US3439210A - Thermionic emission cathode having reduced frontal area and enlarged emission area for ion bombardment environment - Google Patents

Description

April 15, 1969 H jGALLA HE -ETAL 3,439,210

THERMIONIC EMISSION CATHODE HAVING REDUCED FRONTAL AREA AND ENLARGED EMISSION AREA FOR ION BOMBARDMENT ENVIRONMENT Filed Jan. 5, 1966 Sheet I of 5 WKM April 15, 1969 H. E. GALLAGHER ET AL 3,439,210 I THERMIONIC EMISSION CATHODE HAVING REDUCED FRONTAL AREA AND ENLARGED EMISSION AREA FOR ION BOMBARDMENT ENVIRONMENT Fil ed Jam. 3, 1966 Imam 5% April 15; 1969 GALLAGHER ET AL 3,439,210

E1) FRONTAL AREA AND MBARDMENT ENVIRONMENT THERMIONIC EMISSION CATHODE HAVING REDUC ENLARGED EMISSION AREA FOR ION BO 5. 1966 Sheet :2 of3 Filed. Jan.

V llllillillilillllllllu Aime/AK United States Patent Office 3,439,210 Patented Apr. 15, 1969 US. Cl. 313337 6 Claims ABSTRACT OF THE DISCLOSURE The thermionic emission cathode has an open face with emission material deposited upon a thin substrate which is fairly wide. The substrate and emission material are arranged normal to the open face with the thin edge of the emission material on the substrate directed toward the open face. It is sinuously arranged in the cathode structure between shielding walls thereof with adjacent portions of the emission material spaced sufficiently far apart that the plasma sheath can enter therebetween. Thus, the narrow edge faces the open face of the cathode for limited ion bombardment, but the entire side surface of the emission material is accessible to the plasma sheath for emission.

This invention relates to cathode structures and more particularly to improvements in cathode structures which are used in plasma discharge devices.

The component most subject to deterioration in a plasma discharge device, such as an electron-bombardment ion thrustor, is usually the cathode. The cathode is used to provide electrons which bombard the gas within the device to create ions. After the gas has been ionized,

the gas ions are electrostatically expelled at high velocity to create the desired thrust. The proposed use of these thrustors is in space missions which call for operating lifetimes of the engine of from 1,000 hours to over 10,000 hours.

Considerable effort has been made toward producing cathodes which can provide electrons over the interval of required use. The problem that arises is that when the material for emitting electrons is relatively thickly coated over a resistive heater, the resulting emitted current which flows through the layer overheats the layer and causes undue flaking and evaporative losses of emitting material. Another phenomenon causing loss of emitting material from the emitting layer is sputtering by the full ion bombardment density that exists Within the plasma discharge device. The type of cathode of this invention is an oxide coated emitter. Other cathode types are impregnated cathodes and metal emitters. Typically, the impregnated cathodes will consist of a thin barium-barium oxide layer on a porous tungsten surface. The ion bombardment suptters away this layer in a short time and the cathode remains inactive for long periods until the layer can be replenished. Metal emitters must operate at such high temperatures, in the range of 2,000" K. and above, that they are both inefficient and of short life.

An object of this invention is to provide a novel structure for a cathode which substantially eliminates losses of emitting material as a result of resistive heating.

Another object of the present invention is the provision of a construction for a cathode which considerably decreases the sputtering loss which occurs.

Yet another object of the present invention is the provision of a structure for a cathode wherein a substantial portion of any losses occurring as a result of sputtering is deposited on adjacent cathode surfaces for re-use.

Still another object of the present invention is the provision of a novel and relatively long lived construction for a cathode for use within a plasma discharge device.

These and other objects of this invention may be achieved by providing a cathode structure wherein the cathode is made of a thin metal sheet preferably in the form of a metal mesh which is coated with the electron emitting material. The metal sheet or mesh is given a folded geometry such as that of a multiple convoluted form in order to increase the surface area of the cathode, but to reduce the relative area exposed to full ion bornbardment. Because of the use of the mesh in the convolutions thereof, the surface area of the cathode is increased to such a great extent as to enable a thinner layer of the emitting material to be applied thereto than heretofore. This, in turn, operates to eliminate the effects of resistive heating. In view of the disposal of the areas of the emitting material on the cathode, the emitting surface is not subject to bombardment from the full ion density within the plasma discharge and therefore the sputtering loss is decreased. Further, in view of the convoluted arrangement of the cathode, any loss of surface material due to sputtering usually is deposited on adjacent cathode surfaces for re-use.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself 'both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings.

Brief description of the drawings FIGURE 1 is a view looking down at one end of a cathode in accordance with this invention;

FIGURE 2 is a view in section along the lines 2-2 of FIGURE 1;

FIGURE 3 is an enlargement of a section of the cathode structure illustrating the mesh construction of the cathode;

FIGURE 4 shows another heater arrangement for the cathode structure of the type shown in FIGURE 1;

FIGURE 5 shows a front View of another arrangement of a cathode in accordance with this invention; and

FIGURE 6 is a side view of the structure shown in FIGURE 5.

