ISSN 10637850, Technical Physics Letters, 2013, Vol. 39, No. 5, pp. 454–456. © Pleiades Publishing, Ltd., 2013. Original Russian Text © A.A. Zenin, A.S. Klimov, V.A. Burdovitsin, E.M. Oks, 2013, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2013, Vol. 39, No. 10, pp. 9–14.
Generating Stationary Electron Beams by a Forevacuum Plasma Source at Pressures up to 100 Pa A. A. Zenin, A. S. Klimov, V. A. Burdovitsin*, and E. M. Oks Tomsk State University of Control Systems and Radioelectronics, Tomsk, 634050 Russia *email:
[email protected] Received January 14, 2013
Abstract—It is shown that, as the gas pressure in a forevacuum plasma electron source increases, electric breakdown in the accelerating gap is caused by the reverse flow of ions from plasma that is generated by both the electron beam and highvoltage glow discharge (HGD). By modifying the electrode system geometry in the accelerating gap of the electron source, it is possible to provide for a two to threefold decrease in the HGD current. This allows the upper pressure in the electron source to be increased up to about 100 Pa when air is used as the working gas and up to 160 Pa in the source filled with helium. DOI: 10.1134/S1063785013050271
The task of generating electron beams at elevated pressures is important in view of the development of electronbeam technologies for the processing of non conducting materials [1, 2], synthesis of various func tional coatings [3], plasmachemical processes [4], etc. At present, the socalled forevacuum plasma electron (FPE) sources [5] can provide for effective generation of electron beams at pressures up to 20 Pa when air is used as the working gas and up to 30 Pa in the electron beam sources operating on helium. Although the achieved level of working gas pressures is rather high, development of electronbeam technologies poses the task of further increasing this level. The present study was aimed at finding ways to fur ther increase the working gas pressure in FPE beam sources. Physically, the pressure limit of FPE source opera tion is related to finite electric strength (electric break down) of the accelerating gap. At gas pressures above 10 Pa, a decisive role in the development of breakdown is played by highvoltage glow discharge (HGD) [6], the intensity of which determines to a considerable degree the density of plasma formed in the region of electronbeam transport. In order to expand the range of working gas pressures, it is of primary importance to provide conditions for reducing the HGD current. Figure 1 shows a schematic diagram of the modi fied electrode system in the accelerating gap of an FPE source based on the hollowcathode discharge (a more detailed description has been given elsewhere [7]). The emission electrode represents a 1mmthick per forated tantalum plate with 0.6mmdiameter holes. The working gases were air and helium. The gas pres sure was measured by a CMR 362 Pfeiffer active capacitive transmitter. The operation principle of this device makes its readings independent of the kind of a
gas, thus increasing the reliability of measurements. The measured electrical characteristics included volt age Ua on the accelerating gap, discharge current Id, and electronbeam current Ib on the collector. The collector represented a Faraday cup that was situated at a distance of 70 mm from the focusing system. In this work, a decrease in the HGD current was achieved by (i) modifying the electrodesystem con figuration in the accelerating gap and (ii) simulta neously decreasing the interelectrode distance (from 15 to 6 mm) and the emissionaperture diameter (from 20 to 8 mm). Experiments showed that this modification ensured a severalfold reduction in the HGD current and virtually excluded the influence of this parasitic discharge on the electric strength of the accelerating gap.
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3 Fig. 1. Schematic diagram of the electrode system in the accelerating gap of the FPE source: (1) perforated emis sion electrode, (2) anode, (3) accelerating electrode, and (4) insulator.
GENERATING STATIONARY ELECTRON BEAMS Idm, mA
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Fig. 2. Plots of threshold discharge current Idm vs. acceler ating voltage Ua for an FPE source filled with air at (1) 100 and (2) 27 Pa.
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Thus, the electric breakdown of the accelerating gap upon increase in the working gas pressure is related primarily to the parameters of the electron beam formed by electrons taken from the emissive boundary of plasma generated in the FPE source. In HGD, the plasma density determining the electron emission cur rent is directly proportional to Id. Figure 2 shows the dependence of maximum discharge current Idm, above which the electric breakdown of the accelerating gap takes place, on the accelerating voltage Ua. As can be seen, the character of the Idm(Ua) curve is determined by the working gas pressure. In the interval of pressures within 20–50 Pa, the Idm value increases with Ua, while at elevated pressures the breakdown threshold current drops with increasing Ua. This difference in behavior of Idm(Ua) is evidently related to principally distinct mechanisms of electric breakdown in different gas pressure intervals. At low pressures, socalled plasma breakdown takes place, according to which plasma penetrates from the discharge region into the acceler ating gap [5]. At elevated pressures, the influence of a reverse flow of ions from plasma prevails, which leads to the appearance of cathode spots on the emitting electrode. A shift in the pressure, at which the latter mechanism is operative, to higher values as compared to those observed previously [5] is apparently related to modification of the electrode system that provided a sharp decrease in the HGD current. Figures 3a and 3b present the current–voltage characteristics of the modified FPE source operating on helium and air, respectively. The most important result of this work is the ability of the modified elec tronbeam source to operate at pressures up to 100 and 160 Pa on air and helium, respectively. These values are several times higher than those achieved in our recent work [6]. It should also be noted that the mod ification of the electrode system, which provided for
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Fig. 3. Plots of the electronbeam collector current Ib vs. accelerating voltage Ua for an FPE source operating on (a) helium and (b) air at pressures (1, 2) 60, (3, 4) 100, and (5, 6) 160 Pa; discharge currents: (a) Id = 0 (1, 3, 5) and 600 mA (2, 4, 6); (b) Id = 0 (1, 3, 5), 200 mA (2, 4), and Idm (6) (Idm is the maximum permissible current at a given Ua).
a significant increase in the working gas pressure, led to some decrease in the efficiency of FPE source oper ation. Indeed, a markedly greater discharge current is necessary to obtain a certain beam current. However, the achievement of such high working pressures at the expense of some loss in the efficiency may be quite acceptable. Acknowledgments. This study was supported by the Russian Foundation for Basic Research (project no. 120831043) and the Ministry of Education and Science of the Russian Federation (project no. 7.3101.2011 and federal targeted program no. 12B37.21.1162).
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and Chemistry of Inorganic Materials (September 10– 15, 2006, Tomsk, Russia). 2. V. A. Burdovitsin, A. S. Klimov, and E. M. Oks, Tech. Phys. Lett. 35, 511 (2009). 3. N. N. Smirnyagina, A. S. Milonov, and N. S. Kar manov, Proceedings of the 8th Int. Conf. “Electron Beam Technologies” (EBT2006, Varna, 2006), Vol. 2, pp. 55–56. 4. A. I. Pushkarev, G. E. Remnev, and D. V. Ponomarev, High Energy Chem. 40, 105 (2006).
5. V. A. Burdovitsin and E. M. Oks, Laser Particle Beams 26, 619 (2008). 6. V. A. Burdovitsin, A. K. Goreev, A. S. Klimov, A. A. Zenin, and E. M. Oks, Tech. Phys. 57, 1101 (2012). 7. V. A. Burdovitsin, I. S. Zhirkov, E. M. Oks, I. V. Osipov, and M. V. Fedorov, Instrum. Exp. Tech. 48, 761 (2005).
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Translated by P. Pozdeev
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