Cathode-grid knot with hot control grid for vacuum microwave devices

Abstract

FIELD: electronic engineering.SUBSTANCE: design of cathode-grid knots (CGK) for vacuum tube devices, including those in the microwave wavelength range. The CGK contains a thermionic impregnated cathode, shadow and control grids with annular and radial jumpers, permanently attached to cylindrical grid holders. On the annular sections of both grids, beyond the junction of the radial jumpers with them, along a circumference of radius Rsh, approximately coinciding with the radius of the shadow grid fastening, annular slots are cut out, separated by partitions with a total cross-sectional area less than the total cross-sectional area of the radial jumpers.EFFECT: increase in the reliability of the CGK by reducing the parasitic current from the control grid and increasing the electrical strength, as well as by enabling visual inspection of welded points when attaching the shadow grid to the grid holder.1 cl, 5 dwg

Description

[0001] The invention relates to electronic engineering, in particular to the design of cathode-grid units (CGU) for vacuum tube devices, including those in the microwave wavelength range.

[0002] Cathode-grid units are used in microwave vacuum tube devices, such as traveling wave tubes (TWTs) and klystrons, to form an electron flow with a given current and a given transverse size, as well as to control the magnitude of the current emitted by the cathode up to its complete cutoff (pulse devices) by changing the voltage on the control grid [1].

[0003] One of the requirements that grid structures must satisfy is a minimum value of parasitic (undesirable for the device) thermionic emission (TEE) current from the control grid at the moment of current cutoff from the cathode, when the control grid has a potential below the cathode potential. The parasitic TEE current can lead to a loss of electrical strength of the interelectrode gaps and to the occurrence of energy exchange between the grids, which leads to melting of the grid structure and loss of control. [2]

[0004] According to the Richardson-Dashman equation, the current density of the TEE from any surface is determined by the surface temperature and its electron work function. The control grid temperature is also determined by the design of the control system and the thermal radiation of the impregnated hot cathode heated to a high temperature. The current density from the cathode surface is (5-10) A / cm 2is achieved at a cathode temperature of (1080-1100)°C, while the temperature of the control grid bridges decreases from the center to the periphery from 780°C to 710°C. Ba and BaO intensively evaporate from the cathode surface in a ratio of 90 / 9 with an electron work function of 2.5 eV and (1.0-1.6) eV, respectively, are adsorbed on the control grid surface and initiate a TEE current even at the specified control grid temperature [3]. As a result of the chemical interaction of Ba and BaO with the grid material and residual gases from the electron gun region adsorbed on its surface, a film with an electron work function significantly lower than the electron work function of the grid material itself is formed on the control grid surface, which is the main reason for the appearance and development of the TEE current from the control grid.

[0005] To reduce the parasitic current of the TEE during dynamic testing of the TWT, the control grid is subjected to forced additional thermal heating by depositing an electron beam with a continuous power of up to 8 W for 45 minutes, applying a positive potential relative to the cathode to the grid [2]. The power of the electron beam is selected experimentally. It should be such that the grid temperature is sufficient for desorption from its surface of a portion of the adsorbed film (or reduction of its thickness), which has low binding energy and electron work function values, in order to prevent melting of the grid bridges. However, during subsequent operation, this measure does not lead to the complete elimination of parasitic thermionic current from the control grid due to the restoration of a film of the previous thickness on the grid surface by Ba and BaO continuously evaporating from the cathode surface and their adsorption on the grid surface.The parasitic current is restored to its previous level within several hours or tens of hours and continues to increase over time, until the electrical strength of the grid-cathode gap is lost.

[0006] A known design of a vacuum tube with a grid intercepting (single) the cathode current, located near the emitting surface of the thermionic cathode [4]. The disadvantage of this design is the interception of the electron flow emitted by the cathode. The magnitude of the power of the intercepted electron flow dissipated on the grid is directly proportional to the cathode current density, the area of ​​the grid jumpers and its potential. In vacuum tube devices with a cathode current density of more than 1 A / cm 2(with an output power of over 70-80 W), the grid can heat up to a temperature close to the melting point of the grid material and melt [1]. Another drawback of this design is the occurrence of thermal emission from the grid surface when it reaches high temperatures. This creates fundamental limitations, as the resulting parasitic current can lead to device failure.

