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High temperature cathodes for high current density
Nuclear Instruments and Methods in Physics Research A 340 (1994) 204-208 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
High temperature cathodes for high current density G. Kuznetsov Institute of Nuclear Physics, Nocosibirsk, Russian Federation
The features of the high temperature cathodes used at INP installations made of the following materials are presented: 1) LAB6, 2)eutectic LaB6-based alloys with ZrB2, HfB 2 and TiB2, 3)IrCe and IrLa alloys. Their lifetimes, vacuum operating conditions and poisoning are considered.
1. Introduction
In dismountable vacuum systems intended for research work, such as accelerating tubes and accelerator cavities, electron sources for studying plasma, welding etc., cathodes with a high resistance to poisoning and a wide range of current density (from 0.1 to 100 A / c m 2) are required. From this standpoint, neither pure metal cathodes, because of their short lifetime and relatively high heating power, nor low temperature materials such as oxides or dispenser, because of their low resistance to poisoning, can be used. The cathode material that offers some advantages as compared to others in the above-mentioned applications is lanthanum hexaboride (LAB6). After the first investigation of its properties by Lafferty [1], many authors have studied LaB 6 [2-4]. A number of new LaB6-based materials with refractory metals, such as tungsten, molybdenum, tantalum, nickel [5], diborides of zirconium, hafnium and titanium, have appeared recently [6]. Cathode alloys of iridium, rhenium and osmium with lanthanum group elements were subsequently developed [7]. They possess both a low work function and a low evaporation velocity as well as an operation temperature high enough to work in dismountable vacuum systems. A m o n g these IrCe and IrLa stand out. However, only " p u r e " LaB 6 is now in wide use.
2. Some features of thermionic materials
The area of LaB 6 cathodes varies from 1 mm 2 to several hundred em 2. In recent years, single-crystal bars of LAB6, up to 7 mm in diameter, have been produced [8]. The cathode quality strongly depends on the purity of the materials used and varies over a wide range. The best cathodes are manufactured from sin-
gle-crystal powders. A m o n g the other methods of manufacturing LaB 6 cathodes, sintering, hot pressing and plasma coating should be noted. Some of the characteristic properties of LaB 6 are: density 4.0-4.5 g / c m 3 (theoretical density 4.7 g / c m 3) expansion factor (6-8) × 10 ~ work function 2.82-2.86 eV spectral emissivity 0.8-0.7 total emissivity 0.7 #1 Its heat- and electro-conductivity depend on the material density. The specific resistance of a dense sample is about 10 t x f ~ / c m at room temperature and rises to 50 ~ x ~ / c m at T = 1600°C [9]. Different authors present quite different values for the work functions of LaB 6 even for one side of the single crystal [8]. For multi-crystal surfaces, the work function is about 2.82-2.86 eV [10]. It is the author's opinion that the emissivity is usually set too high and is 0.45 for the smooth surface of the dense sample. During operation it increases up to 0.5-0.55. The porosity of LaB 6 plates ranges between 8.5 and 15% (a permissible standard). Therefore, after durable heating, "the sintering" (reduction in size) of LaB 6 cathodes giving rise to problems of its support is observed. Fig. 1 presents the current density versus temperature for some cathodes. Correction for the brightness temperature at a wavelength of 0.65 ixm is shown in Fig. 2. One can read about the methods of producing LaB 6 and its properties more thoroughly in ref. [8]. At the Institute of Nuclear Physics, Siberian Division of the Russian Academy of Science, LaB 6 cathodes of 4, 6, 10, 14, 17, 20, 30, 50, 80, 110 and 120 mm diameter are utilized. Most of them are heated by a radiation method.
#1 0.45 in the author's opinion.
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0 1 6 8 - 9 0 0 2 ( 9 3 ) E 0 9 7 8 - 2
205
G. KuznetsoL' / Nucl. Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
Z u
i0~ I0
ICe
/ /
/ L
/
0,1 1400
1600
1800
/~
/'
10-s
u
10-7
~
lo-~
~_
/
~n
S
/
2000
-~
8400
/
10-o 10-10
2
~_ o_0 10_11
/
qS
8eoo
.Y/oB/, /
10-~
Temperature ca±hodes (°C)
L~
Fig. 1. Current density versus temperature for some cathodes.
