Field-emission cathode electron source for space applications.

Abstract

To prevent deterioration and wall thinning of a cathode due to atomic oxygen and plasma ions in outer space, and prolong the life of a field emission cathode type electron source.SOLUTION: A field emission cathode electron source for outer space includes a cathode 12 that emits electrons 20, an extraction electrode 16 having an opening window 16A through which the electrons 20 pass, which extracts the electrons 20 from the cathode 12, and an extraction power supply 18 that applies an extraction voltage for extracting the electrons 20 from the cathode 12 between the cathode 12 and the extraction electrode 16, and a transparent thin film 30 that allows the electrons 20 to pass through and does not allow substances larger than electrons 20 to pass through is arranged in the opening window 16A of the extraction electrode 16 so as to block the opening window.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a field emission cathode type electron source for space use, and in particular to a field emission cathode type electron source for space use that can prevent cathode deterioration and thinning due to atomic oxygen and plasma ions and thereby extend its life, and to a conductive tether propulsion device for space use and a neutralizer for an electric propulsion for space use that use the same. [Background technology]

[0002] Electron sources that emit electrons into space are used for various purposes in space development, such as to prevent static electricity on artificial satellites, to neutralize tethered thrusters, and to neutralize electric thrusters including ion engines.

[0003] The conductive tether propulsion device used to remove space debris from satellite orbit is a propulsion device that propels the device by generating an electric current due to an induced electromotive force generated when a conductive tether crosses the Earth's magnetic field, and by interacting with the Earth's magnetic field, obtains the Lorentz force to propel the device. In this conductive tether propulsion device, a field emission cathode type electron source is promising as an electron emitter that emits electrons from one end of the tether into the plasma in space because it does not require a working gas and can operate at low power (Patent Document 1, Non-Patent Document 1).

[0004] In electric propulsion devices, such as ion engines, used for orbital control of space structures such as artificial satellites, maintaining north-south-east-west positions, and main propulsion, a neutralizer is used to neutralize the emitted positive ion beam by emitting electrons into space that are electrically equal to the emitted positive ion beam, thereby preventing the satellite or other space structure from becoming charged and maintaining the thrust (Patent Document 2). A field emission cathode type electron source has also attracted attention as an electron source for this neutralizer (Non-Patent Document 2).

[0005] Electron sources can take various forms, but as described above, the form in which the cathode serving as the electron emission source is a field emission type cathode does not require a power source for heating the cathode as in the case of a hot cathode such as a hollow cathode. For this reason, it is considered a promising electron source for ion engines and tether propulsion devices intended for use in outer space where power consumption is severely restricted. In particular, the form in which carbon nanotubes are used on the surface of a field emission type cathode is attracting attention as a technology applicable to ion engines and conductive tether propulsion devices due to the excellent properties of carbon nanotubes, such as high electron emission ability, resistance to residual gas, and long life.

[0006] 1, a field emission cathode type electron source generally comprises, as its main components, a cathode 12 that emits electrons 20, which are disposed on a base 10, an extraction electrode (also called a grid or gate) 16 having an opening window 16A through which the electrons 20 pass, and an extraction power supply 18 for applying a high voltage as an extraction voltage between the cathode 12 and the extraction electrode 16 to extract the electrons 20 from the cathode 12. In the figure, 14 is a mask for obtaining the effect of focusing electrons by shaping the electric field, and the width W indicates the region from which the electrons are emitted.

