US2955229A - Secondary emission suppression in electron beam tubes - Google Patents

Description

Oct. 4, 1960 R. J. BONDLEY SECONDARY EMISSION SUPPRESSION IN ELECTRON BEAM TUBES Filed Nov. 14. 1956 Inventor": FPa/ph dBond/ey, .b W ?74M.

H/s Z t torney SECONDARY EMISSION SUPPRESSION ELECTRON BEAM TUBES Ralph J. Bondley, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 14, 1956, Ser. No. 622,228

'3 Claims. (Cl. $15-$39 This invention relates to improved electric discharge devices of the beam type, particularly with respect to freedom from the detrimental efl ects of unwanted secondary emission during operation.

In high frequency electron beam tubes, .operation depends upon the passage of an elongated electron .beam in energy-exchanging relation with respect to a conductive structure. In one type of velocity modulation discharge device, the beam passes across one or more gaps, each defined by apertured par-ts providing parts, of-the opposed walls of a cavity resonator. Where more than one resonator and associated gap is employed at spaced points along the path of the beam, the beam path is usually enclosed by a drift tube defining a field-free drift space. At various points in the tube and particularly in the region immediately surrounding the gaps, substantial high frequency fields exist and the bombardment of these parts by the beam upon passing through the apertures produces secondary electrons. If the material of the parts defining these gaps emits secondary electrons as a result of the impact of primary electrons and particularly if the emission ratio is greater than one, there is a tendency for a large number of secondary electrons to be liberated in the gap region. Under certain operating conditions, this phenomenon :builds up to ,the

point, where the operating efficiency deteriorates to the point where the tube becomes useless. In another type of high frequency beam device known as a traveling wave tube, the beam interacts with .a conductive slow, wave structure which is often in the form of a spiral. Electrons emitted due to the impact of primary electrons thereon may result in a build-up of secondary electrons which are not in proper phase relation with respect to the high frequency field on the spiral, with a resultant loading of the structure and loss of gain and efficiency. Beam collimating axial magnetic fields are usually used with both types of discharge devices mentioned above and this field tends to collimate the secondary electrons as vwell as the primary electrons so that the problems resulting from secondary electrons are further aggravated in, discharge devices of these types.

, ,I have found that many of the operatingtdiiiiculties I experienced in tubes of this type under what maybe -itermed severe operating conditions with respect to the formation of secondaries, are in fact due to the secondaries and the resultant loading of the associated circuits. I have also found that these difficulties may be avoided by adequately minimizing the productionof sec- .ondary electrons. Severe operating conditions are en- 1 electrons under operating conditions at those points along the beam path where secondaries are most likely to be produced and where due to the combination of fields existing are most apt to build up into a runaway or cumulative phenomenon. Specifically, I .provide a clean States Patent f Pfatented Oct. 4,

metallic titanium surface on those parts defining the beam path which are subjected to substantial high frequency fields during operation of the device.

It is accordingly an important object of this invention to provide an improved electron beam discharge device having improved operating characteristics and exhibiting substantial freedom from the detrimental effects of secondary emission.

It is a further object of my invention to provide an improved electron discharge device of the beam type having those parts along the beam path which are subjected to conditions favorable to the formation of secondary electrons of improved surface characteristics with respect to the emission of secondary electrons.

Further objects and advantages of this invention will become apparent as the following description proceeds,

reference being had to the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. l is an elevational view in -section of an electron discharge device of a type to which, my

.The discharge device is of the three cavity type-including 1 an input cavity 1, an output cavity 2 and an intermediate floating or stabilizing cavity 3. These cavities are spaced 1 alonga beam pathand separated by drift tubes 4 and 5.

' a partof an electron gun structure.

At the input end of the tube is provided a source of electronsin the form of a hemispherical cathode 6 forming The gun structure is supported from the input cavity by means of a beam passage defining tube 7 and is insulated therefromfby means of asuitable cylindrical ceramic insulator 8. The cathode is adapted to be maintained at an emitting temperature by means of a heater element 9, illustrated schematically as having one terminal connected with'the cathode supporting sleeve 10 and the other end joined to a conductor 11 which is sealed through the end of the envelope in insulated relation with respect thereto.

