Hollow Cathode

Abstract

To provide a hollow cathode that has high heat insulation property even in an axial direction and is less likely to deteriorate over time.SOLUTION: A hollow cathode includes: a cathode tube; a heater provided on an inside or outside of the cathode tube; a thermal shield enclosing the cathode tube and the heater; and a keeper electrode enclosing the thermal shield. The thermal shield includes: an inner cylinder made of a ceramic fiber-molded body including a ceramic coating at the surface thereof; and an outer cylinder made of a metallic foil.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hollow cathode. [Background technology]

[0002] The use of electric propulsion in artificial satellites is expanding, and the market is rapidly expanding, with adoption increasing not only in traditional deep space exploration and geostationary satellites, but also in constellation satellites in low orbit, etc. The main electric propulsion technologies used on the market are Hall thrusters and ion engines, and hollow cathodes are used in most of them.

[0003] The function of a hollow cathode is to heat an electron-emitting material (usually called an emitter or insert) to a high temperature of several hundred to several thousand degrees Celsius to emit thermoelectrons, supply a working gas, generate plasma by applying voltage, and extract an electron beam from an orifice. It also plays a role in igniting thrusters and neutralizing ion beams.

[0004] Hollow cathodes pose a high level of technical difficulty and many challenges for the following reasons: (1) The materials that can be used are limited due to wear and contamination caused by the ultra-high temperature and high-density plasma. (2) They must be lightweight and resistant to the mechanical environment of vibration and shock during launch, while also being able to withstand a steep temperature gradient, especially around the heating heater, and they must be able to withstand thermal deformation and stress caused by these thermal cycles for several thousand cycles. For these reasons, hollow cathodes are a key component of electric propulsion.

[0005] The most difficult aspect of hollow cathode design is thermal design. The heater can reach high temperatures of up to 1800°C, while the temperature of the hollow cathode housing needs to be kept at around 400-600°C. Furthermore, from the perspective of power saving and improving the thermal interface with the system, it is important to reduce heat leakage from the heater. For this reason, the heat shield, which plays a role in suppressing radiant heat from the ultra-high temperature heater, is an important component that determines the thermal design.

[0006] Patent Document 1 describes a hollow cathode in which a heater embedded in a ceramic sprayed layer is disposed on the outside of a cathode tube, and the outside of the heater is covered with a heat shield.

[0007] Patent Document 2 describes a hollow cathode comprising a cathode tube having a cylindrical portion at one end, an electron-emitting material disposed inside the cylindrical portion of the cathode tube, and a heater disposed around the cylindrical portion at a distance from the cathode tube. The hollow cathode heats the cathode tube and the electron-emitting material by radiation generated when the heater is heated, causing the electron-emitting material to emit electrons. A heat shield is provided to cover the heater, and a support means is further provided to maintain the heat shield and heater spaced apart. This structure prevents short circuits between the heater and the heat shield.

[0008] Patent Document 3 describes a heat insulating shield having a multilayer structure of thin cylindrical shields made of a high-melting-point metal with high emissivity, such as tantalum. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-74887 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-16795 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-250316 Summary of the Invention [Problem to be solved by the invention]

[0010] However, while the invention described in Patent Document 3 uses a high-melting-point metal for the heat shield, the thin, multi-layer structure near the heater is subject to high temperatures, which can cause softening due to phase changes and deformation due to thermal expansion. This can lead to contact with the heater, causing a short circuit and impairing the hollow cathode's functionality. Furthermore, the heat shield uses metal foil to primarily block radiant heat, and heat is transferred axially to the hollow cathode via thermal conduction, resulting in poor insulation. This results in the heater's heat dissipating to the attachment point and the satellite body, resulting in energy loss and requiring additional power. Furthermore, since the heat shield is a thermal insulator that blocks radiant heat, it is important to maintain a low emissivity on the surface of the thermal insulator in order to maintain its performance. However, when exposed to high temperatures, the metal surface loses its inherent luster due to reactions with ions that have entered the material, which causes a decrease in the thermal insulation performance. As described above, heat-insulating materials that suppress radiation are prone to deterioration over time, even if they have excellent initial performance. Furthermore, while they have high heat-insulating performance in the direction perpendicular to the metal foil, they have poor heat-insulating properties in the plane direction (the axial direction of the hollow cathode).

