Hall thruster magnet exciting coil degassing device
By designing the Hall thrust excitation coil degassing device, using the tank to simulate the space environment, heating and cooling the excitation coil, the problem of difficulty in removing volatiles in the excitation coil in the prior art is solved, and the performance and reliability of the thrust are improved.
Patent Information
- Application Number
- CN202510142756.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-10
- Publication Date
- 2025-05-06
AI Technical Summary
The prior art is difficult to effectively remove volatiles from silicone resin before the excitation coil of Hall thrusts are assembled, resulting in a degradation of thrust performance.
A Hall thrust excitation coil degassing device is designed, including a tank body, a test platform, a heat conductor, a heat source, a cold source and a vacuum pump. By simulating the low temperature and vacuum conditions in the space environment, the excitation coil is heated and cooled to remove volatiles.
It realizes effective removal of volatiles before the excitation coil is assembled into the Hall thrust, improves the discharge efficiency and performance of the thrust, and avoids the damage of the lead outer skin.
Smart Images
Figure CN119943569A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of spacecraft electric propulsion, and in particular to a degassing device for an excitation coil of a Hall thruster. Background Art
[0002] The coil assembly of the Hall thruster mainly includes two coils, an inner coil and an outer coil. Through a specific winding method, its reliable insulation at high temperatures is ensured. When the Hall thruster is working, the heat generated by ions bombarding the inner wall of the discharge channel will be transferred to the excitation coil, and superimposed with the self-heating of the excitation coil when it is energized, the excitation coil is in a high temperature state. However, the excitation coil is formed by winding an inorganic fiber material on a core through a silicone resin. Under high temperature, the silicone resin will produce volatiles, which will enter the discharge channel and participate in the discharge, which will reduce the discharge efficiency of the thruster and contaminate the inner wall of the discharge channel, resulting in a decrease in the performance of the thruster. Therefore, before the excitation coil is assembled to the Hall thruster, it must be subjected to high-temperature degassing treatment to complete the removal of volatiles in advance.
[0003] Currently, how to effectively remove volatiles before the excitation coil is assembled to the Hall thruster has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention
[0004] The present invention provides a degassing device for an excitation coil of a Hall thruster, which is used to solve the problem of how to effectively remove volatiles before the excitation coil is assembled to the Hall thruster.
[0005] The present invention provides a degassing device for an excitation coil of a Hall thruster, comprising: Tank; The test platform is arranged inside the tank; The heat conducting member is installed on the test platform, used to carry the excitation coil of the Hall thruster, and is in contact with the lead wire of the excitation coil; A heat source is arranged outside the heat-conducting member and is used to heat the excitation coil; The cold source is connected to the heat-conducting component and is used to cool the heat-conducting component.
[0006] In some embodiments, the heat source comprises: The cylinder has a vertical axis, and the top and bottom ends are open structures, and the cover is arranged outside the heat conducting member; There are multiple spacer rings installed in the cylinder along the axial direction of the cylinder; There are multiple infrared heating tubes which are evenly installed on the spacer ring along the circumference of the spacer ring.
[0007] In some embodiments, a plurality of first mounting holes are provided on the side wall of the cylinder; the first mounting holes of each layer are oblong holes, and the extending direction is parallel to the axial direction of the cylinder; A plurality of second mounting holes are evenly arranged on each spacer ring along the circumferential direction; each second mounting hole is an oblong hole extending along the radial direction of the spacer ring.
[0008] In some embodiments, the heat source further comprises: The first heat insulation layer is attached to the inner wall of the cylinder.
[0009] In some embodiments, the cold source comprises: The cooling pipe is installed on the test platform and fixedly connected to the heat conducting part; The liquid storage tank is connected with the cooling pipe.
[0010] In some embodiments, the thermally conductive member comprises: A support rod for carrying the excitation coil of the Hall thruster; There are two heat-conducting rods, both of which have vertically arranged axes and contain cavities formed inside. The top ends are fixedly connected to the opposite ends of the support rods, and the bottom ends are fixedly connected to the cooling pipes. The heat-conducting rods are in contact with the leads of the excitation coils. There are two liquid absorbing cores, which are installed in the accommodating cavity in a one-to-one correspondence with the two heat conducting rods.
[0011] In some of the embodiments, a cooling channel is formed inside the side wall of the tank.
[0012] In some of the embodiments, a guide rail is provided at the bottom of the tank; the test platform is movably mounted on the guide rail.
[0013] In some embodiments, it also includes: A vacuum pump is connected to the tank.
