US 3749967 A
An electron beam discharge device for transmitting from the evacuated interior of the device a broad area electron beam through an electron window to a region of higher pressure outside the device. Electron emission means are disposed in an enclosure effective to shield the electron emission means from external electrical influences. Grid means at a high negative potential is disposed between the electron emission means and the electron window to effect the transmission through the electron window of electrons received from the electron emission means through the grid means. The electron window may be carried by an evacuated enclosure which may or may not be at ground potential. Space-charge limited emission together with alignment of parts insure maximum uniformity of the beam.
Description (OCR text may contain errors)
1 11% States Patent 1 91 1111 3,749,967 Douglas-Hamilton et al. 451 J l 31, 1973  ELECTRON BEAM DISCHARGE DEVICE Primary Examiner-Roy Lake  Inventors: Diarmaid II. Douglas-Hamilton, Assistant ErY iF"T -1? 'n9 "111D? Boston; Evan Pugh, Lexington, A ttorney-Charles M. Hogan and Melvm E. both of Mass. derick  Assignee: Avco Corporation, Cincinnati, Ohio  ABSTRACT  Filed: Dec. 23 1971 An electron beam discharge device for transmitting from the evacuated interior of the device a broad area PP 211,375 electron beam through an electron window to a region of higher pressure outside the device. Electron emis- 52 U.S. c1. 315/85 315/157 means disWsed effective  Int. Cl. I101] 1/52 shield the electron emission means from external 58 Field of Search 250/49- 313/63 74- influe'lces' Grid means a high negative Pmen' 315/85 328/255 1, tial is disposed between the electron emission means and the electron window to effect the transmission  References Cited through the electron window of electrons received from the electron emission means through the grid 7 UNITED TE PATENTS means. The electron window may be carried by an 13; B e p 313/74 evacuated enclosure which may or may not be at rews r ground potential. Space-charge limited emission to- 3,0l4,132 12/1961 Goldie 313/63 X gether with alignment of parts insure maximum unit-OF mity of the beam.
18 Claims, 8 Drawing Figures 34 I I I r M5311 PUMP 42 A a? I 33 1 vi: P i? 1 ELEcmou BEAM 11 "E"\S\(\ as i 57 21.2251 22 2. cmcurr men v0 TAGE L 1 SUPPLY PATENIED JUL 3 1 I975 SNEH20F4 MH HNHN DIARM AID DOUGLAS- HAMILTON EVAN R. PUGH INVENTOR.
ATTORNEYS- PATENTED M3 1 73 SHEEI t 0F 4 I I I I I I I I I I I I f I I! mm w DIARMAID DOUGLAS-HAMILTON EVAN R. PUGH INVENTOR.
4 m ZOFrOmJm ATTORNEYS ELECTRON BEAM DISCHARGE DEVICE This invention relates to electron discharge devices.
and, in particular, to electron discharge devices for irradiating exterior of the device a volume having a substantial cross-section with an electron beam uniform over the cross-section irradiated.
The use of ionizing energy in the form of high-energy electrons finds application in a variety of apparatus and processes including those of radiation chemistry, sterilization, preservation, supporting an electrical discharge in a gas, etc. The development of radiation curable coating compositions such as paints and varnishes has made possible advances in the coating field which, aside from the qualitative benefits, provides the advantages of greatly reduced curing times and substantial reductions in space requirements for curing equipment. The degree to which electron-initiated polymerization replaces conventional baking and other curing methods is, however, dependent upon the availability of electron-emission equipment capable of providing efficient utilization of the power required to provide the polymerizatiomeffecting electrons and effective distribution of the resultant energy in a manner such as to provide a production rate compatible with the intended operation.
The use of ionizing energy in the form of high-energy electrons may find application in the field of magnetohydrodynamics to provide electrically conductive ionized gases; it is used in lasers to provide an appropriate medium for lasing action. The present invention is particularly useful in the production of spatially uniform discharges in gas lasers at pressure levels and sizes such that electron-ion pair diffusion to the confining walls is negligible, which is to say where the discharge is not wall dominated. For a further discussion, reference is made to Patent Applications, Ser. No. 50,933 filed June 29, 1970 and Ser. No. 72,982 filed Sept. 17, 1970, now U.S. Pat. No. 3,702,973 and assigned to the same Assignee as this patent application.
