Russian Physics Journal, Vol. 44, No. 9, 2001
GENERATION OF ELECTRON BEAMS IN THE RANGE OF FOREVACUUM PRESSURES Y. A. Burachevskii, V. A. Burdovitsin, M. N. Kuzemchenko, A. V. Mytnikov, and E. M. Oks
UDK 537.533
This paper presents the results of a study on the generation of electron beams at gas pressures ranging from 0.01 to 0.1 Torr. The fact that this range of pressures is attainable with mechanical pumps only has provoked interest in this problem. To generate an electron beam, use is made of a plasma source based on a hollow-cathode discharge in combination with a plane-parallel acceleration gap. In the given range of pressures, the peculiarities of emission and acceleration of electrons are related to the high probability of ionization of the gas in the acceleration gap and to the formation of an ion flow propagating toward the electron beam. This causes a decrease in discharge operating voltage and also an increase in plasma density in the emission region. Two types of breakdown are observed in the acceleration gap: an interelectrode breakdown and a breakdown in the plasma–electrode system. The designed electron source allows one to obtain beams of cylindrical cross section with currents of up to 1 A and energies of up to 10 keV.
INTRODUCTION The advantages of plasma electron sources (PES's) over hot cathode ones are most conspicuous in the range of elevated gas pressures, since the former do not contain hot elements which may fail upon interaction with a gaseous medium [1]. At the same time, plasma electron sources which generate continuous beams are, as a rule, capable of operating in the range of pressures of no greater than 10–4 Torr. First and foremost, this is due to the need for high electric strength of the acceleration gap. The presence of the upper limit of the range of operating pressures places, in turn, certain restrictions on the field and conditions of application of the given sources. Earlier studies [2] have revealed that, in principle, the operating pressures of plasma electron sources can be increased, but for some reason this work has not been continued. The use of the existing electron sources requires complex and expensive systems of differential pumping. In this connection, the development of plasma-chemical technologies based on a beam-plasma discharge offers new promises in the field. The main concern of the present work was to elucidate why the electric strength of the acceleration gap decreases and how this effect can be precluded.
1. PLASMA SOURCES GENERATING ELECTRON BEAMS IN THE POOR RANGE VACUUM The problem of generation and use of electron beams in the range of elevated pressures up to atmospheric has long been studied. This problem can, however, be considered solved only for sources operating in the pulse mode [3]. The initiation of a discharge in the acceleration gap and the subsequent decrease in voltage across the gap impede the generation of continuous beams. This makes the formation of an electron beam impossible, and such a situation is considered as a breakdown. In the present work, we studied the emissive properties of a hollow-cathode discharge, elucidate why the electric strength of the acceleration gap of plasma electron sources decreases in the range of increased pressures, and how this
Tomsk State University of Automated Control and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 85–89, September 2001. 996
1064-8887/01/4409-0996$25.00 2001 Plenum Publishing Corporation
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Fig. 1. Schematic of the experimental model of the electron source.
decrease can be prevented. Another objective of the work was to design a source capable of generating an electron beam at elevated pressures.
2. EXPERIMENTAL PROCEDURE The experiment was performed using an experimental model of an electron source shown schematically in Fig. 1. The emitting medium was the plasma of a hollow-cathode discharge. This type of discharge was chosen as most appropriate to the given range of pressures and providing the required plasma density as well as for reasons of simplicity and absence of additional factors, e.g., magnetic fields, which could affect the purity of the experiment. In this electron source, the electronemitting plasma was produced in the discharge chamber. The chamber consisted of hollow Co cathode 1 and plane anode 2 with central emission hole 3 of diameter 16 mm. To stabilize the plasma boundary and to screen the sag of the accelerating field into the discharge system, the anode hole was covered with a fine metal grid. The mesh size of the grid was varied from 0.25×0.25 to 1.0×1.0 mm. The geometric transparency of the grid reached 70%. In some of experiments, the grid was replaced by a perforated electrode with hole sizes close to the mesh sizes of the grid. An electron beam was obtained by extracting electrons through the emission hole in the anode and by applying voltage to the anode 2 – extractor 4 acceleration gap. The magnetic field in the cathode cavity and in the acceleration gap was induced by a solenoid put on the electron source. The pressure was increased by feeding a gas (air) into the working chamber of the vacuum system. This ensured the equality of pressures in the gas-discharge chamber and in the region of acceleration and transportation of the beam. A more detailed description of the design and characteristics of the source can be found elsewhere [7, 8]. All electrodes of the experimental model were mounted on standard ceramic insulators 5. In some experiments, a Plexiglas insulator was used in the acceleration gap to provide visual observation.