[0014] In some embodiments, it also includes: Thermocouple, installed on the excitation coil, with the lead wire in contact with the heat-conducting member; The second heat insulating layer is coated on the outside of the heat conducting member, the lead wire of the excitation coil and the lead wire of the thermocouple.
[0015] The beneficial effects of the present invention are as follows: The degassing device for the excitation coil of the Hall thruster of the present invention is provided with a cooling channel inside the side wall of the tank body by arranging a tank body, a test platform, a heat conductor, a heat source, a cold source and a vacuum pump. The cooling channel is introduced into a cooling medium such as liquid nitrogen or liquid ammonia, so that the temperature inside the tank body can be reduced to the required temperature. The test platform is arranged in the tank body, and is used to support the heat conductor, the heat source and the cold source. The heat conductor is installed on the test platform, and is used to carry the excitation coil of the Hall thruster, and is in contact with the lead of the excitation coil. The heat source is arranged outside the heat conductor, and can generate heat to heat the excitation coil, so that the excitation coil is maintained at a specified temperature value for a specified period of time, so as to complete the task of removing volatiles in the silicone resin and the surface paint layer. The vacuum pump is connected to the tank body, and can evacuate the tank body. On the whole, the tank body is used to simulate the real environment of the excitation coil of the Hall thruster in space, and the vacuum degree of less than 1×10 - 4 Pa, the temperature is lower than 100K, and the surface temperature of the excitation coil is greater than 400℃. At the same time, during the heating process of the excitation coil, the cold source always cools down the heat-conducting parts and the leads of the excitation coil to ensure that the lead does not exceed 250℃, avoiding the lead part from experiencing additional high temperature and hardening, thereby avoiding the phenomenon of lead skin damage during assembly. It is suitable for degassing the excitation coils of Hall thrusters of different models, different powers and different sizes, and has the characteristics of wide application range, simple installation and convenient maintenance. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure 1 It is a structural schematic diagram of some specific embodiments of a degassing device for an excitation coil of a Hall thruster according to the present invention; Figure 2 yes Figure 1 A schematic diagram of the structure of the heat source in the degassing device of the Hall thruster excitation coil is shown; Figure 3 yes Figure 1 A schematic diagram of the combined structure of the heat conducting member and the cooling pipe in the degassing device of the excitation coil of the Hall thruster is shown; Figure 4 is a cross-sectional view of a heat conducting member; Figure 5 It is a schematic diagram of the temperature change of two sizes of excitation coils over time.
[0017] In the accompanying drawings, 110, tank body; 111, guide rail; 120, test platform; 130, heat conductor; 131, support rod; 132, heat-conducting rod; 133, wick; 140, heat source; 141, cylinder; 1411, first mounting hole; 142, spacer ring; 1421, second mounting hole; 143, infrared heating tube; 144, first thermal insulation layer; 150, cold source; 151, cooling tube; 160, top plate. DETAILED DESCRIPTION
[0018] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than 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.
[0019] As described in the background technology, when the Hall thruster is working, the heat generated by ions bombarding the inner wall of the discharge channel will be conducted to the excitation coil, and superimposed with the self-heating of the excitation coil when it is energized, so that the excitation coil is in a high temperature state. However, the excitation coil is formed by bonding an inorganic fiber material with a silicone resin and winding it on a wire core. At high temperatures, the silicone resin will produce volatiles, which will enter the discharge channel and participate in the discharge, which will reduce the discharge efficiency of the thruster and pollute the inner wall of the discharge channel, resulting in a decrease in the performance of the thruster. Therefore, before the excitation coil is assembled to the Hall thruster, it must be subjected to high-temperature degassing treatment to complete the removal of volatiles in advance. When the excitation coil is in working condition, the temperature of the lead of the excitation coil is relatively low. Therefore, a silicone rubber fiberglass hose is usually used to form a lead outside the wire core. The silicone rubber fiberglass hose has a temperature resistance of less than 300°C. If the excitation coil is heated to above 400°C for degassing as a whole, the lead part will experience additional high temperature and become hard. When the excitation coil is subsequently assembled to the Hall thruster, the outer skin of the lead wire is prone to damage. Therefore, during the high-temperature degassing process of the excitation coil, it is necessary to ensure that the lead wire does not experience additional high temperatures. Currently, how to effectively remove volatiles before the excitation coil is assembled to the Hall thruster has become a technical problem that needs to be solved urgently by those skilled in the art.