As taught in the prior art, a high-energy electron source may be provided by accelerating electrons to high energy in an evacuated tube, and permitting the high energy electrons to issue from the tube through an appropriate electron window. The high-energy electrons may be caused to issue from the tube in the form of a sheet. In one such device, electrons are accelerated as a small beam within an evacuated tube and then a rapid scanning movement is imparted to an electron beam before it passes to the electron window and issues from the tube. In another such prior art device, an electron beam is focused into a sheet from within the tube by a system of cylindrical electron optics. See Robinson, U.S. Pat. Nos. 2,620,751 and 2,680,814. Where precise focusing is not essential, the electronemitting cathode or cathodes are enclosed in a suitable housing which restricts and directs the electron sheet to the electron window. See Trump, U.S. Pat. No. 2,887,599. The Trump electron acceleration tube includes an extended line high-voltage cathode structure within which an electron-emitting filament or filaments are positioned, an evacuated grounded metallic structure which has an extended line window or windows to permit electrons to emerge into air, and one or more equipotential shields interposed between the cathode and the metallic structure and maintained at intermediate voltages.
In a further prior art device, a second window is interposed between the first window and a workpiece or the like causing the electron beam to pass for a distance sufficient to provide the desired degree of spreading through a gaseous medium disposed between the two windows, the average density of the gas being maintained substantially below that of air so as to provide a controlled resistance to uninterrupted flow of electrons that is substantially less than that of one atmosphere of air. See Colvin et a], U.S. Pat. Nos. 3,418,155 and 3,440,466.
In accordance with the present invention, a broad area uniform electron beam is provided without the necessity of a secondary enclosed zone. In a preferred embodiment, there is disposed in an evacuated metallic enclosure at ground potential a further metallic enclosure at a high negative potential and containing electron generating means, one end of the enclosure being covered by a metallic electron permeable grid. Opposite the grid is an electron window at ground potential adapted to permit the emergence out of the evacuated metallic enclosure of electrons received from the electron generating means via the aforementioned grid. Space-charge limit and parallelism of the electron generating means, grid and electron window are provided to insure maximum uniformity of the beam.
It is, accordingly, an object of the invention to provide apparatus for generating a uniform broad area electron beam.
Another object of the invention is to provide an improved electron beam discharge device.
A further object of the invention is to provide an improved electron beam discharge device for providing exterior of the device a uniform broad area beam and which is simple, compact, inexpensive to manufacture and dependable in operation.
The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in conjunction with the accompanying drawings, in which:
FIG. l is a schematic illustration in sectional side view of an electron beam discharge device constructed in accordance with the invention for providing a broad area uniform electron beam exterior of the device through an electron window;
FIG. 2 is a perspective illustration with parts broken away of a sheet of metal foil and support plate comprising the electron window shown in FIG. 1;
FIG. 3 is a perspective illustration with parts broken away of the grid disposed within the device of FIG. 1;
FIG. 4 is a perspective illustration with parts broken away of an alternate embodiment of the electron window;
FIG. 5 is a diagrammatic illustration showing details of the pulse circuit of FIG. 1;
FIG. 6 is a schematic illustration in sectional side view of another embodiment of the invention;
FIG. 7 is a perspective illustration of the grid of FIG. 6; and
FIG. is a perspective illustration of the electron window shown in FIG. 6.
Referring now to FIG. 1, there is shown an electron beam discharge device designated generally by the nu meral and comprising a cylindrical and preferably metallic main housing 11 having an aperture 12 sealably closed by electron window means more fully described hereinafter.
