US2240183A - Electric discharge device - Google Patents

Description

April 29,11. w. "C, HAHN 2,240,183

msm-arc nIscHAReE vsvrcs Filed nec. 1, 193s 2 sheets-sheet 1 Inventor: William C. Hahn His Attorney.

Sass .MME m v2 Sheets-Sheet 2 w. C. HAHN Filed Dec. 1, 19:58V

Pfg. 6.

7/ 'eff ELECTRIC DISCHARGE DEVICE |||4 le .ff m 1M, C iw lo d mm 0 ,td N n /u/u .m sd Lm, .f m o 4 w w E-- -Is m 4, .Q /a P rl Ve n De 0 Il Ii e @n i l m 70 f -Il -lib U \\\\\\m\\\\\\\.m /W ||Il.| ...I :Id Rv. M6 1M e .2M ,wib ,a 7 0 a M B M A 0 m s y c la w y B w e 0 0 l0 A Po COEDUO s -la E 0 0 :O Lmz April 29, 1941.

/(ifa/dea/ V/aa/.' 2 C h/Vaa/a/qf/oa Inventor: WHiamC. Hahn by His Attorney.

atentecl Apr. 29, 1941 'I'he present invention relates to improvements in electric discharge devices and to improved macro nrs William O. Hahn, Scotia, N.

Y.'l to GenralkElectric Company, a corporation oi New or f application December i, 193s, sensi Nb. :caser (Cl. E50-27.)

methods of operating such devices. While not limited thereto, the invention is particularly applicable in connection with ultra-short-wave devices of the general type disclosed and claimed in my copending application Serial No. 153,602 led July 14,1937, Patent No. 2,220,839, dated November.5, 1940 and assigned to the General Electric Co.

Discharge devices of the type to which this invention refers involve the use of means i'or producing an electron beam and means for collecting the beam after it has performed the functions desired of it. Ordinarily the amount of electron current which can be handled by such a device is limited by heating of the collecting electrode. Such heating is a result of the mechanical impact of the individual electrons on the electrode surface and may be quite great, even for a small beam current, if the average beam velocity is high. A

It is one object of the present invention to provide means and a method of operation whereby the heating of theeollecting electrode may-be kept at a low value even where very high cur-v rent densities are employed in the electron beam. 'I'his is accomplished by utilizing an arrangement such that all the beam components maybe received at a relatively low velocity at the collecting surface irrespective of the velocities which may occur in other parts of the beam. The features which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference vto the'following description taken in connection with the drawings in which Fig. 1 illustrates in longitudinal section a complete electronic discharge device suitably embodying my invention; Figs. 2 and 3 are imaginative representations useful in explaining the invention; Figs. 4 and 5 are graphs also of explanatory character,$and Fig. 6 shows an alternative application of the invention.

Referring particularly to Fig. l I have shown a tube of the cathode ray type which comprises an evacuated envelope having an elongated shaft portion It and an enlarged anode containing portion Il. This envelope may be suitably constituted of glass, quartz, or any equivalent insulating material.

The shaft portion IB encloses means, such as a known type of electron gun, for producing an electron beam. The combination shown combiased to a suitable prises a thermionlc cathode Hl which is indicated in dotted outline and a focusing cylinder l5 for co the electrons to a concentrated beam. A

This cylinder may be either connected directly to the cathode or maintained at a few volts negative or positive with respect to it. In order to accelerate the electrons to a desired extent there is provided an accelerating electrode I8 which is spaced from the cathode and which may be positive potential, say, several hundred volts.

In order that the intermediate portion of the beam path may be maintained at a desired potential level there are provided a number of intermediate electrodes I8 which suitably comprise rings of conductingA material applied to the inner wall surface of the envelope. They are provided with lead-in connections la and with external contact-making terminals 2Q. A. number of magnetic focusing coils (not shown) may be placed valong the envelope to prevent dispersion of the electrons and to maintain the beam in focus during its passage through the discharge space. In

some cases electrostatic beam-focusing means may advantageously be used.

