Hollow cathode

Abstract

To provide a hollow cathode which enables increase of an amount of a current of an electronic beam while inhibiting excessive increase in size.SOLUTION: A hollow cathode 1 includes an insert 15 serving as an electron discharge source. The insert 15 includes: an inner peripheral surface 20 and an inner bottom surface 21 forming an internal space 17 which is open in an electron discharge direction; and at least one operation gas G supply hole 19 which is open on at least one of the inner peripheral surface 20 and the inner bottom surface 21.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a hollow cathode.

Background Art

[0002] An electrostatic acceleration type thruster is a type of electric thruster mounted on a spacecraft. The electrostatic acceleration type thruster generates a high-temperature plasma of an operating gas, and obtains thrust by accelerating and discharging ions in the plasma with an electric field. The thruster becomes negatively charged by discharging ions. To prevent this charging, an electron source (i.e., an electron gun) is mounted on the thruster. The electron beam of the electron source irradiates the discharged ions to neutralize the ions. Generally, a hollow cathode that easily obtains an electron beam with a large current is used for this electron source (see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A hollow cathode includes an insert (i.e., an emitter) as an electron emission source. The insert is formed in a hollow cylindrical shape and emits electrons by heating with a heater or the like. The number of electrons emitted per unit area increases as the temperature of the insert rises. However, considering thermal damage, durability, operation stability, etc., there is an upper limit to the heating temperature of the insert. Therefore, when it is desired to increase the current amount of the electron beam within the operating temperature range, it is necessary to increase the surface area of the insert, and the insert inevitably becomes larger.

[0005] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a hollow cathode capable of increasing the current amount of an electron beam while suppressing excessive enlargement.

Means for Solving the Problem

[0006] The hollow cathode according to one aspect of the present disclosure includes an insert as an electron emission source. The insert includes an inner peripheral surface and an inner bottom surface that form an internal space opening in the electron emission direction, and at least one working gas supply hole that opens in at least one of the inner peripheral surface and the inner bottom surface.

[0007] The insert includes a first cylindrical portion including the inner peripheral surface and a second cylindrical portion including the inner bottom surface, and the first cylindrical portion and the second cylindrical portion may be provided separately from each other. The supply hole may be formed as a groove extending toward the inner peripheral surface at an end surface of the first cylindrical portion in contact with the second cylindrical portion. Both the first cylindrical portion and the second cylindrical portion have a cylindrical outer shape, and the outer diameter of the second cylindrical portion may be smaller than the outer diameter of the first cylindrical portion.

Advantages of the Invention

[0008] According to the present disclosure, it is possible to provide a hollow cathode capable of increasing the current amount of an electron beam while suppressing excessive enlargement.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6A

Figure 6B

Figure 7

Figure 8A

Figure 8B

Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present disclosure will be described. In the drawings, the same reference numerals are given to the common parts, and redundant descriptions are omitted. The hollow cathode according to the present embodiment is installed in an electric thruster (for example, a Hall thruster) such as an electrostatic accelerator and an electromagnetic accelerator, and is used as an ion beam neutralizer. However, the application of the hollow cathode is not limited to the propulsion mechanisms of these spacecraft.

[0011] First, an outline of the hollow cathode 1 according to the present embodiment will be described. FIG. 1 is a schematic configuration diagram of the hollow cathode 1. For convenience of explanation, the Z-axis and the Z-direction, which is the extending direction of the Z-axis, are defined. The Z-axis is, for example, the central axis of the hollow cathode 1. Also, the radial direction and the circumferential direction with respect to the Z-axis are defined. Further, in FIG. 1, the left side is defined as "front" and the right side is defined as "rear".

[0012] The hollow cathode 1 shown in FIG. 1 includes a cathode 10 (10A, 10B, 10C), a heater 11 provided radially outward of the cathode 10, a heat shield 12 provided radially outward of the heater 11, and a keeper (keeper electrode) 13 provided in front of the cathode 10. When viewed from the Z-direction, the cathode 10, the heater 11, and the heat shield 12 are arranged substantially concentrically. The cathode 10 is set to a low potential with respect to the anode of the above-described electric thruster by the anode power supply 51.

