Efficient generation of a high-field terahertz pulse train in bulk lithium niobate crystals by optical rectification.

We demonstrate a highly efficient method for the generation of a high-field terahertz (THz) pulse train via optical rectification (OR) in congruent lithium niobate (LN) crystals driven by temporally shaped laser pulses. A narrowband THz pulse has been successfully achieved with sub-percent level conversion efficiency and multi MV/cm peak field at 0.26 THz. For the single-cycle THz generation, we achieved a THz pulse with 373-µJ energy in a LN crystal excited by a 100-mJ laser pulse at room temperature. The conversion efficiency is further improved to 0.77 % pumped by a 20-mJ laser pulse with a smaller pump beam size (6 mm in horizontal and 15 mm in vertical). This method holds great potential for generating mJ-level narrow-band THz pulse trains, which may have a major impact in mJ-scale applications like terahertz-based accelerators and light sources.

Application of a drift compensation low-level radio frequency system based on time-multiplexing pick-up/reference signals.

The high accuracy, low drift low-level radio frequency (LLRF) system is essential for the long-term stability of the accelerator RF and the acquirement of low emittance, high intensity electron beams. A time-multiplexing pick-up/reference signal based LLRF system is proposed to deal with the component temperature related phase drift and has been deployed and applied at the Xi'an Gamma-ray Light Source (XGLS) injector. The long term dual-receiver out-of-loop stability experiments with a continuous wave laser based phase reference distribution system (PRDS) show that the LLRF system can achieve ∼40 fs Root-Mean-Square (rms) phase accuracy and 51 fs/52 fs peak-peak drift (in 7 days/17 h with the high power RF system, respectively) while the reference phase varies both ∼30 ps. An ∼4 h beam-based experiment has also been conducted to evaluate the overall performance of the whole XGLS timing and synchronization system, which shows that the PRDS, LLRF system, high power RF system, and laser oscillator laser-RF synchronization system can keep long-term phase stability.

Superhydrophobic metal organic framework doped polycarbonate porous monolith for efficient selective removal oil from water.

A series of superhydrophobic polycarbonate porous monoliths modified with metal organic framework (Z8/PC) were firstly fabricated through a facile thermally impacted non-solvent induced phase separation method for efficient selective oil/water separation. The performance of the monoliths on oil/water separation was evaluated in terms of selectivity, equilibrium adsorption capacity, corrosion resistance, kinetics, and circulation. The results showed that the use of ZIF-8 significantly compensated for the shortage of pure monolith. Compared with pure PC monolith, the hydrophobic angle of the Z8/PC-2 monolith promoted from 136.18° to 154.25° due to the micro-nano flower surface. Meanwhile, the Z8/PC-2 monolith displayed a more intricate and continuous interconnected 3D hierarchical micro-nano structure, which possessed the monolith a higher specific surface area of 146.84 m2 g-1 and porosity of 89.5%. What's more, more superior oil/water separation abilities of Z8/PC-2 monolith were manifested by the selective removal of oil or organic solvent from water within 30s, high equilibrium adsorption capacity, and excellent corrosion resistance. In addition, the ten-cycle regeneration of porous monoliths via centrifugation or evaporation displayed additional attractiveness. Therefore, porous Z8/PC monolith will be a promising candidate for the efficient selective oil/water separation of oil spills and organic solvents.

Linearly polarized X-ray fluorescence computed tomography based on a Thomson scattering light source: a Monte Carlo study.

A Thomson scattering X-ray source can provide quasi-monochromatic, continuously energy-tunable, polarization-controllable and high-brightness X-rays, which makes it an excellent tool for X-ray fluorescence computed tomography (XFCT). In this paper, we examined the suppression of Compton scattering background in XFCT using the linearly polarized X-rays and the implementation feasibility of linearly polarized XFCT based on this type of light source, concerning the influence of phantom attenuation and the sampling strategy, its advantage over K-edge subtraction computed tomography (CT), the imaging time, and the potential pulse pile-up effect by Monte Carlo simulations. A fan beam and pinhole collimator geometry were adopted in the simulation and the phantom was a polymethyl methacrylate cylinder inside which were gadolinium (Gd)-loaded water solutions with Gd concentrations ranging from 0.2 to 4.0 wt%. Compared with the case of vertical polarization, Compton scattering was suppressed by about 1.6 times using horizontal polarization. An accurate image of the Gd-containing phantom was successfully reconstructed with both spatial and quantitative identification, and good linearity between the reconstructed value and the Gd concentration was verified. When the attenuation effect cannot be neglected, one full cycle (360°) sampling and the attenuation correction became necessary. Compared with the results of K-edge subtraction CT, the contrast-to-noise ratio values of XFCT were improved by 2.03 and 1.04 times at low Gd concentrations of 0.2 and 0.5 wt%, respectively. When the flux of a Thomson scattering light source reaches 1013 photons s-1, it is possible to finish the data acquisition of XFCT at the minute or second level without introducing pulse pile-up effects.

