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High-brightness electron source isolated from the ambient atmosphere
Vacuum 55 (1999) 7}11
High-brightness electron source isolated from the ambient atmosphere L.I. Antonova, V.P. Denissov* Faculty for Applied Mathematics, St-Petersburg State University, Bibliotechnaya pl.2, St-Petersburg, 198904, Russia Received 12 November 1998; accepted 28 December 1998
Abstract A new kind of electron source for electron gun is proposed. It consists of a negative electron a$nity photocathode, separated from the rest of the vacuum chamber with a thin alumina "lm, transparent for electrons and impenetrable for ions and atoms. The cut-o! of the barrier "lm is about 700}900 eV and its transparency is as high as 65% at 1000 eV. The total isolation of two volumes made possible the retaining of the photocathode sensitivity of 1000 lA/lm for 120 h and longer in the large-volume vacuum chamber. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Electron beams are extensively used in many physical and technological devices. Electron}positron colliders and electron-beam welding, electron spectrometers and X-ray tubes are the examples. The heart of any electron gun is a source of charged particles. The best electron source is the source with the most e$cient conversion of external energy into the free electron current, with the smallest energy spread, and "nally the highest stability during storage and operation. Three kinds of electron cathodes are available } thermionic, "eld emitter and photocathode. The "rst one, most traditional and used in almost every application until now, is far from being perfect. Its e!ectivity is low, the power consumption is very high, and the energy spread can reach 0.5}0.7 eV. Perhaps its only distinction is the ability to operate in technical vacuum. The second, "eld emitter, has 100% conversion e$ciency and the monochromatic electron #ux. The main disadvantage of the "eld emitter is the low total current which can be obtained from the simple structure, about 1 lA in continuous mode. More sophisticated structures are very promising but there are many technological problems to be solved [1]. The third one, photocathode, seems to be more satisfactory for many applications [2]. Take a sample of p-doped semiconductor (Fig. 1a) (ntype is less e$cient). In thermodynamic equilibrium the
*Corresponding author.
concentration of electrons in the conduction band is very small, and the number of electrons with energy, su$cient to overcome the surface barrier, is negligible. Under illumination with quanta hl'E #E an excited elec tron can escape into vacuum, but "rst it must preserve its energy during the motion from the excitation point to the surface. Depending on the relation between hl, E , and light extinction coe$cient a, the probability of this process can vary. In any case only hot electrons can be emitted and the e!ectivity of the transport cannot be very high. This process presents a conventional positive electron a$nity (E '0) photoemission [3]. The best samples of these photocathodes can have 40}50% quantum yield, i.e. 40}50 electrons per 100 incident quanta. The shortcomings of these cathodes are a relatively wide energy distribution, and the need for intense white or blue light sources, which are not very convenient for short-pulsed modulation. Let the semiconductor be gallium arsenide or similar. In this case the situation changes drastically after the deposition of the consecutive layers of Cs and Cs#O onto its atomically clean surface (Fig. 1b). It is known that this coverage decreases the work function to 1.0}1.1 eV, i.e., makes it less than the energy gap of the semiconductor [4]. In this case any electron, excited to the conduction band, even thermalized at its bottom, has a signi"cant probability to escape, limited only with the recombination, the energy loss in the band bending region, and tunneling through the surface barrier. The recombination rate is relatively low, the thickness of both layers is small, and the loss is anticipated to be small too. The electron a$nity de"ned as the di!erence between the
0042-207X/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 9 ) 0 0 1 1 2 - 8
8
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11
Fig. 1. Schematic presentation of positive (a) and negative (b) electron a$nity photoemission. After su!ering few scattering events an electron can lose its energy. It will not be able to escape if the a$nity is positive, but have some escape probability if it is negative.
