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Terahertz (THz)-driven acceleration has recently emerged as a new route for delivering ultrashort bright electron beams efficiently, reliably, and in a compact setup. Many THz-driven acceleration related working schemes and key technologies have been successfully demonstrated and are continuously being improved to new limits. However, the achieved acceleration gradient and energy gain remain low, and the potential physics and technical challenges in the high field and high energy regime are still under-explored. Here we report a record energy gain of 170 keV in a single-stage configuration, and demonstrate the first cascaded acceleration of a relativistic beam with a 204 keV energy gain in a two-stages setup. Whole-bunch acceleration is accomplished with an average accelerating gradient of 85 MV/m and a peak THz electric field of 1.1 GV/m. This proof-of-principle result is a crucial advance in THz-driven acceleration with a major impact on future electron sources and related scientific discoveries., 8 pages, 6 figures

Low longitudinal emittance means small energy spread and short bunch length, which makes high-power short-wavelength coherent radiation possible. In a storage ring, due to quantum excitation, the equilibrium longitudinal emittance is mainly the overall contribution of all bend-related elements, such as bends, undulators and laser modulators. By introducing longitudinal Twiss function and analyzing the 6D one-turn map in 3D Twiss form, the longitudinal emittance contribution of all these elements is theoretically studied in this paper, and a general method for minimizing the storage ring equilibrium longitudinal emittance is proposed. An integrated optimization of longitudinal beta function shows, the longitudinal emittance scales as the third power of bending angle, and can be as low as subpicometer with a beam energy of 400 MeV in an ultimate state.

A modified Kombinierte Null Grad Struktur (KONUS) beam dynamics for inter-digital H-mode (IH) drift tube linac (DTL) is proposed in this study. This modified KONUS beam dynamics, which is aperiodic and is suitable for a short ( injector of a synchrotron-based proton therapy system, is divided into three sections, namely, rebunching ( − 80°), 0°acceleration, and debunching (10°). It is slightly different from the conventional KONUS cavity, and no focusing magnets exist in the cavity to reduce the cost of fabrication. The presence of 21 gaps in the cavity can accelerate a 15 mA proton beam from 3 MeV to 7 MeV. The cavity is tapered to adjust the gap voltage distribution, and the inner diameter varies from 196.8 mm to 232.6 mm. A cavity length of 1 m has a theoretical power consumption of 151 kW, whereas the maximum surface electric field is 40 MV/m (2.3 Kp). A new design process, which is similar to alternating-phase focusing method, is proposed. Details of the design method and results are presented in this study.

Distributed X-ray sources provide a solid foundation for the invention of new-generation computed tomography (CT). This study proposes a novel multi-focal spot electron gun scheme for distributed X-ray sources. The source in this study uses a dispenser cathode as the electron emitter, a mesh grid to control emission current, and two electrostatic lenses for beam shaping, focusing, and deflecting. Novel focusing and deflecting electrodes were designed to increase the number of focal spots in the distributed source. It consists of two identical half-rectangle opening electrodes by adjusting the potential of the two electrodes to control the trajectory of the electron beam and then obtain multi-focal spots on the anode target. The number of X-ray sources in the distributed source is limited by the amount of cathodes. The multi-focal spot electron gun can break this limitation and provide more possibilities and options for distributed X-ray sources and stationary CT. Experimental results reflect the excellent repeatability of the multi-spot electron gun. The dynamic effective focal spot sizes of the deflected (deflected amplitude, 10.5mm) and non-deflected electron beams at full with half max (FWHM) are 1.30mm × 0.60mm and 1.15mm × 0.55mm, respectively (anode voltage, 160kV; beam current, 30 mA).

In this paper, a circuit-level model for accurately calculating vertical-cavity surface-emitting laser's (VCSEL) response under pulsed working condition is implemented in the advanced design system computer-aided design environment. New normalization method and thermal leakage current expression are proposed to rate equation to improve numerical convergence of the model's output part, which is critical for pulsed working condition. A self-consistent decoupling method is applied to include the self-heating effects in both input and output parts of the model for getting the accurate waveforms of input current and output light power under pulsed driving conditions. The model's input part is similar to standard SPICE model for p-n junction diode, and is convenient for extracting model parameters to match the device's input impedance profile. A real device's model parameters are extracted, and its voltage–current–light power ( V – I – P ) curves, small-signal transfer function, and transient response under pulsed drive condition are calculated to demonstrate the model's calculation capability. The proposed model is well applicable for designing a high-power electromagnetic sensor including VCSEL and other high-frequency devices.

