US2400976A - Resonator - Google Patents

Description

a. cAssEN RESONATOR origial Filed April' 19, 1941` May 28, 1946.

tux@ INVENTOR Benea//c 6455er).

ATTORNEY two electrodes.

Patented May 28, 1946 ansoNA'roa Benedict Cassen,

Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pitts? burgh, Pa.,

a corporation of Pennsylvania Original application April 19, 1941, Serial No.

389,351. I ary 12, 1942, Serial Divided an d this application Febru- No. 430,601

3 Claims. (Cl. 178--44) Mylinvention relates to electric'discharg'e ap-l paratusand has particular relation to high voltage discharge tubes, such as generators of hard X-rays.v V

This application is a division of my copending application Serial No. 389,351, led April 19, 1941, now Patent No. 2,342,789, Feb. 29, l1944, and assigned to the Westinghouse Electric 8i Manufacturing Company.

In producing -X-rays having a wave length less than .25 Angstrom, a'potential greater than 50,000 volts is impressed between the cathode and the anode. If thel potential is impressed directly from an alternating vcurrent source or from a battery of accumulators, the usual high voltage problems arise. Either the cathode or the anode is at a high potential relative to ground,

and elaborate insulation is required to prevent arcing between the parts of the system in electrical contact with or adjacent to either 'of the The cooling coils, which are essential for such high power apparatus, present Va particularly serious problem in this respect.

Protective equipment of elaborate design is `also required to assure that operators, people who are being treated with the apparatus and others in the vicinity ofthe apparatus are not injured bythe high potentials. l

It is, accordingly, an object of my invention to provide a generator of hard X-rays having a wave length less than .25 Angstrom which shall incorporate no high voltage insulating and protective equipment.

A more general object of my invention is to provide a high voltage electric discharge device which shall incorporate no insulating and protective equipment for high voltages.

Another general object of my invention is to provide an eflicient high voltage electric dis-' charge device.

My invention arises `from the realization that the high voltage necessary to produce hard X-rays may be impressed as the electric component of a high frequency electromagnetic field entirely within a hollow body resonator. Externally such a resonator is throughout .at ground potential, and, therefore, neither the cooling system nor the other 'parts of the apparatus need be elaborately insulated in spite of the high potential impressed within the resonator.

In accordance' with my invention, I provide a discharge device comprisingahollow body resonator within which an electron emitter is disposed. The resonator'is coupled'to a suitable oscillation generator which produces an electro-- magnetic iield within the resonator and under the action of the eld electrons ow from the source through the resonator. An X-ray target vis disposed in the path of the electrons, and as the electrons impnge thereon, X-rays of the desired wave length are produced.

In accordance with the broader aspects of my invention, the resonator may be. an ordinary hollow tank, the internal surface of which is conductive. When such a tank is excited to oscillate, a standing electromagnetic wave or fractions of a wave is produced along adimension of the tank corresponding to the mode of oscillation and the electrons from the emitters flow through the tank along a path determined by the electric component of the eld of' the wave. The wave has potential nodes at conduc-y tive walls of the tank which are Vtransverse to the dimension of oscillator and voltage loops ileld as the electromagnetic A more specic object of my invention is to` 1 provide apparatus for radiating X-rays in precisely timed discrete pulses. v f

An ancillary object of my invention is to provide a circuit for coupling an oscillation generator to a hollow body resonator in lsuch manner that the generator locks into stable oscillation. with the rescnator.

. Another ancillary object of my invention is to provide lan eiicient hollow body resonator.

More concisely stated, it is an object of my invention to provide X-ray system that shall not incorporate the insulating yequipment ordinarily required for the high Voltage involved.

an eilicient high Voltage.

alternating potentials which intermediate the nodes. Hereinafter, I shall refer tothe potential produced by the electromagnetic potential to distinguish it from theusual direct or low frequency maybe used in thel apparatus. The electron iiow is produced by a *field vwhich arises from the diierence of electromagnetic potential between the emitter and the points in the resonator which are at any instant at a higher electromagnetic potential than the emitter. The emitter should be so disposed in the tank that the electrons in passing through the tank are subject to the potential of thefield for an interval of time that is short compared to a period of the field. If the electron path is so long that the electrons are effected by the iield during an interval of time of the order of a period of the iield, the electrons are both accelerated and decelerated while traversing their path and do not have the necessaryenergy when they impinge on thetarget.

protuberance.