[0007] A strong electric field is generated between the cathode 12 and the extraction electrode 16 by the extraction power supply 18, and this strong electric field causes electrons 20 to be emitted from the cathode surface. The emitted electrons 20 flow toward the extraction electrode 16, pass through an opening window 16A of the extraction electrode 16, and are emitted outside the extraction electrode 16. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2007-109514 A [Patent Document 2] Japanese Patent Application Publication No. 11-280640 [Non-patent literature]

[0009] [Non-Patent Document 1] Shimada et al., "Effectiveness Evaluation of Atomic Oxygen Countermeasures for Carbon Nanotube Field Emission Cathode," Proceedings of the 58th Space Science and Technology Conference, November 12-14, 2014, Nagano Brick Hall [Non-Patent Document 2] Yamamoto et al., "Neutralization characteristics of an ion engine using a field emission cathode," Proceedings of 2019 Symposium on Laboratory Experiment for Space Science. (2019). Summary of the Invention [Problem to be solved by the invention]

[0010] However, outer space is an environment of atomic oxygen 22 and plasma ions 24, which collide with the cathode 12 at high speed, causing the cathode 12 to wear out due to reactions and impacts, resulting in a short lifespan. In addition, the electric field is weakened at the opening window 16A, so the electrons 20 emitted from the cathode 12 are accelerated and diverged, and some of them do not pass through the opening window 16A and are lost by colliding with the extraction electrode 16 in the periphery. As a result, more than 20% of the electrons 20 flow into the extraction electrode 16 and are lost, resulting in poor power utilization efficiency. In order to reduce this power loss, the extraction voltage cannot be increased and a low voltage is required. In addition, there are problems such as the electron source potential fluctuating due to the surrounding plasma ions 24, causing large fluctuations in the emission current.

[0011] The present invention has been made to solve the above-mentioned problems in the conventional technology, and has as its object to prevent deterioration and thinning of the cathode caused by atomic oxygen and plasma ions in outer space, thereby extending the life of a field emission cathode type electron source. [Means for solving the problem]

[0012] The present invention provides a method for emitting electrons into space using a wide electron emission region. RukaA field emission cathode type electron source for space use, comprising: a cathode; an extraction electrode having an opening window through which electrons pass in order to extract electrons from the cathode; and an extraction power source for applying an extraction voltage between the cathode and the extraction electrode in order to extract electrons from the cathode, the field emission cathode type electron source being configured to transmit electrons but not transmit through materials larger than the electrons. , conductive or dielectric The transparent membrane is placed through the opening of the extraction electrode. Underside The cathode is then blocked by a blocking member, thereby preventing atomic oxygen and plasma ions coming from outer space from colliding with the cathode, thereby solving the above-mentioned problem.

[0014] where The permeable thin film may be a carbon-based sheet made of carbon.

[0015] The permeable thin film may be a ceramic sheet made of ceramic.

[0016] The transparent thin film may be a ceramic sheet having a conductive layer formed on at least one surface thereof, or a laminate of such layers.

[0017] The extraction voltage is set so that the electrons pass through the thin film with an energy loss of 20% or less. 1 The acceleration voltage can be set to 0 kV or less.

[0018] The present invention also provides a conductive tether propulsion device for space use, comprising the field emission cathode type electron source described above.

[0019] The present invention also provides a neutralizer for an electric propulsion system for use in space, comprising the field emission cathode type electron source described above. Effect of the Invention

[0020] According to the present invention, a thin transmissive film that transmits electrons but does not transmit substances larger than electrons is disposed at the opening of the extraction electrode so as to cover the opening. This blocks atoms and ions larger than electrons and prevents atomic oxygen and plasma ions in outer space from colliding with the cathode, thereby preventing deterioration and thinning of the cathode and extending the life of the field emission cathode type electron source. [Brief description of the drawings]

