US2653271A - High-frequency apparatus - Google Patents

Description

pt- 1953 J. R. WQODYARD 2,653,271

HIGH-FREQUENCY APPARATUS Filed Feb. 5, 1949 3 Sheets-Sheet l A/ OSC/L L4 TOR Fig. 341.

Fig. 36.

IN V EN TOR. JOHN R W000 YARD Sept. 22, 1953 J. R. WOODYARD 2,653,271

HIGH-FREQUENCY APPARATUS Filed Feb. 5, 1949 s Sheets-Sheet 2 INVENTOR. (/OH/V f1. W000 WW0 Z4 TTORY.

5 Sheets-Sheet 3 INVENTOR.

,4 TTOR/VE )4 J. R. WOODYARD HIGH-FREQUENCY APPARATUS Sept. 22, 1953 Filed Feb. 5, 1949 (/0///\/ A. VVOODYARD PULSE GENER/JTOR Patented Sept. 22, 195? HIGH-FREQUENCY APPARATUS John R. Woodyard, Berkeley, Calif., assignor to The Sperry Corporation, a corporation of Delaware Application February 5, 1949, Serial No. 74,837

8 Claims. (01. 315-) The present invention relates to high frequency apparatus and methods for producing high-energy electrically charged particles, and, more particularly, the invention is concerned with such apparatus and methods wherein the electric fields contained within sections of electromagnetic wave guides and/or cavity resonator devices are utilized to energize the discrete particles of a beam of particles.

A principal object of the present invention is to provide new and improved apparatus and. methods for producing a high-energy beam of electrically charged particles.

Another object of the present invention is to provide an arrangement for producing a beam of electrons having an energy content of the order of a billion electron-volts useful in medical therapy or for the production of highly penetrating X-rays or for other purposes.

Heretofore there have been employed diverse types of apparatus for producing beams of highenergy charged particles. The known types of apparatus may be divided, for the present discussion, into two general classes, e. g. (1) roundand-round or curvilinear accelerators, and (2) linear accelerators.

The first of the above-named classes includes that form of apparatus in which either a static or a variable magnetic field is employed, with or without accompanying electric fields, to whirl electrically charged particles in curved trajectories whereby a progressively increasing amount of energy is absorbed by the charged particles.

This class of acceleration apparatus has proved to be generally satisfactory for the production of electrons of energies up to about electronvolts. However, it may be shown that, following the teachings of the prior methods, the production of a beam of electrons having energies about ten times that of the presently attainable energies necessitates electromagnets having pole pieces roughly 80 feet in diameter. Clearly, apparatus of this type is difiicult to construct and very expensive for the production of billion-electronvolt beams of electrons.

A further object of this invention, therefore, is to provide a new and improved electron energizing arrangement of the linear type whereby the need for large electromagnets or other sources of intense magnetic fields is substantially eliminated.

The linear types ofaccelerators heretofore utilized for producing streams of electrically charged particles stemmed originally from the early forms of cathode ray tubes, wherein electrons were continuously accelerated in a unidirectional electric field maintained between a cathode and an anode. Due to inherent limitations, the maximum energies obtainable from such unidirectional fields were of the order of a few million electron-volts. To extend the range of the obtainable energies, alternating field types of linear accelerators were used, amploying means for intermittently applying accelerating impulses to the particles.

In one form, the electrons of the beam were exposed to the alternating field for a time approximately equal to the time of a half-cycle. In another form, the electrons were acted on by the field a plurality of times, but such accelerating impulses were supplied only during the positive half-cycles of the accelerating field, the particles being shielded from the field during the negative half-cycles so that only half the energy supplied to the field was constructively utilized. Later types of alternating current linear accelerators, of both these latter forms, extended the limit of the obtainable energy by using high frequency electric fields provided by tuned circuits of the lumped parameter type, and, later still, in the distributed parameter type of circuit embodied in cavity resonator apparatus.