[0011] The present invention has been made in view of the problems inherent in the prior art, and an object of the present invention is to provide a hollow cathode that has high thermal insulation properties in the axial direction as well and is resistant to deterioration over time. [Means for solving the problem]

[0012] A hollow cathode according to an embodiment of the present invention comprises a cathode tube, a heater provided inside or outside the cathode tube, a heat shield surrounding the cathode tube and the heater, and a keeper electrode surrounding the heat shield, wherein the heat shield includes an inner tube made of a ceramic fiber compact having a ceramic coating on its outer surface, and an outer tube made of metal foil. In the hollow cathode of the present invention, the inside of the heat shield (inner cylinder), which is exposed to high temperatures, is made of a ceramic fiber compact, so it is resistant to softening even when exposed to high temperatures and can avoid short circuits due to contact with the heater. Furthermore, the inner cylinder made of a ceramic fiber compact has high thermal insulation properties in all directions, preventing heat diffusion to the satellite body and attachment section, thereby improving energy efficiency. Furthermore, the inner cylinder made of a ceramic fiber compact has a ceramic coating on the outer surface, which fixes the outer surface and reduces fluffing and particle generation, preventing particle contamination inside the hollow cathode. However, due to their large surface area and optical properties, ceramic fiber moldings generally have high emissivity, and therefore their radiative insulation performance is insufficient. Therefore, it is difficult for an inner tube made of a ceramic fiber molding alone to provide radiative insulation performance. Therefore, by providing an outer tube made of a metal foil with a glossy surface and low emissivity, the radiative insulation performance that is insufficient with the inner tube alone can be compensated for. In addition, because the outer cylinder is made of metal foil, the overall weight of the heat shield can be reduced. Furthermore, the insulating effect of the inner cylinder prevents creep deformation, thermal deformation, and reaction with ions on the surface of the outer cylinder, allowing it to fully demonstrate higher insulating performance than the inner cylinder and ensure heat insulation for the keeper electrode surrounding the heat shield.

[0013] The ceramic fiber is preferably a carbon fiber because of its high heat insulating properties.

[0014] The ceramic coating is preferably a coating made of pyrolytic carbon, as this has a smooth surface and a high emissivity.

[0015] The metal foil is preferably made of Ta, W, Ir, Hf, Nb, Mo, or Ru, which have a high melting point. These metals have a glossy appearance, and therefore can ensure high radiation heat insulation performance.

[0016] The metal foil is preferably made up of multiple layers, with each layer being separated from the other by a support member. Alternatively, the metal foil is preferably made up of multiple layers, with the front and / or back surfaces of each layer having irregularities that separate the foils from one another. In this way, by making the metal foil up of multiple layers and separating the layers, high thermal insulation performance can be ensured. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a hollow cathode that has high thermal insulation properties in the axial direction as well and is resistant to deterioration over time. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a cross-sectional view showing the hollow cathode of the present embodiment. [Figure 2] FIG. 3 is a cross-sectional view showing details of the heat shield of the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] The hollow cathode of this embodiment will be described in detail below with reference to the drawings. Note that the dimensional proportions in the drawings are exaggerated for the sake of explanation and may differ from the actual proportions.

[0020] The hollow cathode of this embodiment includes a cathode tube, a heater provided inside or outside the cathode tube, a heat shield surrounding the cathode tube and the heater, and a keeper electrode surrounding the heat shield. The heat shield includes an inner cylinder made of a ceramic fiber compact with a ceramic coating on its outer surface, and an outer cylinder made of metal foil.

[0021] The hollow cathode of this embodiment will be described with reference to FIG. 1. The hollow cathode 10 shown in FIG. 1 comprises a cathode tube 12, a heater 14, a heat shield 16, and a keeper electrode 18. The cathode tube 12 maintains pressure against the working gas, and the heater 14 is configured as a heat-generating part that heats an electron-emitting material 24 located inside the cathode tube 12. A flange-shaped terminal part 20 is provided on the outer periphery of the open side of the cathode tube 12, and an orifice 22 is provided in the center of the bottom. The terminal part 20 of the cathode tube 12 is fixed to a fixing part 28. The fixing part 28 is part of the housing of the hollow cathode 10, and therefore the cathode tube 12 is fixed and supported within the housing.

[0022] A cylindrical heat shield 16 is provided on the outside of the heater 14, and a cylindrical keeper electrode 18 with a bottom and an orifice 19 at its bottom is provided on the outside of the heat shield 16. The orifice 19 at the bottom of the keeper electrode 18 is positioned opposite an orifice 22 in the cathode tube 12. An electron-emitting material 24 is provided inside the cathode tube 12, and the electron-emitting material 24 is fixed in place while being urged toward the bottom of the cathode tube 12 by a spring member 26.

