ISSN 1063-7850, Technical Physics Letters, 2016, Vol. 42, No. 7, pp. 712–714. © Pleiades Publishing, Ltd., 2016. Original Russian Text © A.A. Zenin, I.Yu. Bakeev, Yu.A. Burachevskii, A.S. Klimov, E.M. Oks, 2016, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2016, Vol. 42, No. 13, pp. 104–110.
Electron Beam Focusing Features in a Plasma Electron Source under Forevacuum Pressures A. A. Zenin*, I. Yu. Bakeev, Yu. A. Burachevskii, A. S. Klimov, and E. M. Oks Tomsk State University of Control Systems and Radioelectronics, Tomsk, 634050 Russia *e-mail:
[email protected] Received November 29, 2015
Abstract—Features of focusing electron beams generated by a forevacuum plasma source under pressures of 10–30 Pa are investigated and the principle possibility of obtaining submillimeter beams is demonstrated. A beam power density of 105 W/cm2 is reached at an electron beam diameter of 0.6 mm. The obtained beam parameters offer opportunities for precision electron beam processing of dielectric materials, including hightemperature alumina ceramics. DOI: 10.1134/S1063785016070142
Plasma electron sources [1], despite the higher emitted electron temperature than in the hot cathode, ensure a power density and focused beam brightness comparable with those of hot-cathode electron sources due to the higher emission current density. Optimization of the accelerating gap geometry and parameters of electron sources with the plasma cathode allows focusing electron beams to diameters of smaller than 1 mm [2], which makes these sources attractive for electron beam welding [3] and some other applications that require precise electron beam action with a high power density. The transition of plasma electron sources to the forevacuum pressure range [4] opened opportunities for electron beam processing of dielectric materials, including welding of ceramic products [5] and welding of metals with ceramics [6]. The efficiency of precise electron beam processing of dielectrics is related, to a great extent, to the possibility of forming focused beams at forevacuum pressures. This Letter presents the results of investigations of the features of focusing the electron beam generated by a forevacuum plasma source. Figure 1 shows a schematic of the experimental setup. In the experiments, we used a forevacuum plasma electron source based on a hollow cathode discharge. The operation principle of the source was described in detail in [7]. Similar to the plasma electron sources used in the traditional pressure range (10‒2–10–1 Pa) [1–3], to enhance the focusing efficiency, electrons were extracted from the plasma through a single emission channel. The emission channel diameter and length were 0.75 and 1 mm, respectively. At accelerating voltages of up to 20 kV and a working gas (helium) pressure of 10–30 Pa,
the source generated a continuous electron beam with a current of tens of mA. The accelerated electron beam was focused by the magnetic field of solenoid 7 and transferred to a distance of 20 cm. To sweep the electron beam from the axis, deflection system 8 was used, which is a square quadripole magnetic deflecting coil with the magnetic core. The beam diameter was measured using a deflection technique described in [1]. The beam was swept in the straight by applying a sinusoidal voltage with an amplitude of 12 V and a frequency of 50 Hz to the magnetic deflecting coil. The electron beam deviated from the axis and alternatively crossed two long gaps in plate 9 of the beam diameter measuring system. The gap width was 0.2 mm and the distance between the gaps was 5.6 mm. When the electron beam crossed the gaps, a current signal was detected on plate 10. The typical oscillogram of this signal is shown in Fig. 1. The signal diameter was determined from the simple relation (1) d = lτ, T where l is the intergap distance, T is the time between the centers of two neighboring peaks, and τ is the full peak width at half maximum (Fig. 1b). The total beam current was measured upon beam deflection to Faraday cylinder 11 shifted by a distance of 40 mm from the axis. The dependences of the electron beam diameter on the accelerating voltage and current in the beam at different gas pressures are presented in Figs. 2a and 2b, respectively. It can be seen in Fig. 2b that the beam diameter decreases with increasing accelerating voltage. The analogous effect of the accelerated electron
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Fig. 1. (a) Experimental setup and (b) typical oscillogram for measuring the beam diameter. (1) Hollow cathode, (2) anode, (3) emission electrode, (4) plasma, (5) accelerating electrode (extractor), (6) electron beam, (7) focusing solenoid, (8) deflection system, (9) upper plate with two gaps, (10) current-receiving plate, and (11) Faraday cylinder.
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energy on the beam focusing conditions was observed at both lower (p < 0.1 Pa) [8] and higher (p > 100 Pa) pressures [9]. Perhaps the increasing electron energy in the beam weakens electrostatic repulsion of electrons at low pressures and reduces their scattering at higher pressures.
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In the forevacuum plasma electron sources, the effect of the beam current on the beam diameter has certain distinguishing features. As a rule, an increase in the beam current degrades beam focusing and leads to an increase in the minimum beam diameter [8, 9].
