RU2446503C2 - Cathode heating unit - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: cathode-heating unit comprises a cylindrical core, in the end of which there is a pellet of an emitting substance, a heater, a circular anisotropic pyrolytic graphite holder, a heater insulator, a support flange, a current lead insulator. At the same time the core holder is made in the form of a sleeve, where the heater insulator is inserted, and the pellet of the emitting substance rests on the core holder via a graphite bushing, and the circular pyrolytic anisotropic holder is extended to the end surface of the core holder.

EFFECT: increased service life of the cathode-heating unit with reduction of consumed capacity, simplified design.

3 cl, 4 dwg

Description

The invention relates to electronic equipment, in particular to an electron source device, and can be used in electron accelerators, the accelerated beam of which is used for radiation technologies, such as wastewater treatment, gas treatment of coal-fired power plants, in linear accelerators used for x-ray diagnostics for medical purposes, and in other technologies.

Known cathode-heating unit of the electron source for linear accelerators (V.A. Polyakov, I.S. Shchedrin "Sources of electrons for linear accelerators and technological goals." "Proceedings of the XV Conference on charged particle accelerators" Protvino October 22 ... October 24, 1996 pg. 24 ... 28). The electron source consists of two nodes: a high-voltage node and a cathode-heating node. The design of the cathode-heating unit of the electron source consists of a holder of a tablet emitting electrons (an emitter made of LaB6) in the form of a hollow truncated cone, a clamp that secures the fixation of the emitter tablet and the auxiliary belt heater cathode, which ensures emitter bombardment. For this, a potential difference is created between the cathode-heater and the emitter. To ensure elastic contact during the entire time of operation of the emitter and the clamp, the clamp, in its supporting part, is made with protrusions that are located outside the high temperature zone, which significantly reduces the rate of inelastic deformation. The tape heater has slots perpendicular to its long side and is insulated from the base with a ceramic insulator. Heat energy is dissipated by heating a large surface of the parts of the cathode-heating unit. The power consumption of the cathode-heating unit, with an emitter diameter of 4 ... 6 mm, is 50 ... 60 watts. Long-term operation of the cathode-heating unit can occur without forced cooling, but it requires a certain volume of liquid dielectric located in the high-voltage unit of the electron source.

Closest to the proposed solution to the cathode-heating unit is the cathode-heating unit (RF patent No. 1156516, H01J 1/20, priority 02.01.84, publ. 30.08.1994, Bull. No. 16, p.211) used in high-voltage electron accelerators. The cathode-heating unit contains a cylindrical core, a core of a substance emitting electrons (emitter) is fixed at the end of the core, installed in an annular anisotropic graphite holder, the material of which axis "C" coincides with its axis. The axis "C" is provided by the technological process of manufacturing parts, it is perpendicular to the plane of deposition of graphite. A carbon material heater is in contact with the emitter tablet, and the heater is in contact with a current lead, which, together with the heater, emitter and anisotropic ring graphite holder, are spring-loaded towards the end face of the core relative to the support sleeve. The contact heater and current lead are insulated from the core body by insulators. The core and the support sleeve are interconnected by holders of thin wire, which provides reduced heat transfer from the emitter to the support sleeve of the cathode-heating unit. A nut is located in the supporting sleeve of the cathode-heating unit, with the help of which the effort of pressing the emitter tablet is regulated, through the annular anisotropic graphite holder, to the end surface of the core. The electric circuit for heating the emitter is thus constituted by an insulated current lead, contact heater, emitter, ring anisotropic graphic holder, core and holders connected to the supporting sleeve.

The well-known solution of the cathode-heating unit described in the analogue solves the problem of heating the emitter, however, to create an accelerating potential difference, an additional power source is required. In addition, in this design of the cathode-heating unit, it is necessary to ensure the correct position of the emitter relative to the holder. At the slightest change in the position of the emitter, the conditions for the formation of an electron beam can change. The correct position of the emitter is determined by the clamp, which is in an elastic state and constantly works in the heating-cooling mode. Over time, this can lead to a loss of the elastic properties of the material - the clamp and, as a consequence, the displacement of the emitter tablet. Deformation and even destruction of the emitter is also possible.

