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Development of new types of oxide cathodes
Applied Surface Science 251 (2005) 64–68 www.elsevier.com/locate/apsusc
Development of new types of oxide cathodes Xianheng Liao *, Xiaoxia Wang, Qinglan Zhao, Mingfen Meng R&D Center Microwave for Device and technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100080, PR China Available online 15 April 2005
Abstract High reliability, long lifetime and low temperature, high emission current density oxide cathodes are presented in this paper. The characteristics of these cathodes, such as dc and pulse emission, evaporation, resistance to poisoning and lifetime, are discussed. They have been widely applied to TWTs and klystrons with high power, long pulse and high duty ratio. The results showed that high emission current density and long lifetime can be realized. # 2005 Elsevier B.V. All rights reserved. Keywords: High reliability; Long lifetime; High emission current density; Oxide cathode
1. Introduction The study and development of the oxide cathode has been ongoing for over 100 years. It has a lot of advantages, such as high current density at short pulse, low operating temperature, simple cathode structure and easy realization for batch-production. It is a widely used thermionic cathode. With the ongoing development of high power microwave devices, the requirements for the emission current density, pulse duration and duty ratio of the cathode have to be enhanced to suit the new applications. A lot of work has been done on the cathode in IECAS in the past 40 years, searching for its operating mechanisms, investigating new experimental structures, and improving emission characteristics with low evaporation rates, high resistance to poisoning and long * Corresponding author. Tel.: +86 10 58887263; fax: +86 10 62548109. E-mail address: [email protected] (X. Liao).
lifetime. Professor Enqiu Zhang found that there were some contradictions between the practicalities of the oxide cathode and the theory. Based on experimental results and careful analyses, he proposed a new theory, the dynamic surface emission center model [1–3]. Professor Zhaohao Wu has always proposed the viewpoint that the hole–surface emitter of the impregnated Ba–W cathode is actually an anamorphosis of oxide cathode [4]. The above-mentioned model and viewpoint are the basis of our research work, and plays an important role in the development of the oxide cathode and the improvement of its characteristics.
2. Background A lot of investigation has been done before the development of new types of oxide cathodes. The
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.119
X. Liao et al. / Applied Surface Science 251 (2005) 64–68
65
following experimental results established the base for the new cathodes: (1) Emission capabilities: Two reservoir oxide cathodes, one sprayed with (BaSrCa)O thin film on its emission surface and the other not, have been studied and found very different emission currents at 730 8C. The emission current of the former is 26 times that of the latter, which shows that the emission ability of the cathode can be greatly increased when excess barium is well combined with (BaSrCa)O. (2) Emission properties after oxide coating removal and emission from Ni-net holes: Following experiments have been made on a reservoir oxide cathode. In the first one, after the cathode was activated, aged and tested to obtain its emission current data, the coated oxide on the surface of the Ni-net was entirely removed and then the cathode was activated, aged and tested again. The result showed that the emission current of the cathode with the oxide coating removed is still high. In the second, a reservoir oxide with Ni-net on its surface was tested, the pores of Ni-net of the cathode were filled with the oxide, but there is no oxide at the warp and woof (the base metal surface) of the cathode Ninet. In this case, the emission current of the cathode is very low. The two experimental results showed that the oxide cathode emits electrons mainly from the thin film on the base metal surface, not from the oxide itself for reservoir oxide cathode.
3. Development of new types of oxide cathodes On the basis of the theoretical model and the results of above-mentioned experiments, we developed two types of oxide cathodes, the high-reliability, long lifetime oxide cathode and the low temperature, highemission current density oxide cathode. 3.1. High-reliability, long lifetime oxide cathode Super-pure Ni alloys, such as Zr–Ni (0.1% Zr), Zr– W–Ni (0.1% Zr, 2% W) and Mg–W–Ni (0.2–0.4%
Fig. 1. The structure of the high-reliability, long lifetime oxide cathode.
