US2608671A - Electron discharge device of the electron velocity modulation type - Google Patents

Description

g- 26, 1952 J. H. FREMLIN ET AL 2,608,671 I ELECTRON DISCHARGE DEVICE OF THE ELECTRON VELOCITY MODULATION TYPE Filed June 19, 1947 2 SHEETS-SHEET 1 FIG. I.

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' IN V EN TOR. JH. FfiEML/N ATTORNEY v m H w MM m ML .m 4 d wu H 2 m mmm m l. 6 HM S ll 2 2 Jimv m Y F B FREMLIN ET AL J. ELECTRON DISCHARGE DEVICE OF THE ELECTRON VELOCITY MODULATION TYPE wil VI Aug. 26, 1952 Filed June 19, 1947 Patented Aug. 26, 1952 'ELE'OTR'ON DISCHARGE DEVICE OFTHE.

ELECTRON TYPE VELOCITY MODULATION JohnnI-Ieaver Fremlin and RogerNorman Hall,- ...London,.. England, assignors to International Standard Electric Corporation, New York,.N..Y.

-ApplicationJ une'19, 1947,: Serial-No. 755,550 .Jn Great Britain February. 8, .1946

'Sectmn 15 Public Law ugust 8,119.46" "Patent expires February 8, 19 5 6: Claims.

.eratl'on of hyper-frequency electromagnetic waves;

It isr'an' object ofthis-invention' 'to provide meanswhereby'thewesonant frequency of a sys- -.tem including an electromagnetic resonator may lie:made'substantially independent of temperaturechanges.

:Ii? i'saiurther object" of the invention to providemeans"whereby the operating frequency of "an'elect'ron discharge device which includesan electromagnetic resonator as part of the constructionthereof may be made substantially independent-hiltemperature variations.

"Yetanother object of the invention is to provide meanswherebythe operating frequency of such electrondis'charge device may be controlled by deliberately altering the temperature of part or thewho'lenfthestructure of the same preferably b'y*electronio means incorporated within the device;

=Iir order to'show'the general nature of the problem and ofits-solntion we may consider a i particular example. *In'Figure i there is illustr'ateddiagrammatically'a length of coaxial line 'short circuited 'at one end and'havingan internal projection I on the outer conductor 2 so as asflormin conjunction with'thepentral cond l1CtO1' 3"af small condenser shunting the line. Inrpracticeit is' common for such condenser to "comprise 'a screw threaded'into'the outer wall.

thelsurgelimpedance. ofl-theli'ne, It the length of the line, 22 the wave propagation velocity therein .the pulsatance, all. quantities being taken at-isomet-arbtitrary. temperature Tb. If new the -quencyof thecombination 'of line and condenser 27 p temperature changes by an infinitesimal 1 amount gT there will result a frequency shift determined in which M is the wavelength correspondingto Now the capacity Co is proportional to the area of the opposing plates of the condenser'andim versely proportional'to'their separation; Winne -Z6 is proportional tolog w m and n 'being the radii of outer and inner'c'onducto'r respectively. Hence, itall conductors 'df the resonator system are of'the same'materiai arid thesystem isheated uniformly we=shall have l i .ldC aCcdT l l a beingtthemoefflcient of. linear expanslomor material. Hence in this case raw 7 -==-adT 3) .lEquationt is quite general whatevertheshape of the resonator. ..From Equation 2 it isse'en that it'would bepos sible to compensatefor thermal effects if dCOrdZt could be made negative.

If now the central conductonwere to be=made of material having a smaller coeflicie'ntof expansionthan that of the material or the-outer conductor'the separation of the plates of the condenser would be greater than" would-be "the case fora resonator system composed throughout of the same materiaha'nd'by the suitablechoiceof these'materials'and' ofthe dimensions of its'parts it is possible to arrange that dC shall be negative and thatthe numerator on theright-han'd sideo'f Equationz be zero. 'Inthis case; however, 20 would increase.

An alternative method would beto keep'the materials of inner and outer conductor the same but to arrange that a part 'ofthe-projection l (Figure 1) be of material having 'a smaller"coefiicient of expansion, so'tha't the separation' of thec'ondenser plates'results ina decrease-oi construction of said resonator and suitably proportioning the dimensions thereof.

In electron discharge devices for the generation of very high frequencies the construction usually includes an electromagnetic resonator within the tube envelope.

It is a common feature of most such devices that there is provided an electro magneticresonator system in which an electro-magnetic field is setup, and an electron beam which passes through, or across one or more gaps of the resonator system, and interacts with the electromagnetic field. The operating frequency approximates to the natural fre uency of the resonator system considered purely as an electromagnetic resonator, while the departure of'the operating frequency from the natural resonator frequency is dependent in general upon three features: external circuit loading, reactive loading due to interaction between the electro-maenetic field and the electron beam and a capacitative load ng due to the c n or gaps across which the electronbeam is projected.

