US2894169A - Electron discharge devices - Google Patents

Description

K. H. KREUCHEN ELECTRON DISCHARGE DEVICES July 7, 19 59 2 Sheets-Sheet 1 Filed March 23, 1954 1. lull-l I III FIG.4

INVENTOR KHgfREL/c-HEN 'ATmRNWS July 7, 1959 K. H'. KREUCHEN ELECTRON DISCHARGE DEVICES 2 Sheet s-Sheet- 2 Filed March 25. 1954 FIG :2

lNVENTOR K H, KREUCHE-N ATTORNE United States Patent 2,894,169 V ELECTRON DISCHARGE DEVICES Karl Heinz Kreuchen, Hounslow, England, assignor to Electric 8: Musical Industries Limited, Hayes, England, aBritish company I Application March 23, 1954, Serial No. 418,198

Claims priority, application Great Britain March 24, 1953 8 Claims. (Cl. SIS-5.38)

The present invention relates to electron discharge devices and is more particularly concerned with electron collecting electrodes therefor.

In certain discharge devices for example high power klystron amplifiers it is necessary to provide an electrode for collecting the electron beam after its utilisation by its passage through a discharge space such as a drift tube and one or more cavity resonators. Such collecting electrodes are usually in the form of hollow elongated structures having one closed end and are constructed from metals which have good heat and electrical conducting properties such as copper or some forms of iron. This is particularly so, when the device is designed for high voltage operation and in these instances such a collecting electrode is usually provided so that it forms a structural part of the envelope of the device whereby it may be maintained cool in operation by the forced circulation of air or fluids over its external surface.

Now copper as well as iron belong to a group of metals which readily form on their surfaces thin films of their corresponding oxides, and since these oxides do not form a strong chemical bond to the metal it is relatively easy to free oxygen therefrom either by gentle chemical reactions or by bombardment thereof with electrons or ions and furthermore such oxide surfaces are relatively good emitters of secondary electrons.

The hollow collecting electrode is mounted coaxially with the other electrodes of the device and the primary electron beam after passing through the various electrodes of the device tends to spread out in all directions so that the majority of the electrons impinge on and are collected by the wall of the hollow electrode. Since the primary electrons have a high velocity they release charged particles and secondary electrons from the wall surface and due to the electric and magnetic field conditions within the hollow electrode these particles and the majority of the secondary electrons move in paths in opposite directions to the primary electrons thereby escaping from the collecting electrode.

The presence of secondary electrons and charged particles, mainly positive ions, impose a damping effect on the resonators which is detrimental to the eflicient operation of the device. The positive ions from the collector will travel in a direction towards the cathode and after their passage through the drift tube they will be accelerated and arrive at the cathode with a high energy. With devices in which large amounts of the metals copper or iron are employed for the construction of the electrodes this positive ion bombardment makes it impractical to employ cathodes of the oxide coated type owing to their consequent very short life.

It is therefore an object of the present invention to provide an improved construction of hollow collector electrode for discharge tubes whereby charged particles and secondary electrons generated therein are substantially prevented from reaching the discharge space.

According to the invention there is provided an elec- 2,894,169 Patented July 7, 1959 tron discharge device comprising means for generating a beam of primary electrons a hollow elongated collecting electrode having a beam entry, opening at one end thereof and means' for. directing said electron beam through said opening into said electrode, said electrodev having arranged therein at least one barrier comprising a plurality of slats disposed at a position within the hollow of said electrode set back from said beam entry opening at a distance of not less than A of the width' of the opening so as to leave an unobstructed hollow portion defined by'the wall of said electrode extending from said barrier to said beam entry opening said unobstructed hollow portion serving substantially to prevent secondary electrons released from said barrier from escaping from said hollow portion.

If desired a series of said barriers may be arranged within the electrode each series of barrier elements being positioned so that the elements of adjacent barriers" are out of alignment with the elements of a previous barrier.

The grid elements are preferably formed of or coated with a material e.g. zirconium which provides an absorber for the charged particles.

