US2579315A - Resonator structure - Google Patents

Description

Filed Nov. 29, '1949 1951 v A. M. GUREWITSCH 2,579,315

RESONATOR STRUCTURE 2 SHEETS-+SI-IEET 1- Inventor: An atole M. Gurewitsch,

37 by Q, His Attorney.

e 1951 A. M. GUREWITSCH 2,579,315

RESONATOR STRUCTURE Filed Nov. 29, 1949 2 SHEETS-SHEET 2 v Fig.3.

Inventor: I

H47 Anatole M. 6u1-ew'itsch,

I"? v I b3 His Attorneg.

Patented Dec. 18, 1951 RESONATOR STRUCTURE Anatale M. Gurewitsch, Schenectady, N. Y., assignor to General Electric Company, a corporationof New York Application November 29, 1949, Serial No. 130,054

7 Claims.

The present invention relates to high frequency resonators, particularly to resonators findin useful application in apparatus for imparting high energy to charged particles.

The invention is applicable in connection with apparatus of the tpe disclosed in United States Patent 2,485,409, patented October 18, 1949, in the names of Herbert C. Pollock and William F. Westendorp and assigned to the assignee of this present invention. Such apparatus comprises means for initially accelerating charged particles by the action of a field produced by a time-varying magnetic flux and for thereafter producing continued acceleration by a localized electric field of cyclically varying character.

In one form of this apparatus, the apparatus employed for producing the localized electric fields comprises a space resonant structure suitably located along the path of the charged particles within the region of the time varying fiuX. This disposition of the resonant structure requires special consideration in view of the space limitations involved and also necessitates the provision of an arrangement for preventing excessive eddy currents from flowing in such structure.

Accordingly, one object of the invention comprises the provision of an electric field-producing device capable of meeting the peculiar space and electrical requirements of the above characterided apparatus. provision of an advantageously constructed form of space resonator adapted to serve as an accelerating device for charged particles.

One form of the invention exemplary of the principles thereof more fully described and defined hereinafter includes a resonator having a plurality of conductive strips supported upon the inner and. outer surfaces of a tubular section of dielectric material. These strips respectively serve as the inner and outer conductors of the resonator. gap across which strong-eleetric fields may be developed when the resonator is excited, and both inner and outer strips are short-circuited at suitable positions to facilitate the procurement of desired operational characteristics.

The features of the invention desired to be protected 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 to the following description, taken in connection with the accompanying drawings, in which Fig.

A more specific object comprises the The inner strips areprovided with a.

1 is a partially sectionalized elevation of an accelerator suitably embodying the invention; Fig. 2 is an enlarged view of the resonator portion of the discharge vessel shown in Fig. 1; Fig. 3 is a schematic representation useful in explaining the invention; Fig. 4 shows in partial section a preferred arrangement for exciting the resonator of Fig. 2; Fig. 5 is a cross section taken on line 5-5 of Fig. 4; Fig. 6 shows a preferred device for tuning the resonator of Fig. 2; and Fig. 7 is a schematic representation of excitation apparatus employed in connection with the device of Fig. 1.

Referring particularly to Fig. 1, there is shown in section a closed rotationally symmetrical glass envelope [0 which defines within its interior an annular chamber. The envelope [0 provides a circular orbit in which charged particles, e. g., electrons derived from a suitably energized source Ill supported from a side-arm Ill, may be accelerated to a high energy level. The envelope is preferably highly evacuated and is provided on its interior surface with a high resistivity, conductive coating, for example, a layer of a metallic salt, in order to reduce the effect of wall charging. Forming a sector of envelope [0 is a resonator structure which does not appear in Fig. 1, but will be shown and described at a later point.

The envelope or vessel l0 lies symmetrically around the axis of a laminated magnetic structure having a central flux path provided by an annular iron core ll. This core is supported at its extremities by attachment to the central portions of opposed pole pieces I 2 and I3 which have planar circular areas I2 and I3 and tapered annular areas l2" and I3". These pole pieces are, in turn, supported by a rectangular frame [5 of laminated iron which surrounds and extends transversely to the envelope It.

