rus
Issue #7-8/2022
I.P.Li, V.I.Kapustin, N.E.Ledentsova, A.V.Shumanov
DESIGN FEATURES OF THREE-MODULE CATHODE HEATING UNITS IN THE INSTANTANEOUSLY READY MAGNETRONS
DESIGN FEATURES OF THREE-MODULE CATHODE HEATING UNITS IN THE INSTANTANEOUSLY READY MAGNETRONS
STATEMENT OF THE TASK
The currently available instantaneously ready magnetrons are characterized by a relatively low, no more than 20–25 kW, pulse power of the generated microwave oscillations. These devices use standard two-module cathode assemblies (Fig.1a) consisting of the alternating auto-electron and secondary emission cathodes [1–4]. The auto-electron cathodes responsible for initiating the onset of generation are tantalum foil washers. Pressed palladium-barium cathodes are used as secondary emission cathodes (SECs) in these assemblies which, along with their main purpose, should provide continuous, pulse to pulse, activation of the auto-electron cathodes. In magnetrons with such design of the cathode-heating units (CHUs) the secondary emission cathode is bombarded by return electrons, which can result in the irreversible changes in both structure and composition of the near surface layer of the emitting surface causing together degradation of the emission properties. In addition, if the energy of the bombarding electrons is high enough, the cathode may overheat and change its geometric dimensions. For this reason the standard dual-module design of the CHU in high-power magnetrons with a pulse output of more than 50 kW may not provide necessary durability.
Microwave oscillators with completely new tactical and technical characteristics, in particular, small-sized magnetrons with instantaneous readiness and pulse power of several hundred kilowatts are necessary for manufacturing modern high-speed complexes, designed for location and tracking of high-speed objects (missiles, aircraft, drones, etc.) with a large action range. The readiness time of commercially available high-power magnetrons, which use single-module CHUs (incandescent magnetrons), is 3–5 minutes and more. Therefore, the task of manufacturing magnetrons with instant availability and an output power exceeding 00–150 kW is extremely relevant and pressing. However, the standard approach to solving this problem by designing a new type of device or improving the design of existing magnetrons is too time consuming and expensive.
The research carried out in recent years by employees of the Technical Centre "Basic Technologies EVP" JSC "Pluton" showed that such a problem, for the first time in world practice, can be successfully solved by commercially available powerful magnetrons with incandescent cathode that include unique cathode unit: a three-module CHU whose operation is based on a combination of three types of emission: thermal emission, auto-electronic, and secondary emission. The following definitions are introduced for the three-module design of the CHU: OEC – the first module; OEC activator being the second module, and SEC – the third module. A fundamental feature of the three-module CHU (Fig.1b) is the division of cathodes into two functional elements, each one performing a well-defined, specific role. One of the elements of the CHU design is the auto-emission block (Fig.1c) which includes the auto-electron cathode with OEC activators symmetrically placed on both sides of the cathode, and the second functional element is a high-efficiency secondary emission cathode. In this case the auto emission units (AEUs) serve as a source of primary electrons, initiating the start of magnetron generation, while the SEC, due to its properties, ensures maintenance of the generation mode throughout the lifetime of the device. The auto-electronic cathode is made of tantalum foil and the activators are made, for example, by pressing and sintering palladium powder or a mixture of palladium powders with the Pd5Ba phase. Different types of SECs characterized by high form stability, stable thermal and secondary-emission properties with σмах ≥ 2.5 – 3.0, sufficient resistance to influence of reverse electron bombardment, operating temperature range within 900–1050 °С, etc. can be used. The proposed design of the three-module CHU consists of alternating auto-emission blocks and secondary emission cathodes – the optimum distance between the AEUs should be within 0.5...1.0 mm.
FUNCTIONAL FEATURES OF AEU OPERATION
Stability of the auto-emission unit is based on electrons emission from the activated barium oxide crystallites of palladium whiskers formed on the end surface of the AEC as a result of special activation of the CHU in electric field [5–10].
The studies carried out in magnetron mockups and electron microscopy instruments confirmed that auto-electron emission current is predominantly generated by the emission current from palladium whiskers formed on the end surface of the AEC during activation of the CHU. Figure 2 shows a microphotograph of a whisker structure on AEC surface, and Fig.3 – signal intensity distribution of tantalum, barium and palladium on the end face obtained by X-ray fluorescent analysis at different values of probing beam electron energy. With increasing probing electron energy the signal intensity of tantalum increases in parallel with a simultaneous decrease of palladium and barium signals indicating the palladium nature of the whiskers.
