US3421039A - Thermionic cathode having emissive material and metallic paths which sputter away at the same rate - Google Patents

Description

Jan. 7. 1969 w. KNAUER ET AL 3,421,039

THERMIONIC CATHODE HAVING EMISSIVE MATERIAL AND METALLIC PATHS WHICH SPUTTER AWAY AT THE SAME RATE Filed Jan. 5. 1966 Sheet of a .45 Fm-JQ Arron/5M 3,421,039 ETALLIC Jan. 7, 1969 w. KNAUER ET AL THERMIONIC CATHODE HAVING EMISSIVE MATERIAL AND M PATHS WHICH SPUTTER AWAY AT THE SAME RATE Sheet Filed Jan.

United States Patent 3,421,039 THERMIONIC CATHODE HAVING EMISSIVE MATERIAL AND METALLIC PATHS WHICH SPUTTER AWAY AT THE SAME RATE Wolfgang Knauer and Hayden E. Gallagher, Malibu, Calif., assignors to Hughes Aircraft Company, Culver, City, Calif., a corporation of Delaware Filed Jan. 3, 1966, Ser. No. 518,081 U.S. Cl. 313-339 9 Claims Int. Cl. H01j 1/20; 19/14 ABSTRACT OF THE DISCLOSURE In a thermionic cathode, the thermionic emissive material is separated by electrically conductive paths. The emissive material is sufliciently thin to prevent excessive resistive heating from current flowing through the metallic paths and thence through the emissive material to the emissive surface. Furthermore, the metallic paths are sufficiently thin that they are sputtered away under ion bombardment at substantially the same rate as the emissive material.

Background This invention relates to thermionic cathode structures for use in gaseous discharges and plasmas and more particularly to improvements therein.

In those gaseous discharges and plasmas in which the ordinary thermionic cathodes cannot be utilized, ion energy exceeds the sputtering threshold so that their active surface layers are removed too rapidly under ion bombardment. Both oxide and dispenser cathodes are adversely aifected in this manner. In the dispenser type cathodes, the active material is sputtered too rapidly to be replenished from the dispenser.

Attempts to increase the lifetime of oxide cathodes by simply increasing the thickness of the oxide layer is ineflectual. The reason for this is that the resistance of the oxide layer rises with layer thickness and eventually Joule heating, caused by passage of the emission currents through the oxide layer, produces local hot spots. As a consequence, the oxide begins to flake off and the lifetime of the cathode is shortened substantially.

It is an object of this invention to provide a thermionic cathode structure suitable for use in gaseous discharges and plasmas having a longer lifetime than hitherto achievable.

Another object of this invention is the provision of a novel thermionic cathode structure which is capable of withstanding heavy ion bombardment for long periods of operation.

Yet another object of the presentinvention is the provision of a novel structure for a thermionic cathode wherein the oxide layer is physically thick and yet electrically and thermally thin.

Summary These and other objects of the present invention are achieved in a thermionic cathode by embedding into the oxide, used for generating electrons, a cell structure of electrically and thermally conductive material. The cathode may have the form of a stack of alternate, oxide and conductive metal disks which are heated from a filament source at their centers. Another form may be a stack of alternate metal and oxide disks which are heated by a filament disposed around their peripheries. A cone may be cut into the centers of the disks whereby electrons are emitted from the centers. Still other arrangements are to fill a heated cup with either felt metal impregnated with oxide material or oxide grains encapsulated in metal. In all of these arrangements it is necessary to provide a sufiiice cient amount of conductive material so that emission currents flow in the conductive structure rather than in the oxide to minimize Joule heating. The conductive material must also provide suflicient thermal conductivity so that the heat energy passed from the heating element to the emitting surface is substantially transported by the conductive material. The thickness of the individual cell walls of the conductive structure must be so small that under ion bombardment it is sputtered away at substantially the same rate as the oxide. Also, the size of the individual cells of the conductive structure has to be so dimensioned that the oxide within each cell does not :become Joule heated when it is utilized as an emission site.

