A non-working fluid thermionic cathode for electric propulsion

Abstract

The application belongs to the field of space electric propulsion, and particularly relates to a propellant-free hot cathode for electric propulsion, which comprises an emitter material, an emitter, a support structure, N heat shields, an extraction grid, a base and an outer shell; the emitter is used to generate heat and heat the emitter material, the emitter material is used to heat and emit electrons; the support structure is used to reduce heat dissipation of the emitter; the N heat shields are used to reduce heat radiation of the emitter to the outside; the cathode emitter heater is integrated with the emitter material, and is isolated by the heat shield design, so that the emitter material can reach a temperature sufficient to meet the emission current under as much heating power as possible; and an extraction grid structure is designed, which can apply a certain electric field to the surface of the emitter, improve the emission current density of the emitter, relieve the space charge effect, and reduce the hindrance of the emitted electrons of the emitter to the process of being extracted to the thruster plume to participate in neutralization.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of space electric propulsion, and particularly relates to a propellantless thermionic cathode for electric propulsion. BACKGROUND

[0002] In the space gravitational wave detection mission, the propellantless thermionic cathode relies on the surface electric field and the thermal electron emission of the material to directly release electrons to the ion plume of the thruster for neutralization; in the miniaturized propulsion system, the propellantless thermionic emission cathode needs to emit electrons under a larger heating power to meet the demand of the ion plume neutralization of the thruster; at the same time, in order to improve the neutralization efficiency of the propellantless cathode to the ion plume, the propellantless cathode also needs to improve the proportion of the current acting on the ion plume in the entire emission current. SUMMARY

[0003] To solve the problem of low neutralization efficiency of the propellantless thermionic emission cathode to the ion plume in the prior art, a propellantless thermionic cathode for electric propulsion is provided;

[0004] A propellantless thermionic cathode for electric propulsion, comprising: an emission body material, an emission body, a support structure, N heat shields, an extraction grid, a base and an outer shell.

[0005] The outer shell is a hollow structure, and one end of the outer shell is provided with a through hole; the outer shell is sleeved outside the emission body material, the emission body, the support structure and the N heat shields, and the other end of the outer shell is fixedly connected with one end of the base; the extraction grid is fixed in the through hole of the outer shell; the extraction grid is a screen structure, and the extraction grid is fixed at a fixed distance from the emission body material; the emission body material is arranged at one end of the emission body, the other end of the emission body is sleeved outside one end of the support structure, and the other end of the emission body is fixedly connected with one end of the support structure; the other end of the support structure is fixedly connected with the other end of the base; the N heat shields are sequentially sleeved outside the support structure and the emission body; the N heat shields are fixedly connected with the support structure; the N heat shields, the emission body material, the emission body and the support structure are arranged on the same central axis.

[0006] The emission body is used to generate heat and heat the emission body material, and the emission body material is used to emit electrons after being heated; when one end of the extraction grid is connected with the positive electrode of an electric field power supply and the emission body material is connected with the negative electrode of the electric field power supply, an electric field is formed between the extraction grid and the emission body material (6), and the electrons emitted by the emission body material pass through the screen mesh apertures of the extraction grid; the support structure is used to reduce the heat conduction of the emission body to the outside of the propellantless thermionic cathode; and the N heat shields are used to reduce the heat radiation of the emission body to the outside of the propellantless thermionic cathode.

[0007] The beneficial effects of the present application: One of the present application for electric propulsion of working fluid-free thermionic cathode aims to integrate the cathode emitter heater with the emitter material, and through the heat shield heat insulation design, to achieve the emitter material to reach a temperature sufficient to meet the emission current at the lowest possible heating power; and design a extraction grid structure, which can apply a certain electric field to the surface of the emitter, improve the emission current density of the emitter, alleviate the space charge effect, and reduce the obstruction of the emitted electrons of the emitter to the process of participating in the neutralization of the thruster plume. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 A perspective view of the main working structure of a working fluid-free thermionic cathode for electric propulsion according to the embodiment of the present application;