The achievable performances of bias-free vertical-cavity surface-emitting laser (VCSEL)-based voltage probe for high-power electromagnetic (HPEM) effects diagnosis of embedded systems are investigated. The minimum driving current needed by VCSEL is calculated to design the probes with high input impedance and high applicable data bit rate (BR) simultaneously. The VCSEL-based probes for 5 V/3.3 V logic family ICs are designed with a kind of 850-nm VCSEL and a family of high-frequency chip resistors. Measurement results show that the VCSEL-based voltage probe's input impedance can maintain higher than 1 kΩ until 300 MHz. The bit error ratio (BER) test shows that the voltage probe is applicable for monitoring digital signals with BR below 280 Mb/s, while the monitoring BER is better than 1E–6 for BR below 200 Mb/s. The processes of disturbance and breakdown effects induced to a minisized drone circuit are successfully captured by the voltage probe in a pulse injection test, demonstrating the compact and fiber output voltage probe's potential value in diagnosing embedded system's HPEM effects in system-level test.

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 $\ensuremath{\sim}42\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ 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.

The use of particle accelerators as photon sources has enabled advances in science and technology1. Currently the workhorses of such sources are storage-ring-based synchrotron radiation facilities2–4 and linear-accelerator-based free-electron lasers5–14. Synchrotron radiation facilities deliver photons with high repetition rates but relatively low power, owing to their temporally incoherent nature. Free-electron lasers produce radiation with high peak brightness, but their repetition rate is limited by the driving sources. The steady-state microbunching15–22 (SSMB) mechanism has been proposed to generate high-repetition, high-power radiation at wavelengths ranging from the terahertz scale to the extreme ultraviolet. This is accomplished by using microbunching-enabled multiparticle coherent enhancement of the radiation in an electron storage ring on a steady-state turn-by-turn basis. A crucial step in unveiling the potential of SSMB as a future photon source is the demonstration of its mechanism in a real machine. Here we report an experimental demonstration of the SSMB mechanism. We show that electron bunches stored in a quasi-isochronous ring can yield sub-micrometre microbunching and coherent radiation, one complete revolution after energy modulation induced by a 1,064-nanometre-wavelength laser. Our results verify that the optical phases of electrons can be correlated turn by turn at a precision of sub-laser wavelengths. On the basis of this phase correlation, we expect that SSMB will be realized by applying a phase-locked laser that interacts with the electrons turn by turn. This demonstration represents a milestone towards the implementation of an SSMB-based high-repetition, high-power photon source. The mechanism of steady-state electron microbunching is demonstrated, providing a basis that will enable its full implementation in electron storage rings to generate high-repetition, high-power coherent radiation.

Physical review accelerators and beams 23(4), 043401 (2020). doi:10.1103/PhysRevAccelBeams.23.043401, In state of the art photoinjector electron sources, thermal emittance from photoemission dominates the final injector emittance. Therefore, low thermal emittance cathode developments and diagnostics are very important. Conventional thermal emittance measurements for the high gradient gun are time-consuming and thus thermal emittance is not measured as frequently as quantum efficiency during the lifetime of photocathodes, although both are important properties for the photoinjector optimizations. In this paper, a single shot measurement of photoemission transverse momentum, i.e., thermal emittance per rms laser spot size, is proposed for photocathode rf guns. By tuning the gun solenoid focusing, the electrons’ transverse momenta at the cathode are imaged to a downstream screen, which enables a single shot measurement of both the rms value and the detailed spectra of the photoelectrons’ transverse momenta. Both simulations and proof of principle experiments are reported., Published by American Physical Society, College Park, MD

An observation of prior-damage behavior inside a high-finesse optical resonator is reported. Intra-cavity average power drops appeared with magnitude and time scale depending on the power level. Increasing further the incident laser beam power led to irreversible damage of the cavity coupling mirror surface. The origin of this phenomenon is investigated with post mortem mirror surface imaging and analysis of the signals reflected and transmitted by the enhancement cavity. Scattering losses induced by surface deformation due to a hot-spot surface contaminant is found to be most likely the dominant physics process behind this phenomenon.

This paper analyzes the sub-10 nm ultra-scaled Si1−xGex fin field-effect transistor (FinFET) and gate-all-around transistor (GAAFET), based on an improved method by combining Slater partial occupation and ab initio density functional theory (DFT). Compared to the existing generalized gradient approximation (GGA) DFT, which is computationally efficient but inaccurate, and the established hybrid functional DFT, which is accurate but computationally demanding, the proposed method achieves both hybrid functional-like high accuracy and GGA-like high computational efficiency concurrently. By using this improved methodology, batch simulations are performed to investigate the size- and composition-dependent quantum confinement and band structure effects in the Si1−xGex FinFET and GAAFET. It is shown that the quantum confinement in ultra-scaled FinFET and GAAFET significantly increases the bandgap of Si1−xGex channel in the sub-10 nm scale. It is quantitatively demonstrated that, due to its two-dimensional quantum confinement, the ultra-scaled GAAFET has larger confinement-induced bandgap increase, lower direct source-drain quantum tunneling, lower off-state transmission and low-bias conductance, and smaller drain-induced barrier lowering, compared to the FinFET which is confined only in one-dimension. These differences jointly indicate that the ultra-scaled GAAFET could offer better device performance than the ultra-scaled FinFET. It is quantitatively shown that, by reducing the Ge composition of the Si1−xGex channel in ultra-scaled FinFET and GAAFET, the bandgap and the threshold gate voltage can be significantly increased; and the transmission coefficients, the low-bias conductance, and the source-drain current can be significantly reduced. These trends can be utilized to optimize device performance by tuning Ge composition in different technology nodes.