An improved electron energizing apparatus is provided by the present invention whereby energies of the order of a billion electron volts are obtained which is, at least, ten times higher than the level previously obtained by prior known means. This high level of electron energy is realized by the projection of an axial electron beam through a cylindrical cavity resonator so excited as to support a standing electromagnetic field whose configuration defines a zero magnetic field in the axial direction and an axial electric field characterized by a large number of halfcycle variations.

The spatial separation of successive half-cycle variations is adjusted to be approximately equal to the free-space half-wavelength at the operating frequency, so that when a beam of electrons, preliminarily accelerated to velocities nearly equal to that of light, is projected axially along the resonator axis, these high velocity electrons traverse the space between successive half-cycle variations in field in substantially the same time that is required for the variations in the electric field to occur. Consequently, a significant portion of the electrons encounter electric fields favorably disposed for imparting energizing impulses to the beam. Hence, after the beam has been subjected to a predetermined high number 3 of such energizing impulses, corresponding to the number of half-cycle variations of the axial electric field, the energy level of the electrons of the beam reaches high values such as electronvolts.

Accordingly, the present invention has for another of its objects the provision oi an improved electron energizing apparatus of the linear type wherein cavity resonator means are excited in such a manner as to subject a beam of high ve-. locity electrons projected. therethrough in an axial direction to a plurality of energizing inrpulses during a single transit of the beam through the resonator. I h

A feature of this invention lies in providing a linear type of electron energizing apparatus with a cavity resonator excited in a resonant mode such that its standing wave pattern defines a zero magnetic field inthe axial direction and anaxial electric field characterized by a plurality 'of half-cycle variations and an arrangement within 'said resonator for adjusting the spatial separation of successive half-cycle variations to a value substantially equal to the frees'pa ce halt-wavelength at the operating freq en y- Another feature of -the present invention is to provide, in an electron energizing arrangement of the above descibed character, a cylindrical resonator means characterized by the fact that the ratio of the length of resonator to its diameter of cross-section is very high.

'A further object of the present invention consists of providing a method of exciting a cavity resonator apparatus whose length is long as 'eompared to its diameter which comprises the step of exciting said cavity resonator in a high order axial electric mode of order (0, 1, n) where 'n is'an integer greater than zero. 7

A specific feature of the present invention lies/in the provision of a plurality of constriotions formed interiorly of'electromagnetic waveenergy guides-or cavity resonators to reduce the interne'dal'spacings of the electromagnetic field variations.

'rhe'onsmccdns may hav'e a spatial separation equal to free space half wavelength at the "operating frequency, or alternatively conitiriu ousloadingfmay be employed by placing a plurality of constrictions along the length of the resonato per half-cycle variation in the 4 standing wave pattern.

additional object'of thepresent invention is to provide a device for the production of highenergy electricallyfcharged particles, wherein the requisite high-frequency excitation energy is impulsively supplied to the device.

.A'still further object is to'provide means within a Wave-energy guide for increasing the velocity of relatively slow-injected electrons so that the'yliiay then be acted upon by the lectro ma'grieticfieldto' provide a high-energy beam of arm 'Still another "object of :the present'invention is to provide an arrangement 'for producing high-energy electron beams as above, wherein means "are I provided for symmetrically coupling highfrequency electromagnetic energy from a source to the utilization device wherein coupling is effected atthe' entrance terminal of the device.

Yet another object of this invention is to provide "a symmetrical coupling for high-energy 4 citation energy is effected at the exit terminal of the device.

A still further object of the present inven tion is to provide an arrangement for coupling high-frequency electromagnetic energy to a utilization device having means for distributively coupling the resonator-excitation energy to the resonator so that the coupling is accomplished substantially uniiormly'alohg the length of the cavity resonator.

Another object of the invention is to provide a novel coupling arrangement for a utilization device employed for the production of a high- ;energy electron beam which comprises a combii i'e'ol symmetrical;distributive coupling arrangement whereby undesired modes of oscillation are substantially eliminated.

lhe invention also relates to the novel features or principles of the instrumentalities described herein, whether or not such are used for the stated objects, or in the stated fields or cembinations. k

Other objects and advantages will beceriie apparent from the specification taken in connection with the accompanying drawings, Where'- in the invention is embodied in concrete form.