In order that the said invention may be clearly understood and readily caried into effect, the invention will now be described applied to a high power device of the klystron type with reference to the accompanying drawings, in which: a

Figure 1 is a section through a klystron amplifier device of known type to which the invention can be applied,

Figure 2 shows in perspective view of a barrier employed in the invention,

Figure 3 is a section on a larger scale of the collecting structure shown in Figure 1, but employing barriers according to the invention, and

Figure 4 is a lay-out showing the disposition of the barriers in the structure of Figure 3.

In order to give an indication of the class of device for which the invention is most suitable, a brief description of a known form of a high power klystron will now be given with reference to Figure l. The device illustrated is not shown to scale and although a single resonator device has been chosen as the example it will be understood that devices of this kind may be provided with more than one resonator. The device comprises a drift tube 1, leading into a hollow resonator 2, through which there is projected an electron beam from a concave cathode 3, to a hollow collecting electrode 4.. The drift tube l or resonator or resonators 2, and collector 4 are usually formed of copper and together they form a major part of the envelope of the device. The vacuum space between the cathode 3 andthe entrance of the collector 4 is termed the discharge space for the purposes of the present description.

In operation of the device a very high potential relative to the cathode 3 for example 100 kv. is applied to the drift tube Land the electrons from the cathode are formed into awell defined beam by an axial mag: netic focussing coil 5, this focussed beam of electrons passing through the discharge space and is then collected by the electrode 4. Theelectron beam after passing through the discharge space is no longersubject to the field of the coil 5 and it thus loses its focus and spreads out towards the wall of the collecting electrode 4 as will be seen from the few electron paths indicated on the'drawing. For this reason, the electrode 4 is con- I lected. re; near to its closed end. The electrode 4 is maintained cool in operation by the circulation of fluid or forced air over its external surface.

The shape of the electrode 4 can be formed for example by thesuccessive drilling of a block of copper with a series of drills ofsuitable diameters, or by direct machining and when so formed the wall of the electrode will have a series of annular steps 6.

It is well .known that copper and also some forms of iron have excellent or at least suflicient electrical and heat conductivity and for this reason they are the metals which are mainly employed in the construction of electrodes for devices of this kind. However, as stated above both of these metals readily form thin films .of oxide on their surfaces and this oxide has only a Weak bond to the main metal, so that, it is relatively easy to free oxygen from the surface by gentle chemical reactions orfby bombardment with electrons or ions.

The surface of the wall of the collecting electrode 4 is subjected to bombardment by electrons and possibly negatively charged particles during operation of the device and such bombardment will release secondary electrons and charged particles such as positive ions from the surface. Due to the electrical field conditions in the electrode 4 those particles with a positive charge will move in paths opposite in direction to the incoming primary electrons of the beam whereby they are able to pass out of the electrode 4 into the region of the resonators 2. Those of the secondary electrons with a velocity component such that they move in a direction towards the drift tube 1 will be focussed by the magnetic field into a similar beam shape as that of the primary beam but in reverse and thereby also will be able to pass out of the electrode 4 into the region of the resonator 2. In crossing the resonator gap both charged particles as well as secondaries cause an unwanted additional beam loading on the cavity leading to reduced efiiciency .of the device. The electrostatic field conditions set up by the flow of electrons will cause positive ions to accumulate and be moved to the centre so as to form a core extending throughout the drift tube 1. The positive ions of this core at the end of the drift tube 1 and nearer the cathode 3 will then be subject to the potential difference between them and the cathode 3 which in effect is of the same order but in reverse of the potential difference employed to accelerate the electrons. The positive ions at the end of tube 1 are accelerated towards and bombard the cathode 3 these ions being constantly replaced in the core. This condition of constant ion replacement is known in the art as an ion drain. Bombardment by positive ions is detrimental to the life of any cathode but particularly in the case of oxide cathodes which are poisoned by the conversion of the ions by the active metal of the cathode e.g. barium and for this reason other less efiicient cathodes such as pure metal cathodes are usually employed.

According to the invention it is proposed to substantially reduce the number of secondary electrons and chargedparticles which are released by bombardment of the collecting electrode from escaping therefrom into the discharge space by extending across the collecting electrode a barrier arrangement which will allow electrons or charged particles travelling in a direction parallel to the axis of the collecting electrode to pass therethrough but will substantially impede electrons or charged particles which reach the barrier from other directions.

