US2602905A - Electron discharge device - Google Patents

Description

July 8, 1952 c. s. BULL ELECTRON DISCHARGE DEVICE Filed Sept. '7, 1946 INVENTOR AMM OV. B

Patented July 8, 1952 ELECTRON DISCHARGE DEVICE Cabot Seaton Bull, Hillingdon, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application September 7, 1946, Serial No. 695,531 In Great Britain October 24, 1944 Section 1, Public Law 690, August 8,1946

Patent expires October 24, 1964 4 Claims. 1

This invention relates to electron discharge devices and has particular but not exclusive reference to electron discharge devices of the type in which electrons from a thermionic cathode are caused to impinge on a secondary-emitting electrode, the emitted secondary electrons being then collected by an anode.

In electron discharge devices it is often desired to cause electrons emanating from a cathode to follow arcuate paths and to cause them to impinge on a further electrode. Forexample, in electron discharge'devices of the abovementioned type it is usually necessary to arrange the secondary-emitting electrode out of sight of the thermionic cathode inorder to prevent particles emitted by the cathode from poisoning the secondary-emitting electrode. With such an arrangement it is necessary to cause the primary electrons to follow arcuate paths in order that they can be caused to impinge on the secondaryemitting electrode. In previously proposed devices the electrons are caused to follow the desired paths by employing one or more electrodes maintained in operation at a suitable potential or potentials said electrode or electrodes causing the electrons to be reflected so that they impinge on the secondary-emitting electrode. Since electrons are emitted from the cathode of the device in widely divergent directions and the electrons are merely reflected to cause them to travel in the required paths, the primary electrons impinge on the secondary-emitting electrode over a large area and hence the secondary-emitting electrode must be likewise of large area. It is then necessary to provide an anode of substantial area in order to collect as much of the secondary emission as is possible. The use of an anode of large area is very undesirable, particularly when the device is intended to operate at very high frequencies owing to the consequent large anode-earth capacity.

The object of the present invention is to provide an improved electron discharge device whereby the electrode on which the electrons are caused to impinge or the anode of said device can be made smaller than heretofore whereby the inter-electrode capacities can be reduced. 7

In the previously proposed devices as mentioned above, the electrons are merely caused to be reflected to follow the desired paths and as far as I am aware no attempt has been made 2- v to follow curved paths and to converge as they approach and impinge on said further electrode. By the provision of such a device the electrons can be caused to impinge on only a small area and hence the size of said further electrode can be suitably reduced and where the invention is applied to a discharge device of the kind referred to in which the further electrode is the secondary-emitting electrode the anode which is then provided to collect the secondary electrons need only be of small area so that the anode-earth capacity of the device can be maintained at a low value. Where the invention is applied to devices in which no secondary-emitting electrode'is provided said further electrode may be the anode of the device in which case it can again be made of small size so that the anode-earth capacity of the device can be maintained at a low value.

The invention is particularly suitable for use in discharge devices in whichthe cathode is of rod-shaped form and electrons are caused to be emitted from susbtantially diametrically-opposite points or sides on said cathode and are caused to impinge on said further electrode at substantially diametrically-opposite points or sides thereon. In this case the focuss ing electrode may comprise a tubular sheet metal electrode of elongated cross section or a similar mesh or wire-wound structure or seriesof rods. While it is preferred that the focussing electrode should completely surround the other electrodes of the device it is, however, possible for the focussing electrode to be composed of a plurality of parts which do not completely surround the further electrodes since spaces may be left between adjacent edges of the plurality of parts suchspaces being disposed in positionswhich 'do not materially afiect the shape of the field necessary to cause focussing of the electrons.