Disposed within the housing is a cylindrical and preferably metallic enclosure 13 having an aperture 14 concentric with the axis of the aperture 12 in the main housing 1 1. Positioned and supported within the enclosure 13 by electrically nonconductive stand-offs l5 and 16 and an electrically conductive circular plate 17 are at least one and preferably a plurality of filaments l8 uniformly spaced one from another, insulated from plate 17, and connected to a source of filament current disposed and arranged in the manner more'fully described below. The filaments 18 are heated in conventional manner by a normally low voltage source to produce thermionic emission. As shown in FIG. 1, the enclosure 13 is supported by an axially disposed tubular extension 19 sealably passing through the rear wall 21 of the main housing and electrically insulated therefrom by insulating material 22 such as Teflon arranged and adapted to withstand not only the provision of a high vacuum in the main housing 11, but also a high potential difference of, for example, 100 or more kilovolts between the main housing 11 and the tubular extension 19. The filaments 18 may be formed of tungsten, thoriated tungsten, or other suitable filament material and spring loaded (not shown) to compensate for expansion and contraction during operation of the device. The tubular extension 19 is suitably sealed as by wall 23 adapted to permit electrical connection to the filaments while permitting a vacuum to be maintained inside of the main housing 1 l. The end of the tubular extension 19 remote from enclosure 13 is electrically connected to and terminates at a further metallic enclosure 24 exterior of the housing 11. Within enclosure 24, the interior of which may be at atmospheric pressure, is disposed the filament power supply 31 and such additional control circuitry or the like such as, for example, an optically actuated pulse circuit 32 where it is desired to operate the device in the pulse mode. Conductors 33, 34, and 35 appropriately couple the filament power supply 31 to the filaments 18 and plate 17.
The interior of the main housing 11 and enclosure 13 is evacuated via pipe 34 by a vacuum pump (not shown) in conventional manner and maintained at a low pressure to prevent electrical breakdown between enclosure 13 and housing 11. Disposed within and covering aperture 14 of enclosure 13 is a metallic screen 35 carried by support means 36 comprising grid means more fully shown in FIG. 3 permeable to electrons generated by the filaments 18.
The screen 35 is in electrical connection with the enclosure 13.
Disposed within and sealably covering aperture 12 in main housing 11 is a thin sheet or foil 37 supported on a reticulated metallic plate 38 more fully shown in FIG. 2 in electrical connection with the main housing 11 and comprising electron window means. As shown in FIG. 3, plate 38 may be provided with a plurality of holes 39 defining the area that the electron beam is intended to encompass. An alternate embodiment wherein slots 41 rather than holes are provided in a plate 38a is shown in FIG. 4. The sheet or foil 37 may, for example, be of aluminum, beryllium, titanium, an alloy or a thin sheet of plastic such as Kapton or Mylar. The foil 37 is positioned so as to completely cover aperture 12 and extend on each side thereof a sufiicient distance to be removably secured against the main housing 11 by a suitable window retaining ring 42. Whereas the foil 37 need not necessarily be (but usually is) in electrical communication with the main housing, plate 38 must be so that it may be maintained at a potential high compared to that of screen 35. The window retaining ring 42 and/or plate 38 may be removably and sealably affixed to the main housing 1 1 by suitable sealing and fastener means; e.g. O-rings, bolts, screws, clamps, or the like.
Returning now to FIG. 1, it will be seen that enclosures 13 and 24, extension 19 and grid 35 are electrically connected to the negative terminal of a conventional high voltage supply 43 adapted to provide a negative potential of, for example, about kilovolts. The positive terminal of the high voltage supply 43 as well as the main housing 11 are grounded to provide a large potential difference of, for example, lOO kilovolts between grid 35 and plate 38.
An important aspect of the invention is the provision of enclosure 13 containing the electron emission means interior of housing 11 and topologically continuous with enclosure 24 containing the filament power supply and associated circuitry exterior of the housing at atmospheric pressure, wherein said enclosures are maintained at a high negative potential with respect to the electron window means to define a Faraday cage effective to shield the filament power supply and the like, and the electron emission means from external fields and influences. Such an arrangement permits the fabrication of electron discharge devices for providing uniform broad area electron beams and having a minimum number of components, an electron discharge device that is dependable in operation wherein as compared to prior art devices the possibility of arcing is greatly reduced, and which permits the electron beam to be dependably and simply controlled with relatively low voltages.
Apparatus in accordance with the invention as shown in FIG. 1 may be used to produce a continuous or pulsed broad area electron beam. In operation the filaments may be negative or positive relative to enclosure 13, the electron emission means may be temperature limited or may be space-charge limited.
