam interactions, using a new code (cabin) that calculates on a graphics processing unit (GPU), allowing for a detailed description of the long-term particle trajectories in 6D phase space. The evolution of the beam emittance and beam intensity has been monitored to study the impact quantitatively, while frequency map analysis has been performed to understand the impact qualitatively. Results from cabin have shown good quantitative agreement with dedicated experiments in the Large Hadron Collider (LHC). For large beam-beam tune shifts, alternatives to the LHC tunes have been found to improve the beam quality. Schemes devised to cancel beam-beam driven resonances, by use of specific intermediate phase advances between the interaction points, work very well with zero crossing angle, and the accuracy required is achievable. Due to lack of symmetry, these schemes have an almost negligible impact with a significant crossing angle. The hourglass effect has been found to reduce the detrimental effects caused by the chromaticity and vice versa. The optimal level of the hourglass effect has been achieved when ${\beta }^{*}=1.5{\sigma }_s$. The ultimate parameters of the Future Circular Hadron Collider (FCC-hh) seem within reach, in absence of residual odd resonances.

We suggest an explanation of the observations of the glowing filaments in a superconducting cavity. We show that the motion of a charged particle in a multicell rf cavity and of a dust particle in a single-cell rf cavity can be stable. The analysis of stability is illustrated by numerical calculations. This effect may also explain the possible failure of high gradient tests of superconducting cavities, which are planned to be used in the linear accelerators for XFEL.

This paper discusses measurements on the stabilization of single bunches with second order chromaticity ($Q^{\prime \prime }$) in the Large Hadron Collider (LHC) at CERN. $Q^{\prime \prime }$ introduces an incoherent betatron tune spread which can produce Landau damping of transverse instabilities. Although the resulting stabilizing effect is similar to that provided by Landau octupoles, the underlying beam dynamics are different. Since the tune spread from $Q^{\prime \prime }$ is based on the longitudinal rather than the transverse action of the particles, it will not be affected by the smaller transverse emittance beams of future machines, such as the High Luminosity LHC or the Future Circular Collider, and may hence provide more efficient Landau damping than magnetic octupoles. This study serves as a proof-of-principle experiment to demonstrate Landau damping from detuning with longitudinal action by means of $Q^{\prime \prime }$ in a carefully prepared and well-understood accelerator environment. The agreement between measurements and pyheadtail tracking simulations shows that $Q^{\prime \prime }$ indeed contributes to the beam stability, that the numerical model of the LHC is accurate, and that the involved beam dynamics mechanisms are understood from both the single- and multiparticle effects points of view. The results also serve as a first experimental validation of the recently proposed radio frequency quadrupole for Landau damping.

Superconducting rf (SRF) photoinjectors are one of the most promising devices for generating continuous wave (CW) electron beams with record high brightness. Ultrahigh vacuum of SRF guns provides for long lifetime of the high quantum efficiency (QE) photocathodes, while SRF technology provides for high accelerating gradients exceeding $10\mathrm{MV}/\mathrm{m}$. It is especially true for low frequency SRF guns where electrons are generated at photocathodes at the crest of accelerating voltage. Two main physics challenges of SRF guns are their compatibility with high QE photocathodes and multipacting. The first is related to a possibility of deposition of photocathode materials (such as Cs) on the walls of the SRF cavity, which can result in increased dark current via reduction of the bulk Nb work function and in enhancing of a secondary electron emission yield (SEY). SEY plays critical role in multipacting (e.g., an exponential growth of the multipactor discharge), which could both spoil the gun vacuum and speed up the deposition of the cathode material on the walls of the SRF cavity. In short, the multipactor behavior in superconducting accelerating units must be well understood for successful operation of an SRF photo-injector. In this paper we present our studies of 1.2 MV 113 MHz quarter-wave SRF photoinjector serving as a source of electron beam for the coherent electron cooling experiment (CeC) at BNL. During three years of operating our SRF gun we encountered a number of multipacting zones. We also observed that presence of ${\mathrm{CsK}}_2\mathrm{Sb}$ photocathode in the gun could create additional multipacting barriers. We had conducted a comprehensive numerical and experimental study of the multipactor discharge in our SRF gun. We had developed a process of crossing the multipacting barriers from zero to the operational voltage without affecting the lifetime of our photocathode and enhancing the strength of multipacting barriers. We found a good agreement between the results of simulations and our experimental data.