This paper proposes the method of using bias-free vertical-cavity surface-emitting lasers (VCSELs) for in-circuit monitoring and effects-diagnosis of digital electronic equipment undertaking HPEM susceptibility test. A circuit level model for typical single mode VCSEL and standard I/O buffer information specification model for I/O ports of digital ICs are built to simulate VCSEL's load effects to the digital IC from specific logic families, as well as VCSEL's light output. Simulation results show that VCSELs connected in parallel to IC's I/O ports can be lighted up and its light output can achieve remote status-monitoring of I/O ports’ states in real time, while its load effects to the I/O ports are light enough not to deteriorate its signal integrity or disturb the circuit's normal operation. An experiment is performed on commercial mini sized drone to validate this study and demonstrate this method's feasibility and advantages in equipment level HPEM susceptibility test.

© 1965-2012 IEEE. Coherent pulse stacking (CPS) is a new time-domain coherent addition technique that stacks several optical pulses into a single output pulse, enabling high pulse energy from fiber lasers. We develop a robust, scalable, and distributed digital control system with firmware and software integration for algorithms, to support the CPS application. We model CPS as a digital filter in the Z domain and implement a pulse-pattern-based cavity phase detection algorithm on an field-programmable gate array (FPGA). A two-stage (2+1 cavities) 15-pulse stacking system achieves an 11.0 peak-power enhancement factor. Each optical cavity is fed back at 1.5kHz, and stabilized at an individually-prescribed round-trip phase with 0.7deg and 2.1deg rms phase errors for Stages 1 and 2, respectively. Optical cavity phase control with nanometer accuracy ensures 1.2% intensity stability of the stacked pulse over 12 h. The FPGA-based feedback control system can be scaled to large numbers of optical cavities.

The Shanghai Soft X-ray Free-Electron Laser facility (SXFEL) is a single-pass FEL facility designed to cover the photon spectrum from 10 nm to 2nm based on self-amplified spontaneous emission (SASE...

Mode instability with different mode excitation has been investigated by off-splicing the fusion point in a 4 kW-level monolithic fiber laser system, which reveals that the fiber systems exciting more high order mode content exhibits lower beam quality but higher mode instability threshold. The static-to-dynamic mode degradation and dynamic-only mode degradation have also been observed in the same high power fiber amplifier by varying the mode excitation, which implicates that the mode excitation plays an important role in mode characteristics in high power fiber lasers. By employing a seed with near fundamental mode beam quality, only dynamic mode degradation-mode instability sets in with negligible static beam quality degradation. Then the fusion point in the seed laser is offset spliced to excite high order mode. As the output power of the main amplifier scales, the beam quality degrades with the beam profile being static, and then the dynamic mode instability sets in, the power threshold of which is higher than that with good beam quality seed. We consider that the static mode degradation is caused by the presence of incoherent supposition of fundamental and high order mode, which leads to that the measured dynamic mode instability threshold is higher.

Thermal emittance and quantum efficiency (QE) are key figures of merit of photocathodes, and their uniformity is critical to high-performance photoinjectors. Several QE mapping technologies have been successfully developed; however, there is still a dearth of information on thermal emittance maps. This is because of the extremely time-consuming procedure to gather measurements by scanning a small beam across the cathode with fine steps. To simplify the mapping procedure and to reduce the time required to take measurements, we propose a new method that requires only a single scan of the solenoid current to simultaneously obtain thermal emittance and QE distribution by using a pattern beam with multiple beamlets. In this paper, its feasibility has been confirmed by both beam dynamics simulation and theoretical analysis. The method has been successfully demonstrated in a proof-of-principle experiment using an L-band radio-frequency photoinjector with a cesium telluride cathode. In the experiment, seven beamlets were generated from a microlens array system and their corresponding thermal emittance and QE varied from 0.93 to $1.14\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ and from 4.6% to 8.7%, respectively. We also discuss the limitations and future improvements of the method in this paper.

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.

Distributed X-ray sources comprise a single vacuum chamber containing multiple X-ray sources that are triggered and emit X-rays at a specific time and location. This process facilitates an application for innovative system concepts in X-ray and computer tomography. This paper proposes a novel electron beam focusing, shaping, and deflection electron gun for distributed X-ray sources. The electron gun uses a dispenser cathode as an electron emitter, a mesh grid to control emission current, and two electrostatic lenses for beam shaping, focusing, and deflection. Novel focusing and deflecting electrodes were designed to increase the number of focal spots in the distributed source. Two identical half-rectangle opening electrodes are controlled by adjusting the potential of the two electrodes to control the electron beam trajectory, and then, multifocal spots are obtained on the anode target. The electron gun can increase the spatial density of the distributed X-ray sources, thereby improving the image quality. The beam experimental results show that the focal spot sizes of the deflected (deflected amplitude 10.5 mm) and non-deflected electron beams at full width at half maximum are 0.80 mm × 0.50 mm and 0.55 mm × 0.40 mm, respectively (anode voltage 160 kV; beam current 30 mA). The imaging experimental results demonstrate the excellent spatial resolution and time resolution of an imaging system built with the sources, which has an excellent imaging effect on a field-programmable gate array chip and a rotating metal disk.