In the case where the filaments are maintained positive or at zero bias with respect to enclosure 13, filament emission will be space-charge limited and the interior of enclosure 13 will contain an electron cloud. The field between enclosure 13 and the foil 37 accelerates electrons escaping through the grid 35 to the required energy for passage through the foil. The transparency of the grid 35 to electrons increases as the aforementioned field is increased because as the field is increased, a larger fraction of the electron cloud within enclosure 13 will be pulled through grid 35 by the external field penetrating there through.
Above a critical positive filament voltage no electrons will escape from enclosure 13 since the electrons will lack the energy required to reach the region adjacent grid 35 where the external field can affect their motion and accelerate them toward the foil. In this case, the dimension of the electron cloud around each filament is therefore limited to thermal distances of about 5 to 10 volts. The electron beam in this case will be uniform provided the voltage drop between filaments 18 and enclosure 13 is less than the aforementioned critical positive filament voltage because for these conditions the electron cloud within enclosure 13 will be smoothed out by its own space-charge repul sion.
Where the filaments are biased negative with respect to enclosure 13, at relatively low filament temperatures, temperature-limited emission will be present, the electron emission being low enough so that spacecharge effects may be ignored. Under these conditions of temperature-limited emission the electrons will drift from the filament to the enclosure 13 and only a fraction of the electrons arriving at the grid 35 will be drawn through.
Under these conditions non-uniformities in the filaments causing non-uniformities in the electron emission will be undesirably reflected in non-uniformities in the resultant electron beam emerging from the device.
In the case where the filament temperature is high enough that the emission rate of electrons exceeds the rate at which they are drawn off the filament by the enclosure 1l3 field, a space-charge limitation on the electron current obtainable will result. Since substantially all of each filament (neglecting end effects) is at a high temperature, any filament non-uniformities are smoothed out by the cloud of electrons surrounding each filament, thereby permitting each filament to be regarded as a uniformly emitting cylinder. After leaving the space-charge electron cloud surrounding each filament, electrons drift toward the enclosure 13 and grid 35. Provision of plate 17 at the appropriate potential insures that most of the electrons leaving the spacecharge electron cloud will arrive at the grid 35. The number of electrons passing through grid 35 then depends on the ratio of the field inside of and external to enclosure 13 up to a saturation value at which every incident electron will be transmitted. At high values of this ratio, there may be a net electron emission from the grid due to secondary electron production at the grid, an effect analagous to that observed in high-power transmission triodes. The electron emission from the grid is about 5 percent of that from the filament under the above circumstances. Uniformity of the electron beam exterior to the device is insured when the separation of the filaments from each other is much less than the scale size of the enclosure in which they are disposed, thus superposing and averaging out beam inhomogeneities due to electron drift. The mesh size of the grid (such as grid 35 for example) must be sufficiently small that the effect of the field external to enclosure 13 remains near the grid structure and does not distort the field in the region between the filaments 18 and the grid 35. Further, to insure a uniform beam care must be taken to insure that the filaments and the electron window means are each parallel to the grid and, hence, parallel to each other. Any departure from the parallel between the filaments and the grid will produce a fractional error in electron beam intensity of about twice that magnitude.
An ultimate limit on the electron beam current exists in the space-charge limit between the enclosure and the foil 37. Childs Law gives a limit of the order of amperes per square centimeter at enclosure 13 for obtainable configurations.
Attention is now directed to FIG. 5 which shows details of a pulse circuit for operation in the pulse mode. Because of the high voltages involved, special precautions must be taken to avoid electrical breakdown. The
interior of housing 11 must be maintained at a low pressure while the filament power supply and associated actuating and control circuitry and the like are designed to and operate more satisfactorily at atmospheric pressure. Further, such circuits and components are readily accessible when so operated, and their disposition in the evacuated main housing would tend to adversely affect the maintenance of a proper vacuum therein. Accordingly, such components and circuits are disposed in enclosure 2d at atmospheric pressure and electrically connected to the filaments in enclosure 13 via extension 19 and separating wall 23.
