ISSN 10642269, Journal of Communications Technology and Electronics, 2013, Vol. 58, No. 4, pp. 357–365. © Pleiades Publishing, Inc., 2013. Original Russian Text © S.P. Morev, N.P. Aban’shin, B.I. Gorfinkel’, A.N. Darmaev, D.A. Komarov, A.E. Makeev, A.N. Yakunin, 2013, published in Radiotekhnika i Elektronika, 2013, Vol. 58, No. 4, pp. 399–408.
ELECTRON AND ION EMISSION
ElectronOptical Systems with Planar FieldEmission Cathode Matrices for HighPower Microwave Devices S. P. Moreva, N. P. Aban’shinb, B. I. Gorfinkel’b, A. N. Darmaeva, D. A. Komarova, A. E. Makeeva, and A. N. Yakuninc aResearch
and Production Enterprise Torii, ul. Obrucheva 52, Moscow, 117393 Russia b VolgaSvet, pr. 50 Let Oktyabrya 101, Saratov, 410052 Russia cInstitute of HighPrecision Mechanics and Control Problems, Russian Academy of Sciences, ul. Rabochaya 24, Saratov, 410028 Russia email:
[email protected] Received November 21, 2012
Abstract—Reasons for limits on application of fieldemission structures in highpower microwave vacuum tubes are studied on the basis of the performed analysis and the methods for their overcoming are discussed. Prospects for application of planar end fieldemission structures (PEFESs) as a longlived current source in highpower vacuum microwave amplifiers are substantiated. The results of design of the proposed electron optical system with cathode matrices consisting of PEFES cells, which forms electron flows with a power over 100 kW in the contiuouswave operating conditions, are presented. DOI: 10.1134/S1064226913040116
INTRODUCTION The advent in 1937 of the first filedemission unit (Müller projector [1]) was preceded by the discovery in 1897 of the fieldemission phenomenon by R. Wood [2], explanation in 1923 of reduction in the work func tion of electrons from metals due to the field emission (Shottky effect [3]), and phenomenological descrip tion in 1928 of the fieldemission current by the Fowler–Nordheim law [4]. First microwave devices using fieldradiating cath odes (femitrons) were designed in 1958 [5]; however, owing to the possibility of obtaining much higher parameters of the devices using hot cathodes, studies in this direction were terminated. An interest in the devices with field emission, which allows one to obtain in principle ultraspeed (up to several nanoseconds) and ultralowvoltage (1–2% of the accelerating potential) control of the beam current in the absence of cathode warmingup loss, was recommenced after publication of C. Spindt’s studies [6] (it is sufficient to look at the materials of the IVM conferences for 1980–2000). Initially, many publications were devoted to the studies of various aspects of formation of emis sion structures, consisting of matrix or edge cathodes. Later, publications on the development of the first electron gun for travelingwave tubes (TWTs) (1986, [7]) and TWTs (1997, [8]) with fieldemission cath odes manufactured according to the Spindt technol ogy appeared. In 2009, the results of studies of pulse (with an relative pulse duration 10) TWTs with field emission cathodes whose electron flow power was 400 W and the perfomance was over 150 hours, were
published [9]. However, up to now, solutions of the problems related to the use of highintensity electron flows formed by the structures with stable field emis sion during at least 1000 hours in microwave devices were not yet obtained. In this paper, physical processes that limit the pos sibilities of wide applications of fieldemission struc tures in the development of highpower electron flows are studied and the ways of removal of these restric tions are analyzed. The structures that are adapted for formation of intense electron flows and intended to ensure stable electron emission in a time interval of about 103 hours under the continuous wave (CW) conditions are proposed. 1. FORMULATION OF THE PROBLEM A variety of designs of cathode structures imple menting the field emission can be conventionally divided into two groups. The first group includes cath ode structures whose elementary cell consists of an axially symmetrical point and an electrode with a hole (Fig. 1a) located coaxially with the point (Spindt structures [6]). As the points these structures use solid tungsten or carbon points and carbon nanotubes. This group also includes structures, in which several tens (or several hundreds) of emitting points are located within one cell (Fig. 1b) of a largestructure control grid (Grigor’ev–Shesterkin structures [10]). The other group includes the socalled planar structures with edge cathodes (Figs. 1c, 1d), which are primarily
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(b)
2
3
3
2
1
1 (c)
(d) 4
2
2
3
3 1
1
Fig. 1. Types of the fieldemission cells with axially symmetrical and planar elements: (a) Spindt cell, (b) Grigor’ev–Shesterkin cell, (c) Chesnokov cell, and (d) Abanshin–Gorfinkel cell; (1) substrate, (2) emitter, (3) control electrode, and (4) anode.