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.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.

The electro-optic (EO) technique detects the Coulomb electric field distribution of relativistic electron bunches to obtain the associated longitudinal profile. This diagnostic method allows the direct time-resolved single-shot measurement and thus the real-time monitoring of the bunch profile and beam arrival time in a non-destructive way with sub-picosecond temporal resolution. In this paper, we report the measurement of the longitudinal profile of an electron bunch through electro-optic spectral decoding detection, in which the bunch profile is encoded into the spectra of the linearly chirped laser pulse. The experimental setup and measurement results of a 40 MeV electron bunch are presented, with a temporal profile length of 527 fs rms (~1.24 ps FWHM) and a beam arrival time jitter of 471 fs rms. Temporal resolution and future experimental improvement are also discussed.

International audience; A new method of laser frequency stabilization using polarization property of an optical cavity is proposed. In a standard Fabry–Perot cavity, the coating layers thickness of cavity mirrors is calculated to obtain the same phase shift for sand p-wave but a slight detuning from the nominal thickness can produce sand p-wave phase detuning. As a result, each wave accumulates a different round-trip phase shift and resonates at a different frequency. Using this polarization property, an error signal is generated by a simple setup consisting of a quarter wave-plate rotated at 45°, a polarizing beam splitter and two photodiodes. This method exhibits similar error signal as the Pound–Drever–Hall technique but without need for any frequency modulation. Lock theory and experimental results are presented in this paper. &

The experimental investigation of mode distortion induced by stimulated Raman scattering (SRS) in a high-power fiber amplifier, which includes the evolutions of optical spectra, spatial beam profiles, and time-frequency characteristics, has been carried out in detail. Temporal-frequency characteristics have been studied for the first time, to the best of our knowledge, by using a low-speed camera and high-speed photodiode traces, which revealed that temporal-frequency characteristics of SRS-induced mode distortion are different from traditional dynamic mode instability (MI). The experimental results show that the output beam profile remains stable before the mode distortion occurs and fluctuates obviously after the onset of SRS-induced MI but on a time scale of seconds, which is much lower than that of Yb-gain-induced MI featuring millisecond-level beam profile fluctuation. It also shows that the mode distortion became measurable in company with the onset of inter-mode four-wave mixing (IM-FWM) when the ratio of Raman light reaches 3%; further, the beam quality factor M 2 degrades gradually from 1.4 to 2.1 as the ratio of Raman light increases. The mode distortion is accompanied by an obvious temperature increase of the output passive fiber, which further confirms that the mode distortion originates from SRS. The cause of the mode distortion induced by SRS has been explained in the context of core-pumped SRS effect, and the investigation on the accompanying IM-FWM effect indicates that the main content of the SRS-induced high-order mode is the LP21 mode.

In recent experiments at Tsinghua University Accelerator Laboratory, the 31 MeV electron beam, which has been compressed to subpicosecond pulse durations, has been used to generate high peak power, narrow band Terahertz (THz) radiation by transit through different slow wave structures, specifically quartz capillaries metallized on the outside. Despite the high peak powers that have been produced, the THz pulse energy is negligible compared to the energy of the electron beam. Therefore, the THz generation process can be complementary to other beamline applications like plasma wakefield acceleration studies and Compton x-ray free electron lasers. This approach can be used at x-ray free electron laser beamlines, where THz radiation can be generated without disturbing the x-ray generation process. In the experiment reported here, a high peak current electron beam generated strong narrow band (∼1% bandwidth) THz signals in the form of a mixture of TM01 and TM02 modes. Each slow wave structure is completed with a mode converter at the end of the structure that allows for efficient (>90%) power extraction into free space. In the experiment, both modes in these two dielectric-loaded waveguides TM01 (0.3 THz/0.5 THz) and TM02 (0.9 THz/1.3 THz) were explicitly measured with an interferometer. The THz pulse energy was measured with a calibrated Golay cell at a few μJ.