The problem of introducing information into the filament control circuitry is solved by utilizing optical techniques to effect control. Thus, for operation in the pulsed mode, pulse circuit means 32 (see FIG. 1) may be actuated separately via electrically insulating light pipes 51 and 52 providing optical communication between suitable light sources 53 and 54 exterior of enclosure 24 and photo cell means 55 and 56 disposed within enclosure 24. Suitable light sources such as, for
example, xenon flash tubes, optical diodes or the like may be sequentially actuated by pulse delay circuit 57 and trigger circuit 58. Upon actuation of the trigger circuit 58 by suitable means (not shown) pulse delay circuit 57 provides a pulse to light source 53 to turn the beam on" and after a desired delay, provide a pulse to light source 5d to turn the beam off." This may be conveniently and simply accomplished by, for example, directing light from light source 53 via a conventional light pipe 51 to photo cell means 55 operative to actuate switching means 59 and apply a suitable negative bias (derived from battery 61) to the filaments. Upon the application of the negative potential or bias to the filaments, as shown, the filaments are biased negative with respect to enclosure 13 (which is at the same negative potential as enclosure 24) and electrons can then escape from enclosure 13 through grid 35, thereby turning the beam on. After the desired delay determined by pulse delay circuit 57, light source 54 is actuated and in turn actuates photo cell 56 via light pipe 52. Switching means 62 controlled by photo cell 56 is ef fective to short circuit capacitor 63 connected across both battery fill and its current limiting resistor, bias the filaments positive with respect to enclosure 13 and thereby turn the beam off.
It is to be understood that the preceding discussion is given only by way of illustration and that the invention is not limited to the specific arrangement shown and described, since other arrangements obvious to those skilled in the art may be optically actuated to achieve the same result.
Referring now to FIGS. 6, 7, and 8, and particularly to FIG. 6, there is shown an alternate embodiment of the invention similar to that shown in FIG. 1, comprising filaments 78 disposed and supported within metallic enclosure I3, all of which are disposed within an evacuated main housing 71 as and for the purposes set forth in connection with FIG. ll. Enclosure 73 is in electrical contact with metallic tubular extension 79, which terminates at and is in electrical connection with metallic enclosure 74 external of the metallic main housing 71.
High voltage insulation is provided between enclosure 73 (which is maintained at a high negative potential) and main housing 71 (which is maintained at a potential positive with respect to enclosure 73-the main housing is normally grounded) by a high voltage bushing disposed on and covering extension 79. As shown in FIG. 6, a suitable high voltage bushing may comprise an electrically non-conductive sleeve 80 covering and extending the length of metallic extension 79, together with a plurality of glass rings 81 separated by metal washers 82, disposed between adjacent glass rings, adjacent washers 82 being electrically connected through resistors 89 to define a voltage divider. Such a high voltage bushing is effective to divide the total voltage of, for example, 100 kilovolts or more, whereby only a relatively small voltage insufficient to cause arcing is present across each glass ring. The high voltage bushing may be removably attached to housing 71, as by screws or the like, to permit repair or replacement and is also sealably attached to prevent loss of the vacuum in main housing 71. A large metal ring 82 is provided adjacent to enclosure 74 to shape the field at this point.
Extending outwardly from main housing 71 and encompassing rectangular aperture 86 is a spacer member 87 at the end of which remote from aperture 86 is sealably mounted an electron window comprising a foil 84 disposed over and covering slotted support plate 85, best shown in FIG. 8. Aperture 86 of housing 71 and aperture 88 of enclosure 73 are each covered by, respectively, grids 91 and 92. Grids 91 and 92, as best shown in FIG. 7, may be identical and formed of small rods 93 uniformly spaced one from another and carried in a frame 94. Grid 91 is in electrical contact with housing 71 and grid 92 is in electrical contact with enclosure 73.
The provision of grid 92 at a high negative potential, the electron window disposed at the extreme end of the spacer member 87 remote from grid 92, and grid 91 at ground potential disposed between the electron window and grid 92, prevents damage to the electron window and especially foil 84, resulting from arcing.
In the event an arc forms within housing 71, it cannot reach and puncture foil 84. Thus, grid 91 performs the dual function of accelerating electrons from enclosure 73, since it is at a positive potential with respect to enclosure 73, and it also protects the electron window from failure due to arcing.
