A hollow cathode high temperature heater

Abstract

The present invention discloses a hollow cathode high temperature heater, which is used to heat the emitter in the cathode tube, and comprises: an insulating liner, a heating element, a skeleton, and a multi-layer carbon felt heat shielding layer, which are coaxially nested and assembled from the inside to the outside; the insulating liner and the skeleton are both high temperature resistant insulating ceramics; the heating section, lead end and lead fixing of the heating element are integrally processed with isostatic high-density graphite, and the cross-section of the lead end of the heating element is significantly larger than that of the heating section; the insulating liner is a thin-walled cylinder, which is completely and tightly fitted with the inner wall of the heating element and the outer wall of the cathode tube, and the insulating liner and the ceramic skeleton realize structural clamping and reliable insulation of the heating element. The present invention uses isostatic high-density graphite as the heating element, which has low cost, high reliability, strong high temperature resistance, high heating efficiency, and long service life. The design of the lead end, insulating liner, skeleton, and heat shielding layer improves the efficiency of inward heat conduction, reduces unnecessary heating and heat dissipation, and has a simple process and is easy to manufacture, assemble and debug.

Description

Technical Field

[0001] The invention belongs to the technical field of space electric propulsion, and in particular relates to a miniaturized assembled graphite high-temperature heater for a hollow cathode. Background Art

[0002] Electric propulsion systems have significant advantages in high specific impulse, can significantly reduce the amount of propellant carried and increase the payload, or significantly extend the working life of spacecraft without increasing the propellant, and are being used more and more. At present, the mainstream ion thrusters and Hall thrusters used at home and abroad rely on hollow cathodes to emit electrons to start normally and work reliably. The prerequisite for hollow cathode ignition is to heat the lanthanum hexaboride or barium tungsten emitter to a high enough working temperature to emit electrons through an external high-temperature heater. Therefore, the high-temperature heater is the key to the start-up of the entire electric propulsion system. Its performance directly affects the normal start-up, long-term operation, repeated start-up life and reliability of the entire system.

[0003] The operating temperature of the hollow cathode emitter can reach up to 1800℃, which places strict requirements on the material selection, structural design and manufacturing process of the heater. The armored heaters and assembled heaters commonly used in traditional hollow cathodes all use high-temperature refractory metal wires as heating elements, such as tantalum wire, tungsten wire, tungsten-rhenium wire, etc. The metal wire becomes brittle after being exposed to high temperatures and is very easy to break; the continuous evaporation of the metal wire at high temperatures will cause the heating wire to become thinner or even break; the gradual penetration or deposition of metal evaporation on the insulating parts will cause the heater to short-circuit; the metal resistivity changes greatly with temperature, and high-temperature power may be out of control, causing the heating wire to melt. In particular, the armored heater is made of coaxial resistance heating wire coiled and molded, and the inner and outer conductors are separated by insulating materials. After the inner and outer conductors at the ends are welded, a loop of heating current from the inner conductor to the outer conductor is formed. The process is difficult, the yield is low, the reliability is poor, and it often becomes a single point failure component. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of low reliability caused by short circuit or fuse of the heating wire in the existing hollow cathode high temperature heater.

[0005] In order to achieve the above object, the present invention provides a hollow cathode high temperature heater for heating an emitter in a cathode tube, comprising:

[0006] With the cathode tube as the center, the cylindrical insulating lining, heating element, frame, and heat shielding layer are nested and assembled from the inside out;

[0007] The insulating lining and the frame are made of high temperature resistant insulating ceramics; the wall thickness of the insulating lining is 0.3-1mm, and its inner surface fits the cathode tube;

[0008] The heating element is made of isostatically pressed high-density graphite; the heating element comprises a heating section and a lead end formed by integral processing; the heating section is a zigzag current channel, closely attached to the outer wall of the insulating liner, axially extending from one end of the insulating liner to the other end, then turning back and repeating the process until the outer wall of the insulating liner is fully covered, so that the current channel is axially laid into several sections, and a gap is provided between two adjacent sections to prevent short circuit; the lead end is formed by continuing to extend the two ends of the current channel along the axial direction, the two lead ends are adjacent and have a gap, and the cross section of the lead end is larger than the cross section of the current channel;

[0009] The frame is coated and fixed to the outer surface of the heating element, and the two ends of the frame extend radially outward to form a groove on the outer surface of the frame; several layers of heat insulation material are laid in the groove to form the heat shielding layer;

[0010] The insulating lining and the frame respectively cover the two sides of the lead end and are used to fix the lead end.