1
10
10z
Current denslty (Alcm 2)
Fig. 3. Evaporation rates of some materials versus current density. Cathodes of iridium-based alloys are manufactured in an arc furnace in an argon atmosphere [7]. An alternative method is the impregnation of an iridium surface with melted salts of metals of the lanthanum group. In Table 1 the properties of alloys of Ir with lanthanum, cerium and presaodymium are shown [7]. O n e can see that the evaporation rate of metals from alloy is more than ten times less than that of LaB 6 at the same temperature. Fig. 3 shows the evaporation rates of some materials versus the current density. The advantage of Ir is obvious. LaB6-based cathodes with ZrB2, TiB 2 and HfB z possess a lower work function when the rate between components is eutectic [6]. In Fig. 4 one can see the curve for the work function versus MeB 2 concentration in alloy. In Fig. 5 the current density at different temperatures is presented. At the same temperature, the eutectical alloys have a current density 4 - 5 times higher than that of pure LAB6, but due to higher emissivity the specific heat characteristics of guns with cathodes made of pure LaB 6 are better than those with cathodes m a d e of LaB 6 + MeB z. It should be mentioned that the latter is highly resistant to poisoning and heat shock. Unfortunately, because of the difference in the evaporation rates of the components the cathode surface is covered with borade, giving an increase in the emissivity which, in turn, reduces the temperature and the specific properties.
3. Methods of cathode heating The cathode lifetime substantially depends on the method of heating. These are: 1) direct heating (used for cathodes in the form of a wire or a hairpin); 2) radiation heating [12]; 3) combination of radiation and electron heating [13]; 4) electron heating (sometimes with a beam scanning through the cathode surface); 5) high-frequency heating. The heating method is chosen in accordance with the requirements of the electron optic system and the possibility of supplying high voltage to the heating system, if the existence of a magnetic field on the cathode surface due to filament current is acceptable. The most natural and simplest m e t h o d is radiation heating. It provides high temperature and high uniformity of temperature along the cathode surface. The heating system efficiency increases for cathodes of larger diameter. Fig. 6 presents the specific heating power as a function of the cathode diameter for the gun fabricated at INP. The horizontal line is the radia-
Cathodes LaB6 + MeB2 ( M e - Tb Zr, HF, V, Nb, Ta) aT(*C)
A
~o 200 -,4
~ 150 o ~ 100 vJ
/
/
%
4,0~
irLal
8 k = 0 , 4 5 ~ 0,4
d
~>3,Lo j 2,9
~o
~.0~
~
/
©
/
/
fJ
2,7
~ 6
10 [ . , , . / ~
8 X=0,8~0,7
1000
1500
Brightness
temperature
}9oox
I.
'LOB6 20 40
zooo T(*C)
x:6soo
60
80 ZrB2 (J
Nass percent
1700K ._.4
oF-, i-i i-"q LaB6 20
40
60
80 ZrB2
Nass percent
Fig. 4. Work function versus MeB 2 concentration in alloy.
Fig. 2. Correction for brightness temperature at a wavelength
of 0.65 txm.
Fig. 5. Current density at different temperatures. II. CONTRIBUTIONS
206
G. Kuznetsou / N u c l . Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
Table 1 Properties of Ir alloys with La, Ce and Pr Alloy
Ir-La Ir-Ce Ir-Pr LaB 6
Weight of rare-earth metal [%]
q5 Work function at 1300 K [eV]
d& / d T
I1
12
Wl a
I/V2 b
at 1100-1500 K [eV/deg.]
Current density at 1700 K [A/cm 2]
Current density at 2100 K [A/cm 2]
Evaporation rate at 2100 K [g/cm 2 s]
Evaporation rate at 2100 K [g/cm 2 s]
8.4 13.0 4.7
2.57 2.57 2.55 2.82
l×10 <5xl0 lxl0 1x10
8.0 10.0 8.0 1.4
130 150 125
1 x l 0 -s 1.6x 10 ') 3x10 s
2x10 a 7xl0 ~ 6x10 7
4 -5 4 4
a Measured using the Becker method. b Measured using the spectral method.