An electron beam discharge device in accordance with the invention, while applicable to many other situations, is particularly useful for laser applications. When a laser application, for example, requires very high power, it is extremely advantageous to use relatively high gas pressure (such as, for example, up to one atmosphere or more) and large transverse dimensions (up to 30 centimeters or more). The use of high pressures and large dimensions avoids the low power density of a low pressure gas laser, which requires extremely long optical paths to produce high power outputs. At such high pressures and dimensions, the discharge is unstable and rapidly transforms into an arc unless an external source of ionization such as an electron beam is used to provide ionization and permit the discharge voltage to be at a magnitude low enough so that discharge instabilities are avoided. An electron beam ionizer in accordance with the present invention need be provided, for example, with only a voltage of the order of 150 kv to achieve useful ionization for such distances and pressures. Further, the provision of electron beam ionization in accordance with this invention permits continuous ionization through such large volumes thereby eliminating the necessity for repetitive pulse ionization in, for example, a laser application. In
addition to the preceding, electron beam ionization can be simply, conveniently and dependably controlled by varying the electron energy or the electron current. The electron current may be varied by changing the filament temperature or the grid to filament voltage in the space-charge limited region. Thus, ionization level and, for example, laser output may be simply and dependably controlled by controlling voltages in low powered, low voltage circuits. This feature of control, along with the ability of the broad area electron beam provided in accordance with the invention to ionize in a truly continuous fashion, makes apparatus in accordance with the invention highly attractive for ionizing a working medium in any application where it is desired or convenient to separate ionization from maintenance of a discharge.
For many laser applications, it has been found that the intensity of the electron beam must be substantially uniform (with variations not exceeding about a few percent) in order to produce a working medium with a substantially uniform ionization necessary to provide uniform gain and optical properties in the lasing medium. The present invention is especially suited for this type of use since it simply, economically and dependably provides a broad area electron beam having the required energy and uniformity to provide uniform gain and optical properties throughout the working region.
The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims:
I. In an electron discharge device for delivering to a region outside said device an electron beam of substantial cross-section, the combination comprising:
a. means defining a Faraday cage including grid means closing one end of said cage adapted to be maintained at a high negative potential, said grid means being adapted to permit the emergence of electrons from said cage;
b. electron emission means disposed within said cage for generating electrons within said cage;
0. housing means having an aperture, said housing means being disposed around said cage and adapted to maintain an evacuated environment in said cage and between said cage and said aperture; and
d. electron window means sealably closing said aperture adapted to permit the emergence out of said housing means of electrons received from said emission means through said grid means said window means being adapted to be maintained at a positive potential with respect to said negative potential.
2. In an electron discharge device for delivering to a region outside said device an electron beam of substantial cross-section and having a longitudinal axis, the combination comprising:
a. enclosure means including electrically conductive grid means closing one end of said enclosure means adapted to be maintained at a high negative potential, said grid means being adapted to permit the emergence of electrons from said enclosure means and disposed substantially symmetrically about the longitudinal axis of said beam;
b. electron emission means disposed within said enclosure means for generating electrons within said enclosure means;
c. housing means having an aperture disposed substantially symmetrically about the longitudinal axis of said beam, said housing means being disposed around said enclosure means and adapted to maintain an evacuated environment in said enclosure means and between said enclosure means and said aperture; and
d. electron window means sealably closing said aperture adapted to permit the emergence out of said enclosure means of electrons received from said emission means through said grid means, said window means being adapted to be maintained at a positive potential with respect to said negative potential.
3. In an electron discharge device for delivering to a region outside said device an electron beam of substantial cross-section, the combination comprising:
a. enclosure means open at one end and adapted to be maintained at a high negative potential, said enclosure means defining a first volume shielded from external electrical influences;
b. electrically conductive grid means permeable to electrons covering said open end and adapted to be maintained at a high negative potential;
0. electron emission means disposed within said enclosure means for generating electrons within said enclosure means;
d. an evacuated metallic housing having an aperture, said housing being spaced from and surrounding said enclosure means and being adapted to be maintained at a positive potential with respect to said high negative potential; and
e. electron window means sealably closing said aperture adapted to permit the emergence out of said housing of electrons received from said emission means through said grid means.
