Cathode heater assembly for vacuum electron device and method of manufacture

Abstract

A cathode heater assembly for use in a vacuum electron device includes a refraction cup having a bottom and sidewalls forming a container, a cathode secured within the container of the refraction cup, and a heater wire coupled to the refraction cup. The cathode heater assembly can be manufactured by providing a refraction cup having a bottom and sidewalls forming a container, inserting a cathode pellet into the container of the refraction cup, impregnating the cathode pellet with an electron-emitting material while the cathode pellet is within the container of the refraction cup, and attaching a heater wire to the refraction cup.

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION Embodiments of the present invention relate generally to vacuum electron devices, and more particularly to providing a cathode heater assembly for use in vacuum electron devices, especially when operating at millimeter-wave frequencies and above. [Background technology]

[0002] Vacuum electron devices utilize the interaction between one or more electron beams and one or more electromagnetic waves generated in an interaction region. A vacuum electron device includes a vacuum chamber or cavity where one or more electron beams are generated and focused, and where the interaction between the one or more electron beams and one or more electromagnetic waves occurs. Examples of vacuum electron devices include, but are not limited to, particle accelerators, klystrons, gyrotrons, gyro-klystrons, traveling wave tubes (TWTs), gyro-TWTs, backward wave oscillators, magnetrons, cross-field amplifiers, free electron lasers, ubitrons, etc.

[0003] Electron beam guns generate electron beams in a vacuum environment. An exemplary electron beam gun is a thermionic emitter, often called a thermionic cathode. When the thermionic cathode reaches a specific temperature, it generates electrons at the cathode surface.

[0004] A variety of cathode materials have proven suitable for thermionic cathodes. Barium oxide-coated tungsten is sometimes used. Alternatively, a porous tungsten matrix impregnated with various oxides, including barium oxide, aluminum oxide, and calcium oxide, may be used. Cathode materials may often be coated with a thin layer of a rare earth material to improve electron emission properties. Electron microscopes often use lanthanum hexaboride, tungsten, or zirconium oxide-coated tungsten as thermionic emitters.

[0005] A heater is used to bring the hot cathode to a design temperature for electron emission. The heater typically includes a wire element located near or in contact with the hot cathode. Applying a voltage to the wire creates a resistive voltage drop, and some of the applied power is dissipated into the wire. With appropriate mechanical and thermal design, this power can be used to heat the hot cathode by conduction and / or radiation. Hot cathodes can also be heated by electron bombardment and laser heating.

[0006] Preferably, the hot cathode comprises a material that provides a suitably low work function so that, when the hot cathode is at a given temperature, the thermal energy of electrons in the material is high enough to generate a desired current density at the cathode surface upon application of a voltage / potential / electric field. The lower the work function, the lower the temperature at which the hot cathode can be operated to achieve the same emission current density at a given surface potential. This can provide advantages to users, including longer product life, simplified design, and / or the highest possible current density at an achievable temperature. Oxide-impregnated porous tungsten cathodes have been found to have significantly lower work functions in the presence of small amounts of scandium or scandium oxide.

[0007] Methods for determining the relationship between work function, temperature, and current density are known to those skilled in the art of hot cathodes. Methods for extracting current from hot cathodes by appropriate application of an electric field are also known.

[0008] Various methods for fabricating hot cathodes have been developed, with varying degrees of performance. Patent document 1, "Method of fabricating tungsten scandate nano-composite powder for cathodes," by Luhmann, Neville C., Gordon Soekland, Diana Gamzina, and Na Li, published July 14, 2020, describes a method for fabricating nano-composite scandate tungsten (NST) cathodes that has been shown to reliably achieve effective work functions of approximately 1.6 to 1.8. Previous methods have achieved work functions as low as 1.1 with similar materials. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 10,714,292 Summary of the Invention [Problem to be solved by the invention]

[0010] Cathode heater assemblies and methods for fabricating cathode heater assemblies having lower work functions and higher current densities are desired. [Means for solving the problem]

[0011] The present invention provides a cathode heater assembly for use in a vacuum electron device, comprising: a refractive cup having a bottom and sidewalls forming a container; a cathode secured within the container of the refractive cup; and a heater wire coupled to the refractive cup.

