US3253180A

US3253180A - Hollow cathodes

Info

Publication number: US3253180A
Application number: US216141A
Authority: US
Inventors: Huber Harry
Original assignee: CSF Compagnie Generale de Telegraphie sans Fil SA
Current assignee: Thales SA
Priority date: 1961-09-27
Filing date: 1962-08-10
Publication date: 1966-05-24
Anticipated expiration: 1983-05-24
Also published as: GB1020233A; FR1308692A; NL283468A; DE1232663B

Description

H. HUBER HOLLOW CATHODES May 24, 19-66 2 Sheets-Sheet 1 Filed Aug. 10, 1962 FIGJ INVENTOR.

H- HUBER Q1/2242 ATTORNEY H. HUBER HOLLOW CATHODES May 24, 1966 2 Sheets-Sheet 2 Filed Aug. 10, 1962 m HERE-55in? nuaiiiinll HHIIIHIHIH FIGIS INVENTOR.

ATTORNEY United States Patent 17 Claims. (01. s1s 3s9 The present invention relates to a cathode structure, and more particularly to so-called hollow cathode structures providing an improved current density.

The so-called hollow cathodes are conventionally realized in the form of a cavity the inner walls of which are entirely or partly lined with an emissive substance, this cavity comprising an orifice with small dimensions for the outflow of the electrons and being heated by any suitable means. Such cathodes are intended to produce an electronic current of great density, this result being based on the emission of electrons by a relatively large surface at the interior of the heated cavity and upon the extraction of these electrons through a relatively small orifice area.

Practice has shown, however, that the density of the current supplied by the hollow cathodes known in the prior art is not as high as one might hope for by simply taking as basis the relationship between the emissive surface and the extraction area and that, in fact, the hollow cathodes known in the prior art do not represent an improvement over other conventional cathodes, from the point of view of current density.

Applicant attributes this shortcoming to the fact that the distribution of the electrons emitted at the interior of the cavity is not homogeneous in volume but that, by reason of the space charge, a minimum of volumetric density of electrons is created in the center of the cavity, and a strong density in proximity to the internal surface thereof.

The result thereof is a minimum of negative potential, i.e., a potential well in the center of the cavity. This well repels the majority of electrons toward the walls of the cavity where they will remain accumulated by forming a strong space charge, and also expels only a small fraction of electrons toward the output orifice through which they may escape under the effect of the electric field due to the voltage applied to the anode of the tube. Thus, the electron output of a hollow cathode is equal to that of a conventional cathodeapart from the effects of the second order-whose emissive surface is equal to the area of the outlet orifice.

The present invention aims at an increase of this output and at a realization of an improved hollow cathode supplying a large current density.

In accordance with the present invention, means are provided to compensate the electronic space charge at the interior of a hollow cathode, the latter thus becoming a palsma cathode.

These means consist-according to the present invention of a reservoir or tank of ionizable material connected to the cavity of the hollow cathode, means being provided for ionizing this material and mixing the ions produced with the electrons at the interior of the cavity.

The ionization may be realized preferably by the phenomenon of surface or contact ionization.

It is, therefore, an object of the present invention to provide a hollow cathode structure which effectively eliminates the shortcomings encountered with the prior art structures.

It is another object of the present invention to provide a hollow cathode structure assuring a relatively large current density and a relatively high overall current intensity.

3,253,189 Patented May 24, 1966 Still another object of the present invention resides in the provision of a hollow cathode structure which eliminates, by simple and inexpensive means, the non-homogeneous electron distribution on the inside of the cathode.

A further object of the present invention resides in the provision of a hollow cathode structure which utilizes a plasma to achieve the aforementioned objects.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, several embodiments in accordance with the present invention, and wherein FIGURE 1 is an axial cross-sectional view through a first embodiment of a hollow cathode arrangement according to the present invention;

FIGURE 2 is an axial cross-sectional view through a second embodiment of a hollow cathode arrangement according to the present invention;

FIGURE 3 is an axial cross-sectional View through a third embodiment of a hollow cathode arrangement according to the present invention, and

FIGURE 4 is a perspective view through a fourth embodiment of a hollow cathode arrangement according to the present invention.

