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Polarized x rays or gamma rays generated from inverse Compton scattering (ICS) sources are high-brightness, quasimonochromatic, and polarization-tunable and are widely applied in medical imaging, material science, and nuclear physics research. In this paper, to simulate ICS between electron beams and polarized laser beams, a Monte Carlo simulation code has been developed and implemented based on the geant4 toolkit. This code considers the polarization as well as the bandwidth of incident laser beams. The code is benchmarked by comparing its simulation results with those from the well-known Monte Carlo tool called cain and experimental results. The comparisons show that this code accurately reproduces the scattered photons, demonstrating its potential for various ICS-based applications.

A light source project named very compact inverse Compton scattering gamma-ray source (VIGAS) is under development at Tsinghua University. VIGAS aims to generate monochromatic high-energy gamma rays by colliding 350-MeV electron beams with 400-nm laser photons within a 12-m beamline. To produce a high-energy electron beam in such a compact space, the system consists of an S-band high-brightness injector and six X-band high-gradient accelerating structures. The goal of the X-band structure is to operate at a high gradient of 80 MV/m. Therefore, we adopts the constant gradient traveling wave approach, where the iris from the first cell to the end cell is tapered. The structure has 72 cells, including 70 cells and 2 couplers, so we named it XT72. The frequency of XT72 is selected to 11.424 GHz, and the 2π/3 mode is adopted. In this paper, we present a comprehensive study covering the detailed design, fabrication, rf tuning, and high-power test results of the first XT72. Additionally, we compare the performance of this structure to that of the previous constant impedance structure. Our results demonstrate that the XT72 is capable of operating at an 80-MV/m gradient with a lower breakdown rate. This advancement paves the way for the development of VIGAS project and contributes to the wider application of X-band room-temperature high-gradient structures in compact accelerator facilities.

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 MV/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.

Pump-probe experiments using x-ray pulses or MeV ultrafast electrons as probes are important for studying ultrafast dynamic processes at the atomic level. Increasing demand is being focused on ultrashort x-ray pulses and electron pulses to research processes down to a few femtoseconds. Due to the limitation of the space charge force, only low charge electron bunches with critical synchronization can achieve such short pulses. A high-precision beam arrival-time monitor for the low charge is especially important. In this paper, a low Q_{l} cavity with high sensitivity is designed to measure the arrival time of charges below 1 pC. The pickup is particularly optimized with high R/Q at 4.76 GHz to provide the maximum signal, and the cold test results are in good agreement with the simulations. An IQ demodulation scheme is developed for the readout electronics. In the beam tests, the measured resolution of the beam arrival-time monitor is 33.46 fs at 0.5 pC.

Emittance is a key figure-of-merit for high brightness electron beams that enable the success of a broad range of modern accelerator-based instruments. With the successful implementation of the emittance compensation scheme and beam-shaping techniques to align each temporal beam slice in phase space, slice emittance starts to play a more dominating role in determining the beam quality. In this paper, we develop an approach, combining simulations and analytical calculations, to systematically study the evolution of the slice emittance in a very high frequency gun photoinjector, which is under intense R&D worldwide for driving the next generation continuous-wave accelerators. Our approach is capable of tracing various sources that introduce curvature in the transverse phase space, corresponding to the growth of the slice emittance. It is shown that nonlinear transverse space charge forces in the vicinity of cathodes and the spherical aberration of the focusing solenoid are the main sources of nonlinear forces, while in a long drift section space charge forces can actually compensate for nonlinear transverse position-momentum correlations and hence recover the emittance. We further demonstrate that the sources of nonlinearities can be controlled by tailoring the transverse beam density profile and the design of solenoid lenses.

High-energy electron radiography (HEER) is a promising diagnostic tool for high-energy-density physics, as an alternative to tools such as X/γ-ray shadowgraphy and high-energy proton radiography. Impressive progress has been made in the development and application of HEER in the past few years, and its potential for high-resolution imaging of static opaque objects has been proved. In this study, by taking advantage of the short pulse duration and tunable time structure of high-energy electron probes, time-resolved imaging measurements of high-energy-density gold irradiated by ultrashort intense laser pulses are performed. Phenomena at different time scales from picoseconds to microseconds are observed, thus proving the feasibility of this technique for imaging of static and dynamic objects.

Coherent transition radiation (CTR) is a widely used approach to characterize the electron beam in accelerator science and technology. Characterization of the CTR with an efficient, versatile and compact setup is in considerable demand for a wide range of applications such as free electron lasers, electron storage rings and other related scientific researches. In this article, we report on spatial-temporal electric field measurements of CTR using the single-shot electro-optic spatial decoding method. A quasi-half-cycle temporal waveform of the THz pulse with peak electric field strength of ∼42 MV/m has been measured. A symmetrical field profile across the transverse center and the distorted wave front of CTR THz pulses near the focal plane were demonstrated. Moreover, timing jitters of the ultrashort electron beams are also monitored via this method with tens of femtosecond accuracy. This comprehensive method provides the temporal waveform and the one-dimensional spatial imaging of THz fields simultaneously, showing it has the potential for single-shot diagnostics of ultrashort electron beams.

