Annular hollow cathode

Abstract

A cathode emitter assembly includes a cathode tube having a gas feed portion and a plasma outflow portion; an outer annular cathode insert in the plasma outflow portion of the cathode tube; an inner cathode insert in the plasma outflow section of the cathode tube; and an annular plasma emission portion defined between an inner surface of the outer annular cathode insert and an outer surface of the inner cathode insert.

Claims

1. A cathode emitter assembly including: a cathode tube having a gas feed portion configured to receive a gas and a plasma outflow portion configured to discharge plasma; at least one cathode insert in the plasma outflow portion of the cathode tube; and at least one plasma emission channel extending about the at least one cathode insert within the cathode tube; wherein the cathode emitter assembly is configured to form the plasma within the at least one plasma emission channel such that gas enters a first end of the at least one plasma emission channel and plasma exits a second end of the at least one plasma emission channel.

2. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises an emissive material.

3. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises a non-emissive material.

4. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises an outer cathode insert, a first inner cathode insert, and a second inner cathode insert in the outflow section of the cathode tube, and wherein the at least one plasma emission channel is defined between an inner surface of the outer cathode insert and respective outer surfaces of the first and the second inner cathode inserts.

5. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises and outer cathode insert, an inner cathode insert, and an intermediate cathode insert in the plasma outflow portion of the cathode tube, and wherein the at least one plasma emission channel comprises an outer channel and an inner channel, the outer channel being defined between an inner surface of the cathode tube and an outer surface of the intermediate cathode insert, and the inner channel being defined between an inner surface of the intermediate cathode insert and an outer surface of the inner cathode insert.

6. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises an outer cathode insert, an inner cathode insert, and first, second, and innermost intermediate cathode inserts in the plasma outflow portion of the cathode tube, and wherein the at least one plasma emission channel comprises a plurality of annular plasma emission channels, and wherein an outermost plasma emission channel is defined between an inner surface of the outer cathode insert and an outer surface of the first intermediate cathode insert, an intermediate plasma emission channel is defined between opposing surfaces of the first and second intermediate cathode inserts, and an innermost plasma emission channel is defined between an inner surface of the innermost intermediate cathode insert and an outer surface of the inner cathode insert.

7. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises an inner cathode insert and an outer cathode insert, and wherein radii of the inner cathode insert and the outer cathode insert are sized from an existing cathode with a conventional tubular emitter using a method of equal pressure drop across a length of the inner cathode insert and the outer cathode insert as compared to the conventional tubular emitter.

8. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises an outer cathode insert and an inner cathode insert, and wherein the outer cathode insert and the inner cathode insert are physically connected, and are configured to permit gas to flow through one or more regions between the outer cathode insert and the inner cathode insert.

9. The cathode emitter assembly of claim 1, wherein the at least one cathode insert comprises an outer cathode insert and an inner cathode insert, and wherein the the outer and inner cathode inserts comprise a monolithic structure, and are configured to permit gas to flow through one or more regions between the outer and inner cathode inserts.

10. The cathode emitter assembly of claim 1, further comprising a sheathed heater positioned about the at least one plasma emission channel.

11. The cathode emitter assembly of claim 1, wherein the at least one plasma emission channel has a uniform size along a length thereof.

12. A method of generating plasma in a cathode emitter assembly including: a cathode tube having a gas feed portion configured to receive a gas and a plasma outflow portion configured to discharge plasma, at least one cathode insert in the plasma outflow portion of the cathode tube, and at least one plasma emission channel extending about the at least one cathode insert within the cathode tube, the method comprising: feeding gas into the gas feed portion of the cathode tube; emitting an electric current from the cathode tube; generating plasma within the at least one plasma emission channel; and discharging the plasma from the at least one plasma emission channel.

13. The method of claim 12, wherein the at least one cathode insert comprises concentric cathode inserts, and the at least one plasma emission channel comprises a plurality of plasma emission channels defined between the concentric cathode inserts.

14. The method of claim 12, wherein the gas is fed at a fixed gas flow rate.

15. The method of claim 12, wherein the at least one cathode insert comprises a non-emitting material.

16. The method of claim 12, wherein the at least one cathode insert comprises an emissive material.

17. The method of claim 16, wherein the emissive material is selected from the group consisting of lanthanum, hexaboride, barium oxide impregnated tungsten, and carbon nanotubes.

18. The method of claim 12, wherein the at least one cathode insert comprises an outer cathode insert, a first inner cathode insert, and a second inner cathode insert in the outflow section of the cathode tube, and the at least one plasma emission channel is defined between an inner surface of the outer cathode insert and respective outer surfaces of the first and the second inner cathode inserts.

