Formation of narrow-focused electron beams generated by a source with a plasma cathode in the forevacuum pressure range

Results are presented from experimental studies of the formation of focused electron beams produced by extracting electrons from the plasma of a steady-state discharge with a hollow cathode in the forevacuum pressure range. Based on the measurements of the energy spectrum and diameter of the electron beam, as well as of the emission parameters of the plasma produced in the course of beam-gas interaction, a conclusion is drawn about the excitation of a beam-plasma discharge that deteriorates the beam focusing conditions. The threshold beam current density for the excitation of a beam-plasma discharge is found to increase with accelerating voltage and gas pressure.     

Formation of a pulsed electron beam in a plasmacathode system under forevacuum pressures

Results of an experimental study of the features of the production of pulsed beams by a plasma electron source operating in the forevacuum pressure range (5–15 Pa) are presented. For this pressure range, the emission properties of the plasma are substantially affected by the backward ion flow generated in the regions of formation and transport of the electron beam. It has been shown that in experimental conditions the ratio of the current of backward ions to the current of the electron beam can reach 10%, which is an order of magnitude greater than the same parameter for electron sources operating in the pressure range that is conventional for devices of this type (0.01–0.1 Pa). The principal factors that impose an upper limit on the beam current in a plasma-cathode source are analyzed.     

Localization of plasma in the extended hollow cathode of a ribbon-electron-beam plasma source at forevacuum pressures

The results of a study of the plasma density distribution in the slit aperture of a right-angled extended hollow cathode used in a ribbon-electron-beam plasma source operating at forevacuum pressures (1–10 Pa) are presented. It is shown that a local peak of plasma density appears in some region of the slit aperture as the slit width is decreased. This results in the appearance of a region of increased current density when the ribbon beam forms. The uniformity of the beam current density distribution is additionally disturbed by the reverse ion flow whose effect on the emission properties of the plasma is significant in the region of elevated pressure. A model which describes the development of plasma density nonuniformity in a hollow cathode is proposed which is based on the idea that the electron current flows predominantly through the slit aperture regions that are associated with local openings of the cathode layer ion sheaths. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 3–9, June, 2007.     

Specific features of the formation of a uniform ribbon electron beam by a plasma source in the forevacuum pressure range

Results are presented from experimental studies of the formation of a ribbon electron beam during the extraction of electrons from the plasma of a steady-state hollow-cathode discharge in the forevacuum pressure range. It is shown that the main reason for the nonuniformity of the current density is the increase in the local nonuniformity of the emission plasma density caused by the return flow of ions from the accelerating gap. Taking this feature into account when developing a system of beam extraction provides for the generation of a ribbon beam with a nonuniformity of the current density along the beam of less than 10%.     

Influence of the longitudinal magnetic field in the accelerating gap on the limiting parameters of a plasma electron source operating in the forevacuum pressure range

Results are presented from experimental studies of the influence of the longitudinal magnetic field in the accelerating gap on the emission current, accelerating voltage, and maximum gas pressure in a plasma electron source generating a continuous electron beam in the forevacuum pressure range. It is shown that the magnetic field in the beam-formation region stabilizes the emitting boundary of the plasma in the accelerating gap, thereby considerably improving the source parameters.     

Charge compensation in an insulated target bombarded by a pulsed electron beam in the forevacuum pressure range

We report on the results of computation of the time dependence of the ion flux and the dielectric target potential at the initial stage of bombardment by an electron beam in forevacuum. It is shown that a satisfactory agreement with experimental data is attained with allowance for the possibility of the discharge between the target and the earthed walls of the vacuum chamber.     

Potential of a dielectric target during its irradiation by a pulsed electron beam in the forevacuum pressure range

The potential induced on the surface of a nonconducting ceramic during irradiation by an electron beam in the forevacuum pressure range (5?C15 Pa) remains negative, but its absolute value is much smaller than the energy of accelerated electrons. The factors affecting the negative potential of a nonconducting ceramic target are determined. The evolution of the charge injected in the ceramic is analyzed by a numerical simulation.     