Mg, 2% W) are always adopted as the bases of long lifetime oxide cathodes. Such cathodes are prepared under super-clean sanitation conditions and applied separately in communication satellite TWTs under ultra-high vacuum conditions. Over 50,000 h lifetime has been reached with 100 mA/cm2 emission current density (for example, M4041, 314H, TL4002, etc.) [5–9]. The high-reliability, long lifetime oxide cathode shown in Fig. 1 was developed by rationally designing the cathode structure, carefully adjusting the prescription of the oxide materials, and looking for the appropriate technology of cathode preparation. The lifetime of the cathode in diode is over 100,000 h with 150 mA/cm2 at 590–750 8C. Meanwhile, over 50,000 h lifetime has been realized when the cathode was TWT for communication satellite applications with 150 mA/cm2 current density. 3.1.1. Conditions of the cathode testing Diode testing was done to check the characteristics of the tested cathodes with diameter of 3 mm. The operating temperature of the cathode was measured by a Ni–Mo thermocouple. After decomposition, activation and aging of the cathode, its emission current, lifetime and resistance to poisoning were tested. 3.1.2. Emission characteristics of the cathode (1) Direct emission current density: The direct emission current density of the cathode is 70 mA/cm2 at 550 8C, 100 mA/cm2 at 600 8C, 200 mA/cm2 at 650 8C, 400 mA/cm2 at 700 8C and over 400 mA/cm2 at 730 8C. (2) Pulse emission current density: The pulse emission current density of the cathode is 3 A/cm2 at 700 8C, 5.5 A/cm2 at 750 8C, 10 A/cm2 at 800 8C and over 20 A/cm2 at 850 8C, with a pulse width of 20 ms and a repetition rate of 50 Hz. (3) Lifetime testing
66
X. Liao et al. / Applied Surface Science 251 (2005) 64–68
(a) Lifetime of the cathodes in diodes: Lifetime testing of the cathodes was done for all diodes or for several diodes randomly selected. No decrease in emission current of the cathode was found after continuous operation for 100,000 h with 150–400 mA/cm2 (DC) at temperatures of 590–750 8C. However, the lifetime of the oxide cathodes with Ni–W–Mg and Ni–W–Zr bases is only 16,000–18,000 h with 250 mA/cm2 at 730 8C. (b) The lifetime of TWTs with this type of cathodes (for communication satellite application): Forty TWTs with 150 mA/cm2 were tracked to obtain lifetime data. The results show that lifetime of the TWT is over 50,000 h, and after the TWT has continuously operated for 20,000 h, the knee-point temperature of roll-off curves of the cathode decreases from 607 to 509 8C, which shows that the activity of the cathode continues to improve over its operating lifetime. 3.1.3. Applications of the cathode in vacuum electron devices (1) This cathode has been applied to TWTs used in communication satellites and operated for over 7 years. (2) The long lifetime cathode was also used in some display devices in the 1980s. The accelerating lifetime test is over 13,000 h for 12-in. black and white CRTs. The same experimental results are obtained for 14-in. black and white CRTs. The above-mentioned results for the experiments and applications of the cathode show that the cathode has good emission characteristics with high reliability and long lifetime. And it also has good resistance to poisoning, low evaporation and good resistance to shaking and shock, which will not be discussed in detail here. 3.2. The low temperature, high-emission current density oxide cathode Usually, the applications of the oxide cathode are only considered in short pulse, low dc density microwave tubes [10], as shown in Table 1, which restricts the development of the oxide cathode. In this
Table 1 Relation of pulse current density (A/cm2) to pulse duration and duty ratio for the oxide cathode used in grid-controlled, high-frequency tubes (850 8C) Duty ratio Pulse width (ms) 1 0.001 0.01 0.1
3
10
20
50
100
200
500
1000
12 12 9 7.5 6.0 5.1 4.3 3.3 2.9 9.5 6.6 4.5 3.5 2.5 2.0 1.5 1.2 1.0 1.5 1.5 1.3 1.1 0.92 0.85 0.75 0.63 0.60
dc density = 0.2 A/cm2.