19. The method of claim 12, wherein the at least one plasma emission channel is defined between an inner surface of a first outer cathode insert and respective outer surfaces of first and second inner cathode inserts.

20. The method of claim 12, wherein the at least one plasma emission channel has a uniform size along a length thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a diagram of a conventional hollow cathode. A 2D cross-section of the hot tip of the cathode showing typical placement of the concentric cathode tube and keeper electrode, with gas source and plasma emission labeled.

(2) FIG. 2 shows a diagram of an exemplary hollow cathode with an annular emission area.

(3) FIG. 3 shows a schematic cross-sectional view of an exemplary hollow cathode with nested intermediate inserts.

(4) FIG. 4 shows a schematic cross-sectional view of an exemplary hollow cathode with multiple inserts.

DETAILED DESCRIPTION

(5) It is desirable for efficient cathode operation to 1) increase the surface area of the emitter, 2) reduce the required gas flow to ignite and sustain the hollow cathode, 3) spread out the plasma as much as possible, 4) provide a low resistance path for electron extraction, and 5) to do so in a compact package as the hot cathode radiates energy and a smaller device is easier to heat shield. These objectives can be achieved by using an annular emitter geometry in a hollow cathode.

(6) Exemplary embodiments adapt the typical tubular emitter geometry of the hollow cathode (see FIG. 1) into an annular, or concentric geometry by placing additional emitter material on axis (see FIG. 2). A cathode emitter assembly 200 includes a cathode tube 210 having a gas feed portion 212 and a plasma outflow portion 214. An outer annular cathode insert 220 is positioned in the plasma outflow portion 214 of the cathode tube 210. An inner cathode insert 230 is positioned in the plasma outflow portion 214 of the cathode tube 210. An annular plasma emission portion 240 is defined between an inner surface 222 of the outer annular cathode insert 220 and an outer surface 232 of the inner cathode insert 230.

(7) Turning now to FIG. 3, an alternative exemplary embodiment is shown in schematic form, omitting exterior structures and showing merely the insert portions of the cathode. FIG. 3 depicts a plurality of intermediate cathode inserts 350 in the outflow section of the cathode tube (although any number of intermediate cathodes may be present, including only 1 such cathode) in addition to an outer 320 and inner 330 insert. Exemplary embodiments of this type have a plurality of annular plasma emission portions 340. The annular plasma emission portions are defined between opposing surfaces of adjacent cathode inserts.

(8) Turning now to FIG. 4, an alternative exemplary embodiment is shown in schematic form, omitting exterior structures and showing merely the insert portions of the cathode. FIG. 4 depicts a cathode assembly 400 having multiple inner cathode inserts 430 which would be positioned in the outflow section of a cathode tube. The annular plasma emission portion 440 is defined between the inner surface 422 of the outer annular cathode insert 420 and outer surfaces 432 of the inner cathode inserts 430.

(9) Consider the internal surface area A.sub.tube of a typical tubular emitter with circular cross section of radius r.sub.0 and length L such that A.sub.tube=2*pi*r.sub.0*L. If we replace this emitter with an emitter of annular cross section with outer radius a and inner radius b, the total surface area of the outer emitter is 2?aL and the total surface area of the inner emitter is 2?bL. The surface area improvement is then A.sub.annular/A.sub.tube=(a+b)/r.sub.0, or expressed in terms of the gap between emitters delta=(a?b)/r.sub.0, A.sub.annular/A.sub.tube=delta+2*b/r.sub.0. For the simple case where additional emitter material is placed on the axis of an existing circular cross section emitter such that a=r.sub.0 we find A.sub.annular/A.sub.tube=2-delta. Since we require b<a (or equivalently, delta>0) for any gap to exist, the limit of area increase in this case is a factor of two. Alternatively, instead of increasing the area, for a fixed current requirement from the cathode the emitter length can be reduced. This is useful to reduce heating power requirements for the cathode.