[0021] [Figure 1] A cross-sectional view showing the basic structure of a conventional field emission cathode type electron source. [Diagram 2] FIG. 1 is a cross-sectional view showing a basic configuration of a first embodiment of the present invention. [Diagram 3] The same figure shows an example of the relationship between acceleration energy and energy loss in a thin film. [Figure 4] FIG. 11 is a cross-sectional view showing a basic configuration of a second embodiment of the present invention. [Diagram 5] The same figure shows an example of the relationship between acceleration energy and energy loss in a thin film. [Figure 6] FIG. 11 is a cross-sectional view showing a basic configuration of a third embodiment of the present invention. [Figure 7] The same figure shows an example of the relationship between acceleration energy and energy loss in a thin film. [Figure 8] FIG. 13 is a cross-sectional view showing the configuration of a ceramic sheet used in a modified example of the third embodiment. [Figure 9] FIG. 13 is a cross-sectional view showing a basic configuration of a fourth embodiment of the present invention. [Figure 10] The same figure shows an example of the relationship between acceleration energy and energy loss in a thin film. [Figure 11] FIG. 1 is a diagram showing an embodiment of a conductive tether propulsion device using a field emission cathode type electron source according to the present invention. [Figure 12] FIG. 1 is a diagram showing an embodiment of an ion engine using a field emission cathode type electron source according to the present invention as a neutralizer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the contents described in the following embodiments and examples. In addition, the constituent elements in the embodiments and examples described below include those that a person skilled in the art can easily imagine, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the constituent elements disclosed in the embodiments and examples described below may be appropriately combined or appropriately selected for use.

[0023] The first embodiment of the present invention is a field emission cathode type electron source having a configuration similar to that of the conventional example shown in FIG. 1, in which a transmissive thin film 30 is disposed in the opening window 16A of the extraction electrode 16 so as to cover it, as shown in FIG.

[0024] The cathode 12 may be made of, for example, carbon nanotubes CNT having a high aspect ratio.

[0025] In this embodiment, the atomic oxygen 22 and plasma ions 24 are blocked by the permeable thin film 30, so that the cathode 12 is not deteriorated or thinned by collisions with these.

[0026] Furthermore, the permeable thin film 30 If is conductive, Compared to the conventional example, a uniform parallel electric field is formed over a wider width W, and the number of electrons that collide with and flow into the extraction electrode 16 and are lost is almost zero. Therefore, by setting an extraction voltage that allows almost all electrons to pass through the permeable thin film 30 and causes no loss of electrons flowing into the extraction electrode 16, it is possible to eliminate power loss from the power supply. Since there is no power loss even when the extraction voltage is increased, it is possible to set the extraction voltage high enough to withstand potential fluctuations in space caused by plasma, and to keep the extraction current stable and without fluctuations.

[0027] In reality, when electrons pass through the thin film 30, there is no decrease in the amount of emitted electrons due to the electrons being trapped in the thin film 30 or being scattered backward, but there is an energy loss of the electrons due to inelastic scattering. 3The calculation results of the acceleration energy (extraction voltage) and the amount of energy loss in the thin film when using the 1000V electron beam are plotted with circles in Figure 3. At an acceleration energy of 10 keV or more (extraction voltage of 10 kV), the electron energy loss is 20% or less.

[0028] In this loss, assuming that the loss is due to radiation cooling into space, the current density of electron emission is 0.5mA / cm 2 The calculated temperature of the thin film in this case is plotted in the same Figure 3 with a triangle mark. The temperature is below 400°C when the acceleration energy is 9 keV or more. The relationship between temperature and acceleration energy can be calculated by multiplying the transmission window exponent (also called the transmission window coefficient) α by the thin film thickness (nm) × material density (g / cm 3 ) x current density (mA / cm 2 ), the same value is obtained if α is the same. In the example of Figure 3, the transmission window index α = 296.

[0029] The triangle plots indicate the maximum temperature rise when almost all inelastic scattering is assumed to have converted into plasmon vibration, i.e., thermal energy. In reality, however, some energy is released as secondary electron emission and does not contribute to the temperature rise of the transparent thin film.

[0030] The amount of secondary electrons emitted and their energy distribution vary significantly depending on the material, surface condition, film thickness, incident electron energy, and other conditions, making it difficult to accurately calculate the amount of emitted energy. However, the temperature of the thin transmissive film calculated based on the assumed secondary electron emission coefficient and secondary electron energy distribution is shown by dashed line A in Figure 3.

