International Vacuum Electronics Conference and Vacuum Electron Sources Conference : IVEC-IVESC 2012 : Monterey, California, April 24-26, 2012

2 Pages·2012·0.212 MB·English

by  IEEE Electron Devices Society

Metal Alloy Cathodes for Application in Vacuum Microwave Devices

Boris Ch. Djubua, Oleg K. Kultashev, Anatoly P. Makarov, Olga V. Polivnikova,
Evgeny M. Zemchikhin
FSUE Istok, Fryazino, Moscow reg., Vokzalnaya str., 2a, 141190, Russia
Fax: [phone], e-mail: [email protected]

Abstract: Metal alloy cathodes based on alloys Pt-Ba, The working temperature of these cathodes in magnetron
Pd-Ba, Ir-La, Ir-Ce, Os-Th and Re-Th are described. amplifiers lies in the range 300÷700 °C that is connected
These metal alloy cathodes have a unique combination of with the necessity of maintenance of the stable value of
emission, thermal and physical properties and found a wide their secondary electron emission in the conditions of
application in vacuum microwave devices (traveling-wave electronic and ionic bombardment of the surface of
tubes, backward-wave oscillators, klystrons, magnetrons). cathodes in these devices. The top value of the working
Cathode design of these metal alloy cathodes is described temperature of these cathodes ensures their functioning
and will be presented. without appreciable presence of thermionic current, and the
bottom value of the working temperature of these cathodes
Keywords: thermionic and secondary electron emission provides their steady work in the conditions of technical
cathodes, metal alloys, cathode design vacuum in a magnetron and evaporation from the anode.
These metal alloy cathodes have a cylindrical form by
Thanks to unique combination of emission capability and design.
thermal properties metal alloy cathodes based on Pt-Ba, Pd- Cathodes based on Ir-La, Os-Th or Re-Th metal alloys
Ba, Ir-La, Ir-Ce, Os-Th or Re-Th alloys have found wide have found wide application as thermionic and secondary
application in vacuum microwave devices of O- and M- electron emission cathodes in magnetron oscillators.
types (traveling-wave tubes, backward-wave oscillators,
Cathodes based on metal alloy Ir-La in magnetron
klystrons, magnetrons). Metal alloy cathodes have high
oscillators provide selection of cathode current in a pulse
thermionic and secondary electron emission properties and
mode from 10÷150 A/cm2 and long life stability of
at the same time have a number of properties characteristic
1000÷10000 hours. Cathodes based on metal alloy Os-Th
for pure metals: high electric conductivity and heat
or Re-Th in magnetron oscillators provide selection of
conductivity, smooth surface, stability to action of electron
cathode current in a pulse mode up to10 А/сm2 and long
bombardment [1-4].
life stability more than 10000 hours. Metal alloy cathodes
The structure of these metal alloys with an additive of based on metal alloys Ir-La, Os-Th or Re-Th have the
0.5÷10% atoms of electropositive metal has two phases. maximum value of secondary electronic emission
One phase consists of solid solution electropositive metal in coefficient (at primary electron energy 600÷800 eV) 2.5;
refractory metal (metal of platinum group or rhenium), and 2.07 and 1.95 accordingly, and the values of work function
the second phase represents chemical compounds of of their exit are equal to 2.7 eV; 3.08 eV and 3.09 eV
refractory metal with electropositive metal. The chemical values accordingly (at temperature of cathodes 1300 °C) .
compound in a metal alloy is a source of active The working temperature of these cathodes in magnetron is
electropositive metal which at working temperature of the 1300÷2000 °C. Such cathodes represent either a cylinder,
cathode covers the surface of the refractory metal with a or a spiral by design.
monatomic film using of diffusion and migration from
Metal alloy cathodes based on Ir alloys with atoms of rare-
alloy volume. The excellent emission capability and long
earth metals (La, Ce) and an additive of refractory metals
life stability of these metal alloy cathodes are attributed to a
(W, Re) are used as a thermionic emitter in vacuum
monolayer of electropositive metal atoms that uniformly
microwave O- type devices. A mechanism of operation and
covers the cathode surface, emission and adsorption
ageing of these metal-alloy cathodes is set which allows
properties of this monolayer and the thermodynamics
calculate the life stability of such cathodes depending on
properties of these metal alloys [5-9].