work function and the band gap is negative, and the phenomenon is termed negative electron a$nity (NEA) photoemission. In NEA photoemission the second requirement to the perfect electron source is determined with the fundamental physics of the process and is low-sensitive to the technological process of the electron source preparation. Pleasingly, the full-width at half-maximum of the energy distribution is about 0.1}0.2 eV at room temperature and less than 0.1 eV at 77 K [5], an acceptable value for many applications. The "rst and third ones can be obtained only with the proper treatment of all elements of the vacuum equipment, from the emitting sample itself to the walls of the bell jar (or glass envelope). This treatment poses a serious technological problem. The most troublesome is the fact that to some extent these requirements are contradictory and mediocre quality cathodes have longer storage and operational lifetime than the highe$cient ones. Many precautions are taken to obtain a satisfactory result in electron tubes, including chemical cleaning procedure, high-temperature bake-out of the tube as a whole, special treatment of the tube with gases and vapors, but still the result is not reproducible. It is evident, that in many applications these technological procedures are unrealizable. In accelerators for instance, the walls and the target cannot be prepared with electronic quality. On the other hand, the products of the photocathode treatment, cesium "rst of all, can be poisonous for the apparatus. In the present paper we present a new approach to the problem. We propose a complete dividing of the vacuum volume into cathode and experimental sub-sections with a "lm. It makes possible to create the most comfortable conditions for the electron source without any special treatment of the experimental device as whole. The separating "lm must be absolutely impenetrable for both atoms and ions, and as transparent for electrons as possible.
Fig. 2. Schematic drawing of the experimental set-up. QMA-quadruple mass-analyzer, CMA-cylindrical mirror analyzer.
2. Experimental set-up The experiments were performed in the super-high vacuum bell jar with the residual gases pressure in 10\ Pa region. The manipulator made possible the heat cleaning of the samples, deposition of cesium from the conventional Russian dispensers, and oxygen adsorption either from the dispenser source, or through the leak valve (Fig. 2). The quadruple mass-analyzer controlled the residuals and the products of evaporation of dispenser sources in 2}150 m/e range. Auger analysis was performed only at few test samples because there is some evidence (though poorly con"rmed by statistical analysis) that the electron bombardment of the surface decreases the quality of the "nal product. The electron energy distributions were studied in the four-grid analyzer with the usual ac-modulating techniques. The photocathode unit was semitransparent glassbonded GaAs layer on the window of the speci"c shape. It was prepared according to [6]. The photocathode
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11
multi-layered substrates AlGaAs}GaAs}AlGaAs were grown by liquid-phase epitaxy techniques. The active diameter of the cathode was 25 mm, and the total diameter of the glass window !50 mm. After the conventional chemical treatment the photocathode window was installed into the bell jar. The barrier "lm must satisfy two contradictory requirements. It is to be su$ciently thin, so that the electrons could cross it without any signi"cant loss, neither in number, nor in energy. On the other hand, it should be su$ciently dense to guarantee the absence of any pinholes which can be the leaks between two sections of the vacuum chamber. So, the speci"c material should be used. One of the best materials for this purpose is alumina. Of course, it cannot be used as a free "lm, and a proper substrate is to be found. We prepared the layer of an organic polymer on the water surface. After polymerization it was deposited onto the platinum covered mesh of the vidicon tube. The layer of aluminum was evaporated by magnetron sputtering and a plasma-discharge oxidation "nished the process [7]. Films as thin as 5 nm could be prepared with this technology though usually the thickness was a little greater. A small number of pin-holes was present sometimes. In this case we repeated the process, so that the double-layer structure was prepared. The vacuum-leak test has shown that there were no holes at the sensitivity limit. After the cathode and barrier units were installed, the bell jar was evacuated and baked out. The usual procedures of thermal GaAs surface cleaning and cesium and oxygen deposition (in &&yo-yo'' procedure) lead to the high-e$ciency photocathode preparation. It had 1500} 2200 mA/lm sensitivity at the test re#ection mode photocathodes (homoepitaxial GaAs layers, for more details see [8, 9] and 900}1200 mA/lm at semitransparent. As the cathode was ready, the "rst cycle of study was performed, including sensitivity and quantum yield spectral distribution curves (QYSDC) measurements. Then the cathode window was sealed with the barrier "lm unit through the casted indium gasket (Fig. 3), and the measurements were repeated. A special attention was paid to the measurement of the emission current vs. anode-voltage dependence (the barrier "lm supporting mesh served as anode) (mode 1), and external anode current as a function of the supporting mesh bias (mode 2). In mode 2 the potential of the anode was supported 30}50 V higher than that of the supporting mesh.
3. Results The QYSDC of the semitransparent and homoepitaxial photocathodes are shown in Fig. 4 (mode 1). The sharp cut-o! at the short wavelength for semitransparent ones is related to the sub-layer optical adsorption
9
Fig. 3. Schematic drawing of the photocathode and barrier "lm units. (1) photocathode window, (2) chromium contact layer, (3) photocathode, (4) casted indium gasket, (5) U-shaped metal ring, (6) ceramic ring, (7) mesh with the barrier ring, (8) mounting metal rings. Arrows show the direction of the applied force.