[0013] The cathode 10 includes an insert 15 (15A, 15B, 15C) which is an electron emission source, and a casing 16 that houses the insert 15. The insert 15 is formed of an electron emission material having a low work function. Such a material is, for example, lanthanum hexaboride (LaB6) or barium oxide (BaO). The insert 15 has, for example, a cylindrical outer shape extending in the Z direction. The insert 15 has an inner peripheral surface 20 and an inner bottom surface 21. The inner peripheral surface 20 and the inner bottom surface 21 form an internal space 17 which is a bottomed space.

[0014] The heater 11 is provided so as to surround the outer periphery of the cathode 10 and heats the insert within the cathode 10. The heater 11 is formed of a heat-generating material containing carbon or the like and generates heat by power supply from a heater power supply 52.

[0015] The heat shield 12 is a hollow cylindrical member that surrounds the outer periphery of the heater 11. The heat shield 12 is formed of a heat-resistant metal (high melting point metal) such as tantalum or molybdenum and shields the radiant heat generated inside the heat shield 12.

[0016] The keeper (keeper electrode) 13 is a plate-shaped conductive member. An aperture 13a that penetrates the keeper 13 in the Z direction is formed in the keeper 13. The central axis of the aperture 13a is located on the Z axis (see FIG. 2). A voltage from a keeper power supply 53 is applied between the keeper 13 and the cathode 10. By this voltage application, the potential of the keeper 13 becomes higher than the potential of the cathode 10, and electrons can be extracted from the plasma P generated within the insert 15.

[0017] The casing 16 is provided with a supply port 18 for the working gas G. The working gas G is a gas having little chemical reactivity and is, for example, a noble gas such as xenon. Further, the insert 15 is provided with a supply hole 19 for the working gas G. The supply hole 19 opens to at least one of the inner peripheral surface 20 and the inner bottom surface 21.

[0018] The driving gas G is introduced from the supply port 18 into the internal space 22 of the casing 16 (i.e., the external space of the insert 15) and the internal space 17 of the insert 15 through the supply hole 19. On the other hand, the insert 15 is heated by the heater 11, and a voltage is further applied between the insert 15 and the keeper 13. When the insert 15 is heated, electrons (so-called thermoelectrons) are emitted from the inner peripheral surface 20 and the inner bottom surface 21 of the insert 15 into the internal space 17. The emitted electrons collide with the driving gas G and ionize the gas. As a result, plasma P is generated in the internal space 17. Further, the electric field generated by the keeper 13 reaches the internal space 17 through the orifice 23, and as a result, electrons are drawn from the plasma P toward the front of the hollow cathode 1 (i.e., the left side in FIG. 1). That is, an electron beam is emitted forward from the hollow cathode 1.

[0019] Next, some examples of the configuration of the cathode according to the present embodiment will be described. FIG. 2 is a cross-sectional view of a cathode 10A according to the first example of the present embodiment. FIG. 3 is a side view of an insert 15A according to the first example of the present embodiment. FIG. 4 is a front view of a second support portion 25 according to each example of the present embodiment.

[0020] As shown in FIG. 2, the casing 16 is a tubular member that extends in the Z direction with the Z axis as the center. The casing 16 has an inner peripheral surface 16a. The inner peripheral surface 16a forms a cylindrical internal space 22. The casing 16 is formed of a heat-resistant metal (high melting point metal) such as tantalum or molybdenum.

[0021] As shown in FIG. 2, in the internal space 22 of the casing 16, an orifice 23, a first support portion 24, an insert 15A, a second support portion 25, and a biasing member 26 are arranged in order from the front to the rear. The orifice 23 functions as an outlet for electrons and the driving gas G. The biasing member 26 is, for example, a compression spring, and biases the orifice 23, the first support portion 24, the insert 15A, and the second support portion 25 forward to prevent them from shifting excessively in the Z direction.

[0022] Instead of the biasing member 26, a non-elastic component (not shown) that fills the space where the biasing member 26 is disposed may be arranged. However, even in this case, it is necessary to ensure the supply of the working gas G to the internal space 17 and prevent the displacement of the insert 15A or the like.

[0023] As shown in FIG. 3, the insert 15A has an outer peripheral surface 27, an outer bottom surface 28 located behind the outer peripheral surface 27, an inner peripheral surface 20, and an inner bottom surface 21 located behind the inner peripheral surface 20. The outer peripheral surface 27 and the outer bottom surface 28 form a cylindrical outer shape.