[0020] To solve the above problems, refer to Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5The present invention provides a degassing device for the excitation coil of a Hall thruster, comprising a tank body 110, a test platform 120, a heat conductor 130, a heat source 140, a cold source 150 and a vacuum pump. The tank body 110 provides a place for removing volatiles. A cooling channel is formed inside the side wall of the tank body 110. By introducing a cooling medium such as liquid nitrogen or liquid ammonia into the cooling channel, the temperature inside the tank body 110 can be reduced to a required temperature. The test platform 120 is arranged in the tank body 110, and is used to support the heat conductor 130, the heat source 140 and the cold source 150. The heat conductor 130 is installed on the test platform 120, and is used to carry the excitation coil of the Hall thruster, and is in contact with the lead wire of the excitation coil. The heat source 140 is arranged outside the heat conductor 130, and can generate heat to heat the excitation coil. The cold source 150 is connected to the heat conductor 130, and is used to cool the heat conductor 130. The vacuum pump is connected to the tank body 110 and can perform a vacuum process on the tank body 110 .
[0021] The working process and principle of the Hall thruster excitation coil degassing device are as follows: First, the excitation coil of the Hall thruster is mounted on the heat conductive member 130 through a metal wire or other fasteners, and the excitation coil is in contact with the heat conductive member 130. Then, the heat source 140 is placed outside the excitation coil and the heat conductive member 130. Next, a cooling medium is introduced into the cooling channel to make the temperature inside the tank 110 lower than 100K, and the tank 110 is evacuated to make the vacuum degree inside the tank 110 less than 1×10 -4 Pa. After that, the heat source 140 generates heat to heat the excitation coil to above 400°C, and the excitation coil is maintained at a specified temperature for a specified period of time to complete the task of removing volatiles in the silicone resin and the surface paint layer. Overall, the tank 110 simulates the real environment of the Hall thruster excitation coil in space, and restores the vacuum degree of less than 1×10 -4 Pa, the temperature is lower than 100K, and the surface temperature of the excitation coil is greater than 400℃. At the same time, during the heating of the excitation coil, the cold source 150 always cools down the heat conductor 130 and the lead of the excitation coil to ensure that the lead does not exceed 250℃, avoiding the lead part from experiencing additional high temperature and hardening, thereby avoiding the phenomenon of lead skin damage during assembly. It is suitable for degassing the excitation coil of Hall thrusters of different models, different powers and different sizes, and has the characteristics of wide application range, simple installation and convenient maintenance.
[0022] Preferably, the cooling channel is spirally arranged along the circumference of the tank body 110 to ensure the cooling effect and efficiency.
[0023] Preferably, there are more than one tank 110, test platform 120, cold source 150 and vacuum pump, and there are multiple heat conductors 130 and heat sources 140, and the multiple heat sources 140 correspond one to one with the multiple heat conductors 130. In this way, multiple excitation coils can be degassed at the same time to achieve mass production.
[0024] Preferably, a plurality of top plates 160 are disposed on the top of the test platform 120. Each top plate 160 is used to carry a heat source 140. Each top plate 160 is provided with a clearance hole for the heat conductor 130 to pass through, so that the top plate 160 does not contact the heat conductor 130, thereby reducing the loss of heat generated by the heat source 140.
[0025] Specifically, in the example, Figure 1 and Figure 2 As shown, the heat source 140 includes a cylinder 141, a plurality of spacer rings 142 and a plurality of infrared heating tubes 143. The axis of the cylinder 141 is vertically arranged and placed on the top plate 160. The top and bottom ends of the cylinder 141 are both open structures to facilitate the use of the cylinder 141. The cylinder 141 is covered on the outside of the heat-conducting member 130 and the excitation coil to be degassed. A plurality of spacer rings 142 are installed in the cylinder 141 along the axial direction of the cylinder 141. A plurality of infrared heating tubes 143 are evenly installed on the spacer rings 142 along the circumference of the spacer rings 142, and can heat the excitation coil to be degassed in multiple directions, ensuring that the temperature of each part of the excitation coil except the lead part is uniform, thereby ensuring the degassing effect of each part.
[0026] Preferably, a reinforcing rod is installed on the outer wall of each cylinder 141 to ensure the rigidity and strength of the cylinder 141 .
[0027] Preferably, multiple layers of first mounting holes 1411 are provided on the side wall of the cylinder 141. The first mounting holes 1411 of each layer are oblong holes, and the extending direction is parallel to the axial direction of the cylinder 141. Each layer of spacer rings 142 is installed through each layer of first mounting holes 1411. Since the first mounting holes 1411 are oblong holes, the height of each layer of spacer rings 142 in the vertical direction can be adjusted, and then the relative position of each infrared heating tube 143 and the excitation coil in the vertical direction can be adjusted to adjust the heating and degassing conditions of various parts of the excitation coil.
