Thermionic cathode heated by electron bombardment

Abstract

A structure wherein a cathode, which is made of a material, such as lanthanum hexa-borides, prone to react with metals at high temperatures and having a high electron emissivity, is held by a supporter which is made of an electrically insulating material. Concentric metal cylinders are arranged at the outer circumference of the supporter to surround a part of the cathode and a heating coil is arranged in the interstice between the metal cylinders. An electron-emissive metal oxide layer is formed on the inside surface of the inner metal cylinder. Thermions created from the oxide layer strike the cathode, and the cathode is heated by the heat of the electron bombardment.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a heating device for a cathode in an electron tube and, more particularly, to a heating device for a cathode which is made of a material, such as lanthanum hexa-borides (LaB.sub.6) and yttrium hexa-borides (YB.sub.6), prone to react with metals at high temperatures.

Description of the Prior Art

In general, borides such as lanthanum hexa-borides (LaB.sub.6) and yttrium hexa-borides (YB.sub.6) have a small work function, and are suitable as cathode materials. However, because they are liable to react with metals at high temperatures, it has been difficult to heat these borides to working temperatures of approximately 1,300.degree.-1,800.degree.C. During the heating of the cathode, the direct heating type requires high power. Indirect heating type is, therefore, effective in reducing power comsumption to as low a value as possible.

FIG. 1 is a schematic diagram showing an example of a prior-art cathode heating device, which is constructed such that a heating coil 2, made of tungsten or the like, is held in a space surrounding cathode 1, made of lanthanum hexa-boride (LaB.sub.6) and heating power is supplied from a power source 3 to the coil 2. Between the cathode 1 and the heating coil 2, an accelerating power source 4 is connected for electron bombardment.

With such a construction, when the coil 2 is heated, the cathode 1 is heated by the radiant heat. Simultaneously therewith, thermions emitted from the coil 2 are drawn to the cathode 1 by the voltage of the accelerating power source 4, and the cathode 1 is heated by heat which is generated by the electron bombardment. In order to increase the temperature of the cathode 1 to working temperature the heating coil 2 must be heated to a temperature of at least about 2,500.degree.-2,800.degree.C when employing a tungsten wire. However, when the coil 2 is heated to the above-mentioned temperature, heat losses due to thermal conduction from the leads at both ends of the coil 2 to the exterior and the heat loss due to the thermal radiation from the coil 2 become so great as not to be negligible.

FIG. 2 shows a sketch for roughly estimating the heat losses due to thermal conduction and due to thermal radiation. Referring to the figure, leads 5 of stainless steel are connected to both ends of the heating coil 2 of tungsten in order to diminish the loss due to the thermal conduction. Now, let T.sub.3 denote the temperature of the central part of the heating coil 2 which is uniform, T.sub.2 denote the temperature of the point of contact between the coil 2 and the lead 5, and T.sub.1 denote the temperature of the end of the lead 5 remote from the coil 2. It is assumed that the diameter of the coil 2 is 0.02 cm, that the length of the coil extended in a straight line is 2.0 cm, that the sectional area of the lead 5 is 4.45 .times. 10 .sup..sup.-2 cm.sup.2, that T.sub.3 = 2,800.degree.C, that T.sub.2 = 1,500.degree.C and that T.sub.1 = 1,000.degree.C. When, although not shown in the figure, a heat shielding plate is provided around the coil 2, the temperature of the heat shielding plate is assumed to be 1,000.degree.C. Then, the radiation heat loss and the condition heat loss can be respectively calculated to be approximately 40 W and 20 W.

In this manner, where the coil 2 is heated to a high temperature, the heat loss due to thermal radiation becomes very large, since it is proportional to the fourth power of the temperature in accordance with Boltzmann's law. When mounting a plurality of heat shielding plates in the space surrounding the coil 2 in the form of concentric cylinders, the radiation heat loss can be reduced. In this case, however, the heat capacity of the shielding plate assembly itself becomes large, and the time required for increasing the cathode temperature becomes large, resulting in the disadvantage that the heating device is very difficult to use.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a device for heating a cathode made of a material such as lanthanum hexa-borides (LaB.sub.6), in which the heat loss by the thermal radiation is small and the heating efficiency is high.

In order to accomplish this object, the present invention includes a heating coil provided within an indirect heating case and an electron-emissive metal oxide layer formed on the surface of the indirect heating case facing to a cathode.