In the drawings,

Fig. l is a diagrammatic representation of the electric field distribution within a metal cylinder;

Fig. 2 is a longitudinal cross sectional View of one embodiment of the present. invention together with a schematic showing of the 'e'ner gizing circuit associated therewith;

Figs. 3c and 3b are explanatory graphs of voltages existinga't various points of the "circuit of Fig.2.

Fig. l is a similar view of another modification of a portion of the interior construction of the resonator of Fig.2;

1 Fig. 5 is a longitudinal cross sectional view of a modified form of the device shown in Fig. 2 in which the excitation energy is coupled to the resonator at the entrance terminal thereof;

Fig. 6 is a similar view showing the coupling of the excitation energy at the exit terminal'oi the resonator;

Fig. '7 is a longitudinal cross sectional view of a modified embodiment or the present'invention in which the "excitation energy iscoupied to the resonator at a plurality of points uniformly distributed along the length of the resonator;

Fig. 8 'is acros'ssectional view taken along the line'88 or Fig. 7;

Fig. 9 is amodifi'c'ationof the arrangement shown in Fig. 7 and shown in longitudi'nal cross Similar characters .of reference are used throughout the figures to indicate corresponding parts.

Referring now' to Fig. 1 ofthe drawing, which is a diagrammatic representationof the instantaneous electromagnetic field distribution 0f a wave propagated in *a wave guide, the reference numeral 2 i design-ates a hollow conducting 'cylindrical tube through Whichielectroma'gnetic waves are regardedas being transmitted in 1a mode of propagation commonly indicated as TMo,1=. The distributions of the electric and magnetic fields are here shown as lines 22 (corresponding to the electric field distribution) and the circles 24 and dots 26 (corresponding to the magnetic field distribution, the circles 24 indicating lines of magnetic force directed toward the reader and the dots 26 indicating lines of magnetic force directed away from the reader).

It is apparent that this mode of propagation is ideal for accelerating electrons or other charged particles for, if a beam of such particles is projected axially along the tube 2| with a velocity such that the particles in the beam traverse a distance 3 between the transverse planes indicated by the dashed lines p and q corresponding to regions of zero axial electric field, in a time equal to a half-cycle of the frequency of the energy, then clearly, the electrons, assuming that they are introduced into the guide in phase with the electrical field, will always experience a field favorable for abstraction of energy therefrom.

When a wave guide, such as tube 2!, is closed at both ends there results one form of a resonator. Energy which is propagated through such a resonator encounters, in the first instance,

one of the conducting end walls interposed in its path, whereupon a refiected wave is set up which travels in an opposite direction. As a result, the incident and reflected waves give rise to a standing wave pattern. In such case, although the electric and magnetic field intensities of the incident wave are in time and space phase coincidence as are the electric and magnetic field intensities of the reflected wave, the resultant electric and magnetic field intensities are in time and space quadrature. From this it is apparent that when electromagnetic energy is propagated in a wave guide terminated in its characteristic impedance, the maxima of the electric and magnetic field intensity variations occur at the same points, as is evidenced by the arrangement of the lines of force of Fig. 1. On the other hand, where the guide is operating as a resonator a 90 spatial separation of the respective maxima of electric and magnetic lines of force diagrammatically represents the field distribution.

Now, it is also known that if the tube 2| be closed by a pair of conducting walls perpendicularly disposed with respect to the axis of the tube 2!, one of the walls being placed at the plane 1) and the other at q, a high-Q cavity resonator is thereby formed having a resonant mode of oscillation known as the TMo,1,1 mode. Consequently, in considering the wave guide of Fig. 1 modified by the proposed conducting walls positioned at planes p and q, the location of the lines of force will be altered in the manner described above.

