Yonggui Li

Sensitivity and stability of substituted phthalocyaninato-polysiloxane Langmuir–Blodgett films and its gas sensor

Abstract The monolayer and Langmuir–Blodgett (LB) film deposition behavior of tetrakis-4-(2,4-di-tert-amylphenoxy) phthalocyaninato-polysiloxane (R4PcPS) were studied. LB film gas-sensitive device was prepared by depositing R4PcPS LB films on a glass substrate with aluminum interdigital electrodes. The results on dynamic gas sensing response characteristics showed that the R4PcPS LB film gas-sensitive device has a higher detecting sensitivity on exposure to trace ammonia (1 ppm of NH3) in air, as well as comparatively lower detecting sensitivity to NO2 and I2 (100 ppm) in air. On the other hand, the R4PcPS LB film gas-sensitive device shows excellent stability.

Numerical study on improvements of Beijing FEL lasing performances through modifications of the beam-duct geometry

In January 1997, the first lasing at 8-18 mu m was observed at Beijing FEL (BFEL). However, the output powers were found not large enough especially in the longer wavelength regime, where the laser power loss on the relatively narrow beam-duct wall is not expected to be small, and actually, the saturation of the laser power was not seen. In order to evaluate and to improve the effects due to the beam duct, we have modified our three-dimensional FEL code to accommodate the finite beam-duct size. It is predicted that more than three times as large as the output power is theoretically achieved at an 18 mu m wavelength with newly designed beam ducts

Upgrade of Beijing IR-FEL project

Beijing IR Free-Electron laser facility has been under upgrade in several aspects. For system modification, a new undulator beam line parallel to the old one that permits BFEL to simultaneously operate at two different wavelengths and a beam line to produce coherent synchrotron radiation and Compton back-scattering soft X-ray are being added. Optical beam pipe from undulator output to the experiment rooms is under fabrication. For improvement of the stability of operation, much work had been done to reduce the fluctuations of the electron beam parameters and the time jitter of the RF pulses. For extension of wavelength of operation, Au plated copper mirrors with axial out-coupling hole were used instead of the original multi-layer dielectric mirrors. For improving the laser performance, a new 31/2 cavity RF gun is pending installation, and a self-adaptive, feed-forward control system for beam loading compensation of the microwave electron gun to avoid longitudinal emittance blow-up has been successfully tested. Separate, independent RF power feeds of the RF gun and linac section will be implemented to facilitate the optimization of the operating conditions

Beam-Loading Compensation for RF Gun by Feed-Forward Control System

A feed-forward automatic control system based on a novel control principle is developed, in order to compensate the amplitude and phase fluctuations of the microwave field in the thermionic RF gun cavity. The fluctuations, which are mainly caused by beam-loading effect, can be effectively restrained through this method. It is experimentally demonstrated that the novel control system has the excellent characteristics of stability and reliability.

Analysis About SASE Experiment on BFEL Facility

Investigation on SASE experimental demonstration on BFEL facility is presented in this paper. Both linear theory and simulation method are used in order to get useful results for guiding the future experiments. The requirements on electron beam and long wiggler are proposed and the possibility of realization is explored.

Compositional change in human enamel irradiated with MIR free electron laser

The purpose of this study was to investigate compositional changes in human enamel irradiated with the free electron laser (FEL). The exposure on dental enamel at the wavelength of 9.64 μm was observed with the Beijing free electron laser. The distribution of elements in the irradiated or non-irradiated enamel was measured by scanning electron microscope (SEM) with energy-dispersive spectroscopy and synchrotron radiation X-ray fluorescence (SRXRF) in Beijing Synchrotron Radiation Facility (BSRF). The results showed that the P/Ca ratio in the ablation region of enamel at the maximum wavelength of infrared absorption was obviously smaller than that at the non-maximum wavelength. In the ablation region the ratios of P/Ca and Ca/Sr were smaller than those in the non-ablation region. The distributions of P, Ca and Sr in the ablation region were heterogeneous due to the element change caused by FEL irradiation.

Applications of the Beijing IR-free electron laser facility

We report on the upgrade of the Beijing TR free electron laser (BFEL) facility, including improvements of the system stability, performance upgrade and optical wavelength range extension. Applications in the field of material sciences, such as the development of mid- and far-infrared detectors made from YBa2Cu3O7 high T-c thin films, and laser isotope separation, as well as applications in the life sciences, such as protein folding studies, and plant denaturation observations, are described

Free electron laser induced two-photon photoconductivity in Hg1-xCdxTe

The Beijing free electron laser ( BFEL) has been employed for the first time to study the nonlinear photoconductivity characteristics of the typical infrared photoelectronic material Hg1-xCdxTe Taking advantage of the high photon flux density of BFEL, we have investigated the photoconductivity characteristics in Hg1-xCdx Te induced by two-photon absorption by means of the photoconductivity technique, observed the photoconductivity signals saturation, and studied the two-photon photoconductivity characteristics on different bias voltages across the sample.

Dynamic desynchronization of BFEL optical resonator

The start-up of a free electron laser driven by short electron bunches is lethargic when the cavity roundtrip time is perfectly synchronized to the electron bunch repetition rate. In order to restore the gain, it is necessary to slightly desynchronize the resonator, which is usually done by reducing the resonator length. At the desynchronism resulting in the largest small-signal gain per pass during the build-up stage, the largest laser power output cannot be obtained simultaneously. Dynamic desynchronization of a FEL optical resonator by linear ramping of the operating frequency is not only to restore the gain, but also to optimize the laser output of a FEL oscillator. We use an arbitrary variation of desynchronism instead of the linear variation at FELIX in the experiment of the Beijing FEL to optimize its performance. The fast electronic phase shifter in the RF system is used to adjust the desynchronism of the optical resonator of the Beijing FEL oscillator. In the paper, the basic principle of operation is briefly introduced first. Then, the schematic diagram of the control system is presented. Finally, typical experimental results are given to illustrate the effectiveness of the method.