In state of the art photoemission electron sources, the transverse emittance of the electron beam is approaching its intrinsic value from the cathode. To reduce the intrinsic emittance, it is straightforward to tune the photon energy of the drive laser toward the photocathode emission threshold. This paper aims at constructing an improved model which takes into account an intermediate energy level to better explain the near threshold photoemission for semiconductors. A dynamic model is also proposed to evaluate the cathode degradation process due to surface oxidation. The models agree well with published results for different materials under various conditions, including electric field, temperature and vacuum condition. The dynamic model predicts that near threshold emission of an oxidized Cs_{3}Sb cathode outperforms UV photoemission of typical metal cathodes in terms of both quantum efficiency and intrinsic emittance. With the oxidization layer, the robustness of semiconductor cathodes to gun vacuum condition may be comparable to a metal cathode. This, however, needs further testing.

The Xian gamma-ray light source (XGLS) is a proposed novel high brightness gamma-ray source based on inverse Compton scattering (ICS) between the high brightness electron and laser beams. The photoinjector of the accelerator is one of the main components that determines the electron beam quality and related gamma-ray parameters [Phys. Rev. Accel. Beams 21, 030701 (2018)PRABCJ2469-988810.1103/PhysRevAccelBeams.21.030701; 20, 080701 (2017)PRABCJ2469-988810.1103/PhysRevAccelBeams.20.080701; 8, 100702 (2005)PRABFM1098-440210.1103/PhysRevSTAB.8.100702]. In order to reduce the risk of this project, a photoinjector including a laser-driven photocathode rf gun, two S-band tubes, and a diagnostic section was constructed and installed from 2016, serving as a pilot plant for the development of the key technologies for the projects. The injector is designed to deliver high quality electron bunches of typically 500 pC charge and ∼120 MeV energy with low emittance and jitter at 10 Hz repetition. Initial system commissioning with an electron beam was finished in the end of 2017, with a goal of a stable 0.6–0.7 mm mrad emittance in a 500-pC bunch clearly demonstrated. In this paper, we report the experimental results and experience obtained in the commissioning, including rf gun, drive laser system, timing and synchronization system, and beam diagnostics.

TTX-II (Tsinghua Thomson Scattering X-Ray Source Phase II) is a Thomson scattering based x-ray source currently under development at the accelerator laboratory in Tsinghua University. The circumference of the storage ring is 5.668 m. The compact nature of the ring gives rise to several design challenges such as chromaticity correction and collective instability, which can be suppressed by carefully optimizing ring parameters. The lattice design and the beam dynamics of TTX-II are presented in this work.

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 ≪-100 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 degrees to -120 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.

High-energy charged particle radiography has been used for diagnostics of high-energy density matter, and electrons can serve as a promising radiographic probe that acts as a complement to commonly used proton probes. Here we report on an electron radiography experiment using 45 MeV electrons from an S-band photoinjector, where scattered electrons, after interacting with a sample, are collected and imaged by a quadrupole imaging system. We achieve a spatial resolution of a few microns (∼4 μm) and a thickness resolution of a few percent for a silicon target of 300–600 μm in thickness. With additional dark-field images captured by selecting electrons with large scattering angles, we show that complementary information for determining external details such as outlines, boundaries and defects can be obtained.

Premodulated comblike electron bunch trains are used in a wide range of research fields, such as for wakefield-based particle acceleration and tunable radiation sources. We propose an optimized compression scheme for bunch trains in which a traveling wave accelerator tube and a downstream drift segment are together used as a compressor. When the phase injected into the accelerator tube for the bunch train is set to ≪-100°, velocity bunching occurs in a deep overcompression mode, which reverses the phase space and maintains a velocity difference within the injected beam, thereby giving rise to a compressed comblike electron bunch train after a few-meter-long drift segment; we call this the deep overcompression scheme. The main benefits of this scheme are the relatively large phase acceptance and the uniformity of compression for the bunch train. The comblike bunch train generated via this scheme is widely tunable: For the two-bunch case, the energy and time spacings can be continuously adjusted from +1 to -1 MeV and from 13 to 3 ps, respectively, by varying the injected phase of the bunch train from -220° to -140°. Both theoretical analysis and beam dynamics simulations are presented to study the properties of the deep overcompression scheme.