The known solution of the cathode-heating unit indicated in the prototype solves the problem of heating the emitter, however, in the cathode-heating unit there should be constant contact between the emitter and the heater throughout the entire service life. This is difficult to accomplish because of the possible core deformation, because the force of the spring is transferred to it, and also because the spring is in the high temperature zone. With a large number of cyclic temperature changes (the working temperature of the tablet emitting electrons ~ 1500 ° C), the elastic properties of the spring decrease, which leads to a weakening of the contact between the emitter and the heater. These factors reduce the durability of the cathode-heating unit.

In fact, the same factors reduce the durability of the devices.

The main task solved by the inventive device is to increase the service life of the cathode-heating unit while reducing the power consumed by it, simplifying the design.

The essence of the invention lies in the fact that in the cathode-heating unit containing a cylindrical core, in the end of which a tablet of emitting substance is fixed, a heater, an annular anisotropic pyrolytic graphite holder, a heater insulator, a support flange, a current lead insulator, a core holder made in the form cup, into which the insulator of the heater is inserted, and the tablet of the emitting substance, is supported on the core holder through a sleeve of graphite, while the annular pyrolytic anisotropic the holder is extended to the end surface of the core holder.

In this case, between the heater and the end surface of the core holder, a screen with protrusions towards the core holder can be introduced, and the heater is made in the form of an ellipse spiral; the size of the spiral is determined by the ratio:

4d <H <0.6D / 3.2,

B = 3.2 · N

where d is the diameter of the heater wire;

N - heater winding height;

D is the diameter of the tablet emitting substance;

In - the width of the coil heater.

Figure 1 shows the cathode-heating unit (longitudinal section along the mounting of the heater), Figure 2 shows the cross-section of the cathode-heating unit at the plane of the heater, Figure 3 shows the cathode-heating unit prepared for molding the heater, Fig. 4 shows the temperature distribution over the diameter of the emitter.

The cathode-heating unit consists of a cylindrical core 1, tablet 2, emitting electrons (emitter), an annular anisotropic pyrolytic graphic holder 3 (Fig.1, 2). The emitter tablet 2 is supported through the sleeve 4 on the core holder 5. In the gap 6 between the emitter tablet 2 and the end surface 7 of the core holder 5, a heater 8 is placed, the output ends 9 and 10 of which pass through the holes 11 in the core holder 5 and the holes 12 of the insulator 13. The insulator 13 is inserted inside the core holder 5 and secured in it with tabs 14. The holes 12 of the insulator 13 are larger in diameter than the holes 11 in the core holder 5. The core holder 5 is attached to the protrusion 15 of the support flange 16. One end 9 of the heater 8 is attached to the current lead 17, which from th e from the flange 16 isolators 18,19, the other end 10 of the heater 8 is connected through opening 20 to the support flange 16.

The assembly sequence is as follows: first the parts interacting with the heater 8 are assembled and the heater 8 is mounted on the support flange 16 and the current lead 17 (Figure 3). Then, heater 8 is molded. Tungsten heaters are molded by heating it in a hydrogen medium by passing an electric current through the heater during visual inspection of the product. Then the core 1 with the emitter 2 is installed on top of the annular holder 3 with the sleeve 4 inserted into it, at the point of contact of the core 1 and the core holder 5, the parts are electrically and mechanically connected into a single unit.

In a particular solution, the cathode-heating unit may be provided with a screen 21 with holes 22 having protrusions 23. The screen 21 and protrusions 23, providing a gap 24 between the core holder 5 and the screen 21, create additional thermal resistance, reducing thermal energy loss through the lower end surface .

The core holder 5 in the lower part has windows 25 (Figure 3), which increases the thermal resistance from the core holder 5 to the protrusion 15 of the support flange 16, and also provides access to the current lead 17. Through the holes 26 in the support flange 16, the cathode-heating unit is attached to the flange of the electron source.

The proposed design solutions of the cathode-heating unit allow assembly operations to be carried out with maximum access to all parts of the assembly and operational assembly control, which is the solution to the problem.

The device operates as follows.

When the necessary vacuum is reached in the device where the cathode-heating unit is installed, the heating circuit of heater 8 is turned on. The heater 8 is heated by an electric current passing through this circuit through an electric circuit composed by a current lead 17 by a flange 16, output ends 9 and 10 and a heater 8. Heat is transmitted by radiation from the heater 8 to the emitter pellet 2.