tion of LaB 6 from 1 cm z at 1800°C t e m p e r a t u r e a n d • = 0.55. O n e can c o m p a r e these data with those given in ref. [13], in which the LaB 6 c a t h o d e with a 2.87 cm 2 area is h e a t e d by a b o m b a r d m e n t a n d radiation m e t h o d . T h e total heating r e q u i r e d is 202 W p e r square c e n t i m e t e r of the c a t h o d e area at 1626°C and 259 W / c m 2 at 1755°C. This is two times higher t h a n for the guns p r o d u c e d at INP, Novosibirsk. This gun has a very big filament c h a m b e r . It is the a u t h o r ' s opinion t h a t the n o n u n i f o r m i t y of the c a t h o d e t e m p e r a t u r e of a b o u t 60°C across the d i a m e t e r is unacceptable. In ref. [11], the specific power for c a t h o d e heating is approximately 2.7 times higher t h a n t h a t for the analogous c a t h o d e m a d e at INP, Novosibirsk, because of cooling in the filament region. T h e c a t h o d e t e m p e r a t u r e d e p e n d s on t h e r m i o n i c p r o p e r t i e s of the c a t h o d e material. Fig. 7 p r e s e n t s the curves for c a t h o d e t e m p e r a t u r e versus the specific heating power for t h r e e c a t h o d e s 120 m m in d i a m e t e r m a d e of different metals. O n e can see that the best result is o b t a i n e d for a M o substrate 1 m m thick. T h e filament materials are tungsten, t a n t a l u m and graphite. F o r plate c a t h o d e s less t h a n 20 m m in diameter a t u n g s t e n wire is used. T h e filament has a cylindri-
cal spiral form a n d the total filament surfacc is 3 - 5 times larger t h a n the c a t h o d e surface. T h e heating a n d t e m p e r a t u r e characteristics for the cathodes, 6 a n d 10 m m in diameter, are p r e s e n t e d in Fig. 8. T h e cathodes, from 30 to 120 m m diameter, have the t a n t a l u m r i b b o n filament a t t a c h e d tightly to the insulators which are placed outside the filament chamber. T h e choice of c u r r e n t rate a n d voltage of the filam e n t is also important. A high c u r r e n t m e a n s a large filament cross-section, which leads to high losses of heating power t h r o u g h feeders. A high voltage (up to 100 V) leads to b r e a k d o w n due to dust, needles, or the s h a r p edge of the filament. In this case, the alternating c u r r e n t reduces the possibility of breakdown.
4. Lifetime of electron guns T h e factors that define the lifetime of the gun are: 1) e v a p o r a t i o n of c a t h o d e materials: 2) soiling of the c a t h o d e surface; 3) destruction of the c a t h o d e surface with breakdowns; 4) destruction of the c a t h o d e support;
LaB6 CATHODES INP CATHODE AREA 120cm2 u
T(°C)
E
2000
{ 1500 45
100 ~
u -q_
PlBO0*C ~0,55
~
.
1BOD'C = ~
[5 I
10
1600"C 1400"5
102
(/3
Cathode area (cm2)
Fig. 6. Specific heating power as a function of cathode diameter.
I000
¢¢s~ °
/
/K"
La3 ~,
/ Mo+LaB6
50
100
Speci{'ic heatln 9 power, P(W/cm 2)
Fig. 7. Cathode temperature versus specific heating power for three cathodes.
207
G. Kuznetsov /NucL Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
IH(~)
T Br (oc) "Z/
12
¢6M~/
2000
10
1800
8
1600
OMM
1400 1200 1000
2 4
6 a 1o 12 UH(V)8oo
20 40 60 80 tooz2o P(Wt)
Fig. 8. Heating and temperature characteristics for LaB 6 cathodes of 6 and 10 mm diameter.
5) change of cathode geometry. 1) The roughness of the cathode surface increases because of nonuniform evaporation from different sides of single crystals forming the multi-crystal. As a result, the emissivity factor becomes higher and, in turn, raises the heating power. Electron and ion bombardment give the same result. Filament evaporation leads to the formation of a film on the back side of the cathode. Since the emissivity factor of the coating is less, heat exchange between the filament and the cathode decreases. The temperature of the cathode decreases in places where film exfoliation takes place. The mentioned effect defines the durability of a cathode-filament couple. There is another effect due to evaporation of the filament. The voltage on the filament increases by about 1% during every thousand hours at 1600°C. 2) Coating of the cathode surface with materials from the walls of the vacuum chamber, or with carbon of destroyed organic compounds, or with material of the focusing electrode evaporated during breakdowns, increases the value of the work function and heating power. In this case, ion and electron bombardment play a positive role by cleaning the surface from dust. 3) During breakdowns between the electrodes and the cathode surface at high power, the temperature at the place of the breakdown is so high that the cathode materials melts and cracks. The fixtures of the other metal seem to be useless because boron will easily destroy them. 4) The process of fast destruction is very typical for the point of connection of LaB 6 with refractory metals, such as tantalum and rhenium, because of the boron penetrating into them. The lifetime of the connection is only 200-300 h at a cathode temperature of 1600°C for tantalum and even less for the other materials. High-quality graphite is clamped between a LaB 6 plate and a holder to increase the lifetime of the gun. This type of connection operates at 1600°C for 3000 h. It should be mentioned that, in this case, graphite im-
pregnated with boron will easily fail on contact with air. The use of graphite fabric [11] seems to be attractive, but it does not make the gun lifetime much longer because boron can move over the graphite surface or even through a narrow gap between two closely connected graphite parts. 5) The nonuniformity of temperature distribution over the surface leads to a change in the geometrical shape of the cathode. For the cathode, 30 mm in diameter a temperature difference exceeding 30°C over the surface is unsuitable. A cathode made of material based on Ir requires especially good uniformity of temperature distribution because of the thickness (or, to be more precise, thinness) of the cathode due to the high cost of Ir itself. The lifetime of the gun is dependent on a combination of several factors. Fig. 9 illustrates the lifetime of the gun with LaB 6 and Ir-based cathodes that worked in practice at INP installations. For LaB 6 cathodes of large diameter, evaporation of a layer 200 t~m thick is permissible. During operation the heat characteristics change by several percent, the emissivity of the surface rises, and the cathode itself and its support deform. Then its time to change the gun.
