Generation of subnanosecond electron beams in air at atmospheric pressure

Optimum conditions for the generation of runaway electron beams with maximum current amplitudes and densities in nanosecond pulsed discharges in air at atmospheric pressure are determined. A supershort avalanche electron beam (SAEB) with a current amplitude of ∼30 A, a current density of ∼20 A/cm², and a pulse full width at half maximum (FWHM) of ∼100 ps has been observed behind the output foil of an air-filled diode. It is shown that the position of the SAEB current maximum relative to the voltage pulse front exhibits a time shift that varies when the small-size collector is moved over the foil surface.     

Effective regimes of runaway electron beam generation in helium,hydrogen, and nitrogen

Runaway electron beam parameters and current-voltage characteristics of discharge in helium, hydrogen, and nitrogen at pressures in the range of several Torr to several hundred Torr have been studied. It is found that the maximum amplitudes of supershort avalanche electron beams (SAEBs) with a pulse full width at half maximum (FWHM) of ∼100 ps are achieved in helium, hydrogen, and nitrogen at a pressure of ∼60, ∼30, and ∼10 Torr, respectively. It is shown that, as the gas pressure is increased in the indicated range, the breakdown voltage of the gas-filled gap decreases, which leads to a decrease in the SAEB current amplitude. At pressures of helium within 20–60 Torr, hydrogen within 10–30 Torr, and nitrogen within 3–10 Torr, the regime of the runaway electron beam generation changes and, by varying the pressure in the gas-filled diode in the indicated intervals, it is possible to smoothly control the current pulse duration (FWHM) from ∼100 to ∼500 ps, while the beam current amplitude increases by a factor of 1.5–3.     

Ultrashort electron beams generated on the flat part of a voltage pulse in nitrogen and helium

In the case of voltage pulses with a small amplitude, an ultrashort avalanche electron beam (UAEB) in a diode filled with nitrogen or helium is generated on a flat part of the pulse. UAEBs obtained at a voltage of 25 kV have a full width at half maximum (FWHM) of about 200 ps and are delayed relative to the voltage pulse front by a time reaching tens of nanoseconds. Waveforms of the electron beam current pulse with several peaks of subnanosecond duration have been observed. At elevated pressures in a gas-filled diode, the voltage across the gap decreases by 10–20% during the electron beam generation.     

Neural Network Modeling and System Simulating for the Dynamic Process of Varied Gap Pulsed GTAW with Wire Filler

As the base of the research work on the weld shape control during pulsed gas tungsten arc welding (GTAW) with wire filler, this paper addressed the modeling of the dynamic welding process. Topside length Lt, maximum width Wt and half-length ratio Rh1 were selected to depict topside weld pool shape, and were measured on-line by vision sensing. A dynamic neural network model was constructed to predict the usually unmeasured backside width and topside height of the weld through topside shape parameters and welding parameters. The inputs of the model were the welding parameters (peak current, pulse duty ratio, welding speed, filler rate), the joint gap, the topside pool shape parameters (Lt, Wt, and Rh1), and their history values at two former pulse, a total of 24 numbers. The validating experiment results proved that the artificial neural network (ANN) model had high precision and could be used in process control. At last, with the developed dynamic model, steady and dynamic behavior was analyzed by simulation experiments, which discovered the variation rules of weld pool shape parameters under different welding parameters, and further knew well the characteristic of the welding process.     

High-current-density subnanosecond electron beams formed in a gas-filled diode at low pressures

The formation of electron beams in a gas diode filled with various gases at low and medium pressures under the action of nanosecond voltage pulses has been studied. It is shown that subnanosecond pulses of the beam current in helium, hydrogen, neon, nitrogen, argon, methane, sulfur hexafluoride, krypton, and xenon can be obtained both at atmospheric pressure and at a pressure of several units or dozens of Torr. In particular, a beam current density above 2 kA/cm² behind the foil at a pulse duration (FWHM) of 250 ps has been obtained in helium-filled diode. On the passage from the regime of ultrashort avalanche electron beam formation to the vacuum diode regime, the beam current pulse amplitude decreases, while both the beam pulse duration (FWHM) and the pulse front width increase.     

Discharge current and ultrashort avalanche electron beam current in a volume nanosecond gas discharge in inhomogeneous electric field

The moment of generation of an ultrashort avalanche electron beam (UAEB) relative to the discharge current pulse front has been determined in a volume discharge formed upon nanosecond breakdown of an air gap at atmospheric pressure in an inhomogeneous electric field. The UAEB current reaches maximum on the front of the discharge current pulse, ～100 ps before the peak of this current. Bias currents with amplitude above 1 kA have been observed. The amplitude of this current increases due to the charging of a capacitor formed by the flat metal anode and a dense discharge plasma expanding from the cathode.     