After traversing the envelope the electron beam is collected by an anode 23 which consists of graphite or other suitable material. electrode 2li in the nature of a suppressorl grid serves to prevent secondary electrons emitted by the anode from returning to the discharge space. This electrode has also another function which will be described more fully hereinafter.

In the operation of the device the intermediate electrodes i8 may be maintained at ground potential, the cathode ifi at 'one thousand to several thousand volts below ground land the anode 23 at one thousand to several thousand volts above the cathode. 'I'he suppressor grid 2t may be biased iifty to several hundred volts negative with respect to the anode 23. These potential relationships may be established, `for examplaby means of a battery 2i connected as shown.

The combination of elements so far described comprises means for producing a unidirectional electron beam of 'substantially constant average intensity and velocity. As pointed out in my aforementioned application Serial No. 153,602, an electron beam of this type may be modulated either as to "electron velocity or as to charge density. The nrst type of modulation involves the production of. systematic irregularities in electron velocity from point to point along the beam. The second involves the production of charge density variations, such variations being A tubular in my aforementioned application Serial No..

153,602 and which need not be elaborated here it is desirable that such modulation be accomplished without the simultaneous production of substantial charge density variations. This may be done by the use of a modulating space which is so shielded from the cathode that variations of the modulating potential are prevented from reacting directly on the cathode emission. Although numerous velocity modulating structures are'shown in my prior applicatiomonly one is illustrated in the present case.

This comprises a modulating chamber 30 formed by a grounded conducting structure which is outside the discharge envelope. It is provided with transversely extending wall portions 3i and 3l which extend relatively close to the outer surface of the envelope and which serve to fix the potential level of theboundaries of the modulating space. Within this space there is provided a modulating electrode comprising a conducting tube 33 which surrounds the beam path so as to be coupled to the beam. By alternately raising and lowering the potential of this electrode with respect to the boundaries of the modulating space (as by applying a high frequency input potential to the electrode), variable potential gradients are produced which act longitudinally on the electron beam as it traverses the approach spaces between the wall members 3l and 3i and the extremities of the electrode '33. v

The modulating effect thus produced will be most pronounced if the length of the tubular electrode 33 is so -correlated to the velocity of the beam that the electron transit time therethrough corresponds at least approximately to a half cycle of the control potential or to an odd number of such half cycles. If this condition is fulfilled, an electrn which enters the modulating space when the potential of the control electrode 33 is a maximumwill be accelerated first by the gradients existing between wall 3| and the electrode and again as it leaves the electrode one half-cycle later while the electrode potential is at a minimum with respect to the boundary wall 3|. Similarly an electron which enters the modulating space in'such time phase as to be retarded as it approaches the control electrode, will also be retarded as it. leavesthe electrode. As a result of these alternate accelerating and retarding effects, the electron beam leaving the chamber 30 will be made up of alternate elements, some of which have a velocity ab'ove the average of the beam and others a velocity below such average.

Modulating potential may be supplied to the controlf electrode 33 from any desired input source such, for example, as a high frequency oscillation generator (not shown).l As a means for connecting the exciting source to the control electrode structure, there is provided a concentric transmission line comprising an inner conductor 35 and an outer conductor 36, these being shown partly broken away.

If only weak control potentials are available,

the velocity modulation produced by the control electrode may be relatively slight. However, it maybe converted into charge density modulation of a higher order ofl magnitude by means now to be described. The mechanism by which such conversion may be accomplished will best be understood by a consideration of the following explanation:

In Fig. 2'the beam is shown as it is assumed to issue from the modulating chamber 30. It will be seen that at this point it comprises alternate groups of fast and slow electrons, the former being indicated by black dots a and the latter by light dots b. So far, the beam is still substantially uniform as far as chargevdensity or electron grouping is concerned.

In Fig. 3 the condition of the same beam is indicated at a somewhat later time when the `more rapidly moving electrons have caught up with the slower electrons. The electrons have now become grouped so that the beam is charge density modulated in the sense that systematic irregularities in charge density occur from point to point along the beam. The change that has taken place is in its very nature one that requires only the elapse of time and the absence of extraneous influences which might tend adversely to effect conditions within the beam. These requirements may be fulfilled by the provision of an electrostatically shielded drift space in which sorting of the electrons can take place. This may comprise, for example, simply a long section of the discharge envelope which is shielded from any but static potentials.