The other objects and features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view depicting a prior-art heating arrangement;

FIG. 2 is a schematic view for calculating losses due to thermal radiation and thermal conduction from a heating coil;

FIG. 3 is a constructional view of an embodiment of the present invention;

FIG. 4 is a schematic view for calculating thermal conduction;

FIG. 5 is a structural view of a cathode;

FIG. 6 is a constructional view showing another embodiment of the present invention;

FIG. 7 is a sectional view of a portion of still another embodiment of the present invention;

FIGS. 8a to 8d are sectional views of supporters for use in the present invention; and

FIG. 9 is a structural view in the case where the heating device of the present invention is applied to an actual cathode.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 3, a cathode 1 made of a boride material such as lanthanum hexa-boride (LaB.sub.6) and yttrium hexa-boride (YB.sub.6) is positioned in a hollow core portion of a supporter 6 made of a high temperature-resistant material, such as boron nitride (BN), which is electrically insulating. Outside the supporter 6, there are arranged concentric cylinder bodies 10a and 10b made of nickel or the like. The end parts of the cylinders on one side are connected by a metal sheet 10c. Formed on the interior surface of the cylinder 10b is an electron-emissive wall made of a coating 9 which is made of an electron-emissive metal oxide such as barium oxide (BaO), calcium oxide (CaO) and strontium oxide (SrO). An electrode 7 of graphite or the like is mounted on one end of the cathode 1, and is connected through a lead 8 to the positive terminal of an electron accelerating power source 4 for electron bombardment. The negative terminal of the power source 4 is connected to the cylinder 10a. A heating coil 2 of tungsten or the like insulated by an alumina coating layer is arranged between the concentric cylinders 10a and 10b, and is supplied with heating power from a power source 3.

With such a construction, when current flows through the coil 2 by means of the heating power source 3 and the coil 2 is thus heated, the cylinders 10a and 10b are also heated. As a consequence, the oxide layer 9 is subjected to the indirect heating. When the oxide layer 9 is heated to approximately 800.degree.C, thermions are emitted. Since the cathode 1 is applied with a positive potential through the electrode 7, which is made of graphite or the like material and does not readily react with the cathode at high temperatures the thermions emitted from the oxide layer 9 are attracted toward the cathode 1 and impinge thereon. The cathode 1 is then heated to approximately 1,300.degree.-1,800.degree.C by the heat of the electron bombardment. Since, in this case, the cylinder 10b has a temperature lower than the cathode 1, heat losses due to thermal conduction and thermal radiation from the cathode 1 are mostly fed-back to the cylinder 10b as is apparent from the construction shown in the figure. Accordingly, once the cylinders 10a and 10b have been heated, the power required for maintaining them at 800.degree.C may be very slight. Heat losses due to thermal radiation from the cylinder 10a are extremely small, because the temperature of this cylinder is as low as 800.degree.C.

As explained above, according to the cathode heating device of the present invention, the cathode 1 is not heated directly by the radiant heat of the heating coil 2, but is heated by the electron bombardment heat in such way that the metal cylinders 10a and 10b are heated and that thermions emitted from the oxide layer 9 formed on the inside surface of the cylinder 10b are accelerated and impinge upon the cathode 1, so that the cathode 1 can be heated to a desired high temperature in the state in which the metal cylinders 10a and 10b are at a temperature lower than that of the cathode 1. From the heating by the electron bombardment heat, the heat loss components are those due to thermal conduction from the cathode 1 through the supporter 6 to the cylinder 10b and these directly escaping due to thermal radiation from the cathode 1. Since both these heat loss components are fed-back to the cylinder 10b surrounding the cathode, there is essentially no heat-loss, and a highly efficient heating can be effected.

In the foregoing embodiment, the material of the cylinders 10a and 10b is not restricted to nickel, but may be any metal having deoxidizing properties at a high temperature of about 800.degree.C. The coating layer is not restricted to electron-emissive metal oxide layer 9 but a layer of a sintered body or an impregnation layer of a porous metal may be adopted, insofar as it has an electron emissivity equivalent to that of the coating layer 9.

Although the disadvantages of the prior-art device have been eliminated by the device of the embodiment shown in FIG. 3, even the improved device has a few problems. One of them is that as the loss heat from the cathode 1 is effectively fed-back to the cylinder 10b, the temperature of the metal oxide layer 9 increases more than is necessary, with the result that the deterioration of the oxide layer 9 is hastened.

The excessive increase in the temperature of the oxide layer 9 in FIG. 3 is attributable to the fact that the thermal feedback from the cathode 1 is too great. As already explained, the causes for the thermal feedback are 1 -- the thermal radiation from the cathode 1 and 2 -- the thermal conduction through the supporter 6. Rough estimates will be hereunder explained for both mechanisms of thermal feedback.