in its broad aspects with a resonator in the form of a completely closed hollow cylinder. principal mode of oscillation of such a cylinder the resonant electric field'is in and has ai wave length approximately equalto t twice the diameter. Conducting leads carrying an electron emissive filament are sealed through one base of the cylinder and a target is supported from the opposite base. The height of the cylinder or the length of the filament leads should be such that the path over which the electrons are accelerated by the eld is short compared to a quarter-wave length of the resonant eld. When the cylindrical resonator is excited to oscillate the potential wave has a node at the circumference of the cylinder and a loop on the axis; the potentiall difference within the cylinder isaxial. The maximum potential difference at any instant is impressed between the bases along the axis. Current ows betweenthe emitter and the target under the action of the axial potential diierence. The return path of the current is made up of a cylindrical shell with a reentrant cylindrical protuberance through which the emitter projects. The height of the protuberance is such that its upper terminal is disposed adjacent to the opposite base of the resonator. The emitter extends just through the end of the protuberance adjacent the resonator base and the target is mounted within the base. Such a resonator has a principal resonant wave length approximately equal to four times the height of the shell.

In the an axial direction 5 Conveniently my invention may be practiced` an area'as is practicable is desirable at the internal end of the protuberance. of the electromagnetic voltage produced is dependent on the characteristic impedance -of the resonator in the region of the voltage loop. The latter impedance is, the greater, the greater the ratio of the area of the base of the outer shell -to that of the protuberance in the region of the loop. Where the shell and the protuberance are of circular cross section the characteristic impedance is proportional to the logarithm of the ratio of the diameter of the outer shell to the diameter of the protuberance in the region of the loop. For a given shell diameter, the ratio The lines of force of the electric eld produced when the resonator is excited to oscillate extend radially between points on the protuberance and corresponding points on the shell. The distance between the protuberance and the opposite base of the resonator is such that the time during which the electrons are subject to the field between the emitter and the base is short compared to a, quarter period of the resonance frequency.

When the resonator is excited to oscillate, a wave having a potential node in the region where the protuberance joins the shell and a loop near the opposite base of the shell `is produced. In this case the return `path for the current which flows between the emitter and the target isalong the internal surface of the outer cylinder, along the surface joining the outer cylinder and the protuberance, and along the internal surface of the In addition to the discharge current there is a large high frequency resonant current which is greatest along the joining surface andtends to concentrate at the junction of the protuberance and the surface.

To power loss, it is desirable that the area bounded by the curve which is dened-by the junction (the diameter of a circle of the protuberance is a circular cylinder as is usual) be as large as is practicable. This desideration cannot be achieved simply by providingv aprotuberance having bases of large area because as smalll and, therefore, the electromagnetic voltage, is a maximum if the protuberance diameter at the internal end is a minimum.

The conditions for minimum power loss and maximum electromagnetic voltage may both be satisfied by providing a resonator having a reentrant conical protuberance. The protuberance is preferably of circular cross section .and tapers from a large diameter base at one base of the shell substantially to a point near the other base of the shell. The junction curve between the protuberance and the former base of the outer shell is a circle of large diameter, and the losses at the junction are minimized. The ratio of the diameter of the shell near its other base to the diameter of a point is high and therefore the electromagnetic voltage is large for the dimensions involved.

The ratio which governs-the characteristic impedance is a maximum if the shell is cylindrical. However, a conical shell with a conical protuberance is less cumbersome and weighs less than a cylindrical shell with a conical protuberance.

pSine for resonators most frequentlyv encountered in practice, thecharacteristic impedance ratios are ,not 'appreciably different for the two structures, lI prefer to use a conical resonator with a reentrant conical protuberance in the practice of my invention. My invention is not, of course, limited to this preferred practice.

In accordance with the broader aspects of my invention, the resonator may have any desired dimensions However, from a practical standpoint I have found that certain dimensions are to be preferred.

The resonant wave length of a conical (or cylindrical) resonator having a reentrant protuberance is roughly four times the distance between the bases of the outer shell. Since X-rays are produced in a high vacuum the resonator is highly evacuated and to minimize vacuum problems, it is desirable that the resonator be small. Aside from physical limitations, the exiguity of the dimensions of the resonator' is limited by the practical consideration that oscillators for exciting a resonator sufficiently to produce X-rays are not available at small wave lengths. 'I have found that an oscillator capable of supplying more than 200 watts steady power and l0 to 50 kilowatts in discrete pulses is necessary for X-ray purposes. Such an oscillator is s conveniently available only at wave lengths greater than two meters. To avoid vacuum difculties, the resonance wave length of the resonator should not be greater than l2 meters. In accordance with a specific aspect of my invention, the resonant wave length of the resonator should lie between 2 and l2 meters and should preferably be of the 'order of 4 meters.

