US3454806A - System for the production of a flat electron beam for a traveling wave tube with purely electrostatic focusing - Google Patents

Description

y 3, 1969 1-1. BITTORF 3,454,806

SYSTEM FOR THE PRODUCTION OF A FLAT ELECTRON BEAM FOR A TRAVELING WAVE TUBE WITH PURELY ELECTROSTATIC FOCUSING Filed July 11, 1966 Fig.3-

1 Fig.1 1 5 35.-

a 1. o L 10 [1 [1,5 1,0 dlmm) H INVENTOR HANNJORG BITTORF aha Mfii ATTORNEYS United States Patent 3,454,806 SYSTEM FOR THE PRODUCTION OF A FLAT ELECTRON BEAM FOR A TRAVELING WAVE TUBE WITH PURELY ELECTROSTATIC FOCUS- ING Hannjiirg Bittorf, Munich, Germany, assignor to Siemens Aktiengesellschaft, a corporation of Germany Filed July 11, 1966, Ser. No. 564,335 Claims priority, application Germany, July 15, 1965, S 98,199 Int. Cl. H01j 29/74, 25/34 US. Cl. 31375 Claims ABSTRACT OF THE DISCLOSURE An electron beam of flat cross-section is provided in a traveling wave tube, the electron beam being directed at 90 from the cathode to an aperture in an anode and having beam deflecting electrodes disposed between a deflection plate and cathode and the cathode and the anode for focusing the electron beam.

The invention concerns an electron beam production system for a traveling wave tube with purely electrostatic focusing, which contains a cathode, a diaphragmshaped acceleration anode and at least one beam forming electrode which cooperate to produce an electron beam of flat cross section which is directed into a helical course on which the electron beam travels along a delay line in the field of a cylindrical condenser.

Traveling wave tubes are known, in which an electron beam is directed purely electrostatically on a helical course between a delay line and an outer electrode. In this case the delay line is charged with a high positive direct potential in comparison with that of the outer electrode whereby the outwardly directed centrifugal force of the electron beam is compensated by an electrostatic counter force.

Traveling wave tubes of this kind with purely electrostatic focusing of the electron beam are normally designated as E-type tubes, and generally operate with a flat electron beam, in which case the problem arises of directing the first beam, the production of which is supposed to take place in a purely electrostatic manner, exactly tangentially to a certain radius into the desired helical course. In the prior known E-type tubes, electron guns have been utilized which consist of a cathode and several slit diaphragms, in which the emission plane of the cathode is located in a plane perpendicularly to a tangent of the helical electron beam course. However, such electron guns exhibit only a small beam density. Beyond this, such known flat beam guns for E-type tubes offer only few possibilities to influence the focusing of the electron beam during the discharge thereof into the field of the cylindrical condenser.

It is therefore the purpose of the invention to create an electrostatically focusing electron gun which produces a highly concentrated flat electron beam and makes possible an angle correction of the flat beam at the point of discharge into the drift space of an E-type tube. In order to fulfill this purpose, it is proposed for an electron beam production system of the kind initially mentioned, according to the invention, that the effective cathode surface is laterally rotated by 90 in relation to the acceleration anode which, at the entrance point of the electron beam extends perpendicularly to the helicallyshaped electron beam course into the field of the cylindrical condenser and separates the electrostatic field existing in the space of the beam production system from the field of the cylindrical condenser, while on that side of the system which is opposite to the cathode surface, there 3,454,806 Patented July 8, 1969 is present a deflection electrode having approximately cathode potential applied thereto in which arrangement beam forming electrodes are disposed laterally from the cathode in the space between the cathode and acceleration anode, and also between the cathode and deflecting electrode. Such electrodes have such a voltage, lying between cathode potential and acceleration anode potential, that the electron flow proceeding from the cathode is deflected by in the direction towards the diaphragm opening of the acceleration anode.

An electron beam production system according to the present invention presents, first of all, the advantage that for a relatively small cathode loading very high current densities of the electron beam leaving the acceleration anode may be obtained. For example, with a cathode loading of approximately 1 a./cm. a flat beam with a current density of approximately 10 a./cm. may be obtained without difficulty. A further important advantage of the invention resides in the fact that by regulation of the voltages which are applied to the individual electrodes, the electron beam may be tilted and simultaneously shifted in radial direction to the helical course. In this case, the gun field is shielded with respect to the field of the cylindrical condenser by the diaphragm-shaped acceleration anode so that a voltage regulation at the gun does not have any disturbing effects on the beam course in the actual drift space of the tube. In the event disturbances still occur in the beam course during the transition between the gun field and the field of the cylindrical condenser, in a further development of the invention a small plate condenser which provides a further possibility for the beam correction may be additionally arranged behind the acceleration anode.

The invention will be explained in greater detail in connection with the drawing, wherein like reference characters indicate like or corresponding parts, and in which:

FIG. 1 is a schematic sectional view of an electron beam producing system embodying the invention;

FIG. 2 is a sectional view similar to FIG. 1, illustrating the use of a condenser structure; and

FIG. 3 is a graph illustrating the current density distribution of an electron beam with utilization of the present invention.