4. The combination as defined in claim 3 wherein said enclosure means is provided with a second aperture and additionally including:
a. hollow extension means coupled to said enclosure means at said second aperture and electrically isolated from and sealably extending through said housing, said extension means defining a second volume shielded from external electrical influences;
b. means sealably disposed in said extension means for coupling said electron emission means to a source of current; and
c. further enclosure means electrically coupled to said extension means for defining a third volume shielded from external electrical influences.
5. The combination as defined in claim 4 wherein said grid means is substantially flat and said electron window means is substantially uniformly spaced from said grid means.
6. The combination as defined in claim 5 wherein substantially all points at which electrons are generated by said emission means are substantially equally spaced from said grid means.
7. The combination as defined in claim 6 and additionally including energizing means disposed in said third volume for actuating said electron emission means to generate electrons.
8. The combination as defined in claim 7 wherein said means disposed in said further volume actuates said electron emission means to generate electrons at a rate sufficient to produce a negative space charge adjacent said emission means.
9. The combination as defined in claim 6 wherein said grid means, and said window means each define a substantial cross-sectional area.
10. The combination as defined in claim 7 and additionally including circuit means disposed in said further volume for controlling the potential of said emission means with respect to the potential of said enclosure means.
11. The combination as defined in claim 10 wherein said further conductive means defines said third volume at atmospheric pressure.
12. In an electron discharge device for delivering to a region outside said device an electron beam of substantial cross-section and having a longitudinal axis, the combination comprising:
a. enclosure means including electrically conductive first grid means comprising one end of said enclosure means adapted to be maintained at a high negative potential, said first grid means being adapted to permit the emergence of a substantially uniform electron beam from said enclosure means and disposed substantially symmetrically about the longitudinal axis of said beam, said enclosure means defining a first volume shielded from external electrical influences;
b. electron emission means disposed within said enclosure means for generating electrons within said enclosure means;
c. an evacuated metallic housing having an aperture, said housing being spaced from and surrounding said enclosure means and adapted to be maintained at a positive potential with respect to said high negative potential, said aperture being disposed substantially symmetrically about the longitudinal axis of said beam;
(1. second electrically conductive grid means disposed in said aperture adapted to be maintained at said positive potential and substantially uniformly permeable to electrons;
e. a hollow spacer member open at opposite ends, carried by said housing and extending outwardly from said aperture substantially symmetrically about the longitudinal axis of said beam; and
f. electron window means sealably closing the end of said hollow member remote from said aperture adapted to permit the emergence out of said housing of electrons received from said emission means through said first and second grid means.
13. The combination as defined in claim 12 wherein said electron window means comprises a thin foil disposed on and covering a reticulated metal support plate, and said support plate is maintained at at least about the same potential as said second grid means.
14. The combination as defined in claim 13 wherein said enclosure means is provided with an aperture and additionally including:
a. electrically conductive hollow extension means coupled to said enclosure means at said aperture and electrically isolated from and sealably extending through said housing, said extension means de- 1 1 l2 fining a second volume shielded from external 17. The combination as defined in claim 7 and addielectrical influences; tionally including: b. means sealably disposed in said extension means a. photo cell means disposed in said third volume for for coupling said electron emission means to a actuating said energizing means; source of current; and b. light transmission means operatively coupled to c. further enclosure means electrically coupled to said photo cell means and extending exteriorly of said extension means for defining a third volume said third volume and said device; shielded from external electrical influences. c. light source means operatively coupled to said light 15. The combination as defined in claim 14 wherein transmission means exterior of said device; and said first and second grid means and said electron win- (1. means exterior of said device for actuating said dow means are substantially flat and coplanar one with light source means. another. 18. The combination as defined in claim 17 wherein 16. The combination as defined in claim wherein actuation of said light source means biases said elecsubstantially all points at which electrons are generated tron emission means first negative and then positive by said emission means are substantially equally spaced 15 with respect to said enclosure means.
from said first grid means.