[0007] A known design of a control grid control system for high-power devices is one in which the control grid jumpers are protected from direct current interception from the cathode by shadow grid jumpers placed between the emitting surface of the cathode and the control grid [5]. The shadow grid jumpers are located near the emitting surface of the cathode. A disadvantage of this design is that evaporation products from the cathode surface, such as Ba and BaO with an electron work function of 2.5 eV and (1.0-1.6) eV, respectively, freely migrate to the shadow grid jumpers, reducing the work function of the jumper surface, which becomes a source of electrons bombarding the control grid jumpers in the operating mode of the device, which can lead to their melting.

[0008] The closest technical solution to the proposed invention is a KSU design with a thermionic impregnated cathode with shadow and control grids made of hafnium. In the operating mode of the KSU, the shadow grid is separated from the emitting surface of the cathode by a distance of 30-50 μm [7].

[0009] The shadow and control grids are disk-shaped with a spherical portion concave toward the cathode, bounded by an annular portion at its periphery with an inner diameter equal to the cathode diameter. Before forming the bridges (electrical discharge machining of the grids), the cups were detachably attached by laser welding to cylindrical molybdenum grid holders. Next, the grids were electrically discharge machined together to form self-aligned bridges, resulting in the control grid bridges being positioned in the geometric and electrical shadow of the shadow grid bridges. In the operating mode of the control grid control system, the grids are heated by energy emitted by the high-temperature cathode. Heat is transferred from both grids to the annular portion of the grid and then to the grid holder through radial bridges at their junctions with the annular portion of the grid.

[0010] A disadvantage of using this design of the control system in high-power (output pulse power of 20 kW or more) pulsed TWTs is the excess current of the parasitic thermionic emission from the control grid, which leads to device failure. Thus, in an X-band TWT with an output pulse power of 20 kW, the maximum permissible value of thermionic emission from the control grid is 0.75 mA. A temporary reduction in the parasitic current of the thermionic emission from the control grid by 1-2 orders of magnitude was achieved by forcibly heating it with the power of the electron beam from the cathode to 8 W for 45 minutes, which is equivalent to an energy of ≈2.2⋅10 4 J.

[0011] The decrease in the TEE current after forced grid heating is apparently due to the desorption of Ba atoms and an increase in the electron work function from the adsorbed film remaining on the grid surface. After thermal treatment, the TEE current from the control grid increased again within a few hours to its previous value (lower in some devices). However, the trend of a slow increase in the TEE current from the grid persisted for most devices. The reason for the restoration of parasitic TEE from the control grid was the restoration of the film thickness due to the adsorption of barium atoms continuously evaporating from the cathode surface.

[0012] Another drawback of the KSU design under consideration is the location of the shadow mesh weld attachment points under the control mesh flange. Imperfect weld surfaces can cause electrical breakdown between the meshes, reducing the dielectric strength of the interelectrode gaps. It is also impossible to visually inspect the quality of the shadow mesh weld attachment points after installing the control mesh, which completely covers the welding zone. This lack of inspection creates the risk of inserting a component with a hidden defect into the product.

[0013] The objective of the present invention is to eliminate or reduce to an acceptable level and further stabilize the parasitic current of the control grid in a thermal emission impregnated cathode control unit (TEC) during the operating time of the device.

[0014] The technical result of this invention is to increase the reliability of the control system by reducing the parasitic current from the control grid and increasing the electrical strength, as well as by enabling visual inspection of welded points when attaching the shadow mesh to the mesh holder.

[0015] The technical result is achieved by increasing the control grid temperature and increasing the desorption rate of barium atoms from the surface during the operating mode of the control grid. This is achieved by reducing heat transfer from the grid bridges to its annular portion and then to the grid holder. This heat transfer is achieved by forming annular slits located circumferentially and separated by partitions beyond the bridge-to-grid interface. Furthermore, the desorption rate of barium atoms should be comparable to or greater than the adsorption rate of barium atoms on the grid surface throughout the entire device's operation. This results in the formation of a film on the grid surface with a higher electron work function than that of barium atoms, which is maintained throughout the device's operation. This reduces the parasitic current from the control grid.In addition, due to the presence of annular gaps, an increase in the electrical strength of the mesh structures is achieved by eliminating the localization of breakdown, as well as an increase in the reliability of fastening the mesh structures, due to the control of the welded points of the shadow mesh during the assembly of the control system.

