ISSN 0020-4412, Instruments and Experimental Techniques, 2017, Vol. 60, No. 5, pp. 705–709. © Pleiades Publishing, Ltd., 2017. Original Russian Text © S. Gao, S.X. Chen, K. Chen, Z.C. Ji, J. Chen, 2017, published in Pribory i Tekhnika Eksperimenta, 2017, No. 00000, pp. 00000–00000.
GENERAL EXPERIMENTAL TECHNIQUES
A Long Pulse Width and High Extraction Rate Arc Plasma Electron Beam Source1 S. Gao, S. X. Chen*, K. Chen, Z. C. Ji, and J. Chen School of Electrical Engineering Wuhan University, Hubei, 430072 China *e-mail:
[email protected] Received October 12, 2016
Abstract− Based on the principle of vacuum arc discharge under magnetic field, a novel plasma cathode electron-beam source was designed. This device can be used to regulate electron-beam current so as to improve the extrication efficiency of electron beam through regulating the exciting current and thus controlling the density of the plasma electron beam source. Experiment results showed that the arc current change with the magnetic field, to be specific, the stronger the magnetic field was, the smaller the arc current will be, then the density of plasma that penetrated the anode hole to serve as electron beam will be higher. From this experiment, it can be seen that under the condition of 10−3 Pa air pressure, 100 V arc voltage, 30 A exciting current, we can obtain the electron beam of 40 ms pulse width, and 828 mA current in the extraction rate of 6.1%. DOI: 10.1134/S0020441217050050
1. INTRODUCTION In the research and application of high power impulse technology, there is an increasing requirement for beam source of strong current and long pulse width [1]. As we know that the conventional thermal cathode electron beam can produce long pulse beam, but the beam current is not strong; also the field emission beam source can produce strong current, but the pulse width of the beam is short. As a result, the vacuum arc plasma cathode electron beam source is the ideal choice, which can solve not only the problem of low output current density of thermal cathode electron gun, but also overcome the disadvantage of short pulse width of field emission electron gun. As a kind of electron beam source, the vacuum arc plasma cathode has been researched by many researchers, and been proved to be feasible [2– 5]. Comparing to the method of producing plasma by hollow-cathode glow discharge, vacuum arc can provide high density plasma, which is an important condition for producing electron beam of strong current and long pulse width. Therefore, in this paper, the vacuum arc serving as an electron beam source has been studied. According to references [6], a novel hollow-anode plasma cathode structure is proposed. Based on the electrostatic focusing method without magnetic field induction, the beam size is regulated by adjusting the parameters of charge-discharge circuit. The electron 1The article is published in the original.
beam extraction efficiency yield of this structure is lower than 4%. According to reference [7], it proposed to use solenoid as anode structure, and extract about 10 A of ion current through the magnetic field produced by the coils. The ion beam extraction efficiency yield of this structure is approximately 2.5%. Cothran [8] realized low voltage circuits extraction method using MOSFET structure, and extricated 100 mA electron beam from hollow cathode discharge plasma. Oks and Brown [9] used pulsed arc as electron beam source and extricated 20 A electron beam using 30 kV voltage. Although this method can achieve considerable extraction efficiency (40%), the according electron beams are narrow pulse width (only hundreds of microseconds), which does not provide electron beam with long pulse width in some cases. In order to acquire long pulse beam, under the condition of strong-current beam, we should further increase the electron beam extrication rate. Through the analysis of vacuum arc characteristics under the condition of longitudinal magnetic field, we adopted a new method where magnet coil was applied. Major characteristics of this method are as follows: (1) The arc plasma density is adjusted by the magnetic field produced by coils. (2) Due to the 90°curved structure of magnet exciting coils, the positive ions and electrons in plasma comes out of the end coil sole, while the neutral atoms escape from gaps of the coil due to not subjecting magnetic force. (3) The charged parti-
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R Electron collector 2 1
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Fig. 1. Schematic of vacuum arc plasma cathode. (1) Cathode, (2) support, (3) trigger electrode, (4 and 5) insulator, and (6) anode.