subsection, a new kind of oxide cathode is introduced. Further increases in emission current density at low temperature have been obtained with this kind of cathode. Moreover, this oxide cathode is also promising in higher duty ratio applications. The characteristics of the cathode, such as dc and pulse current emission, evaporation, resistance to poisoning, as well as its application to microwave devices, are presented below. 3.2.1. Emission characteristics of the cathode (1) Direct emission current density: The direct emission current density of the cathode is 250– 300 mA/cm2 at 600 8C, 500–600 mA/cm2 at 650 8C, 900–1000 mA/cm2 at 700 8C, 1500 mA/ cm2 at 750 8C and over 2500 mA/cm2 at 800 8C. (2) Pulse emission current density (a) Emission at short pulse width (25 ms): The pulse emission current density of the cathodes is shown in Table 2 for five different operating temperatures and four diode samples with pulse duration of 25 ms and a repetition rate of 400 Hz. (b) Emission at long pulse width (500 ms): Fig. 2 shows the relation of emission current to pulse widths. The marks of ‘‘(&), (*), (~)’’ correspond to the new type of low temperature, high current density oxide cathode, and marks of ‘‘(!), (^)’’ correspond to the conventional oxide cathode. It is clear that the pulse emission current of the cathode of new type is very steady with the increase of pulse duration. In contrast, the pulse emission current of the conventional oxide cathode increases with the increase of its pulse width, which shows that the temperature of some parts of the cathode surface increases with the
X. Liao et al. / Applied Surface Science 251 (2005) 64–68 Table 2 Pulse emission current density (A/cm2) Diode no.
1 2 3 4
67
Table 3 Threshold pressures of O2 poisoning for the cathode
Temperature (8C) 750
780
800
820
850
6.7 7.7 6.9 7.8
8.2 9.5 8.2 10.5
10.8 10.8 10.9 12.8
14.4 13.5 13.7 15.2
17.5 16 16 18.2
increase of its pulse width due to the high interface resistance. The two different results indicate that the new type of low temperature, high current density oxide cathode has good emission capability at long pulse because of its low interface resistance. (c) Pulse emission for different duty ratios: The pulse emission of the such cathode changes very little after 1329 h of lifetime testing when the duty ratio is changed from 0.05% to 2% with corresponding pulse width and repetition rate of 1 ms, 50 Hz and 50 ms, 400 Hz, respectively. The test results for dc and pulse emission show that this cathode has high dc emission current density at high duty ratio and long pulse duration.
Temperature (8C)
Emission current density (A/cm2)
O2 pressures (Pa)
690 740 790
0.5 0.8 1.0
1.3 10 6.1 10 1.0 10
4 4 3
These characteristics are particularly useful for microwave device applications. 3.2.2. Resistance to poisoning for the cathode (a) Resistance to O2 poisoning: The threshold pressures of O2 poisoning and tested emission current are shown in Table 3 for the cathode at several different temperatures. The cathode tests show that its emission current can quickly come back to the value before O2 poisoning when the test system returns to original vacuum state. (b) Resistance to air exposure: When this kind of cathode was applied to grid-controlled TWTs and high power klystrons, we have observed that no obvious effect on the electron emission and lifetime was ever found after this cathode was exposed to air several times (4–5 times). 3.2.3. Evaporation of the cathode The experimental data for the evaporation rate of the cathodes are shown in Table 4. It can be seen that the cathode evaporation rates decrease by an order of magnitude when the cathode temperature decreases 50 8C. 3.2.4. Application of the cathode in microwave devices In order to rationally appraise the characteristics of a cathode, the cathode should be applied to a vacuum device so as to test how much current density it can provide, how long a lifetime it has in practical applications and so on. Several application examples Table 4 Relation of cathode evaporation rates to temperature evaporation rate (g/cm2)
Fig. 2. The emission current versus pulse width of the low temperature, high-emission current density oxide cathode. (&),(*) and (~ ) corresponds to the new type of low temperature, high current density oxide cathode, (!) and (^) corresponds to the conventional oxide cathode.
Temperature (8C)
Evaporation (g/cm2s)
950 900 850
1.1 10 8 1.9 10 9 1.98 10 10
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X. Liao et al. / Applied Surface Science 251 (2005) 64–68
for the cathode in microwave devices are given in this subsection. (1) In TWT (a) Grid-controlled TWT for communication satellite application: The cathode provides an emission current density of 5–6 A/cm2 with 1% duty ratio at 750–800 8C. (b) Moderate power TWT: The cathode provides an emission current density of 6 A/cm2 with 1% duty ratio at 750–800 8C and a lifetime of over 4000 h. (c) TWT with high duty ratio: The cathode provides an emission current density of 3– 4 A/cm2 with 4.5% duty ratio at 750–800 8C and a lifetime of over 3000 h. (2) Transmitting tube: The cathode provides an emission current density of 3 A/cm2 with 3% duty ratio at 820 8C and a lifetime of over 3000 h. (3) Klystrons (a) High power klystrons (>500 kW): The cathode provides an emission current density of 10 A/cm2 with 0.3–0.5% duty ratio at 850 8C. (b) Moderate power klystrons: The cathode provides an emission current density of 9 A/cm2 with 0.3% duty ratio at 800– 850 8C and a lifetime of over 4000 h. (c) High power klystrons with high duty ratio: The cathode provides an emission current density of 3 A/cm2 with 1.5% duty ratio and over 50 ms pulse width at 750–800 8C and a lifetime of 6000–10,000 h. The above applications have shown that this cathode has high reliability and long lifetime, and up to
now, few devices failure has occurred due to a cathode problem.