[0012] The heater wire may be bonded to the outer bottom surface of the bottom of the refractive cup. The refractive cup may include one or more openings for managing excess material flow. The heater wire may be shaped as a ribbon. The cathode may include a nano-scandate tungsten (NST) pellet impregnated with an electron-emitting material. The cathode may be impregnated with the electron-emitting material while in the refractive cup. The emitting surface of the cathode and the top of the sidewall of the refractive cup may be coplanar. The heater wire may be bonded to the refractive cup using laser welding or electron beam welding. The vacuum electronic device may include a linear beam tube, a traveling wave tube, a klystron, or a backward wave oscillator.

[0013] The present invention further provides a method of manufacturing a cathode heater assembly, the method including the steps of providing a refraction cup having a bottom and sidewalls forming a container; inserting a cathode pellet into the container of the refraction cup; impregnating the cathode pellet with an electron emitting material while the cathode pellet is within the container of the refraction cup; and attaching a heater wire to the refraction cup.

[0014] The step of attaching the heater wire to the refraction cup may include attaching the heater wire to a bottom surface of a bottom of the refraction cup. The step of attaching the heater wire to the refraction cup may occur before the step of inserting the cathode pellet into the container. The step of attaching the heater wire to the refraction cup may occur after the step of inserting the cathode pellet into the container. The method may further include fixing the cathode pellet in the container. Fixing the cathode pellet in the container may include using a brazing material, welding, and / or using an electron-emitting material. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram illustrating an exemplary vacuum electron device, such as an exemplary traveling wave tube (TWT), in accordance with an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating an exemplary electron beam gun according to an embodiment of the present invention. [Figure 3] FIG. 3 illustrates an exemplary cathode heater assembly according to an embodiment of the present invention. [Figure 4] FIG. 4 shows the temperature rise in a pure tungsten material when 0.7 Joules is applied to the exposed surface in a semi-infinite model. [Figure 5] FIG. 5 is a process diagram illustrating a method of fabricating the laser-welded cathode heater assembly of FIG. 3 in accordance with an embodiment of the present invention. [Figure 6a] FIG. 6a shows a side view of a coiled heater wire according to some embodiments of the present invention. [Figure 6b] FIG. 6b shows a top view of a coiled heater wire according to some embodiments of the present invention. [Figure 7] FIG. 7 illustrates an exemplary cathode heater assembly including the coiled heater wire of FIGS. 6a and 6b, in accordance with an embodiment of the present invention. [Figure 8] FIG. 8 illustrates a perspective view of an exemplary cathode heater assembly in accordance with an embodiment of the present invention. [Figure 9] FIG. 9 is a diagram illustrating a side view of the cathode heater assembly of FIG. 8 in accordance with an embodiment of the present invention. [Figure 10] FIG. 10 is a diagram illustrating a top view of the cathode heater assembly of FIG. 8 in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] This document describes an exemplary cathode heater assembly with a low work function and high current density, and a method for fabricating the cathode heater assembly. This approach provides a cathode heater assembly that achieves a desired cathode temperature with low applied power and simple components. The techniques described herein are particularly beneficial for vacuum electron devices such as TWTs and klystrons, as well as electron microscopes and lithography machines. The low power consumption of the design allows for deployment in power-limited platforms. The simplicity of the components and assembly allows for cost reduction.

[0017] 1 is a diagram illustrating an exemplary vacuum electron device 100, such as a traveling wave tube (TWT) 100, in accordance with an embodiment of the present invention. Although FIG. 1 is illustrated with respect to a TWT, the cathode heater assemblies described herein can be used in a variety of vacuum electron devices 100.