TiO2 pillared montmorillonite in-situ growth of CeOx/MnOy nanoparticles for effective arsenic (III) adsorption in wastewater.

Compared with As(V), As(III) is a tricky issue worldwide for its higher toxicity and more difficult to remove in aqueous solution. In present study, a novel CeOx/MnOy nanoparticles anchored layered structural TiO2 pillared montmorillonite (TiO2-Mt-Ce-Mn) was fabricated and applied as an efficient absorbent for As(III) removal. Under the condition of the initial As(III) concentration = 20 mg/L and adsorbent dose = 0.4 g/L, TiO2-Mt-Ce-Mn with a high specific surface area (148.099 m2/g) has an outstanding adsorption capacity (46.58 mg/g) for As(III) at pH 4.2, and the effect of oxy-anions on adsorption efficiency is slight except for H2PO4-. Interestingly, the layered structure provides sufficient attachment space for CeOx/MnOy nanoparticles, while CeOx/MnOy nanoparticles in turn endows TiO2-Mt a high redox potential, which further facilitates the oxidation of As(III), and this significantly reduces the toxicity of wastewater. The adsorption mechanism includes the oxidation of As(III) to As(V) by both CeOx/MnOy nanoparticles and TiO2 and effective adsorption of the residual As(III) and the formed As(V) subsequently. In addition, the adsorption efficiency of TiO2-Mt-Ce-Mn can still maintain 79.6% after five cycles through a facile regeneration method. Thus, the nanocomposite with low-cost synthesis process, high adsorption capacity, and regenerability is a promising candidate for As(III) treatment of wastewater.

A low level radio frequency system drift compensation technique by time-multiplexing pick-up/reference signals.

The Low Level radio frequency system long-term stability is critical for the operation of accelerator facilities. The RF cavity field phase drift observed at the Tsinghua Thomson scattering X-ray source showed the correlations with devices temperature characteristic. We proposed a drift compensation technique by time-multiplexing cavity pick-up and phase reference signals, which guaranteed that they shared the same route with the same change. The preliminary ∼84 h Dual-Receiver out-of-loop stability test showed phase drift of 100 fs peak-peak (∼45 fs rms) when the reference signal phase changed ∼40 ps peak-peak.

Experimental feasibility of dual-energy computed tomography based on the Thomson scattering X-ray source.

Unlike large-scale and expensive synchrotron radiation facilities, the Thomson scattering X-ray source can provide quasi-monochromatic, energy-tunable and high-brightness X-ray pulses with a small footprint and moderate cost, making it an excellent candidate for dual-energy and multi-energy imaging at laboratories and hospitals. Here, the first feasibility study on dual-energy computed tomography (CT) based on this type of light source is reported, and the effective atomic number and electron-density distribution of a standard phantom consisting of polytetrafluoroethylene, water and aluminium is derived. The experiment was carried out at the Tsinghua Thomson scattering X-ray source with peak energies of 29 keV and 68 keV. Both the reconstructed effective atomic numbers and the retrieved electron densities of the three materials were compared with their theoretical values. It was found that these values were in agreement by 0.68% and 2.60% on average for effective atomic number and electron density, respectively. These results have verified the feasibility of dual-energy CT based on the Thomson scattering X-ray source and will further expand the scope of X-ray imaging using this type of light source.

Proposal of a femtosecond megahertz repetition-rate electron diffraction instrument based on the Chinese Academy of Engineering Physics terahertz free electron laser beamline.

We report the design and optimization of a femtosecond electron diffraction instrument with megahertz repetition-rate. The proposed instrument is mainly based on the photoinjector of the Chinese Academy of Engineering Physics Terahertz free electron laser facility, which merges a 320 kV DC photocathode gun with two 4-cell super-conducting linac operating at 1.3 GHz. Simulations show that high quality electron probes with fC charge, ultralow emittance, and femtosecond scale bunch duration can be generated. Together with the capability of operating at continuous wave mode, this instrument is well matched with the demand of future electron diffraction.

Non-perturbing THz generation at the Tsinghua University Accelerator Laboratory 31 MeV electron beamline.

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.

Necrosis of osteosarcoma cells induces the production and release of high-mobility group box 1 protein.