[0011] Preferably, the frame is respectively abutted against and fixed to two ends of the outer surface of the heating section, and a gap is provided between the frame and other areas of the outer surface of the heating section.

[0012] Preferably, the frame and the insulating liner clamp and fix two sides of the lead end, and the insulating liner abuts against a partial area of ​​the lead end, and a gap is provided with other areas of the lead end.

[0013] Preferably, a side of the frame close to the lead end is stepped, and a gap is provided between the frame and a part of the lead end.

[0014] Preferably, the thermal insulation material is carbon felt.

[0015] Preferably, the lead end is provided with a lead fixing piece for connecting to an external circuit; the material of the lead fixing piece is selected from isostatically pressed high-density graphite.

[0016] Preferably, the high temperature resistant insulating ceramic is one of alumina ceramic and boron nitride ceramic.

[0017] Preferably, the gap between two adjacent sections of the current channel is 0.2-1 mm, and the width of the current channel is 0.3-2 mm.

[0018] The technical effects of the present invention include:

[0019] (1) Using isostatically pressed high-density graphite material instead of traditional refractory metal wire as the heating element. Graphite is extremely resistant to high temperatures and can withstand sudden changes in temperature without cracking or damage. It has a small thermal expansion coefficient and has good electrical and thermal conductivity and chemical stability. The strength of graphite increases with increasing temperature and can meet the requirements for the use of hollow cathode heaters. In addition, graphite has low hardness, low price, and is easy to process into precision-sized parts, reducing material and processing costs.

[0020] (2) This heater adopts a mechanical assembly structure based on isostatically pressed high-density graphite material and high-temperature resistant ceramics. It has a simple process and is easy to manufacture, assemble and debug. It has low cost, high reliability, strong high-temperature resistance, high heating efficiency and long service life. It can be widely used in the main cathode and neutralizer cathode of electric thrusters, as well as the hollow cathode of plasma contactors.

[0021] (3) The cross-section of the lead end of the heating element is significantly larger than that of the heating section; the insulating lining is a thin-walled cylinder, which fits tightly with the inner wall of the heating element and the outer wall of the cathode tube. The insulating lining and the ceramic skeleton realize structural clamping and reliable insulation of the heating element. The design of the lead end, insulating lining, skeleton, and thermal shielding layer improves the efficiency of inward heat conduction and reduces unnecessary heating and heat dissipation. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the overall structural assembly of the hollow cathode high temperature heater of the present invention;

[0023] Figure 2 It is the rear view of the heating element;

[0024] Figure 3 is a top view of the heating element;

[0025] Figure 4 It is the expansion diagram of the heating element;

[0026] Figure identification: 1- cathode tube; 2- frame; 3- heat shielding layer; 4- lead fixing piece; 5- heating element; 51- lead end; 52- heating section; 53- through cut; 6- insulating lining. DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] In the description of the present invention, it should be noted that the terms "left", "right", "vertical", "horizontal", "inside", "outside", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they cannot be understood as limitations on the present invention.

[0029] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0030] like Figure 1 As shown, the present invention discloses a hollow cathode high temperature heater, comprising a skeleton 2, a heat shielding layer 3, a lead fixing part 4, a heating element 5, and an insulating lining 6. With the cathode tube 1 as the center, the insulating lining 6, the heating element 5, and the skeleton 2 are coaxially nested and assembled in sequence, and all are cylindrical structures. The insulating lining 6 and the skeleton 2 are made of high temperature resistant insulating ceramics, which are used to ensure reliable insulation between the heating element 5, the cathode tube 1, and the heat shielding layer 3. The cathode tube 1 is provided with a fixing portion abutting the right side of the insulating lining 6, which is used to limit the position of the insulating lining 6.