It is further known that a resonator may be constructed of such length that a plurality of half-cycle variations of field intensity may be present normal to the transverse field. In other words, in conventional symbolism the configuration of the field may be defined as the TMo,1,n mode, where n denotes the number of such half-wave patterns. As has been indicated in the prior art, this mode of operation is ideal for the accelerating or energizing of electrons.

A simple numerical example illustrates the nature of the problems encountered and the limits thereby established in prior art devices.

6 energies of 10 electron-volts by means of a cylindrical cavity resonator excited in the TMo,1,o mode; operations in such a mode would necessitate very high power of excitation. With such magnitude of excitation energy field emission or so-called low-temperature emission assumes an important role in the successful maintenance of the standing wave pattern within the resonator. Electron emission that occurs mainly as a result of a strong external field effect on the work function has been frequently described as field emission. As it is well known, field emission imposes an upper limit on the voltage gradient which can be supported in an evacuated chamber before breakdown. This limit may conservatively be placed at about 10 volts per centimeter. Thus, a resonator so excited and energized for the production of 10 electron-volt particles would necessarily be about 10 centimeters or about 30 feet long.

Theoretically, a cylindrical resonator excited in the TM0,1,0 mode may be expanded to such a length since the resonant wavelength is independent of the length of the chamber. However, to be an efficient linear accelerator, it is necessary that the electrons traverse the length of the resonator in a period corresponding to that of a half-cycle of the excitation frequency, so that the electrons may have a maximum net energy gain. To accomplish this, the resonator would necessarily be operated at a wavelength approximately equal to 60 feet, and in accordance with the well-known relationship between the resonant wavelength of such a resonator and its diameter of cross section, the diameter of cross section of such a resonator would necessarily be about e6 feet. Obviously, the utilization of an evacuated cavity resonator of such proportions presents constructional and'maintenance problems which raise serious doubts as to the practicability of the device.

In accordance with the present invention it is proposed to operate a cylindrical resonator at a high-order mode designated as the 'IM rn mode where n is the number of half-cycle variations of the axial electric field. For the present purpose 1L may be any integer greater than zero and preferably is of the order of 100. Under such circumstances, as pointed out more fully below, it is not necessary for the electrons to traverse the length of the resonator in a time interval corresponding to a half-cycle variation in the excitation frequency. When excited in the TMO,l,n the standing Wave configuration within the resonator comprises nodes in the longitudinal component of the electric field, successive ones of which are spaced a distance comparable to, although somewhat greater than the free-space half-wavelength at the operating frequency.

For modes of oscillation of order in which n is greater than zero, it is known that the resonant Wavelength is dependent on the length of the resonator as well as on the diameter of crosssection thereof. It may be shown that for a mode characterized by n=l00, the required diameter of cross section for a resonator of length equal to 30 feet is only several inches provided the wavelength is suitably chosen. Or, assuming that a resonator of a predetermined small diameter as compared with its length is selected for use in the accelerating or energizing apparatus, it has been found that a mode of excitation of sufliciently high order may be found which satisfied the conditions for the maintenance of the desired field.

ayeaaati be equal to the distanceibetweemsuccessive nodes in the resonator so thatithe .electrons .may alwaysencounteriacceleratingelectric forces. 'fFor energies 01 the order here contemplated :the velocity oi the electrons :is i very znearly'zequal toxc thevelocity fofl light. Z'Tnus, r the necessary condition tobe satisfied maybe-restated inxthefollowing manner: the. distance between successive nodes-ishouldabe equal .:to the free-space "halfwavelength at the operatingfrequency.

Inasmuch as the internodal .distance in the cavity resonate: is':s.omewhat-- greater than the free-space.shalf wavelength and,- since thewelectrons :cannotxhave: velocities greater than c, itis .cle'ar that unless t e interncdal distance within thesresonator ..-betreduced to substantially the "value :oi' the=-free-space shall-wavelength, electrons :will be subjectedat timestozdecelerating 'iforce :fieldsr as we1l=as accelerating-force fields, xtherebyeseriously reducing the efiiciency of the apparatus.