A generally conical collecting electrode 12 is supported from the output cavity 2 at the other end of the discharge device in insulated relation with respect thereto bya seal j construction illustrated generally by the numeral -13 and includingan annular ceramic member 14bonded to thin metal flanges 15. e

The three cavities are in general of similar construction'and are hollow cylinders in shape. Each cavity is (are screwed together.

provided with a'tuning member terminating in 'a ringlike portion 16, movable in the interaction gap of the cavity. The ring is movable from the exterior of the tubeby means of a suitable bellows arrangement'17. The input cavity is provided with a concentric line input connection 18 terminating in an input loop 19. "The output cavity is provided with a Waveguide output 20 communicating with the output cavity 2 through a dielectric window 21. The rigidity of the assembly including .the three cavities is enhanced by means of supportingabolts 22 and secured toflanges on the three .cavitie's, only one of which is illustrated in the drawing. Inorder to facilitate the fabrication and assembly of the tube, it

is made sectional and the drift tubes, drift tube 5, "for example, are made in two parts 5a and 5b which are screwed together and then hermetically sealed by means of flanges 23 which are welded together at their outer edges. These flanges are previously bonded to the drift tube sections 5a and 5b in such position that they come essentially into contact :as these parts of thedrift tube In the operation of the device described above as an amplifier, the electron beam emanates from the cathode and is accelerated by a suitable apertured accelerating anode'lo'ca'ted in a space between thecathode and the beam pathgdefiningtube'l This electrode is not visible in the drawing'but is well understood by those skilled inthe'art; The accelerated beam passes across the gap in the input cavity and is modulated in accordance with :aniinput signal coupled to the cavity'by the coupling loop 19. .The modulated beam then traverses the es sentially field-free drift space defined by the drift tube .5 and enters the floating orstabilizi'ng cavity 3 where'the electron ibunches resulting from the modulation. and

driftspac'ej are accentuated'and thesebunches 'then'pass througha' further drift space defined by the tube 4 :and across thegap 'ofthe. output cavity to excite high frequency'voltages' across that gap andprovide for the supply ofmodulated and amplified waves in the output waveguide 20. v The electrons of the beam are collected ion the inner wall ofthe conical electrode 12. In operation, the cavities, the drift tubes, and also the collector,

sifdesire'd, may all be operated at a. high positive po- *tential with respect to the cathode. It is common practhe to operate the structure at ground potential withthe cathode at a high negative potential. As is well understood, 'an 'a'xial'magnetic field for collimating the electron beam is provided. This field may be produced by suitable permanent magnet or electromagnet means which,

in the interests of simplifying the drawing, havenot been illustrated;

In the operation of devices of this character under certain combinations of high voltage and high power conditions, particularly at high frequencies, there is a tendency for power conversion efliciency to fall ofi seri- "ously'. I have found that actually this decrease in efiiciency is due to loading of the cavity resulting from the emission of secondary electrons particularly from the areas surrounding the gaps in each of the cavities where high frequency fields of high strength exist. When these surfacesare bombarded by the electrons of the forward electron-beam they tend to liberate secondary electrons and 'some of these are liberated in proper phase relation with respect to the high frequency fields to'travel back to the other side of the gap where more secondaries are released. This initiates a cumulative action resulting in a-large number of secondary electrons" improperly phased with respect to the desired function ofthe gap with a resultant loss inefficiency and output of thedevice. As previously mentioned, this condition is aggravated by the presence of the axial magnetic field 1 used for collimating the primary electrons. I have found that this undesirable condition may be eliminated by I coating the parts of the interior of the tube, particularly those regions surrounding the gaps, which are subjected to high fields with a material which not'only has the property of exhibiting a low yield of secondarieswhen inits clean state, but alsoa material which may be readi- 1y maintained in a clean state under operatin'g'conditions in an electric discharge device. I have found that the operation may be materially improved by providing a 1 surface of clean titanium. 7 Titanium has the property of absorbing gases at high temperatures and, in this way tends to keep itself clean by absorbing any gases or oxides which tend to be present on its surface. 7

In Fig. 2 I have shown an enlarged view of a subassembly of the device of Fig. 1, including the input cavity and portion b of theidrift tube.- This isa suitable subassembly of the device :to;subject to a coating treatment in accordance with the present invention. The