From the foregoing it might seem that with l an appropriate length of drift space even the slightest amount of velocity modulation could be converted into 100% charge density modulation, or, in other words, that the maximum obtainable charge density modulation is independent of the velocity modulation. That this is not the case is due primarily to the action of space charge, (that is, of the mutual repulsion of electrons) in opposing electron grouping which is characteristic of a charge density modulated beam. What actually takes place may best be understood by comparing the electron beam to an elongated tube of a highly elastic solid material such as rubber.

In this connection. let it be assumed that such a tube is being moved longitudinally through space so as to simulate an electron beam having constant average velocity. Ii.' a momentary retarding force is applied to one end of the tube anda momentary accelerating force to the other end, a process of compression is initiated. Although the average velocity of the tube as a whole may not be affected, the forces in question start process the tube as a whole is continuing its moa relative motion of certain elements of the tube from its ends toward its center. After a certain time this motion ceases as a result of compression of the intermediate region of the tube.

After the maximum compression is reached, the elasticity of the medium produces a restorative motion of the displaced elements toward the ends of the tube. Here again compression and cessation of such-motion occurs and the whole process is repeated. If the medium in question is perfectly elastic, an indefinite number of repetitions are possible, periods of maximum compression being alternated with periods of maximum mobility. It will be understood that during this tion through space.

This is considered t'o be the sort of thing which happens in an electron beam which en subjected to velocity modulation. With the passage of time (that is, with the Be of the velocity modulated beam through space), the action ofthe faster electrons in overtaking the slower ones. produces compressions or localized increases in electron density. As soon as lmaximum charge density is attained, i. e., as soon as the mutually repulsive forces o the electrons become suiiicient to prevent their further compression, electron dispersion is initiated. This, in turn, overshoots. so to speak, thus producing further compressions, and so on. The varia tions of compression which occur at any given point along the beam path may be referred to as the charge density modulation of the beam at such point, while the variations of velocity comprise its vvelocity modulation.

Taking into consideration the space. charge factor, it may be shown that the distance (drift space) required to be traversed by the beam for conversion of the initial velocity modulation into the maximum possible charge density modula tion is substantially independent o the magnitude of the velocity modulation at least ior relatively small values of the latter. It is, however, a iunctio'nof the beam velocity, the average charge density of the beam, the modulating fre'- quency, the diameter of the beam', the diameter of metal and glass parts surrounding the beam, and the magnitude of any external electrostatic or magnetic force acting on the beam. An approximate formula which has proven useful in determining the proper length of the drift space is as follows:

LM=9.O2 lO-5a%x Where a LM represents the maximum length 'ci the drift space in centimeters (measured from center to center of the grids at each end ci the space). represents the in volts. represents the beam current in milliamperes. represents the wave length oi the applied signal in centimeters (in vacuum). is a constant whose value is determined by the dimensions of the envelope and electrode parts. For most cases it will fall between 1.0 and 2.0 and may be assigned an average value of 1.3.

4average velocity of the beam Once the physical length o f the drift space has been ixed by utilization of this formula, a nal optimum adjustment may be obtained by varying the beam velocity from the value used in the computation. The best adjustment may be determined objectively by continuously varying the potential applied between the cathode and the intermediate electrodes 2i until maximum output is obtained from the discharge devicey as a whole. (The signicance of this last statement will be better understood when the remaining elements of the device have been described.)

Referring again to the particular structure of Fig. l, it is to be considered that the drift mace of the illustrated device is roughly co-extensive with the tubular conducting section 3S which extends 'from the boundary wsu el'. rt is fin-tuer assumed that the right-hand extremity of the drift space marks a point of maximum charge density modulation of the beam for the intended condition of operation of the discharge device. Since the magnitude of the charge density modulation may be appreciably in excess oi that of the velocity modulation produced by the input electrode it is clear that an ampliiication eiect can be obtained through the electron-sorting" edect o! the drift space.