[0024] The inner peripheral surface 20 is a cylindrical surface centered on the Z axis. The inner peripheral surface 20 extends in the Z direction from the front end 15a of the insert 15A to the inner bottom surface 21. The inner bottom surface 21 is continuous with the inner peripheral surface 20. The inner bottom surface 21 is a plane intersecting the Z axis and is, for example, perpendicular to the Z axis. Therefore, the inner peripheral surface 20 and the inner bottom surface 21 form a bottomed cylindrical internal space 17 that opens forward (i.e., in the electron emission direction).

[0025] The insert 15A is divided into a first cylindrical portion 30 having the inner peripheral surface 20 and a second cylindrical portion 31 having the inner bottom surface 21 and the outer bottom surface 28. The first cylindrical portion 30 is located in front of the second cylindrical portion 31. In the insert 15A of the first example, the first cylindrical portion 30 and the second cylindrical portion 31 are formed as an integral part. Also, the outer diameter of the second cylindrical portion 31 in the first example is set to a value equal to or less than the outer diameter of the first cylindrical portion 30.

[0026] As shown in FIG. 3, the insert 15A has at least one supply hole 19 for the working gas G that opens to the inner peripheral surface 20. The supply hole 19 is a through hole that extends in the radial direction from the outer peripheral surface 27 of the insert 15A to the inner peripheral surface 20. Therefore, the internal space 17 of the insert 15A communicates with the external space of the insert 15A (i.e., the internal space 22 of the casing 16) through the supply hole 19. Note that the number and position of the supply holes 19 are arbitrary as long as the area of the inner peripheral surface 20 does not decrease excessively and sufficient supply of the working gas G to the internal space 17 is possible. For example, four supply holes 19 are provided at an angular interval of 90° along the circumferential direction.

[0027] As shown in FIG. 2, the first support portion 24 and the second support portion 25 support the insert 15A and form an annular gap 32 between the outer peripheral surface 27 of the insert 15A and the inner peripheral surface 16a of the casing 16. The gap 32 functions as a flow path for the working gas G following the supply hole 19.

[0028] The first support portion 24 is a disk member formed of carbon and having a predetermined thickness. The outer diameter of the first support portion 24 is substantially equal to the inner diameter (diameter) of the inner peripheral surface 16a (internal space 22). A through hole 24a is formed in the first support portion 24. The through hole 24a is formed around the Z axis. The diameter of the through hole 24a is substantially equal to the diameter (inner diameter) of the internal space 17 as viewed from the Z direction.

[0029] A countersink groove 24b shown in FIG. 2 is formed in the first support portion 24. The countersink groove 24b has a predetermined depth in the Z direction and is formed over the entire circumferential direction. The front end 15a of the insert 15A is inserted into the countersink groove 24b. If the insert 15A can be positioned by the second support portion 25 or other components (not shown), the countersink groove 24b may be omitted.

[0030] The second support portion 25 is a cylindrical member formed of carbon. The outer diameter of the second support portion 25 is substantially equal to the diameter of the internal space 22. A through hole 25a, a countersink groove 25b, and a slot 25c are formed in the second support portion 25. The through hole 25a is formed around the Z axis and penetrates the second support portion 25 in the Z direction.

[0031] The countersink groove 25b has a predetermined depth and is formed over the entire circumferential direction. The rear end 15b of the insert 15A is inserted into the countersink groove 25b.

[0032] As shown in FIG. 4, the slot 25c has a predetermined width in the circumferential direction and extends in the Z direction. The slot 25c extends from the inner peripheral surface 25d forming the through hole 25a to the outer peripheral surface 27 of the second support portion 25. Therefore, the second support portion 25 has a C-shaped cross-sectional shape when viewed from the Z direction. Note that if sufficient flow of the working gas G can be obtained through the slot 25c, the through hole 25a may be omitted.

[0033] As described above, the front end 15a of the insert 15A is inserted into the countersink groove 24b, and the rear end 15b of the insert 15A is inserted into the countersink groove 25b. As a result, the central axis of the internal space 17 of the insert 15A substantially coincides with the Z axis, and the relative position of the insert 15A with respect to the casing 16 is defined. Further, the gap 32 between the insert 15A and the inner peripheral surface 16a of the casing 16 is maintained, and the occurrence or promotion of a chemical reaction due to the contact between the two is suppressed. Also, excessive temperature drop of the insert 15A due to the contact between the two can be prevented.

[0034] Further, a slot 25c is formed in the second support portion 25. As a result, the internal space 17 of the insert 15A communicates with the supply port 18 through the internal space 22 including the supply hole 19, the gap 32, and the slot 25c. That is, a supply path for the working gas G from the supply port 18 to the internal space 17 is constructed.