As a solution of the foregoing problem, the -.present inventiontprovides means within-a cavity resonator for "precisely fixing spacing :ata value substantially-equal to the free-space half-wavelength thereby insuring that the internodal the electrons, traveling with a velocity almost equal to that of lightyaresubstantially always in anaoceler-ating force field.

--More specificallyas-wlll appear in the detailed description hereinbelow, a preferred form of this feature comprises a type of-alteration of the electromagnetic field existing :withinthe cavityresonator which is accomplished by providing constriations within: the otherwise smooth-wall resonator, the. constrictions being so; arranged as to reducethe diameter of the resonator by .a predeterminedzamount and through-a predetermined lengththereof. Toproduce the desired value of the internodal spacing, the constrictions aredisposed in the' median planes ofthe longitudinal electric. field, in the region where the radial comv ponent of the electric field is. a maximumend the axial component .isiia-minimum. A convenient means for producing such constrictions-has been found tube in the iormof conductive rings mounted within the resonator and extending peripherally therearound.

It will be understood that it. isvwithin the. contemplation of the present :inve'ntion 1 that the energy absorbed by the electronsismainlymanifest by increased-:massof the electrons. The

changein the velocity, at such high velocities; is practically negligible. The overall result mainly resides in theincreased energy-or" the particles.

However, the apparatus of the presentinvention will sometimes be calledranelectron accelerator,"

even when the acceleration :is negligible, in: ac-

- cordance with conventionalusage of .the'aword.

To determine the magnitudezof. .the radio-irequencypower' required to obtain electrons having energies; of electron. volts, :let: lusazconsider... a

simple...numerical1 example. We have previously foundithat a cylindrical cavity resonator excited in the TM0,1,0 mode twould have .an approximate length of 10 centimetersatotprovide operating frequency, .as discussed above.

ehavingithezaboveespecifiedwenergy- This results Efrem :theconservative :estimate of the possible voltage .;gradient (10 volts 1 per centimeter), .WlEIlOh.1CB.ILbB successfully maintained in such a resonator. "Where the excitation; employed is in the lTMoamimodeyit. is apparent that a similar overall length of resonatorwould lee-required for :the'production ofelectrons having a billion electronzyoltsef .zenergy. ,If theexcitation energy .zsupplied to tuber l has, a wavelength equal to .120 ;cm., arcavity resonator-.sectioniormed by. conducting-wallspositioned at the points-p an'dg will rlriaveia shunt impedance:otapproximately -.l-() .aohms .and :willxbe .a .halfewavelengthuor 110 cm. long. ..:riccordinglypeach. of these-1.0. 0111;;5881710115 .-.providesl0 volts;andthe peakpower: per; section -;is:..given.:by

1, R 10 Watts pable of delivering power in excess of this quan- .tity.

To obtainelectrons.having the same energy,

. it.is.possible to reduce the requisite amount of ex- ..citation.energy.by .the simple expedient of employing. a longertube'tl. Withinlimits, the re quired powerandtube length are inverselypro- .p-ortional. At the same operatingfrequenoy electrons having the specified energy are. obtainable with atube length of 1& cm. or approximately ..300.ft. with .an input power of kw. "l/Vith a .constant tube length, the power required varies .substantiallyas thesquare of the electron energy. .Electrons having an energy of approximately 19 .eleotronvolts are obtainable with a 30. it. tube withan averagepower of 10 kw. or a peak power of 10 megawatts.

Theabove calculations includecertain approximations. Inarriving at the shunt impedance,

.copper' losses in the assumed end walls of each .10 ...cm.-section were included, whereas these .-.losses..wo.uld not occur with thecontemplated cavity. resonator such as tube 2! having a. length equal to numerous half-cycle variations at the Furthermore, no account was taken of the fact that the electrons are not exposed to the peak value of the. .electriofield all of the time in their passage through the tube 25. However, these two errors tend to cancel each other.