The measurement of electron bunch temporal profile is one of the key diagnostics in accelerators, especially for ultrashort bunches. The electro-optic (EO) technique enables the precise longitudinal characterization of bunch electric field in a single-shot and nondestructive way, which can simultaneously obtain and analyze the time jitter between the electron bunch and the synchronized laser. An EO monitor based on spatial decoding for temporal profile measurement and timing jitter recoding has recently been demonstrated and analyzed in depth for low-energy bunches at the Tsinghua Thomson scattering X-ray source. A detailed description of the experimental setup and measurement results are presented in this paper. An EO signal as short as 82 fs (rms) is observed with 100 μm gallium phosphide for a 40 MeV electron bunch, and the corresponding length is 106 fs (rms) with 300 μm zinc telluride. Owing to the field-opening angle, we propose a method to eliminate the influence of energy factor for bunches with low energy, resulting in a bunch length of ∼60 fs (rms). The monitor is also successfully applied to measure time jitter with approximately 10 fs accuracy. The experiment environment is proved to be the main source of the slow drift, which is removed using feedback control. Consequently, the rms time jitter decreases from 430 fs to 320 fs.

We propose a method based on the slice energy spread modulation to generate strong subpicosecond density bunching in high-intensity relativistic electron beams. A laser pulse with periodic intensity envelope is used to modulate the slice energy spread of the electron beam, which can then be converted into density modulation after a dispersive section. It is found that the double-horn slice energy distribution of the electron beam induced by the laser modulation is very effective to increase the density bunching. Since the modulation is performed on a relativistic electron beam, the process does not suffer from strong space charge force or coupling between phase spaces, so that it is straightforward to preserve the beam quality for terahertz (THz) radiation and other applications. We show in both theory and simulations that the tunable radiation from the beam can cover the frequency range of 1–10 THz with high power and narrow-band spectra.

Radio frequency breakdown is one of the fundamental phenomena that limits the operational performance of most of high power and high gradient vacuum rf devices. We report on experimental results of rf breakdown in an S-band photocathode gun triggered by an intensity controlled laser. Through measurement and analysis of the time dependence of the collected current at the gun exit and the stored rf energy in the cavity, one can gain insight into the time evolution of the rf breakdown process. Multiple breakdowns were observed within one rf pulse due to power flow between cells after the initial emission. Similarities of the laser-triggered breakdowns to those occurring in the course of cavity conditioning and normal operation are found by comparing the postbreakdown signals in both cases. It is shown that an intense laser can offer a more controllable and flexible method for rf breakdown studies.

Characterizing the rf-to-laser jitter in the photocathode rf gun and its possible origins is important for improving the synchronization and beam quality of the linac based on the photocathode rf gun. A new method based on the rf compression effect in the photocathode rf gun is proposed to measure the rf-to-laser jitter in the gun. By taking advantage of the correlation between the rf compression and the laser injection phase, the error caused by the jitter of the accelerating field in the gun is minimized and thus 10 fs time resolution is expected. Experimental demonstration at the Tsinghua Thomson scattering x-ray source with a time resolution better than 35 fs is reported in this paper. The experimental results are successfully used to obtain information on the possible cause of the jitter and the accompanying drifts.

C-band low-energy electron linear accelerators are preferable for applications which require compact structure and accurate positioning. During the past two years, the accelerator laboratory of Tsinghua University has developed a C-band 6 MeV standing-wave biperiodic on-axis coupled linear accelerator. The weight of the accelerator is 7.4 kg and the length is about 39 cm, which includes the thermal-cathode gun and the flange for tests. High capture ratio, narrow energy spectrum, and small spot size have been achieved in the high power rf test, which shows good performance of this accelerator and its great potential for industrial and medical applications.

Choke-mode damped structures are being studied as an alternative design to waveguide damped structures for the main linac of the Compact Linear Collider (CLIC). Choke-mode structures have the potential for lower pulsed temperature rise and simpler and less expensive fabrication. An equivalent circuit model based on transmission line theory for higher-order-mode damping is presented. Using this model, a new choke geometry is proposed and the wakefield performance is verified using Gdfidl. This structure has a comparable wakefield damping effect to the baseline design which uses waveguide damping. A prototype structure with the same iris dimensions and accelerating gradient as the nominal CLIC design, but with the new choke geometry, has been designed for high-power tests.

We present the lattice design for the pulse mode of the compact laser-electron storage ring for a Compton x-ray source. The lattice is optimized to suppress the intrabeam scattering (IBS), which is the dominant factor that leads to emittance growth. To better simulate the beam dynamics in the pulse mode, we developed a macroparticle 3D Monte Carlo algorithm. The IBS module can perform an element-to-element calculation to evaluate the emittance change due to IBS, while the Compton scattering module can provide the spatial and temporal information of the x-ray photon yield. Parameters of the storage ring, electron beam, and scattered photon yield of the pulse mode of this Compton x-ray source are presented.