[0031] like Figures 2 to 4 As shown, the material of the heating element 5 is isostatically pressed high-density graphite. The melting point of graphite is as high as 3850°C, which is extremely resistant to high temperatures, has good thermal stability and thermal shock resistance, and can withstand drastic temperature changes without cracking and damage. It has a small thermal expansion coefficient and good electrical and thermal conductivity and chemical stability. Its electrical conductivity is more than one hundred times higher than that of general non-metals, and its thermal conductivity exceeds that of metal materials such as steel, iron, and lead. It is resistant to acid, alkali, and organic solvent corrosion. The strength of graphite increases with increasing temperature. At 2000°C, the strength of graphite is doubled compared to normal temperature, which can meet the use requirements of hollow cathode heaters. The graphite structure heating element does not have the inherent shortcomings of refractory metal heating wires, and can always provide a firm and reliable heating element material and structural guarantee for the long-term stable operation of the hollow cathode. In addition, isostatically pressed high-density graphite has excellent properties such as uniform blank structure, high density, high strength, low hardness, and isotropy. It is easy to process into parts of precise size, reducing material and processing costs.

[0032] The heating element 5 comprises a heating section 52 and a lead end 51. The lead end 51 is formed integrally with the heating section 52. The processing of the heating element is specifically as follows: the heating element 5 is staggeredly cut along the axial direction, and the cuts are densely and evenly distributed along the axial direction. The remaining graphite forms an axial tortuous, elongated and evenly distributed current channel to form the heating section 52, so that the resistance is increased to ensure sufficient heating value. The heating element 5 is also provided with an axial through cut 53, and the two ends of the corresponding heating section 52 are symmetrical on both sides of the through cut 53 and along the through cut 53, and the two ends of the current channel are further extended along the axial direction to form two lead ends 51. It can be seen that the heating section 52 is a narrow and long current channel, which is close to the outer wall of the insulating lining 6, extends axially from one end of the insulating lining 6 to the other end, then turns back and repeats the process until the outer wall of the insulating lining 6 is covered. The gap between two adjacent sections of the current channel is 0.2-1mm, and the width of the current channel is 0.3-2mm, which not only ensures the heating effect of the heating section 52, but also avoids short circuits. The cross-section of the lead end 51 is significantly larger than the cross-section of the current channel to increase the structural strength of the lead end 51, while reducing the resistance and voltage division of the lead end 51, reducing unnecessary heating, and improving the thermal efficiency of the heating element 5. A lead fixing part 4 is provided on each lead end 51 for connecting the leads of the external circuit. The lead fixing part 4 can be selected from a fixed threaded hole, etc., and the leads are connected and fixed by isostatically pressed high-density graphite screws and gaskets. The material of the lead fixing part 4 is selected from isostatically pressed high-density graphite, which is the same material as the heating element 5, has the same thermal expansion coefficient, and is not easy to be damaged.

[0033] like Figure 1 As shown, the insulating lining 6 is a thin-walled cylindrical structure with a wall thickness of 0.3-1 mm, which is used to maximize the heat transfer efficiency of the insulating lining 6 while ensuring structural strength. Its inner wall surface is in contact with the cathode tube 1, and its outer wall surface is in contact with the heating element 5, which is used to insulate the heating element 5 from the cathode tube 1 to prevent short circuit and enable the heat of the heating element 5 to be transferred to the cathode tube 1 to the greatest extent. Figure 1 From the perspective, corresponding to the position of the lead end 51, the insulating liner 6 also extends radially outward, abutting against a portion of the right side of the lead end 51. The cathode tube 1 is provided with a fixing portion at the right end of the insulating liner 6, which clamps and fixes the insulating liner 6 and the lead end 51 with the frame 2 to fix and support the insulating liner 6 and the lead end 51. In some embodiments, there is a gap between the insulating liner 6 and other areas on the right side of the lead end 51 instead of direct contact, so as to reduce the heat dissipation of the heating element 5 to the outside.

[0034] The frame 2 covers and is fixed to the outer surface of the heating element 5. The frame 2 is respectively abutted and fixed to the two ends of the outer surface of the heating section 52 and the partial area on the left side of the lead end 51, and there is a gap between the frame 2 and the outer side of the heating element 5 and other areas to reduce the heat conduction between the heating element 5 and the outside, so as to reduce the heat dissipation. The insulating lining 6 and the frame 2 clamp the two sides of the lead end 51 together. Figure 1 From the perspective, according to the increase in the cross-section of the lead end 51, the right end of the skeleton 2 is designed to be stepped, and a gap is provided between the skeleton 2 and the lead end 51 in the stepped area to prevent the skeleton 2 from overfitting with the lead end 51 and to avoid damage to the lead end 51. The left and right sides of the skeleton 2 extend outwards respectively, thereby forming a groove on the outside of the skeleton 2, and several layers of carbon felt are laid in the groove as a heat shielding layer 3 to shield heat and reduce heat conduction and radiation losses.