o•10~ 10~
% ~
J
10z
10 -1
1
LaBe ctB6 100ur~ ~ O ~ m 10
10 2
Current; density (A/c~ 2)
Fig. 9. Gun lifetime with LaB 6 and Ir-based cathodes. II. CONTRIBUTIONS
208
G. Kuznetsov / Nucl. Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
Ir-based c a t h o d e s n e e d a good vacuum. T h e straight line in Fig. 9 displays the data relate to an Ir cathode, 30 m m in diameter, operating in vacuum at a b o u t 10 7 Torr, with a short c u r r e n t pulse regime without ion b a m b a r d m e n t . T h e separately displaced points show the data for the gun working u n d e r conditions similar to those for LAB6: v a c u u m a b o u t 10 6 Torr, pulse durability 12 ixs, a n d ion b o m b a r d m e n t . T h e lifetime of such a c a t h o d e is 2 - 3 times longer t h a n that of the LaB 6 cathode. To conclude, it should be n o t e d that the lifetime of these c a t h o d e s is similar to the lifetime of oxide and dispenser c a t h o d e s t h a t o p e r a t e in b a k e o u t systems.
[6]
[7]
[8]
[9]
References [1] J. Lafferty, J. Appl. Phys. 22 (1951) 299. [2] E.K. Storms and B.A. Mueller, J. Appl. Phys. 50 (1979) 3691. [3] M. Futamoto, M. Nakazava, K. Usami, S. Hosoki and U. Kowabe, J. Appl. Phys. 51 (1980) 3869. [4] E.K. Storms, J. Appl. Phys. 50 (1979) 4450. [5] G.M. Kuznetsova, G.A. Kudintseva and L.I. Suchkov, High-performance Cathodes Based on LaB 6. Electronic
[10] [11] [12]
[13]
Technique, Ser. 16, Generatornye, Modulatornye i Rentgenovskiye Pribory, vol. 1, 1969, p. 76. S.S. Ordanjan, E.E. Nikolaeva, I.K. Horoshilova and E.N. Ostrovsky, in: Sixth All-Union Conf. on High Current Electronics, vol. 1 (Tomsk, 1986) p. 58. S.E. Rozhkov, O.K. Kultashev and A.A. Gugnin, Technical Characteristics of Thermionic Emitters Based on Alloy Ir with La, Ce, Pr. Electronic Technique, Ser. 16, Generatornye, Modulatornye i Rentgenovskiye Pribory, vol. 2, 1969, p. 81. V.S. Kresanov, N.P. Malahov, V.V. Morozov, N.N. Semashko and V.Ja. Shljuko, High-performance Electron Emitter Based on LaB~ (Energoatomizdat, Moscow, 1987). V.S. Neshpor, B.A. Fridlender and Ju.B. Paderno, The Heat- and Electro-conductivity of Monolithic LaBs, at High Temperature. Teplofizika Vysokikh Temperatur, 1976, p. 903. V.S. Fomenko, Thermionic Emission Properties of Materials (Naukova dumka, Kiev, 1981). M.D. Goebel et al., Rev. Sci. Instr. 56 (1985) 1717. G.A. Kudintseva, A.I. Melnikov, E.V. Morozov and B.P. Nikonov, Thermionic Cathodes (Energiya, Moscow, 1966). M.E. Herniter and W.D. Getty, IEEE Trans. Plasma Sci. PS-15 (1987) 351.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
High temperature cathodes for high current density G. Kuznetsov Institute of Nuclear Physics, Nocosibirsk, Russian Federation
The features of the high temperature cathodes used at INP installations made of the following materials are presented: 1) LAB6, 2)eutectic LaB6-based alloys with ZrB2, HfB 2 and TiB2, 3)IrCe and IrLa alloys. Their lifetimes, vacuum operating conditions and poisoning are considered.