[0028] Preferably, a plurality of second mounting holes 1421 are evenly arranged along the circumferential direction on each spacer ring 142. Each second mounting hole 1421 is an oblong hole, extending along the radial direction of the spacer ring 142. It should be noted that each infrared heating tube 143 is respectively connected to each spacer ring 142 through the second mounting hole 1421. Since the second mounting hole 1421 is an oblong hole, the inclination angle of each infrared heating tube 143 and the distance between each infrared heating tube 143 and the excitation coil can be adjusted, and then the relative position of each infrared heating tube 143 and the excitation coil in the horizontal direction can be adjusted to adjust the heating and degassing conditions of various parts of the excitation coil.
[0029] Preferably, the heat source 140 further includes a first heat insulating layer 144. The first heat insulating layer 144 is attached to the inner wall of the cylinder 141 to provide heat insulation and reduce heat exchange between the infrared heating tube 143 and the environment inside the tank 110.
[0030] Specifically, in the example, Figure 1 and Figure 3 As shown, the cold source 150 includes a cooling pipe 151 and a liquid storage tank. The cooling pipe 151 is installed at the lower part of the test platform 120 and is fixedly connected to the bottom of the heat-conducting member 130. The top of the heat-conducting member 130 extends to the top of the top plate 160 through the clearance hole on the top plate 160. The excitation coil to be degassed is installed on the top of the heat-conducting member 130. The liquid storage tank is used to store cooling media such as water. The liquid storage tank is connected to the cooling pipe 151 through a circulating pump, and the cooling medium can be input into the cooling pipe 151 to cool the leads of the heat-conducting member 130 and the excitation coil.
[0031] In some of the applications, the heat conductor 130 is made of metal and is in an "n" shape. It cooperates with the cooling tube 151 and can cool the lead wire of the excitation coil to below 250° C. during the heating process of the excitation coil.
[0032] In other applications, such as Figure 3 and Figure 4As shown, the heat conducting member 130 includes a support rod 131, two heat conducting rods 132 and two wicks 133. The support rod 131 is used to carry the excitation coil of the Hall thruster and is located above the top plate 160. There are two heat conducting rods 132, and the axes are both vertically arranged. The two heat conducting rods 132 are respectively formed with accommodating cavities. The top ends of the two heat conducting rods 132 are respectively fixedly connected to the opposite ends of the support rod 131, and the bottom ends are respectively fixedly connected to the cooling pipe 151. The heat conducting rod 132 is in contact with the lead wire of the excitation coil. The two wicks 133 are installed in the accommodating cavity in a one-to-one correspondence with the two heat conducting rods 132. It should be noted that the support rod 131 can be installed with multiple excitation coils at the same time to realize mass production. Each heat conducting rod 132 is made of metal. The free end of the lead wire of the excitation coil extends downward along the heat conducting rod 132. Each wick 133 is a porous material. Low-boiling-point liquid is adsorbed in each wick 133. The heat from the lead causes the low-boiling-point liquid in the wick 133 to vaporize, and when it reaches the bottom of each accommodating cavity, it is cooled to generate liquid. With the help of the wick 133, it climbs to the top of the accommodating cavity and is heated again. In this cycle, the heat of the lead can be quickly transferred to the cooling tube 151, greatly improving the cooling efficiency and cooling effect, and can cool the lead of the excitation coil to below 100°C.
[0033] Preferably, a guide rail 111 is provided at the bottom of the tank body 110 . The test platform 120 is movably mounted on the guide rail 111 , so that the test platform can be moved into and out of the tank body 110 .
[0034] Preferably, the Hall thruster excitation coil degassing device also includes a thermocouple, a second thermal insulation layer and a controller. The thermocouple is mounted on the excitation coil by bonding to detect the temperature of the excitation coil. The lead of the thermocouple is in contact with the heat conductive member 130. The heat conductive member 130 is used to cool the lead of the thermocouple. It should be noted that in order to improve the cooling effect, the lead of the thermocouple extends downward along the heat conductive rod 132. The second thermal insulation layer is coated on the outside of the heat conductive member 130, the lead of the excitation coil and the lead of the thermocouple to play a heat insulating role. The controller is electrically connected to the thermocouple and each infrared heating tube 143 respectively, and can receive the temperature signal detected by the thermocouple, and control each infrared heating tube 143 to work according to the temperature signal. It can be dynamically configured according to the position of the temperature measurement point of the excitation coil to form a corresponding control loop, and a multi-loop PID control algorithm is used to control the temperature of the excitation coil at a specified temperature value to ensure the temperature uniformity of all temperature measurement points on the excitation coil.