Referring now to the drawing wherein like reference numerals are used throughout the various views to designate like parts, and more particularly to FIGURE 1, there is illustrated in this figure a cavity 1 having, for example, a spherical shape, lined at the inside thereof at 2 with a layer of emissive material and equipped at 3 with an outlet or output orifice, this cavity 1 being heated indirectly by a filament 4 thermally insulated within the envelope 5. A suitable anode voltage is applied between the connection 6 connected to the cavity 1 and an anode (not shown).

According to the present invention, the cavity 1 is connected by means of a tubulure 7 with a reservoir or tank 8 filled with an ionizable substance, such as cesium. For instance, a porous body 9 may be arranged and immersed in the cesium reservoir liquid 10. The reservoir or tank 8 is heated by means of a small heating element 11, thermally insulated within the envelope 12, so that the cesium may evaporate.

If it is assumed that the emissive material 2 has, in this example, a work function larger than the ionization function of cesium, for example, that it is constituted by tungsten, rhenium, tantalum, niobium, or the like, it is additionally necessary to provide for means in order that the cesium atoms fall or impinge on the emissive surface rather than pass through the orifice 3. For this purpose, for instance, a shield or screen 13 may be arranged in the cavity 1, as shown in FIGURE 1, this shield consisting of any suitable metal and being interposed between the tubulure 7 and the orifice 3. It is also possible to simply provide the outlet orifice at right angles to the direction of the cesium injection.

Operation The above-described hollow cathode structure operates as follows:

The emissive layer 2 which is indirectly heated by the filament 4 emits an electron cloud filling the cavity 1. At the same time, the cesium vapor obtained by heating of reservoir or tank 8 escape-s through the pores of the body 9 and :penetrates through the tubulure 7 into the cavity 1. The cesium atoms are prevented by the shield 13 from being oriented directly toward the orifice 3, and are, instead, reflected by said shield 13 and directed toward the surface of the layer 2. It is known that, when a gas or vapor atom falls on a sufficiently hot surface and when the ionization potential of such atom is lower than the working potential of the material of such surface, the 'atom is ionized and the electrons resulting from the ionization are absorbed by the surface and flow through the external circuits, whereas the positive ions escape from the surface. This phenomenon is known under the name of surface or contact ionization. The cesium ions obtained in this manner mix with the electrons emitted by the surface 2 and the heating of the cesium. reservoir and/ or of the cavity is regulated in such a manner that the electronic space charge is just about compensated by the quantity of ions produced by surface ionization. A plasma is thus obtained within the cavity which is submitted to the action of the electric field created by-the anode voltage applied between the anode (not shown) and the connection 6 (certain force lines of this field penetrating into the cavity 1 through orifice 3). The action of this field is such that the effusion current of the elec trons contained in the plasma is attracted by the anode and leaves through orifice 3 'while the ions are repelled by the same field and remain in the interior of the cavity.

The hollow plasma cathode according to the present invention offers the advantage as compared to the known hollow electronic gas cathodes of no longer presenting a potential well at the interior thereof but instead operates with an essentially homogeneous distribution of particles which permits the establishment of an effusion current entraining the electrons from the entirety of the cavity space. As a result thereof, it is possible to obtain a great current density at the output which may be of the order of 100 A./cm. in C.W. (continuou wave) operation.

In the embodiment shown in FIGURE 2 in which the same reference numerals have been used as in FIGURE 1 to designate like parts, it has been assumed that the emissive layer 2 is of a substance the work function of which is lower than the ionization function of the ionizable matter used, for example, of an alkaline-earth oxide, if the ionizable material is cesium. In this case, surface ionization by the impact of atoms on the surface of the cavity is no. longer possible, and it is necessary, therefore, to provide a special conductor made of a material having a high work function, the surface of which will be utilized for the impact of atoms with a view toward ionization thereof.

In this spirit, the difiference between FIGURES 1 and 2 essentially consists in that a shield or screen 13 no longer is present in FIGURE 2, but that an ionizer is interposed instead in the path of the cesium atoms, for example, in the form of a tungsten filament 14 energized through the insulating passages 15 by a heating current.

The cesium vapor is then ionized upon the impact on the ionizer 14. Otherwise, the operation of this cathode is the same as that shown in FIGURE 1.