their operating parameters. The low evaporation velocity of
Metal alloy cathodes based on Pt-Ba, Pd-Ba alloys are used rare earth metal atoms from these metal alloy cathodes at
as "cold" secondary electron emission cathodes in working temperature is connected with the high energy of
magnetron amplifiers. These metal alloy cathodes have the formation of chemical compounds of these rear-earth
maximum value of secondary electron emission coefficient metals with iridium.
(at primary electron energy 600÷800 eV) 3.0 and 2.6
The energy of formation of the compounds Ir La and IrCe
accordingly and the values of the work function are equal 2 5
has values of 2.2 eV per one La atom and 2.5 eV per one
to 2.2 eV and 2.5 eV accordingly at the temperature
Ce atom [5]. The excellent emission capabilities of these
1000°C of cathodes [3].
978-1-4673-0369-9/12/$31.00 © 2012 IEEE 177
metal alloy cathodes are also provided by the following References
properties of such metal alloys:
1. Djubua, B. Ch., L. A. Ermolaev, O. K. Kultashev,
 low work function of monolayer film of lanthanum or “Emission properties of alloys Pt-Th, Ir-Th, Os-Th,
cerium on iridium substrate (2.7÷2.9 eV within Re-Th,” Radiotekhnika i Elektronika, Vol. 11, no.
1600÷1800 °C temperature range), 11, p.1149, 1966 (in Russian).
 high adsorption or energy of rare earth atoms near 2. Vasileva, E.V., B. Ch. Djubua, L. A. Ermolaev, O.
monolayer concentration on the surface of these K. Kultashev, “Emission properties of alloys Pt-La,
alloys (e.g., 6 eV for cerium atoms on the surface of Ir-La, Os-La,” Radiotekhnika i Elektronika, Vol.
its alloy with iridium), 11, no. 11, p. 1150, 1966 (in Russian).
3. Djubua, B. Ch., L. A. Ermolaev, N. P. Esaulov, I.
 high stability of emission properties of these cathodes
P. Starodubov, “Electron emission of alloys Pt-Ba,
to electron and ion bombardment of the emitting
Pd-Ba, Au-Ba,” Radiotekhnika i Elektronika, Vol.
surface of these alloys.
12, no. 8, pp. 1523, 1967 (in Russian).
These metal alloy cathodes provide an emission current
4. Rozhkov, S. E., O. K. Kultashev, A. A. Gugnin,
with 100 A/cm2 density and life stability more than 1000
“Emission properties investigation of emitters
hours and at the emission current density 10 A/cm2 the life
based on alloys of iridium with rare earth metals,”
stability of cathodes is more than 10000 hours [4-6].
Electronics technique, S. Generators and X-ray
The metal alloy cathodes based on Ir alloys with atoms of devices, Vol. 16, no. 2, pp. 81-83, 1969 (in
rare-earth metals (La, Ce) and an additive of refractory Russian).
metals (W, Re) are either directly heated designs or
5. Rozhkov, S. E., and O. K. Kultashev,
electron heated cathode designs. The directly heated
“Thermodynamic properties of metal alloys,” Baku,
cathodes have a miniature cathode body, filament from W
pp. 95-96, 1975 (in Russian)
or W-Re wire and ceramic mount with four heater leads.
The cathode body emitting surface usually is a circle or 6. Kultashev, O. K., and B. Ch. Djubua, “Miniature
square with 0.5÷3 mm size. The filament is connected with Metal Alloy Thermionic Cathodes,” Physica
the cathode body and four heater molybdenum leads of Spectra, Vol. 71, pp. 127-129, 1997.
metal ceramic leg by laser welding. Metal alloy cathodes
7. Djubua, B. Ch., V. N. Ilyin, O.V. Polivnikova, E.
with 4÷6 mm emitting surface diameter are heated by
M. Zemchikhin, “Spiral cathodes for microwave
electron bombardment method. The electron heated
heating magnetrons,” Appl. Surface Science, Vol.
cathode units have a main cathode and a supplementary
111, pp. 99-100, 1997.
cathode. The supplementary cathode is also manufactured
from metal alloys Ir-La or Ir-Ce. The supplementary 8. Djubua, B. Ch., E. M. Zemchikhin, F. P. Makarov,
cathode is usually a directly heated cathode. The anode O. V. Polivnikova, “Secondary electron emission,
potential is applied between the main and supplementary surface composition and modes of activation of
cathodes. The electron beam is concentrated onto the main metal alloy cathodes,” Appl. Surface Science, Vol.
cathode back surface resulting in cathode heating. 111, pp. 285-287, 1997.
9. Djubua, B. Ch., O. V. Polivnikova, “Stratum-like
In the recent years these metal alloy cathodes were also
structured metal alloy cathode,” Appl. Surface
used as miniature electron sources for electron lithographic
Science 215, pp. 242-248, 2003.
processes, in electron scanning microscopes and Auger
spectrometers, in devices for getting multiple-charged ions
and as filed emission cathodes [9].

178