It is convenient to construct the device so that the potential applied to the focussing' electrode is the same as the potential applied in operation to the cathode of the device; namely, zero'potential. In such case the shape of said focuss'ing electrode in cross-section is substantially elliptical in form and the axes of said rod-shaped cathode and the further electrode, which will also be preferably of rod-shaped form, will be disposed in a plane containing the minor axis of said elliptical focussing electrode,

The invention can also be applied 'to devices employing a rod-shaped cathode'and further electrode in which electrons are emitted from only one side of the cathode in which case the focussing electrode would'be'approximately one-half of the focussing electrode employed in the case where electrons are emitted from diametricallyopposite points or'sides of the cathode; for example, where an elliptical; iocussing electrode is provided in the latter kind of device the shape of 3 the focussing electrode in a device in which electrons are emitted from only one side of the oathode will be semi-elliptical.

Further, it is not necessary for the cathode and the further electrode to be of rod-shaped form since the cathode may be, in efiect, a point source of electrons, in which case the focussing electrode will be of spheroidal form or of a form equivalent to the shape of the other focussing electrodes referred to above.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 is a plan view of an electrode structure of an electron discharge device in accordance with one embodiment of the invention,

Fig. 2 diagrammatically illustrates the shape of equi-potential surfaces required to produce focussing of electrons, and

Figures 3, 4 and 5 are plan views of certain oi the electrodes shown in Figure l, and in accordance with further embodiments of the invention.

Referring first to Figure 1 of the drawings, the reference numeral 5 indicates a rod-shaped thermionic cathode which is surrounded by a control electrode 6 in turn surrounded by a screening electrode I. The electrodes 6 and I may be wirewound grids of elliptical form in cross-section as shown in the drawing each grid being supported by support rods 8 and 9 respectively mounted parallel to the cathode 5. The reference numeral I indicates a further rod-shaped electrode spaced from the cathode. Electrode II) is provided with a coating of suitable material, such as magnesium, so that the electrode will have a secondary emission ratio substantially greater than unity and will emit a large number of secondary electrons when a primary beam of electrons impinges thereon. The secondary-emitting electrode I0 is surrounded by an anode I I in the form of a wire-wound grid supported by a single support rod I2, the cathode 5, grid support means or support rods 8 and 9, the electrode III and the support rod I2 all being preferably disposed parallel to one another and in thesame plane. Surrounding the afore-mentioned electrodes is a tubular focussing electrode I3 of elongated elliptical shape in cross-section arranged with its minor axis coincident with the plane containing the aforesaid electrodes. In operation of the device shown in Figure 1, the cathode may be maintained at zero potential, the electrode 6 at two volts negative with respect to the cathode, the electrodes I, I0 and I I at 100, 250 and 350 volts positive respectively with respect to the cathode. The focussing electrode I3 may bemaintained at the same potential as the cathode 5. The provision of the focussing electrode I3 serves to cause electrons emitted in widely divergent directions from the cathode 5 to follow arcuate paths somewhat as indicated by the dotted lines in Figure 1 and to bring them to a sharp focus on the secondaryemitting electrode I0. Since the electrons are brought to a sharp focus the electrode II! can be made of only small superficial area and consequently the anode II can also be made of small superficial area 'so reducing the anode-earth capacity. The actual shape of the focussing electrode I3 can best be ascertained experimentally by rolling balls on a stretched rubber sheet. It is, however, not necessary for the focussing electrode I3 to be a true ellipse since electrodes of rectangular or other elongated shapes approxi- 4 mately an ellipse will be found to serve the same purpose. Also it is not necessary for the focussing electrode I3 to surround completely the other electrodes since said electrode can be composed of a plurality of parts which substantially surround the other electrodes. Good results can be obtained if the ratio of minor to the major axis of the electrode I3 is 8.75:9.75 or 18:22 and if the distance between the centres of the cathode 5 and and the secondary-emitting electrode I0 is about one-third to one-half of the minor axis of the electrode I3. The electrode I3 may be formed of sheet metal or alternatively it may be in the form of a mesh, a closely-wound grid, or a series of rods.