[0016] The proposed invention is explained by drawings Figs. 1-5.

[0017] Fig. 1-4 show the design of meshes with annular slits in the KSU. The following positions are indicated on the drawings: 1 - annular bridges; 2 - radial bridges; 3 - annular part of the mesh; 4 - annular slits; 5 - partitions between the slits; 6 - mesh holder.

[0018] Fig. 1 and 2 show the geometry of the grid structure in the CCU, consisting of a spherical part concave towards the cathode with radial and annular bridges and a flat annular part: Fig. 1 - top view; Fig. 2 - section.

[0019] Fig. 3 shows the directions of propagation of the thermal energy flow from the radial jumpers of the mesh to its annular part and to the mesh holder through the partitions between the gaps.

[0020] Fig. 4 shows photographs of the control grid with annular slots in the control system at a cathode temperature of 1080°C: a) - without forced heating; b) - with forced heating of the grid by an electron beam with a power of 8 W.

[0021] The proposed invention is a heat-transfer system (HTS) comprising a thermionic impregnated cathode, shadow grids, and control grids with annular and radial bridges. The grids are permanently attached to cylindrical grid holders. Annular slits are cut into the annular sections of the control grid. The slits are located on the annular portion of the control grid outside the area where the radial bridges adjoin the annular portion of the grid, at a circumference of radius Rщ, which approximately coincides with the shadow grid attachment radius. The radial bridges mate with the annular portion of the grid at a radius of Rк. The width of the annular slits Δ does not affect the amount of heat dissipated from the grid bridges to the grid holder. The width of the slits and the radius of the circle on which they are located are selected such that, on the one hand, they are not located on the mesh holder: Rс.д.>Rщ+Δ / 2, where Rс.д. is the radius of the mesh holder.On the other hand, the width of the annular part of the mesh between the edge of the gap and the radius of the radial jumpers Rk ensures the mechanical strength of the central part of the mesh with jumpers: Rk. <rщ-δ 2.

[0022] The number of annular slits is selected such that the total cross-section of the partitions between the slits is less than the total cross-section of the radial bridges. Since the mesh thickness is the same over its entire surface, this condition can be formulated as follows: the total width of all partitions between the annular slits nd≤ND, where: n is the number of partitions between the annular slits; d is the width of the partition; N is the number of radial bridges; D is the width of the radial bridges. This condition ensures an increase in the temperature of the mesh bridges, since the entire heat flux from the radial bridges is distributed to the mesh holder through the partitions between the annular slits, which have a smaller total area than the total cross-sectional area of ​​all radial bridges.This is precisely the physical meaning of increasing the temperature of the mesh due to the increase in resistance to the heat flow from the mesh bridges to the mesh holder through the partitions between the annular gaps.

[0023] The effect of increasing the temperature of the control grid due to the presence of annular gaps in the design is visually observed at a higher temperature, which is created during the process of forced heating of the grid by the power of the electron beam.

[0024] In Fig. 4a, in the mode without forced heating, the temperature of the central annular bridge increased to 837°C, the temperature of the second annular bridge - to 830°C, and the temperature of the radial bridges at the place of their junction with the annular part of the grid - to 772°C. During forced heating of the grid with an electron beam with a power of 8 W, the temperature distribution along the grid radius was as follows: central annular bridge - 1118°C; central radial bridges - 1105°C; the second annular bridge and radial bridges to the middle of their length - 1054°C and the temperature of the radial bridges from the middle of their length to the junction with the annular part of the grid 1050°C (Fig. 4b).

[0025] For comparison, Fig. 5 shows photographs of the control grid without annular slots in a control grid control system of a traditional design - an analogue (without annular slots) at a cathode temperature of 1080°C: a) - without forced grid heating; b) - with forced grid heating with a power of 8 W. The control grid temperature at a cathode temperature of 1080°C from the center to the periphery is from 780°C to 710°C (Fig. 5).