The desired temperatures are approximately 1,500.degree.C for the cathode 1 made of LaB or the like and approximately 800.degree.C for the oxide layer 9. For the sake of simplicity, therefore, those quantities of heat flow, for the respective heat transfer mechanisms when the temperature difference between the specified values is assumed may be determined.

FIG. 4 indicates the dimensions necessary for such calculations. Let Qr be the quantity of heat which is imparted from the cathode 1 to the oxide layer 9 (the temperature of which is equal to that of the cylinder 10b) by thermal radiation, and Qc be the quantity of heat which is imparted through the supporter 6 by thermal conduction. T.sub.10 and T.sub.20 are the temperatures of the cathode 1 and the oxide layer 9 respectively. .epsilon. is the emissivity, .sigma. is Stephen-Boltzmann's constant, and K the coefficient of heat transfer of the supporter 6. Then, the equations of heat transfer due to thermal conduction and radiation are respectively given as follows: ##EQU1##

Now let T.sub.10 = 1,500.degree.C, T.sub.20 = 800.degree.C, .epsilon.= 0.5 and .sigma.= 5.67 .times. 10.sup..sup.-12 Joule /sec - cm.sup.2 - .degree.K.sup.4 . When employing boron nitride for the supporter 6, the coefficient of heat transfer becomes K = 0.63 Joule/cm-sec-.degree.C. The dimensions in FIG. 4 are d.sub.1 = 0.1 cm, d.sub.2 = 0.4 cm, l.sub.1 = 0.2 cm and l.sub.2 = 0.3 cm. Then, there are obtained:

It is understood that for the dimensions d.sub.1, d.sub.2, l.sub.1 and l.sub.2 given above, thermal conduction through the supporter 6 is very great when using boron nitride.

When the temperature difference between the cathode 1 and the oxide layer 9 is to be kept at about 700.degree.C, a calculated value of power corresponding to the heat loss component (about 400 Watts) must be applied to the cathode 1. In actuality, thermal contact resistances exist between the cathode 1 and the supporter 6 and between the supporter 6 and the cylinder 10b, respectively, and the coefficient of heat transfer K apparently becomes smaller by one order or so. The power to be applied to the cathode 1 is, actually, less than 10 Watts. Under this condition, the temperature difference of 700.degree.C is not established, and the temperature of the oxide layer 9 of approximately 1,300.degree.C has been observed when the temperature of the cathode 1 is 1,500.degree.C. Consequently, if a temperature difference of 700.degree.C is to be maintained without changing the dimensions, an insulating material having a smaller coefficient of heat transfer K by approximately one order must be employed. (According to calculations, the heat loss is about 400 W when employing boron nitride for the supporter 6, and hence, the coefficient of heat transfer must be decreased by two orders for restraining the loss to less than 10 Watts. As previously explained, however, thermal contact resistances exist between the adjacent ones of the cathode 1 -- supporter 6-- cylinder 10b, and hence, the decrease of the coefficient of heat transfer by approximately one order suffices. ) In this regard, alumina has a coefficient of heat transfer of K = 0.07 Joule/ sec - cm - .degree.C (at 800.degree.C), which is about one order smaller than the coefficient of heat transfer of boron nitride. With a cathode of the above dimensions, therefore, the use of alumina is preferable to boron nitride. According to experiments, the value 700.degree.C has been observed as the temperature difference between the cathode 1 and the oxide layer 9, and the life of the oxide layer 9 has been extended.

Where the supporter 6 is specified beforehand, the desired temperature difference can be established by appropriately determining the dimensions d.sub.1, d.sub.2, l.sub.1 and l.sub.2 or by making the areas and shapes of the contact surfaces between the cathode 1 and the supporter 6 and between the supporter 6 and the cylinder 10b different.

The power consumption of the cathode can thus be minimized by selecting the insulating material of the supporter 6 in dependence on the shape and dimensions of the cathode 1 with reference to equations (1) and (2) for Qc and Qr respectively.

Where the cathode 1 is heated to a temperature (for example, 2000.degree.C) considerably higher than the usual working temperature, the temperature of the contact part between the supporter 6 and the cathode 1 increases, and both members chemically react with each other in some cases. In that event, a layer 11 made of graphite powder or a sintered body thereof may be formed between the cathode 1 and the supporter 6, as shown in FIG. 5.

Another problem of the embodiment in FIG. 3 is that when the cathode 1 is used at high temperatures for a long period of time, a thin film which is electrically conductive is formed on the surface of the supporter 6 of the high temperature-resistant insulating material by the vaporization of the cathode material such as LaB.sub.6, resulting in a degradation of the insulation between the cathode 1 and the oxide layer 9.