used for displays (Chesnokov [11] and Abanshin– Gorfinkel structures [12]). The analysis of Russian and foreign scientific pub lications shows that difficulties impeding wide appli cations of fieldemission structures in formation of highpower electron flows are basically explained by two reasons. The first reason is the low stability of operation of the cell, which uses the field emission at the residual gas pressure (about 10–6–10–8 mmHg) characteristic of most commercial devices [13]. At this residual pressure, the emitting part of the cathode can collapse under the bombardment of the emitter with ions of the residual gases formed in 1
the gap between the cathode and the anode [14]. In addition, efforts to increase the averaged density of the current from a cathode consisting of a periodic struc ture (see [6] or [10]), lead to the thermal overload of the emitter point, and, when some limiting value the 1 We
should only note that, in our opinion, an individual lonely axially symmetrical point of the autoelectronic emitter collapses sooner than a monolithic and elongated edge autoelectronic emitter.
fieldemission current is exceeded, to the develop ment of the explosive electron emission [15, 16]. The other reason is the large angular scatter of elec trons, which is typical of the cells of this type. For example, according to the estimates of the authors of the paper [17], the main contribution to the field emission current is given by the electrons with angles in an interval of 0°–60° with respect to the axis of the point. As the calculations performed in [18] have shown, the scatter of the transverse components of 2
electron velocities in the cells is especially high [10]. Similar processes are also characteristic of edge struc tures (see [11] or [12]). Thus, the development of elec tronoptical systems (EOSs) of highpower microwave devices based on fieldemission structures is related to simultaneous solution of a number of problems. First, it is necessary to form an electron flow with a small scatter of transverse velocity components of electrons. 2
Due to the potential sagging in the holes of the largestructure control grid, the field emission basically occurs from the points of the peripheral (with respect to the center) cells. In this case, electrons emitted by the peripheral points are disturbed by the electrostatic lens of the cell of the largestructure control grid so that transverse velocity components whose values are compara ble with their longitudinal components appear in the cells.
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Fig. 2. Behavior of nitrogen ions in the region of the Spindt cell: U T = 0.026 V (Т = 300 K).
Second, the required value of the beam current should be ensured at a relatively low averaged current density of the cathode (at which the thermal overload of auto electronic emitters does not occur). Third, reliable protection of the cathode against the ion bombard ment should be provided. In addition, taking into account the twoscale structure of these EOSs, these requirements should be met at the microlevel of the cell with the autoelectronic emitter and at the mac rolevel of the electron gun as a whole. 2. RESULTS OF CALCULATIONS OF THE CELLS OF FIELDEMISSION STRUCTURES AND THEIR DISCUSSION As an example (Fig. 2a), the cell of a fieldemitting cathode with an axially symmetrical point and a hole in the extracting electrode was selected. The parame ters of the cell are as follows [14]: (i) the distance from the substrate to the extracting electrode is 0.08 µm; (ii) the thickness of the extracting electrode is 0.02 µm; (iii) the radius of the point is 0.01 µm; (iv) the potential of the extracting electrode is 200 V. Equipotential surface S with a potential of 245 V was placed at 500 µm from the extracting electrode. For convenience, we name the selected object the microgun.
In accordance with [13], the main residual gas in a TWT under a pressure of 10–6–10–8 mmHg is nitrogen M (the nitrogentoelectron mass ratio nitrogen = 5.16 × M el 104), and hydrogen and helium are present in much smaller amounts. If the average thermal velocity is rep resented by its equivalent potential, at a temperature of 300 K, we obtain U T = 0.026 V. The dynamic equi librium, at which the number of formed ions is equal to the number of outgoing (or recombining) ions in a unit volume, starts several microseconds after the beam switching. Positively charged nitrogen ions located in the electric field of the microgun consisting of the Spindt sell or the Grigor’ev–Shesterkin cell, are intensely attracted to the points of field emitters, which act as a usual lightning protector (Figs. 2b, 2c). In this case, as follows from the analysis of Figs. 2b, 2c, the nature of ion movements weakly depends on their initial energy, even when it changes by almost two orders of magnitude. Large angular scatter in the Spindt cell can be decreased by applying focusing lenses in the area of the microgun. Fig. 3a shows one of possible configura tions of microgun electrodes, which permits, in authors' opinion [20], to obtain at the output from the 3
microgun a weakly divergent electron flow. However, 3 The configuration of electrodes was taken from [19].