The novel features that are considered characteristic of this invention are set forth with particularity of 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 cross-sectional View of one embodiment of the invention;

FIGURE 2 is a cross-sectional view of another embodiment of the invention;

FIGURE 3 is a cross-section of a disk which may be used in an embodiment of the invention of the type shown in FIGURE 1 or FIGURE 2;

FIGURE 4 is a cross-section of another disk which may be used in an embodiment of the invention of the type shown in FIGURE 1 or FIGURE 2;

FIGURE 5 is an enlarged cross-section of a cathode structure in accordance with this invention;

FIGURES 6 and 7 respectively are cross-sectional views of two other embodiments of this invention; and

FIGURE 8 is an enlarged cross-sectional view of one of the elements of the cathode structure shown in FIGURE 7.

Description of the preferred embodiments Referring now to FIGURE 1, there may be seen a crosssection of a thermionic cathode in accordance with this invention. The cathode structure comprises a core or central hub 10 of a pyroceramic material having wound therein heater filament windings 12. Electrical current is applied to the filament windings to generate the heat required for causing the oxide cathode to liberate electrons. The central core 10 hasa flange 14 in order to hold a plurality of disks which are alternately composed of oxide emitting material 16 and conductive metal 18. By way of example, the oxide material may be barium oxide, or calcium oxide, or strontium oxide, and the conductive metal may be nickel. On top of the stack of disks is another flange 20 which may be threaded or fastened through the hub 10 by any suitable means.

With the multiple disk cathode shown in FIGURE 1, electrons are emitted radially. FIGURE 2 is a crosssection of another embodiment of the invention wherein, instead of the heater being at the center surrounded by the oxide-metal disks, the heater structure 22 is cupshaped and the alternate oxide disks 24, conductive metal disks 26 are disposed within the cup. In addition, a conical opening 28 having its axis aligned with the axis of the disks is cut into the disks and extends from the uppermost disk down to the lowest disk. This cathode emits electrons axially and has the virtue that the entire cathode is shielded except for the emitting region.

While the embodiment of the invention described thus far indicates that the oxide disks are alternate with conductive disks, it is to be understood as being included within the scope of this invention to employ conductive disks which are coated with an oxide coating.

FIGURE 3 is an enlarged cross-section illustrative of a disk which may be used in FIGURE 1 or FIGURE 2 instead of alternate nickel-oxide layers wherein the center 32 is a conductive metal, such as nickel, and the outer surfaces are coated with a suitable oxide layer 34, 36.

FIGURE 4 is a cross-sectional view of an oxide coated metal disk wherein the metal disk 38 has holes therein so that the amount of oxide 40 available for producing electrons is increased by virtue of the fact that it fills the holes in the disk.

The dimensions of the cell structures in the cathode as described depends substantially upon the type of plasma used. To indicate the procedure by which the pertinent dimensions are determined, a specific situation is analyzed below.

As a first step in the computation of the cellular cathode dimensions, one has to know the average ion energy and the rate at which ions impact at the cathode. This determines the maximum electron emission rate (in accordance with the laws governing double sheaths). The maximum emission current density in turn established the maximum dimensions for the cells, containing the oxide. Then, from the sputtering rates of the oxide and of the conductive material, the cell wall thickness can be determined. With cell size and wall thickness known, both electrical and thermal impedance can be computed and it can be seen whether both satisfy the requirements for low voltage and low temperature gradients.

According to Langmuir, the amount of electron current density that can be drawn from an emitter into a plasma whereby is the ion current, m+ is the plasma ion mass, m is the electron mass flowing toward the oathode. For a given average energy eV of the ions, and a plasma density 11, (1 becomes Hence, the maximum electron current density is [26V 1/2 a =6n m.

For a mercury plasma, for example with a plasma density n of 3x10 particles per cm. and with an average ion energy of 10eV, the maximum electron current density r becomes 1 A./cm.

A. H. W. Beck in an article in the Proc. Inst. El. Eng. B. 106, 372 (1959), and H. A. Pike in an article in the M.I.T. Lincoln Lab. Techn. Rpt. 356 (1964) have given criteria for the limiting thickness of an oxide layer, up to which overheating can be prevented. If is the work function of the cathode, p the resistivity of the oxide and o' the emission current density, the layer thickness 7 must be where 6 6 are the sputtering rates and P P are the densities of the cell material and of the oxide. With nickel as cell material and mercury ions of 402V (10eV thermal energy and 30eV from the voltage drop in the cathode sheath) the sputtering rates are 6CE1O 4 atoms/ ion, 6 510 atoms/ion. The densities are P =8.9 g./cm. and P l g./cm. Hence, the cell walls have to be approximately 5 X 10- cm. thick.