- coating with titaniumm'ay to advantage be accomplished I in a vacuum .by'heating a filament of titanium wire posiwtioned in the region betweenthetunerl element 16 and the end -of thetube 3!, :in Otherwords, in 'ther'gap of cavity 1 across the electrons travel. This is the region of high field during operation of the device. The filament may be heated by passage of electric current therethrough to a temperature in the order of 1400-1600" C. and at these temperatures in vacuum evaporation readily takes place and the clean titanium deposits on the parts defining the gap and to a lesser extent on the surrounding parts of the discharge device. The coating process is carried on until at least the parts defining the gap are covered with a continuous layer of titanium. Additional titanium does no harm, it being essential, however, that a rather continuous surface beprovided at 'the gaps. After the coating has been accomplished the tube component is baked out, preferably in vacuum, to clean up the surface and dissolve any oxide that may have been formed. The temperature and time of this heating operation are related and as the temperature is increased the time decreases. For example, the bakeout of 400 C. wouldvrequire around eight hours, whereas. an .800 (3. baking out might be accomplished in a few minutes. Actually a temperature of around 600 C. for a period of 1 or 2 hours is very-desirable. It will be understood that. the interior of the other cavities, in particular the gap} regionsthereof, are treated in a similar manner. Having once been treated in this manner, the tube may .be let down to air since it may be readily cleaned up again as it is evacuated and baked out at a subsequent time.

While I have described my invention as applied to a klystron type of device to which it is particularly suited, it may also be applied to other discharge devices of the high frequency beam type where a large field exists between ratherclosely spaced metal parts. For example, it .may be applied to the adjacent turns of the helix or other slow wave structure of a traveling wave tube. V

It is also possible to make these parts of titanium rather than providing the surface by cladding or coating a base. metal. However, large bodies of titanium are expensive and, in addition, the evaporated coating pro- .vides the bright surface which is desired when applied contaminants. This facilitates obtaining an uncontaminated layer of titanium on the finished tube.

- It should be pointed out that not all materials which vyield a low number of secondaries upon bombardment with primary electrons are suitable for this operation. For example, aluminum, whichwould appear to be well suited for such an application, does not work. Apparently, it :is not commercially possible to obtain an aluminum surface on the parts which remains sufficiently clean metal to exhibit the properties usually attributed to aluminum as far as secondary electrons are concerned. 1 -In-the foregoing description, titanium alone has been mentioned as the surface material employed in accordance with the present invention. It should be noted that zirconium may be substituted in whole or in part for the titanium and that in general these materials are effective in absorbing gases at lower temperatures than the titanium alone.

v While I have shown and described particular embodifIIIGHlSOf-TDY invention, it will be apparent to those skilled the art that changes and modifications may be a made without departing from my invention in its broader aspects and I aim, therefore, to cover in the appended claims all such changes and modifications as come within the truespirit and scope of my invention.

What -I claim as new and desire to secure by Letters Patentof the United States is:

-1. A high frequency electric discharge device comprising means providing an electron beam having a predetermined path, means providing a collimating mag- 7-5:;flfiti6 field along said path, a conducting structure sur- .,;:rounding:the pathofthe beam including spaced parts across which a high frequency field is produced during operation of the device for interaction with the beam, and means for improving the efliciency of said device by minimizing secondary emission electrons from said spaced parts comprising a surface of clean titanium metal on said spaced parts.

2. A high frequency electric discharge device of the beam type including means producing an electron beam having a predetermined path, electrode means positioned along said path including means defining a plurality of interaction gaps and a plurality of field-free drift spaces therebetween, the parts defining said gaps being subjected to high frequency fields during operation of said device and means for minimizing beam loading due to secondary electrons during operation of said device comprising a surface of clean titanium metal on said parts.

3. A high frequency electric discharge device of the beam type including means producing an electron beam having a predetermined path, means providing a collimating magnetic field along said path, electrode means po- 20 sitioned along said path including means defining a References Cited in the file of this patent UNITED STATES PATENTS 2,233,917 De Boer et a1. Mar. 4, 1941 2,263,164 Dailey Nov. 18, 1941 2,409,608 Anderson Oct. 22, 1946 2,491,284 Sears Dec. 13, 1949 2,754,448 Vanlperen July 10, 1956 2,846,609 Espersen Aug. 5, 1958 2,852,715 Rich Sept. 16, 1958 FOREIGN PATENTS 934,919 France Jan. 19, 1948

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