As is more clearly shown in Figure 2 each barrier is constructed in the form of a grid having a series of slats 7,; arranged across a circular frame 8, said slats 7 being arranged with their major surfaces normal to the plane of the frame 8, so that when mounted in the device their front edges are presented to the electron beam, A sin le r s ver l gr l k ar i s m y onv nien ly be mount d wit n t device y se n the frame 8 to he annular steps 6 provided along the wall of the selectrode 4. The first of the barriers constituted by the slats 7 in the frame 8 is thus provided within the electrode 4 at a position away from the beam entry opening thereof, at a distance of not less than /1 of the width of the opening.

A modified electron collecting electrode 4 of the device of Figure 1 having a barrier arrangement mounted therein, is illustrated in greater detail in Figure 3 in which it will be seen that more than one grid like barrier is provided. .Eour barriers are employed in the device shown in Figure 3 and the slats 7 .of these barriers are arranged so that they are out of alignment with the slats of adjacent barriers. For this purpose it is preferable to arrange them so that slats 7 in adjacent barriers are oriented with regard to one another so that :they assume dilferent angular positions relatively to the axis of the electrode 4. The electrode 4 diflers from that previously shown in Figure 1 only in that the inner wall nea to he open end i pr vided with a coni a Wall portion 9, the .first barrier ,8 being mounted on thefirst step 6 so that this barrier is disposed away from the beam entry opening of the electrode 4. The few electrons of the beam which strike the edges of the slats 7 of the first barrier will give rise to secondary electrons but these secondaries will be effectively trapped by the conical wall 9 and substantially none will escape from the opening of the electrode '4. The angular positioning of the various barriers will be clearly seen from the layout illustrated in Figure 4.

Gharge particles and secondary electrons released from the wall of the electrode 4 by the impingement thereon of primary electrons will travel mainly in a direction towards the axis of the electrode 4 and will thus in, all probability strike one of the slats 7 of a barrier and be collected thereby. Although collection of said charge particles will substantially prevent them escaping from the collector electrode 4 a more eflicient device is ob tained when said charge particles are fixed or absorbed as by a gettering action and this can be achieved by providing the barrier with a suitable getter material such as zirconium, thorium, tantalum, molybdenum or hafnium. The slats 7 of the barrier may therefore be either formed of or coated with one or more of the above metals.

The above mentioned getter materials are most effective when they are operated over a range of temperatures of from 600 to 1000 C. and since the frames 8 holding the slats 7 are directly connected to the cool wall of the electrode 4 it can be arranged that the surface area of each slat 7 in the axial direction of the electrode 4 is such that the centres of the slats can attain a temperature of about 1000" C. by electron bombardment thereof by the primary beam but that the portions of these slats which are near the frame 8 are at a lower temperature, there is thus set up across the slats a temperature gradient.

Since the primary beam in its passage to the collector electrode 4 is magnetically focussed only a very few electrons of the beam will be able to strike the metal parts of the device other than the edges of the slats 7 and the wall of the electrode 4 so that by substantially prevent ing secondary electrons and charge particles generated at the collector space from reaching the discharge space, damping of the resonator 2 by secondary electrons and charge particles is very substantially reduced and also bombardment of the cathode by positive ions is substantially prevented so that the life of the cathode is increased. Since only very few positive ions will be able to reach the cathode of the device it is therefore made possible to employ an oxide coated cathode in a high power device. t 7

Although the Present invention has been described with reference to high power devices of the klystron type it is in no way limited to such since barriers of the described may be employed in devices of other form provided that the electron collecting electrode is in the form of a hollow structure, for example travelling wave tubes.

What I claim is:

1. An electron discharge device comprising means for generating a beam of primary electrons a hollow elongated collecting electrode having a beam entry opening at one end thereof and means for directing said electron beam through said opening into said electrode, said electrode having arranged therein at least one barrier comprising a plurality of slats disposed transversely of said electrode disposed at a position within the hollow of said electrode set back from said beam entry opening at a distance of not less than of the width of said opening so as to leave an unobstructed hollow portion defined by the wall of said electrode extending from said barrier to said beam entry opening said unobstructed hollow portion serving substantially to prevent secondary electrons released from said barrier from escaping from said hollow portion.