[0031] Taking into account this secondary electron emission, the temperature of the thin transmissive film shows a temperature distribution that is almost constant at 400°C at an acceleration energy of 5 keV and approximately 330°C at 8 keV, and indicates that there may be an optimal voltage value at voltages lower than 10 kV.

[0032] Furthermore, for a power source to be used in space, such as on a satellite, a voltage of 10 kV or less is preferable in terms of size, weight, etc., and the transparent thin film can be made of various materials, from light metals to ceramics, and has a heat resistance of 500 to 1000°C or more, but considering durability and heat resistance, a voltage of 400°C or less is considered preferable. Therefore, the thin film thickness, material density, and current density conditions that result in a transmission window index α of 300 or less are preferable, and the thin film thickness and material density conditions that result in an accelerating voltage of 10 kV or less and an electron transmission energy of 80% or more are preferable.

[0033] In this embodiment, metals with low material density, such as beryllium, aluminum, silicon, and titanium, and ceramics with high strength and high heat resistance, such as carbon nitride, silicon nitride, silicon oxide, alumina, and zirconia, can be used as the transmission thin film 30. As shown in the transmission window index α, materials that can be made thin can increase the current density or reduce the acceleration voltage, thereby making it possible to make the power source more compact, more powerful, and less expensive.

[0034] As specific condition ranges, the thickness of the transparent thin film 30 is 0.3 nm to 500 nm, and the density of the thin film material is 1.8 g / cm 3 ~6g / cm 3 The transmitted electron current density is 0.5mA / cm 2 ~500mA / cm 2 is feasible.

[0035] The second embodiment of the present invention is a field emission cathode type electron source having a configuration similar to that of the first embodiment shown in FIG. 2, in which a graphene sheet 31, which is a carbon-based sheet, is disposed in the opening window 16A of the extraction electrode 16 so as to cover the opening window 16A, as shown in FIG.

[0036] The cathode 12 may be made of, for example, carbon nanotubes CNT having a high aspect ratio.

[0037] In this embodiment, the graphene sheet 31 blocks the atomic oxygen 22 and plasma ions 24, so that the cathode 12 is not deteriorated or thinned due to collisions with these.

[0038] Furthermore, since the graphene sheet 31 is conductive, a uniform parallel electric field is formed over a wider width W than in the conventional example, and the number of electrons that collide with and flow into the extraction electrode 16 and are lost is almost zero. In reality, electron energy loss occurs when passing through the graphene sheet 31. The calculation results of the acceleration energy and energy loss in the transmission thin film when a graphene sheet with a thickness of 0.34 nm is used are plotted in Figure 5 with circles. The electron extraction energy rate (=100% - energy loss%) is 96.4% at 500 eV and 99.9% at 3.5 keV or more. At a current density of 50 mA / cm with a transmission window coefficient α = 37, 2 The calculated temperatures of the thin transmitting film at high current densities are plotted in the same Figure 5 as triangle marks, and are below 400°C at 500 eV and below 200°C at 3.5 keV.

[0039] Furthermore, similar to the first embodiment shown in Figure 3, the temperature of the transmitting thin film taking into account the secondary electron emission energy is shown by dashed line B in Figure 5, indicating the possibility that there is a region where there is almost no temperature rise between 2 and 4 keV acceleration energies.

[0040] From the above, when the graphene sheet 31 is applied to the transparent thin film 30, a high-output and compact electron source system is possible because a low acceleration voltage is possible at a high current density, and the temperature rise of the graphene can be reduced, so that a system with excellent life and durability can be achieved by suppressing reaction and deterioration. Even if 2 to 5 layers of the graphene sheet 31 are stacked to improve durability, there is almost no problem because the thickness is thin.

[0041] In the present embodiment, the graphene sheet 31 is used as the transmissive thin film, which is conductive, has high strength, and can be thinned, and is therefore particularly preferable.

[0042] The carbon-based transparent thin film made of carbon is not limited to a graphene sheet, and for example, a sheet of graphite or CNT can also be used.