[0009] Figure 2 A cross-sectional view of a working fluid-free thermionic cathode for electric propulsion according to the embodiment of the present application;

[0010] Figure 3 A structure diagram of the emitter and support structure according to the embodiment of the present application;

[0011] Figure 4 A cross-sectional view of the emitter and support structure according to the embodiment of the present application;

[0012] Figure 5 A schematic diagram of a working fluid-free thermionic cathode for electric propulsion according to the embodiment of the present application. DETAILED DESCRIPTION

[0013] Embodiment I: The technical solutions of the present application will be described below in conjunction with the drawings of the embodiments of the present application, and the present embodiment will be described, so as to clearly and completely describe the technical solutions in the embodiments of the present application: Figure 1 to the drawings Figure 5 of the present application, and the present embodiment will be described, so as to clearly and completely describe the technical solutions in the embodiments of the present application:

[0014] A working fluid-free thermionic cathode for electric propulsion, comprising: an emitter material 6, an emitter 5, a support structure, N heat shields, an extraction grid 1, a base 4 and an outer shell 8;

[0015] The outer shell 8 is a hollow structure with a through hole at one end. The outer shell 8 is fitted over the emitter material 6, emitter 5, support structure, and N heat shields. The other end of the outer shell 8 is fixedly connected to one end of the base 4. The lead-out grid 1 is fixed inside the through hole of the outer shell. The lead-out grid 1 is a screen structure, and it is spaced at a fixed distance from the emitter material 6. The emitter material 6 is positioned at one end of the emitter 5, and the other end of the emitter 5 is fitted over one end of the support structure. The other end of the emitter 5 is fixedly connected to one end of the support structure. The other end of the support structure is fixedly connected to the other end of the base 4. N heat shields are sequentially fitted over the support structure and emitter 6. The N heat shields are fixedly connected to the support structure. The N heat shields, emitter material 6, emitter 5, and support structure are arranged along the same central axis.

[0016] The emitter 6 is used to generate heat and heat the emitter material 7, which emits electrons after being heated. When one end of the lead-out gate 1 is connected to the positive terminal of the electric field power supply and the emitter material 7 is connected to the negative terminal of the electric field power supply, an electric field is formed between the lead-out gate 1 and the emitter material 6, and the electrons emitted by the emitter material 6 pass through the mesh of the lead-out gate 1. The support structure is used to reduce the heat conducted from the emitter 5 to the outside of the non-working medium hot cathode. N heat shields are used to reduce the heat radiated from the emitter 5 to the outside of the non-working medium hot cathode.

[0017] Specifically, during the operation of the cathode emitter, the relationship between the work function of the emitter material surface and the emission current density (emission current magnitude / emission surface area) is as follows:

[0018] In the formula, j e Thermoemission current density, in A·m -2 A is the thermal emission constant, which is 1204000 A·m. -2 ·K -2 ;k B The value is Boltzmann's constant, which is 1.38 J·K. -1 T is the temperature of the metallic material, in Kelvin; D is the thermionic emission current correction factor; W f Let be the surface work function (work function) of the emitter material, representing the minimum energy at which an electron leaves the material surface; exp(·) represents an exponential function.

[0019] The formula above describes thermionic emission from a material without an electric field at the emitter surface. When an electric field is present at the cathode surface, the work function of the material decreases; this effect is known as the Schottky effect. Therefore, the thermionic emission current density when thermionic electrons are extracted from the emitter under the influence of an electric field can be expressed as:

[0020] Where E is the electric field strength, with units of V·m. -1 ε0 is the vacuum permittivity, taken as 8.85 × 10⁻⁶. -12 F·m-1 e represents the unit charge, which is 1.6 × 10⁻⁶. -16 C (Coulomb);

[0021] These two formulas show that there are two main ways to improve the electron emission performance of an emitter, or a combination of these two methods: increasing the temperature of the emitter and increasing the electric field strength applied to the surface of the emitter, or a combination of both. All of these can effectively increase the emission current density of the emitter.

