ISSN 00204412, Instruments and Experimental Techniques, 2011, Vol. 54, No. 2, pp. 226–229. © Pleiades Publishing, Ltd., 2011. Original Russian Text © Yu.G. Yushkov, V.A. Burdovitsin, A.V. Medovnik, E.M. Oks, 2011, published in Pribory i Tekhnika Eksperimenta, 2011, No. 2, pp. 85–88.

GENERAL EXPERIMENTAL TECHNIQUES

A Forevacuum Plasma Source of Pulsed Electron Beams Yu. G. Yushkov, V. A. Burdovitsin, A. V. Medovnik, and E. M. Oks Tomsk State University of Control Systems and Electronics, pr. Lenina 40, Tomsk, 634050 Russia Received August 20, 2010

Abstract—A plasma electron source designed for generation of a pulsed wideaperture electron beam in the forevacuum pressure range (5–20 Pa) is described. The source is based on the use of a hollowcathode glow discharge. At an accelerating voltage of 20 kV, a current pulse length of 100 µs, and a pulse repetition rate of 10 Hz, the electron beam current is 100 A, and the maximum density of the beam pulse power is 10 J/cm2. The obtained parameters of the electron beam and the features of the source functioning in the forevacuum pressure range show that this source can be used to good effect to modify the surface properties of noncon ducting materials. DOI: 10.1134/S0020441211010283

INTRODUCTION Materials treatment with pulsed highcurrent low energy electron beams, as a result of which only a thin surface layer is heated or fused, modifies substantially the surface properties of a material at great depths, and, finally, increases its microhardness and corrosion resistance and reduces the friction coefficient [1, 2]. This technology uses, as a rule, plasma sources of elec trons with unstable [3] or quasistable [4] emission plasma boundary. In this case, a variety of materials treatable by pulsed electron beams has been reduced to metals, alloys, and other conducting materials. Considerable progress has been recently observed in development of socalled forevacuum plasma sources of electrons [5]. The distinction of these kinds of devices consists in their ability to generate beams in the formerly inaccessible pressure range of 5–20 Pa, which can be attained using solely a mechanical (for evacuum) pumping stage. Though this advantage is undoubtedly significant, nevertheless, the ability to directly treat nonconducting materials is one of the main merits of a forevacuum electron source. Experi ments at forevacuum pressures show that, when an accelerated electron beam with an energy of a few keV is incident on an isolated target, the steadystate float ing potential of this target appears to be close to the ground potential [6]. This provides a means for effi cient treatment of dielectrics with an electron beam, the energy of which is almost equal to the accelerating potential. The use of forevacuum plasma electron guns for nonconducting materials treatment has been suc cessfully demonstrated by welding aluminum oxide ceramics by means of a continuous focused electron beam [7]. Nevertheless, in order to use pulsed electron beams for direct surface treatment of nonconducting materi als (first and foremost, various ceramics), one should ensure generation of a pulsed beam with a large cross

section in the forevacuum pressure range. In addition, the specific parameters of this beam, in particular, the pulse beam energy density, must be sufficient for high efficiency surface treatment. In the microsecond range of pulse lengths, this density must be ~10 J/cm2 [8]. In this paper, we describe the design, the operating principle, the characteristics, and the parameters of the plasma electron source designed for pulsed wide aperture beam generation in the forevacuum pressure range. PLASMA SOURCE OF ELECTRONS The design of the forevacuum plasma source of a pulsed electron beam is schematically shown in Fig. 1. In the forevacuum pressure range, it is almost impos sible to produce a significant pressure difference between the regions of emission plasma generation and electron beam formation, which is characteristic of plasma electron sources operating in the range of traditionally lower pressures of ~0.1 Pa. Therefore, under the isobaric operating conditions of plasma electron sources, the mutually exclusive problems of generating a dense emission plasma and, simulta neously, retaining a high breakdown strength of the accelerating gap can be solved by combining a hollow cathode discharge system with a plane–parallel accel erating gap of the smallest possible length. In this case, special design features of the acceler ating gap unit prevent initiation of a breakdown over the socalled “long paths.” These design principles of forevacuum plasma electron guns were used to good effect earlier in our continuously operating devices [9]. They have also formed a basis of plasma electron sources for pulsed wideaperture electron beam gener ation in the forevacuum pressure range. Copper cathode 1 contains a cylindrical cavity 93 mm in diameter and 70 mm in height. Cylindrical