In order that this result may be effectively utilized, a third electrode d2, appropriately designated an output electrode, is coupled to the portion of the beam issuing from the drift space. This electrode is enclosed within a chamber d3 similar to the modulating chamber previously described. It comprises end walls de and ed' which approach relatively close to the outer wall oi the envelope and which are'adapted to cooperate with the extremities of the tubular electrode d2 to produce potential gradients in the spaces between them.

In my application Serial No. 153,602, previously referred to, it has been shown that an electrode such as the electrode i2 may be used to abstract energy from a modulated beam traversing it.

s is a result of the fact that the charge density variations in the beam will induce similarly varying conduction currents in the electrode i2 andthe circuit elements associated therewith.

This ee'ct is' most pronounced when the length of the tubular electrode #i2 corresponds approximately to the spacing between adjacent charge density maxima and minima in the beam. Under these conditions the approach of a chargedensity maximum will coincide with the recession of a charge density minimum and a resultant current pulse will be produced in the electrode. Another current pulse of opposite sign will be induced one-half cycle later when the approach of a charge density minimum coincides with the recession of a charge density maximum.

The electrode lZmay be coupled to a suitable output circuit (not shown) by means of a cone centric transmission line comprising conductors di and d so that these current pulses will be transformed into voltage phenomena in such circuit. The characteristics of the transmission line di, it should preferably be matched 'to those l oi the output circuit and of the electrode system.

.after the beam has passed through the tubular electrode i2 it contains components of charge density modulationv and velocity modulation which vary in relative magnitude from point to point along the beam. lf the beam is collected at a point at which appreciable velocity modulation exists, the collecting element must be biased to a potential at which even the lower velocity beam components are able to reach the surface of the element; Under these conditions the higher velocity components of the beam will `be moving at considerable speed when they strike the collecting surface, and will cause substantial heating thereof. Not only does the heat thus generated represent a power loss and therefore a diminution of the efiiciency of the system but it also presents a considerable problem as to its dissipation. In order to take care of this latter factor the volume of the collecting electrode must be relatively large and the total beam current must be limited to a relatively low value unless articial cooling is to be employed.

In order to overcome this dimculty and to I avoid excessive heating of the collecting element I propose to collect the beam at a point at which the received electrons are moving with substantially uniform velocity. yIf this condition isy fuliilled, the collecting member may be biased to such a potential that substantially all the electrons are collected with approximately zero velocity as to produce no heating of the memto locate such a point and to place the collect ing element at it. y

The significance of the foregoing statement may be better understood by reference to the graphical representation of Fig. 4. in which the curves A and B show respectively the variations of velocity modulation and of charge density modulation from point to point along the tube. It will be seen that at a (that is, at the right- Vhand boundary of the modulating space 30) the velocity modulation. (A) is at a maximum and the charge density maximum (B) is substantially zero. However, as one proceeds along the drift spaced provided within the conducting tube 39,

In order to vary the tuning of the transmission line there is provided a movable device comprising a hanged disk i3 which is interntted within the tubular conductor 6I so as to be axially slidable therein. A second disk 64 slidably engages the conductor SII and provides a capacitive coupling between that conductor and the tubular member Il. This coupling is substantially a short circuit as far as high frequency currents are concerned but is open circuited with respect to direct currents by virtue of the interposition of an insulating member 56. A similar connection is provided between the tubular conductor Si and the anode lead-in conductor 51 by means of a disk 58 which slidably engages y the conductor 5l. 'I'he disk assembly may be the charge density increases and the velocity f A lecting element to be placed at the optimum point. 'In order to avoid this difilculty it is possible to shorten the drift space and to place the output electrode 42 at a point d where the charge density modulation is something less than a maximum. This will decrease the possible amplication but will still yield a satisfactory output. 'I'he collecting electrode may then be placed at a point e, slightly displaced from b,

where the velocity modulation is low, although not precisely a minimum.