During one-ration energy from the electrode polarising sup lies is inevitably dissipated as heat and the structure ofthe device will normally be subject to appreciable temperature rise resulting in chan e of dimensions and conse uent frequency drift. For example, if a copper resonant circuit of. any shape designed for use at a frequency of 3,000 mc./s. were to, suffer a temperature chan e of 100 C., a fre uency change of the'order of 5 mc./s. would result. Fractional changes in ower in ut to the system will lead to corresponding fractional changes of frequency which may well be serious if the oscillator is being used, for exam le, as beating oscillator in a narrow-band receiver."

The effect of temperature rise is to cause a change in the dimensions of the resonator as such, thereby tending to increase the wave length, but the change of gap dimensions is such as to increase the separation between opposite faces with a consequent tendency to reduce the capacitative loading, although this is usually more than oifset by the increase of the effective area of the plates of the condenser formed by the gap.

If now means could be devised for increasing the gap dimensions in such manner that the reduction of capacity thereby more than offsets the increase of capacity due to increase of surface area, together with increase in the natural wave-length of the resonator system as such, it would be possible not only to offset any frequency drift: due to temperature rise, but also by controlling the temperature to change the operating frequency at Will. Such a solution may be obtained by utilising materials of different rates of thermal expansion in the construction of the electron discharge device.

In accordance with this aspect of the invention there is provided an electron discharge device having means for projecting a beam of electrons through an eleotro-magnetic resonator system associated with said field in which materials having difierent coefficients of thermal expansion are used in the construction of said resonator system so that the relative expansion of parts of said resonator system are so controlled as to maintain the operating frequency substantially constant in spite of thermal variations in said system.

According to another aspect of this invention there is provided an electron discharge device, having means for projecting a beam of electrons through an electromagnetic field, thereby exciting an electromagnetic resonator system associated with said field, in which materials having diiferent coefficients of thermal expansion are used 'in the construction of said resonator system so that the relative expansions of parts of .said resonating system are so controlled as to vary the operating frequency in sympathy with the potential difference between electrodes controlling a beam of electrons which heats one or Figure 3.

Figures 5and 6 are similar to FiguresB and 4 respectively, but show preferred means for achieving one of the objects of this invention While Figure 7, corresponding to Figures 3 and 5, shows alternative means for carrying out this object.

Figure 8 is an elevational view in part section of a high frequency electron discharge device in accordance with the invention in which. thermal frequency drift has been eliminated.

Figure 9 is a section through the line 9-Q-9 of Figure 8. i a v Figures 10 and 11 are diagrammatic views of vertical and horizontal sections respectively of a device according to the invention in which pre determined change of frequency is accomplished by utilisation of the thermal effect. I

Referring now to Figures 3 and 4 which are representativeof the coaxial type of resonator systems, 4 is the outer conductor carrying fins 5 which extend inwardly to reduce high frequency leakage; 6 is the inner conductor carry.- ing fins which enclose the oscillator .drift? space and 8 is an antenna which can be coupled to an external and tunable cavity circuit-of-the well known wave-guide type.

When such a system is heated by the electron beam used in operation, expansion of all the parts Will take place. If this expansion were uniform the proportional extension of the gaps 9 would be equal to that of the length and diameter of the whole structure and, by the laws of similarity, the relative increase of wavelength would be numerically the same. It will be noted, however, that the extension of the gaps will reduce the capacity loading introduced by the fins and hence reduce. the wavelengthwere it not more than balanced by the effector the iri crease of both fin thicknesses and fin length along the axis of the tube.

If, then, the relative expansion of the gaps could be increased, the increase of wavelength spasms consequent spun-teasgiant ss reduced or eliminated.

In Figures 5 and 6 is illustrated;aomeansby which this..may.be done... .Aislat -flo, is cut in each fin "1' along thelength of the centre line to a suiiicient length fora small degree 'ofrelativ'e movement-ofthe two sides-flto he -possible without the introduction 511 excessive for'ces. "The two *sides -are thenz joined in the middle "by' strips: H of material, for: example tu ngsten, of lower coefficientiofs expansionrthan the rest. It can be readily seen that the gaps 9 will then i a m rapidly "t a o ns-11in to the-increase of' length-entire system andthis will lead to compensation of the tendency for the frequency to change if the dimensions of the system are properly chosen.

The possibility of complete elimination of the frequency change by the system shown in Figure depends on the relative dimensions along the direction of electron flow of the gaps 9 and the fins 5, the proportional change of the gaps 9 of course being greatest when their original length is least.

If the compensation obtainable is inadequate, owing to the fact that the lengths of the gaps are large, the modification shown in Figure 7 may be used. Here the low-expansion-coeflicient strips l2 are arranged diagonally so that when the centre conductor expands longitudinally, by a form of lazy tongs effect, the effective width of the inner conductor may actually be reduced by heating.