FIGURES 3 and 4 illustrate modifications of possible geometric forms. In FIGURE 3, the spherical cavity has been replaced by a cylinder 16 lined at the inside thereof with emissive material 2, as hereinabove. The ionizer 14 is spirally wound. Again in this figure, the same reference numerals as in the previous figures are used to designate like elements. The heating systems have been omitted in FIGURE 3 in order to simplify the showing. The operation, however, is the same as before.

In FIGURE 4, the cavity has the shape of a parallelepiped 17 and the ionizer is a plate 18. Several systems have also been shown connected in such a manner as to obtain a laminar structure, susceptible of supplying not only a high density but also a strong total current intensity. The other deatils regarding structure and operation are the same as in the previous figures.

The present invention is not limited to the details shown and described hereinabove, but is susceptible of all modifications and variations within the scope of a person skilled in the art. For example, the heating of the cavity may be achieved also by electronic bombardment or by any other known equivalent means. It may possibly be achieved also simply by thermal radiation from the ionizer. The choice of materials both for the emissive substance and for the ionizable material is left to the discretion of the designer according to the respective requirements. The ionization means indicated are not limitative but may be substituted by any equivalent means. Additionally, the geometric configurations shown may be modified in different ways so as to adapt the same to each particular case. It is understood that all of these changes and modifications fall within the broad concept of the present invention and are to be considered as encompassed thereby.

Thus, while I 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 many changes and modifications within the spirit and scope of the present invention, and I, 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.

I claim:

1. In a high vacuum electron tube, a hollow cathode structure, comprising wall mean-s forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surfaces of said wall means, heating means positioned adjacent the outer surface of said wall means and operatively associated with said electron-emissive layer to emit electrons therefrom operable to escape through said aperture, and

, charge-compensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity by establishing therewith a plasma including reservoir means in the form of an enclosure providing internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material, and duct means operatively connecting said reservoir means with said cavity.

2. In a high vacuum electron tube, a hollow cathode structure, comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surfaces of said wall means, heating means positioned adjacent the outer surface of said wall means and operatively associated with said electron-emissive layer to emit electrons therefrom operable to escape through said aperture, and chargecompensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity by establishing therewithin a plasma including reservoir means in the \form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material of the type providing ions by contact with a heated surface, and duct means operatively connecting said reservoir means with said cavity.

3. In a high vacuum electron tube, a hollow cathode structure, comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surfaces of said wall means, first heating means positioned adjacent the outer surface of said wall means and operatively associated with said electron-emissive layer to emit electrons there-from operable to escalpe through said aperture,.and charge-compensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity to thereby form a plasma therein including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material essentially consisting of cesium, and duct means operatively connecting said reservoir means with said cavity.

4. In a high vacuum electron tube, a hollow cathode structure, comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer of a material selected from the group consisting of tungsten, tantalum, rhenium and columbium and covering substantially all of the internal surfaces of said wall means, first heating means operatively associated with said electron-emissive layer to emit electrons therefrom operable to escape through said aperture, and charge-compensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity to thereby form a plasma therein including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material essentially consisting of cesium, and duct means operatively connecting said reservoir means with said cavity.

5. In a high vacuum electron tube, a hollow cathode structure, comprising wal-l means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer of a material selected from the group consisting of tungsten, tantalum, rhenium and columbiu'm and covering substantially all of the internal surfaces of said wall means, first heating means operatively associated with said electron-emissive layer to emit electrons therefrom operable to escape through said aperture, and charge-compensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity to thereby form a plasma therein including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material essentially consisting of cesium, duct means operatively connecting said reservoir means with said cavity, and further means for producing vaporization in said ionizable material.

6. In a high vacuum electron tube, a hollow cathode structure, comprising Wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surfaces of said wall means positioned adjacent the outer surface of said wall means and heating means operatively associated with said electron-emissive layer to emit electrons therefrom operable to escape through said aperture, and charge-compensating means operatively associated with said cavity for compensating for the electron spacecharge within said cavity by establishing therewithin a plasma including reservoir means in the form of an en closure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material of the type providing ions by contact with a heated surface, duct means operatively connecting said reservoir means with said cavity, and further means effectively vaporizing said ionizable material.

7. In a high vacuum electron tube, a hollow cathode structure having wall means forming at least one cavity provided with an aperture in the walls of the cavity, the aperture being of dimensions substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering at least in part the internal surface of said wall means, and means for effectively producing emission of electrons from said electron-emissive layer to escape through said aperture, the improvement essentially consisting of charge-compensating means operatively associated with said cathode structure for compensating for the electron space charge existing in the cavity by ef- 'rfectively establishing therewithin a plasma while maintaining a high vacuum outside of said cavity.