Fig. 4. Quantum yield spectral distribution curves of the GaAs negative electron a$nity photocathodes, (1) homoepitaxial (re#ection mode) and (2) semitransparent.
and can be shifted to the blue spectral region with the increase of the aluminum concentration (up to 90}95%), though the quality of the cathode deteriorates and the maximum e$ciency decreases. These QYSDCs are conventional for this kind of photocathodes and demonstrate the high quality of the epitaxial layers. The electron di!usion length determined with the conventional methods [4] is 3}5 mm and the escape probability varies from 30 to 40%. The full-width at half-maximum of the energy distribution is about 0.18}0.20 eV. The stability of the cathode is far from being perfect (Fig. 5), squares (mode 1). Two main reasons determine the decay process, namely, the cesium evaporation from the surface and the activation layer poisoning with the
10
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11
Fig. 5. White light sensitivity decay after a stay in vacuum bel jar, (1) conventional coverage, (2) antimony-containing coverage (both for open photocathode), (3) sealed-o! photocathode with conventional coverage.
residual gases [10]. We cannot discuss this dilemma in details here, but the fact is that after a quick decay with minutes time constant, a slower one begins with hours duration. The process can be made less pronounced with special cesium treatment or with antimony deposition, but this gives only a partial-success [8], Fig. 5, triangles (mode 1). Signi"cant changes in emission current or anode voltage rise can destroy the quasi-equilibrium and sharply decrease the sensitivity. The electron discharge in electron optics elements is disastrous for the cathode. The radical change in the cathode behavior was obtained with closing the cathode with the barrier "lm. We could not observe any decay of the sensitivity for more than 120 h after a small fall at the closing moment (Fig. 5), circles (mode 1 and 2). The reason for this fact is obvious. The volume of the near-cathode space is very small, few cubic millimeters, and it becomes cesium saturated and active gases free very soon. In any case no factor of the instability remains active. Increased pressure up to 10\ Pa pressure made no pronounced e!ect onto the sensitivity. Naturally, electron discharges outside the photocathode area have no e!ect too. The main question remains, how transparent is the barrier "lm to electrons. To study this problem we measured the output current as function of the cathode-"lm bias (Fig. 6) (mode 2). In the same picture the voltagecurrent plot for mode 1 is shown. Two facts are obvious from the illustration. First, there exist a pronounced threshold for electrons escape through the barrier "lm.
Fig. 6. White light photoemission current of the sealed- o! photocathode as function of the mesh potential, (1) mode 1, (2) mode 2 (for explanation see text).
Evidently, it must depend on the "lm thickness. Zero current at lower bias makes obvious that there are no holes in the barrier layer. Second, the "lm is thin enough for being transparent for 800}1000 eV electrons. The transmissivity remains almost constant at higher energies. The absolute value of the transmissivity was about 60}65%, evidently due to the electron re#ection at the mesh grid. The preliminary experiments show that no serious deterioration of the electron spin polarization takes place.
4. Conclusion To summarize, the proposed technology of NEA photocathode seal-o! is similar to that of the proximity tubes production. It is well known, that the mean operating time before failure of these tubes is as long as many thousands hours. The operating bias between the cathode and MCP is usually in 500}1000 V range, close to the value necessary for the initial stages of electron beam formation, too. Our experiments con"rm the fact, that this technology can give satisfactory results in preparing the stable source of electrons.
References [1] Jensen KL, Abrams RH, Parker RK. J Vac Sci Technol B 1998;16:749.
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11 [2] Tang H et al. SLAC-PUB-6167, October 1993. [3] Parmigiani P, Ferrini G, Michelato P. Sol. State Commun 1998;106:21. [4] Bell RL. Negative electron a$nity devices. Oxford: Clarendon Press, 1973. [5] Feigerle CS, Pierce DT, Seiler A, Celotta RJ. Appl Phys Lett 1984;44:866.
11
[6] Antypas GA, Escher JS, Edgecumbe J, Enck RS. J Appl Phys 1978;49:1302. [7] Smirnov BN. Vacuum 1991;42:57. [8] Antonova LI, Denissov VP. Appl Surf Sci 1997;111:237. [9] Antonova LI et al. Sov Technol Phys Lett 1985;11:250. [10] Wada T, Nitta T, Nomura T, Miyao M, Hagino M. Jap J Appl Phys 1990;29:2087.
