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.     

The temporal structure of a runaway electron beam generated in air at atmospheric pressure

Supershort avalanche electron beams (SAEBs) generated in air at atmospheric pressure have been studied with picosecond time resolution. It is established that an SAEB has a complicated structure that depends on the interelectrode gap width and cathode design. In a gas-filled diode with a small gap width, an SAEB current pulse with a full width at half maximum (FWHM) of ??25 ps has been observed behind a collimator with a hole diameter of 1 mm. As the gap width is increased or decreased relative to the optimum value that corresponds to the maximum beam current, the SAEB current pulse shape changes and pulses with two peaks are more likely detected. The two-peak SAEB current pulse shape is retained behind aluminum foil with a thickness of 60 and 110 ??m.     

Emissive characteristics of electric discharge initiated by high-current electron beam in atmosphere

Spectral and kinetic characteristics of electric discharge induced by a high-current electron beam in air at atmospheric pressure have been experimentally studied. The luminescence spectrum of a volume (diffuse) discharge generated under the conditions studied displayed emission bands due to the second positive system of nitrogen molecules. The cathode region of discharge exhibited bright cathode spots, which emitted short-term (nanosecond) pulses with continuous spectrum and long-term line radiation from atoms entering into the cathode material composition.     

Volume x-ray emission in gas diodes at atmospheric pressure

The generation of x-rays and high-energy electron beams in gas diodes filled with air and nitrogen at atmospheric pressure has been studied by experimental and theoretical methods. It is established that soft x-ray radiation is not only generated in the region of dense discharge, but is predominantly emitted from a weak-current discharge region. For a high-energy electron beam formation in the gap, the role of the voltage pulse front is not less important than that of the voltage amplitude; the electric field strength at the cathode has an optimum value for the electron beam formation.     

High-current electron beam in a hollow-cathode gas discharge at elevated pressures (∼100 Torr)

A discharge with plasma filling a flat-bottom cavity of depth δ in the cathode, partly closed by a dielectric plate with a hole (determining the aperture of the discharge between the cavity bottom and the anode), has been studied. In a discharge cell of type 1 with δ = 0.5 mm and a hole diameter of 22 mm, a pulsed electron beam was obtained with a duration of t _EB = 700 ns and a beam current j _EB approximately 10 times greater than that (j _AD) of the equivalent anomalous discharge (at fixed discharge voltage U and gas pressure p _He = 3.5 Torr). An electric field with the direction opposite to the field of applied voltage appeared at the cathode that was related to a space charge formed at the cathode plasma boundary, which could not follow a rapid drop of voltage across the discharge gap. In a discharge cell of type 2 with δ = 0.5 mm and a narrow slit (S = 0.1 × 5 cm²) in the dielectric plate, a pulsed electron beam was obtained with a duration of t _EB = 2 ns and a beam current of j _EB = 0.7 kA/cm² (j _EB/j _AD = 1.5) at U = 4.2 kV and p _He = 50 Torr.     

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.     

The formation of accelerated electrons and their effect on the structure of nanosecond discharge with slot cathode

Experimental investigations are performed of the current-voltage characteristics and spatial distribution of optical radiation of plasma within the cathode cavity and in the discharge gap under conditions of transverse nanosecond electric discharge with a slot cathode. The investigations are performed in helium in the gas pressure range from 1 to 100 torr. It is demonstrated that the cathode layer is formed at the initial stage of discharge and that the electrons accelerated in the narrow cathode region pick up energy of the order of 1 keV. The efficiency of electron beam generation is estimated, and the mechanisms of generation of fast electrons are identified. The free paths of accelerated electrons in plasma under different initial conditions are determined, and the mechanisms of relaxation of energy of this group of electrons are analyzed. The correlation is established between the special features of relaxation of energy of fast electrons and the formation of the structure of beam-plasma discharge with a slot cathode.     

Protecting the anode foil of a high-current electron accelerator against arc-discharge damage

The mechanism of anode foil damage during the extraction of a high-power pulsed electron beam from a high-current diode has been experimentally studied on a TEU-500 electron accelerator [1]. It is established that the breakage of the anode foil is caused by the appearance of cathode spots on its surface, the intense electron emission from these spots during positive voltage pulses (postpulses following the main negative pulse of accelerating voltage), and the formation of arc discharge in the interelectrode gap. The improvement of diode matching to the pulse-forming line of the accelerator and the use of an auxiliary electrode (anode) forming additional vacuum discharge gap (crowbar) with the cathode practically excludes the anode foil breakage by arc discharge and significantly increases the working life of the foil (up to ∼10⁵ electron beam pulses).     

丝束同轴冷阴极电子枪的研制

与传统基于热阴极电子枪的熔丝增材制造技术相比,基于丝束同轴冷阴极电子枪的熔丝增材制造技术具有明显优势。为了缩短与世界先进电子束熔丝增材制造技术的差距,通过分析丝束同轴冷阴极电子束产生机理,采用模拟仿真技术优化影响束流品质的阴极、阳极参数,根据仿真结果研制了一套具有完全自主知识产权的丝束同轴冷阴极电子枪。所研制的丝束同轴冷阴极电子枪输出束流呈现"X"形状,熔滴过渡模式可以调控,熔丝成形效率达到5.02kg/h。     

Experimental study of the converging density wave excitation in a cylindrical anode of a high-current diode

Density waves excited in a cylindrical anode irradiated by a short pulsed high-current electron beam in a relativistic vacuum diode have been experimentally studied. The electron trajectories are almost perpendicular to the side surface of a cylindrical anode coaxial with the cathode. As a result of the discharge, the surface layer of the continuous anode exhibits no visible changes, whereas an inner region of the anode is evaporated. The mechanism of the isentropic wave excitation is elucidated by experiments with a coaxial anode comprising a tube tightly fit to the central rod. According to the most probable explanation, the electron beam induces collapse in the subsurface layer of the anode.     