Description Reference is now made to FIGURES 1 and 2 which respectively show a top view of an embodiment of this invention and a view in section along the lines 2-2 of FIGURE 1. A cathode structure in accordance with the invention comprises at least one and preferably three heat shields respectively 10, 12 and 14 which are formed into a cylindrical cup or housing having closed sides and an open axial face. Disposed within the cup is the electron emitting structure 16. While this structure may be made of a thin metal sheet a metal mesh 18 (as shown in the enlarged section in FIGURE 3) is preferred for reasons which will become apparent as this description progresses. The metal mesh is coated with a suitable material which emits electrons when it is heated.

By way of illustration and not as a limitation of the invention, the metal mesh is given a convoluted form which, as may be seen from the end view shown in FIGURE 1, may be considered as a flower pattern. The oxide coated metal mesh is bent or formed in a flower pattern usinga plurality of aluminum oxide posts 21, 23 to establish the radius of each bend. Each post is normal to the open face and is directed toward the open face, and is held in place by the bent oxide coated metal mesh and serves to strengthen the mesh against lateral pressures. The oxide coated metal mesh is supported within the heat shield cup 10 by a plurality of aluminum oxide insulators respectively 20, 22, 24, 26, 28, which are mounted on the bottom of the cup and extend adjacent to the cathode and support it centrally within the cup. If desired, one or more of these insulators may also be placed at the bottom of the cup and the cathode may be spaced from the bottom by these.

As shown in FIGURES 1 and 2, the heater leads respectively 30, 32 extend up from the bottom of the cup and are connected to the ends of the wire mesh by any suitable attaching means. An arrangement preferred is shown in FIGURE 1. Two nickel metal strips respectively 29, 31 and 33, 35 are bent to clamp over the respective heater leads 30, 32 and the ends of the metal mesh. These are then spot welded to provide the required electrical connections as well as mechanical rigidity.

By way of illustration and not to serve as a limitation upon the invention, it is preferred that the metal mesh be made of a nickel mesh, 70 wires per inch in two dimensions using 0.0045 inch diameter wire. The Width of the mesh is on the order of one-half inch and a six inch length of the mesh is convoluted into the flower pattern. A coating of a barium oxide emitting material is applied. The amount of material applied is 10 milligrams per square centimeter. The heat shields may also be made of nickel and have a thickness of 0.004 inch.

From the structure described in FIGURES 1 and 2, it will be appreciated that the heat shields surround all but one end of the emitting structure of the cathode and the emitting structure is convoluted. As a result, the emitting surface area of the cathode structure is on the order of ten times the area actually directed toward the open face and facing the ion chamber. As a result, the sputtering losses are decreased by about one-tenth from those values obtained for a cathode whose total emitting surface faces the ion chamber. The basic reasoning behind this is that, regardless of how large the actual cathode emitting surface is, the total number of energetic ions impacting on the cathode is determined by the front area of the cathode facing the plasma. This is true because most of the ion generation is distributed throughout the whole volume of the discharge chamber. In other words, the relative number of ions generated in the immediate vicinity of the cathode is relatively small. In a folded geometry of the type shown in FIGURES 1 and 2, the density of the plasma inside the folds of the cathode mesh is expected to adjust itself to the rate of ion arrival I available from the main plasma volume. It will therefore be lower in the immediate vicinity of the cathode surface Within the folds than in the rest of the discharge chamber. As a consequence, the ratio of the ion current density bombarding the cathode emitting surface to the ion current density available in the discharge chamber at the frontal area of the cathode is expected to be on the order of the ratio of frontal cathode area to total cathode emitting surface area. With the ratio of ten to one, which is quite practical, the oxide coating required for 10,000 hour life can be reduced by a factor of ten from what it is presently.

By using a wire mesh instead of a sheet, additional storage of electron emitting material is made available between the individual wires of the cathode mesh. Therefore, the thickness can be reduced by another factor of two, so that the ultimate thickness of emitting material required may be on the order of 2 /2 X 10- centimeters. This is a quite satisfactory value from the point of view of Joule heating particularly since the average electron current density is also low because of the large total surface area. Finally, in a folded geometry of the type shown in FIGURE 1, a large fraction of the sputtered active material is not lost but is redeposited on the useful cathode surface.

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The success of the cathode in accordance with this invention is critically dependent upon Whether or not the plasma penetrates the individual cathode spaces. Sufficient electron emissions will be obtained only if the plasma sheath follows the individual cathode emitting surfaces. This may be said to occur when the distance between neighboring folds is considerably larger than the plasma sheath width. The plasma sheath width may be estimated to be equal to the Debye length. The latter is (in c.g.s. units) where kT represent thermal energy of ions, 11 represents the plasma density, and e represents the electron charge.

With a plasma density of 5 10 particles per centimeter cubed, and a thermal ion energy equal to thirty electron volts, the Debye length is approximatel 10- centimeters. Thus, if the separation between the cathode folds is greater than 10 centimeters, the plasma should be able to penetrate these folds easily.