[0023] The heat shield 16 is disposed to surround the heater 14 and blocks the heat from being emitted from the heater 14 to the outside. That is, the heat from the heat-generating portion of the heater 14 is directed not only toward the cathode tube 12 but also to the outside, but the presence of the heat shield 16 can reduce the heat emission to the outside. Therefore, the inside of the heat shield 16 is kept warm. Details of the heat shield 16 will be described later.

[0024] The electron-emitting material 24 may be, for example, a metal material with a relatively low work function such as tungsten, a sintered material such as lanthanum hexaboride, or a base metal such as porous tungsten impregnated with barium oxide, aluminum oxide, calcium oxide, or the like, and these may be molded into a hollow shape.

[0025] The keeper electrode 18 surrounds the heat shield 16 and has an orifice 19 located opposite the orifice 22 of the cathode tube 12. Therefore, electrons emitted from the electron-emitting material 24 inside the cathode tube 12 are emitted from the orifice 22 of the cathode tube 12, and the electrons emitted from the orifice 22 are further emitted from the orifice 19 of the keeper electrode 18.

[0026] In the hollow cathode 10 of this embodiment, the heat generated by the heat generating portion of the heater 14 is transferred to and heated by the electron-emitting material 24 provided inside the cathode tube 12. When the temperature of the electron-emitting material 24 reaches a certain high temperature or higher, application of a voltage to the keeper electrode 18 causes the electrons to be emitted.

[0027] The power supply that supplies power to the keeper electrode 18 has a function of switching from constant voltage control to constant current control, and the power supply that supplies power to the heater 14 is controlled by constant current control.

[0028] The operation of the hollow cathode 10 of this embodiment will be described below. Ionized gas (e.g., xenon) is supplied into the cathode tube 12. The gas passes through the electron-emitting material 24 and flows out of the orifice 22 of the cathode tube 12. At this time, a pressure difference occurs between the gas pressure at the electron-emitting material 24 and the orifice 22.

[0029] The power supply for the keeper electrode 18 operates under constant voltage control, applying a high voltage of approximately 100 V to the keeper electrode 18. A current is also passed from the power supply to the heater 14. At this time, the heater power supply operates under constant current control or constant voltage control. When the heat-generating portion of the heater 14 generates heat, the electron-emitting material 24 disposed inside the cathode tube is heated. As the temperature rises, the electron-emitting material 24 begins to emit thermoelectrons, causing a breakdown between the electron-emitting material 24 and the keeper electrode 18. The resulting discharge generates plasma through collision ionization between electrons and the ionized gas. After plasma generation, the electron-emitting material 24 is heated autonomously by the plasma generated inside due to the so-called hollow cathode effect, so the heater is deenergized. That is, the heater is controlled under constant current or constant voltage control at least until plasma is generated in the electron-emitting material 24. After plasma generation, the keeper power supply switches to constant current control.

[0030] In the above, the inner diameter of the heater 14 is larger than the outer diameter of the cathode tube 12, and the heater 14 is configured to surround the cathode tube 12 on the outside of the cathode tube 12 while being spaced apart from the side surface of the cathode tube 12. However, the opposite may also be true. That is, the inner diameter of the cathode tube 12 may be larger than the outer diameter of the heater 14, and the cathode tube 12 may be configured to surround the cathode tube 12 on the outside of the heater 14 while being spaced apart from the side surface of the cathode tube 12.

[0031] The hollow cathode of this embodiment is characterized by the heat shield, which includes an inner cylinder made of a ceramic fiber compact having a ceramic coating on its outer surface, and an outer cylinder made of metal foil.

[0032] 2 is a cross-sectional view of the heat shield 16 taken along the central axis of the hollow cathode 10. As shown in FIG. 2, an outer cylinder 32 is located outside an inner cylinder 30. The dashed dotted line in FIG. 2 indicates the central axis of the hollow cathode 10.