Pressure-produced ionization of nonideal plasma in a megabar range of dynamic pressures

The low-frequency electrical conductivity of strongly nonideal hydrogen, helium, and xenon plasmas was measured in the megabar range of pressures. The plasmas in question were generated by the method of multiple shock compression in planar and cylindrical geometries, whereby it was possible to reduce effects of irreversible heating and to implement a quasi-isentropic regime. As a result, plasma states at pressures in the megabar range were realized, where the electron concentration could be as high as n _e ≈2×0²³ cm^?3, which may correspond to either a degenerate or a Boltzmann plasma characterized by a strong Coulomb Γ _D =1–10) and a strong interatomic Γ _a =r _a n _a ^1/3 ～1) interaction. A sharp increase (by three to five orders of magnitude) in the electrical conductivity of a strongly nonideal plasma due to pressure-produced ionization was recorded, and theoretical models were invoked to describe this increase. Experimental data available in this region and theoretical models proposed by various authors are analyzed. The possibility of a first-order “phase transition” in a strongly nonideal plasma is indicated.     

Inhomogeneous extended hollow cathode discharge for raising the current density in a forevacuum plasma source of a ribbon electron beam

The plasma parameters and the emissivity of a ribbon electron beam source based on a discharge with an inhomogeneous extended hollow cathode are measured. A constriction in the cathode cavity increases the plasma density near the emitting area boundary, which adds to the electron current density in the beam. The reason for the above effect is the formation of the plasma density distribution nonuniform across the cavity with a maximum in the middle. This maximum is caused primarily by a plasma electron flow from the constriction, which is generated by the electric field and is directed toward a slit emission-extracting aperture.     

Relaxation of the electron temperature in an inert-gas afterglow plasma at elevated pressures

The relaxation of the electron temperature T _e in helium and neon afterglow at elevated pressures is studied theoretically and experimentally. It is shown that the processes in which fast electrons are produced are accompanied by the heating of thermal electrons. The high-energy part of the electron energy distribution function is studied in the intermediate regime (between the local and nonlocal regimes) of its formation. It is shown that, in this case, the calculated effective energy transferred from the fast electrons to the thermal electrons depends substantially on the wall potential of the discharge tube. Comparison of these calculations with experiments testifies to the reliability of the probe technique for measuring T _e in an afterglow at elevated pressures.     

Self-consistent multidimensional electron kinetic analysis forinductively coupled plasma sources

Based upon the kinetic equations coupled with electromagnetic analysis for the recently developed inductively coupled plasma sources (ICPS), a self-consistent electron kinetic model is presented for 2-D (r, z) in a cylindrically symmetric configuration space and 2-D (ν _∂, ν_z) in the velocity space, The EM model is based on the mode analysis, while the kinetic analysis gives the perturbed Maxwellian distribution of electrons by solving the Boltzmann-Vlasov equation. The kinetic analysis shows that the RF energy in an ICPS is extracted by a collisionless dissipation mechanism, once the electron thermovelocity is close to the RF phase velocities determined by the reactor height and mode indexes. In this context, the effect of varying the reactor geometry is reported in terms of the electron energy distribution function. The analytical results are compared to the experimental data of Barnes et al. (see Appl. Phys. Lett., vol.62, no.21, p.2622-4 (1993)), which shows qualitative agreements in many aspects     

Expansion of a laser plume from a silicon wafer in a wide range of ambient gas pressures

Expansion of the laser plume from a silicon wafer into surrounding gas is considered in the range of ambient gas pressure from 0.1 to 1 bar using a kinetic approach. The plume is generated by a nanosecond Gaussian laser pulse. Absorption of laser radiation and heating and melting of the target are described by a two-dimensional thermal model. Axisymmetric flow in the laser plume is calculated by the direct simulation Monte Carlo method. It was found that diffusion of mixture components is significant in the considered time scale, flow is non-equilibrium, and regions of high rarefaction temporally appear in the flow. In atmospheric pressure, the re-deposition of the silicon vapor was observed only in the vicinity of the laser spot.     

Parameters of the beam plasma formed by a forevacuum plasma source of a ribbon beam in zero-field transportation system

We have studied the generation of the beam plasma formed by a forevacuum plasma source of a ribbon electron beam in the conditions of its transportation without an accompanying magnetic field. The ignition conditions in the beam transportation region of the beam–plasma discharge producing a plasma formation of the plasma sheet type with a plasma concentration of ~10¹⁶ m^–3 and an electron temperature of 1–2.5 eV have been determined. The attained values of parameters and the sizes of the plasma formation make it possible to use it in technologies of the surface modification of planar extended articles.     