In order to solve such a problem, an evaporation preventing plate 12 may be provided in proximity to the supporter 6 as illustrated in FIG. 6. Since the evaporation preventing plate 12 functions as a mask for the vaporization of the cathode material and prevents the material from adhering to the surface of the supporter 6, there is good electrical insulation between the cathode 1 and the oxide layer 9 for a long period of time.

Even when a supporter 6' of a shape as shown in FIG. 7 is employed instead of the provision of the preventing plate 12, the same effect is achieved. An part 6a of the supporter 6' becomes the so-called shade portion with respect to the cathode 1, and the cathode material such as LaB.sub.6 cannot be readily deposited on the portion. Even when supporters 6' of shapes illustrated in FIGS. 8a to 8d are used, the same effect is achieved. Any of the shapes has an extended portion which defines a shade with respect to the cathode 1 and on which LaB.sub.6, or the like cannot be readily deposited, so that there is good electrical insulation between the cathode 1 and the oxide layer 9 is even during the use of the cathode for a long period of time.

FIG. 9 shows an overall concrete structure of the heating device of the present invention. In the figure, the same constituent parts as in FIG. 3 are assigned with the same symbols. The aperture of the metal sheet 10c for connecting the end parts of the cylindrical bodies 10a and 10b is made smaller than the inside diameter of the cylindrical oxide layer 9. This serves to prevent thermions for the electron bombardment, emitted from the oxide layer 9, from being mixed into the thermions which are emitted from the cathode 1 towards the opening portion of a Wehnelt electrode 16 or a grid electrode. The heating coil 2 has power supplied thereto through lead wires from a power source (not shown) which is disposed outside a cathode base 14 of glass or the like. The cylinder 10a is connected to electrode terminals by lead wires 13.

While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art and we, therefore, do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

Claims

We claim:

1. A thermionic cathode structure comprising:

a cathode made of a material having a high electron emissivity;

a supporter made of an electrically-insulating and high temperature-resistant material for supporting a part of said cathode;

a first cylinder held in contact with said supporter and arranged so as to surround a part of said cathode;

a second cylinder arranged concentrically with said first cylinder;

a cathode heating coil arranged in an interstitial space between said first and second cylinders;

an electron-emissive metal oxide layer formed on an inside surface of said first cylinder;

flange means, extending from said first cylinder toward said cathode past said oxide layer, for preventing electrons emitted from said oxide layer from being mixed with electrons emitted from a part of said cathode;

a power source connected to said coil for supplying heating power to said coil; and

a power source connected between said cathode and said first cylinder in order to cause electrons, emitted from said oxide layer, to strike against said cathode.

2. The structure according to claim 1, wherein said cathode is made of a boride.

3. The structure according to claim 1, wherein said oxide layer is made of at least one member selected from the group consisting of barium oxide, strontium oxide and calcium oxide.

4. In a thermionic cathode structure having a cathode;

an insulating body for insulatingly supporting said cathode; and

a heating coil, disposed adjacent said cathode;

the improvement comprising:

means, disposed between said heating coil and said cathode and surrounding at least part of said cathode, for emitting thermions which impinge upon said cathode to heat said cathode, in response to the heating of said heating coil, wherein said means comprises a first cylinder supported by said insulating body and surrounding said cathode and a layer of electron-emissive material coated on the interior surface of said fist cylinder facing said cathode; and

a flange extending from said first cylinder toward said cathode past said layer of electron-emissive material, said electron-emissive layer being confined between said flange and said insulating body.

5. The improvement according to claim 4, wherein said means comprises a wall made of an electron-emissive material which, when heated by said coil, emits thermions which impinge upon said cathode to heat said cathode.

6. The improvement according to claim 4, wherein said means further includes a second cylinder surrounding said coil, with said coil disposed between said second cylinder and said first cylinder.

7. The improvement according to claim 6, wherein said means further includes a first power source for applying a potential between said first cylinder and said cathode and a second power source for applying a heating current to said heating coil.

8. The improvement according to claim 4, wherein said layer of electron-emissive material is made of a material selected from the group consisting of a metal oxide, a sintered body, and a porous metal-impregnated layer.

9. The improvement according to claim 8, wherein said metal oxide is made of at least one material selected from the group consisting of barium oxide, strontium oxide and calcium oxide.

10. The improvement according to claim 4, wherein said cathode is made of a boride.

11. The improvement according to claim 4, further including a layer of graphite disposed between said insulating body and said cathode.

12. The improvement according to claim 4, further including an evaporation preventing plate disposed between said insulating body and said electron-emissive wall.

13. The improvement according to claim 4, wherein said insulating body has an extended portion defining a shade area between said wall and said cathode to prevent the formation of an electrically conductive film between said wall and said cathode.