In either case, when the expansion coefficients of the materials available are known, it is easy to calculate the correct lengths and points for fixing of the strips H of Figure 5 or 12 of Figure '7.

It may further be noted that the slot ID of Figure 5 does not harm the operation of the oscillator as the lines of current flow in a coaxial system will be parallel to this slot.

In Figure 3 is illustrated a practical assembly of a velocity modulation device of the above described type. The details already mentioned are indicated by like reference numerals. The complete device comprises a glass envelope suitably based with pins [3 for connection to external supply circuits. I 4 is an annular metallic disc bonded to the envelope l5 and to a glass can l6 surrounding the antenna 8 and provided with an exhaust tabulation IT. The resonator tube 4 is fixed to this annular disc I, which also provides a convenient means for mounting the assembled tube onto the wall of a wave guide so that the antenna 8 projects inside the cavity. The remaining electrodes are mounted on support wires l8 which connect with the pins 13. In addition to the resonator system already described, the electrode system comprises an indirectly heated cathode l9 behind which is mounted a screen 20; control grid 2| and screen grid 22 control and screen grids assist in focussing a ribbon-shaped beam of electrons which is projected through the resonator system onto an anode 23 at the far end of the gap system. The primary electron focussing means is a magnetic field provided by suitably placed field coils external to the tube structure.

An electron discharge device as described above would be compensated for temperature changes. It would not be a matter of great difficulty to provide such a device with a second controlled electron beam designed to heat the resonator system either directly or indirectly by means of electron bombardment. The cathode could wen be'v I enlarged] and T independent: focussing means and control electrcdesi -added to.1iomnitlre said secondLbeamL 'Tlre -temperaturew'of tm resonator: system, and by proper proportioning 'of the parts thereof, hence the oscillatioirfrequency;

could be adjusted by variation :ofipolarisidgpotentialsapplie'd' to the i control electrodes. of :said second electron beam. The further modification 'about'to be described; nowevenisrtorbe preferred, as thereby 1 a greater" range. of. frequency ivariaetion can be obtained for the same change ofisaiid' polarisingpotentials- In Figureslilandrlltpartaof a modified form of resonator system is illustrated diagrammatically. Instead of a slit in the central conductor, the drift tube 24 is made in two halves, and instead of the central conductor 4' (Figure 8) of the lower part of the coaxial line resonator system, we have two separate supports 25 performing the same electrical function. The two portions 24 of the drift tube are arranged to overlap, leaving small gaps 26 between them. At the very high frequencies for which these tubes are designed the capacity across the gaps 26 is sufficiently large to present negligible impedance to alternating current at the operating frequency. Furthermore, the characteristic impedance of a resonator system in which the central conductor takes the form of a pair of ribbon shaped members differs but little from that of the conventional coaxial line, so that the electrical design of the resonator system is not fundamentally altered by the modification. Each of the supports 25 is constructed of two lengths of bimetallic strips end-to-end, but reversed in their direction of bend. When the bimetallic strips are heated the two halves bend in opposite directions so that the associated half of the drift tube does not depart sensibly from the vertical but is moved bodily towards the wall of the resonator tube thus decreasing the gap 23. The bimetallic supports are arranged to be heated by a second electron beam in the manner previously described. Arrangements for this are not shown in the drawings, but in one particular application it is arranged that the support nearer the cathode is bombarded by a portion of said second beam and has holes therein arranged to allow a further portion of this beam to impinge on the other bimetallic support. In this manner the temperature of the bimetallic strips, and hence the working gap dimensions and the oscillation frequency of the device, is controlled in sympathy with the potential variation applied to control electrodes of the aforementioned second electron beam.

What is claimed is:

1. In a cavity resonator for a velocity modulation tube, a pair of conductors disposed in coaxial relation to form a resonant space, said conductors having aligned apertures therethrough to provide a path for flow of electrons, the portions of said conductors defining said apertures forming narrow gaps between said path and said resonant space, the inner one of said conductors having two relatively movable parts to control the width of said gaps, and means connected to said parts having a given coefiicient of expansion different from the ooefiicient of expansion of the material of said parts to control in response to changes in temperature the relative movement of said parts.

2. In a cavity resonator according to claim 1, wherein said means includes an element having said coefiicient of expansion interconnecting said parts.

3. In a cavity resonator according to claim 1, wherein said means includes a pair of elements wherein said means includes a bimetallic ele- 10 2,424,805

inent.

JOHN HEAVER FREMLIN. ROGER NORMAN HALL.

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

UNITED STATES PATENTS Number Name Date 1,884,591 Davis Oct. 25, 1932 2,374,810 Fremlin May 1, 1945 2,413,364 McCarthy Dec. 31, 1946 DeWalt July 29, 1947 FOREIGN PATENTS Number Country Date Great Britain Jan. 21, 1935

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