'8. In a high vacuum electron tube, a hollow cathode structure comprising wallmeans forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electronemissive layer covering substantially all of the internal surfaces of said wall means, heating means operatively associated with said electron-emissive layer to emit electrons therefrom operable to escape through said aperture, and chargecompensating means operativelyassociated with said cavity for compensating for the electron space-charge Within said cavity by establishing therewithin a plasma including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material adapted to be vaporized, connecting means operatively connecting said reservoir means with said cavity, the work function of said electron emissive layer bein g higher than the work function of said ioniz-able material, and vapor deflector :means within said cavity between said connecting means and said aperture for effectively directing the flow of vapor of said ionizable material, admitted through said connecting means, against the hot surface of said wall means to thereby produce ions of said ionizable material by contact ionization and therewith compensate the space-charge of the electrons by the charge of the ions, whereby the mixture of said ions and electrons form a plasma.

9. In a high vacuum electron tube, a hollow cathode structure comprising Wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surfaces of said wall means, heating means operatively associated with said electron-emissive layer to emit electrons therefrom adapted to escape through said aperture, and chargecompensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity by establishing therewithin a plasma including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material, connecting means operatively connecting said reservoir means with said cavity, the work function of said electron-emissive layer being lower than the work function of said ionizable material, ionizing means within said cavity between said connecting means and said aperture having a material with relatively high work function, and further means for effectively heating said ionizing means.

10. In 'a high vacuum electron tube, a hollow cathode structure comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer of an earth-oxide material and covering substantially all of the internal surfaces of said wall means, heating means operatively associated with said electron-emissive layer to emit electrons therefrom adapted to escape through said aperture, and charge-compensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity by establishing therewith a plasma including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material essentially consisting of cesium, connecting means operatively connecting said reservoir means with said cavity, the work function of said electron-emissive layer being lower than the work function of said ionizable material, ionizing means within said cavity between said connecting means and said aperture having a material with a relatively high work function, and further means for effectively heating said ionizing means.

11. In a high vacuum electron tube, a hollow cathode structure comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron eX- tracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surfaces of said wall means, heating means operatively associated with said electron-emissive layer to emit electrons therefrom adapted to escape through said aperture, and charge compensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity by establishing therewithin a plasma including reservoir means in the form of an enclosure providing an intern-a1 space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material, connecting means operatively connecting said reservoir means with said cavity, the work function of said electron-emissive layer being lower than the work function of said ionizable material, ionizing means within said cavity between said connecting means and said aperature having a [filamentary material with a relatively high" work function, and further means for effectively heating said ionizing means by passing an electric current through said filamentary material.

12. -In a high vacuum electron tube, a hollow cathode structure comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron eX- tracting anode facing said aperture, an ele'ctron-emissive layer covering substantially all of the internal surfaces of said wall means, heating means operatively associated with said electron-emissive layer to emit electrons therefrom adapted to escape through said aperture, and charge-compensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity by establishing therewithin a plasma including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material, connecting means operatively connecting said reservoir means with said cavity, the work function of said electron-em-issive layer being lower than the work function of said ionizable material, ionizing means within said cavity between said connecting means and said aperture having a filamentary material made of'tun-gsten, and further means for eifectively heating said ionizing means by passing an electric current through said filamentary material.

'13. In a' high vacuum electron tube, a hollow cathode structure comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surfaces of said Wall means, heating means operatively associated with said electron-emissive layer to emit electrons therefrom adapted to escape through said aperture, and chargeco-mpensating means operatively associated with said cavity for compensating for the electron space-charge within said cavity by establishing therewithin a plasma including reservoir means in the form of an enclosure providing an internal space essentially isolated from the space under high Vacuum within said tube outside of said hollow cathode, said internal space containing an ionizable material, connecting means operatively connecting said reservoir means with said cavity, the work function of said electron-emissive layer being lower than the work function of said ionizable material, plate-shaped ionizing means within said cavity between said connecting means and said aperture having a material with a relatively high work function, and further means for effectively heating said ionizing means.