[0035] Preferably, the first thermal insulation layer 144 and the second thermal insulation layer are made of high-temperature resistant high-silica glass fiber.
[0036] For example, the degassing requirements for the excitation coil of a certain model 300W / 600W Hall thruster are as follows: the vacuum degree should be less than 6.5×10 -3 Pa, the excitation coil undergoes a heating stage, a heat preservation stage and a cooling stage. In the heating stage, the temperature rises from room temperature (25℃±10℃) to high temperature (400℃), and the heating rate is 3℃-5℃ / min. In the heat preservation stage, the temperature is maintained at 400℃±20℃, and the temperature of the lead part of the excitation coil is controlled within the range of 200℃±20℃, and the heat preservation time is 12h±0.1h. In the cooling stage, the temperature drops from high temperature to room temperature at a cooling rate of 3℃-5℃ / min. The resistance value measurement accuracy is better than 10mΩ. Before and after the degassing treatment, the excitation coil is weighed and the data is recorded, and the weight loss of each excitation coil is calculated, and the weighing accuracy is better than 0.01g. After the degassing treatment, the appearance of the excitation coil should be free of oxidation discoloration, rust, bumps, and wire skin damage. Figure 5 The temperature of the excitation coils of two sizes varies with time. It should be noted that the heating temperature is controlled at 405° C. The size of the first excitation coil is larger, and the size of the second excitation coil is smaller.
[0037] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, “counterclockwise”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.
[0038] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.
[0039] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection, an electrical connection, or communication with each other; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.
[0040] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.
[0041] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.
Claims
1. A degassing device for the excitation coil of a Hall thruster, characterized in that: include: Tank; A test platform is arranged in the tank; A heat conducting member, installed on the test platform, used to carry the excitation coil of the Hall thruster and in contact with the lead wire of the excitation coil; A heat source, disposed outside the heat-conducting member, for heating the excitation coil; A cold source is connected to the heat conducting member and is used to cool the heat conducting member.
2. The degassing device for the excitation coil of a Hall thruster according to claim 1, characterized in that: The heat source comprises: The cylinder has a vertical axis, and the top and bottom ends are open structures, and is covered outside the heat conducting member; A plurality of spacer rings are installed in the cylinder along the axial direction of the cylinder; There are multiple infrared heating tubes which are evenly installed on the spacer ring along the circumference of the spacer ring.
3. The degassing device for the excitation coil of a Hall thruster according to claim 2, characterized in that: The side wall of the cylinder is provided with multiple layers of first mounting holes; the first mounting holes of each layer are oblong holes, and the extending direction is parallel to the axial direction of the cylinder; A plurality of second mounting holes are evenly arranged on each of the spacer rings along the circumferential direction; each of the second mounting holes is an oblong hole extending along the radial direction of the spacer ring.
4. The degassing device for the excitation coil of a Hall thruster according to claim 2, characterized in that: The heat source also includes: The first heat insulation layer is attached to the inner wall of the cylinder.
5. The degassing device for the excitation coil of a Hall thruster according to any one of claims 1 to 4, characterized in that: The cold source comprises: A cooling pipe, installed on the test platform and fixedly connected to the heat conducting member; A liquid storage tank is communicated with the cooling pipe.
6. The degassing device for the excitation coil of a Hall thruster according to claim 5, characterized in that: The heat conducting member comprises: A support rod for carrying the excitation coil of the Hall thruster; There are two heat-conducting rods, both of which have vertically arranged axes and are respectively formed with accommodating cavities inside, and the top ends are respectively fixedly connected to the opposite ends of the support rod, and the bottom ends are respectively fixedly connected to the cooling pipe; the heat-conducting rods are in contact with the leads of the excitation coil; There are two liquid absorbing cores, which are installed in the accommodating cavity in a one-to-one correspondence with the two heat conducting rods.
7. The degassing device for the excitation coil of a Hall thruster according to any one of claims 1 to 4, characterized in that: A cooling channel is formed inside the side wall of the tank body.
8. The degassing device for the excitation coil of a Hall thruster according to any one of claims 1 to 4, characterized in that: A guide rail is arranged at the bottom of the tank body; the test platform is movably mounted on the guide rail.
9. The degassing device for the excitation coil of a Hall thruster according to any one of claims 1 to 4, characterized in that: Also includes: A vacuum pump is connected to the tank.
10. The degassing device for the excitation coil of a Hall thruster according to any one of claims 1 to 4, characterized in that: Also includes: A thermocouple is mounted on the excitation coil, and a lead wire is in contact with the heat conductive member; The second heat insulating layer is coated on the outside of the heat conducting member, the lead wire of the excitation coil and the lead wire of the thermocouple.