[0035] As described above, the internal space 17 of the insert 15A that opens forward is formed by the inner peripheral surface 20 and the inner bottom surface 21. Therefore, when the insert 15A is heated, the inner peripheral surface 20 and the inner bottom surface 21 become electron emission surfaces. Since the inner bottom surface 21 functions as an electron emission surface, the total area of the electron emission surface is increased as compared with the case where the internal space of the same size is formed only by the inner peripheral surface. That is, the number of electrons emitted can be increased without extending the length of the inner peripheral surface along the Z direction (and thus the length of the insert). That is, according to the present embodiment, it is possible to provide a hollow cathode that can increase the current amount of the electron beam while suppressing excessive enlargement.

[0036] FIG. 5 is a cross-sectional view of the cathode 10B according to the second example of the present embodiment. FIG. 6A is a side view of the insert 15B according to the second example of the present embodiment. FIG. 6B is a front view of the insert 15B. The second example is different from the first example in the configuration of the insert. Other configurations are the same as those in the first example, and overlapping descriptions are omitted.

[0037] As shown in FIG. 6A, the insert 15B according to the second example, like the insert 15A according to the first example, has an outer peripheral surface 27 that forms a cylindrical outer shape, an outer bottom surface 28 that is located at the rear and surrounded by the outer peripheral surface 27, and an inner peripheral surface 20 and an inner bottom surface 21 that open forward and form a cylindrical bottomed inner space 17.

[0038] Also, like the first example, the insert 15A is divided into a first cylindrical portion 30 having an inner peripheral surface 20 and a second cylindrical portion 31 having an inner bottom surface 21 and an outer bottom surface 28. In the insert 15B of the second example as well, the first cylindrical portion 30 and the second cylindrical portion 31 are formed as an integral part (in other words, a single part).

[0039] However, compared to the first example, the position of the supply hole 19 is different in the second example. That is, the supply hole 19 opens on the inner bottom surface 21. Specifically, the supply hole 19 penetrates between the inner bottom surface 21 and the outer bottom surface 28. Also, the supply hole 19 is located in a region that overlaps with the through hole 25a of the second support portion 25 on the inner bottom surface 21 when viewed from the Z direction. The number of supply holes 19 is arbitrary. For example, as shown in FIG. 6B, four supply holes 19 are formed at equal angular intervals on the same circle when viewed from the Z direction.

[0040] The second support portion 25 according to the second example may or may not have a slot 25c (see FIG. 4) similar to the second support portion 25 according to the first example. In either case, the through hole 25a of the second support portion 25 functions as a part of the supply path of the operating gas G. Therefore, as shown in FIG. 5, the operating gas G is supplied to the inner space 17 via the through hole 25a and the supply hole 19.

[0041] The inner bottom surface 21 of the second example functions as an electron emission surface in a region other than the location where the supply hole 19 opens. Therefore, similar to the first example, the number of electrons emitted can be increased without extending the length of the inner peripheral surface along the Z direction.

[0042] Note that the insert 15B according to the second example may further have a supply hole 19 (see FIG. 3) that opens to the inner peripheral surface 20, similar to the first example. That is, the supply hole 19 is formed in both the inner peripheral surface 20 and the inner bottom surface 21. When the supply hole 19 opens to the inner peripheral surface 20 in the second example, the second support portion 25 has a slot 25c shown in FIG. 4.

[0043] FIG. 7 is a cross-sectional view of the cathode 10C according to the third example of the present embodiment. FIG. 8A is a side view of the insert 15C according to the third example of the present embodiment. FIG. 8B is a front view of the first cylindrical portion 30 of the insert 15C according to the third example. The third example has a different insert configuration compared to the first example. Other configurations are the same as those of the first example, and duplicate explanations are omitted.

[0044] Similar to the first and second examples, the insert 15C according to the third example also includes a first cylindrical portion 30 having an inner peripheral surface 20 and a second cylindrical portion 31 having an inner bottom surface 21. However, as shown in FIG. 7, in the insert 15C of the third example, the first cylindrical portion 30 and the second cylindrical portion 31 are provided as separate bodies from each other.