Acknowledgements We are deeply indebted to Professor Enqiu Zhang and Professor Zhaohao Wu for their work at the 100th anniversary of invention of the oxide cathodes. Their contributions to the theory of the oxide cathode have promoted the development of the highreliability, long lifetime oxide cathode, as well as the low temperature, high-emission current density oxide cathode, and show that the oxide cathode not only has a long lifetime at short pulse, but can also provide high direct emission current density, long pulse duration and high duty ratio, and has widely applicable prospects.
References [1] [2] [3] [4]
[5] [6] [7] [8] [9]
[10]
Z. Enqiu, Acta Phys. Sin. 23 (1974) 1. Z. Enqiu, Acta Phys. Sin. 23 (1974) 9. Z. Enqiu, Acta Phys. Sin. 25 (1976) 1. W. Zhaohao, in: Proceedings of Fourth National Annual Conference on Vacuum Electronics, Xianyang, China, 1982, p. 133. G.E. Moore, L.A. Wooten, J. Morrison, J. Appl. Phys. 26 (1955). H.E. Ken, Bell Lab Record, December1960, p. 452. K.M. Olsen, Bell Lab Record, January 1961. H.B. Frost, Bell Lab Record, January 1962. H.E. Korn, Beaction Phenamemao Report on 23rd Annual Conference on Physical Electronics (Massachnetts Institute Technology, 1963), p. 106. J.W. Rush, Microw. J. (1969) 88.
Development of new types of oxide cathodes Xianheng Liao *, Xiaoxia Wang, Qinglan Zhao, Mingfen Meng R&D Center Microwave for Device and technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100080, PR China Available online 15 April 2005
Abstract High reliability, long lifetime and low temperature, high emission current density oxide cathodes are presented in this paper. The characteristics of these cathodes, such as dc and pulse emission, evaporation, resistance to poisoning and lifetime, are discussed. They have been widely applied to TWTs and klystrons with high power, long pulse and high duty ratio. The results showed that high emission current density and long lifetime can be realized. # 2005 Elsevier B.V. All rights reserved. Keywords: High reliability; Long lifetime; High emission current density; Oxide cathode
1. Introduction The study and development of the oxide cathode has been ongoing for over 100 years. It has a lot of advantages, such as high current density at short pulse, low operating temperature, simple cathode structure and easy realization for batch-production. It is a widely used thermionic cathode. With the ongoing development of high power microwave devices, the requirements for the emission current density, pulse duration and duty ratio of the cathode have to be enhanced to suit the new applications. A lot of work has been done on the cathode in IECAS in the past 40 years, searching for its operating mechanisms, investigating new experimental structures, and improving emission characteristics with low evaporation rates, high resistance to poisoning and long * Corresponding author. Tel.: +86 10 58887263; fax: +86 10 62548109. E-mail address: [email protected] (X. Liao).
lifetime. Professor Enqiu Zhang found that there were some contradictions between the practicalities of the oxide cathode and the theory. Based on experimental results and careful analyses, he proposed a new theory, the dynamic surface emission center model [1–3]. Professor Zhaohao Wu has always proposed the viewpoint that the hole–surface emitter of the impregnated Ba–W cathode is actually an anamorphosis of oxide cathode [4]. The above-mentioned model and viewpoint are the basis of our research work, and plays an important role in the development of the oxide cathode and the improvement of its characteristics.