1. Introduction
In dismountable vacuum systems intended for research work, such as accelerating tubes and accelerator cavities, electron sources for studying plasma, welding etc., cathodes with a high resistance to poisoning and a wide range of current density (from 0.1 to 100 A / c m 2) are required. From this standpoint, neither pure metal cathodes, because of their short lifetime and relatively high heating power, nor low temperature materials such as oxides or dispenser, because of their low resistance to poisoning, can be used. The cathode material that offers some advantages as compared to others in the above-mentioned applications is lanthanum hexaboride (LAB6). After the first investigation of its properties by Lafferty [1], many authors have studied LaB 6 [2-4]. A number of new LaB6-based materials with refractory metals, such as tungsten, molybdenum, tantalum, nickel [5], diborides of zirconium, hafnium and titanium, have appeared recently [6]. Cathode alloys of iridium, rhenium and osmium with lanthanum group elements were subsequently developed [7]. They possess both a low work function and a low evaporation velocity as well as an operation temperature high enough to work in dismountable vacuum systems. A m o n g these IrCe and IrLa stand out. However, only " p u r e " LaB 6 is now in wide use.
2. Some features of thermionic materials
The area of LaB 6 cathodes varies from 1 mm 2 to several hundred em 2. In recent years, single-crystal bars of LAB6, up to 7 mm in diameter, have been produced [8]. The cathode quality strongly depends on the purity of the materials used and varies over a wide range. The best cathodes are manufactured from sin-
gle-crystal powders. A m o n g the other methods of manufacturing LaB 6 cathodes, sintering, hot pressing and plasma coating should be noted. Some of the characteristic properties of LaB 6 are: density 4.0-4.5 g / c m 3 (theoretical density 4.7 g / c m 3) expansion factor (6-8) × 10 ~ work function 2.82-2.86 eV spectral emissivity 0.8-0.7 total emissivity 0.7 #1 Its heat- and electro-conductivity depend on the material density. The specific resistance of a dense sample is about 10 t x f ~ / c m at room temperature and rises to 50 ~ x ~ / c m at T = 1600°C [9]. Different authors present quite different values for the work functions of LaB 6 even for one side of the single crystal [8]. For multi-crystal surfaces, the work function is about 2.82-2.86 eV [10]. It is the author's opinion that the emissivity is usually set too high and is 0.45 for the smooth surface of the dense sample. During operation it increases up to 0.5-0.55. The porosity of LaB 6 plates ranges between 8.5 and 15% (a permissible standard). Therefore, after durable heating, "the sintering" (reduction in size) of LaB 6 cathodes giving rise to problems of its support is observed. Fig. 1 presents the current density versus temperature for some cathodes. Correction for the brightness temperature at a wavelength of 0.65 ixm is shown in Fig. 2. One can read about the methods of producing LaB 6 and its properties more thoroughly in ref. [8]. At the Institute of Nuclear Physics, Siberian Division of the Russian Academy of Science, LaB 6 cathodes of 4, 6, 10, 14, 17, 20, 30, 50, 80, 110 and 120 mm diameter are utilized. Most of them are heated by a radiation method.
#1 0.45 in the author's opinion.
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0 1 6 8 - 9 0 0 2 ( 9 3 ) E 0 9 7 8 - 2
205
G. KuznetsoL' / Nucl. Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
Z u
i0~ I0
ICe
/ /
/ L
/
0,1 1400
1600
1800
/~
/'
10-s
u
10-7
~
lo-~
~_
/
~n
S
/
2000
-~
8400
/
10-o 10-10
2
~_ o_0 10_11
/
qS
8eoo
.Y/oB/, /
10-~
Temperature ca±hodes (°C)
L~
Fig. 1. Current density versus temperature for some cathodes.