FIGURE 5 is an enlarged end view in cross-section of several coated conductive disks. As previously indicated, the coating may be barium oxide and the conductive metal upon which the barium is coated may be nickel. It will be seen that the upper and lower surfaces of the nickel disk 44 are coated by the respective oxide coatings 42, 46. There is a small space left between adjacent oxide coatings. The plasma sheath 48 established between the plasma 50 and the surface of the layers of the cathode does not penetrate into the spaces between the oxide coatings since these are smaller than the Debye length. It will be noted that the metal disks extend beyond the termination of the oxide coatings thereon.

Both the barium oxide and nickel which are used in these disks are gradually sputtered under ion bombardment. In order for the decrease in diameter of the disks to be substantially identical for both materials, a suitable ratio of the layer thicknesses must be chosen. For an estimate of this ratio, the following assumptions are made. First, the nickel layer is permitted to stand out over the barium oxide layer by a distance d which is approximately equal to the width b of the barium oxide layer. With a Debye length x smaller than the width of the oxide layer, the plasma sheath will follow the contour of the emitter surface and thus provide the fields necessary to draw current from the recessed barium oxide surfaces. Furthermore, the ions impinge with equal rates on the barium oxide and nickel surfaces. It is realized that the ion velocities are not isotropically distributed, and the majority of the ions probably move radially. The higher impact rate on the barium oxide layer however is expected to be more or less compensated for by the larger sputtering rate for impact under a small angle on the nickel surface.

If during a specific time interval layers of thickness t and 1 are removed from the barium oxide and nickel surfaces, as shown in FIGURE 5, the ratio of width 0 to height a (of the nickel layer) must be determined by Since the ratio t /t is equal to the ratio of sputtering yields y and 'y the nickel layer thickness 0 becomes With sputtering rates at 40V of 210" atoms/ion and 713%?! 10- atoms/ ion and with a barium layer thickness of b=2.5 10 cm., the thickness of the nickel disks becomes c l] 10 cm.

This thickness appears to be reasonable from a mechanical point of view. It also can be shown that with respect to heat and current conduction the small thickness is tolerable. For a net disk height of 0.5 cm., for a heat conductivity of 0.37 W/ cm. C. at 1000" K., and for a heat radiation from the cathode surface of 1 W/cm. the temperature differential between inner and outer disk radius becomes ATEZOO C., which appears to be quite tolerable. Furthermore, with a resistivity of 1.8 10- 'Q-cm. at 1000 K. and with an emission current density of 0.5 a./cm. the voltage drop across the nickel disk becomes AEIWQV, which is completely negligible. Thus, the nickel disks can indeed fulfill their role as heat and electric conductors. It is apparent from the above calculations that the disk could be perforated to improve the storage of active material without detriment to cathode performance.

FIGURE 6 is a cross-sectional view of another embodiment of the invention. There may be seen a cupshaped heating structure 52 wherein the cup is filled with felt metal strands 54, preferably made of nickel, which have been impregnated with oxide. Felt metal is the designation given to porous metal strands which are made by compressing powdered metal or by sintering metal and then grinding it into slivers. The felt metal has microporous openings. The dissolved oxide is mixed with the porous metal and the microporous openings absorb the oxide material. After the oxide has solidified, the metal impregnated with the oxide is placed in the cup-shaped structure 52 and heated. The conductive metal provides both the thermal and electrical paths necessary to avoid undue loss of the oxide material. The oxide material emits electrons. The effects of the ion bombardment are minimized by virtue of the structure provided.

Another variation of the arrangement in FIGURE 6 is shown in FIGURE 7. Here the cup-shaped heating structure 56 is loaded with small spheres 58. Each of the spheres is shown in FIGURE 8 and comprises a central gain 60 of oxide material surrounded by a metal coating 62, such as nickel. The oxide material is encapsulated in the metal coating. The metal coating is not perfect, there are sufiicient cracks so that upon heating, the oxide material can emit electrons which will pass through these cracks. Undue heating of the oxide is avoided by the conductive metal encapsulating material.