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Under the experimental conditions, at a pressure of 10 Pa, the electron current almost did not affect the beam diameter, while at a pressure of 30 Pa the current growth even somewhat decreased the diameter (Fig. 2b). One of the possible reasons for a decrease in the beam diameter at large currents is the additional beam focusing caused by the distortion of the accelerating field as a result of penetration of beam plasma in the accelerating gap. Generation of the high-density beam plasma and its noticeable effect on the emission properties of the plasma and conditions of the electron beam formation is one of the characteristics features of forevacuum plasma electron sources.
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A decrease in the beam diameter with increasing current can be caused also by compression of the beam by its own magnetic field. The importance of taking into account the own magnetic field of the electron beam generated by a plasma source in the low-pressure range was mentioned in [10].
Fig. 2. Dependences of the beam diameter on (a) accelerating voltage and (b) beam current. (1) 5 mA and 10 Pa, (2) 15 mA and 10 Pa, (3) 5 mA and 30 Pa, (4) 15 mA and 30 Pa, (5) 14 kV and 10 Pa, (6) 18 kV and 10 Pa, (7) 14 kV and 30 Pa, and (8) 18 kV and 30 Pa. TECHNICAL PHYSICS LETTERS
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plasma electron source for burning a hole in a 1.5-cm alumina ceramic cube. Thus, the results of our investigations are evidence for the possibility of effective focusing electron beams generated by a plasma electron source at forevacuum pressures. The attained power density of the beam is sufficient for size processing of high-temperature ceramics. Acknowledgments. This study was supported by the Russian Foundation for Basic Research (project no. 15-08-00871_a) and the Ministry of Education and Science of the Russian Federation (project no. 3.49.2014/K). E.M. Oks works within the state order Research Engineering of the Ministry of Education and Science of the Russian Federation, project no. 783.
Fig. 3. Hole burnt by the electron beam (accelerating voltage 18 kV and beam current 15 mA) in aluminum ceramics.
REFERENCES At higher pressures beyond the forevacuum range, an increase in the beam current always leads to an increase in the beam diameter. In this pressure range, the beam broadening processes caused by electron scattering on the residual gas apparently prevail over beam focusing by the own magnetic field. An increase in the beam diameter with increasing gas pressure is expected and can be attributed to electron scattering on residual gas molecules. Nevertheless, as can be seen from the experimental data presented in Fig. 2, despite higher pressures, even under these conditions the plasma electron source allows focusing electron beams to submillimeter sizes. In this case, the electron beam power density in the crossover attains 105 W/cm2. In the forevacuum pressure range, the electron beam power density and, consequently, beam brightness are lower than the corresponding parameters of the electron beam generated by the plasma sources in the traditional pressure range (10–2–10–1 Pa) by a factor of 3–4 [1–3]. However, the electron beam power density level attained in the forevacuum plasma sources is sufficient for precise processing of dielectrics, including high-temperature ceramics. As an example, Fig. 3 shows the result of using a forevacuum
1. S. Yu. Kornilov, N. G. Rempe, A. Beniyash, N. Murray, T. Hassel, and C. Ribton, Tech. Phys. Lett. 39 (10), 843 (2013). 2. S. Yu. Kornilov and N. G. Rempe, Tech. Phys. 57 (2), 236 (2012). 3. S. Yu. Kornilov, I. V. Osipov, and N. G. Rempe, Instr. Exper. Tech. 52 (3), 406 (2009). 4. V. A. Burdovitsin, A. S. Klimov, A. V. Medovnik, E. M. Oks, and Yu. G. Yushkov, Forevacuum Plasma Electron Sources (Izd. Tomsk. univer., Tomsk, 2014) [in Russian]. 5. V. A. Burdovitsin, A. S. Klimov, and E. M. Oks, Tech. Phys. Lett. 35 (6), 511 (2009). 6. A. A. Zenin and A. S. Klimov, Dokl. Tomsk. Gos. Univ. Sist. Upravl. Radioelektron. 1 (27), 10 (2013). 7. V. A. Burdovitsin, I. S. Zhirkov, E. M. Oks, and I. V. Osipov, Instr. Exp. Tech. 48 (6), 761 (2005). 8. S. Yu. Kornilov, I. V. Osipov, and N. G. Rempe, in Proceedings of the 3rd Int. Kreindel Seminar “Plasma Emission Electronics” (PEE’2009, June 23–30, 2009, UlanUde, Republic of Buryatiya), pp. 112–117 [in Russian]. 9. A. I. Golovin, M. M. Golubev, E. K. Egorova, A. V. Turkin, and A. I. Shloido, Tech. Phys. 59 (5), 670 (2014). 10. V. N. Devyatkov, N. N. Koval’, and P. M. Shchanin, Tech. Phys. 43 (1), 39 (1998).
TECHNICAL PHYSICS LETTERS
Translated by E. Bondareva
Vol. 42
No. 7
2016