The ends of the core H are separated from the pole pieces [2 and It by narrow gaps I1 and I8 which are so proportioned as to cause the core to saturate at a predetermined level of the ma netic flux passing through it. The annular faces l2" and I3" of the two pole pieces each have a double taper as shown, the purpose of this configuration being explained at a later point. An opening l6, which extends continuously through frame l5, pole pieces 12 and I3, and core ll, permits cooling air to be circulated through these parts.

The magnetic structure is excited by means 0 a pair of series connected coils 25 and 26 which surround the pole pieces and which may be energized in such a manner as to provide a cyclically varying flux in the magnetic circuit. Electrons produced within the envelop l9 are affected in two ways by the variations in magnetic flux thus obtained. In the first place, since the magnetic flux traversing core II links the circular path provided by envelope l0, any variation of such fiux necessarily produces an electric field tending to accelerate electrons projected along such path. In this latter respect, the apparatus is comparable to a transformer with a secondary comprising a circular path along which the various electrons are accelerated. In general, although the voltage per turn in such a transformer may be low, within a practically attainable range of flux variation the electrons can be mad to achieve very high energies, e. g. several million electron volts, because of the tremendous number of turns which they may execute during a single cycle of magnetic flux variation. In addition to the acceleration produced by flux linking the electron path, the flux produced by the annular pole pieces l2" and I3" in the region of the electron orbit tends 'to cause the electrons to follow an inwardly spiraling path. It has been shown that by a proper design of the magnetic structure the centripetal 'fo'rce produced by the magnetic field existing at the electron orbit may be caused to balance the centrifugal tendencies of the accelerated electrons. In general, this 'result requires that the following erla'tionship be satisfied:

where is the flux included in the electron orbit, r is the radius of the orbit and Br is the field strength at the orbit. This equation indicates that the flux must be twice as strong as that which would be produced by a homogeneous field equal to the field Br extending over the entire area enclosed by the orbital electron path. This condition may be realized by making the reluctance per unit of cross-sectional area of the magnetic path at the electron orbit greater by an appropriate amount than its average reluctance within the orbit. In order to maintain the desired proportionality between the enclosed flux and the guide field, 'i. e., the field Br, at all times during an accelerating period, one may adjust the air gap existing between .pole pieces [2" and I3" to the appropriate value. It is readily practicable to control the dimensions of the gap from point to point over the pole piece in such a fashion as to eifect the balanced relation of guide field and enclosed fiux which is desired for the purpose specified above and which is further necessary 7 for radial and axial stability of the electron orbit. This may be done, for example, by a construction such as that shown in Fig. l in which the pole pieces are doubly tapered. The principles governing the proper space distribution of the guide flux are more fully set forth in United States Patent No. 2,394,070, granted February 5, 1946, to D. W. Kerst.

When all the conditions specified in the foregoing are fulfilled, electrons introduced into chamber [0 in a period when the magnetic field is increasing may be expected to be drawn into the particular orbit in which a balance of centripetal and centrifugal forces exist and to be continuously accelerated along such orbit as long as the magnetic field increases in value. Assuming that the peak value of the magnteic field is sufiiciently high, a total energy on the order of several million electron volts may be acquired by the accelerated electrons in a small fraction of a second.

It may be noted, however, that when an electron has obtained a velocity corresponding to an energy level of about 3 million electron volts it is already within about 1% of the velocity of light. Accordingly, further gains in energy result primarily in an increase in the mass of the affected electrons and only insignificantly in further gain in electron velocity, this result being consistent with the Einstein mass-energy equivalence formula. Accordingly, electrons which have attained a velocity within a few percent of the velocity of light will gyrate with a relatively constant periodicity, or at a relatively fixed frequency, provided they can be confined to an orbit of relatively fixed radius. As is pointed out in the aforementioned Patent 2,485,409 of Pollocl: and Westendorp, this consideration makes it possible to impart further energy to the electrons by means of a localized electric field of fixed frequency acting repetitively on the electrons as they continue their gyrations within the envelop :0. By this means extremely high energy levels can be reached through a mechanism which avoids difficulties associated with any attempt to achieve corresponding energy levels by a magnetic accelera'tioh alone. With this in mind, the apparatus of Fig. 1 is so constructed that saturation of core ll occurs after a sufficient acceleration of the electrons has been obtained by magnetic means, and an electric field accelerating device to be hereinafter described is brought to play. However, the guide field produced between pole pieces 12" and I3" continues to increase as a result of continued energization of coils 25 and 26 in order that the accelerated electrons may still be confined'to the desired orbit.