There has accordingly been described herein a novel, useful thermionic cathode structure suitable for use in gaseous discharges and plasmas. This cathode is longer lived than the presently known cathode structures.

What is claimed is:

1. A thermionic cathode structure comprising heating means, and cathode means having one surface to which heat from said heating means is applied for emitting electrons from another surface thereof, said cathode means including a body of material having the property that when it is heated it emits electrons, said body of material having one surface disposed to receive heat from said heating means and another surface disposed to emit electrons, and metal conductive material means in contact and substantially coextensive with said body of material for conducting a substantial amount of the emission currents and heat from the surface of said body which is exposed to said heating means to the surface which emits electrons, the improvement comprising: said body of material and said metal conductive material means having relative thicknesses such as to be sputtered away under ion bombardment at substantially the same rate.

2. A thermionic cathode structure comprising heating means, alternate layers of material for emitting electrons in response to heat energy and metal conductive material, means for applying heat energy from said heating means to one layered surface of said alternate layers whereby electrons are emitted from said material for emitting electrons at the opposite layered surface, the improvement comprising: said material for emitting electrons and said metal conductive material having a relative thickness to prevent ohmic heating of said material for emitting electrons due to current flow and to sputter away said material for emitting electrons and said conductive material at substantially equal rates under ion bombardment.

3. A thermionic cathode as recited in claim 2 wherein said layers of metal conducting material comprise nickel and said layers of material for emitting electrons comprises one of the group consisting of barium, strontium and calcium oxides.

4. A thermionic cathode structure comprising a disk of material having the property that it will emit electrons in response to heat, metal conductive disk means in contact with said disk of material, for protecting said disk of material from adverse effects due to emission currents and temperature, the thickness of said disk of material and said metal conductive disk means being established to provide substantially the same sputtering rate to ion bombardment, and means for applying heat to one edge of said disk of material for causing electrons to be emitted from the other edge thereof.

5. A thermionic cathode structure as recited in claim 4 wherein said cathode structure comprises a plurality of said disks of material and metal conductive material means disposed adjacent one another, said plurality of disks having an opening in the center thereof and said means for heating being disposed within said central openmg.

6. A thermionic cathode structure as recited in claim 4 wherein said cathode structure comprises a stack of said disks of metal conductive material means and material for emitting electrons responsive to the application of heat, said heating means comprises means for applying heat to the outer edges of said stack of disks, and said stack of disks have a conical shaped hole formed therein with the base of said conical shaped hole being adjacent one end of said stack of disks and the axis of said conical shaped hole being coincident to the axis of said stack of disks.

7. A thermionic cathode comprising a cup-shaped means for generating heat and a plurality of means within the cup of said cup-shaped means for emitting electrons responsive thereto to said heat, each of said plurality of means comprising material for emitting electrons and metal conductive material in contact therewith, the thickness of said material for emitting electrons and the thickness of said conductive material being established to provide substantially the same sputtering rate under ion bombardment.

8. A thermionic cathode as recited in claim 7 wherein each of said plurality of means comprises a nucleus of said material for emitting electrons encapsulated in a metal conductive material shell having openings therethrough.

9. A thermionic cathode as recited in claim 7 wherein each of said plurality of means comprises a porous strand of said metal conductive material with said material for emitting electrons within the pores thereof.

References Cited UNITED STATES PATENTS 1,701,356 2/1929 Bruckel et al 313-346 X 2,014,539 9/1935 Stansbury 313-337 2,173,208 9/1939 Lecorguillier 313-346 X 2,459,841 1/1949 Rouse 313-346 2,888,592 5/1959 Latl'erty 313-346 2,937,304 5/1960 Goldberg et al. 313-346 X 3,147,362 9/1964 Ramsey et al 313-346 X 3,149,253 9/ 1964 Luebke 313-346 X 1,416,623 5/1922 Donath 313-345 X 1,891,074 12/1932 Winter 313-339 2,107,945 2/1938 Hull et al. 313-343 X 2,177,703 10/ 1939 Francis 313-343 X 2,420,014 5/1947 Rayfield 313-345 3,210,575 10/1965 Podolsky 313-346 X FOREIGN PATENTS 952,543 11/ 1956 Germany.

JOHN W. HUCKERT, Primary Examin r.

A. J. JAMES, Assistant Examiner.

US. Cl. X.R.

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