An estimation of AEU emissivity was carried out using a special pulse high-voltage power supply operating in the following modes: amplitude value Uа = 5,0 kV; duty cycle Q = 1000; speed of a linear sweep of high-voltage 200 V/s, pulse duration of voltage discrete t = 0.5, 1, and 6 μs.
The measurement of the auto-electron emission current was carried out using the electrical circuit (device) shown in Fig.4, and a typical volt-ampere characteristic (VAC) of the magnetron is shown in Fig.5.
Stability of the auto-emission properties of the three-module CUU was evaluated by measuring the emission current decay value at fixed voltage, for example, at Uа = 4.5 kV (Fig.6).
Figure 7 shows dependencies of the emission current decay rates of the three-module CHU (1) and two-module CHU (2).
Based on the analysis of the characteristics shown in Figs.6 and 7, it follows that the three-module CHUs have a distinct advantage over the two-module units. In particular, other things being equal, the rate of auto-electron emission current decrease in three-module CHU is 6 times slower than in the two-module, and the auto-electron emission current level in the characteristic point of this node is more than twice higher than in its counterpart.
SECONDARY EMISSION CATHODE
In addition to palladium-barium cathodes, platinum-barium cathodes, metal-porous impregnated or pressed tungsten-aluminate cathodes (matrix-type cathodes, MTC), similar cathodes treated by pulsed plasma currents can be used in three-module CHUs, pressed oxide-nickel cathodes (PONC) with agglomerated emission-active component 6, production technology of which doesn’t provide for use of binders based on organic compounds and others.
It should be noted that while standard dual-module CHU with palladium-barium SEC has temperature limit not exceeding 950–970 °С, the temperature of three-module CHU can be increased up to 1050–1070 °С. This is explained by the fact that the three-module CHU uses compounds as the secondary emission cathode material, which have high form stability, and the change of geometric dimensions of the activator, up to 5–7%, does not imply worsening of the electric parameters of magnetrons. This is the most important property of three-module CHUs which allows of full activation of cathode nodes where MTC, PONC or other types of SECs are used as the third module.
TESTING TRIPLE-MODULE CHU IN MAGNETRONS
The component production technology and assembly procedures of the three-module CHU for the different magnetron types are standard.
Two devices of 2–3 centimetre wavelength range based on a self-heated magnetron with a standard dual-module CHU of the nominal pulse power of 7.5–8.0 kW were fabricated for the study. Tantalum foil washers of standard thickness were used as AEC, and activators of 0.2 mm thickness were made by powder technology from a mixture of palladium powders and Pd5Ba phase with Ba concentration of ~ 2% wt. The secondary emission cathode module was made of porous tungsten with porosity 28±1% impregnated with aluminate of composition 3BaO · 0,5CaO · Al2O3, obtained by co-deposition method. Compliance tests of the devices allowed of obtaining the following results: impulse power 9.5 kW (the first device) and 9.7 kW (the second device), the fluctuations of microwave oscillations is less than 1%. The obtained parameters of the devices are fully compliant with the necessary requirements.
In order to assess a possibility of creating high-power magnetrons with a self-heated start, an experimental device based on a heated magnetron with a metal-porous single-module cathode with a rated pulse power of 110–150 kW, in which the incandescent MTC was replaced by a three-module CHU, was produced. In this node the activators made of PdB-2 alloy were used. Metal-porous tungsten-aluminate emitters were also used as SECs. When studying electrical parameters, the following results were obtained: readiness time less than 0.5 s; pulse power P = 65–70 kW; complete absence of doubling of the UHF oscillation spectrum; no faulty start of device in the generation mode under cold conditions at temperature T = 60 °C.
By increasing the anode voltage to its nominal value, the pulse power was increased to 140 kW, but it resulted in doubling of the microwave spectrum. This undesirable factor occurred due to insufficient cathode temperature, which heating is only provided by return electrons bombardment in the absence of a preheater filament. Based on the CHU design and manufacturing technology, the problem could be solved, for example, by reducing the heat loss due to introduction of thermal isolators at the core of the cathode, or by replacing the MTC by a highly efficient pressed nickel oxide cathode.