3. EMISSIVE PROPERTIES OF A HOLLOW-CATHODE DISCHARGE It is well known that the gas-discharge plasma is rather sensitive to electron losses [9]. In this connection, studying the influence of the electron emission both on the plasma parameters and on the main characteristics of the discharge is important both for the elucidation of the mechanism of this phenomenon and for practical implementation of the plasma electron source. In spite of the fact that the hollow-cathode discharge in its pure form as well as in combination with other types of discharge finds wide application in different devices, its emissive properties, particularly, for the range of elevated
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Fig. 2. Emission current Ie and plasma density n versus accelerating voltage Ve: 1, 2 – Ie, 3 – n. р, mTorr: 2, 3 – 28, 1 – 40. Fig. 3. Discharge voltage Vd versus accelerating voltage Ve. р, mTorr: 1, 2 – 10; 3, 4 – 90. V, mT: 1, 3 – 0; 2, 4 – 34.
pressures, have not been adequately investigated. In this work, we study the relationship between the electron emission and the discharge operating voltage as well as the plasma parameters. The results shown in Fig. 2 indicate that in the conditions of stabilization of the discharge current measured in the circuit of the hollow cathode, an increase in accelerating voltage causes an increase both in emission current and in plasma density in the emission region. The behavior of the discharge voltage Vd with increasing Ve, appears to depend on the magnetic induction as well as on the gas pressure. The increase in Vd at low pressures is replaced by a decrease in Vd at pressures higher than 60 mTorr. The magnetic field suppresses the increase in Vd (Fig. 3). The emission current increases with increasing gas pressure p. This fact is explained within the framework of the concept of formation of gas ions in the acceleration gap, which move toward the electron beam [10]. Upon entering the discharge plasma, these ions change their charge at gas molecules, and this results in an increase in plasma density and, consequently, in emission current. The possibility of realization of this mechanism was pointed out earlier in [6]. The presence of the ion flow also explains the decrease in discharge voltage with increasing accelerating voltage, since part of the ions finds its way into the discharge region through the emission holes and has a possibility to participate in the γ-processes at the cathode immediately or upon charge exchange. Since these ions appear to be "excess" ions for the discharge, the latter becomes not quite self-sustained and can be sustained at lower discharge voltages. In our opinion, the effect of the magnetic field lies in additional retention of both fast and plasma electrons within the cathode cavity.
4. ELECTRIC STRENGTH OF THE ACCELERATION GAP As already noted, the main factor that prevents the generation of an electron beam at elevated pressures is the low electric strength of the acceleration gap. This may result in a breakdown manifesting itself in a rapid decrease in voltage and an abrupt increase in Ie in the circuit of the high-voltage rectifier. To elucidate why the acceleration gap loses its electric strength and to find out how this loss could be prevented, we measured the highest achievable accelerating voltage Vem depending on the gas pressure, emission current, and geometric factors. Visual observation has revealed that to maintain the accelerating voltage, it is necessary to eliminate the emergence of periphery breakdowns along long paths. Once special measures were taken, breakdowns were observed only in the region of electron emission. The measurements allowed us to conclude that we dealt with breakdowns of two types. They differed in the conditions of their occurrence and in the 998
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Fig. 4. Limiting accelerating voltage Vem versus emission current Ie. Anode – grid, h = 0.3 mm; р = 75 ( ) and 68 mTorr ().
character of interrelation of the main parameters. A breakdown of the first type, which was conventionally called an "interelectrode" breakdown, occurred between the acceleration electrode and the anode. In this case, a rapid increase in current was detected only in the circuit of the high-voltage rectifier. A breakdown of the second type occurred between the discharge plasma and the acceleration electrode. As this took place, the breakdown current flowed through the acceleration electrode − plasma − hollow cathode circuit and was detected both in the circuit of the high-voltage rectifier and in that of the discharge power supply (DPS). The type of breakdown occurring in a specific situation was determined for the most by the size h of the emission holes of the PES and by the gas pressure in the acceleration gap. The relatively small size of the holes and the low pressure led to an "interelectrode" breakdown, whereas the large-size holes and the high pressure were responsible for the "plasma" mechanism of breakdown. In our experiments, a breakdown of the first type was initiated by increasing the voltage across the acceleration gap at a specified value of the emission current. The experiments performed using various anode electrodes (grids and plates with holes differing in number) have shown that the emission current is just the factor affecting the breakdown. Figure 4 shows the breakdown voltage of the acceleration gap, Vem, in relation to the electron emission current, Ie, for different gas pressures. Here, the emission current was measured immediately before the breakdown. The results of the experiments make it possible to conclude that the electron beam promotes the breakdown of the acceleration gap. At the same time, when the emission current exceeds some threshold value, the breakdown voltage Uem is found to increase. We managed to initiate a breakdown of the second