Subnanosecond electron beams formed in a gas-filled diode

We have studied the conditions for the formation of a pulsed beam of runaway electrons in a diode filled with air at atmospheric pressure, whereby the current and voltage pulses in the system were measured with a subnanosecond time resolution. It is experimentally demonstrated for the first time that the electron beam appears on the leading front of the voltage pulse at a relatively small voltage on the discharge gap. At atmospheric pressure, a full width at half maximum of the current pulse does not exceed 0.3 ns.     

Primary electron pulse shape evaluation in an EBT system

The time resolution of an electron beam testing system (EBT) is mainly related to the primary electron (PE) sampling pulse width. Signal deconvolution techniques are available to enhance the time resolution of the system, provided the PE pulse shape is known with high accuracy. While for high energies this shape has already been evaluated, for the low energies commonly used in MOS IC testing, some additional difficulties must be accounted for, such as increased PE beam spot dispersion, charge trapping into passivation oxides, and lower SIN ratio at the detector. Here, we describe the direct measurement of the PE current used to sample internal voltage waveforms through the use of a fast avalanche photodiode. A numerical simulation has also been performed to help in the correct interpretation of the results. Using a known signal as an input to a matched-impedance microstrip line, a numerical deconvolution technique has been applied to the signal sampled by finite-duration current pulses to evaluate the goodness of the restoration of the original signal     

Generation of subnanosecond 10-GHz pulses in high peak and high average power mode

The regime of excitation of microwave pulses in a 10-GHz range at a pulse duration of 0.8 ns and a peak power of ～2 GW has been studied in a relativistic backward wave oscillator with an extended periodic slow-wave system. A pulsed electron accelerator generating high-current electron beams (3 GW, ～600 keV, ～5 kA, 7 ns) at a repetition rate of 700 Hz and a pulse train width of 1 s has been developed based on a high-voltage generator with inductive energy storage, a semiconductor current interrupter, and a pulse-sharpening hydrogen-filled discharge gap. Optimization of the regime of the field-particle interaction allowed an average microwave power of 2.5 kW to be obtained at a transport magnetic field strength reduced below the cyclotron resonance value.     

Runaway electron beam generation by a plasma cathode in atmospheric air discharge

Electron beam generation in atmospheric air discharge with a flat cathode has been experimentally studied. The discharge was excited by voltage pulses of negative polarity with an amplitude of 220 kV, a full width at half maximum of 2 ns, and a leading front width of 0.7 ns. The electron beam was monitored using luminescence of a layer of ZnS-CdS:Ag phosphor placed behind a 20-μm-thick foil anode. It was found that the intensity and homogeneity of luminescence of the phosphor layer increased when a dielectric tube with a length smaller than half of the interelectrode distance was placed in the near-cathode part of the air gap. It is concluded that a plasma cathode is formed within the volume confined by the tube and electrons emitted from this region are accelerated in the open part of the gap. In addition, the dielectric tube decreases the divergence of the electron beam.     

Effect of nitrogen pressure on the energy of runaway electrons generated in a gas diode

The energy spectra of runaway electrons generated in a gas diode under the action of voltage pulses with a front width of ∼300 ps and amplitude of ∼140 kV have been studied using a time-of-flight spectrometer at nitrogen pressures in a range of 0.1–760 Torr. The delay of runaway electron beam pulse relative to the driving voltage pulse has been determined. The electron energy depends in a complicated manner on the nitrogen pressure in the gas diode and on the cathode geometry. A minimum breakdown voltage for a gap between tubular cathode and flat anode has been observed at a nitrogen pressure of ∼100 Torr. A decrease in the nitrogen pressure below 100 Torr leads to an increase in the maximum of voltage drop on the gap and the energy of the main fraction of electrons.     

Evaluation of laser drilling of holes in thermal barrier coated superalloys

Abstract

Laser drilling of precise holes in thermal barrier coated Ni based superalloys has been studied. The interplay between various hole geometrical features such as hole shape, taper, barrelling, undercut, etc. and laser parameters such as pulse energy, pulse width and pulse repetition rate have been examined. The hole diameters are seen to follow a linear dependence on the incoming laser power densities for pulse width up to 2·0 ms. However, such a linear dependence was not observed for a pulse width of 3·0 ms. It was found that high pulse energy and short pulse width (high power density) gave crack free recast layer, whereas low pulse energy and longer pulse width (low power density) gave microcracks in the heat affected layer of superalloy. The significant barrelling observed in IN718 material at low power density values is due to multiple reflection of the incident beam from the cavity in combination with plasma formation at the evaporation front and trapping of the incident radiation causing excessive heating in that region.     