High-brightness electron source isolated from the ambient atmosphere L.I. Antonova, V.P. Denissov* Faculty for Applied Mathematics, St-Petersburg State University, Bibliotechnaya pl.2, St-Petersburg, 198904, Russia Received 12 November 1998; accepted 28 December 1998
Abstract A new kind of electron source for electron gun is proposed. It consists of a negative electron a$nity photocathode, separated from the rest of the vacuum chamber with a thin alumina "lm, transparent for electrons and impenetrable for ions and atoms. The cut-o! of the barrier "lm is about 700}900 eV and its transparency is as high as 65% at 1000 eV. The total isolation of two volumes made possible the retaining of the photocathode sensitivity of 1000 lA/lm for 120 h and longer in the large-volume vacuum chamber. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Electron beams are extensively used in many physical and technological devices. Electron}positron colliders and electron-beam welding, electron spectrometers and X-ray tubes are the examples. The heart of any electron gun is a source of charged particles. The best electron source is the source with the most e$cient conversion of external energy into the free electron current, with the smallest energy spread, and "nally the highest stability during storage and operation. Three kinds of electron cathodes are available } thermionic, "eld emitter and photocathode. The "rst one, most traditional and used in almost every application until now, is far from being perfect. Its e!ectivity is low, the power consumption is very high, and the energy spread can reach 0.5}0.7 eV. Perhaps its only distinction is the ability to operate in technical vacuum. The second, "eld emitter, has 100% conversion e$ciency and the monochromatic electron #ux. The main disadvantage of the "eld emitter is the low total current which can be obtained from the simple structure, about 1 lA in continuous mode. More sophisticated structures are very promising but there are many technological problems to be solved [1]. The third one, photocathode, seems to be more satisfactory for many applications [2]. Take a sample of p-doped semiconductor (Fig. 1a) (ntype is less e$cient). In thermodynamic equilibrium the
*Corresponding author.
concentration of electrons in the conduction band is very small, and the number of electrons with energy, su$cient to overcome the surface barrier, is negligible. Under illumination with quanta hl'E #E an excited elec tron can escape into vacuum, but "rst it must preserve its energy during the motion from the excitation point to the surface. Depending on the relation between hl, E , and light extinction coe$cient a, the probability of this process can vary. In any case only hot electrons can be emitted and the e!ectivity of the transport cannot be very high. This process presents a conventional positive electron a$nity (E '0) photoemission [3]. The best samples of these photocathodes can have 40}50% quantum yield, i.e. 40}50 electrons per 100 incident quanta. The shortcomings of these cathodes are a relatively wide energy distribution, and the need for intense white or blue light sources, which are not very convenient for short-pulsed modulation. Let the semiconductor be gallium arsenide or similar. In this case the situation changes drastically after the deposition of the consecutive layers of Cs and Cs#O onto its atomically clean surface (Fig. 1b). It is known that this coverage decreases the work function to 1.0}1.1 eV, i.e., makes it less than the energy gap of the semiconductor [4]. In this case any electron, excited to the conduction band, even thermalized at its bottom, has a signi"cant probability to escape, limited only with the recombination, the energy loss in the band bending region, and tunneling through the surface barrier. The recombination rate is relatively low, the thickness of both layers is small, and the loss is anticipated to be small too. The electron a$nity de"ned as the di!erence between the
0042-207X/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 9 ) 0 0 1 1 2 - 8
8
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11
Fig. 1. Schematic presentation of positive (a) and negative (b) electron a$nity photoemission. After su!ering few scattering events an electron can lose its energy. It will not be able to escape if the a$nity is positive, but have some escape probability if it is negative.
work function and the band gap is negative, and the phenomenon is termed negative electron a$nity (NEA) photoemission. In NEA photoemission the second requirement to the perfect electron source is determined with the fundamental physics of the process and is low-sensitive to the technological process of the electron source preparation. Pleasingly, the full-width at half-maximum of the energy distribution is about 0.1}0.2 eV at room temperature and less than 0.1 eV at 77 K [5], an acceptable value for many applications. The "rst and third ones can be obtained only with the proper treatment of all elements of the vacuum equipment, from the emitting sample itself to the walls of the bell jar (or glass envelope). This treatment poses a serious technological problem. The most troublesome is the fact that to some extent these requirements are contradictory and mediocre quality cathodes have longer storage and operational lifetime than the highe$cient ones. Many precautions are taken to obtain a satisfactory result in electron tubes, including chemical cleaning procedure, high-temperature bake-out of the tube as a whole, special treatment of the tube with gases and vapors, but still the result is not reproducible. It is evident, that in many applications these technological procedures are unrealizable. In accelerators for instance, the walls and the target cannot be prepared with electronic quality. On the other hand, the products of the photocathode treatment, cesium "rst of all, can be poisonous for the apparatus. In the present paper we present a new approach to the problem. We propose a complete dividing of the vacuum volume into cathode and experimental sub-sections with a "lm. It makes possible to create the most comfortable conditions for the electron source without any special treatment of the experimental device as whole. The separating "lm must be absolutely impenetrable for both atoms and ions, and as transparent for electrons as possible.