type by increasing the discharge current, while keeping the specified voltage across the acceleration gap. Figure 5 shows the limiting discharge current Idm versus the accelerating voltage Ve. The increase in the transparency of the anode electrode by increasing the number of holes shifted the current Idm toward higher values. The experiments performed provide evidence that it is precisely the discharge current rather than the emission current that determines the onset of breakdown in this case. The difference in the conditions of occurrence and in the character of the above discharges suggest that their mechanisms are different. In analyzing the interelectrode breakdown, it should be taken into account that in the absence of emission, i.e., for Id = 0, the application of the accelerating voltage results in a well-known high-voltage glow discharge with currents of a few milliamperes in the acceleration gap. Actually, the electrical breakdown is the transition from the high-voltage to low-voltage form of the discharge. The reason for this transition is the appearance of an additional ionizer whose role in this case is played by the electron beam. The results obtained with a breakdown of the second type can be explained using a model described elsewhere [8]. The central point of this model is the proposition that the breakdown of an acceleration gap occurs when the plasma from the discharge region gets into the gap. There are two prerequisites for this effect. The first one lies in the fact that the thickness of the near-anode layer of space charge, which separates the discharge plasma from the anode, becomes considerably lower than the size of the emission holes. The second one is that the plasma − acceleration electrode distance estimated from the "3/2 power" law should be smaller than the width of the acceleration gap. When these conditions are 999
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fulfilled, the discharge current switches from the anode to the acceleration electrode, which is accompanied by a drastic decrease in voltage across the acceleration gap. We consider this phenomenon as a breakdown. The decrease in the thickness of the near-anode layer results mainly from the increase in plasma density due to the increase in discharge current as well as to the inflow of gas ions from the acceleration gap. As already noted, the electron beam can provide not only a decrease, but also an increase, in the limiting accelerating voltage (Fig. 4). This effect, unexpected at first glance, is observed when the emission current exceeds some threshold value and solely in the case of an interelectrode breakdown. We consider two plausible mechanisms responsible for the given effect. The first mechanism is local heating of the gas during the passage of the electron beam. The mechanism of heating is treated in the following manner. The electrons of the beam ionize gas molecules in inelastic collisions in the acceleration gap. The ions formed are accelerated by the electric field and, when colliding elastically with the gas molecules, transmit their momentum to the latter, thus heating the gas. Our estimates show that at an electron current of 1 А the given phenomena may result in a decrease in neutral gas density by a factor of 1.5–2 which, in turn, should decrease the probability of ionization processes. At the same time, the authors are aware of some contradiction involved in the mechanism considered. So, it is not inconceivable that there exists another reason for the effect at hand. This reason can be associated with accumulation of positive ions in the acceleration gap, which provides a nonuniform potential distribution and is, thus, equivalent to the shortening of the acceleration gap. Nevertheless, for both reasons, there should be an increase in the electric strength of the acceleration gap according to the character of the left branch of the Paschen curve.
CONCLUSION The studies performed have made it possible to reveal two main reasons for the decrease in the electric strength of the acceleration gap in plasma electron sources. The first reason for the effect is related to an abrupt transition of the highvoltage glow discharge to a low-voltage discharge in the acceleration gap. This transition is promoted by the presence of the electron beam. The second reason lies in the possibility of the plasma getting into the acceleration gap from the discharge region. This causes the discharge to switch from the anode to the acceleration electrode, which involves a drastic decrease in accelerating voltage. Based on the investigations performed, we have managed to optimize the design of the electron source and to obtain an electron beam with a current of up to 1 А and an energy of up to 10 keV at a gas pressure of up to 0.1 Torr.
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Yu. E. Kreindel, Plasma Electron Sources [in Russian], Energoatomizdat, Moscow (1977). S. I. Belyuk, Yu. E. Kreindel, and N. P. Rempe, Zh. Tekh. Fiz., 50, 203–205 (1980). S. P. Bugaev, Yu. E. Kreindel, and P. M. Schanin, Electron Beams of Large Cross Section [in Russian], Energoatomizdat, Moscow (1984). A. A. Novikov, Electron Sources Based on a High-Voltage Glow Discharge with the Anode Plasma [in Russian], Energoatomizdat, Moscow (1983). Yu. E. Kreindel and V. A. Nikitenskii, Zh. Tekh. Fiz., 41, 2378–2382 (1971). V. A. Gruzdev, Yu. E. Kreindel, and Yu. M. Larin, Ibid., 43, 2318–2323 (1973). V. A. Burdovitsin and E. M. Oks, Rev. Sci. Instr., 70, 2975–2778 (1999). V. A. Burdovitsin, Yu. A. Burachevskii, A. V. Mytnikov, and E. M Oks, Zh. Tekh. Fiz., 71, 48–50 (2001). V. L. Galanskii, Yu. E. Kreindel, E. M. Oks, et al., Ibid., 57, 877–882 (1987). V. Burdovitsin, D. Danilishin, A. Mytnikov, and E. Oks, 12th Symp. on High Current Electronics, September, 24– 29, Tomsk, Russia, 2000.
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