[0018] As shown, the vacuum electron device 100 includes an electron beam gun 102 configured to generate one or more electron beams (transmitted in the z-direction). The electron beam gun 102 can be used for sheet beams, hollow beams, pencil beams, diverging beams, multi-beams, etc. The vacuum electron device 100 further includes an interaction circuit, which includes an RF input window 104, an RF output window 106, and a magnet array 110 configured to direct and shape the one or more electron beams through the interaction circuit. The vacuum electron device 100 further includes a collector 108 configured to collect the one or more electron beams transmitted through the vacuum electron device 100.

[0019] 2 illustrates an exemplary electron beam gun 102 in accordance with an embodiment of the present invention. The electron beam gun 102 includes a heater 202, a cathode 204 in contact with the heater 202, and an anode section 206 (which may include one or more pre-accelerating anodes, one or more focusing anodes, and / or one or more accelerating anodes). A voltage V1 can be applied to the heater 202 to heat the cathode 204. A voltage V2 can be applied to the anode section 206 to focus and direct electrons in an electron beam 208 passing through an interaction circuit. The heater 202 and the cathode 204 are referred to herein as a cathode heater assembly 210.

[0020] 3 illustrates an exemplary cathode heater assembly 210 according to an embodiment of the present invention. The cathode heater assembly 210 includes a heater wire 302 bonded to a back surface 306 of a nanoscandate tungsten (NST) cathode (cathode pellet) 304. The cathode 304 includes an emission surface 308 on its front surface. The heater wire 302 may be bonded to the back surface 306 of the cathode 304 using laser welding or electron beam welding.

[0021] The heater wire 302 can be made of any electrically conductive material with a suitably high melting point and structural rigidity. Exemplary materials include tungsten, tungsten rhenium, molybdenum-rhenium, molybdenum-ruthenium, molybdenum-cobalt, molybdenum-nickel, and / or combinations of these materials. The material of the heater wire 302 may be selected based on its melting point, resistivity, workability at room temperature, and lifetime at operating temperatures. The ends of the heater wire 302 may be secured to an insulating structure to mechanically locate the cathode emission surface 308. The diameter of the heater wire 302 may be selected to achieve a desired power drop, thereby achieving a desired temperature when the cathode heater assembly 210 is placed in its operating environment.

[0022] The cathode (cathode pellet) 304 may be composed of a 10% to 50% porous NST sintered matrix impregnated with a barium-calcium-aluminate (a mixture of BaO, CaO, and Al2O3) impregnant to form a composite emitter. The NST sintered matrix may be made of an NST powder composed of micron-sized (100 nm to 10 microns) tungsten particles and nano-sized scandium oxide (10 nm to 2 microns), which is sintered at elevated temperatures to create the desired porous matrix. An exemplary cathode 304 may be fabricated as described in U.S. Patent No. 6,233,399, entitled "Method for Fabricating Tungsten-Scandate Nanocomposite Powder for Cathode Applications," published July 14, 2020, by Luhmann, Neville C., Gordon Soekland, Diana Gamzina, and Na Li.

[0023] In some embodiments, the type of attachment of the heater wire 302 to the cathode 304 meets certain criteria. This attachment type preferably withstands the high operating temperatures of the heater wire 302, which are often several hundred degrees Celsius higher than the temperature of the cathode emission surface 308, and can withstand many cycles from room temperature to the operating temperature. Meeting this condition eliminates many attachment types. The attachment type, in turn, preferably does not compromise the low work function requirement of the cathode 304. Given the small size of the cathode 304, not much power is required to heat the cathode 304, including the cathode emission surface 308, to a temperature that can cause irreversible changes. For example, without any cooling, a 5 mg cathode pellet 304 made of pure tungsten can reach 1500°C with the uniform and instantaneous application of 1 Joule of power.

[0024] To achieve a good attachment, power may be applied to bring the heater wire material, the cathode material, or both to their melting points without causing the emission surface 308 of the cathode 304 to reach an unacceptable temperature. The use of laser welding or electron beam welding allows a precise amount of heat to be applied at a precise location, in this case the interface between the heater wire 302 and the back surface of the cathode 304. By properly selecting the welding parameters, a securement of the two components can be achieved while maintaining an acceptable temperature of the emission surface 308.