Figure 2 of the drawings indicates the equipotential surfaces of the electric field necessary to obtain focussing of the electrons in the device shown in Figure I. It will be seen that the elliptical focussing electrode I3 has a shape corresponding to the zero equi-potential surface shown in Figure 2. This shape is preferable since it permits the electrode I3 to be maintained in operation at the same potential as the cathode 5. It is, however, possible to employ a focussing electrode corresponding in shape to the 5 or 10 equi-potential surfaces. Electrodes of these forms will require to be maintained in operation at +5, -5 or -10 volts respectively with respect to the cathode potential. The shape of the electrodes I and I I conform substantially to the shape of the equi-potential surfaces in their vicinity. 7 It will be observed from Figure 1 that the secondary-emitting electrode I0 is at least partially shielded from the cathode 5 by being arranged in the shadow area of the support rods 8 and 9 and I2 so that it is unlikely that a substantial number of particles evaporated or emitted by the cathode 5 will be deposited on the secondaryemitting electrode I0 so as to poison the latter. In the arrangement shown in Figure 1 the anode II is in the form of a wire-wound grid and is preferably constructed of tungsten wire to withstand the heat generated by the impact of secondary electrons. Theanode should be constructed so that only a small proportion of the primary electrons from the cathode 5 is intercepted by the anode, the quantity of such current being preferably less than twenty-five per cent of the total amount, this consideration therefore determining the pitch and diameter of the anode wires.

It is not necessary for the cathode and secondary emitter to be equally spaced from the centre of one ellipse along the minor axis. Good focus can be obtained where, for example, the secondaryemitting electrode It ,is disposed closer to one side of the electrode I3 than the cathode 5 is to the opposite side of the electrode I 3.

If the grid support means or rods 8 and 9 and rod I2 do not shield the secondary-emitting electrode I0 sufficiently from material evaporated from the cathode, additional shielding means in the form of a narrow metal strip It may be interposed between the cathode 5 and secondaryemitting electrode It, as shown in Figure 3. This strip I 4 is arranged parallel to the cathode 5 and may be attached to the adjacent support rod 9 of the screen grid 1. Instead of the strip It being spaced from the electrode 7 as shown in Figure 3 it may be arranged to contact with the wire of the electrode 1 as shown in Figure 4 and a further strip I5 may be provided as shown attached to the support rod I2 of the anode. Alternatively, the strip I4 or I5 of Figure 3. or 4.

5 may be supported independently of the screening electrode 7 or the anode II, and connected to either electrode inside or outside the envelope of the device.

Although in Figures 1, 3 and 4 the anode H is shown in the form of a wire-wound grip the focussing obtained by the use of the electrode I3 is sufficiently good to permit, if desired, the use of an anode comprising one or two hat or curved metal strips l6, Figure 5, arranged parallel to the secondary-emitting electrode 19 and in the plane containing the electrode land the cathode 5..

With such an arrangement the primary current collected by the anode can be reduced to less than one-tenth of the total primary current without loss in the heat-dissipating properties of the anode. If the heat dissipation is not required to be high a small gain with respect to the anode/earth capacity can be obtained by employing only one strip It. In this case it is preferable to remove the strip 15 situated between the electrode I and the screening electrode '2 since thereby the anode-screening electrode capacity is somewhat reduced.

A discharge device constructed with the elliptical electrode I 3 of Figure 1 and the electrode structure shown in Figure 4 has given good results with electrodes of the following dimensions: Cathode diameter 1.14 mm.; rods 8, 0.5 mm. in