[0043] In the graphene sheet 31 of the second embodiment, when the concentration of atomic oxygen 22 is high, there is a concern that reaction degradation may occur.

[0044] Therefore, in the third embodiment of the present invention, as shown in FIG. 6, a ceramic sheet 32 ​​made of a ceramic material (such as silicon nitride) that is not oxygen-reactive is disposed in the opening window instead of the graphene sheet 31 of the second embodiment.

[0045] Other points are the same as those in the second embodiment, and therefore the description will be omitted. Unlike the second embodiment, the third embodiment is not conductive, but acts as a dielectric to provide a uniform parallel electric field over a wider width W than the conventional example.

[0046] In this embodiment, the permeable thin film is a ceramic sheet 32, so it does not react with the atomic oxygen 22 and does not deteriorate or become thin.

[0047] Incidentally, the ceramic sheet 32 ​​has a larger thickness than the graphene sheet 31 of the second embodiment, and therefore the loss is larger.

[0048] In this embodiment, the ceramic material is silicon nitride (Si 3 N 4 In the past, silicon dioxide (SiO) was used as the ceramic material, but the ceramic material is not limited to this. For example, instead of nitride, oxide, such as silicon dioxide (SiO), may be used. 2 It is also possible to use other ceramic materials.

[0049] In this embodiment, the calculation results of the acceleration energy and the energy loss in the transmission thin film when silicon nitride (SiN) with a thickness of 10 nm is used as the ceramic sheet 32 ​​are shown by the plot of circles in FIG. 7. The electron extraction energy rate is 84.4% at 2 keV and 98.9% at 10 keV. Even if the thickness is increased to 20 to 30 nm to improve strength and durability, the loss is 10% or less at an acceleration voltage of 5 kV or more. A current density of 5 mA / cm with a transmission window coefficient α=165 is 2The calculated temperature of the transmissive thin film at 2 keV (plotted with triangle marks) is below 500°C at 2 keV and below 400°C at 4 keV, which is significantly below the thermal shock temperature of silicon nitride of 800°C. Furthermore, as shown in Figs. 3 and 5 for the first and second embodiments, the temperature of the transmissive thin film taking into account the secondary electron emission energy is indicated by dashed line C in Fig. 7, which is below 300°C at an acceleration energy of 5 keV, indicating the possibility of a minimum temperature region around 7 keV.

[0050] In the third embodiment, since the ceramic sheet 32 ​​is not conductive, there is a concern that a loss due to charging up of the permeable thin film 30 may occur at high currents. As a modification of the present invention that solves such a problem, a ceramic sheet 34 in which a conductive Si base is left by controlling doping can be used as the permeable thin film 30, as shown in FIG.

[0051] This ceramic sheet 34 can be produced, for example, by forming a SiN layer 34b on a base Si layer 34a by N-doping, for example, by ion beam or plasma, from the surface that will become the outer side of the permeable thin film 30, and then thinning the Si layer 34a from the surface that will become the inner side of the permeable thin film 30 by, for example, etching.

[0052] According to this embodiment, it is possible to suppress deterioration due to atomic oxygen concentration and reduce loss due to charge-up at the same time.

[0053] Next, a fourth embodiment of the present invention is shown in Fig. 9. In this fourth embodiment, a thin conductive material, such as a graphene sheet, having a thickness of several nm is bonded to the inside of a ceramic sheet 32 ​​similar to that in the third embodiment to form a conductive layer 36.

[0054] According to this embodiment, it is possible to suppress deterioration due to atomic oxygen concentration and reduce loss due to charge-up at the same time.