The oscillator which is used in the practice ofmy invention to excite the resonator comprises The magnitude tial produced `terial such as copper.

` tube issec'ured. The tube |9 electric discharge valve means, the output loop to the resonator. The resonator is a massive structure, and its dimensions 'cannot conveniently be varied for tuning purposes. A problem,

therefore, arises in adjusting the oscillator to between the loop and the discharge valve means is varied until the oscillator and the circuit of which is connected through a couplin posed of Monel metal, and the cover 2| has The inner surface of the top 'is faced with lead resonator lock into stable oscillation. I have i found that stability is attainable over a substantial range of the impedance and precise adjustment to a critical point is not essential.

It is desirable that the flow of electrons from the emitter to'the target be limited to the time intervals during which the electromagnetic poassembly comprises a rin tential tending to draw electrons from the emitter is high. If electrons flow from the emitter during the intervals when the potential'is relatively low, they, to a large extent, fail to reach the target and to circulate in the resonator causing excessive loss. To limit the ilowof electrons in the high potential intervals, the emitter is shielded by a screen conductively connected` to the resonator and a biasing potential of several h'undred volts is interposed between the emitter and the resonator.l 'I'he `potential prevents the electrons from penetrating the screen into electromagnetic eld and the target, unless the poten- The novelfeatures that I consider characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and its method of operation, together with additional objects vand advantages thereof, derstood from the following description of specific embodiments when read in connection withthe accompanying drawing, in which:

Figure 1 is a diagrammatic view showing an embodiment of my invention; and e Fig. 2 is a graph illustrating the operation of my invention.`

The apparatus shown in Fig. l comprises a hollow body resonator 3 made up of an external conical shell 5 and a reentrant conical protuber-v ance 1. The protuberance l tapers in the same direction as the shell 5 and is preferably coaxial with the shell. In accordance with the broader aspects of my invention, the protuberance and an electronic emitter assembly 33 extends.

a thickness of the order of one-tenth of an inch.

or any other, high-atomic-weight material. The tubeis provldedwlth ears A"425 and" is ,held tightly against a,g'asket"21 disposed over the opening I1 in the shell 5 by bolts 29 which passA through; the ears land engage the smallbase of fthe shell. A` suitablewater-cooling jacket 3| is` provided aboutthe tube |9 andits top 2|. l

The small diameter base 9 of the protuberance 1 is rounded olf lso that it, in eiect,` constitutes a point rather `than a surface of Aextensive area. It is provided ywith an opening 32 through which The g shaped lament 35 of tungsten or any other suitably emissive material 31. A pair of rigid conducting leads 39 are welded to thefllament 35. The leads and the filament are enclosed within a narrow electrically conductive cylinder 4| .open at the end remote from the 'ilament 31. The open end4 is sealed vacuumtight by an insulating plug 43 preferably comf posed of glass and the leads 39 are sealed vacuumby the field is of large magnitude.

the shell need not be coaxial, but their axes should preferably be parallel. The distance between the small diameter base 9 of the protuberancel and the small diameter vbase of the shell 5 is short compared to the height of the shell. The. ratio between the shell height and the distance between Ithe small diameter bases should be of the order of 5:10 10:1. Cooling coils |3 are provided about the surface `|5 joining the large diameter ends of the protuberance and the shell.

The shell 5, the protuberance 1 and their joining surface |5 of the embodiment shown in Fig. 1 are composed of an electrically conductive ma- In accordancewith the broader aspects of my need not be composed of conductive material but it is essential that the innerl surface of the resonator be conductive.

tight through the-plug 43. The leads are of such length that the filament is adjacent the unsealed end of 'the tube 4|. The end 45 of the tube 4| near the filament is closed but has a central opening 41 of small diameter. Insulating spacers 49 are provided Within the tube 4| to prevent the leads 39 from contacting the tube. The tube extends a short distance through the 'small diameter base 9 of the protuberance 1 and is hardsoldered vacuum-tight to the protuberance by a solder ring 5| melted at the junction between the protuberance and the tube.