Referring to FIG. 1, the reference numeral 1 designates a cathode, for example an MK cathode having, for example, a diameter of 3 mm. which is surrounded by a Wehnelt electrode 2, which is provided with a slit 3, by means of which an electron bundle 4, for example with 1.8 mm. width and 5 mm. height, is derived from the electron flow emitted from the cathode 1 by means of the diaphragm action of the slit 3. Disposed at an angle of 90 with respect to the effective cathode surface, is a diaphragm-shaped accelerating anode 5, which extends perpendicularly to the electron beam direction at the entrance point of the electron beam 4 into the field of a cylindrical condenser (not illustrated), which follows the acceleration anode. The acceleration anode 5 is, at its end adjacent to the cathode 1, provided with an extension, such as a flange 6 which runs parallel to the effective cathode surface and extends to within the proximity of the electron flow from the cathode 1. On the side of the system opposite to the cathode surface is disposed a plate-like deflection electrode 7 which is to be at a potential approximately equal to that of the cathode. The deflection electrode 7 is provided at its end, adjacent to the acceleration anode 5, with an extension 8 which is disposed opposite to the extension 6 of the acceleration anode 5. Such an angle-shape of the deflection electrode is desirable as the upper boundary of the electron beam 4 must be bent more than the lower boundary thereof during the deflection. In the space between the part 6 of the acceleration anode 5 and the Wehnelt electrode 2, there is disposed a first beam forming electrode 9 which is to be at an electrical potential lying between that of the cathode and that of the acceleration anode. On the other side of the electron beam 4 is disposed a second beam forming electrode 10 which is positioned at approximately equal distances from the Wehnelt electrode 2 and the deflection electrode 7, said second beam forming electrode having a cylindrical shape and having a potential which is substantially the same as that of the first beam forming electrode 9. In addition thereto, disposed below the cylindrical beam forming electrode 10 is a further plate-shaped beam forming electrode 11 which is disposed laterally of the electron course and extends substantially perpendicularly to the plane of the cathode.

Approximately the same potential as that of the Wehnelt electrode 2 is to be applied to this electrode.

The electrode system illustrated in FIG. 1 with the described potentials creates a potential distribution as indicated by the potential lines 12 in FIG. 1. The deflection and the simultaneous concentration of the electron flow proceeding fram the cathode 1 in the direction towards the diaphragm opening 13 of the acceleration anode is based upon such potential distribution. Although, in this case, the influence of the individual electrodes may not be isolated from each other, the following statements may be made with respect thereto: The Wehnelt electrode 2 with the Wehnelt slit 3 serves, in particular, for the defining and limiting of the beam cross section. It also prevents, in known manner, a spreading of the electron beam through space charge forces. The potential of the Wehnelt electrode 2 lies between cathode potential and a negative tenth part of the acceleration anode potential, while the voltage at the two beam forming electrodes 9 and preferably amounts to one tenth to four tenths of the acceleration anode potential. The size of the latter voltage primarily determines the strength of the emission current and the positioning of the focus of the fiat electron beam 4. In this system, the cross sectional shape of the beam forming electrode 10 does not necessarily have to be circular. It is only important that the field distribution indicated by the lines 12 is substantially achieved.

The additional plate-shaped beam shaping electrode 11 which is to lie on a potential between cathode potential and a cathode potential reduced 'by one tenth of the acceleration anode potential, together with the beam forming electrode 9 and the part 6 of the acceleration anode 5 makes possible a precise correction of the beam course. In particular by adjustment of the voltage at the plateshaped beam shaping electrode 11, an overlapping of the beam boundaries may be avoided because the electrode 11 principally influences the upper boundary of the beam while the course of the lower boundary of the beam is determined by the beam forming electrode 9 and the extension 6 located at the acceleration anode 5. The deflection electrode 7, lying approximately on cathode potential principally causes the deflection of the electron beam 4. This electrode also enables an influencing of the angle of emergence of the electron beam 4 from the diaphragm opening 13 of the acceleration anode 5 during which process the beam may also be shifted somewhat laterally. The shape of the acceleration anode is, on the one hand, determined by the fact that the gun field is supposed to be strictly separated from the field of the actual drift space in order to avoid a reciprocal influencing of the two fields. On the other hand, the angled extension 6 at the .acceleration anode 5 is provided for cooperation with the two essential beam forming electrodes 9 and 10, to obtain the desired beam shape. In connection therewith, it should be noted that the acceleration anode may, of course, also consist of two separate parts, namely, the plate 5 with the diaphragm opening 13 and a separate plate perpendicular to the first plate, corresponding to the extension 6.