Detection of defects in multiple-layer structures by using surface plasmon resonance

A method for diagnosing a silver-dielectric interface is proposed. This method relies on surface plasmon resonance at the Ag-dielectric interface. It yields a good estimate of an air gap between a thin Ag film and a polymer (dielectric). Detection of an air gap thickness of approximately half of the incident probing beam wavelength was monitored. Probing a small air gap is also discussed in the case of poor adhesion at the Ag-polymer interface.     

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.     

Discharge characteristic of nanosecond-pulse DBD in atmospheric air using magnetic compression pulsed power generator

Dielectric barrier discharge driven by repetitive nanosecond pulses can offer highly efficient non-thermal plasma at atmospheric pressure and is widely used for plasma applications. In this paper, the discharge is generated using a compact pulsed power generator based on one-stage magnetic compression. The output pulse can be up to 30 kV with a rise time of about 40 ns and a full width at half maximum of 70 ns. The electrical characteristics of the discharge parameters are studied by the measurement of voltage and current waveforms. The effects of applied voltage amplitude, voltage polarity, pulse repetition frequency, and barrier dielectric on discharge characteristics are investigated, respectively. The experimental results show that the discharge current, discharge power and electron density increase with the increase of the applied voltage, and the pulse repetition frequency has a slight effect on the electrical parameters. Moreover, the discharge current is influenced by the dielectric barrier, but it is not varied with the polarity of applied pulses.     

Hysteresis of the current-voltage characteristic of a continuous electron-beam-controlled discharge in a nitrogen flow

A hysteresis in the current-voltage characteristic of continuous electric discharge controlled by an electron beam in a nitrogen flow has been experimentally observed. It is suggested that the transition from a self-sustained glow discharge regime (typical of ionization chamber) to the electron-beam-controlled discharge is accompanied by the transition from the laminar to turbulent boundary layer at the cathode surface.     

YAG投影管中多层干涉过滤膜的研制

提出在YAG投影管的荧光层与YAG面板之间研制多层干涉过滤膜的原理 ,并阐述了干涉膜的设计和制作。测试结果表明 ,带有干涉膜的YAG投影管图像色纯度有所改善 ,投影屏幕图像中央白场亮度约提高了 6 0 %。     

Operating characteristics of the hollow anode glow discharge ion source

In this work, a new shape of a glow discharge ion source with axial extraction has been designed and constructed. High output ion beam current can be extracted axially in a direction normal to the discharge region without using extraction system. Optimization of the distance between the anode and the cathode has been determined using argon gas. It is found that the optimum gap distance between the anode and the cathode is equal to 3.5 mm, where stable discharge current and maximum output ion beam current can be obtained. The discharge characteristics of the ion source at different operating gas pressures have been measured at this optimum distance between the anode and the cathode. A disk of Teflon insulator has been put between the anode and the cathode. This disk was covering the cathode area and reducing the discharge area on the cathode surface for discharge confinement, therefore, a higher output ion beam current could be obtained.     

Some High Intensity Flash Light Sources of the Luminescent Tube Type

The design and performance of vacuum flash tubes of the cathode ray tube type are described in which short-duration impulses of a high-energy electron beam are used to excite a phosphor screen and thus to produce luminescent radiation pulses of high intensity. Triode type tubes are shown which may be operated with 2kW pulses, and a diode type tube is described where the screen may be energized with I MW pulses. Either single-pulse radiation or trains of pulses with short pulse intervals can be produced, the duration of an individual radiation pulse ranging from about 0.1 microsec to the order of 100 microsec according to the rated value of the respective flash tube. The spectrum of the radiation is continuous with a form of the spectral curve depending on the type of phosphor used for the screen.     

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.     

Anomalous acceleration of interelectrode-plasma ions in the spark stage of vacuum discharge

A new method of obtaining high-energy ion beams using vacuum spark discharge is proposed. It is shown that ions are accelerated in the presence of a plasma cloud in the interelectrode gap, where the development of strong electron instability during the propagation of a cathode electron beam leads to potential buildup to a level exceeding the applied voltage. The appearance of anomalously accelerated ions of the interelectrode plasma is accompanied by a jump in the diode current.     

Special features of mechanisms of formation of transverse nanosecond discharge with slot cathode, confined by dielectric walls

Experiments are performed and comparison is made of the electrical and optical characteristics of open and confined transverse nanosecond discharges with a slot cathode. It is found that the duration and shape of current pulse under conditions of open and confined discharges are significantly different and, under some conditions, the current density of confined discharge exceeds that of open discharge by more than an order of magnitude. Estimates are obtained of the coefficient of electron emission from cathode plasma, which significantly exceed the coefficient of electron emission in an abnormal discharge. It is demonstrated that the beam mode of primary electrons in the cathode layer during motion in the anode direction gradually transforms to hydrodynamic mode. The free paths of accelerated electrons in plasma are determined, and it is demonstrated that they reach the anode and have a significant impact on the dynamics of development and on the structure of optical radiation of transverse nanosecond discharge with a slot cathode.