Fig. 2. Schematic drawing of the experimental set-up. QMA-quadruple mass-analyzer, CMA-cylindrical mirror analyzer.
2. Experimental set-up The experiments were performed in the super-high vacuum bell jar with the residual gases pressure in 10\ Pa region. The manipulator made possible the heat cleaning of the samples, deposition of cesium from the conventional Russian dispensers, and oxygen adsorption either from the dispenser source, or through the leak valve (Fig. 2). The quadruple mass-analyzer controlled the residuals and the products of evaporation of dispenser sources in 2}150 m/e range. Auger analysis was performed only at few test samples because there is some evidence (though poorly con"rmed by statistical analysis) that the electron bombardment of the surface decreases the quality of the "nal product. The electron energy distributions were studied in the four-grid analyzer with the usual ac-modulating techniques. The photocathode unit was semitransparent glassbonded GaAs layer on the window of the speci"c shape. It was prepared according to [6]. The photocathode
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11
multi-layered substrates AlGaAs}GaAs}AlGaAs were grown by liquid-phase epitaxy techniques. The active diameter of the cathode was 25 mm, and the total diameter of the glass window !50 mm. After the conventional chemical treatment the photocathode window was installed into the bell jar. The barrier "lm must satisfy two contradictory requirements. It is to be su$ciently thin, so that the electrons could cross it without any signi"cant loss, neither in number, nor in energy. On the other hand, it should be su$ciently dense to guarantee the absence of any pinholes which can be the leaks between two sections of the vacuum chamber. So, the speci"c material should be used. One of the best materials for this purpose is alumina. Of course, it cannot be used as a free "lm, and a proper substrate is to be found. We prepared the layer of an organic polymer on the water surface. After polymerization it was deposited onto the platinum covered mesh of the vidicon tube. The layer of aluminum was evaporated by magnetron sputtering and a plasma-discharge oxidation "nished the process [7]. Films as thin as 5 nm could be prepared with this technology though usually the thickness was a little greater. A small number of pin-holes was present sometimes. In this case we repeated the process, so that the double-layer structure was prepared. The vacuum-leak test has shown that there were no holes at the sensitivity limit. After the cathode and barrier units were installed, the bell jar was evacuated and baked out. The usual procedures of thermal GaAs surface cleaning and cesium and oxygen deposition (in &&yo-yo'' procedure) lead to the high-e$ciency photocathode preparation. It had 1500} 2200 mA/lm sensitivity at the test re#ection mode photocathodes (homoepitaxial GaAs layers, for more details see [8, 9] and 900}1200 mA/lm at semitransparent. As the cathode was ready, the "rst cycle of study was performed, including sensitivity and quantum yield spectral distribution curves (QYSDC) measurements. Then the cathode window was sealed with the barrier "lm unit through the casted indium gasket (Fig. 3), and the measurements were repeated. A special attention was paid to the measurement of the emission current vs. anode-voltage dependence (the barrier "lm supporting mesh served as anode) (mode 1), and external anode current as a function of the supporting mesh bias (mode 2). In mode 2 the potential of the anode was supported 30}50 V higher than that of the supporting mesh.
3. Results The QYSDC of the semitransparent and homoepitaxial photocathodes are shown in Fig. 4 (mode 1). The sharp cut-o! at the short wavelength for semitransparent ones is related to the sub-layer optical adsorption
9
Fig. 3. Schematic drawing of the photocathode and barrier "lm units. (1) photocathode window, (2) chromium contact layer, (3) photocathode, (4) casted indium gasket, (5) U-shaped metal ring, (6) ceramic ring, (7) mesh with the barrier ring, (8) mounting metal rings. Arrows show the direction of the applied force.