(10) In addition to improving the scaling of the total current, there exist analytical scaling laws for pressure drop making reasonable assumptions of Poiseuille flow that connect the standard tube configuration to the new annular design. For a tubular design with radius and fixed volumetric flow rate, the ratio of the change in pressure is given by

(11) $\begin{array}{cc}\frac{?P_{\mathrm{tube}}}{?P_{\mathrm{annular}}}=\frac{{\left(\frac{r_0}{b}\right)}^4}{{\left(\frac{a}{b}\right)}^4-1-\frac{{\left({\left(\frac{a}{b}\right)}^2-1\right)}^2}{\ln \left(\frac{a}{b}\right)}},& \left(1\right)\end{array}$

(12) Therefore, given a standard hollow cathode tube design with known r.sub.0 and ?P.sub.tube, one can devise a similar annular cathode configuration by choosing any one of the desired inner emitter radius, desired change in pressure ?P.sub.annular, ratio of a/b or gap spacing delta. Similarly, there also exists an expression for the flow velocity:

(13) $\begin{array}{cc}\frac{u_{\mathrm{tube}}}{u_{\mathrm{annular}}}=\frac{\left(r_0^2-r^2\right)/b^2}{1-{\left(\frac{r}{b}\right)}^2+\left({\left(\frac{a}{b}\right)}^2-1\right)\frac{\ln \left(\frac{r}{b}\right)}{\ln \left(\frac{a}{b}\right)}}.& \left(2\right)\end{array}$

(14) When the ratio a/b is greater than 1, then the velocity of the gas down the tube has decreased.

(15) Using these scaling laws, an exemplary annular hollow cathode may be based, for example, on an existing tubular cathode. In this example, the pressure drop matches that of the conventional existing cathode (for a fixed gas flow rate) and a convenient inner annulus radius b=r.sub.0 may be used in the exemplary design. From Eqn. (1) it follows that b/a=1.8, producing an increase in surface area and decrease in the mean flow velocity by the same factor 1+b/a=2.8.

(16) This technique may naturally be applied to reduce the temperature and thus power consumption of a cathode operating at a given current, increase the current available at a given operating temperature, or reduce the size of a cathode operating at a given current.

(17) Embodiments of the invention naturally spread out the plasma by moving plasma production off axis, which is correlated with improved cathode performance due to reduced plasma instability.

(18) Exemplary embodiments also reduce gas flow velocity while maintaining pressure profile, increasing neutral residence time in cathode for better gas utilization efficiency. Exemplary embodiments have an increased cross-sectional exit plane area with same pressure gradient and total mass flow rate due to reduced velocity, enabling 1) reduced insert exit plane current density when operating at same total current as a conventional design, and 2) increased total current at matched exit plane current density as a conventional design.

(19) Exemplary embodiments have an increased electron emission area packed into the same volume as a conventional design, permitting a more compact electron emission source of comparable total current capacity.

(20) Exemplary embodiments also have backwards compatibility with conventional cathode design envelopes. This type of design can be retrofit onto existing cathode and keeper designs, providing a route to delta qualification and comparison of performance benefits with the existing design.

(21) Exemplary embodiments are compatible with other recently invented techniques: Intricate internal keeper geometries attainable by 3D printing for radiation shielding. Altered keeper orifice geometry using multiple orifice or non-circular orifice keepers.

(22) The exemplary embodiments described above include a single annulus comprising two emitting surfaces. The same principles would apply to a plurality of nested emitting surfaces, for example two annuli comprising an outermost emitting surface facing inward, an innermost emitting surface facing outward, and intermediate surface(s) oriented appropriately forming concentric rings.

(23) Exemplary embodiments include a circular ring or annulus for ease of fabrication but any form topologically the same as a ring (i.e., stretchable into a ring shape) such as a square or oval ring would perform similarly, and annular as used herein refers to topology and should not be understood to refer only to circular cross-sectional shapes.

(24) The inner and outer annulus surfaces of the cathode need not be identical, or even both be emissive. Either the inner or the outer surface of the annulus might be replaced by a non-emitting thermionic material such as graphite, or by a different type of emitting material such as lanthanum hexaboride, barium oxide impregnated tungsten, carbon nanotubes, or others. For example, an exemplary cathode assembly may include a conventional circular tube hollow cathode with a plain graphite rod inserted on axis into the tube, as it may provide several plasma spreading and velocity-reducing benefits even without the increase in emission area.

(25) An annulus may be formed for only part of the length of an emitter instead of its full length. For example, a solid cylinder of emitter material may have an annulus bored into one half of its length, with slots milled into the other half to allow gas to pass through to the annular portion, enabling a single emitter of complicated shape to achieve the benefits of an annular emitter. In general we have shown a particular manner of mounting the central portion of the annulus in our diagrams, but this is not intended to be an exclusive configuration.

(26) Exemplary annular designs can be combined with other state-of-the-art methods, including the multiple orifice keeper or a keeper with integrated radiation shielding.

(27) Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a means) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.