The following ,radditional illustrations provide an indication. as to the approximate interrelationship of the various-design factors. It is assumed that the'excitationenergyhas awave- *lengthof 20 cm. and the average power, based ona pulsing time natioof 1000 to 1, will be discussed. Eleotrons of 100 in. e. v. are obtainable -.with a 30 ft. resonator operating with a. voltage gradient of 10 volts per cm. and requiring l0 kw. It'lwilLbe noted that this arrangement produces .anzenergy equal to that. of thelargest existing .f-betatron and could'be constructed at-a frac- :tiOlLflfqthfl cost of the; latter. Tor-procure an ener y D I electron voltsatube .lengthof 3,00

9 it. maintaining a voltagegradient of 10 volts :per cm. with a required power of 100 kw. represents at the present state of the art the best compromise between tube length and input power. For a relatively small installation a tube length of 3 ft. having a gradient of volts per cm. with a 1 kw. power input provides an energy of 10 m. e. v. Such an arrangement has numerouscommercial possibilities, such as an industrial -X-ray equipment for photographing holes in castings and the like. Moreover, inasmuch as such a resonator is only three feet. long it can readily be sealed off and the combined equipment will weigh but a few hundred pounds.

In Fig. 2 there is illustrated one embodiment of the invention together with a schematic showing of the energizing circuit associated therewith. A wave guide 2! partially enclosed at both ends with apertured discs 28, 28 and made preferably of copper, copper-plated steel or brass provides a suitable cavity resonator. The internal surfaces of the resonator thus formed are made highlyconducting to minimize power losses in the device. Throughout the length of the guide 1 constrictions may be arranged in the form of annular rings 29 located normally with respect to the longitudinal axis of the guide 27. To avoid spurious field emissions, the inner edges 3! of the rings 29 are preferably rounded. The center-tocenter spacing of successive rings 29 is made equal to a half-cycle of the free-space wavelength at the operating frequency so that numerous half-wave sections 32 are formed for the purpose discussed above. At each end of the guide 21 an end section 33 is formed on one side by a ring 29 and on the other by an apertured disc 28. A source of electrons 34, providing a beam which is concentric with the guide 21, is supported within a glass bell or press 36 by a lead-in strut 31. The glass bell 36 is, in turn, connected by a met'al-to-glass seal 38 to the end of guide 27. Similarly, at the other end of the guide 2'! another glass bell or press 36 isconne'cted with a suitable seal 38 to facilitate the maintenance of the internal vacuum of the assembly.

The electron gun 34 may include the following elements (not shown): a cathode with an electron-emitting surface, an indirect heater and a focusing or current control'electrode.

For'the control and supplyof energy to the equipment, a generator may be employed having the desired output frequency and power intensity, suchas a magnetron oscillator 42, pulsed by a pulse generator 43, which is also connected to a delay line 4 3. The delay line 44 has two leads 46, 41, connected to the cathode lead-in strut 37 and the outer wall of the guide 21, respectively. A coupling device such as an input wave guide 48 having avacuum-tight seal 39 is employed to transmit energy from the oscillator 42 to the .guide 2?. In place of the input wave guide 48, a concentric line may be used,l in which case it :should be properly formed at the point of con- :nection to the guide 21, in a coupling loop, probe or antenna means to effect a transfer of energy to the guide 21.

' It is, of course, understood that several oscillators, depending on the desired power input to the guide 27, be arranged in parallel to augment the energy supply to the input wave guide 48 or other coupling device. Moreover, a plurality of coupling devices making connection with the'guide 2'! along its length may .be fed by one or more oscillators.- Where a plurality of coupling devices are used, care shouldbe-taken to 10 insure that the proper phase relationship among the energy introduced into the guide 21 is maintained. To achieve correct balance in this respect, suitable phase shifting apparatus may be employed in conjunction with the various coupling devices.