[0025] 4 shows the temperature rise in pure tungsten material when 0.7 Joules are applied to the exposed surface in a semi-infinite model. If the material surface is spaced at least 500 microns from the back surface 306 where the welding occurs, the temperature rise of the cathode emission surface 308 will be less than 100° C. Maintaining the emission surface 308 at a low temperature prevents contamination of the cathode emission surface 308 during welding.

[0026] 5 is a process diagram illustrating a method of fabricating cathode heater assembly 210 according to an embodiment of the present invention. In step 504, NST powder (or pure tungsten powder) 502 is pressed to form NST pellets 304. In step 506, the NST pellets 304 are sintered. In step 508, the NST pellets 304 are impregnated with an electron-emitting material, such as barium calcium aluminate. In step 510, the heater wire 302 is formed. In step 512, the heater wire 302 is laser welded (or electron beam welded) to the NST pellets 304 to form the cathode heater assembly 210. In some embodiments, the laser welding comprises a single 500 microsecond pulse at 1.5 kW.

[0027] Figure 6a shows a side view of a coiled heater wire 602 according to some embodiments of the present invention. Figure 6b shows a top view of a coiled heater wire 602 according to some embodiments of the present invention. In some embodiments, the coiled heater wire 602 can be formed by winding the wire around a mandrel. The coil of the coiled heater wire 602 can be used to mechanically secure itself to the cathode 604 with or without laser welding and / or electron beam welding.

[0028] FIG. 7 illustrates a coiled heater cathode assembly 700 according to some embodiments of the present invention. The coiled heater cathode assembly 700 can be formed by inserting a cathode 604 into a coiled heater wire 602. In some embodiments, the cathode 604 can be the same or similar (same or similar material, same or similar shape, etc.) as the cathode 304. In some embodiments, the cathode 604 and the coiled heater wire 602 can be attached without welding. In some embodiments, the heater wire 602 can be wrapped directly around the cathode 304 to form an intimate mechanical bond therebetween. In some embodiments, the coiled heater wire 602 can be attached to the cathode 604 using laser welding or electron beam welding.

[0029] FIG. 8 is a diagram illustrating a perspective view of a cathode heater assembly 800 according to an embodiment of the present invention.

[0030] The cathode heater assembly 800 includes a heater wire 806, a refractive cup 802 attached to the heater wire 806, and a cathode 804 contained within the refractive cup 802. In some embodiments, the heater wire 806 may be ribbon-shaped, although other shapes are possible. The refractive cup 802 may include a bottom and sidewalls, which together form a container. In some embodiments, the top surface of the sidewall of the refractive cup 802 and the emission surface of the cathode 804 form a single surface in a single plane. In other embodiments, they form different surfaces in different planes. In some embodiments, the different planes are parallel, but in other embodiments, they may not be parallel.

[0031] To manufacture the cathode heater assembly 800, the cathode pellet may be formed to a specific size. The cathode 804 may be the size of the container of the refraction cup 802, or slightly longer or shorter, depending on the purpose of the electron emitter. The cathode pellet may be secured within the refraction cup 802 using brazing or electron-emitting material or welding techniques. In some embodiments, the electron-emitting material is placed directly into the refraction cup 802 to impregnate the cathode pellet, thereby forming the cathode 804 and securing the cathode 804 within the refraction cup 802. In some embodiments, the surface may be polished to achieve a highly uniform emission surface.

[0032] In some embodiments, the refraction cup 802 is made of metal. In some embodiments, the refraction cup 802 is made of an alloy of titanium-zirconium-molybdenum (TZM), molybdenum, and / or tungsten. A heater wire 806 may then be secured to the refraction cup 802. The shape of the heater wire 806 can be optimized by machining, chemical etching, or laser / electron beam cutting techniques to provide focused heating while optimizing power efficiency. The heater wire 806 may be made of molybdenum-rhenium.