[0055] The calculation results of the acceleration energy and energy loss in the transmitting thin film when a 10 nm thick SiN sheet is laminated with a 0.34 nm thick graphene sheet are plotted with circles in Figure 10. The electron extraction energy rate is 79.3% at 2 keV and 98.9% at 10 keV. Even if the thickness is increased to 20-30 nm to improve strength and durability, the loss is less than 10% at an acceleration voltage of 5 kV or more. Transmission window coefficient α = 202, current density 5 mA / cm 2 The calculation result of the temperature of the transmissive thin film at 4 keV (plot with triangle mark) is below 400°C at 4 keV. Furthermore, similar to the third embodiment shown in Fig. 7, the temperature of the transmissive thin film considering the secondary electron emission energy is shown by dashed line D in Fig. 10, which is below 300°C at an acceleration energy of 5 keV, indicating the possibility of a minimum temperature region around 7 keV.

[0056] It is also possible to bond a CNT sheet or a graphite sheet instead of the graphene sheet, or to deposit a metal such as beryllium (Be), aluminum (Al), silicon (Si), or titanium (Ti) to form the conductive layer 36. In order to further suppress charge-up, it is also possible to place them on both sides of the film, sandwich them between ceramic films, or laminate them.

[0057] FIG. 11 shows a schematic diagram of an example in which the above embodiment is applied to a conductive tether propulsion device for space use.

[0058] In the figure, 100 is a conductive wire (tether), 110 is a field emission cathode type electron source (FEC) according to the present invention, 120 is an acceleration gate, 122 is an acceleration power supply, and 130 is a deceleration gate.

[0059] FIG. 12 is a schematic diagram of an example in which the above embodiment is used in a neutralizer for an ion engine for space propulsion.

[0060] In the figure, reference numeral 140 denotes an ion source using microwaves, 142 denotes an acceleration power supply for the ion source, 150 denotes an acceleration gate, 152 denotes an acceleration power supply, 160 denotes a deceleration gate, 170 denotes a neutralizer using the FEC according to the present invention, and 172 denotes an acceleration power supply for the neutralizer.

[0061] In the above embodiment, the extraction voltage is set to an acceleration voltage of 10 kV or less at which electrons pass through the thin film with an energy loss of 20% or less, but the extraction voltage is not limited to this.

[0062] The cathode 12 is not limited to one using CNTs. [Explanation of symbols]

[0063] 12…Cathode 16...Extraction electrode 16A…Opening window 18…Pull-out power supply 20...electronic 22...Atomic oxygen 24...Plasma ion 30…Transparent thin film 31...Graphene sheet 32, 34...Ceramic sheet 34a...Si layer 34b...SiN layer 36...Conductive layer 100...Conductive wire (tether) 110... Field emission cathode type electron source (FEC) 140…Ion source 170…Neutralizer (FEC)

Claims

1. a cathode having a wide electron emission area for emitting electrons into space; an extraction electrode having an opening window through which electrons pass, for extracting electrons from the cathode; an extraction power supply for applying an extraction voltage between the cathode and the extraction electrode for extracting electrons from the cathode; A field emission cathode type electron source for space use, comprising: A field emission cathode type electron source for use in outer space, characterized in that a thin conductive or dielectric film, which allows electrons to pass through but does not allow substances larger than electrons to pass through, is disposed below the opening window of the extraction electrode so as to cover the opening window, thereby preventing atomic oxygen and plasma ions coming from outer space from colliding with the cathode.

2. 2. A field emission cathode type electron source for use in space according to claim 1, wherein said transparent thin film is a carbon-based sheet made of carbon.

3. 2. A field emission cathode type electron source for use in outer space according to claim 1, wherein said transmitting thin film is a ceramic-based sheet made of ceramic.

4. 4. A field emission cathode type electron source for use in space according to claim 3, wherein said transparent thin film is a ceramic sheet having a conductive layer formed on at least one surface thereof, or a laminate of such ceramic sheets.

5. 5. A field emission cathode type electron source for use in space according to claim 1, wherein the extraction voltage is an acceleration voltage of 10 kV or less that allows electrons to pass through the transparent thin film with an energy loss of 20% or less.

6. 6. A conductive tether propulsion device for space use, comprising the field emission cathode type electron source according to claim 1.

7. 6. A neutralizer for an electric propulsion system for use in space, comprising the field emission cathode type electron source according to claim 1.

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