1
10
10z
Current denslty (Alcm 2)
Fig. 3. Evaporation rates of some materials versus current density. Cathodes of iridium-based alloys are manufactured in an arc furnace in an argon atmosphere [7]. An alternative method is the impregnation of an iridium surface with melted salts of metals of the lanthanum group. In Table 1 the properties of alloys of Ir with lanthanum, cerium and presaodymium are shown [7]. O n e can see that the evaporation rate of metals from alloy is more than ten times less than that of LaB 6 at the same temperature. Fig. 3 shows the evaporation rates of some materials versus the current density. The advantage of Ir is obvious. LaB6-based cathodes with ZrB2, TiB 2 and HfB z possess a lower work function when the rate between components is eutectic [6]. In Fig. 4 one can see the curve for the work function versus MeB 2 concentration in alloy. In Fig. 5 the current density at different temperatures is presented. At the same temperature, the eutectical alloys have a current density 4 - 5 times higher than that of pure LAB6, but due to higher emissivity the specific heat characteristics of guns with cathodes made of pure LaB 6 are better than those with cathodes m a d e of LaB 6 + MeB z. It should be mentioned that the latter is highly resistant to poisoning and heat shock. Unfortunately, because of the difference in the evaporation rates of the components the cathode surface is covered with borade, giving an increase in the emissivity which, in turn, reduces the temperature and the specific properties.
3. Methods of cathode heating The cathode lifetime substantially depends on the method of heating. These are: 1) direct heating (used for cathodes in the form of a wire or a hairpin); 2) radiation heating [12]; 3) combination of radiation and electron heating [13]; 4) electron heating (sometimes with a beam scanning through the cathode surface); 5) high-frequency heating. The heating method is chosen in accordance with the requirements of the electron optic system and the possibility of supplying high voltage to the heating system, if the existence of a magnetic field on the cathode surface due to filament current is acceptable. The most natural and simplest m e t h o d is radiation heating. It provides high temperature and high uniformity of temperature along the cathode surface. The heating system efficiency increases for cathodes of larger diameter. Fig. 6 presents the specific heating power as a function of the cathode diameter for the gun fabricated at INP. The horizontal line is the radia-
Cathodes LaB6 + MeB2 ( M e - Tb Zr, HF, V, Nb, Ta) aT(*C)
A
~o 200 -,4
~ 150 o ~ 100 vJ
/
/
%
4,0~
irLal
8 k = 0 , 4 5 ~ 0,4
d
~>3,Lo j 2,9
~o
~.0~
~
/
©
/
/
fJ
2,7
~ 6
10 [ . , , . / ~
8 X=0,8~0,7
1000
1500
Brightness
temperature
}9oox
I.
'LOB6 20 40
zooo T(*C)
x:6soo
60
80 ZrB2 (J
Nass percent
1700K ._.4
oF-, i-i i-"q LaB6 20
40
60
80 ZrB2
Nass percent
Fig. 4. Work function versus MeB 2 concentration in alloy.
Fig. 2. Correction for brightness temperature at a wavelength
of 0.65 txm.
Fig. 5. Current density at different temperatures. II. CONTRIBUTIONS
206
G. Kuznetsou / N u c l . Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
Table 1 Properties of Ir alloys with La, Ce and Pr Alloy
Ir-La Ir-Ce Ir-Pr LaB 6
Weight of rare-earth metal [%]
q5 Work function at 1300 K [eV]
d& / d T
I1
12
Wl a
I/V2 b
at 1100-1500 K [eV/deg.]
Current density at 1700 K [A/cm 2]
Current density at 2100 K [A/cm 2]
Evaporation rate at 2100 K [g/cm 2 s]
Evaporation rate at 2100 K [g/cm 2 s]
8.4 13.0 4.7
2.57 2.57 2.55 2.82
l×10 <5xl0 lxl0 1x10
8.0 10.0 8.0 1.4
130 150 125
1 x l 0 -s 1.6x 10 ') 3x10 s
2x10 a 7xl0 ~ 6x10 7
4 -5 4 4
a Measured using the Becker method. b Measured using the spectral method.