Where the production of X-rays is desired, a suitable target positioned in the path of the highenergy beam of electrons may be employed. For convenience, an X-ray producing medium, such as target 39 may be mounted within press 39 by means of a supporting member 41. The physical orientation of the target 39 may be selected to provide the most effective utilization. If desired, a positioning mechanism (not shown) may be used in conjunction with the target 39 and associated parts to permit the altering of the position of the target 39 during or preparatory to the operation of the apparatus.

It is, of course, understood that, in the event it is desired to employ directly the high-energy beam of electrons for medical therapy, or other purposes, not only the target 39 but the bell 36 may be dispensed with. In such case, to maintain the internal vacuum, the aperture in end wall 29' may be covered by an electron-permeable substance or the entire end wall may be made of such substance. A light metal, such as magnesium, aluminum or beryllium constitutes such electron-permeable substance. In this connection it should be noted that the electron beam when passing through the exit end of the guide 21 is possessed of an accumulation of considerable energy.

However, whereit is desired to obtain a fine control of the energy of the exit beam, both the type and thickness of material may be selected to alter the magnitude of the energy of the exit beam. Indeed, such a substance interposed in the path of the exit beam may be employed independently of any'sealing off of the end of guide 7 and solely for energy control purposes. In addition, the entrance end of the guide 27 may be modified by eliminating the glass bell 36. In its place an electron-permeable substance may be used to cover the aperture of end wall zfi'with a source of electrons positioned adjacent the inpute end of the guide 21.

In operation, a beam of electrons emanating from the electron source 34 is periodically projected through the apertured disc 28 and made to traverse the longitudinal axis of the guide 21.

Upon entering the guide 21, the electron beam is subjected to the accelerating electromagnetic field maintained therein. With the internodal spacings of the field pattern being maintained substantially equal to the free-space half-wavelength at the operating frequency, electrons entering the guide with a velocity approaching that of light will traverse each half-wavelength section 32 in a time interval equal to the half-cycle variation in the electromagnetic field. As a result, the electrons will always be present in a force field favorable for their increased energy, as previously described in connection with the introductory remarks and Fig. 1.

In order to reduce the power required for the operation of the device, a pulse generator 43 is employed to control or activate the supply of electromagnetic energy from oscillator 42 to the guide 21. At the same time, the pulse generator is employed for activation or energizing the cathode circuit through a delay line 44. Thus, electrons emanating from the electron source 34 are injected into the guide 21 periodically and at aesaaan such times when the voltage of the electromagnetic field contained thereinahasattainedja sufe ficient magnitude. The attainingof sufficient' electromagnetic voltage within-the gui'deizil prion to the injection OfithB electronsds achievediby reason of the delay line 44 interposedzbetween-the: electron source 3t and the pulse; generaton'dfii? While in preceding paragraphs.it;has.;been :in= dicated that a right; circular: cylindrical waveguide is eifective =for.-propagating an electric node. of the type TMmmwhich type -ofnode=is=par-- ticularly suitable for the acceleration of:,e1e,c trons, it is of coursetunderstood thatthexwave energy guide zlrma-y. assume other shapes with-:- out departing from the: spiri-tof theninvention':v Furthermore, apart from the conductive annular rings 29, the wave energy guide 21, 96 :(the latter in connection with Fig. 11) has been described as being hollow, however, dielectric material may be used in part to obtain desired electrical characteristics. Thus, such a dielectric material may be used to fill merely aupartofthe-guideor the whole thereof.

In the introductory remarks itahaslibeenindicated that the electrons may beenergizedby passage through a resonator. It is, of course; understood that the aoceleration'of the electrons. may be achieved by projecting them through a traveling wave electromagnetic field. In such a;

case, electrons should be made to ride-thewave crest or, stated somewhat differently, have. a: phase relative to the phase of .theelectromagneti'c field such that they are continually.positioned for the absorption of energy from the field;