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19mmdiameter copper rod 2 is fixed in place at the center of the cavity. Caprolon insulator 3 is attached to this electrode on the side of the emission hole and is used to fix the position of the mesh anode 4. Mesh anode 4 is composed of two superimposed stainless steel grids with a mesh aperture of 0.3 × 0.3 mm and a geometrical transparency of 60%. The mesh anode 4 is fastened on flange 5. The assembled hollowcathode unit is attached to insulator 6. The plane–parallel accelerating gap is formed by two mesh electrodes—anode 4 and extractor 7. The spacing between these electrodes, which was 25 mm, remained constant in all experiments. The mesh of extractor 7 (2.5 × 2.5 mm, 70%) is also made of stainless steel. Insulators 6 and 8 of the discharge and highvoltage gaps are made of caprolon. The diameter and height of the insulators are 100 and 70 mm, respectively, for the discharge gap and 146 and 40 mm for the accelerating gap. The discharge current pulse was produced by an artificial forming line charged to 2–8 kV and switched by a ТГИ500/16 thyratron. The current pulse length was 100 µs, and the maximum discharge current with out a change to an arc was as high as 100–120 A. The repetition rate of the discharge current pulses was limited by the temperature conditions of the elec trodes in the source, being 10 Hz or under. A dc volt age of up to 20 kV was applied to select and accelerate electrons. The electron source was placed on the vacuum chamber pumped down by a mechanical pump to a limiting pressure of 1 Pa. The operating pressure (5– 15 Pa) was regulated by supplying gas (argon or air) directly into the vacuum chamber. The pulse parameters of the discharge and the beam were measured using the standard methods (voltage dividers, current transformers, and a Faraday cup). A linear array of ten 2.5mmdiameter probes separated by 5 mm from each other was used to inves tigate the current density distribution over the beam cross section. CHARACTERISTICS AND PARAMETERS OF THE ELECTRON SOURCE From the previous section, it is evident that the power supply system of the electron source does not contain special circuits for initiation of a plasmapro ducing discharge (in our case, a hollowcathode glow discharge). As a rule, this is an auxiliary source of shortduration highvoltage pulses. For plasma elec tron sources functioning in the forevacuum pressure range, initiation of a discharge is substantially facili tated by a reverse ion flux from the plasma of the “par asitic” highvoltage lowcurrent discharge produced in the acceleration gap upon application of a high volt age [10]. Since the discharge current exceeds considerably the threshold current of cathode spot formation, pre INSTRUMENTS AND EXPERIMENTAL TECHNIQUES

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Fig. 1. Schematic diagram of the forevacuum plasma source of the pulsed electron beam: (1) hollow cathode, (2) central cathode insert, (3) central insulator, (4) anode mesh, (5) anode flange, (6) discharge gap insulator, (7) extractor mesh, and (8) acceleratinggap insulator.

vention of the discharge from changing to the arcing mode becomes an important condition for stable per formance of the electron source. Though the design of the source does not allow forced distribution of the current over the hollow cathode, the required value of the diffuse discharge current is attained by condition ing (training) the electrodes in a discharge. In this case, the time interval in which the discharge currents reach the required values is strongly dependent on the initial conditions. After the source is depressurized, this process may last for a few hours; nevertheless, under vacuum conditions, the time it takes for the dis charge parameters to attain the maximum values does not exceed 10–20 min. A change to the forevacuum pressure range almost does not affect the high stability and repeatability of pulsed electron beam parameters, which are the gen eral characteristics of plasma electron sources. The typical oscillograms of the discharge and electron emission currents (i.e., the total beam current) are presented in Fig. 2. Note that the efficiency of extrac tion of electrons from the plasma is rather high (the beam current reaches values close to the dis charge current). During a pulse, the voltage applied to the accelerating gap decreases by 1–2 kV, which is determined by the voltage drop across the ballast resis tor that limits the acceleratinggap breakdown cur rent. The current–voltage characteristics of the electron source are presented in Fig. 3. In the characteristics, one can easily discern the current saturation region, which explicitly indicates that secondary electrons knocked out of the emission mesh by the reverse ion flux from the region of electron beam acceleration and Vol. 54

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Fig. 2. Oscillograms (1) of the discharge current pulses and (2) electron emission current at an accelerating voltage of 10 kV and a gas (air) pressure of 12 Pa.

Fig. 3. Current–voltage characteristics of the source at dis charge currents (1) of 80, (2) 60, and (3) 40 A and a gas (air) pressure of 15 Pa.

transportation make a minor contribution to the total beam current. Though the contribution of secondary electrons may considerably increase the electron beam current [11], this operating mode of the plasma source seems to be inefficient, since secondary electrons apply a positive feedback to the primary beam current, the existence of which gives rise to the emission cur rent instability resulting in a breakdown of the acceler ating gap.

the beam structure near the extractor corresponds to the plasma emission surface. The transformation of the radial distribution with an increase in the distance from the extractor indicates that the generated beam is weakly convergent. This effect may be caused by the beam plasma that penetrates through the extractor mesh into the accelerating gap and distorts the plane parallel shape of the accelerating gap. In this case, as the beam moves away from the region of its formation and acceleration, the total current remains constant within the measurement accuracy. Under the experi mental conditions, the maximum current density at the axis, which was 5 A/cm2, was attained at a distance

The radial distributions of the electron beam cur rent density are shown in Fig. 4 for different distances from the extractor mesh 7 (Fig. 1). One can see that Current density, A/cm2 2.5

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Fig. 4. Radial distributions of the electron beam current density at an accelerating voltage of 10 kV. The distances from the extractor mesh are (1) 5, (2) 17, (3) 22, and (4) 33 cm.

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Fig. 5. Microhardness profile of the ceramic sample treated with the pulsed electron beam.

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of 17 cm from the extractor. At a beam current pulse length of 100 µs and an accelerating voltage of 20 kV, the maximum beam current was 100 A, and the pulse beam energy density was as high as 10 J/cm2. The beam focusing by the magnetic field reduced the beam diameter approximately by a factor of 2 and, therefore, increased the pulse energy density. The obtained electron beam was used for surface treatment of alumina ceramics. Melting of the ceramic surface is evidence of its pulse heating to a temperature of >2000°C (the melting point of Al2O3 is 2050°C). Melting of the ceramics caused its surface properties to change. The depth profile of the micro hardness of the ceramic sample is shown in Fig. 5 as an illustration. At the same time, no failure of the ceramic sample due to the temperature gradient was observed during its treatment.

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ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, grant no. 100800257a.

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