US2825838A - Electron sources - Google Patents

Description

March 4, 1958 D. CHARLES ELECTRON SOURCES 2 Sheets-Sheet 2 Filed June 2, 1954 I a I I Ill/11111111 H l l l l l hl lfl FIG.3

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ELECTRQN SQURCES Daniel Charles, Paris, France, assignor to Compagnie Genet-ale de Telegraphic Sans Fil, a corporation of France If an electron discharge tube is to have a reasonably useful life, the density of emission of its electron source must be limited.

Thus, for example, in the case of tungsten or tantalum cathodes, the current density compatible with a good cathode life is of the order of 0.5 ampere per square centimetre and in the case of oxide cathodes of the order of 0.2 ampere per square centimetre.

It would be natural to think that the'total current could be increased, without exceeding the admissible current density, simply by increasing the dimensions of the emitting surface. But, in certain important cases, this is not possible. Indeed, with certain electron optical systems designed to form and direct the beam produced by the cathode, the latter should not exceed certain maximum dimensions.

The only way to obtain high current densities is to operate with pulse emitting sources. With pulses of relative- 1y short duration, it is possible to attain very high densities, for example, or" the order of 20 to 50 amceres per square centimetre. However, it may be extremely desirable to have electronic tubes with a beam which is continuous and constant in time and still has a density of the above magnitude.

It is an object of the present invention to provide such a tube. More particularly the invention has for its object an electron source characterized in that it comprises a cathode comprising means for causing only a fraction of its emissive area to emit electronsat a. given instant and means for renderingsuccess'ively emissive the various portions of this area, whereby the beam ismade continuous notwithstanding the fact that each portion of the cathode, considered separately, functions as a pulse emitting source. I

It is obvious that while the invention may be carried out in many ways, the simplest is to combine a cathode, having a relatively large area that is preferably as large as the other considerations governing the construction of the tube in question allow, with a screen having a restricted opening, for example a slit, and to impart to the cathode and/ or the screen a relative movement such that the whole of the cathode passes before the restricted opening of the screen. For example, the cathode or the screen may be rotated, or they may be given a relative alternating movement.

According to one embodiment of the invention, the cathode carrier may be given for example the shape or" a cylinder, the emissive substance of the cathode covering the latter as a layer. The cathode is surrounded by a screen including a very narrow slit and a suitable motor drives it in rotation within this screen, in such manner that, during one rotation of the cathode, the whole of its surface passes before the slit in the screen, which is suitably biased. Each portion of the area of the cathode therefore emits electrons only during the time when it is in front of the slit.

The construction of the motor which drives the cathode carrier and the stator winding being disposed outside the tube. in any case it is necessary to take precautions to prevent the held of the motor from interfering with the operation of the tube.

Other features and advantages of the invention will appear from the ensuing description with reference to the accompanying drawings, given merely by way of example and in which,

Fig. l is a sectional view, with a portion cut away, of a traveling wave tube having a magnetic field incorporating a cathode according to the invention;

Pig. 2 is a diagrammatic sectional view, along line 22 of Pig. 1, of the driving means for the tube show in Fig. l;

Fig. 3 is a diagrammatic sectional view of a modification of the cathode according to the invention.

According to the embodiment shown in Fig. 1, a traveling wave tube of the type described in the U. S. Patent No. 2,694,783 issued November 16, 1954 to Daniel Charles is provided with a cathode of the invention. Within the envelope 1 of this tube are housed a delay line 2, a hat negative electrode 3 and a collector 4. Coaxial coupling lines 5 and s are connected to the line 2. A magnetic field created by polar pieces (not shown) is diagrammatically illustrated at 7. The operation of this type of tubes is well known and it is unnecessary to describe the same.

The cathode layer 3 is carried by a cylinder 9, driven in rotation in the manner shown in Fig. 2. The cathode 8 is surrounded by a screen electrode 10 which forms a rectangular-sectioned envelope brought to the same voltage as the cathode 8. The latter is provided with a slit 11 along the entire length of the cylinder 9. Opposite the wall 12 of the electrode it? is disposed a flat electrode 13. The cathode 8 is heated by a filament 14, fed from a source 15. The electrode 13 is connected by a connection 16 to an intermediate point of a source 17 whose positive pole is earthed and whose negative pole is connected to the screen 10, the latter being connected to the electrode 3. As described in the above application, the electric field between the electrodes 10 and 13 is equal to E/2, E being the electric field between the electrodes 2 and 3.