It is possible, however, and it constitutes a further aspect of my invention that the point of occurrence of a velocity modulation minimum may be varied arbitrarily so as to coincide with the position of the collecting element. This may b e done in one way by exerting a demodulating" effect on the electron beam after its passage through the output electrode system. One structureV for accomplishing this result is shown in Fig. l.

This comprises a variable oscillatory circuit connected to the suppressor grid 24. As illustrated, this circuit comprises the combination of a lead-in connection 50 connected to the suppressor grid and a tubular conductor 5I surrounding the suppressor grid and the conductor 50. These elements in combination comprise a transmission line which may be tuned to be resonant at a desired frequency. In view of the form of the said line it will ordinarily be resonant when its length approximates that of a.v

quarter wave (or an odd number of quarter waves) at the frequency involved.

` trode structures.

reached -the eil'ect at the collecting electrode of moved as desired by means of a handle 59 which projects from the end of the tubular member 5I.

The action of the charge density modulated beam in approaching the tubular electrode 24 is to induce pulses of conduction current therein and to excite the transmission line connected to the electrode. Such excitation may result in the establishment of a continuously variable potential gradient between 'the electrode 24 and the juxtaposed conducting wall 5i, which gradient acts to modify-the velocity modulation of the beam. By adjusting the position of the slid-able disk 53 one may vary the phase relation between this additional velocity modulation and the initial modulation produced b y the preceding elec- When a proper adjustment is this additional modulation may be such as precisely to offset the initial modulation. This phenomenon is explained by the graphical representation of Fig. 5 wherein thecurves A and B are similar to those shown in Fig. 4, and the curve C indicates the spatial variation of the velocity modulation produced in the region g (Fig. l) by the electrode 24 when its associated circuit is properly adjusted. The curve D indicates the result and modulation as determined by.the difference between 4A and C. It will be seen Ithat at the point h, which corresponds to the surface of the electrode 23, the additional modulation C is precisely'equal and opposite to the initial modulation indicated by the curve A. Consequently at this point Ithe velocity of the beam is uniform and is invariable with time. It is, therefore, possible to bias the electrode 23 to such a potential that it is capable of collecting the entire beam with zero or very low velocity. As previously explained, this results in correspondingly low heating.

The effect of the presence of the additional lead-in conductor 51 is to modify somewhat the action of the electrode 24. However, this factor K merely changes the adjustment of the member 53 for which the desired results will be obtained. It does not prevent its occurrence.

Since the result described in the foregoing is accomplished without any appreciable absorption of power by the modulating system it is apparent that the reduced heating of the collecting element increases the overall elciency of Ithe system. Furthermore it may serve greatly to increase the amount of electron current which may be employed in the beam without producing excessive heating of the collecting electrode.

Jn Fig. 6 I have shown a modification of the invention in which the demodulating effectv is obtained by an electrode system which does not involve the suppressor electrode 24. In this case a demodulatingv chamber 65 is provided which fllows immediately after the energy-abstracting electrode system comprising the chamber 62 and the electrode 63. but is entirely separate .therefrom. This chamber contains a tubular electrode S-which is connected to a resonant oscillatory system provided, for example, by concentric quarter-wave conductors 68 and 69.

The portion of the electron beam which issues from the energy-abstracting chamber 62 is still highly charge density modulated. It is, therefore, effective to induce substantial currents in the tubular electrode 66 and. in its associated -resonant circuit. These currents will in turn produce voltage variations in the circuit' connected to the electrode 66 and corresponding potential gradients between the extremities of the electrode S6 and the adjacent boundary walls of the chamber 65. By this means additional velocity modulation will be produced in the beam, such modulation being adjustable as to phase and magnitude by a disk-like member 1| which is adapted to be moved toward and away from the tubular electrode S6. 'Ihe position of this member controls the capacitive end loading imposed on Ithe transmission line formed by the conductors 68 and 69, and thus serves to modify its state of resonance. The additional modulation produced within the chamber 65 may be made to oset the initial modulation of the beam in such away as to produce a state of uniform electron velocity at the collecting surface 'of the electrode 23. 'I'his is brought about in the same manner as was explained in connection with Fig. 1.