CONCLUSIONS
The results confirmed a principle possibility of creating the series of small magnetrons with a self-heated start and a wide range of pulse power (from tens to several hundred kW) without any substantial redesign or upgrade of the design of devices by replacing the cathode preheating units used in the magnetrons with three-module cathode heating units provided a certain selection of modules was accomplished.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The currently available instantaneously ready magnetrons are characterized by a relatively low, no more than 20–25 kW, pulse power of the generated microwave oscillations. These devices use standard two-module cathode assemblies (Fig.1a) consisting of the alternating auto-electron and secondary emission cathodes [1–4]. The auto-electron cathodes responsible for initiating the onset of generation are tantalum foil washers. Pressed palladium-barium cathodes are used as secondary emission cathodes (SECs) in these assemblies which, along with their main purpose, should provide continuous, pulse to pulse, activation of the auto-electron cathodes. In magnetrons with such design of the cathode-heating units (CHUs) the secondary emission cathode is bombarded by return electrons, which can result in the irreversible changes in both structure and composition of the near surface layer of the emitting surface causing together degradation of the emission properties. In addition, if the energy of the bombarding electrons is high enough, the cathode may overheat and change its geometric dimensions. For this reason the standard dual-module design of the CHU in high-power magnetrons with a pulse output of more than 50 kW may not provide necessary durability.
Microwave oscillators with completely new tactical and technical characteristics, in particular, small-sized magnetrons with instantaneous readiness and pulse power of several hundred kilowatts are necessary for manufacturing modern high-speed complexes, designed for location and tracking of high-speed objects (missiles, aircraft, drones, etc.) with a large action range. The readiness time of commercially available high-power magnetrons, which use single-module CHUs (incandescent magnetrons), is 3–5 minutes and more. Therefore, the task of manufacturing magnetrons with instant availability and an output power exceeding 00–150 kW is extremely relevant and pressing. However, the standard approach to solving this problem by designing a new type of device or improving the design of existing magnetrons is too time consuming and expensive.
The research carried out in recent years by employees of the Technical Centre "Basic Technologies EVP" JSC "Pluton" showed that such a problem, for the first time in world practice, can be successfully solved by commercially available powerful magnetrons with incandescent cathode that include unique cathode unit: a three-module CHU whose operation is based on a combination of three types of emission: thermal emission, auto-electronic, and secondary emission. The following definitions are introduced for the three-module design of the CHU: OEC – the first module; OEC activator being the second module, and SEC – the third module. A fundamental feature of the three-module CHU (Fig.1b) is the division of cathodes into two functional elements, each one performing a well-defined, specific role. One of the elements of the CHU design is the auto-emission block (Fig.1c) which includes the auto-electron cathode with OEC activators symmetrically placed on both sides of the cathode, and the second functional element is a high-efficiency secondary emission cathode. In this case the auto emission units (AEUs) serve as a source of primary electrons, initiating the start of magnetron generation, while the SEC, due to its properties, ensures maintenance of the generation mode throughout the lifetime of the device. The auto-electronic cathode is made of tantalum foil and the activators are made, for example, by pressing and sintering palladium powder or a mixture of palladium powders with the Pd5Ba phase. Different types of SECs characterized by high form stability, stable thermal and secondary-emission properties with σмах ≥ 2.5 – 3.0, sufficient resistance to influence of reverse electron bombardment, operating temperature range within 900–1050 °С, etc. can be used. The proposed design of the three-module CHU consists of alternating auto-emission blocks and secondary emission cathodes – the optimum distance between the AEUs should be within 0.5...1.0 mm.
FUNCTIONAL FEATURES OF AEU OPERATION
Stability of the auto-emission unit is based on electrons emission from the activated barium oxide crystallites of palladium whiskers formed on the end surface of the AEC as a result of special activation of the CHU in electric field [5–10].
The studies carried out in magnetron mockups and electron microscopy instruments confirmed that auto-electron emission current is predominantly generated by the emission current from palladium whiskers formed on the end surface of the AEC during activation of the CHU. Figure 2 shows a microphotograph of a whisker structure on AEC surface, and Fig.3 – signal intensity distribution of tantalum, barium and palladium on the end face obtained by X-ray fluorescent analysis at different values of probing beam electron energy. With increasing probing electron energy the signal intensity of tantalum increases in parallel with a simultaneous decrease of palladium and barium signals indicating the palladium nature of the whiskers.