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R, μm
300 V
1
0V
6
300 V
θ 0 0
1
2
z, μm
(b) B/B0
R, cm
0.3
0.8 0.2 0.4
0.1
0 0
0.2
0.4
0.6
0.8
Z, cm Fig. 3. Distribution of the potential and behavior of the electron flow in (a) the Spindt cell with an additional electrostatic lens and (b) the EOS with Grigor’ev–Shesterkin cells.
this cell is also unprotected against the ion flow from the macrogun anode and the drift channel of the device. Thus, the use of the cathode matrix of field emission structures (see [6] or [10]) in macroguns of highpower microwave vacuum tubes is possible only when the emitting points are efficiently protected against the the ion flow and at relatively small current densities averaged over the cathode area at which there 4
is no transition to the explosive emission process. In the Piercetype electron guns with a spherical or flat cathode and small area compression, which form tra ditional forward intense flows in an extended drift channel (Fig. 3b), application of fieldemission struc 4 Here,
a macrogun is meant as a traditional electron gun includ ing the focusing (or control) electrode, anode, and cathode, and containing a great number of cells with fieldemission emitters.
tures (see [6] or [10]) at device residual pressures of 5
10–6–10–8 mmHg is not very promising. The following parameters of the cell (Fig. 4) were 6
accepted for the edge structure [12]: (i) the distance from the extracting electrode to the edge is 0.8 µm; (ii) the radius of the point is 0.01 µm; 5 The figure depicting of the electron gun was taken from [18]. 6
The advantage of the considered structure is the simplicity of its manufacturing on a standard process equipment with design rules of 1.0 µm and the fact that, for the selected dimensions and values of the potential, the experimentally reached average cur rent density in the CW conditions, which is 0.2 A/cm2 with no current degradation, was ensured in this structure for over 1000 hours.
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I1 Iem
I+
I2
I–
Fig. 4. Behavior of the electron flow in the Abanshin–Gorfinkel cell (for obviousness, the flow from the right edge emitter is absent).
(iii) the distance from the edge to the anode is 20 µm; (iv) the extracting electrode potential is 120 V; and (v) the anode potential is 200 V. Molybdenum was used as the material of the extracting electrode and the substrate of the edge, and silicon dioxide was used as the material of the insulat ing layer. The material of the edge of the autoelec tronic emitter was αcarbon. The period of the struc ture was 10 µm. The analysis of Fig. 4 (for conve nience, the behavior of trajectories from one edge of the planar structure is represented) has shown that the current from the edge can be divided into three com ponents (I1, I2, and Iem). Here, I1/Iem ≈ 10–6 and I2 /Iem ≈ 10–6. Due to this fact, currents I1 and I2 (which give the most angular scatter) can be disre garded. The calculated value of the linear current from the point of the edge emitter was Iem = 298 µA/cm (the edge length was equal to 0.2 µm). Note that up to 80% of the current (I – ≈ 0.8Iem) deposits on the extracting electrode and only 20% of the emitter current (I + ≈ 0.2Iem) goes to the anode. The electron tilt angle scat ter is also very significant (the maximum tilt angles are ~45°). In the absence of a coating that protects the edge of the autoelectronic emitter, the ion flow arrives directly at the autoelectronic emitter as the electrode having the minimum potential [21]. Due to the above
properties, it is inexpedient to use such structures and the Spindt structures as current sources when high power microwave vacuum devices are designed. To increase the current flowing to the anode, a special potential distribution in the microgun region, which can form motion of the electron beam in the necessary direction, is required. Figure 5 shows the paths of electron trajectories in a planar cell with two additional layers made of a con ducting material, which can ensure the electron flow completely directed toward the anode. However, the scatter of the transverse components of electron veloci ties is still sufficiently high, which points at the need for more thorough formation of the electron flow in the microgun region. Figure 6 shows the configuration of the microgun electrodes for a gun that formed an elec tron flow with a relatively small scatter of the transverse components of the electron velocities (for 99% of the beam current, electron tilt angles scatter from 0 to 15° at the output from this tetrode cell). This scatter proved to be comparable with the scatter of velocities of beam electrons transmitted through the grid structure con sisting of the shadow and control grid, which is located near the hot cathode [22]. It follows from the analysis of Fig. 7 that, in a wide interval of initial energies, ions starting from the anode surface arrive primarily at the control electrode having
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Table 1. Parameters of the cathode Beam current, A
12.4
Radius of the outer cylindrical surface of the cathode r1, mm
73.7
Radius of the inner cylindrical surface of the cathode r2, mm
54.7
Radius of curvature of the cathode R, mm
45.6
Radial offset of the center of curvature r0 with respect to the EOS axis, mm
40.0
Current density from the toroidal surface of the cathode jtor, A/cm2
0.133
Current density from the conical surface of the cathode jcon, A/cm2
0.135
Current density from the end of the cylindrical surface of the cathode jcyl, A/cm2
0.165
the cathode potential. Owing to the specific potential distribution in the cell, the ions fail to fall on the auto electronic emitter even at the tilt of the ion flow reach ing 45° with respect to the direction from the control electrode to the anode. Thus, in the considered tet rode cell with chosen electrode configuration, in addi
tion to small scatter of electron tilt angles in the formed electron flow, the autoelectronic emitter is reliably protected against the ion flow at the microlevel. This allows us to consider this cell as an element of a planar edge fieldemission structure (PEFES) forming the cathode of the electron macro gun for producing the intense flow of a highpower microwave tube. As in [23], for a relatively low density of the current from the cathode, the required current from the cathode consisting of PEFES cells can be obtained by increasing the cathode area. 3. RESULTS OF CALCULATIONS OF THE ELECTRONOPTICAL SYSTEM FOR A HIGHPOWER MICROWAVE DEVICE
8 6 3
7 5
An electron gun with an oxide cathode, which is used in commercial Otype TWTs (Table 1), was
2
selected as the basic design.