As has been mentioned heretofore, an electric field producing device employed to impart further energy to electrons gyrating within envelope l0 must meet the peculiar space and electrical requirements of the above described apparatus. In order to secure maximum efficiency, it is desirable to keep the volume of the gap between pole pieces l2" and I3" small, and, therefore, the electric field producing device should be as sociated with envelope ID in a manner requiring a minimum gap volume for a maximum volume available for the electron beam. In order to secure maximum e'fficiency of energy transfer from the electric field producing device to the electron beam, such device should be closely coupled to the electron beam. And, since these considerations, as a practical matter, require conductors within the strong cyclically varying fiux traversing. envelope Ii), such conductors should be of sufiiciently small width and thickness to prevent the generation of excessive eddy currents therewithin.

All these requirements are 'efiiciently met by the resonator structure shown in Fig. 2 which comprises a tubular section as of dielectric material. Section 30 is of approximately elliptical cross-section conforming to the cross-section of envelope [0 and forms a sector thereof. In practice, envelope H3 is generally constructed as a unit in a suitable mold, and a sector thereof may be removed to obtain spacefor section 36. Since it is preferable to utilize a dielectric material having a small high-frequency loss coefiicient;

section 30 may be formed of such material and shaped similarly to the sector removed from envelope I0. Section 30 may be attached to envelope III- in vacuum tight relation by means of suitable rubber gaskets in a manner well known to those skilled in the art.

In order to provide means for conducting high frequency currents, tubular section 30 is coated upon all its exposed surfaces with a layer 30' of conductive material, such as silver. Portions 3| of coating 30' are removed from the exterior surface of section 30 to form a plurality of approximately evenly spaced conductive strips 32 extending longitudinally along both sides thereof, as appears in Fig. 5. Portions 3| are discontinuous near one end of section 30 in order to provide a peripheral conductive strip 33. In a like manner, portions 34 are removed from coating 30 on the inner surface to form a plurality of conductive strips 35 extending longitudinally along section 30. Portions 34 are discontinuous to provide peripheral conductive strips 36 and 31. A peripheral gap 38 is provided by removing a portion between strips 36 and 31. Portions 39 are removed from the end surfaces (one of which is not shown) of section 3'0 to provide'a plurality of conductive strips 40. Portions 3i, 39 and 34, removed respectively from the coating upon the exterior surface, end faces and inner surface of section 30, are spatially interrelated so that each strip 32 will be conductively connected to the oppositely disposed strip 35 upon the adjacent inner surface of section 30.

One convenient means of forming conductive layer 36 is as follows. A layer of silver paint may be placed upon the inner and outer surfaces and end faces of section 36 and subsequently baked to secure adherence thereto. The thickness of this layer may be increased by means of electroplating, or by repeated painting and baking, for the purpose of obtaining a layer having a thickness greater than the depth of penetration of the radio frequency current which flows therein when the resonator is excited. As an order of magnitude, the layer may have a final thickness of about 1 to 1 mils when the resonator is to be excited at 160 me. The various portions removed from the conductive layer as above described may be made by masking during the painting operation, or by burning in grooves after baking and electroplating with a tungsten disc which is rolled along the conductive surface carrying a heavy current through the contact area.

With the proper selection of longitudinal dimensions, the structure shown in Fig. 2 operates as a quarter-wave, closed concentric line resonator at a particular excitation frequency. The space between the conductive coating on the exterior surface of section 30 and the coating on the interior surface constitutees in effect av space resonant system comprising a quarter-wave transmission line section. Accordingly, if section 36- is excited at the proper frequency, a cyclically reversible electric field of high intensity may be made to appear across gap 38. By choosing the frequency of reversal of this field to correspond to the frequency of rotation of electrons moving within envelope l6 and section 30, an increase in the energy level of such electrons may be effected in accordance with the principles previously outlined.