The effective anode-cathode gap in an ion diode operating in a bipolar-pulse regime

The effective anode-cathode gap (ACG) in a self-magnetically insulated ion diode operating in a double (bipolar) pulse regime has been studied. In this diode, the ACG is bounded by a plasma layer at the anode surface and by electrons drifting near the cathode surface. Analysis of the system operation showed that, during the first voltage pulse, the effective ACG decreases at a constant velocity of 1.5 ± 0.1 cm/μs from 9 to 1–2 mm (depending on the pulse duration) and is not completely bridged by plasma. After reversal of the voltage polarity, the effective gap width is restored for 10–20 ns on a nearly initial level. During the second pulse, electrons drift within a 1- to 1.5-mm-thick layer near the anode, while the thickness of a plasma layer on the anode surface does not exceed 0.5 mm.     

Dielectric barrier discharge pumped N(2) laser

A pulsed transverse dielectric barrier discharge has been used to excite N(2)/He (1:10) gas mixtures. Laser action in 337.1 nm was attained at pressures between 400 mbars and 1.3 bars. The maximum pulse energy was 1.4 μJ, with an efficiency of 0.002% and a pulse width of 8 ns.     

超快大功率SiC光导开关的研究

选用钒掺杂浓度为0.2at％的高质量6H-SiC晶体, 电阻率为7.0×10⁸Ω·cm, 研制出超快大功率SiC光导开关. 在脉冲宽度为20ns的光源激发下, 分别测试了在不同的偏置电压和光能条件下开关的电脉冲输出特性. 结果表明: 1mm电极间隙的SiC开关器件的性能优越, 耐偏压高, 光导电脉冲的上升时间快(6.8ns), 脉宽<20ns, 稳定性好. 负载为40Ω的电阻上输出线性电脉冲电压随开关的偏置电压和光强增大而增大, 在2.5kV的偏置电压, 最大瞬时电流约为57.5A, 瞬时功率高达132kW.     

Cathode-initiated vacuum breakdown in a pulse regime

Based on an analysis of the Joule heating of emitters by thermionic current, new methods and criteria are proposed for evaluating vacuum breakdown cathode-initiated in a pulse regime for an arbitrary cathode shape. The applicability of the proposed methods and validity of the new criteria are confirmed by the results of experiments using nanosecond pulses applied to coaxial copper electrodes with a gap width of d=0.2 mm and an area of S=2500 mm².     

High-power source of 200–350 nm spontaneous emission excited by unipolar current pulses

Spectral, energy, and temporal characteristics of pulse discharge in xenon have been experimentally studied. Upon passing from an oscillatory regime to unipolar pulses of discharge current, the power of emission in the wavelength interval 200–350 nm increases, while the emission pulse full width at half maximum (FWHM) decreases. A quartz pulsed discharge lamp excited by a generator based on high-current high-voltage diodes radiates in the 200–350 nm interval at a peak radiant intensity above 65 kW/sr and at a pulse FWHM ∼ 2 μs.     

Electron source and acceleration regime of a picosecond electron beam in a gas-filled diode with inhomogeneous field

It is experimentally demonstrated that, upon the application of a subnanosecond high-voltage pulse to the gap of a diode filled with air at atmospheric pressure, a bunch of runaway electrons is formed in a sharply inhomogeneous electric field near the cathode. The bunch duration does not exceed 50 ps, which is shorter than the electron flight time through the interelectrode gap in the continuous acceleration regime. This duration remained unchanged when the gap width was varied between 6 and 26 mm. The electron energy in the picosecond electron beam, as determined from the time-of-flight measurements in the drift channel behind the anode foil of the diode, agree with the results of numerical calculations of the electron acceleration dynamics in the vacuum diode approximation.     

Optimisation of FDM process parameters by Taguchi method for imparting customised properties to components

Customisation of material properties by route of controlling the process parameters is a landmark ability of the additive manufacturing (AM) processes. Parametric optimisation of fused deposition modelling process using Fortus 250mc modeller is accomplished for conical primitives of constructive solid geometry in the present research. Experiments were designed according to the Taguchi technique for four factors at three levels each – slice height (SH), contour width (CW), raster width and air gap (AG). Analysis of signal-to-noise ratios is utilised for establishing the optimal process parameters and the relative percentage contribution of factors is estimated using ANOVA. Optimal levels of process parameters are found to vary with the variation in the type of basic shape of primitive. It has been established that AG has a maximum impact over the part build volume followed by CW and SH. Also, it can be safely concluded that the interaction effect of parameters is relatively less important.