[0033] The heat shield according to this embodiment has a dual structure consisting of an inner and outer cylinder, and heat emitted from the heater is blocked first by the inner cylinder and then by the outer cylinder. That is, the inner cylinder is directly exposed to heat from the heater, but because it is made of a ceramic fiber molded body, it is resistant to deterioration over time. Furthermore, because the inner cylinder has a ceramic coating on its outer surface, it is possible to improve the optical properties of the surface and suppress fuzzing. Furthermore, the outer cylinder is made of metal foil, and the heat received from the heater is reduced by the heat insulation provided by the inner cylinder, improving resistance to deterioration over time such as creep deformation, lifespan, and thermal cycle resistance. On the other hand, when using insulation with metal foil, heat leakage is proportional to the fourth power of the absolute temperature. In contrast, when using insulation with ceramic fiber compacts, heat leakage is proportional to the first power of the absolute temperature. Therefore, by combining insulation with ceramic fiber compacts with insulation with metal foil, it is possible to improve the insulation performance even with the same size as metal foil alone. Furthermore, by reducing the heat received by the metal foil of the outer tube, the conductive heat leakage in the axial direction due to the metal foil is reduced, and the inner tube also exhibits high insulation performance in the axial direction, so the heat shield as a whole can suppress heat leakage not only in the radial direction but also in the axial direction. The inner and outer cylinders will be described in detail below.

[0034] [Inner cylinder] The inner tube is made of a ceramic fiber compact with a ceramic coating on the outer surface. Ceramic fiber has lower conductivity than metal foil, so current does not easily flow even in the event of a short circuit, allowing the heater's performance to be maintained. Examples of ceramic fibers that can be used include SiC fiber, alumina fiber, and carbon fiber. Of these, carbon fiber is preferred due to its high heat resistance. Carbon fibers include graphite carbon fibers and carbonaceous carbon fibers, but carbonaceous carbon fibers are preferred because carbonaceous carbon fibers have low thermal and electrical conductivity, high thermal insulation properties, and are less likely to allow current to flow even in the event of a short circuit, which is less likely to damage the heater.

[0035] The thickness of the ceramic fiber molded body having a ceramic coating on the outer surface is preferably 0.5 to 100 mm.

[0036] Ceramic coatings used to coat ceramic fibers include pyrolytic carbon, glassy carbon, and CVD-SiC. Pyrolytic carbon is made from hydrocarbon gas and the ceramic coating is formed by chemical vapor deposition, which allows for the formation of a dense ceramic coating and effectively prevents fluffing and particle generation. In addition, pyrolytic carbon coatings have a smooth surface and high emissivity, making them highly effective in radiative insulation. "Having a ceramic coating on the outer surface" means that the ceramic coating is on the outer surface that defines the shape of the molded body, and in the case of a cylindrical shape, this means both end surfaces, the outer peripheral surface, and the inner peripheral surface. The thickness of the ceramic coating is, for example, 10 μm to 2 mm. When the thickness of the ceramic coating is 10 μm or more, it is highly effective in covering the surface of the molded body and preventing the ceramic fibers from scattering, and when the thickness of the ceramic coating is 2 mm or less, the high heat insulating properties inherent to the ceramic fibers can be fully exhibited.

[0037] The ceramic fibers can be bonded to each other by glassy carbon obtained by applying a resin that serves as a carbon precursor, curing, and baking the resin. The diameter of the ceramic fibers is preferably 5 to 30 μm. Furthermore, the thermal insulating material made of ceramic fibers is preferably in the form of a mat, a paper product, or the like. The mat can be obtained, for example, by laminating ceramic fibers with a length of 5 to 300 mm. The paper product can be obtained by papermaking ceramic fibers with a length of 0.1 to 10 mm.

[0038] [Outer cylinder] The outer cylinder is made of a metal foil, such as Ta, W, Ir, Hf, Nb, Mo, or Ru. The thickness of the metal foil is preferably 5 to 1000 μm.

[0039] The metal foil is preferably composed of multiple layers to ensure high radiation heat insulation performance. In this case, the layers are preferably spaced apart to suppress conductive heat transfer. When the layers are spaced apart, they are preferably arranged so that the gap between the layers is 0.01 to 5 mm. To space the layers apart, it is preferable to arrange them via a support member as a spacer, or to form irregularities on the front and / or back surfaces of each layer. The irregularities can be formed by dimple processing (also called embossing) on ​​each layer. The number of metal foil layers is optimized from the viewpoints of heat insulating performance and weight reduction. For example, it is preferably 2 to 50 layers. Within the above range, both heat insulating performance and weight can be kept within desirable ranges. To ensure radiant heat reflectance, these metal foils preferably have a surface roughness (Ra) of 1 μm or less. When the surface roughness is 1 μm or less, the wavelength is sufficiently smaller than the infrared rays that are the center of radiant heat in a particular temperature range, and the foil exhibits a metallic luster against infrared rays, resulting in less scattering of reflected light and ensuring high thermal insulation performance.