[0045] As shown in FIGS. 8A and 8B, the first cylindrical portion 30 is a hollow cylinder without an inner bottom surface 21. The first cylindrical portion 30 has a groove 34 in an annular end surface (rear surface) 33 that contacts the second cylindrical portion 31. The groove 34 extends from the outer peripheral surface 27 to the inner peripheral surface 20. The number of grooves 34 is arbitrary, for example, four as shown in FIG. 8B. The four grooves 34 are arranged at an angular interval of 90° around the Z axis, for example.

[0046] The second cylindrical portion 31 is a cylinder (disk) having a front surface (first surface) 35 including the inner bottom surface 21 and a rear surface (second surface) 36 as the outer bottom surface 28 of the insert 15C. The length (height) of the second cylindrical portion 31 in the Z direction is set to a value that can at least maintain mechanical strength. Note that this value is the same for the first example and the second example as well.

[0047] The front surface 35 of the second cylindrical portion 31 is in contact with the end surface 33 of the first cylindrical portion 30 (see FIG. 7). Thereby, the inner surface of the groove 34 of the first cylindrical portion 30 and the front surface 35 of the second cylindrical portion 31 of the second cylindrical portion 31 constitute the supply hole 19 for the working gas G.

[0048] In this way, by dividing the insert 15 according to the present embodiment into the first cylindrical portion 30 and the second cylindrical portion 31, the structure of each is simplified. Therefore, the mechanical strength of the insert can be improved.

[0049] The outer diameter of the second cylindrical portion 31 may be equal to the outer diameter of the first cylindrical portion 30, or may be smaller than the outer diameter of the first cylindrical portion 30. In the latter case, as shown by the dotted line and hatching in FIG. 8B, the front surface 35 of the second cylindrical portion 31 is in contact with the inner peripheral side portion of the end surface 33. Also, the outer peripheral side portion of the end surface 33 is exposed to the external space of the insert 15A (that is, the internal space 22 of the casing 16). Therefore, the supply path of the working gas G passing through the second cylindrical portion 31 can be expanded without changing the shape of the casing 16. For example, the concern that the supply amount of the working gas G to the internal space 17 is restricted by the gap 32 can be eliminated.

[0050] Note that the supply hole 19 is not limited to being constituted by the groove 34 shown in FIG. 8A. That is, the supply hole 19 may be a through hole exemplified in FIG. 3. Also, the supply hole 19 may be formed only in the second cylindrical portion 31, or may be formed in both the first cylindrical portion 30 and the second cylindrical portion 31, as exemplified in FIG. 6A. That is, also in the third example, the supply hole 19 is formed in at least one of the inner peripheral surface 20 and the inner bottom surface 21.

[0051] Note that the present disclosure is not limited to the above-described embodiments, but is shown by the description of the claims, and further includes all changes within the meaning and scope equivalent to the description of the claims.

Description of Reference Numerals

[0052] 1…hollow cathode, 10(10A, 10B, 10C)…cathode, 11…heater, 12…heat shield, 13…keeper (keeper electrode), 13a…aperture, 15(15A, 15B, 15C)…insert, 16…casing, 17…internal space, 18…supply port, 19…supply hole, 20…inner peripheral surface, 21…inner bottom surface, 22…internal space, 23…orifice, 24…first support portion, 24a…through hole, 24b…counterbore groove, 25…second support portion, 25a…through hole, 25b…counterbore groove, 25c…slot, 25d…inner peripheral surface, 26…biasing member, 27…outer peripheral surface, 28…outer bottom surface, 30…first cylindrical portion, 31…second cylindrical portion, 32…gap, 33…end face (rear face), 34…groove, 35…front face (first face), 36…rear face (second face), 51…anode power supply, 52…heater power supply, 53…keeper power supply, G…working gas, P…plasma

Claims

1. An insert as an electron emission source, wherein the insert has an inner peripheral surface and an inner bottom surface that form an internal space opening in the electron emission direction, and at least one supply hole for at least one working gas that opens in at least one of the inner peripheral surface and the inner bottom surface and includes a hollow cathode.

2. The insert includes a first cylindrical portion having the inner peripheral surface and a second cylindrical portion having the inner bottom surface, and the first cylindrical portion and the second cylindrical portion are provided separately from each other. The hollow cathode according to Claim 1.

3. The supply hole is formed as a groove extending toward the inner peripheral surface at an end surface of the first cylindrical portion in contact with the second cylindrical portion, both the first cylindrical portion and the second cylindrical portion have a cylindrical outer shape, and an outer diameter of the second cylindrical portion is smaller than an outer diameter of the first cylindrical portion. The hollow cathode according to Claim 2.

Images