Osteosarcoma is among the commonly observed malignancies worldwide. High-mobility group box 1 protein (HMGB1) is a highly conserved protein and is involved in the progression of various types of human cancer. The aim of the present study was to explore whether the level of HMGB1 was involved in the necrosis of osteosarcoma cells. Doxorubicin (DXR), as an inducer of necrosis, was administered to human osteosarcoma cell lines (MG63, Saos-2 and U2OS), and the results indicated that 0.5 µg/ml DXR significantly induced the necrosis of MG63 cells (P<0.01), while 0.5 and 1.0 µg/ml DXR suppressed the viability of MG63 and U2OS cells (P<0.05), relative to untreated controls. Additionally, treatment with DXR was observed by western blot analysis to markedly increase the expression levels of HMGB1 in MG63 cells, and to significantly increase the levels of secreted HMGB1 in the supernatants of MG63 and U2OS cells (P<0.01). In conclusion, cell necrosis increased the level of HMGB1 in osteosarcoma cells, as well as the level of secreted HMGB1 in cell supernatants. Therefore, HMGB1 may be a potential target in molecular therapy for patients with osteosarcoma.

Development of sub-100 femtosecond timing and synchronization system.

The precise timing and synchronization system is an essential part for the ultra-fast electron and X-ray sources based on the photocathode injector where strict synchronization among RF, laser, and beams are required. In this paper, we present an integrated sub-100 femtosecond timing and synchronization system developed and demonstrated recently in Tsinghua University based on the collaboration with Lawrence Berkeley National Lab. The timing and synchronization system includes the fiber-based CW carrier phase reference distribution system for delivering stabilized RF phase reference to multiple receiver clients, the Low Level RF (LLRF) control system to monitor and generate the phase and amplitude controllable pulse RF signal, and the laser-RF synchronization system for high precision synchronization between optical and RF signals. Each subsystem is characterized by its blocking structure and is also expansible. A novel asymmetric calibration sideband signal method was proposed for eliminating the non-linear distortion in the optical synchronization process. According to offline and online tests, the system can deliver a stable signal to each client and suppress the drift and jitter of the RF signal for the accelerator and the laser oscillator to less than 100 fs RMS (∼0.1° in 2856 MHz frequency). Moreover, a demo system with a LLRF client and a laser-RF synchronization client is deployed and operated successfully at the Tsinghua Thomson scattering X-ray source. The beam-based jitter measurement experiments have been conducted to evaluate the overall performance of the system, and the jitter sources are discussed.

Measurement of pre-bunched beam's longitudinal form factor based on radiation from a tunable-gap undulator.

The form factor, representing the statistical characteristics of a bunch's longitudinal distribution, is one of the most essential properties of a pre-bunched electron beam and is used for many types of frontier accelerator applications. We demonstrated the measurement of a pre-bunched beam's longitudinal form factor component based on coherent radiation from a widely tunable-gap undulator. The radiation energy from bunches with different longitudinal properties was measured as a function of undulator gap. The root-mean-square length of a 60 pC ultrashort quasi-Gaussian bunch generated by linac and chicane compression ranged from 75 fs to 240 fs, as obtained by fitting the radiation energy curve. Furthermore, the form factor component of the bunch train based on nonlinear longitudinal space charge oscillation was measured, and a higher-order harmonic component was observed with the proposed method than with the widely used coherent transition radiation method. The proposed method may satisfy the requirements of sub-fs bunch length measurement with proper undulator design.

A pulse-to-pulse timing jitter measurement between two synchronized amplified laser beams for TTX.

In China, Tsinghua Thomson Scattering X-ray Source (TTX) is the dedicated hard X-ray source based on the Thomson scattering between a terawatt ultrashort laser and a relativistic electron beam. In the TTX, two synchronized Ti: sapphire laser systems generate the terawatt ultrashort infrared scattering laser and the ultraviolet driving laser for the photocathode RF gun to produce the electron beam; measuring the timing jitter between the electron beam and the laser beam is an essential task for the X-ray source. In the present study, we report on a single shot, non-collinear cross correlator with fs resolution and measured the timing jitter between the two synchronized laser systems with a pulse-to-pulse method, which is beneficial to estimate the jitter of the X-ray yield in the TTX system. Although it is more important to synchronize the scattering laser to the electron beam and not of the driving laser, the laser-laser jitter measurement would be a good first step towards that goal, and the result generated can be considered as the error signal for the potential feedback stabilization.

Diffraction based method to reconstruct the spectrum of the Thomson scattering x-ray source.

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.

Tunable High-Intensity Electron Bunch Train Production Based on Nonlinear Longitudinal Space Charge Oscillation.