[0026] During forced heating with an 8W electron beam, the central annular bridge reached a temperature of 1054°C, while the second annular bridge reached 1052°C. The bridge temperature decreased radially from the center to the periphery. At the midpoint of the radial bridges, behind the second ring, the temperature reached 1050°C.

[0027] Thus, the implementation of the present invention in a CCU design with a control grid with annular slits allowed for an increase in the control grid temperature by approximately 57°C-62°C compared to the control grid temperature without annular slits. This temperature increase was sufficient to increase the rate of barium desorption from the grid surface compared to the rate of barium adsorption, which continuously evaporates from the cathode surface. This allowed for a reduction in the adsorbed film thickness, an increase in the electron work function, a decrease in the grid current, and maintenance of this current during long-term TWT operation.

[0028] During dynamic tests of the TWT with a control grid having 6 annular slots with a width of Δ=300 μm and a partition width of d=1.2 mm, the parasitic current of the TEE did not exceed 60 μA and did not increase, and forced heating of the grid was not required.

[0029] List of sources

[0030] 1. Yu. A. Grigoriev., B. S. Pravdin., V. I. Shesterkin. Electron-optical systems with grid control / / Reviews of electronic engineering. Ser. 1. Microwave electronics. 1987. Issue 7 (1246). Moscow: Publishing house of the Central Research Institute of Electronics. 71 p. IT

[0031] 2. Bogachev R.Yu., Zhuravlev S.D., Krachkovskaya T.M., Rzhevin N.V., Tikhomirov D.A., Shesterkin V.I. Thermionic emission from control grids made of hafnium and anisotropic pyrolytic graphite in the control system of a high-power pulsed TWT / / Microwave Electronics and Microelectronics. Collection of articles from the XIV All-Russian Conference. St. Petersburg: ETU "LETI" Publishing House, June 2-6, 2025, pp. 195-199.

[0032] 3. L.J. Zhuge, H.M. Wu, FJ Qu, WB Wang, BY Jiang, XH Liu Study on the composition and phase of BaO-Hf-Mo under high temperature / / Journal of Physics and Chemistry of Solids 67 (2006) 1394-1398

[0033] 4. Pallakoff OE To grid or not to grid? / / Microwave J. 1974. Vol. 13. No. 5. P. 52-56

[0034] 5. Patent No. 3594885. 1971. George V. Miram. Method for fabricating a dimpled concave dispenser cathode incorporating a grid

[0035] 6. Belov AC, Sakhadzhi GV, Babanov Zh.N., Popov I.A. Cathode-grid units / / BULLETIN OF SARATOV STATE TECHNICAL UNIVERSITY Vol.4, No.1 (49), 2010, pp. 161-165.

[0036] 7. Glukhova O.E., Kolosov D.A., Shesterkin V.I., Krachkovskaya T.M., Zhuravlev S.D., Bogachev R.Yu. Influence of barium adsorption on the work function of hafnium grids of the cathode-grid unit / / JTF. 2024. Vol. 94. Issue 10. P. 1747-1755.

Claims

A cathode-grid unit containing a thermionic impregnated cathode, shadow and control grids with annular and radial jumpers permanently attached to grid holders in the form of a cylinder, characterized in that on the annular sections of both grids beyond the junction of the radial jumpers along a circumference of radius R щ , approximately coinciding with the radius of the shadow mesh fastening, annular slots of width Δ are cut out, separated by partitions of width d with a total cross-sectional area less than the total cross-sectional area of ​​the radial jumpers, and the radius at which the annular slots are formed, their width Δ and the width of the partitions d between the annular slots satisfy the relations: R с.д. >R щ +Δ / 2; R к <R щ -Δ / 2; nd≤ND; where: R с.д. - radius of the mesh holder, R щ - the radius at which the gaps are located, R к- radius of the junction of the radial bridges, n - number of partitions between the annular gaps; d - width of the partition; N - number of radial bridges; D - width of the radial bridges.

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