The sub-dividing of coating 30' into longitudinal strips as described provides a convenient and direct means of reducing eddy currents induced therein by the cyclically varying magnetic flux. However, unless these strips are properly interconnected, some eddycurrents will still flow with the undesirable consequences of heating the strips and interfering with the distribution of the varying flux. Moreover, such subdivision of coating30 seriously affectsthe highfrequency excitation of thestructure. If strips 32 arenot connected to each. other, and similarly if strips are not connected to each other, the structure presentselectrically a number of coupled conductive members which are sufliciently loosely coupled to have a great number of undesirable modes. The provision of peripheral conductive strips 33, 36, and 31,- as hereinbefore described, furnishes a simple and effective means of surmounting the problems simultaneously, as will now appear by reference to Fig. 3.

The representation in Fig. 3 schematically illustrates the relative positions of adjacent strips upon section 36, such strips being identified by reference characters identical with those employed in connection with Fig. 2 for corresponding elements. It will be observed that, since the varying flux passes through strips 32 and 35 in the same direction, eddy currents flowing in the same direction are induced in these strips. For convenience, it is assumed that the varying flux is changing in such a direction as to induce the currents indicated by the conventional arrows and characters i1. i2, is, and i4. Strips 46 are essentially parallel to thedirection of the varying flux, and, hence, the eddy currents induced therein will be negligible. It is now apparent that two closed conductive loops a and b are formed; loop b including strips 32, 33, 46, 35 and 31, and loop a including strips 32, 33, 40, 35 and 36. If the strip 33 is positioned opposite gap 38, the areas enclosed by the upper and lower portions of each of these loops are essentially equal and, therefore, an essentially complete compensation of eddy currents is accomplished. Moreover, strips 33, 3'6 and 31 provide sufficiently effective coupling of strips 32 and 35 respectively to suppress undesirable modes which might be generated when the structure of Fig. 2 is excited.

If complete compensation of eddy currents is desired, strip 33 should be approximately as wide as the combined widths of strip 36, gap 38 and strip 31 positioned opposite thereto. In practice, however, it may be found desirable to make strip 33 approximately as wide as gap 36 and to position it opposite thereto, such being effective to counter-balance irregularities in strip width and thickness, etc. Although it is preferred to employ strips 33, 36 and 31 as above described, sufiiciently tight coupling may be obtained for some purposes by utilizing only peripheral strips 33 and 36. Insuch event, these strips may be positioned at any convenient position along section 30 so long as they are approximately opposite each other.

As will be observed from Fig. 2, portions 3| are not removed to form longitudinally extending strips 32 over the entire exterior surface of section 30. It is not necessary to sub-divide coating 30 in this manner in portions which are approximately parallel to the direction of the varying flux because eddy currents generated in such portions are small. Therefore, portions of coating 30 adjacent the inner and outer circumferences of section 3|]v maybe employed as a means of coupling high frequency energy into the resonator, as may be seen by reference to Figs. 4 and 5 wherein like reference characters are employed to designate elements corresponding to similar elements shown in Fig. 2. A portion 4| is removed adjacent the outer circumference of section 30 to provide a longitudinally extending conductive strip 42. Although the strip 42 is 7 shown as being joined at its left hand extremity to the remainder of coating- ','it may in some cases be made entirely separate therefrom by extending portion 4|. -Energy may be supplied to the resonator structure by the combination of a conductor 43-- which connects with the conductive strip 42 and a tubular element 44 surrounding the conductor and forming with it a concentric transmission line; At. the point at which conductor 44 approaches the resonator it merges into a channel-shaped enlargement 45 which is of sufficient longitudinal andjlateral extententirely to cover removed portion 41 and thus to afford high frequencyshielding. While conductor 43 and-the enlargedmember 45 are shown as being of-metallic cross-section, it will ordinarily be preferable to-form them of a nonconducting dielectric internally coated with a thin metal layer inorderto minimize currents induced by the varying'magnetic-field. By choosing the correct dimensions and an appropriate produce optimum tuningand matching of the system.