1
7
The tetrode PEFES obtained in the previous sec tion was placed on the end surface of five coaxial cylin drical rings forming the emitting surface of the macro gun cathode. The cylindrical rings were shifted one relative to another so that the centers of the end sur faces of the cylindrical rings proved to be on the spher ical surface of the cathode (Fig. 8). The macrogun used to form the tubular electron flow was equipped with a special electrode with the potential equal to the cathode potential (Fig. 8); it receives the ion flow 8
4
Fig. 5. Distribution of the potential and behavior of the electron flow in the tetrode PEFES cell: (1, 3, 6, 8) con ducting layers; (2, 5, 7) insulating layers; (4) edge emitter made of αcarbon; the potential at the control electrode 1 and focusing electrodes and cathode 3, 4 is Ucat = 0; and potentials at the intermediate anodes 6 and 8 are Uа1 = 120 V and Uа2 = 0.
arriving from the drift channel. To transport a tubular electron flow, a magnetic focusing system with unipo lar distribution of the axial component of the magnetic induction (Table 2) was used in the EOS and the elec tron gun was partially shielded from the magnetic field. 7 The
diameter of the cathode was selected to be no larger than 6.5 inches, since the planar structures [12] are currently manu factured by the use industrial equipment that does not allow one to obtain larger dimensions. 8 The advantage of a macrogun is the possibility of the ultralow voltage (0.8% from the anode potential) control of the beam current with virtually instantaneous readiness for operation.
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9 8 1
7
6 5 3
4 2
Fig. 6. Distribution of the potential and behavior of the electron flow in the tetrode PEFES cell with optimized geometry: (1, 3, 5, 7, 9) conducting layers; (2, 6, 8) insulating layers; (4) edge emitter made of the αcarbon; the potential of the control electrode 1 and focusing electrodes and cathode 3, 4, 5 is Ucat = 0; and intermediate anodes 7 and 9 potentials are Uа1 = Uа2 = 180 V.
(a)
(b)
(c)
(d)
Fig. 7. Behavior of the nitrogen ions in the region of the tetrode PEFES cell: (a) U T = 0.026 V (Т = 300 K), (b) U T = 2 V, (c) U T = 20 V, and (d) U T = 200 V.
As it follows from the analysis of Fig. 8, in this case, the cathode of the electron gun is reliably protected at the macrolevel from the ion flow due to displacement of the emitting surface of the annular cathode in the radial direction outside the deposition zone of the ion flow. In spite of strong potential disturbances near the
cathode of the electron gun, a compact (in the radial direction) tubular flow with a high perveance (Рμ = 3.0 µA/V3/2) was formed owing to partial shielding of the electron flow from the magnetic field. The behav ior of the strongly “magnetized” electron flow formed by the PEFES in the drift channel is little different
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2
1
3
Fig. 8. (1) Trajectories of nitrogen ions, (2) force lines of the magnetic field, and (3) trajectories of electrons in region of the mac rogun (U0 = 25000 V).
from transport of the electron flow in the basic EOS with a toroidal oxide cathode (Fig. 9), and, when the flow power is ~300 kW, this flow arrives without
CONCLUSIONS
Table 2. Parameters of the MFS Amplitude of the magnetic field in the MFS, Gs Value of the magnetic field on the cathode, Gs Brillouin radius of the beam, mm
900 80 2.76
Brillouin filling of the drift channel by the beamdrift 0.081 Filling with consideration for the magnetic field on the cathode
losses at the EOS collector. This EOS is entirely suit able for design of microwave amplifiers with an out put power of several tens of kilowatt in CW.