[0022] To effectively increase the temperature of the emitter while maintaining the same heating power, it is necessary to reduce the heat loss of the emitter's heating structure. Heat transfer from an object mainly occurs through three mechanisms: conduction, convection, and radiation, described by the following equations:

[0023] φ=-λSΔT+hSΔT+εSσT 4 Where S is the heat transfer area in m²; h is the convective heat transfer coefficient; ΔT is the temperature gradient; φ is the total heat flux of the three heat transfer methods; λ is the thermal conductivity of the material in W / (m·K); and σ is the blackbody radiation constant 5.67·10⁻⁸ W / (m²·K). 2 ·K 4 ε is the emissivity coefficient of the material surface radiation, which is a dimensionless coefficient and is related to the surface roughness and the material itself.

[0024] Conduction, convection, and radiation correspond to the three terms on the right side of the formula, respectively. In the high-vacuum environment of space electric propulsion, convection is almost non-existent; therefore, the main forms of heat transfer are radiation and conduction, as shown in the attached diagram. Figures 1-2 As shown, this application reduces heat loss of the emitter material by increasing the heat conduction path through the support structure, and reduces the outward heat radiation of the emitter through N heat shields, thereby achieving a higher emitter temperature at a lower heating power and improving the electron emission capability of the emitter. The working fluidless hot cathode of this application for electric propulsion also includes an extraction grid, which is a screen structure. The extraction grid is fixed in the through hole of the shell, and the electrons emitted by the emitter material pass through the screen pores of the extraction grid. Compared with the throttling orifice of the hollow cathode in the prior art, the setting of the extraction grid enables the hollow cathode to eject more negatively charged plasma outward per unit time, thereby increasing the emission current density of the emitter.

[0025] Furthermore, the support structure includes a support body 3 and a fixed base 9; the support body 3 is a cylindrical shell; one end of the cylindrical shell is fixedly connected to the launcher 5, and the other end of the cylindrical shell is fixedly connected to the fixed base 9, which is also fixedly connected to the other end of the base 4; the surface of the cylindrical shell is provided with multiple rectangular through holes.

[0026] Specifically, for conductive heat dissipation, the main insulation method is to use a thermal labyrinth in the form of the support structure of the emitter, as shown in the attached figure.Figures 3-4 As shown, the cylindrical shell of the support body has multiple rectangular through holes forming a thermal labyrinth structure. When the integrated emitter is continuously heated, heat is generated and conducted to the emitter material at the front end, causing the emitter material to emit electrons outward. The support structure can effectively increase the length of the heat conduction path, which is beneficial to increase the heat conduction path to the lower temperature part, reduce heat transfer loss, and reduce heating power.

[0027] Furthermore, the cylindrical shell surface is provided with x·y rectangular through holes, where x is the number of rows of rectangular through holes, and each row includes y rectangular through holes;

[0028] The spacing between any two adjacent rectangular through holes in any row is the same, and the spacing between any two adjacent rectangular through holes in each row is the same.

[0029] The shape center of the j-th rectangular through hole in row i is located on the perpendicular bisector of the line connecting the shape centers of the two rectangular through holes adjacent to the j-th rectangular through hole in row i-1.

[0030] Specifically, experiments have shown that rectangular through holes are easy to process in the fabrication of support structures and allow for the longest possible heat conduction path. Multiple rectangular through holes arranged in an orderly manner increase the heat conduction path.

[0031] Furthermore, all N heat shields are open-top cylindrical bodies with circular holes at the bottom. The support structure passes through the circular holes at the bottom of the N heat shields and is fixedly connected to them. The heat shield with the smallest distance from the support structure or the emitter 5 is designated as the first heat shield, and the cylindrical wall of the first heat shield is spaced apart from the support structure or the emitter 5 by a first predetermined distance. The cylindrical wall of the nth heat shield is spaced apart from the cylindrical wall of the (n-1)th heat shield by a second predetermined distance, where 1 < n ≤ N.