tion of LaB 6 from 1 cm z at 1800°C t e m p e r a t u r e a n d • = 0.55. O n e can c o m p a r e these data with those given in ref. [13], in which the LaB 6 c a t h o d e with a 2.87 cm 2 area is h e a t e d by a b o m b a r d m e n t a n d radiation m e t h o d . T h e total heating r e q u i r e d is 202 W p e r square c e n t i m e t e r of the c a t h o d e area at 1626°C and 259 W / c m 2 at 1755°C. This is two times higher t h a n for the guns p r o d u c e d at INP, Novosibirsk. This gun has a very big filament c h a m b e r . It is the a u t h o r ' s opinion t h a t the n o n u n i f o r m i t y of the c a t h o d e t e m p e r a t u r e of a b o u t 60°C across the d i a m e t e r is unacceptable. In ref. [11], the specific power for c a t h o d e heating is approximately 2.7 times higher t h a n t h a t for the analogous c a t h o d e m a d e at INP, Novosibirsk, because of cooling in the filament region. T h e c a t h o d e t e m p e r a t u r e d e p e n d s on t h e r m i o n i c p r o p e r t i e s of the c a t h o d e material. Fig. 7 p r e s e n t s the curves for c a t h o d e t e m p e r a t u r e versus the specific heating power for t h r e e c a t h o d e s 120 m m in d i a m e t e r m a d e of different metals. O n e can see that the best result is o b t a i n e d for a M o substrate 1 m m thick. T h e filament materials are tungsten, t a n t a l u m and graphite. F o r plate c a t h o d e s less t h a n 20 m m in diameter a t u n g s t e n wire is used. T h e filament has a cylindri-
cal spiral form a n d the total filament surfacc is 3 - 5 times larger t h a n the c a t h o d e surface. T h e heating a n d t e m p e r a t u r e characteristics for the cathodes, 6 a n d 10 m m in diameter, are p r e s e n t e d in Fig. 8. T h e cathodes, from 30 to 120 m m diameter, have the t a n t a l u m r i b b o n filament a t t a c h e d tightly to the insulators which are placed outside the filament chamber. T h e choice of c u r r e n t rate a n d voltage of the filam e n t is also important. A high c u r r e n t m e a n s a large filament cross-section, which leads to high losses of heating power t h r o u g h feeders. A high voltage (up to 100 V) leads to b r e a k d o w n due to dust, needles, or the s h a r p edge of the filament. In this case, the alternating c u r r e n t reduces the possibility of breakdown.
4. Lifetime of electron guns T h e factors that define the lifetime of the gun are: 1) e v a p o r a t i o n of c a t h o d e materials: 2) soiling of the c a t h o d e surface; 3) destruction of the c a t h o d e surface with breakdowns; 4) destruction of the c a t h o d e support;
LaB6 CATHODES INP CATHODE AREA 120cm2 u
T(°C)
E
2000
{ 1500 45
100 ~
u -q_
PlBO0*C ~0,55
~
.
1BOD'C = ~
[5 I
10
1600"C 1400"5
102
(/3
Cathode area (cm2)
Fig. 6. Specific heating power as a function of cathode diameter.
I000
¢¢s~ °
/
/K"
La3 ~,
/ Mo+LaB6
50
100
Speci{'ic heatln 9 power, P(W/cm 2)
Fig. 7. Cathode temperature versus specific heating power for three cathodes.
207
G. Kuznetsov /NucL Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
IH(~)
T Br (oc) "Z/
12
¢6M~/
2000
10
1800
8
1600
OMM
1400 1200 1000
2 4
6 a 1o 12 UH(V)8oo
20 40 60 80 tooz2o P(Wt)
Fig. 8. Heating and temperature characteristics for LaB 6 cathodes of 6 and 10 mm diameter.
5) change of cathode geometry. 1) The roughness of the cathode surface increases because of nonuniform evaporation from different sides of single crystals forming the multi-crystal. As a result, the emissivity factor becomes higher and, in turn, raises the heating power. Electron and ion bombardment give the same result. Filament evaporation leads to the formation of a film on the back side of the cathode. Since the emissivity factor of the coating is less, heat exchange between the filament and the cathode decreases. The temperature of the cathode decreases in places where film exfoliation takes place. The mentioned effect defines the durability of a cathode-filament couple. There is another effect due to evaporation of the filament. The voltage on the filament increases by about 1% during every thousand hours at 1600°C. 2) Coating of the cathode surface with materials from the walls of the vacuum chamber, or with carbon of destroyed organic compounds, or with material of the focusing electrode evaporated during breakdowns, increases the value of the work function and heating power. In this case, ion and electron bombardment play a positive role by cleaning the surface from dust. 3) During breakdowns between the electrodes and the cathode surface at high power, the temperature at the place of the breakdown is so high that the cathode materials melts and cracks. The fixtures of the other metal seem to be useless because boron will easily destroy them. 4) The process of fast destruction is very typical for the point of connection of LaB 6 with refractory metals, such as tantalum and rhenium, because of the boron penetrating into them. The lifetime of the connection is only 200-300 h at a cathode temperature of 1600°C for tantalum and even less for the other materials. High-quality graphite is clamped between a LaB 6 plate and a holder to increase the lifetime of the gun. This type of connection operates at 1600°C for 3000 h. It should be mentioned that, in this case, graphite im-
pregnated with boron will easily fail on contact with air. The use of graphite fabric [11] seems to be attractive, but it does not make the gun lifetime much longer because boron can move over the graphite surface or even through a narrow gap between two closely connected graphite parts. 5) The nonuniformity of temperature distribution over the surface leads to a change in the geometrical shape of the cathode. For the cathode, 30 mm in diameter a temperature difference exceeding 30°C over the surface is unsuitable. A cathode made of material based on Ir requires especially good uniformity of temperature distribution because of the thickness (or, to be more precise, thinness) of the cathode due to the high cost of Ir itself. The lifetime of the gun is dependent on a combination of several factors. Fig. 9 illustrates the lifetime of the gun with LaB 6 and Ir-based cathodes that worked in practice at INP installations. For LaB 6 cathodes of large diameter, evaporation of a layer 200 t~m thick is permissible. During operation the heat characteristics change by several percent, the emissivity of the surface rises, and the cathode itself and its support deform. Then its time to change the gun.