14. In a high vacuum electron tube, a hollow cathode structure having wall means forming at least one cavity provided with an aperture in the walls of the cavity, the aperture being of dimensions substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive layer covering substantially all of the internal surface of said wall means, and means for effectively producing emission of said electrons from said electron-emissive layer and adapted to escape through said aperture, the improvement essentially consisting of charge-compensating means operatively associated with said cathode structure for compensating the electron space-charge in the cavity by establishing therewithin a plasma while maintaining a high vacuum outside of said cavity including ionizing means producing ions and provided with an ionizable material, the electron-emissive layer material and the ionizabrle material having different work functions.

15. In a high vacuum electron tube, a hollow cathode structure, comprising wall means forming at least one cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller than the transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive material extending over at least a part of the internal surfaces of said wall means, means operatively associated with said electron-emissive material and operable to produce emission of electrons therefrom adapted to escape through said aperture, and charge-compensating means operatively associated with said cavity for compensating for the electron space charge within said cavity by effectively establishing therewithin a plasma While maintaining a high vacuum outside of said cavity.

16. In a high vacuum electron tube, a hollow cathode structure, comprising wall means forming at least one spherical cavity, an aperture being provided in the wall means of said cavity and having a size substantially smaller thanthe transverse dimension of said cavity, an electron extracting anode facing said aperture, an electron-emissive material extending over substantially all of the internal surfaces of said wall means, means operatively associated with said electron-emissive material and operable to produce emission of electrons therefrom adapted to escape through said aperture, and chargecompensating mean-s operatively associated with said cavity for compensating for the electron space-charge within said cavity by effectively establishing therewithin a plasma while maintaining a high vacuum outside of said cavity.

17. In a high vacuum electron tube, a hollow cathode structure, comprising wall means forming a plurality of cavities adjacent one another, an aperture being provided in the wall means of at least some out said cavities and each having a size substantially smaller than the transverse dimension of a respective cavity, an electron extracting anode facing said aperture, an electron-emissive material extending over substantially all of the internal surfaces of said wall means, means operatively associated with said electron-emissive material and operable to produce emission of electrons therefrom adapted to escape through said aperture, and charge-compensating means operatively associated with at least some of said cavities for compensating the electron space-change within the respective cavity by establishing therewithin a plasma 15 while maintaining a high vacuum outside of said cavity.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Nuclear Physics, Green, page 85 relied on, Mc- Graw Hill (1955).

GEORGE N. WESTBY, Primary Examiner.

DAVID J. GALVIN, Examiner.

Claims

1. IN A HIGH VACUUM ELECTRON TUBE, A HOLLOW CATHODE STRUCTURE, COMPRISING WALL MEANS FORMING AT LEAST ONE CAVITY, AN APERTURE BEING PROVIDED IN THE WALL MEANS OF SAID CAVITY AND HAVING A SIZE SUBSTANTIALLY SMALLER THAN THE TRANSVERSE DIMENSION OF SAID CAVITY, AN ELECTRON EXTRACTING ANODE FACING SAID APERTURE, AN ELECTRON-EMISSIVE LAYER COVERING SUBSTANTIALLY ALL OF THE INTERNAL SURFACES OF SAID WALL MEANS, HEATING MEANS POSITIONED ADJACENT THE OUTER SURFACE OF SAID WALL MEANS AND OPERATIVELY ASSOCIATED WITH SAID ELECTRON-EMISSIVE LAYER TO EMIT ELECTRONS THEREFROM OPERABLE TO ESCAPE THROUGH SAID APERTURE, AND CHARGE-COMPENSATING MEANS OPERATIVELY ASSOCIATED WITH SAID CAVITY FOR COMPENSATING FOR THE ELECTRON SPACE-CHARGE WITHIN SAID CAVITY BY ESTABLISHING THEREWITH A PLASMA INCLUDING RESERVOIR MEANS IN THE FORM OF AN ENCLOSURE PROVIDING INTERNAL SPACE ESSENTIALLY ISOLATED FROM THE SPACE UNDER HIGH VACUUM WITHIN SAID TUBE OUTSIDE OF SAID HOLLOW CATHODE, SAID INTERNAL SPACE CONTAINING AN IONIZABLE MATERIAL, AND DUCT MEANS OPERATIVELY CONNECTING SAID RESERVOIR MEANS WITH SAID CAVITY.