The described possibilities-to be able to influence the beam characteristics, and in particular the emergence angle in the diaphragm opening 13 of the acceleration anode 5are of primary importance for E-type tubes. A shoot-in error of approximately 3 with respect to the tangent of the drift course of the electron beam in the field of the cylindrical condenser would lead to a radial fluctuation of 17% and to a voltage fluctuation of :15% with respect to the equilibrium course. In spite of the control of the electron beam undertaken by means of the deflection electrode 7, beam oscillations and current density variations during the traveling of the electron beam may still occur in the actual drift of an E-type tube. This results from the fact that between the gun and the drift space of the tube, there is always an electrical transition field which represents a certain field disturbance for the electron beam. However, most of this field disturbance may be compensated if the gun space and the field of a cylindrical condenser present in the drift space are additionally separated from each other by the insertion of a small plate condenser, as illustrated in FIG. 2. In this figure the reference numeral 14 designates the plate condenser, while the electrode 5 corresponds to the acceleration anode of FIG. 1. The arcuate electrodes 15 and 16 limit the drift space of the tube laterally, the electron beam to be directed electrostatically onto a helically-shaped course between these electrodes, in which system the inner electrode 15 is expediently constructed in known manner as a delay line.

FIG. 3 illustrates the measured current density distribution of a fiat electron beam which has been produced by means of a gun .according to FIG. 1. In this figure d designates the width of the beam cross-section, with its height amounting to 5 mm. The measurements were taken with the following voltages at the individual electrodes. The potential at the cathode 1, the Wehnelt electrode 2, the deflection electrode 7 and the additional beam forming electrode 11 respectively .amounted to 0 volt. Applied to the beam forming electrode 9 was a potential of 53 v. and to the cylindrical beam forming electrode 9 a potential of 41 v., while the potential of the acceleration anode 5 amounted to 410 v. The beam was concentrated at a perveance of 0.7- 10 a./v. in the ratio of 10:1, with its emergence angle after the passage from the acceleration anode amounting to approximately 5.

The invention is not limited to the illustrated example of construction. In particular the cathode surface may be concavely formed and the Wehnelt electrode may have a larger dimension so that the electron beam initially becomes more convergent whereby an even higher beam concentration may be obtained. In this case, the arrangement of the other electrodes may be the same as in FIG. 1. However, the individual beam forming electrodes may also have a different shape if by this feature the illustrated potential distribution in the gun space is essentially retained.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. An electron beam production system for a traveling wave tube which utilizes purely electrostatic focusing in which an electron beam of flat cross-section is to be brought into a helical course on which the electron beam is to travel along a delay line in the field of a cylindrical condenser, the production system comprising:

a cathode including a cathode surface for emitting a flow of electrons;

a Wehnelt electrode surrounding said cathode and having a rectangular aperture therein for deriving an electron beam of rectangular cross-section from the electron flow emitted from said cathode;

an acceleration anode having a diaphragm opening therein, said acceleration anode being positioned at the entrance point of the electron beam into the field of such a cylindrical condenser for separating the electrostatic field present in the space of the beam production system, said cathode surface being disposed laterally of said acceleration anode and at substantially 90 thereto, said acceleration anode including an end adjacent said cathode and an end portion which extends substantially parallel to said cathode surface and to within the proximity of the electron flow from said cathode surface;

a deflection electrode oppositely disposed With respect to said cathode for operating at substantially the same potential .as said cathode, said deflection electrode being of generally planar shape and including an end which faces said acceleration anode and extends at a right angle to the plane of said deflection electrode in the direction toward said end portion of said acceleration electrode; and

beam forming electrodes operatively disposed between said cathode and said acceleration anode, and between said cathode and said deflection electrode, for operating at such a potential between that of said cathode and said acceleration anode so as to deflect the electron flow from said cathode surface by 90 in the direction toward the diaphragm opening of said acceleration anode.

2. An electron beam production system according to claim 1, wherein one of the beam forming electrodes is in the form of a plate and extends into the space between the Wehnelt electrode and the extended end of the acceleration anode, another of said beam forming electrodes having a cylindrical form and disposed at the opposite side of the electron flow and spaced equi-distant from the Wehnelt electrode and the deflection electrode.

3. An electron beam production system according to claim 2, wherein the two beam forming electrodes are disposed to receive substantially the same potential, with such potential amounting to one tenth of four tenths of the acceleration anode potential.

4. An electron beam production system according to claim 3, wherein on that side of the cylindrical beam forming electrode facing away from the deflection electrode, there is disposed a further beam forming electrode of plate-like shape which is laterally spaced from the electron flow and extends substantially perpendicularly to the effective cathode plane and which, together with the Wehnelt electrode, is to be at substantially the potential of said cathode.

5. An electron beam production system according to claim 4, wherein there is provided a plate condenser which surrounds the discharge course, and with respect to the discharge direction extends behind the diaphragm opening of the acceleration anode.

References Cited UNITED STATES PATENTS 2,442,848 6/1948 Gardner 313- 2,807,739 9/1957 Berterottiere et al. 315-3.5 X 2,857,548 10/1958 Kompfner et al. 315-3.5 2,900,558 8/1959 Watkins 31539.3 X 3,189,785 6/1965 Kluver 31539'.3

FOREIGN PATENTS 209,957 8/ 1957 Australia.

HERMAN KARL SAALBACH, Primary Examiner.

SAXFIELD CHATMON, JR., Assistant Examiner.

U.S. Cl. X.R.

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