o•10~ 10~
% ~
J
10z
10 -1
1
LaBe ctB6 100ur~ ~ O ~ m 10
10 2
Current; density (A/c~ 2)
Fig. 9. Gun lifetime with LaB 6 and Ir-based cathodes. II. CONTRIBUTIONS
208
G. Kuznetsov / Nucl. Instr. and Meth. in Phys. Res. A 340 (1994) 204-208
Ir-based c a t h o d e s n e e d a good vacuum. T h e straight line in Fig. 9 displays the data relate to an Ir cathode, 30 m m in diameter, operating in vacuum at a b o u t 10 7 Torr, with a short c u r r e n t pulse regime without ion b a m b a r d m e n t . T h e separately displaced points show the data for the gun working u n d e r conditions similar to those for LAB6: v a c u u m a b o u t 10 6 Torr, pulse durability 12 ixs, a n d ion b o m b a r d m e n t . T h e lifetime of such a c a t h o d e is 2 - 3 times longer t h a n that of the LaB 6 cathode. To conclude, it should be n o t e d that the lifetime of these c a t h o d e s is similar to the lifetime of oxide and dispenser c a t h o d e s t h a t o p e r a t e in b a k e o u t systems.
[6]
[7]
[8]
[9]
References [1] J. Lafferty, J. Appl. Phys. 22 (1951) 299. [2] E.K. Storms and B.A. Mueller, J. Appl. Phys. 50 (1979) 3691. [3] M. Futamoto, M. Nakazava, K. Usami, S. Hosoki and U. Kowabe, J. Appl. Phys. 51 (1980) 3869. [4] E.K. Storms, J. Appl. Phys. 50 (1979) 4450. [5] G.M. Kuznetsova, G.A. Kudintseva and L.I. Suchkov, High-performance Cathodes Based on LaB 6. Electronic
[10] [11] [12]
[13]
Technique, Ser. 16, Generatornye, Modulatornye i Rentgenovskiye Pribory, vol. 1, 1969, p. 76. S.S. Ordanjan, E.E. Nikolaeva, I.K. Horoshilova and E.N. Ostrovsky, in: Sixth All-Union Conf. on High Current Electronics, vol. 1 (Tomsk, 1986) p. 58. S.E. Rozhkov, O.K. Kultashev and A.A. Gugnin, Technical Characteristics of Thermionic Emitters Based on Alloy Ir with La, Ce, Pr. Electronic Technique, Ser. 16, Generatornye, Modulatornye i Rentgenovskiye Pribory, vol. 2, 1969, p. 81. V.S. Kresanov, N.P. Malahov, V.V. Morozov, N.N. Semashko and V.Ja. Shljuko, High-performance Electron Emitter Based on LaB~ (Energoatomizdat, Moscow, 1987). V.S. Neshpor, B.A. Fridlender and Ju.B. Paderno, The Heat- and Electro-conductivity of Monolithic LaBs, at High Temperature. Teplofizika Vysokikh Temperatur, 1976, p. 903. V.S. Fomenko, Thermionic Emission Properties of Materials (Naukova dumka, Kiev, 1981). M.D. Goebel et al., Rev. Sci. Instr. 56 (1985) 1717. G.A. Kudintseva, A.I. Melnikov, E.V. Morozov and B.P. Nikonov, Thermionic Cathodes (Energiya, Moscow, 1966). M.E. Herniter and W.D. Getty, IEEE Trans. Plasma Sci. PS-15 (1987) 351.