2. Background A lot of investigation has been done before the development of new types of oxide cathodes. The
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.119
X. Liao et al. / Applied Surface Science 251 (2005) 64–68
65
following experimental results established the base for the new cathodes: (1) Emission capabilities: Two reservoir oxide cathodes, one sprayed with (BaSrCa)O thin film on its emission surface and the other not, have been studied and found very different emission currents at 730 8C. The emission current of the former is 26 times that of the latter, which shows that the emission ability of the cathode can be greatly increased when excess barium is well combined with (BaSrCa)O. (2) Emission properties after oxide coating removal and emission from Ni-net holes: Following experiments have been made on a reservoir oxide cathode. In the first one, after the cathode was activated, aged and tested to obtain its emission current data, the coated oxide on the surface of the Ni-net was entirely removed and then the cathode was activated, aged and tested again. The result showed that the emission current of the cathode with the oxide coating removed is still high. In the second, a reservoir oxide with Ni-net on its surface was tested, the pores of Ni-net of the cathode were filled with the oxide, but there is no oxide at the warp and woof (the base metal surface) of the cathode Ninet. In this case, the emission current of the cathode is very low. The two experimental results showed that the oxide cathode emits electrons mainly from the thin film on the base metal surface, not from the oxide itself for reservoir oxide cathode.
3. Development of new types of oxide cathodes On the basis of the theoretical model and the results of above-mentioned experiments, we developed two types of oxide cathodes, the high-reliability, long lifetime oxide cathode and the low temperature, highemission current density oxide cathode. 3.1. High-reliability, long lifetime oxide cathode Super-pure Ni alloys, such as Zr–Ni (0.1% Zr), Zr– W–Ni (0.1% Zr, 2% W) and Mg–W–Ni (0.2–0.4%
Fig. 1. The structure of the high-reliability, long lifetime oxide cathode.
Mg, 2% W) are always adopted as the bases of long lifetime oxide cathodes. Such cathodes are prepared under super-clean sanitation conditions and applied separately in communication satellite TWTs under ultra-high vacuum conditions. Over 50,000 h lifetime has been reached with 100 mA/cm2 emission current density (for example, M4041, 314H, TL4002, etc.) [5–9]. The high-reliability, long lifetime oxide cathode shown in Fig. 1 was developed by rationally designing the cathode structure, carefully adjusting the prescription of the oxide materials, and looking for the appropriate technology of cathode preparation. The lifetime of the cathode in diode is over 100,000 h with 150 mA/cm2 at 590–750 8C. Meanwhile, over 50,000 h lifetime has been realized when the cathode was TWT for communication satellite applications with 150 mA/cm2 current density. 3.1.1. Conditions of the cathode testing Diode testing was done to check the characteristics of the tested cathodes with diameter of 3 mm. The operating temperature of the cathode was measured by a Ni–Mo thermocouple. After decomposition, activation and aging of the cathode, its emission current, lifetime and resistance to poisoning were tested. 3.1.2. Emission characteristics of the cathode (1) Direct emission current density: The direct emission current density of the cathode is 70 mA/cm2 at 550 8C, 100 mA/cm2 at 600 8C, 200 mA/cm2 at 650 8C, 400 mA/cm2 at 700 8C and over 400 mA/cm2 at 730 8C. (2) Pulse emission current density: The pulse emission current density of the cathode is 3 A/cm2 at 700 8C, 5.5 A/cm2 at 750 8C, 10 A/cm2 at 800 8C and over 20 A/cm2 at 850 8C, with a pulse width of 20 ms and a repetition rate of 50 Hz. (3) Lifetime testing
66
X. Liao et al. / Applied Surface Science 251 (2005) 64–68