Recently a MeV quasi-monochromatic compact gamma-ray source with high peak spectral density based on the inverse Compton scattering (ICS) has been proposed in the Department of Engineering Physics, Tsinghua University. This type compact gamma-ray source will be used for advanced X/gamma-ray imaging application based on the nuclear resonance Fluorescence (NRF) [1]. The machine size and the peak spectral density of scattered photons are the most important parameters for such applications. In order to make the source compact enough, a compact commercial narrow bandwidth Nd: Yag laser system with ~50 fs FWHM duration and ~1.5 J maximum energy per pulse is selected as the scattering laser, and the linac is proposed to combine a photo-injector and an X-band main linac to obtain high quality 250 MeV maximum energy electron beam with high charge (~ 200 pC) and low transverse and longitudinal emittance. In ICS, the properties of the generated photons are determined by the parameters of the incident laser and electron beam, and also their interaction geometry. In this paper, we will present the optimization of the linac design. We systematically simulate and optimize the linac design with Matlab, Astra [2] and Cain [3]. In the simulations and optimizations, we use differential evolution algorithm for simultaneous optimization of multiple parameters. Three possible types of photo-injector, S-band photocathode RF(radio frequency) gun with S-band booster, C-band photocathode RF gun with C-band booster, X-band photocathode RF gun with X-band booster, are systematically optimized and compared. We also analyzed wakefield effect on the electron beam quality.

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 $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ gallium phosphide for a 40 MeV electron bunch, and the corresponding length is 106 fs (rms) with $300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ 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 $\ensuremath{\sim}60\text{ }\text{ }\mathrm{fs}$ (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.

Cold cathodes have attracted a lot of attention in the field of accelerators in recent years. While the development of suitable cold cathodes is in progress, attempts have been made to combine the cold cathode with a rf structure. Due to the strong dependence on the electric field, field emissions peak at the wave crest, which is not the best injection phase, during a rf cycle. To make the injection phase adjustable, a flexible double-frequency rf gun is designed. The addition of a 3rd-harmonic field to the fundamental one in the half cell will move the wave crest toward a better injection phase and make the initial bunch length shorter. The full cell is resonant at the fundamental frequency. Since only the half cell is resonant at two frequencies, the gun can be easily tuned. Simulations show that the time-dependent rf effects on the transverse and longitudinal phase spaces of the electron bunch can be reduced by choosing proper rf parameters and the space charge effects can be compensated for by using an external solenoid field. Therefore, the gun is able to provide low emittance, low energy spread and short electron bunches with high average current.

Based on the collision of intense laser and relativistic electrons, a Thomson scattering x-ray source can produce quasi-monochromatic x-ray pulses with high brightness in the tens keV or even higher energy regime, which can eliminate the beam hardening effect encountered in computed tomography (CT) by using polychromatic x-rays generated through Bremsstrahlung and make it possible to relate the reconstructed linear attenuation coefficients to the composition of a material. In this paper, we demonstrate the capacity of quantitative CT measurement based on Tsinghua Thomson scattering X-ray source (TTX) and the potential of anatomical segmentation using quantitative linear attenuation coefficient analysis. A peanut sample (Arachis hypogaea L.) was chosen for this study. According to the reconstructed CT image, all anatomical structures except for the testa (i.e. the seed coat) of peanut were identified clearly in terms of the shape and size, and there were high similarities between reconstructed linear attenuation coefficients of cotyledon and its theoretical values. After quantitative analysis of the reconstructed linear attenuation coefficients, the hull can be peeled off the core at the threshold of 0.31 cm -1 . Our results pave the way towards fundamental researches and practical applications based on quantitative CT at TTX.

A novel method using high-power laser as a circular deflector is proposed for the measurement of femtosecond (fs) and sub-fs electron beam. In the scheme, the electron beam interacts with a laser pulse operating in a radially polarized doughnut mode (TEM_{01^{*}}) in a helical undulator, generating angular kicks along the beam in two directions at the same time. The phase difference between the two angular kicks makes the beam form a ring after a propagation section with appropriate phase advance, which can reveal the current profile of the electron beam. Detailed theoretical analysis of the method and numerical results with reasonable parameters are both presented. It is shown that the temporal resolution can reach up to ∼100 attosecond, which is a significant improvement for the diagnostics of ultrashort electron beam.

We propose a method based on the slice energy spread modulation to generate strong subpicoseond 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 further applications, such as terahertz (THz) radiation and resonant excitation of plasma wakefield. 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.

As Thomson scattering x-ray sources based on the collision of intense laser and relativistic electrons have drawn much attention in various scientific fields, there is an increasing demand for the effective methods to reconstruct the spectrum information of the ultra-short and high-intensity x-ray pulses. In this paper, a precise spectrum measurement method for the Thomson scattering x-ray sources was proposed with the diffraction of a Highly Oriented Pyrolytic Graphite (HOPG) crystal and was demonstrated at the Tsinghua Thomson scattering X-ray source. The x-ray pulse is diffracted by a 15 mm (L) ×15 mm (H)× 1 mm (D) HOPG crystal with 1° mosaic spread. By analyzing the diffraction pattern, both x-ray peak energies and energy spectral bandwidths at different polar angles can be reconstructed, which agree well with the theoretical value and simulation. The higher integral reflectivity of the HOPG crystal makes this method possible for single-shot measurement.