Crab crossing is essential for high-luminosity colliders. The high-luminosity Large Hadron Collider (HL-LHC) will equip one of its interaction points (IP1) with double-quarter wave (DQW) crab cavities. A DQW cavity is a new generation of deflecting rf cavities that stands out for its compactness and broad frequency separation between fundamental and first high-order modes. The deflecting kick is provided by its fundamental mode. Each HL-LHC DQW cavity shall provide a nominal deflecting voltage of 3.4 MV, although up to 5.0 MV may be required. A proof-of-principle (POP) DQW cavity was limited by quench at 4.6 MV. This paper describes a new, highly optimized cavity, designated the DQW SPS series, which satisfies dimensional, cryogenic, manufacturing, and impedance requirements for beam tests at the Super Proton Synchrotron (SPS) and operation in the LHC. Two prototypes of this DQW SPS series were fabricated by U.S. industry and cold tested after following a conventional superconducting radio-frequency surface treatment. Both units outperformed the POP cavity, reaching a deflecting voltage of 5.3–5.9 MV. This voltage—the highest reached by a DQW cavity—is well beyond the nominal voltage of 3.4 MV and may even operate at the ultimate voltage of 5.0 MV with a sufficient margin. This paper covers fabrication, surface preparation, and cryogenic rf test results and implications.

Betatron phase measured from excited oscillation can be used to correct the beta function and the nonlinear lattice. With an improved algorithm the measurement precision has achieved 1 mrad (rms), or equivalently, 0.3% of beta beat at NSLS-II. The beta beating can be corrected to 1% in less than half an hour. This precise technique can be applied to the nonlinear lattice correction as well. The betatron phase probed by an off-axis beam provides information on the sextupole strength. In comparison with the model the nominal strength can be restored. It was shown that the proposed scheme treated all the leading order resonance driving terms generated by sextupoles, as well as the detuning terms and the nonlinear chromaticity. At NSLS-II a half percent sextupole correction led to a 15% increase of the dynamic aperture.

An all-optical centimeter-scale laser-plasma positron accelerator is modeled to produce quasimonoenergetic beams with tunable ultrarelativistic energies. A new principle elucidated here describes the trapping of divergent positrons that are part of a laser-driven electromagnetic particle-shower with a large energy spread and their acceleration into a quasimonoenergetic positron beam in a laser-driven plasma wave. Proof of this principle using analysis and particle-in-cell simulations demonstrates that, under limits defined here, existing lasers can accelerate hundreds of MeV pC quasi-monoenergetic positron bunches. By providing an affordable alternative to kilometer-scale radio-frequency accelerators, this compact positron accelerator opens up new avenues of research.

Based on a quasioptical approach and direct particle-in-cell simulations, we study dynamics of oversized relativistic surface-wave oscillators (SWOs) of the Cherenkov type with 2D periodical corrugated structures of cylindrical geometry. Such corrugation allows significant rarefication of the spectrum of modes with different azimuthal indices. As a result, selective excitation of a mode with a given azimuthal index is possible. Azimuthal index of the generated mode depends on the voltage rise time. For short (nanosecond scale) rise time, generation of an azimuthally symmetric mode can be realized. For longer (hundreds nanoseconds to microseconds) rise time, the modes with high azimuthal indexes would be excited. These conclusions are supported by the experiments where Ka-band SWOs with 2D corrugated structures were realized based on the $300\mathrm{keV}/100\mathrm{A}/4\mu \mathrm{s}$ thermionic accelerator SATURN. For an oversize factor of 16, stable narrow-band generation with output power of 1.5–2 MW was obtained at the frequency of 32.5 GHz corresponding to the mode with an azimuthal index of $m=3$. The project of Ka-band subgigawatt power SWOs operating at the azimuthally symmetric mode based on $500\mathrm{keV}/4\mathrm{kA}/20\mathrm{ns}$ high current explosive-emission accelerator SINUS-6 is under development.