FIGURE 4 is a view looking into the cathode cup of an embodiment of the invention identical to that shown in FIGURE 1 except that the flower pattern of the cathode (shown fragmentarily) is completely closed and the heater wires respectively 36, 38 may contact with the metal mesh of the cathode at opposite sides. As a result, the current supplied by the heater wires 36, 38 divides and flows through the two halves of the metal mesh. A central insulated ring 39 provides support and keeps the cathode centrally located.

FIGURE 5 and FIGURE 6 are respectively top and side views of an alternate arrangement for the cathode structure. Here, the cathode 40 has the pattern of a waffle grid. A heat shield 42 is at the sides and bottom of the waffle grid. The leads providing heater current, respectively 44, 46 extend through insulators, respectively 4-5 and 47 in the heat shield 42 to connect to the ends of the grid and also serve to support it above the heater shield. Here again, care must be taken to see that the adjacent folds are sufficiently far apart to enable the plasma sheaths to reach into the folds.

Since the smallest area of the wafile shaped cathode faces into the ion chamber the same reasoning as expressed previously applied to this cathode, namely that the thickness of the emitting coating required to produce the longevity may be reduced by the ratio of the area facing the ion chamber to the total area of the cathode. Further, because of the use of a metal mesh and the additional storage available within the mesh, another reduction by approximately one-half of the required coating may be made.

From the foregoing description it should be appreciated that a cathode structure has been provided whereby the amount of emitting material coating on the cathode required to provide a predetermined amount of electron emission over a predetermined interval is considerably reduced in thickness to the point where emitting currents no longer cause such heating as to cause flaking or cracking or evaporation of the emitting material. Furthermore, the shape of the cathode and its disposition is such that the effects of ion bombardment are minimized. While certain specific geometries such as the flower and waflie patterns for the cathode structure have been shown herein, it should be appreciated that these are by way of example only. Those skilled in the art will readily appreciate that other multiply convoluted forms may be used without departing from the scope and spirit of this invention.

What is claimed is:

1. A cathode for thermally emitting electrons for ionizing a gas to form a plasma within a chamber wherein said cathode structure is subject to ion bombardment, said cathode comprising a housing and a cathode structure therein, said housing having an open face and closed sides, said cathode structures comprising a multiply convoluted metal sheet, said convolutions being disposed to provide a regular pattern, adjacent folds of said metal sheet being spaced sutficiently far from one another to permit the penetration between said folds of a plasma sheath, said folds further being disposed to expose a surface area of said cathode structure toward said open face of said housing which is less than the total emitting surface area, a layer of electron emitting material coating said metal sheet so that the electron emitting material on the sides of said multiply metal sheet is protected from excessive ion bombardment, and heater means connected to said metal sheet for enabling the thermal heating thereof.

2. A cathode structure as recited in claim 1 wherein said multiply convoluted metal sheet comprises a metal mesh.

3. Apparatus as recited in claim 1 wherein said multiple convolutions are arranged in a flower patter symmetrically disposed about a central axis, and there is included a shield surrounding all but one end of said metal mesh.

4. The cathode structure of claim 1 wherein the thickness to width ratio of said multiply convoluted metal sheath with said layer of electron emitting material coating thereon is substantially 1:10.

5. A cathode structure for use in a sputtering environment for thermally emitting electrons, the improvement comprising:

a cylindrical cup, said cylindrical cup having an open axial face, said cylindrical cup acting as a heat shield when said cathode structure is operatively emitting electrons;

heater leads positioned within the interior of said housing;

a metal mesh, said metal mesh being positioned within said housing so that one edge of said metal mesh is directed toward the open face of said housing, said metal mesh being electrically connected to said heater leads, said metal mesh being convoluted within said housing, emissive material means coated upon said metal mesh, said emissive material means emitting electrons when heated so that electrons are discharged out of the open end of said housing, said emissive material means being edgewise directed to the open face of said housing to minimize sputtering damage to said emissive material, said emissive material means on adjacent convolutions of said metal mesh being spaced at least equal to the Debye length so that a plasma sheath may enter between the convolutions so that the entire surface of emissive material means is available for the emission of electrons. 6. The cathode of claim 5 wherein posts are positioned within the convolutions of said support to maintain said convolutions spaced at least equal to the Debye length.

References Cited UNITED STATES PATENTS 1,981,669 11/1934 Ronci et al. 313-345 X 2,001,548 5/1935 Romhild 313-333 X 2,246,162 6/1941 Benjamin 313345 X 2,416,661 2/1947 Lawton 3l3-346 X 2,937,301 5/1960 Germeshausen et a1. 313-346 X 2,107,945 2/1938 Hull et al 313---346 X 2,119,913 6/1938 Holst et al 3l3337 X 2,154,298 4/1939 Ayer 3l3---337 X 2,212,827 8/1940 Etzrodt 313-346 X 2,452,075 10/ 1948 Smith a 313-346 X 2,456,649 12/ 1948 Rouse 313-346 X 2,459,841 l/ 1949 Rouse 313-346 FOREIGN PATENTS 74,010 11/ 1944 Czechoslovakia. 1,165,545 6/ 1958 France.

JOHN W. HUCKERT, Primary Examiner.

A. I. JAMES, Assistant Examiner.

U.S. C1. X.R. 313-270, 345, 346

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