Non-paraxial perturbation wave equations are solved for general astigmatic Gaussian beams for the first time, to the best of our knowledge, in the angular spectrum representation by taking into account generic boundary conditions. Expressions for second-order corrections are derived and exemplified with an optical cavity made of two cylindrical mirrors. Non-paraxial corrections can lead, depending on the choice of boundary conditions, to a transverse S-shaped beam mode, which has been qualitatively been observed in a highly divergent non-planar four-mirror cavity.

A theoretical model is established to simulate the stimulated Brillouin scattering (SBS) effect in fiber amplifiers using white noise signal (WNS) phase-modulated seed. Based on the theoretical model, the influences of amplifier structure and cut-off frequency of WNS on SBS threshold are discussed in detail. Through theoretical analysis, optimized amplifier structure and phase modulation signal are achieved to suppress SBS, and a fiber laser based on a main oscillator power amplifier with counter-pumped scheme is established. The oscillator, with 3 dB spectral linewidth of 48 GHz, is produced by a WNS phase-modulated single-frequency seed. The amplifier uses a 915 nm diode laser pump and 25/400 Yb-doped optical fiber. The output power reaches 3.02 kW at 1064 nm, with 3 dB root-mean-square linewidth of 48 GHz. Amplified quality is obtained with M, M at maximum power.

Shanghai soft X-ray free-electron laser facility (SXFEL) is the first coherent X-ray light source in China with the shortest wavelength down to 2 nm. SXFEL is based on a 1.5 GeV C-band linac, including one seeded FEL line, one self-amplified spontaneous emission FEL line and five experimental stations. The development of SXFEL is in two stages, the test facility SXFEL-TF and the user facility SXFEL-UF. SXFEL-TF is based on a 0.84 GeV linac, aiming to master the key technologies of a cascaded FEL with high gain harmonic generation (HGHG) and echo-enabled harmonic generation (EEHG) scheme. Meanwhile, the object of SXFEL-UF is to start user operation at the end of 2019. This paper presents the design, construction and commissioning status of SXFEL. © 2019, Chinese Lasers Press. All right reserved.

Higher order modes self-excitation can seriously degrade klystron performance and cause pulse shortening in the high-gain relativistic klystron amplifier. In order to suppress higher order modes self-oscillation in the device, some special measures are taken to increase the threshold current of such modes. An output power of 1.02 GW with an efficiency of 24.3% and a gain of 53.7 dB is obtained in the simulation with an input power of 4.3 kW and a frequency of 2.88 GHz. The experiment is performed on the linear transformer driver. Under conditions of an RF power of 30 kW, the peak output power has achieved 1.2 GW, with an efficiency of 28.6% and a gain of 46 dB; the corresponding pulse length is 120 ns.

The radiation of high-gain short-wavelength free-electron laser depends on the slice transverse emittance of the electron bunch. This essay introduces the method of slice emittance measurement, and shows the brief setup of this experiment using the solenoid scanning and RF deflecting cavity at Tsinghua University. The preliminary experimental results show that the slice rms emittance of the electron bunch generated by photocathode RF gun has considerable variations along the bunch and is typically less than 0.55 mm mrad for the laser rms radius of 0.4 mm.

X-ray sources based on Thomson-scattering require strict synchronization between the laser pulse and the electron bunch. Jitter between the laser and the RF phase should be kept within a femtosecond scale to guarantee electron beam quality and the time of flight before impact with the terawatt laser pulse. A laser–RF synchronizer that uses the 36th harmonic of the laser repetition rate was built at Tsinghua University, and it has tested to have a less than 100-fs [10-Hz, 100-kHz] rms time jitter. An RF deflector cavity is used to verify the electron bunch pulse-to-pulse time jitter with respect to the RF accelerating phase in the same condition, and the result shows a 540-fs rms jitter. This paper discusses the synchronization experiment setup, as well as the time jitter measurements under different conditions.

A scheme, spontaneous emission high-gain harmonic generation (SEHG) free-electron laser (FEL), is proposed and analyzed for generating the X-ray FEL. The SEHG scheme works in a similar mechanism as high-gain harmonic generation (HGHG), but without the need for a seed laser. The scheme requires two undulators. The 1st undulator must be sufficiently long so that the energies of electrons are modulated within the bunch, but still away from saturation. A dispersion section is followed to transfer energy modulation into density modulation. The 2nd undulator simply serves as a radiator. A simple, one-dimensional, analytical estimation of SEHG is given to show the process of energy modulation and optimize the system parameters. The three-dimensional FEL simulation code, GENESIS, has been used to simulate, verify, and optimize the SEHG scheme for the soft X-ray free-electron laser (SXFEL) project in China. The simulation results are presented in comparison with the self-amplified spontaneous emission (SASE) and HGHG schemes. At 9 nm radiation wavelength, up to 120 MW of output power can be achieved by the SEHG scheme, with a total length of 47.3 m long undulators. Though the undulator length is comparable with the SASE scheme, the output bandwidth of the SEHG scheme is smaller. Moreover, it is tunable and does not require a seed laser. The SEHG scheme offers an attractive alternative option for the X-ray FEL.