(a) Lifetime of the cathodes in diodes: Lifetime testing of the cathodes was done for all diodes or for several diodes randomly selected. No decrease in emission current of the cathode was found after continuous operation for 100,000 h with 150–400 mA/cm2 (DC) at temperatures of 590–750 8C. However, the lifetime of the oxide cathodes with Ni–W–Mg and Ni–W–Zr bases is only 16,000–18,000 h with 250 mA/cm2 at 730 8C. (b) The lifetime of TWTs with this type of cathodes (for communication satellite application): Forty TWTs with 150 mA/cm2 were tracked to obtain lifetime data. The results show that lifetime of the TWT is over 50,000 h, and after the TWT has continuously operated for 20,000 h, the knee-point temperature of roll-off curves of the cathode decreases from 607 to 509 8C, which shows that the activity of the cathode continues to improve over its operating lifetime. 3.1.3. Applications of the cathode in vacuum electron devices (1) This cathode has been applied to TWTs used in communication satellites and operated for over 7 years. (2) The long lifetime cathode was also used in some display devices in the 1980s. The accelerating lifetime test is over 13,000 h for 12-in. black and white CRTs. The same experimental results are obtained for 14-in. black and white CRTs. The above-mentioned results for the experiments and applications of the cathode show that the cathode has good emission characteristics with high reliability and long lifetime. And it also has good resistance to poisoning, low evaporation and good resistance to shaking and shock, which will not be discussed in detail here. 3.2. The low temperature, high-emission current density oxide cathode Usually, the applications of the oxide cathode are only considered in short pulse, low dc density microwave tubes [10], as shown in Table 1, which restricts the development of the oxide cathode. In this
Table 1 Relation of pulse current density (A/cm2) to pulse duration and duty ratio for the oxide cathode used in grid-controlled, high-frequency tubes (850 8C) Duty ratio Pulse width (ms) 1 0.001 0.01 0.1
3
10
20
50
100
200
500
1000
12 12 9 7.5 6.0 5.1 4.3 3.3 2.9 9.5 6.6 4.5 3.5 2.5 2.0 1.5 1.2 1.0 1.5 1.5 1.3 1.1 0.92 0.85 0.75 0.63 0.60
dc density = 0.2 A/cm2.
subsection, a new kind of oxide cathode is introduced. Further increases in emission current density at low temperature have been obtained with this kind of cathode. Moreover, this oxide cathode is also promising in higher duty ratio applications. The characteristics of the cathode, such as dc and pulse current emission, evaporation, resistance to poisoning, as well as its application to microwave devices, are presented below. 3.2.1. Emission characteristics of the cathode (1) Direct emission current density: The direct emission current density of the cathode is 250– 300 mA/cm2 at 600 8C, 500–600 mA/cm2 at 650 8C, 900–1000 mA/cm2 at 700 8C, 1500 mA/ cm2 at 750 8C and over 2500 mA/cm2 at 800 8C. (2) Pulse emission current density (a) Emission at short pulse width (25 ms): The pulse emission current density of the cathodes is shown in Table 2 for five different operating temperatures and four diode samples with pulse duration of 25 ms and a repetition rate of 400 Hz. (b) Emission at long pulse width (500 ms): Fig. 2 shows the relation of emission current to pulse widths. The marks of ‘‘(&), (*), (~)’’ correspond to the new type of low temperature, high current density oxide cathode, and marks of ‘‘(!), (^)’’ correspond to the conventional oxide cathode. It is clear that the pulse emission current of the cathode of new type is very steady with the increase of pulse duration. In contrast, the pulse emission current of the conventional oxide cathode increases with the increase of its pulse width, which shows that the temperature of some parts of the cathode surface increases with the
X. Liao et al. / Applied Surface Science 251 (2005) 64–68 Table 2 Pulse emission current density (A/cm2) Diode no.
1 2 3 4
67
Table 3 Threshold pressures of O2 poisoning for the cathode
Temperature (8C) 750
780
800
820
850
6.7 7.7 6.9 7.8
8.2 9.5 8.2 10.5
10.8 10.8 10.9 12.8
14.4 13.5 13.7 15.2
17.5 16 16 18.2
increase of its pulse width due to the high interface resistance. The two different results indicate that the new type of low temperature, high current density oxide cathode has good emission capability at long pulse because of its low interface resistance. (c) Pulse emission for different duty ratios: The pulse emission of the such cathode changes very little after 1329 h of lifetime testing when the duty ratio is changed from 0.05% to 2% with corresponding pulse width and repetition rate of 1 ms, 50 Hz and 50 ms, 400 Hz, respectively. The test results for dc and pulse emission show that this cathode has high dc emission current density at high duty ratio and long pulse duration.
