Effective generation of electron beams in an abnormal discharge with increased photoelectron emission from a cathode

An abnormal discharge is realized in which electron beams with energies in a kiloelectronvolt range are generated at an efficiency of η∼1. The results are explained within the framework of a photoelectron discharge model. A transition to this mechanism is possible due to a significant increase in the power of self-illumination from a large electron beam drift volume. The passage to a discharge with predominant photoemission leads to a decrease in the cathode potential fall region, a drop in the ion current to the cathode, and an increase in the efficiency.     

Anwendung der Hohlkatodenplasmaquelle in der Vakuumtechnik

Various effective and simple constructed plasma sources can be developed by using the hollow cathode effect. The improve of the plasma efficiency is caused by the penetration of two or more glow regions of different parts of the cathode. The equatation p × d ∽ 1 mbar cm is a rule of thumb for the appearance of the hollow cathode effect. The hollow cathode arc discharge is caused by an additional thermal electron emission besides the effectes of the hollow cathode glow discharge. All this effects are the reason for the higher degree of ionization for a hollow cathode source in comparison with the most other plasma sources. Application of the hollow cathode glow discharge for surface finish are technics like plasmacleaning, electron beam ablation or gas flow sputtering. Latters are new technics which allow the development of new sources for the coating technology. The application of the hollow cathode are discharge is possible by using the thermal energy generated by the electron beam (electron beam heating, electron beam melting, and electron beam evaporation) and by using the different possibilities of coating technologies whith the hollow cathode arc source (plasma enhanced evaporation, plasma enhanced reactive PVD and plasma CVD).     

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.     

High-current wide-aperture electron beam formation in discharge with increased gas ionization at pressures up to several dozen Torr

Pulsed discharge in a gas with additional ionization produced by an auxiliary electron beam has been studied. The auxiliary beam is emitted from the wall of a protrusion on the cathode and propagates along the cathode surface. The additional ionization ensures that the main wide-aperture beam current to the anode and the total discharge current exceed that of an equivalent anomalous discharge by up to an order of magnitude. The proposed modified discharge operates in a broad range of pressures (up to several dozen Torr) without other changes in the system design.     

On the mechanism of electron beam generation in an open discharge with anomalously high efficiency

The parameters of an open discharge operating without anode grid at elevated beam current densities are studied. It is demonstrated that the discharge is characterized by an almost 100% efficiency of electron beam generation in both pulsed and continuous regimes. A mechanism is proposed that accounts for such a high efficiency, according to which a zone of low potential gradient formed near the cathode blocks the ion current to cathode, while the electron emission proceeds by the photoemission mechanism.     

Plasma cathode for a broad-beam electron accelerator

We propose a grid-stabilized plasma cathode based on a slit-contracted low-pressure glow discharge with hollow anode. The area of the plasma cathode is one order of magnitude higher than that in systems where electrons are extracted immediately from plasma in the cathode cavity. Conditions for the discharge initiation, the current switching to the hollow anode, and the obtaining of uniform emission from the plasma cathode are determined. At an accelerating voltage of 160 kV, an electron beam with a 1000 × 180 mm cross section, a total current of several amperes, and a current pulse duration of up to 10⁻³ s was obtained. The plasma cathode operates under technical vacuum conditions (air, 0.1 Pa) and ensures high stability and reproducibility of the beam current pulses.     

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.     

The application of the glow discharge to material processing

The mechanism and characteristics of the glow discharge are outlined. Energetic electrons and ions, which are generated in the cathode fall region, may be focused or guided to form a beam by geometrical arrangements of the discharge electrodes and by magnetic fields. Two practical arrangements are described: a spherical hollow cathode device, which produces a hot zone at its centre; and a hollow anode device, which produces wellcollimated electron and ion beams. Glow discharge beam devices work at rough vacuum pressures, in the range 10^–2 to 1 mm Hg, and are particularly suited to the processing of glass and oxide ceramics since, owing to the presence of plasma, no electrical charging of target insulating materials takes place. Limitations are set on the operating conditions of glow discharges by the heat input to the electrodes, by erosion due to sputtering, and by the glow-to-arc transition. Proper engineering, however, should allow cathode current densities of the order of 1 A/cm² when operated at an anode voltage of the order of 10 kV, and focusing should realise electron beam power densities of up to the order of 1 MW/cm², with efficiency of about 50%. Thus, the glow discharge may have a wide range of applications to material processing, from etching by sputtering at low power to processing by the most intense heating. New methods of forming and fabricating oxide bodies based on condensation of vapour and on powder deposition are discussed.     