The development, design, and construction of the Tsinghua Thomson scattering X-ray (TTX) source have been ongoing since 2001. The TTX source is based on the Thomson scattering of a femtosecond laser pulse by a relativistic electron beam, which aims at an ultra-fast, high-flux, monochromatic, and tunable X-ray source for scientific, medical, and industrial applications. A recent experiment sought to generate a soft X-ray pulse through Thomson scattering. In this experiment, a 3-eV electron beam generated from a photocathode radio-frequency gun is focused by quadrupole magnets to several hundred microns and collided with a 120-mJ, 60-fs laser beam. The generated X-ray is detected by a micro-channel plate. The energy, pulse duration, and number of X-ray photons is estimated to be 290.4 eV, 1 ps, and 6.4×10 3 , respectively.

With the recent rapid development of x-ray imaging using Thomson scattering x-ray sources, there is an increasing demand for knowledge of the focal spot characteristics of this type of light source. This is because imaging quality is closely related to focal spot characteristics. In this paper, an analytical expression of the focal spot size and beam parameters at arbitrary interaction angles is derived. Based on this, the focal spot can be optimized in a straightforward manner by adjusting the laser and electron focusing at a fixed interaction geometry. As the beam focusing increases, a smaller focal spot and a higher photon yield are obtained, while the bandwidth of scattered x rays deteriorates, leading to a compromise of the above three physical parameters. Considering the beam position and time fluctuations, the focal spot will be broadened and the photon yield will be reduced. Using typical beam parameters at the Tsinghua Thomson scattering x-ray source, it is shown that a micrometer-sized focal spot can be easily achieved. The results will help one to develop the x-ray imaging field, especially in micro-computed tomography applications based on Thomson light sources.

kW-level 1030 nm polarization-maintained fiber laser with narrow linewidth and near-diffraction-limited beam quality is demonstrated. Theoretical simulations based on the power balance equation are first performed to optimize the system parameters of the 1030 nm ytterbium-doped fiber laser for the maximum suppression of amplified spontaneous emission (ASE). With the optimized parameters, both the copumped and counterpumped MOPA lasers are implemented to obtain an output power of >1 kW. In both cases, the ASE suppression ratio reaches 40 dB with a 3 dB linewidth of about 0.14 nm, and the polarization extinction ratio is about 12 dB at 1 kW of output power. The beam quality starts degrading at 900 W of output power in the copumped structure, but maintains nearly single mode (Mx2,My2)=(1.07,1.12) until power is over 1 kW in the counterpumped structure.

In this paper, we present an improvement of Martin–Puplett interferometer (MPI) for terahertz frequency measurement, in which flat mirrors replace the roof ones. To understand the function of the improved MPI, its basic principle is described and the characteristics of electric field are analyzed. With the improved MPI, the operation frequency of a far-infrared laser is conveniently measured, and the experimental result is in good agreement with that of the far-infrared laser theoretical reported.

© 2018 Chinese Optics Letters. Coherent pulse stacking (CPS) is a new time-domain coherent addition technique that stacks several optical pulses into a single output pulse, enabling high pulse energy and high average power. A Z-domain model targeting the pulsed laser is assembled to describe the optical interference process. An algorithm, extracting the cavity phase and pulse phases from limited data, where only the pulse intensity is available, is developed to diagnose optical cavity resonators. We also implement the algorithm on the cascaded system of multiple optical cavities, achieving phase errors less than 1.0°(root mean square), which could ensure the stability of CPS.

We proposed the Tsinghua Thomson scattering X-ray (TTX) source as an ultra-fast, high flux source for advanced X-ray imaging studies and applications. A linac system, which consists of an S-band photocathode RF gun, a SLAC type 3 m traveling wave tube and two X-band structures, generates ultra-short, high brightness electron pulses to scatter with tera-watt femto-second laser pulses. A compact low energy electron storage ring is also designed to dramatically enhance the average X-ray flux. In this paper, we present the simulation studies and optimized parameters of the electron and X-ray pulses. The construction and commissioning status of TTX is also reported.

A mode analysis is carried out with the MAFIA TS3 module to study the high-power microwave generation in the inward-emitting coaxial vircator with two opposed nonaxisymmetric emitting surfaces. The central angle thetas of the emittors, and the axial distance D between the cathode and anode reflector, are verified to have a large influence on the mode component. For a fixed diode voltage of 250 kV, the coaxial diode current is 13.7 kA when the beam injection is axisymmetric (thetas = 180deg). In this situation, the simulation gives the highest efficiency of 8.3% for D = 5.0 cm and 10.9% for D = 10.0 cm , comparing with the nonaxisymmetric beam injection. For the case of two opposed nonaxisymmetric emitting surfaces with central angle thetas, there exists a range for thetas to acquire a rather pure output of the TE11 mode (80% of the total power), which is 80deg < thetas < 160deg. However, the efficiency decreases with smaller thetas. The polarized direction of the main TE11 mode is consistent with the directional layout of the two opposed emitters. FFT transformation shows that the dominant frequency for thetas < 80deg can be estimated by the equations derived from the 2-D theoretical analysis of the symmetric coaxial vircator.