Temperature (8C)
Emission current density (A/cm2)
O2 pressures (Pa)
690 740 790
0.5 0.8 1.0
1.3 10 6.1 10 1.0 10
4 4 3
These characteristics are particularly useful for microwave device applications. 3.2.2. Resistance to poisoning for the cathode (a) Resistance to O2 poisoning: The threshold pressures of O2 poisoning and tested emission current are shown in Table 3 for the cathode at several different temperatures. The cathode tests show that its emission current can quickly come back to the value before O2 poisoning when the test system returns to original vacuum state. (b) Resistance to air exposure: When this kind of cathode was applied to grid-controlled TWTs and high power klystrons, we have observed that no obvious effect on the electron emission and lifetime was ever found after this cathode was exposed to air several times (4–5 times). 3.2.3. Evaporation of the cathode The experimental data for the evaporation rate of the cathodes are shown in Table 4. It can be seen that the cathode evaporation rates decrease by an order of magnitude when the cathode temperature decreases 50 8C. 3.2.4. Application of the cathode in microwave devices In order to rationally appraise the characteristics of a cathode, the cathode should be applied to a vacuum device so as to test how much current density it can provide, how long a lifetime it has in practical applications and so on. Several application examples Table 4 Relation of cathode evaporation rates to temperature evaporation rate (g/cm2)
Fig. 2. The emission current versus pulse width of the low temperature, high-emission current density oxide cathode. (&),(*) and (~ ) corresponds to the new type of low temperature, high current density oxide cathode, (!) and (^) corresponds to the conventional oxide cathode.
Temperature (8C)
Evaporation (g/cm2s)
950 900 850
1.1 10 8 1.9 10 9 1.98 10 10
68
X. Liao et al. / Applied Surface Science 251 (2005) 64–68
for the cathode in microwave devices are given in this subsection. (1) In TWT (a) Grid-controlled TWT for communication satellite application: The cathode provides an emission current density of 5–6 A/cm2 with 1% duty ratio at 750–800 8C. (b) Moderate power TWT: The cathode provides an emission current density of 6 A/cm2 with 1% duty ratio at 750–800 8C and a lifetime of over 4000 h. (c) TWT with high duty ratio: The cathode provides an emission current density of 3– 4 A/cm2 with 4.5% duty ratio at 750–800 8C and a lifetime of over 3000 h. (2) Transmitting tube: The cathode provides an emission current density of 3 A/cm2 with 3% duty ratio at 820 8C and a lifetime of over 3000 h. (3) Klystrons (a) High power klystrons (>500 kW): The cathode provides an emission current density of 10 A/cm2 with 0.3–0.5% duty ratio at 850 8C. (b) Moderate power klystrons: The cathode provides an emission current density of 9 A/cm2 with 0.3% duty ratio at 800– 850 8C and a lifetime of over 4000 h. (c) High power klystrons with high duty ratio: The cathode provides an emission current density of 3 A/cm2 with 1.5% duty ratio and over 50 ms pulse width at 750–800 8C and a lifetime of 6000–10,000 h. The above applications have shown that this cathode has high reliability and long lifetime, and up to
now, few devices failure has occurred due to a cathode problem.
Acknowledgements We are deeply indebted to Professor Enqiu Zhang and Professor Zhaohao Wu for their work at the 100th anniversary of invention of the oxide cathodes. Their contributions to the theory of the oxide cathode have promoted the development of the highreliability, long lifetime oxide cathode, as well as the low temperature, high-emission current density oxide cathode, and show that the oxide cathode not only has a long lifetime at short pulse, but can also provide high direct emission current density, long pulse duration and high duty ratio, and has widely applicable prospects.
References [1] [2] [3] [4]
[5] [6] [7] [8] [9]
[10]
Z. Enqiu, Acta Phys. Sin. 23 (1974) 1. Z. Enqiu, Acta Phys. Sin. 23 (1974) 9. Z. Enqiu, Acta Phys. Sin. 25 (1976) 1. W. Zhaohao, in: Proceedings of Fourth National Annual Conference on Vacuum Electronics, Xianyang, China, 1982, p. 133. G.E. Moore, L.A. Wooten, J. Morrison, J. Appl. Phys. 26 (1955). H.E. Ken, Bell Lab Record, December1960, p. 452. K.M. Olsen, Bell Lab Record, January 1961. H.B. Frost, Bell Lab Record, January 1962. H.E. Korn, Beaction Phenamemao Report on 23rd Annual Conference on Physical Electronics (Massachnetts Institute Technology, 1963), p. 106. J.W. Rush, Microw. J. (1969) 88.