[0032] Specifically, in practical applications, the first set distance between the first heat shield and the supporting structure or emitter 5 can be adjusted according to the actual situation, and the distance between two adjacent heat shields can be adaptively adjusted according to the actual situation; for example... Figures 1-2 As shown, this embodiment sets up three heat screens, namely the first heat screen, the second heat screen and the third heat screen. The axial length of the heat screen is set according to actual needs. The first heat screen and the second heat screen have the same length and are fixedly connected to the support structure respectively. Due to space requirements, the axial length of the third heat screen in this embodiment is shorter than the axial length of the first heat screen and the second heat screen. The third heat screen is fixedly connected to the second heat screen.

[0033] Furthermore, the emitter 5 includes a cylindrical shell, a filling ceramic, and a hot wire 7; the filling ceramic is filled inside the cylindrical shell, and the hot wire 7 is disposed inside the filling ceramic; the cylindrical shell is sleeved on the outside of one end of the support structure and is fixedly connected to the support structure; the hot wire 7 is used to generate heat by electric heating.

[0034] Furthermore, the length of the fixed distance between the lead-out gate 1 and the emitter material 6 is in the range of 0.2 mm to 2 mm.

[0035] Specifically, in addition to ensuring the emitter's temperature meets the requirements for electron extraction, an electric field needs to be applied to the emitter's surface to extract the emitted charge. Therefore, we designed an extraction gate structure with a fixed distance (0.2-2 mm) between it and the emitter material. This ensures that the electric field between the extraction gate and the emitter material is sufficient to reduce the difficulty of electron emission. The extraction gate is a high duty cycle grid. When the cathode is working, a voltage higher than the emitter's potential is applied. Thus, a stable and uniform electric field is formed between the grid and the emitter, pointing towards the emitter. This electric field effectively reduces the difficulty of electrons detaching from the emitter surface and increases the emission current density. Compared to simple thermal emission of electrons from the material, this greatly improves the emitter's extraction performance. Furthermore, this electric field can accelerate electrons that have already detached from the surface, reducing the density of suspended electrons above the emitter surface and mitigating the space charge effect.

[0036] Furthermore, the outer casing 8 is a hollow structure made of metallic material. Specifically, the outer casing is made of metallic material to apply a potential to the lead-out gate.

[0037] In this embodiment, the N heat shields are cylindrical shells of different diameters. The heat shields are made of heat-resistant metals with low emissivity and high reflectivity. In specific implementations, tantalum or molybdenum can be used as the heat shield material. The purpose is to withstand the radiative heat transfer when the emitter is heated to the working temperature and to isolate and reflect the radiated heat to reduce heat loss. Since any object with a temperature will continuously radiate heat outward, at lower temperatures it emits infrared light, and as the temperature rises, the object begins to turn red and then yellow. This outward energy transfer is not negligible for achieving cathode performance with low power consumption. Therefore, a multi-layered heat-resistant metal heat shield structure is added. When the emitter is working, some of the emitted radiation is reflected and some is absorbed by the first heat shield, which heats the first heat shield. As its temperature rises, it also radiates heat. The second heat shield is used to reflect the heat radiated by the first heat shield.

[0038] like Figure 5As shown, this application discloses a working fluidless hot cathode for electric propulsion. By integrating the emitter with the heater, a support structure and a heat shield are added. The support structure reduces conductive heat dissipation, and multiple heat shields reduce radiative heat transfer. The support structure and heat shield ensure that the internal hot wire of the emitter can reach a sufficiently high temperature after heating, allowing more heat to be used to heat the emitter material. At the emitter temperature that achieves sufficient electron emission capability, the electron emission capability of the emitter is improved with the lowest possible heating power. In addition, a voltage with a higher potential than that of the emitter is applied to the lead-out grid, so that an electric field is formed between the grid and the emitter, reducing the surface electric field of the emitter material, drawing out electrons, and allowing the electrons to pass through the grid and be emitted into space. The combination of the support structure, lead-out grid, and heat shield enables the emitter, the core component, to work normally. As a whole, it is a working fluidless cathode that is different from traditional hollow cathodes and can emit electrons into space.