An estimation of AEU emissivity was carried out using a special pulse high-voltage power supply operating in the following modes: amplitude value Uа = 5,0 kV; duty cycle Q = 1000; speed of a linear sweep of high-voltage 200 V/s, pulse duration of voltage discrete t = 0.5, 1, and 6 μs.
The measurement of the auto-electron emission current was carried out using the electrical circuit (device) shown in Fig.4, and a typical volt-ampere characteristic (VAC) of the magnetron is shown in Fig.5.
Stability of the auto-emission properties of the three-module CUU was evaluated by measuring the emission current decay value at fixed voltage, for example, at Uа = 4.5 kV (Fig.6).
Figure 7 shows dependencies of the emission current decay rates of the three-module CHU (1) and two-module CHU (2).
Based on the analysis of the characteristics shown in Figs.6 and 7, it follows that the three-module CHUs have a distinct advantage over the two-module units. In particular, other things being equal, the rate of auto-electron emission current decrease in three-module CHU is 6 times slower than in the two-module, and the auto-electron emission current level in the characteristic point of this node is more than twice higher than in its counterpart.
SECONDARY EMISSION CATHODE
In addition to palladium-barium cathodes, platinum-barium cathodes, metal-porous impregnated or pressed tungsten-aluminate cathodes (matrix-type cathodes, MTC), similar cathodes treated by pulsed plasma currents can be used in three-module CHUs, pressed oxide-nickel cathodes (PONC) with agglomerated emission-active component 6, production technology of which doesn’t provide for use of binders based on organic compounds and others.
It should be noted that while standard dual-module CHU with palladium-barium SEC has temperature limit not exceeding 950–970 °С, the temperature of three-module CHU can be increased up to 1050–1070 °С. This is explained by the fact that the three-module CHU uses compounds as the secondary emission cathode material, which have high form stability, and the change of geometric dimensions of the activator, up to 5–7%, does not imply worsening of the electric parameters of magnetrons. This is the most important property of three-module CHUs which allows of full activation of cathode nodes where MTC, PONC or other types of SECs are used as the third module.
TESTING TRIPLE-MODULE CHU IN MAGNETRONS
The component production technology and assembly procedures of the three-module CHU for the different magnetron types are standard.
Two devices of 2–3 centimetre wavelength range based on a self-heated magnetron with a standard dual-module CHU of the nominal pulse power of 7.5–8.0 kW were fabricated for the study. Tantalum foil washers of standard thickness were used as AEC, and activators of 0.2 mm thickness were made by powder technology from a mixture of palladium powders and Pd5Ba phase with Ba concentration of ~ 2% wt. The secondary emission cathode module was made of porous tungsten with porosity 28±1% impregnated with aluminate of composition 3BaO · 0,5CaO · Al2O3, obtained by co-deposition method. Compliance tests of the devices allowed of obtaining the following results: impulse power 9.5 kW (the first device) and 9.7 kW (the second device), the fluctuations of microwave oscillations is less than 1%. The obtained parameters of the devices are fully compliant with the necessary requirements.
In order to assess a possibility of creating high-power magnetrons with a self-heated start, an experimental device based on a heated magnetron with a metal-porous single-module cathode with a rated pulse power of 110–150 kW, in which the incandescent MTC was replaced by a three-module CHU, was produced. In this node the activators made of PdB-2 alloy were used. Metal-porous tungsten-aluminate emitters were also used as SECs. When studying electrical parameters, the following results were obtained: readiness time less than 0.5 s; pulse power P = 65–70 kW; complete absence of doubling of the UHF oscillation spectrum; no faulty start of device in the generation mode under cold conditions at temperature T = 60 °C.
By increasing the anode voltage to its nominal value, the pulse power was increased to 140 kW, but it resulted in doubling of the microwave spectrum. This undesirable factor occurred due to insufficient cathode temperature, which heating is only provided by return electrons bombardment in the absence of a preheater filament. Based on the CHU design and manufacturing technology, the problem could be solved, for example, by reducing the heat loss due to introduction of thermal isolators at the core of the cathode, or by replacing the MTC by a highly efficient pressed nickel oxide cathode.
CONCLUSIONS
The results confirmed a principle possibility of creating the series of small magnetrons with a self-heated start and a wide range of pulse power (from tens to several hundred kW) without any substantial redesign or upgrade of the design of devices by replacing the cathode preheating units used in the magnetrons with three-module cathode heating units provided a certain selection of modules was accomplished.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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