diameter the centres of which were spaced apart by 2.5 mm.; rods 9, 0.75 mm. in diameter the centres of which were spaced apart by 5.2 mm. the minor axis of the grid 6 was 1.6 mm. and was wound with wire of 0.05 mm. diameter with 144 turns per inch; the minor axis of the grid 1 was 2.95 mm. and was wound with wire of 0.075 mm. diameter with 96 turns per inch, the width of the strip M was 3.5 mm. and the width of the strip was 2 mm.; the rod 12 was 0.75 mm. in diameter and the electrode It was 2.5 mm. in diameter; the distance apart of the centres of the rod 12 and electrode ID was 2.6 mm.; the diameter of the circular portion of the anode winding was 4.15 mm. and the anode was wound with 0.1 mm. diameter wire with 49 turns per inch. The major axis of the electrode I 3 was 22.5 mm. and the minor axis 16.5 mm. The centre of the cathode was spaced along the minor axis of the electrode I3 a distance of 3.975 mm.; and was disposed at a distance of 5.425 mm. from the centre of the rod I2, the centre of the latter being 2.6 mm. from the centre of the electrode I0. The length of the various electrodes was 20 mm. The device had an anode/earth capacity of 5 t, a control grid/earth capacity of 5 urf. and an anode/control capacity of 0.006 t. The electrons from the cathode are formed into a line focus on the electrode I0 approximately 0.5 to 1.0 mm. wide.

Where it is desired to employ a cathode which emits electrons from only one side, the discharge device may be constructed similarly to that shown in Figures 1, 3, 4 and 5, but the electrode l3 in this case may be semi-elliptical. Also the electrode [3 where the cathode 5 is effectively a point of source may be of spheroidal form. In both latter forms of the invention the shape of the focussing electrode may be modified to conform to the shape of the equi-potential surfaces as potentials other than that of the cathode, as referred to in connection with Figure 2.

Although the invention has been described above as applied to an electron discharge device of the multiplier type it will be understood that the invention is not limited thereto, since it can be applied to other devices where a sharplyfocussed beam is required. For example, the device shown in the drawings can be employed as a screened grip valve in which case the electrode 10 will form the anode and the anode l I could be omitted, or the pitch of the winding of the anode ll could be suitably changed so as to enable it when maintained at a suitable potential to function as the suppressor grid in a pentode valve.

What I claim is:

1. An electron discharge device having a cathode, a further electrode on which electrons from said cathode are caused to impinge, an anode and a focusing electrode elliptical in section and which surrounds said cathode and said further electrode and said anode, said further electrode comprising a secondary emitting electrode mounted within said anode, said cathode, further electrode and anode being spaced apart and disposed in substantially the same plane coincident with the minor axis of said focusing electrode.

2. An electron discharge device according to claim 1, wherein said cathode and said further electrode are rod-like electrodes.

3. An electron discharge device including a cathode, an electron receiving electrode spaced from said cathode, shielding means interposed between said cathode and said electron receiving electrode, and means for causing electrons from said cathode to follow curved paths around said shielding means to said electron receiving electrode; said last named means comprising a tubular focusing electrode of elongated cross section surrounding the other electrodes; said cathode, shielding means and electron receiving electrode being mounted in a plane containing the minor axis of said focusing electrode, said cathode and said electron receiving electrode being disposed on opposite sides of and spaced from the major axis of said focusing electrode.

4. An electron discharge device including a cathode, an electron receiving electrode spaced from said cathode and having a secondary emission ratio substantially greater than unity, means adjacent said electron receiving electrode for collecting secondary electrons emitted thereby, shielding means interposed between said cathode and said electron receiving electrode, and means for causing electrons from said cathode to follow curved paths around said shielding means to said electron receiving electrode; said last named means comprising a tubular focusing electrode of elongated cross section surrounding the other electrodes; said cathode, shielding means and electron receiving electrode being mounted in a plane containing the minor axis of said focusing electrode.

CABOT SEATON BULL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,146,607 Van Oberbeek Feb. 7, 1939 2,159,774 Veenemans et a1. May 23, 1939 2,173,267 Strutt et a1 Sept. 19, 1939 2,281,274 Dallenbach et a1. Apr. 28, 1942 2,390,701 Ferris Dec. 11, 1945 FOREIGN PATENTS Number Country Date 110,573 Australia May 10, 1939

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