0.65
It has been shown that, when the EOSs with low voltage control beam currents intended for high power microwave amplifiers with a small readiness time are created, cathode matrices with PEFESbased fieldemission cells can be proposed as electron sources. In the EOSs with annular cathode matrices, appli cation of PEFESbased cells allows one to ensure pro tection at the macrolevel against bombardments of ions arriving at the gun region from the drift channel of the devices and, at the microlevel, against ions formed in the PEFES region.
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(b)
Fig. 9. (1, 2) Distribution of the magnetic field and trajec tories of the electron flow in the region of the drift channel: (a) the EOS with an oxide hot cathode and (b) the EOS with the PEFES cells cathode; (1) calculation and (2) experiment.
The use of the cathode matrices with the PEFES can allow one to design (at the existing technology level in Russia) microwave amplifiers performing more than 1000 hours in CW with the output power of several tens of kilowatt. ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research, project nos. 1007 00526a and 120712066ofi_m. REFERENCES
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3. W. Schotky, Z. Phys. 14, 63 (1923). 4. R. H. Fowler and L. W. Nordheim, Proc. R. Soc. Lon don, Ser. A 119 (781), 173 (1928). 5. F. M. Chabonnier, J. P. Barbour, L. F. Garret, and W. P. Dyke, Proc. IEEE 51, 991 (1963). 6. C. A. Spindt, J. Appl. Phys. 39, 3504 (1968). 7. P. M. Lally, C. D. Mack, and C. A. Spindt, in Proc. 8th Microwave Power Tube Conf., Monterey, 1988 (IEEE, New York, 1988). 8. H. Makishima, H. Imurat, M. Takahashi, et al., in Techn. Dig. 10th Int. Vacuum Microelectron. Conf. (IVMC’97), Kyongju, Aug. 12–17, 1997 (IEEE, New York, 1997), p. 194. 9. D. R. Whaley, R. Duggal, C. M. Armstrong, et al., IEEE Trans. Electron Devices 56 896 (2009). 10. Yu. A. Grigor’ev, S. V. Vasil’kovskii, V. I. Shesterkin, and Z. A. Yartseva, RF Patent No. 1738013 (1993). 11. V. V. Chesnokov, Author’s Certificate No. 174726. 12. N. Abanshin and B. Gorfinkel, Patent No. 320 (2000). 13. I. V. Alyamovskii, Electron Beams and Electron Guns (Sovetskoe Radio, Moscow, 1966) [in Russian]. 14. D. I. Trubetskov, A. G. Rozhnev, and D. V. Sokolov, Lectures on Microwave Vacuum Microelectronics (Gos UNTs “Kolledzh”, Saratov, 1996) [in Russian]. 15. G. A. Mesyats and D. I. Proskurovskii, Pis’ma Zh. Eksp. Teor. Fiz., No. 4, 243 (1974). 16. F. Charbonnier, Tech. Dig. 10th Int. Vacuum Microelec tron. Conf. (IVMC’97), Kyongju, Aug. 12–17, 1997 (IEEE, New York, 1997), p. 7. 17. C. A. Spindt, I. Brodie, L. Humphrey, and E. R. West erberg, J. Appl. Phys. 47, 5248 (1976). 18. A. I. Petrosyan and V. I. Rogovin, Prikl. Fiz., No. 2, 86 (2008). 19. V. P. Sazonov, Priority of Russia in Vacuum Microwave Electronics in XX Century (MedpraktikaM, Moscow, 2012) [in Russian]. 20. V. P. Sazonov, I. I. Golenitsky, and S. A. Rumyantsev, in Proc. Int. Vacuum Microelectron. Conf. (IVMC’94), Grenoble, 1994 (IEEE, Piscataway, 1994), p. 285. 21. N. P. Aban’shin, B. I. Gorfinkel’, and A. N. Yakunin, Tech. Phys. Lett. 32, 896 (2006). 22. Yu. A. Grigor’ev, B. S. Pravdin, and V. I. Shesterkin, Obz. Elektron. Tekh. Ser. 1: Elektron SVCh, No. 7 (1987) [in Russian]. 23. A. I. Petrosyan, S. P. Morev, and V. I. Rogovin, in Tech. Dig. 12th Int. Vacuum Microelectron. Conf. (IVMC’99), Darmstadt, Germany, July 6–9, 1999 (American Vac uum Society 1999).
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