It is known that in this kind of tube the cathode must not exceed certain dimensions. Assuming for example that its width must not exceed 1 mm., with a tungsten ribbon having this width and a length of 3 cm., there is obtained an emitting surface of 0.3 square centimetre. If the admissible maximum current density is 0.5 ampere per square centimetre, a maximum current of milliamperes is obtained. Furthermore, it will be assumed, by way of example, that the diameter of the cylinder 9 is 2 cm. (Fig. l), the slit 11 is 1 mm. wide and this cylinder is rotated at 3,000 R. P. M. Under these conditions, only the portion of the cathode in front of the slit 11 emits electrons, since the electrode 10 and the cathode 8 are at the same potential. The circumference of the cathode 5's being equal to Zn cm., each point of the cathode during each rotation of the cylinder 9 will be exposed to the slit Ill for only of a revolution. As the cylinder 8 is assumed to be rotated at 50 R. P. 8., it can be seen that each point is disli atented Mar. 4, 1%58 posed in front of the slit, for each revolution, i. e. for each of a second, during 1 ms 207r 50 1r 'lfhusyeverything occurs as though each point of the cathode- 8. emitted electrons by pulses having a duration of 1 ms. t T these pulses recurring 50 times per second.

Thus, the tube functions under normal conditions of pulse emission in which a current density, much higher than :that in the case of continuous emission, is possible. If: I represents the instantaneous current and I the mean current:

If I is equal to 100 milliamperes per square. centimetre, which may be considered as acceptable as concerns the useful life of the cathode, I is equal to 6.3.amperes per square centimetre.

In this example the area of the aperture in the screen is 0:3 cm. Thus there is obtained a total permanent current of 1.9 ampere instead of 150 milliamperes obtainable with. a fixed cathode.

With an oxide cathode the difference would be even greater.

There has been diagrammatically illustrated in Fig. 2 an embodiment of meansfor driving the cathode 8 in rotation. The envelope 1 of the tube shown in Fig. 1 comprises a lateral extension 18 of a nonmagnetic material. Within this extension is housed a rotor 19 which rotates the cylinder 9. The stator winding, diagrammatically shown at 19a, is disposed outside the envelope and is fed by a source 20.

It will be understood that, since cathode 8 is rotated rapidly, it is necessary to employ a sliding contact 21 for the current. On the other hand, the heating element 14, situated within the cylinder 9, may be fixed.

It may be inconvenient to pass a relatively high current through-the sliding contact 21 and the invention provides a very simple means for avoiding this. The device employed is shown in Fig. 3,. wherein the sliding contact 21 merely-serves to bring the cathode 3 to the same potential as. that in. the-example shown in Fig. 1. However, the cathode'current-does not pass through the contact 21, but

through. an internal fixed. cathode 22 negatively biased withirespect to the cathode 8; provided with a heating filament 23 and disposed within the cylinder 9. The rotating cathode 8 receivesthe current emitted by the fixed cathode22.

4 In the arrangement shown in Fig. 3, the bombardment of the electrons issuing from the internal cathode may even be used for maintaining the external cathode at the desired temperature. In this case, a grid 24 may be provided and negatively, or positively, biased so as independently to regulate the current and the heating.

It is obvious that the invention is not limited to the described and illustrated embodiments which are given merely by way of examples. It is obvious that the oathode'according to the invention may be utilized with any other discharge tube. For example, the principle of the invention is applicable when the emitting source is an ion generating source.

Further, the screen electrode 10, instead of being at the same potential as the cathode 8, may be negatively biased with respect to the latter (Fig. 3). In this case the slit would be wider than the slit 11 shown in Fig. 1. The electrode 10 behaves in this case as a Wehnelt electrode and the variations in its potential have for efiect to vary the dimensions of the emitting surface of the cathode and hence modify the emission.

What I claim is:

1. An electron emissive source for electron discharge tubes comprising: a first emissive thermionic cathode rotatably mounted; energized connections comprising a sliding contact for biassing said cathode; a fixed screen surrounding said cathode and having a slot facing the latter; energized connectionsfor bringing said screen to a potential which is at most equal" to the potential of said first cathode; means for bringing in continuous succession all the surface portions of said cathode in front of said slot; a second cathode coaxial with said first cathode and located within the latter; energized connections for bringing said second cathode to a negative potential with respect to said first cathode, thereby heating said first cathode by electron impact ofsaid second cathode emission; an anode electrode facing said slot on opposite side of said screen with respect to said first cathode, and terminal connections for positively biassing said anode with respect to said screen and thereby extracting electrons through said slot from the portion of said first cathode instantaneously facing said slot.

2. An electron emissive source according to claim 1 further comprising a grid between said first and second cathode.

References Cited in the file of this patent UNITED STATES PATENTS 1,323,304. Mauclaire' Dec. 2, 1919 2,085,576 Du Mont June 29', 1937 2,175,582 VOgel Oct. 10, 1939 2,427,203 Essig Sept. 9, 1947

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