Characteristic of the ultrawideband electromagnetic radiation generated by PCSS

A novel ultra-wideband electromagnetic pulse generating method based on the photoconductive semiconductor switches (PCSS) is presented. Gallium arsenide is used to develop the PCSS for an ultrashort electromagnetic pulse source. The pulse generated by such PCSS is within picosecond (ps) time scale, and can yield power pulse with an voltage over 10 kV. The experimental results show that the pulses are stable, with the peak-peak amplitude change of 6% and the time jitter within several picoseconds. The radiations of the PCSS triggered by the picosecond laser and fenitosecond laser pulse series illustrate that the electromagnetic pulses would have high repetition of more than 80 MHz and frequency bandwidth of DC-6 GHz. The radiations of "lock-on " mode of the PCSS are also analyzed here. (c) 2007 Wiley Periodicals, Inc.

Homoepitaxial Growth of 4H-SiC and Ti/4H-SiC SBDs

Homoepitaxial growth of4H-SiC on off-oriented Si-face （0001） 4H-SiC substrates was performed at 1500℃ by using the step controlled Epitaxy. Ti/4H-SiC Schottky barrier diodes （SBDs） with blocking voltage over lkV have been made on an undoped epilayer with 32μm in thick and 2-5 × 10^15 cm^-3 in carrier density. The diode rectification ratio of forward to reverse （defined at ± 1V） is over 107 at room temperature and over 10^2 at 538K. Their electrical characteristics were studied by the current-voltage measurements in the temperature range from 20 to 265 ℃. The ideality factor and Schottky barrier height obtained at room temperature are 1.33 and 0. 905eV, respectively. The SBDs have on-state current density of 150A/cm^2 at a forward voltage drop of about 2.0V. The specific on-resistance for the rectifier is found to be as 7.9mΩ · cm^2 and its variation with temperature is T^2.0.

Scaling relationships in indentation of power-law creep solids using self-similar indenters

We use dimensional analysis to derive scaling relationships for self-similar indenters indenting solids that exhibit power-law creep. We identify the parameter that represents the indentation strain rate. The scaling relationships are applied to several types of indentation creep experiment with constant displacement rate, constant loading rate or constant ratio of loading rate over load. The predictions compare favourably with experimental observations reported in the literature. Finally, a connection is found between creep and 'indentation-size effect' (i.e. changing hardness with indentation depth or load).

Fluctuation characteristics of arc voltage and jet flow in a non-transferred dc plasma generated at reduced pressure

A torch with a set of inter-electrode inserts between the cathode and the anode/nozzle with a wide nozzle exit was designed to generate plasma jets at chamber pressures of 500–10 000 Pa. The variation of the arc voltage was examined with the change in working parameters such as gas flow rate and chamber pressure. The fluctuation in the arc voltage was recorded with an oscilloscope, and the plasma jet fluctuation near the torch exit was observed with a high-speed video camera and detected with a double-electrostatic probe. Results show that the 300 Hz wave originated from the tri-phase rectified power supply was always detected under all generating conditions. Helmholtz oscillations over 3000 Hz was detected superposed on the 300 Hz wave at gas flow rates higher than 8.8 slm with a peak to valley amplitude lower than 5% of the average voltage value. No appreciable voltage fluctuation caused by the irregular arc root movement is detected, and mechanisms for the arc voltage and jet flow fluctuations are discussed.

A scaling law of photoionization in ultrashort pulses

By means of the numerical solution of time-dependant Schrodinger equation, we verify a scaling law of photoionization in ultrashort pulses. We find that for a given carrier-envelope phase and duration of the pulse, identical photoionizations are obtained provided that when the central frequency of the pulse is enlarged by k times, the atomic binding potential is enlarged by k times, and the laser intensity is enlarged by k(3) times. The scaling law allows us to reach a significant control over direction of photoemission and offers exciting prospects of reaching similar physical processes in different interacting systems which constitutes a novel kind of coherent control.

Fiber Dispersion and Nonlinearity Influences on Transmissions of AM and FM Data Modulation Signals in Radio-Over-Fiber System

Transmission properties of data amplitude modulation (AM) and frequency modulation (FM) in radio-over-fiber (RoF) system are studied numerically. The influences of fiber dispersion and nonlinearity on different microwave modulation schemes, including double side band (DSB), single side band (SSB) and optical carrier suppression (OCS), are investigated and compared. The power penalties at the base station (BS) and the eye opening penalties of the recovered data at the end users are both calculated and analyzed. Numerical simulation results reveal that the power penalty of FM can be drastically decreased due to the larger modulation depth it can achieve than that of AM. The local spectrum broadening around subcarrier microwave frequency of AM due to fiber nonlinearity can also be eliminated with FM. It is demonstrated for the first time that the eye openings of the FM recovered data can be controlled by its modulation depths and the coding formats. Negative voltage encoding format was used to further decrease the RF frequency thus increase the fluctuation period considering their inverse relationship.

Abnormal phenomenon of the dependence of the retention factors for uncharged species on applied voltage in capillary electrochromatography

Polymethacrylate-based monolithic columns were prepared for capillary electrochromatography (CEC) by in situ copolymerization of butyl methacrylate (BMA), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), and ethylene dimethacrylate (EDMA) in the presence of a porogen in fused-silica capillaries of 100 mum I.D. The abnormal phenomenon that retention factors for neutral species decreases with applied voltage in CEC was observed. Capillary electrophoresis (CE) instruments usually require a period of time to increase voltage from 0 kV to desired value, which is called as ramp time. Such ramp time and any error in the determination of dead time should be taken into account during the accurate calculation of retention factors. After the correction of the retention factors, the plots of the corrected factors for alkylbenzene versus applied voltage were made, the absolute value of the plot slopes are less than 1.8 X 10(-4), Which indicates that the corrected retention times for neutral species do not show any dependence on applied voltage. Further, the plots of the corrected retention times for acidic and basic compounds versus the reciprocal of applied voltage were drawn, where the target compounds were eluted in neutral form. The very nice linearity of the plots was obtained. The linear correlation coefficients are over 0.999. Here, the slopes of the plots represent