[0035] In some embodiments, the insulating lining 6 and the frame 2 are made of high temperature resistant insulating ceramics, such as alumina ceramics, boron nitride ceramics, etc.

[0036] In the working state, voltage is applied to the heating element 5 through the lead end of the heater, and the current generates heat through the tortuous, slender and evenly distributed heating element 5. The heat generated by the heating element 5 is protected by the skeleton 2 and several layers of heat shielding layers and is conducted to the emitter in the cathode tube 1 through the insulating lining 6, heating the emitter to a sufficiently high temperature so that the emitter starts to emit electrons, successfully igniting the hollow cathode, and the hollow cathode enters self-sustaining stable operation. The heater is powered off and waits for the next ignition.

[0037] In summary, the present invention uses isostatically pressed high-density graphite material instead of traditional refractory metal wire as the heating element. Graphite is extremely resistant to high temperatures and can withstand drastic temperature changes without cracking and damage. It has a small thermal expansion coefficient and has good electrical and thermal conductivity and chemical stability. The strength of graphite increases with increasing temperature and can meet the use requirements of hollow cathode heaters. In addition, graphite has low hardness, low price, and is easy to process into parts of precise size, reducing material and processing costs. The heater adopts a mechanical assembly structure based on graphite material and high-temperature resistant ceramics, with a simple process, easy to manufacture, assemble and debug, low cost, high reliability, strong high-temperature resistance, high heating efficiency, long service life, and the cross-section of the lead end of the heating element is significantly larger than that of the heating section; the insulating lining is a thin-walled cylinder, which is completely and tightly fitted with the inner wall of the heating element and the outer wall of the cathode tube. The insulating lining and the ceramic skeleton realize the structural clamping and reliable insulation of the heating element. The design of the lead end, insulating lining, skeleton, and thermal shielding layer improves the efficiency of inward heat conduction and reduces unnecessary heating and heat dissipation. The invention can be widely used in the main cathode and neutralizer cathode of electric thrusters, as well as the hollow cathode of plasma contactors, etc.

[0038] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A hollow cathode high temperature heater for heating an emitter in a cathode tube, characterized in that: Include: With the cathode tube as the center, the cylindrical insulating lining, heating element, frame, and heat shielding layer are nested and assembled from the inside out; The insulating lining and the frame are made of high temperature resistant insulating ceramics; the wall thickness of the insulating lining is 0.3-1mm, and its inner surface fits the cathode tube; The heating element is made of isostatically pressed high-density graphite; the heating element comprises a heating section and a lead end formed by integral processing; the heating section is a zigzag current channel, closely attached to the outer wall of the insulating liner, axially extending from one end of the insulating liner to the other end, then turning back and repeating the process until the outer wall of the insulating liner is fully covered, so that the current channel is axially laid into several sections, and a gap is provided between two adjacent sections to prevent short circuit; the lead end is formed by the two ends of the current channel continuing to extend axially and expanding radially, the two lead ends are adjacent and have a gap, and the cross section of the lead end is larger than the cross section of the current channel; The frame is coated and fixed to the outer surface of the heating element, and the two ends of the frame extend radially outward to form a groove on the outer surface of the frame; several layers of heat insulation material are laid in the groove to form the heat shielding layer; The insulating lining and the frame respectively cover the two sides of the lead end and are used to fix the lead end; The frame is respectively abutted against and fixed to two ends of the outer surface of the heating section, and a gap is provided between the frame and other areas of the outer surface of the heating section; The frame and the insulating lining clamp and fix the two sides of the lead end, the insulating lining abuts against a part of the lead end, and a gap is provided with other areas of the lead end; The side of the frame close to the lead end is stepped, and a gap is provided between the frame and a part of the lead end.

2. The hollow cathode high temperature heater according to claim 1, characterized in that: The heat insulating material is carbon felt.

3. The hollow cathode high temperature heater according to claim 1, characterized in that: The lead end is provided with a lead fixing piece for connecting an external circuit; the material of the lead fixing piece is selected from isostatic high-density graphite.

4. The hollow cathode high temperature heater according to claim 1, characterized in that: The high temperature resistant insulating ceramic is one of alumina ceramic and boron nitride ceramic.

5. The hollow cathode high temperature heater according to claim 1, characterized in that: The gap between two adjacent sections of the current channel is 0.2-1 mm, and the width of the current channel is 0.3-2 mm.

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