The heating of the tablet 2 emitter should be brought before the emission of electrons from the material of the tablet. This heating can be obtained due to a large current or due to a lower current, but using various techniques that localize the heat created by the heater. These include the use of materials with low thermal conductivity, the use of materials with different heat conductivity in the structure of the material, and the use of screens.

The heat generated by heater 8 is held by core 1, emitter 2, core holder 5, insulator 13, which has an anisotropic property, namely that the thermal conductivity across the insulator is less than the thermal conductivity along the insulator, the annular anisotropic graphic holder 3 and the sleeve 4. The latter are made of anisotropic pyrolytic graphite, which has lower thermal conductivity in the transverse direction, which, together with the extension of the holder 3 to the core holder 5 improves the thermal characteristics of the cathode-heating about the node.

In addition, the core holder 5 in the lower part has windows 25 that reduce the cross-section of the material of the core holder 5, which could cause heat leakage due to heat conduction from the core holder 5 to the support flange 16. In this case, the support flange 16 has a smaller thickness at the attachment point the core holder 5, and the support flange 16 has a thin protrusion 15.

The improvement of the thermal characteristics of the cathode-heating unit is affected by the use of the screen 21 with projections 23 towards the screen 21, which reduce the contact area with the core holder 5 and create a gap 24 between them, which reduces the thermal conductivity of the connection of the screen 21 of the core holder 5.

The heater 8 is made in the form of an ellipse spiral. This is due to the fact that the largest possible area of the emitter 2 is captured by the heat flux from the heater 8, while the heat flux goes to the emitter 2 both from the upper branch and from the lower one. The same flow goes in the opposite direction, but there is thermal shielding in the form of a core holder 5, an insulator 13 and a screen 21, which quite effectively prevent the loss of thermal energy in this direction. This design solution was tested on a prototype of the cathode-heating unit, which showed that the temperature difference between the emitter 8 and the outer surface of the core 1 is ~ 200 ° C at the working temperature of the emitter. Moreover, the dimensions of the heater are related by the ratio:

4d <H <0.6D / 3.2,

B = 3.2 · N

where d is the diameter of the heater wire;

N - heater winding height;

D is the diameter of the tablet emitting substance (emitter);

In - the width of the coil heater.

The height of the winding H cannot be less than four diameters of the heater wire 8, otherwise the wire will break, and it does not make sense to make a height greater than the indicated value since The internal volume of the device is increasing. The width of the coil B is determined based on the optimal ratio of the area of the emitter 2 and the area of the heat flux generated by the heater 8.

On the model of the cathode-heating unit, a study was made of its temperature characteristics. The study was conducted on a cathode-heating unit with an emitter with a diameter of 15 mm thermal imager brand "Tangem VS60". The result of measuring the temperature distribution along the diameter of the emitter is presented in Figure 4. The graph shows that the temperature on the surface of the emitter is almost the same, there are no overheated places that could affect the life of the device.

This design solution allows you to simplify the design of the electron source, because it can be used without additional cooling devices, and there is no effort that would affect the emitter. The cathode-heating unit is simple in technological design, it is convenient to carry out assembly of parts on it, to carry out and control the complex molding operation of heater 8, and it is also easy to carry out the final assembly with high geometry accuracy.

Thanks to this technical solution, when prototyping the cathode-heating unit, good results were obtained: the power consumption, at the same emitter temperature, decreased by half, the exit time to the operating mode was reduced by 40% due to the rapid achievement of the temperature at which emission begins.

Claims (3)

1. A cathode-heating unit containing a cylindrical core, in the end of which a tablet of emitting substance is fixed, a heater, an annular anisotropic pyrolytic graphite holder, a heater insulator, a current lead insulator, a support flange, characterized in that a core holder is inserted into the cathode-heating unit made in the form of a glass into which a heater insulator is inserted, a tablet of emitting substance rests on a core holder through a graphite sleeve, and the annular anisotropic pyrolytic The th graphite holder is extended to the end surface of the core holder. 2. The cathode-heating unit according to claim 1, characterized in that between the heater and the end surface of the core holder a screen is introduced with protrusions towards the core holder. 3. The cathode-heating unit according to claim 1, characterized in that the heater is made in the form of an ellipse spiral, and its dimensions are determined by the ratio:
4d <H <0.6D / 3.2
B = 3.2 · N, where
d is the diameter of the heater wire;
N - heater winding height;
D is the diameter of the tablet emitting substance;
In - the width of the coil heater.

Images