Fig. 4. Quantum yield spectral distribution curves of the GaAs negative electron a$nity photocathodes, (1) homoepitaxial (re#ection mode) and (2) semitransparent.
and can be shifted to the blue spectral region with the increase of the aluminum concentration (up to 90}95%), though the quality of the cathode deteriorates and the maximum e$ciency decreases. These QYSDCs are conventional for this kind of photocathodes and demonstrate the high quality of the epitaxial layers. The electron di!usion length determined with the conventional methods [4] is 3}5 mm and the escape probability varies from 30 to 40%. The full-width at half-maximum of the energy distribution is about 0.18}0.20 eV. The stability of the cathode is far from being perfect (Fig. 5), squares (mode 1). Two main reasons determine the decay process, namely, the cesium evaporation from the surface and the activation layer poisoning with the
10
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11
Fig. 5. White light sensitivity decay after a stay in vacuum bel jar, (1) conventional coverage, (2) antimony-containing coverage (both for open photocathode), (3) sealed-o! photocathode with conventional coverage.
residual gases [10]. We cannot discuss this dilemma in details here, but the fact is that after a quick decay with minutes time constant, a slower one begins with hours duration. The process can be made less pronounced with special cesium treatment or with antimony deposition, but this gives only a partial-success [8], Fig. 5, triangles (mode 1). Signi"cant changes in emission current or anode voltage rise can destroy the quasi-equilibrium and sharply decrease the sensitivity. The electron discharge in electron optics elements is disastrous for the cathode. The radical change in the cathode behavior was obtained with closing the cathode with the barrier "lm. We could not observe any decay of the sensitivity for more than 120 h after a small fall at the closing moment (Fig. 5), circles (mode 1 and 2). The reason for this fact is obvious. The volume of the near-cathode space is very small, few cubic millimeters, and it becomes cesium saturated and active gases free very soon. In any case no factor of the instability remains active. Increased pressure up to 10\ Pa pressure made no pronounced e!ect onto the sensitivity. Naturally, electron discharges outside the photocathode area have no e!ect too. The main question remains, how transparent is the barrier "lm to electrons. To study this problem we measured the output current as function of the cathode-"lm bias (Fig. 6) (mode 2). In the same picture the voltagecurrent plot for mode 1 is shown. Two facts are obvious from the illustration. First, there exist a pronounced threshold for electrons escape through the barrier "lm.
Fig. 6. White light photoemission current of the sealed- o! photocathode as function of the mesh potential, (1) mode 1, (2) mode 2 (for explanation see text).
Evidently, it must depend on the "lm thickness. Zero current at lower bias makes obvious that there are no holes in the barrier layer. Second, the "lm is thin enough for being transparent for 800}1000 eV electrons. The transmissivity remains almost constant at higher energies. The absolute value of the transmissivity was about 60}65%, evidently due to the electron re#ection at the mesh grid. The preliminary experiments show that no serious deterioration of the electron spin polarization takes place.
4. Conclusion To summarize, the proposed technology of NEA photocathode seal-o! is similar to that of the proximity tubes production. It is well known, that the mean operating time before failure of these tubes is as long as many thousands hours. The operating bias between the cathode and MCP is usually in 500}1000 V range, close to the value necessary for the initial stages of electron beam formation, too. Our experiments con"rm the fact, that this technology can give satisfactory results in preparing the stable source of electrons.
References [1] Jensen KL, Abrams RH, Parker RK. J Vac Sci Technol B 1998;16:749.
L.I. Antonova, V.P. Denissov / Vacuum 55 (1999) 7}11 [2] Tang H et al. SLAC-PUB-6167, October 1993. [3] Parmigiani P, Ferrini G, Michelato P. Sol. State Commun 1998;106:21. [4] Bell RL. Negative electron a$nity devices. Oxford: Clarendon Press, 1973. [5] Feigerle CS, Pierce DT, Seiler A, Celotta RJ. Appl Phys Lett 1984;44:866.
11
[6] Antypas GA, Escher JS, Edgecumbe J, Enck RS. J Appl Phys 1978;49:1302. [7] Smirnov BN. Vacuum 1991;42:57. [8] Antonova LI, Denissov VP. Appl Surf Sci 1997;111:237. [9] Antonova LI et al. Sov Technol Phys Lett 1985;11:250. [10] Wada T, Nitta T, Nomura T, Miyao M, Hagino M. Jap J Appl Phys 1990;29:2087.