While I have described my invention in con; neetion with panticular structural embodiments it will be understood. that many modifications may be made by those skilled in the art' without departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent modications as fall within the true spirit and scope of the foregoing disclosure.

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

tioned to receive the beam at low velocity whereby heating of the collectingelement is substantially avoided.

l. The method which comprises producing an electron beam, effecting initial .velocity modulation of the beam, abstracting power from the beam at the modulation frequency, thereafter` producing a secondary velocity modulation of the beam so related to the initial modulation as substantially to offset the effects thereof and to provide a region of relatively uniform electron velocity at some point along the beam, and nally collecting the beam at such point.

2. In a discharge device, the combination which includes means for producing a stream of moving charges, means for producing velocity variations of the stream, means dependent on the existence of such variations in the stream for abstracting energy therefrom, an electrode for collecting the stream after such abstraction of .energy therefrom and means interposed between the energy abstracting means and the collecting electrode for minimizing the velocity variations in the stream at the point of its impingement on the said electrode.

3. In a discharge device, the 'combination which includes'means for producing an electron beam, means for modulating the beam at high frequency' to produce velocity variations therein, means for abstracting power from the beam at the modulating frequency, means acting subsequently on the beam materially to reduce the velocity variations therein at a particular region ofthe beam, and means for collecting the beam at such region, said last-named means being electrically condilated beam and in which successive conversions between states of maximum velocity modulation and of maximum charge density modulation can occur, means associated with the said space and coupled to the beam at a region of maximum charge density modulation for abstracting power therefrom, and means for thereafter collecting the beam at a region of minimum velocity modulation, said last-named means being. electrically conditioned lto receive the beam at low velocity whereby excessive heating of the collecting element is avoided.

5. A discharge device including means for producing an electron beam, means for velocity modulating the beam at high frequency, means .providing a drift space to be traversed by the electrode vand excitable by charge density variai tions in the beam to produce additional velocity beam modulation thereof, said last-introduced means being effective to assure a condition of substantially uniform velocity of the beam at the region of its' impingement on the collecting electrode.

6. A. discharge device including means for producing an electron beam, means for velocity modulating the beam at high frequency, means providing a drift space to be traversed by the beam after velocity modulation thereof, said space being of sufficient length to permit effective conversion of the velocity modulation of the beam into charge density modulation, energyabstracting means coupled to the portion of the beam issuing from the drift space for supplying power from the beam to a high frequency utilization circuit, an electrode for collecting the beam after its passage through the energy-abstracting means, and circuit means coupled to a portion of the beam between the energy-abstracting means and the collecting electrode for producing additional velocity modulation of the beam, said circult means being adjustable to provide a relationship between the said additional velocity modulation of the beam and the initial modulation thereof such that a region of minimum electron velocity variation exists at the collecting electrode.

7. A high frequency discharge device including means for producing an electron beam, means for velocity modulating the beam at high frequency, energy-abstracting means for supplying power from the modulated beam to a high frequency' utilization circuit, an electrode for collecting the alfer its passage through the energyabstract g means, and means for assuring a condition of substantially uniform electron velocity at the collecting electrode, said last-named means including a combination of conductive members traversed by the beam and circuit ele- 'beam after its passage. the energy-abments associated with sala manners for prducing additional velocity modulation of the beam by mutual reaction therewith. the said additional modulation being in opposition to the initial f modulation of the beam so as to exert a velocity equalizinz eilect thereon.

8. A high frequency discharge device including means for producing an electron beam,v means for velocity modulatinz the beam at high frequency. energy-obstructing mean; for m9171118 power fromthe modulated beam to a hiel: frequency utilization circuit. an electrode for collecting the stracting means. and means-tor assuring a condition of substantially uniform electron velocity at thecollectinz electrode, said last-named means comprising;r an oscillatory circuit coupled to the beam and eiiective by reaction therewith to produce additional velocity modulation thereof, theI said additional modulation being ln opposition to theinitinlmodulationotthebecmlontoexert avelocityequalizingeitectthereon.'

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