High-intensity trains of electron bunches with tunable picosecond spacing are produced and measured experimentally with the goal of generating terahertz (THz) radiation. By imposing an initial density modulation on a relativistic electron beam and controlling the charge density over the beam propagation, density spikes of several-hundred-ampere peak current in the temporal profile, which are several times higher than the initial amplitudes, have been observed for the first time. We also demonstrate that the periodic spacing of the bunch train can be varied continuously either by tuning launching phase of a radio-frequency gun or by tuning the compression of a downstream magnetic chicane. Narrow-band coherent THz radiation from the bunch train was also measured with µJ-level energies and tunable central frequency of the spectrum in the range of ∼0.5 to 1.6 THz. Our results pave the way towards generating mJ-level narrow-band coherent THz radiation and driving high-gradient wakefield-based acceleration.

Studying the Polarization Switching in Polycrystalline BiFeO3 Films by 2D Piezoresponse Force Microscopy.

For rhombohedral multiferroelectrics, non-180° ferroelectric domain switching may induce ferroelastic and/or (anti-)ferromagnetic effect. So the determination and control of ferroelectric domain switching angles is crucial for nonvolatile information storage and exchange-coupled magnetoelectric devices. We try to study the intrinsic characters of polarization switching in BiFeO3 by introducing a special data processing method to determine the switching angle from 2D PFM (Piezoresponse Force Microscopy) images of randomly oriented samples. The response surface of BiFeO3 is first plotted using the piezoelectric tensor got from first principles calculations. Then from the normalized 2D PFM signals before and after switching, the switching angles of randomly oriented BiFeO3 grains can be determined through numerical calculations. In the polycrystalline BiFeO3 films, up to 34% of all switched area is that with original out-of-plane (OP) polarization parallel to the poling field. 71° polarization switching is more favorable, with the area percentages of 71°, 109° and 180° domain switching being about 42%, 29% and 29%, respectively. Our analysis further reveals that IP stress and charge migration have comparable effect on switching, and they are sensitive to the geometric arrangements. This work helps exploring a route to control polarization switching in BiFeO3, so as to realize desirable magnetoelectric coupling.

In-line phase-contrast imaging based on Tsinghua Thomson scattering x-ray source.

Thomson scattering x-ray sources can produce ultrashort, energy tunable x-ray pulses characterized by high brightness, quasi-monochromatic, and high spatial coherence, which make it an ideal source for in-line phase-contrast imaging. We demonstrate the capacity of in-line phase-contrast imaging based on Tsinghua Thomson scattering X-ray source. Clear edge enhancement effect has been observed in the experiment.

Generation of first hard X-ray pulse at Tsinghua Thomson Scattering X-ray Source.

Tsinghua Thomson Scattering X-ray Source (TTX) is the first-of-its-kind dedicated hard X-ray source in China based on the Thomson scattering between a terawatt ultrashort laser and relativistic electron beams. In this paper, we report the experimental generation and characterization of the first hard X-ray pulses (51.7 keV) via head-on collision of an 800 nm laser and 46.7 MeV electron beams. The measured yield is 1.0 × 10(6) per pulse with an electron bunch charge of 200 pC and laser pulse energy of 300 mJ. The angular intensity distribution and energy spectra of the X-ray pulse are measured with an electron-multiplying charge-coupled device using a CsI scintillator and silicon attenuators. These measurements agree well with theoretical and simulation predictions. An imaging test using the X-ray pulse at the TTX is also presented.

Note: Single-shot continuously time-resolved MeV ultrafast electron diffraction.

We have demonstrated single-shot continuously time-resolved MeV ultrafast electron diffraction using a static single crystal gold sample. An MeV high density electron pulse was used to probe the sample and then streaked by an rf deflecting cavity. The single-shot, high quality, streaked diffraction pattern allowed structural information within several picoseconds to be continuously temporally resolved with an approximately 200 fs resolution. The temporal resolution can be straightforwardly improved to 100 fs by increasing the streaking strength. We foresee that this system would become a powerful tool for ultrafast structural dynamics studies.

Experimental demonstration of high quality MeV ultrafast electron diffraction.

The simulation optimization and an experimental demonstration of improved performances of mega-electron-volt ultrafast electron diffraction (MeV UED) are reported in this paper. Using ultrashort high quality electron pulses from an S-band photocathode rf gun and a polycrystalline aluminum foil as the sample, we experimentally demonstrated an improved spatial resolution of MeV UED, in which the Debye-Scherrer rings of the (111) and (200) planes were clearly resolved. This result showed that MeV UED is capable to achieve an atomic level spatial resolution and a approximately 100 fs temporal resolution simultaneously, and will be a unique tool for ultrafast structural dynamics studies.