[0040] In the heat shield according to this embodiment, the inner and outer cylinders may be in contact with each other or may be spaced apart. Spaced apart is preferable because the presence of a gap improves the heat insulating performance. However, when spaced apart, it is important to maintain a constant distance between the inner and outer cylinders to prevent temperature variations in the circumferential direction. Therefore, it is preferable to place a support member between the inner and outer cylinders as a spacer, or to form irregularities on the outer surface of the inner cylinder and / or the inner surface of the outer cylinder. The irregularities can be formed by applying dimple processing (also called embossing) to the outer surface of the inner cylinder and / or the inner surface of the outer cylinder. When the inner cylinder and the outer cylinder are spaced apart, the distance between them (the distance from the outer circumference of the inner cylinder to the inner circumference of the outer cylinder) is preferably 0.01 to 5 mm.

[0041] The heat shield according to this embodiment can be obtained, for example, by first fabricating an inner tube and wrapping metal foil around it, or by fabricating the inner tube and outer tube separately and then inserting the inner tube into the hollow portion of the outer tube to integrate them.

[0042] Although the above description has been given with reference to a cylindrical shape as an example of the shape of the heat shield, the shape is not limited to a cylindrical shape and may be a rectangular tube having a polygonal cross section. [Example]

[0043] The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to the following examples.

[0044] [Example 1] To confirm the heat insulating performance of the heat shield in the hollow cathode of this embodiment, a hollow cathode similar in configuration to the hollow cathode 10 shown in FIG. 1 was first fabricated. The heat shield consisted of an inner cylinder made of a cylindrical carbon fiber molded body with a pyrolytic carbon coating on the outer surface, and an outer cylinder made of tantalum foil processed into a cylindrical shape. The outer cylinder was obtained by winding embossed tantalum foil with a thickness of 0.1 mm to a thickness of approximately 1 cm. The heater 14 was then heated to approximately 1800°C, and the temperature of the outer cylinder (keeper electrode 18) was measured. The measurement result was 403.6°C.

[0045] [Comparative Example 1] The temperature of the outer cylinder was measured in the same manner as in Example 1, except that a hollow cathode was used in which the entire heat shield was composed only of cylindrical tantalum foil. The measurement result was 417.5°C.

[0046] A comparison between Example 1 and Comparative Example 1 shows that the use of the heat shield according to this embodiment provides excellent heat insulating performance. Furthermore, when the tantalum foil of the heat shield was observed after use, the innermost peripheral portion of the tantalum foil of Comparative Example 1 was exposed to the heat of the heater and lost its original metallic luster, whereas the innermost peripheral portion of the tantalum foil (outer cylinder) of Example 1 maintained its metallic luster before the test. From the above, the inner cylinder blocked the heat from the heater, thereby suppressing deterioration of the outer cylinder made of tantalum foil. Furthermore, FEM analysis modeling the entire hollow cathode, including the heat shield, revealed that the maximum temperature of the metal foil had decreased by more than 300 K, and it was estimated that the anisotropy of the thermal insulation performance of the inner cylinder was smaller than that of the metal foil outer cylinder, confirming improvements in lifespan and thermal cycle resistance.

[0047] Although the present embodiment has been described above using examples, the present embodiment is not limited to these examples, and various modifications are possible within the scope of the gist of the present embodiment. [Explanation of symbols]

[0048] 10 Hollow Cathode 12 cathode tube 14 Heater 16 Heat Shield 18 Keeper electrode 20 Terminal Section 22 Orifice 24 Electron-emitting materials 26 Spring member 30 Inner cylinder 32 outer cylinder

Claims

1. a cathode tube; a heater provided inside or outside the cathode tube; a heat shield surrounding the cathode tube and the heater; and a keeper electrode surrounding the heat shield; The hollow cathode has an inner tube made of a ceramic fiber molded body having a ceramic coating on its outer surface, and an outer tube made of a metal foil spaced apart from the inner tube.

2. 2. The hollow cathode of claim 1, wherein the ceramic fibers are carbon fibers.

3. 3. The hollow cathode according to claim 1, wherein the ceramic coating is a pyrolytic carbon coating.

4. 4. The hollow cathode according to claim 1, wherein the metal foil is made of Ta, W, Ir, Hf, Nb, Mo, or Ru.

5. 5. The hollow cathode according to claim 1, wherein the metal foil is made up of a plurality of layers, and the layers are spaced apart from one another via a support member.

6. 5. The hollow cathode according to claim 1, wherein the metal foil is made up of a plurality of layers, and the front and / or back surfaces of each layer have irregularities that separate the layers from one another.

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