An RF deflecting structure has been designed and fabricated for a transverse-to-longitudinal emittance exchange experiment at Argonne National Laboratory (ANL) [Y.-E. Sun, et al., Design study of a transverse-to-longitudinal emittance exchange proof-of-principle experiment, in: Proceedings of PAC-2007, Albuquerque, NM, USA, 2007, pp. 3441–3443 [1] ]. The structure is a 1300 MHz normal conducting RF cavity consisting of three cells operating in a dipole ( TM 110 -like) mode. As high as 3.4 MV deflecting voltage is required for the experiment. In this paper, we present detailed RF design of the cavity, particle tracking simulations in the cavity and low power RF measurement results in comparison with numerical simulations. The cavity cell geometries (gaps in particular) are optimized to giving a zero orbit offset for particles crossing the cavity with zero-phase.

This paper gives the representation of the electric field for cyclotron accelerating gap with general dee geometry. A special case of this representation is the cosine gradient theory of central region electric field developed by Kost and Mackenzie [IEEE NS-22 (3) (1975) 1922] in TRIUMF for 180° dees. A general expression of energy gain and a formula to determine the vertical electric focusing, which improves the alternating-gradient focusing proposed by Gordon and Marti [Part. Acceler. 11 (1980) 161] using a simple electric-field model, are obtained. In our theory, the velocity-variation π/2 focusing and radius-change effect have been predicted to perfect the calculation of the vertical electric focusing. We take the 100 MeV cyclotron under design and construction at China Institute of Atomic Energy, CYCIAE-100 [T. Zhang, X. Guan et al., in: Proceedings of the Third Asia Particle Accelerator Conference, Korea, 2004, p. 267], as an application of this theory in two-dee cyclotron running at push-push mode.

This work concentrated on studying the relationship between electron emission and materials properties. Ferroelectric (PLZT 7/65/35) and paraelectric (PLZT 12/65/35) compositions of lead lanthanum zirconate titanate (PLZT) system were chosen to study for their widely different ferroelectric and dielectric properties. The experimental results proved the wrong of the spontaneous polarization switching model which based on the ferroelectric properties of the cathode. The mechanism of electron emission from PLZT ceramics was ascertained to field emission and surface plasma emission. The emission current depends slightly on the ferroelectric properties and depends significantly on the dielectric constant.

Two closely spaced electron bunches have attracted strong interest due to their applications in two color X-ray free-electron lasers as well as witness bunch acceleration in plasmas and dielectric structures. In this paper, we propose a new scheme of delay system to vary the time delay up to several picoseconds while not affecting the bunch compression. Numerical simulations based on the Linac Coherent Light Source are performed to demonstrate the feasibility of this method.

The roughness of a photocathode could lead to an additional uncorrelated divergence of the emitted electrons and therefore to an increased thermal emittance. To calculate the emittance growth due to the cathode roughness, people usually choose a simple 2D sinusoidal surface model to avoid mathematical complexity. In this paper, we demonstrate an analytical method, which is inspired by the point spread function that has been widely used in radiation imaging field, to accurately evaluate the emittance growth due to the random roughness of a real-life cathode. Both analytical and numerical studies are performed. Our analytical formulas clearly reveal the relationship between the surface roughness and the emittance growth. Both analytical and numerical results surprisingly show that in the typical 3D random surface case, the influence of the surface roughness on the emittance growth is much smaller than the 2D sinusoidal case with typical roughness properties, however with roughness properties which are matched to the 3D case, the emittance growth conditions in these two cases are very similar. Even with applied electric field strength up to $120\text{ }\mathrm{MV}/\mathrm{m}$, the total emittance growth is still below 10%. It implies that the large emittance growth (50%--100%) observed on metallic cathodes in some experiments, which is generally believed to be the result of the electric field on the rough surface, might be due to some other reasons.

Strong electron emission and dense plasma generation have been observed in a modulator electron gun with a Ba 0.67 Sr 0.33 TiO 3 ferroelectric cathode. Parameter of the modulator electron gun and lifetime of the ferroelectric cathode were investigated. It was shown that electron emission from Ba 0.67 Sr 0.33 TiO 3 cathode with a positive triggering pulse is a sort of plasma emission. Electrons were emitted by the co-effect of surface plasma and non-compensated negative polarization charges at the surface of the ferroelectric. The element analyses of the graphite collector after emission process was performed to show the ingredient of the plasma consist of Ba, Ti and Cu heavy cations of the ceramic compound and electrode. It was demonstrated the validity of the Child–Langmuir law by introducing the decrease of vacuum gap and increase of emission area caused by the expansion of the surface plasma.