Auxiliary tuning and trimming of the resonator may be accomplished by removing a portion 48 adjacent-the inner circumference of section 38 to provide a longitudinally extending conductive strip which is joinedto the remainder of coating 30' at its-right hand extremity, as is shown in Fig. 6. The tuning adjustment is obtained by moving ametallic or metalized strip 48 along strip 4'! in order to connect varying lengths of strip 47 tocoating- 30. Strip 48 may be retained in a desired position in any convenient manner, such as byan adhesive material.

The operational correlation ofnthe resonator structure and the other various elements of the accelerator heretofore considered may best be understood by reference. to Fig. 7 which shows diagrammatically the acceleratingstructure as a whole in combinatio'n with schematically illustrated excitationequipment: In thisfigure, parts which have been describedbearnumbers corresponding to those by which they have already beenidentified."

- Referring'particularly to Fig.- '7, there is shown cycles, to coils'25and 2.6 --by which the mag netic system of the accelerator is energized. A second power source 51, which is assumed to be appropriately connected to source Hl' of Fig. 1, and which may be an intermittently energized circuit of the type described in Pollock and Westendorp application Serial No. 639,462, serves to inject electrons into the envelope at appropriate intervals correlated with the cyclical reversals of the magnetic field, Finally, a'high frequency power source'52' which may consist, for example, of an electronic oscillator, supplies, when energized high frequency potential through transmission line conductors 43 and 44 to the inner and-outer conductors of a resonator 60 which is identical with the resonator shown in Figs. 2, 4 and 5. Properly cor-related energization of the magnetic field, the electroninjecting means and the high frequency-power supply for the resonator is accomplished by means of a timing circult indicated schematically by the block 55, it

being indicated'thatthe timing circuitis conaction of the timing circuit, the system as a whole is controlled in such fashion that initial acceleration of the injected electrons is accomplished by variation of the magnetic field up to the point where the electrons have attained an energy level of several millcn electron volts. Thereafter, by saturation of the core II, the accelerating effect of the magnetic field is substantially eliminated and subsequent acceleration of the elec-' trons to high energy levels is accomplished by bringing into operation the resonator 60.

While the above description of the invention has referred to a conductive layer of a material such as silver being placed upon tubular section 30 and thence subdivided to form the desired resonator structure, it will be readily appreciated that similar structure may be formed by securing strips of a conductive material to the surface of section 30 by means of a suitable adhesive material, for example, an alkyd resin prepared by reacting a polybasic acid and a polyhydric alcohol, such as a resin being prepared from glycerol and phthalio anhydride. It will also be observed that conductive strips 35 may be placed around the entire inner periphery of section 36, as is indicated in Figs. 4 and 5, even though such is not essential in portions adjacent the inner and outer circumferences as has been heretofore explained.

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

1. In apparatus for the acceleration of charged particles in an orbital path which is enclosed by an annular envelope of dielectric material and traversed by a time varying magnetic field, a res onator forming a sector of said annular en-- velope and including an inner conductor comprising a conductive layer supported upon the inner surface of said sector of said envelope and having portions thereof removed to form a plurality of spaced apart conductive strips extending longitudinally along said sector, said sector having a gap extending around the periphery thereof, and a first conductive strip extending around the periphery of said sector, and an outer conductor comprising a conductive layer supported upon the outer surface of said sector of said enevelope and having portions thereof removed to form a plurality of spaced apart conductive strips extending longitudinally along said sector and a second conductive strip extending about the periphery of said sector opposite said first conductive strip.

2. In apparatus for the acceleration of charged particles in an orbital path which is enclosed by an annular envelope of dielectric material and traversed by a time varying magnetic field, a resonator forming a sector of said annular envelope and including an innerconductor comprising a conductive layer supported upon the inner surface of said sector of said envelope and having portions thereof removed to form a plurality of spaced apart conductive strips extending longitudinally along said sector, said sector -having a gap extending around the periphery thereof, and a conductive strip on each side of said gap extending around the periphery of said sector, and an outer conductor comprising a conductive layer supported upon the outer surface of said sector of said envelope and having portions thereof removed to form a second plurality of spaced apart conductive strips extending longitudinally along said sector and a conductive strip extending about the periphery of said sector opposite said gap and said conductive strips extending around the periphery of the inner surface of said sector.