迈克耳孙干涉仪应用功能的扩展

通过对迈克耳孙干涉仪的简易改装,实现了光速测量、透明固体和液体的折射率测量、金属丝弹性模量测量以及多普勒效应的演示,拓展了迈克耳孙干涉仪的应用功能.     

Operation of the EGA electron gun at high gas pressures

Summary This paper presents experimental results on the effects of increasing gas pressure on electron gun operations. The electron gun used is the gun EGA which has been developed for the TSS-1 mission. Various gases have been used with pressures in the range from 10⁻⁶ to a few times 10⁻³ mbar. The measurements, taken in a vacuum chamber, show a steady increase in the slope of theI–V characteristics of the gun when the pressure is increased, as a consequence of ionization phenomena induced by the electron beam. Further measurements have been taken of the overall current on a target at some distance from the gun itself. An attempt is also made at a theoretical calculation of the effects observed, based on simplified models.     

Problems in expanding the working range of wavelength-tunable light filters

The possibilities for expanding the range of tuning of the wavelength isolated by a tunable interference light filter, based on a combination of a nonuniform multilayer and a reflecting prism, are investigated. The degree of influence of the immersion liquid on the lower limit of wavelength tuning is studied. That influence is shown to be limited. A connection between the scheme for obtaining a nonuniform multilayer and the maximum wavelength of light reflected by the multilayer is demonstrated theoretically. The working range of a wavelength-tunable filter, the nonuniform multilayer of which is based on holographic photographic plates, is expanded by changing the scheme for obtaining the multilayer. The possibility of advancing into the near ultraviolet by using other materials (such as bichromized gelatin) is demonstrated experimentally. V. D. Kuznetsov Physics and Engineering Institute of Siberia at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 63–69, April, 1997.     

Optical performance of carbon-nanotube electron sources

The figure of merit for the electron optical performance of carbon-nanotube (CNT) electron sources is presented. This figure is given by the relation between the reduced brightness and the energy spread in the region of stable emission. It is shown experimentally that a CNT electron source exhibits a highly stable emission process that follows the Fowler-Nordheim theory for field emission, fixing the relationship among the energy spread, the current, and the radius. The performance of the CNT emitter under realistic operating conditions is compared with state-of-the-art electron point sources. It is demonstrated that the reduced brightness is a function of the tunneling parameter, a measure of the energy spread at low temperatures, only, independent of the geometry of the emitter.     

Microwave heating of electrons of a dense plasma column at frequencies higher than electron cyclotron frequency

In this experimental study the absorption of plasma waves, excited in a dense plasma column, was investigated and the localization of the regions of efficient heating of electrons were determined in a broad range of parameters: 1>_ce/>0·1, 0·3<n/n _UH <25, 10^–5<v/< <10^–3. The heating of electrons near the second electron cyclotron harmonic was investigated in greater detail.We are grateful to the members of the Staff of the Institute of Plasma Physics Drs. P.unka, R.Klí'ma, V.Kopecký, J.Musil for discussion of the results. We also thank Drs. J.Musil, F.áek for the help and advice in the early stages of experimental work during construction of different diagnostic devices. Our thanks are due to the director of the Institute Dr. J.Váa for encouraging support of this experimental program.     

用于真空电子太赫兹器件的微型热阴极电子束源研究

太赫兹波辐射源是太赫兹(THz)波技术的关键.真空电子太赫兹器件在高频、大功率太赫兹源发展中较其他技术有明显的优势,微米尺度高电流密度微型电子束源则是研制真空电子太赫兹器件的核心之一.本文在研制低温、大电流纳米粒子氧化钪掺杂含钪扩散阴极(nanosized-scandia doped dispenser cathode)的基础上,采用发射抑制膜沉积与聚焦离子束(FIB)刻蚀技术,研制无需压缩直接提供高电流密度的微型电子束的电子源.所研究的电子束源直径400μm,在工作温度950fiC,提供空间电荷限制电流密度50 A/cm2时,已稳定工作1000 h以上,并且层流性良好.本文阐述了阴极制备工艺、电子发射特性、微米尺度电子束源的获得和特性,介绍了发射抑制膜的结构和抑制特性的评估.并探讨了镀膜和刻蚀对发射的影响机理.这一电子束源在常规毫米尺度电子源的基础上产生微米尺度的微区高电流密度的电子束,为真空电子太赫兹辐射源的研制提供了新的途径.