The tube 4| is thus conductively connected to the resonator 3 but the lament 31 is insulated from the resonator. The perforated end 45 of the tube 4|, in eiect, constitutes a screen about the lament 31 and the flow of electrons from the filament may be controlled by controlling the potential between the end 45 (i. e., the resonator) and the lament.. y

The resonator is supported from hollow tubular arms 53 and 55 extending from diametral openings near the large diameter end of the outer shell 5 in a direction perpendicular to Vthe axis 0f the shell. The arms are soldered vacuumtght to thev outer shell. A sleeve 51 issecured to each of the arms and the sleeves are rotatably supported on ball-bearing wreaths 59 which'are suspended from axed supporting bracket 6| 'The resonator l3 may thus be rotated about an axis determinedby the arms 53 and 55. l

One of the arms 53'communicates with` a pump systemtS through ka Sylphon 65. 'Ihe system comprises a tubular oil-dilusion pump 61. the

suction tube es of which is secured vacuum-tight 'I'he sleeves 15.are rotatably supported on ball e bearing wreaths 19 suspended from a rigid bracket invention, the resonator A central opening I1 is provided in the base of the shell `5. `Over the opening a closed cylindrical tube I9 having a atcover 2| at an angle of approximately 45 to the axis of the is preferably com- 8|. 'I'he diffusion pump 61 may thus rotate with the resonator 3. .sThe tube 83 from the suction opening 19 -of the mechanical pump 1| is suiliciently long and ilexible to permit free rotation of the diffusion cylinder 61 and the resonator 3, and thenSylphon 65 takes Aup any dilerence in displacement between the diilusion pump and the resonator arising from the rotation.

'Ihe remaining tubular supporting arm 55 is conductors and the bridging bar have a v the resonator.

4- sealed vacuum-tight by a plug 8E of insulating material, such as glass. tending from terminals 83 sealed through the plug 35 projects into the space between the protuberance 1 and the shell 5 adjacent the supporting arm 53. The loop 81 should be so positioned as to be threaded by a portion of the flux of vthe magnetic component of the field produced within the resonator 3 when the latter is excited. When the resonator 3 is rotated, the position of the loop relative to the resonator :field should not change, and, therefore, the loop should be rotated with the resonator. To provide for the free rotation ofthe loop while the parts to which it is connected externally are fixed, the terminals 09 of the loop are connected to movable brushes 9| which engage fixed conductive rings 33.

The resonator 3 is excited from a high fre- A coupling loop 81 exquency oscillation generator 95. The generator comprises a pair of three-electrode high-vacuum discharge tubes 91 and 03, each having an anode IOI, a cathode |03, and a control electrode |05. The control electrodes are connected together by a short conductor |01. To the center |09 of the conductor or terminal, a biasing network consisting of` a resistor III and a capacitor II3 in parallel is connected. The other terminal of the network is connected to a conductor II5 which connects the cathodes |03.

From each of the anodes IOI of the discharge devices 91 and 99 rigid conductor II1, extends. Each of the conductors engages a ring 93 to which the loop terminals 89 are connected. The anode conductors |I1 are bridged by a conducting bar II9, the position of which along the conductors may be adjusted.

The bridging bar I I9 is connected to a grounded line conductor I2I of an alternating' current source (not shown). The other line conductor |23 of the source is connected to the conductor II5 which connects the cathodes. The oscillator 95 may be enclosed within a shielding container |25 provided with a neck |21 within which the rings 93 connected to the loop 31 are mounted. The resonator 3 is grounded but the loop 01 and vits terminals 89 are insulated from the resonator.

The anode conductors |I1,'the loop terminals 89, the loop 81 and the bridging conductor IIS constitute the output circuit of the oscillator 95. The character of the interaction of the oscillator and the resonator 3 may be determined by adjusting the position of the bridging bar I I9 along the anode conductors I I1. The bridging bar may be so disposed that the portions of the anode conductors between the bar and the anodes IOI are shorter than that required for stable oscillation of the generator 05 and the resonator 3 as a unit. This situation arises when the anode low impedance compared to the equivalent impedance of the loop 81 and the resonator 3, and in such an event the oscillator operates independently of The bridging bar II9' may be so positioned that the portion of the anode-conductors .between the bar and the anodes are too long, The resistance of the anode conductors and the bridging bar is then so high that no oscillations are produced by the generator 95. Intermediate' these extreme` positions of the bridging bar Ils there is an extended region. for which the resonator 3 and theoscillator 05 lock into stable oscillation. of electrical efciency Q hollow resonator, there is a nite range tions of be; ."f fr of posiwhich the frequency of In other words, because the index is so high for the the oscillations produced by the generator is controlled by the dimensions of the resonator and not by the position of bar II9. Over this range, the shorting bar IIS may be positioned to act as a means of supplying the necessary inductance for the oscillator 95 to look into for emcient power transfer. v y

When the resonator is properly excited by the oscillator 95, an electromagnetic eld the electric component of which is radial is produced Within the resonator and a difference of electromagnetic potential exists between points along the pro- 'tuberance and points along the shell radially displaced from the protuberance points. Between the apex 0 of the protuberance andthe opposite base of the shell the lines of force of the electric component of the electromagnetic eld extend at gradually decreasing angles to the axis of resonator as the axis is approached and they are along the axis between the center of the apex and the center of the base II. An electromagnetic potential difference is therefore impressed between a point just outside of the screen 65 and a central point near the base II of the shell 5 opposite the filament.