cles in plasma will be focused by magnetic field in some degree. 2. EXPERIMENT APPARATUS AND APPROACH The cathode structure of vacuum arc plasma is shown in Fig. 1. In terms of the component materials of the vacuum arc plasma source, the cathode is made of a brass rod 1, copper collar 2 is used as a support, 3 is adopted as trigger electrode, the ceramic tube 4, 5 and brass ring 6 are used as the insulator and anode, respectively. The anode bore diameter as well as the distance between cathode surface and anode are both 10 mm. The support can serve as thermal sink, cathode holder, and electrical contact at the same time. The ceramic tube 5 is used to restrain cathode spots within cathode emission surface. At ignition of the arc, the trigger electrode provides flashover on the cathode surface and arc forms, then arc go through the anode hole and into the coil. A thin graphite layer is required on the surface of ceramic tube 4 between the trigger electrode and cathode to reduce the resistance and facilitate the first ignition of the source. The coil is made of a conductor wire having a diameter of 4 mm. The filter coils are made of 5 layers, for each layer, the winding number is 32 and, the diameter of internal duct is about 35 mm. Moreover, one end of the filter coil is connected with the anode. The schematic of the test circuit is shown in Fig. 2. It can be seen that the collector current is measured using resistance R, and the waveforms of voltage and current are collected by a digital oscilloscope.
Trigger current generator Fig. 2. Power supply scheme.
shape is a 60° cone where cathode spots serve as vertex while anode serves as undersurface. After the formation of arc plasma, due to the annular shape of anode, one part of plasma in arc column will bombard anode surface, while the other part of plasma will penetrate into excitation coils through anode hole before serving as plasma electron beam source. Figure 3 shows the measured curve of arc current which varies with exciting current under the different arc voltage. The stronger the magnetic field was, the smaller the arc current would be. With the increase of the axial magnetic field, the diffusion of the vacuum arc is restrained, and the axial current density increases with the decrease of the arc radius [10–12]. With the increase of current density, the electron temperature and the kinetic energy at the cathode are increased; the motion of cathode spot is more intense, as shown in Fig. 4. Arc current, A 14 12 10
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3. INFLUENCE OF MAGNETIC FIELD ON ARC PLASMA Generally, when the arc current is relatively low (normally below several thousand amperes), the vacuum arc is diffusion-type vacuum arc, of which the
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Fig. 3. Arc current with different exciting current under the different arc voltage.
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Fig. 4. Cathode spot with different exciting current.
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Fig. 5. Electron beam at filter outlet with different exciting Fig 4 and 5.
4. INFLUENCE OF MAGNETIC FIELD ON ELECTRON BEAM
waveform under the condition of the arc voltage is 100 V and the exciting current is 30 A.
Figure 5 shows the picture of plasma electron beam sources at coil outlets under the exciting current. In Fig. 5, the exciting current is 0, 15, 30, and 40 A. According to the figures, it showed that the greater the exciting current is, the larger the plasma electron beam density will be. Figure 6 is the electron beam current
Figure 7 shows the electron beams current under the different exciting currents. The greater the exciting current is, the larger the corresponding electron beam current will be. Figure 8 shows the relation between electron beam extrication efficiency and exciting current. Under the same voltage, the increase of magnetic field will decrease arc current and significantly Electron beam, mA 800 600 400 200 0 10
Fig. 6. Electron beam current waveform. INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
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Fig. 7. Electron beam current with different exciting current. Vol. 60
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the corresponding electron beam current will be. Figure 10 shows the relation between electron beam extrication efficiency and arc voltage. The increase of arc current enhances the density of plasmas that entered into the coil, but increasing the electron loss accordingly under the condition of constant exciting current, which is the main reason for the decreased extrication rate of electron beam.
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Fig. 8. Extrication efficiency with different exciting current.
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As the exciting current increases, the resulted magnetic field increase accordingly. The constraint effect of magnetic field makes more and more plasma penetrate through anode hole into excitation coils, leaving a small part of plasma moving onto anode surface, which is the factor for forming arc current. Therefore, under the same arc voltage, as magnetic field increases, the plasma penetrating anode holes increases while the plasma density reaching anode surface decreases, thus the measured arc current gradually decreases. Correspondingly, under the condition of same voltage, a higher magnetic field represents more plasma entering into excitation coil, thus the extraction efficiency of electron beam will be higher. This method, through changing magnetic field to control plasma density, can increase the utilization ratio of arc plasma, and avoid the low efficiency of conventional method which increases plasma density through increasing the voltage between cathode and anode. ACKNOWLEDGMENTS
Fig. 9. Electron beam current with different arc voltage.
Extrication efficiency, % 9
This work is supported by Fundamental Research Funds for the Central Universities (no. 2042016kf1139). REFERENCES
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Fig. 10. Extrication efficiency with different arc voltage.
increase electron beam extrication efficiency. Figure 9 shows the electron beams current under the different arc voltage. The greater the arc voltage is, the higher
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