2. An electron discharge device according to claim 1 wherein said barrier has an exposed surface thereof constituted by an ion gettering material.

3. An electron discharge device comprising means for generating a beam of primary electrons, an elongated hollow electrode the hollow in which decreases in size from its open to its closed end, and means for directing said electrons into the open end of said electrode a plurality of barriers in the form of slats arranged across the hollow of said electrode, with slats in any one barrier positioned so that they are out of alignment with the slats of an adjacent barrier, the first of said barriers being disposed at a position within said hollow electrode set back from said beam entry opening at a distance of not less than of the width of said opening so as to leave an unobstructed hollow portion defined by the wall of said electrode extending from said first barrier to said beam entry opening, said unobstructed hollow portion serving substantially to prevent secondary electrons released from said first barrier from escaping from said hollow portion.

4. An electron discharge device comprising means for generating a beam of primary electrons a hollow elongated collecting electrode having a beam entry opening at one end thereof and means for directing said electrons into said opening, the diameter of the hollow in said electrode decreasing from its open to its closed end so as to form annular steps, and a plurality of barriers each comprising a series of slats mounted transversely of said electrode said barriers being positioned within and along the length of said hollow by securing said barriers to said annular steps, the first of said steps securing the first of said barriers in point of distance from said beam entry opening being set back from said opening at a distance of not less than of the width of said opening so as to leave an unobstructed hollow portion defined by the wall of said electrode extending from said first barrier to said beam entry opening, said unobstructed hollow portion serving substantially to prevent secondary electrons released from said first barrier from escaping from said hollow portion.

5. An electron discharge device according to claim 4 wherein said barriers are angularly displaced relatively to one another whereby the slats of any one barrier are not directly in the shadow area of the slats of an adjacent barrier.

6. An electron discharge device comprising means for generating electrons an elongated hollow collecting electrode having a beam entry opening and means for focussing said electrons into a beam directed towards said opening, said electrode having at least one barrier arranged within said hollow said barrier comprising a series of slats arranged transversely of said hollow electrode so as to present a minimum interception for the electron beam, said barrier being disposed at a position within the hollow of said electrode set back from said beam entry opening at a distance of not less than of the width of said opening so as to leave an unobstructed hollow portion defined by the wall of said electrode extending from said barrier to said beam entry opening, the inner portion of said unobstructed hollow portion widening out between the beam entry opening thereof and said barrier to provide an electron trap whereby secondary electrons emitted from said slats by impingement thereon of said beam are substantially prevented from escaping from said opening.

7. A high power discharge device comprising a thermionic oxide coated cathode for generating a beam of electrons, and a hollow collecting electrode for said electrons, said collecting electrode having arranged within the hollow thereof a series of members an exposed surface of which is of ion gettering material, said members being disposed at a position within the hollow of said electrode set back from said beam entry opening at a distance of not less than of the width of said opening so as to leave an unobstructed hollow portion defined by the wall of said electrode extending from said 'barrier to said beam entry opening, and said members being in heat conducting connection with said electrode whereby in operation of the device a temperature gradient is set up across said members.

8. A high power klystron having an oxide cathode for generating a stream of electrons, a drift tube, a hollow resonator and a hollow collecting electrode having a beam entry opening said collecting electrode having disposed within the hollow thereof and away from the beam entry opening at least one barrier comprising a plurality of slats arranged so as to lie transversely of said electrode and present a minimum interception for primary electrons, said barrier having an exposed surface of ion gettering material, said barrier being disposed at a position within the hollow of said electrode set back from said beam entry opening at a distance of not less than A of the width of said opening so as to leave an unobstructed hollow portion defined by the wall of said electrode extending from said barrier to said beam entry opening said unobstructed hollow portion serving substantially to prevent secondary electrons released from said barrier from escaping from said hollow portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,829,299 Beck Apr. 1, 1958

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