[0033] The heater wire 806 may be attached to the refraction cup 802 using laser welding and / or electron beam welding. In some embodiments, the heater wire 806 may be attached to the outer bottom surface of the bottom of the refraction cup 802. In other embodiments, the heater wire 806 may be attached to the outer surface of the sidewall of the refraction cup 802. In some embodiments, the heater wire 806 may be attached within the container, for example, to the inner bottom surface of the bottom or the inner surface of the sidewall. Attaching the heater wire 806 to the refraction cup 802 rather than directly to the cathode 804 prevents the cathode 804 from being affected by the attachment process (e.g., welding) which can reduce its emission parameters and lifetime. In some embodiments, this attachment occurs before positioning the cathode 804 within the refraction cup 802. In some embodiments, this attachment occurs after inserting the cathode into the refraction cup 802.

[0034] In some embodiments, the deflector cup 802 can have additional openings to allow free flow of excess brazing or emitting material or to provide additional securing tabs for the cathode 804. The deflector cup 802 can help form a focusing electrode, solving the long-standing problem of edge emission in thermionic emitters. Edge emission creates stray electrons that cause interference, reduce efficiency, and add thermal load to the anode and circuitry.

[0035] The use of the refractive cup 802 results in a robust, lightweight cathode heater assembly 800 that provides high-quality emission, ease of manufacturing, and extended product life. While a simple refractive cup shape is shown here, more complex geometries can be employed to create / increase electron focusing. For example, Pierce angles can be used at the edges to help shape the beam and radius corners to reduce gradients.

[0036] 9 is a diagram illustrating a side view of a cathode heater assembly 800 according to an embodiment of the present invention. As described above, the cathode heater assembly 800 includes a heater wire 806, a refractive cup 802, and a cathode 804 (not shown) contained within the refractive cup 802. As shown, in some embodiments, the height of the refractive cup 802 is 0.9 mm. Other dimensions are possible.

[0037] 10 is a diagram illustrating a top view of a cathode heater assembly 800 in accordance with an embodiment of the present invention. As described above, the cathode heater assembly 800 includes a heater wire 806, a refractive cup 802, and a cathode 804 contained within the refractive cup 802. As shown, in some embodiments, the refractive cup 802 has a diameter of 1.2 mm and the cathode 804 has a diameter of 0.9 mm. Other dimensions are possible.

[0038] In some embodiments, the present invention provides a cathode heater assembly comprising: a nanoscandate tungsten cathode made of a high-temperature sintered matrix of 10% to 50% porous NST powder containing or consisting of micron-sized (100 nm to 10 microns) tungsten particles and nano-sized scandium oxide (10 nm to 2 microns) and impregnated with a barium-calcium-aluminate (a mixture of BaO, CaO, and Al2O3) impregnant to form a composite emitter; a piece of conductive material as a heater wire with a cross-section selected to dissipate adequate power to heat the cathode; and attachment of the cathode and heater to each other by laser welding or electron beam welding.

[0039] In some embodiments, the present invention provides a cathode heater assembly that includes a nanoscandate tungsten cathode made of a high-temperature sintered matrix of 10% to 50% porous NST powder composed of micron-sized (100 nm to 10 microns) tungsten particles and nano-sized scandium oxide (10 nm to 2 microns) impregnated with a barium-calcium-aluminate (a mixture of BaO, CaO, and Al2O3) impregnant to form a composite emitter; a piece of conductive material as a heater wire with a cross-section selected to dissipate adequate power to heat the cathode; and attachment of the cathode and heater to each other by mechanical fastening using a helically wound wire.

[0040] NST powder is made of micron- and nano-sized particles that form a porous matrix with interconnected micron-sized pores. This is important for allowing the electron-emitting material to flow to the cathode surface during operation. Because the cathode is sintered and made of small particles, prolonged exposure to high temperatures closes the pores, forming a denser matrix and blocking the pathways for electron-emitting material to flow to the surface. Direct welding, specifically laser welding or electron beam welding, does not affect the porous structure of the cathode because it stores energy only for a very short time in a very limited location on the back surface. Mechanically securing a heat wire to the cathode, such as by spiral wrapping, avoids the need to heat the cathode. The use of a cup also reduces the application of heat to the cathode, preserving the porosity of the NST material.