The standing electromagnetic potential wave which is produced along the resonator 3 is illustrated graphically at any instant in Fig. 2.V Distance is plotted vertically and potential horizontally towards the right. `To illustrate how the potential varies along the resonator, the outline of the resonator is shown in broken lines. The full-line quarter-sine Vr represents the electromagnetic potential difference between corresponding radial points along the protuberance 1 and shell 5. For example the potential difference between point Pp onthe protuberance and pointvPs on the shell is given by the potential coordinate of the point Vp. The potential difierence between the'screen 45 andthe base II Iis given by the maximum voltage coordinate of the curve Vi- (at the point L) The potential wave has a node N at the large diameter end of the resonator and a loop L near the small diameter base II of the shell. A current loop coincides with the potential node N and lthe current is, therefore, large along the large diameter surface I5 of the resonator.v It is for this reason that the surface is provided with cooling coils I3.

The potentials along the resonator 3 represented by the curve Vr is an alternating potential having ahigh frequency determined by the resonator 3; the corresponding wave length being approximately four times the length of the resonator. Since the anode-cathode potential impressed on the tubes 91 and 99 of the generator which excites the resonator is of the alternating type, the electromagnetic potential within the resonator is modulated at the frequency of the alternating current potential impressed on the tubes. The alternating current potential frequency is small compared to the resonant frequency of the resonator.

A potential |23 is interposed between the lament 36 and the resonator 3 maintaining the direct current potential of the resonator negative relative to that of the filament. The potential functions as a negative bias between the perforated cover 66 of the tube 4I which encloses the lament and the leads 39 and controls the ilow of electrons from the filament to the small diameter base of the shell.

Electrons now from the filament 35 under the combined action of the .e potential B23 and aecomo Although I have shown and described certain the modulated electromagnetic potential. The electron now from the filament is blocked by the biasing potential until the electromagnetic potential exceeds a predetermined magnitude. At

. .this point the electrons are drawn from the eld of the screen 45 into the high electromagnetic eld between the screen and the small diameter quency and amplitude and the biasing potential.

In a system constructedin accordance with my invention, the length of the resonator 3 is 48 inches. The largest diameter of the outer shell 5 is 32 inches. The smallest diameter of the outer shell is 14 inches. The distance between the small diameter base li of the outer shell and the small diameter base 9 of the protuberance 'l is 9 inches. The large diameter of the protuberance is 18 inches. Its small base is a rounded-ofi surface 4originally 4 inches vin diameter. The discharge devices 91 and 90 used in the oscillation generator 05 in an embodiment of my invention are Westinghouse W13-889 tubes. The anode-` cathode potential impressed on each of the devices is an ordinary commercial 60cycle potential having a peak magnitude of 12,000 volts. The resistor lil in thebiasing network is 5,000 ohms,

specific embodiments of my invention, I am fully laware that many modifications thereof are possible. My i invention, therefore, is not to be restricted except in so far as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

l. A hollow body resonator comprising a hollow conical enclosure having a refentrant conical protliberance with the axis of said protuberance substantially parallel to the axis of said enclosure,

said conical protuberance tapering in the same direction as said conical enclosure. the internal surface of said enclosure including said protuberance being electrically conductive.

2. A hollow body resonator comprising 'a hollow V conical enclosure having a re-entrant conical protuberance, said enclosure and protuberance being Y lcoaxial and tapering in the same direction, the internal surface of said enclosure including said protuberance being electrically conductive.

3. A hollow body resonator compng a. hollow enclosure having a re-entrant conical protuberf ance with the axis oi.' said protuberanoe extendtricallyv conductive.

and the capacitor in the network is the distributed capacity of the resistor.

ing in substantially the same direction 'as the axis' of said enclosure, said enclosure and protuberance being taperedv in the same direction, said protuberance and the 'remainder of said exiciosure bounding an annular space between their side walls and a conical space of substantiallsr smaller height than the height'of said enclosure' between their ends. the internal surface oi said enclosure including said protuberance being eiec BENEDICT Cassani

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