Influence of laser pulse parameters on characteristics of a source of multicharged metal ions based on laser-induced medium-power spark discharge

The dynamics of laser-induced vacuum spark discharge with storage energy not exceeding 25 J has been studied in a broad range of laser pulse energies and power densities. It is shown that, using discharge-initiating laser pulse of nanosecond duration, it is possible to obtain stable single pinching of cathode jet plasma at a storage voltage above 10 kV. Plasma pinching is accompanied by the generation of a beam of multicharged ions of the cathode material (aluminum) up to Al⁸⁺. The maximum energies of ions obey the scaling relation E _max = 5ZeU ₀ that has been obtained previously for a low-voltage discharge. An increase in the laser pulse energy leads to growth of the average beam charge and a sharp decrease in the ion energy.     

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。     

The effect of compensation currents of the open discharge

Previous conclusions concerning a general mechanism of electron beam formation in glow discharges of various types, including the open discharge, are confirmed. The discharge behavior is determined by ionization processes and by the bombardment of cathode with fast heavy particles, in agreement with well-established notions about the glow discharge. Neither compensation currents nor the electron beam current can account for the photoelectron mechanism of the open discharge, despite still existing opposite assumptions.     

Specific features of wide-aperture hollow cathode discharge in helium

The characteristics of wide-aperture discharge with a hollow cathode in helium have been studied under the conditions of predominating electron generation via photoemission from the cold cathode. In this regime, currents exceeding by an order of magnitude those for the conventional abnormal discharge have been obtained. The current-voltage characteristics are independent of the helium pressures in a range of 5–15 Torr, which is a distinctive feature of the photoemission dominated discharge with electron beam in the system under consideration.     

Sparkless discharge in dense gases with preionization by a low-energy electron beam of a barrier open discharge

A highly stable sparkless discharge can be obtained using preionization by an electron beam, photons, and plasma electrons formed in a barrier open discharge, for which a grid electrode (that also serves as a cathode for the main discharge) is situated immediately on a dielectric-coated electrode surface. In the standard gas mixture for an ArF laser at a pressure of 2.5 bar, an energy deposition of 2.5 J/cm³ in a 12-ns pulse at a specific power of 210 MW/cm³ was achieved. The results are of interest for the development of technological excimer lasers operating at a high pulse repetition rate.     

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.     

Formation and energy relaxation of a fast electron beam in cathode regions of a glow discharge in helium

Results of a three-dimensional direct statistical simulation of formation and energy relaxation of a group of high-energy electrons in cathode regions of the low pressure (p < 1 Torr) glow discharge in helium are given. It is demonstrated that the electron distribution function at the exit from the cathode sheath contains a group (beam) of high-energy electrons which did not suffer inelastic collisions and had energy close to the cathode fall potential. The beam is shown to be a main source of charged particle production in the negative glow region. The calculated electron energy distribution function is compared with experimental data.     

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.     

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.     

A compact ion source using a hollow cathode discharge and its application to thin film formation

A compact and simple ion beam source was developed and its application to ion beam sputter deposition of thin films was studied. The ion source consisted of a hollow cathode, an anode and an extraction electrode. Two acceleration electrodes were mounted at the edge of the cathode. Since the gas pressure in the cathode should be kept at a relatively high value (more than 0.05 Torr) to maintain the discharge, the ion source employed a differential pumping system. The fundamental discharge characteristics of the ion source were studied using argon or helium as the working gas.The ion source was applied to the ion beam sputter deposition of copper and ruthenium oxide films. The crystallinity of the copper films increased with an increase in the acceleration voltage, while all the ruthenium oxide films appeared to be amorphous or of very small grain size, regardless of the acceleration voltage. The effect of the acceleration voltage on the film resistivity was also studied.     

A magnetron injection gun with a reduced filament temperature and elongated cathode lifetime

An optoelectronic gyrotron system that makes it possible to efficiently heat a cathode by electrons reflected from a magnetic mirror has been developed. The parameters of the primary electron beam are retained at a level that is acceptable for efficient microwave power generation. The results of a trajectory analysis of electrons and the cathode temperature conditions are reported. The corresponding decrease in power and temperature of the cathode filament suggests a significant (severalfold) increase in the filament lifetime, which is one of the main parameters determining the device lifetime.     

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).