[0039] In practical use, under a heating power of 4W, the working fluidless hot cathode of this application for electric propulsion begins to show a current of 0.01 mA that can be detected by instruments, and the temperature of the emitter reaches 900-1000℃ at this time, and finally achieves an emission current of 4mA within 10W.

[0040] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A fluidless thermal cathode for electric propulsion, characterized in that, include: The emitter material, emitter, support structure, N heat shields, lead-out grid, base and outer shell; The outer shell is hollow with a through hole at one end. It is fitted over the emitter material, the emitter, the support structure, and N heat shields. The other end of the outer shell is fixedly connected to one end of the base. An outlet grid is fixed inside the through hole of the outer shell. The outlet grid is a mesh structure, spaced at a fixed distance from the emitter material. The emitter material is positioned at one end of the emitter, and the other end of the emitter is fitted over one end of the support structure, which is fixedly connected to that end. The other end of the support structure is fixedly connected to the other end of the base. N heat shields are sequentially fitted over the support structure and the emitter. The N heat shields are fixedly connected to the support structure. The N heat shields, emitter material, emitter, and support structure are arranged along the same central axis. The emitter generates heat and heats the emitter material, which then emits electrons after being heated. When one end of the lead-out grid is connected to the positive terminal of the electric field power supply, and the emitter material is connected to the negative terminal of the electric field power supply, an electric field is formed between the lead-out grid and the emitter material. Electrons emitted by the emitter material pass through the mesh of the lead-out grid. The support structure reduces the heat conducted from the emitter to the outside of the non-working fluid hot cathode. N heat shields reduce the heat radiated from the emitter to the outside of the non-working fluid hot cathode. The support structure includes a support body and a fixed chassis; The support body is a cylindrical shell; one end of the cylindrical shell is fixedly connected to the launcher, and the other end of the cylindrical shell is fixedly connected to the fixed base, which is also fixedly connected to the other end of the base; multiple rectangular through holes are provided on the surface of the cylindrical shell; All N heat shields are open cylindrical bodies with round holes at the bottom. The support structure passes through the round holes at the bottom of the N heat shields and is fixedly connected to the N heat shields. The first heat screen is defined as the heat screen with the smallest distance from the supporting structure or the emitter. The cylindrical wall of the first heat screen is spaced at a first predetermined distance from the supporting structure or the emitter. The cylindrical wall of the nth heat screen is spaced at a second predetermined distance from the cylindrical wall of the (n-1)th heat screen, where 1 < n ≤ N.

2. The working fluidless thermionic cathode for electric propulsion according to claim 1, characterized in that: The surface of the cylindrical shell is provided with A rectangular through hole, in which x The number of rows for the rectangular through holes, each row including y A rectangular through hole; The spacing between any two adjacent rectangular through holes in any row is the same, and the spacing between any two adjacent rectangular through holes in each row is the same. No. i Line number j The center of the shape of the rectangular through hole is located at the... i -1 line and the i Line number j The perpendicular bisector of the line connecting the centers of two adjacent rectangular through holes.

3. The working fluidless thermionic cathode for electric propulsion according to claim 1, characterized in that: The emitter includes a cylindrical shell, a filling ceramic, and a hot wire; the filling ceramic is filled inside the cylindrical shell, and the hot wire is disposed inside the filling ceramic; the cylindrical shell is fitted onto the outside of one end of the support structure and is fixedly connected to the support structure. The heating element is used to generate heat by passing electricity through it.

4. A fluidless thermionic cathode for electric propulsion according to claim 1, characterized in that: The length of the fixed distance between the lead-out grid and the emitter material is in the range of 0.2 mm to 2 mm.

5. A fluidless thermionic cathode for electric propulsion according to claim 1, characterized in that: The outer shell is a hollow structure made of metal.

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