[0041] The above description of preferred embodiments of the present invention is by way of example only, and other variations and modifications of the above-described embodiments and methods are possible in light of the above teachings. The embodiments described herein are not intended to be exhaustive or limiting. The present invention is limited only by the following claims.

Claims

1. 1. A cathode heater assembly for use in a vacuum electron device, said cathode heater assembly comprising: an inflective cup having a bottom and a sidewall forming a container; a cathode secured within the container of the refraction cup; and a heater wire coupled to the deflector cup; a cathode heater assembly comprising:

2. The cathode heater assembly of claim 1 , wherein the heater wire is bonded to an outer bottom surface of the bottom portion of the refraction cup.

3. The cathode heater assembly of claim 1 , wherein the deflector cup includes one or more openings for managing excess material flow.

4. The cathode heater assembly of claim 1 , wherein the heater wire is formed as a ribbon.

5. 10. The cathode heater assembly of claim 1, wherein the cathode comprises a nano-scandate tungsten (NST) pellet impregnated with an electron emitting material.

6. 6. The cathode heater assembly of claim 5, wherein the cathode is impregnated with the electron emissive material within the refractive cup.

7. 2. The cathode heater assembly of claim 1, wherein the cathode emission surface and the top of the sidewall of the refractive cup are coplanar.

8. The cathode heater assembly of claim 1 , wherein the heater wire is coupled to the refraction cup using laser welding or electron beam welding.

9. The cathode heater assembly of claim 1 , wherein the vacuum electron device comprises a linear beam tube.

10. The cathode heater assembly of claim 1 , wherein the vacuum electronic device comprises a traveling wave tube.

11. The cathode heater assembly of claim 1 , wherein the vacuum electron device comprises a klystron.

12. The cathode heater assembly of claim 1 , wherein the vacuum electronic device comprises a backward wave oscillator.

13. 1. A method of fabricating a cathode heater assembly for use in a vacuum electron device, the method comprising: providing an inverted cup having a bottom and a sidewall forming a container; inserting a cathode pellet into the container of the refraction cup; impregnating the cathode pellet with an electron emitting material while the cathode pellet is in the container of the refractive cup; and Attaching a heater wire to the refractive cup A method comprising:

14. The method of claim 13 , wherein the step of attaching the heater wire to the refraction cup comprises attaching the heater wire to a bottom surface of the bottom of the refraction cup.

15. The method of claim 13 , wherein the step of attaching the heater wire to the deflector cup occurs before the step of inserting the cathode pellet into the container.

16. The method of claim 13 , wherein the step of attaching the heater wire to the refraction cup occurs after the step of inserting the cathode pellet into the container.

17. The method of claim 13 further comprising securing the cathode pellet within the container.

18. The method of claim 17 , wherein the step of securing the cathode pellet within the container comprises the use of a braze material.

19. The method of claim 17 , wherein the step of securing the cathode pellet within the container comprises welding.

20. 20. The method of claim 17, wherein the step of securing the cathode pellet within the container includes using the electron emitting material.

21. 1. A cathode heater assembly comprising: a nanoscandate tungsten (NST) cathode made of a high-temperature sintered matrix of NST powder with a porosity of 10% to 50% containing micron-sized tungsten particles and nano-sized scandium oxide, impregnated with a barium-calcium-aluminate impregnant to form a composite emitter; and a heater wire having a cross-section configured to dissipate power to heat the NST cathode and cause it to emit electrons, the heater wire being attached to the NST cathode by at least one of laser welding or electron beam welding. a cathode heater assembly comprising:

22. 1. A cathode heater assembly comprising: a nanoscandate tungsten (NST) cathode made of a high-temperature sintered matrix of NST powder with a porosity of 10% to 50% containing micron-sized tungsten particles and nano-sized scandium oxide, impregnated with a barium-calcium-aluminate impregnant to form a composite emitter; and a heater wire having a cross-section configured to dissipate power to heat the NST cathode and cause it to emit electrons, the heater wire being attached to the NST cathode by mechanical fixation by helically wrapping the heater wire around the NST cathode; a cathode heater assembly comprising:

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