3. In apparatus for the acceleration of charged particles in an orbital path which is enclosed by an annular envelope of dielectric material and traversed by a time varying magnetic field, a resonator forming a sector of said annular envelope and including an inner conductor comprising a conductive layer supported upon the inner surface of said sector and having portions thereof removed to form a plurality of spaced apart conductive strips extending longitudinally along said sector, said sector having a gap extending around the periphery thereof, and a conductive strip on each side of said gap extending around the periphery of said sector, and an outer conductor comprising a conductive layer supported upon the outer surface of said sector and having portions thereof removed to form a plurality of spaced apart conductive strips extending longitudinally along said sector and a conductive strip extending about the periphery of said sector opposite said gap.

4. In apparatus for the acceleration of charged particles in an orbital path which is enclosed by M an annular envelope of dielectric material and traversed by a time varying magnetic field, a resonator forming a sector of said envelope and having inner, and outer conductors comprising conductive layers supported respectively upon the inner and outer surfaces of said sector of said envelope, said inner layer having a gap near one end extending around the inner periphery of said sector, and a plurality of spaced apart slots extending longitudinally along said sector and terminating adjacent said gap to form a conductive strip extending around the periphery of said sector on each side of and contiguous to said gap, said outer layer having a plurality of spaced apart slots extending longitudinally along said sector but terminating opposite said gap to form a conductive layer extending around the periphery of said sector.

5. A resonator comprising a first conductive layer supported upon the inner surface of a tubular section of dielectric material and having a peripheral portion removed to form a gap near one end of said section and longitudinal portions removed to form a plurality of inner conductive strips extending along said tubular section, said longitudinal portions being discontinuous adjacent said gap to form first peripheral conductive strips on both sides of said gap, and a second conductive layer supported upon the outer surface of said tubular section and having longitudinal portions removed opposite at least some of the longitudinal portions removed from said first conductive layer to form a plurality of outer conductive strips opposite at least some of said inner conductive strips, each of said outer conductive strips being conductively joined across the end faces of said tubular section to the inner conductive strip opposite thereto, and said longitudinal 10 portions removed from said second conductive layer being discontinuous opposite said gap to form a second peripheral conductive strip.

6. A resonator comprising a first conductive layer supported upon the inner surface of a tubular section of dielectric material and having a peripheral portion removed t form a gap near one end of said section and longitudinal portions removed to form a plurality of inner conductive strips extending along said tubular section, said longitudinal portions being discontinuous at a selected position along said section to form a first peripheral conductive strip, and a second conductive layer supported upon the outer surface of said tubular section and having longitudinal portions removed opposite at least some of the longitudinal portions removed from said first conductive layer to form a plurality of outer conductive strips opposite at least some of said inner conductive strips, each of said outer conductive strips being conductively joined across the end faces of said tubular section to the inner conductive strip opposite thereto, and said longitudinal portions removed from said second conductive layer being discontinuous at a position along said section opposite the position of the discontinuity in the longitudnal portions removed from said first conductive layer to form a second peripheral conductive strip.

7. A resonator comprising an outer conductor comprising a plurality of first conductive strips supported in peripherally spaced apart relation upon the outer surface of a tubular section of dielectric material, and an innerconductor comprising a plurality of second conductive strips supported opposite said first conductive strips upon the inner surface of said tubular section and extending along the end faces of said section to connect each of said first conductive strips to the second conductive strip supported oppositely thereto, said second conductive strips being Joined together near one end of said tubular section to form a conductive portion extending around the inner periphery of said tubular section, and said portion having a peripheral section removed to form a gap across which a strong electric field may be developed when said resonator is excited, said first conductive strips being joined together opposite said gap to form a conductive strip extending about the outer periphery of said tubular section.

ANATALE M. GUREWITSCH.

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

UNITED STATES PATENTS Number Name Date 2,193,602 Penney Mar. 12, 1940 2,480,169 Westendorp Aug. 30, 1949 2,485,409 Pollock et a1 Oct. 18, 1949

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