It will be noted that there is asection 33 havin a length substantially equaltotheiree-space quarter-wavelength at the operating frequency located at the input end of the guide 21; In the event that the electrons emanatingfrom the electron source. 34 are injected with some velocity relatively low-with respeet-to'that of lightpand which is properly relatedtothe level of the radio frequency excitation power; the average-velocity of the electrons during the first quarter-wave-= length may be approximately one-half thevee. locity of light. As a consequence, the electrons will arrive at the first node at a time when the cyclically-varying electromagnetic fieldreverses. Moreover, where the magnitude ofthe-radio frequency energy employed-is substantially high; the electrons will arrive at thefirst node: with a'velocity very nearly approaching= that of Wight; Once such a velocity is attained by the electrons, there will occur relatively-small velocity changes; so that uniform loading, providedhysuitably positioned constrictions maybe used throughout the remainder of thelength of theguide-Tli These considerations are equally applicable to any of the other modifications of the presentin vention described herein.

In Figs. 3a and 3b. graphs of the volta-ges ex* isting at various points in the circuit of Fig. 2 are illustrated. In Fig. 3a thereisplotted a wave form or the voltage appearing at the output ter-' minals of the pulsegenerator 43.- Asa resultof' the connection of the pulse generator -43Cto the magnetron oscillator 47., the output wave-formnf the oscillat-or has the form illustratedby Fig. 3b? As previouslydescribed, owing to the: delay'linewhich is interposed between the pulsetgeneraton 3 and the electron source-34; electrons-are not projected through the waveguide 21' until the excitation voltage therein-has assumed a suficient amplitude. In this connection an explana tory numerical example mayprove useml As 1 2 previously discussed; .withatime ratio of 1000 to l,iit:is:possible to obtainelectrons having the desired energy without employing an excessive highspowerz excitation. energy source. In the events that: the' time duration of each pulse is madeiequalito one-microsecond, with an excitation" wave having a-frequen'oy of 15x10 cycles perrsecond; therewilloccur approximately 1500 cyclicalv variations ofxthe electromagnetic field forreachpulse Novattemphhasw been made to show thexbuilding; up; of the-ivoltage of oscillator 42 it to. its. normal: operating j output: voltage, nor excitation:voltage;-withirr.the:i-guide27. With a .05 microsecond. delayr introduced by: delay line 44, 15cyclica'1Nariationsiwillloccur':(based on the above-mentioned operating-frequency and pulse durations) prior: to .theinjection of electrons into the guide 21. Thisdelay will compensate for both theivoltage; Off. the: oscillator 42' as well as the f radio frequency excitation voltage withinthe excitationenergy.suppliedpto the guide 2'! during each? pulse; will'necessarily be lost'as far as chargedpartiole energization is concerned. I-Iowever,:this'. is a negligible quantity of power which does.not:impair the functioning or the device.

Whilethe above numerical example was based on a timerratio iof .10001to .=1 for convenientrepresentation,..a:ratio of 25'tor1'hasbeen employed in.Figs..3d"an"d:3b;=.

Whereitisxdesired totemploy a smaller magnitude of. radio; frequency power and a source ofs'relatiyely slow moving-t. electrons, i. e., electrons whichishave a velocity substantially lower than that of :li'ght prior to'being' introduced into-the energizingvportioniofithe apparatus, a wave guidel'l :may .be .formed with tapered constrictions as shown in Fig.4; To permit the injectedtelectrons :whichlare:relatively slow movingsto'z attain: substantially the velocity of light, several sections. 54,.56;;51,:58,3 59 of unequal length are provided adjacentrthe:entrance/end of the guide :2 'l;,'ras.indicated by the arrow fi l. Similarly, wherei.the-inputtradio:.frequency power to the guide 2'l is :relatively small: in magnitude, the tapering of the successive lengths of the several sections .54.; 56, 51,53 ,159: may be advantageously employed; Thei-sections 54;: 5t, 5?, 58, 59 are formed with tapered annular rings ti, 6! having increasing centen-to-centert spacings along the lengthzofrthe. guide 21 .until 'the axial extent of the 'lastisectionnfiz isa substantially equal to the free space half-wavelength at the operating frequency whereupon the constrictions are arranged as described inconnectiorr with the. major portionsof the guide 21 of Fig-2-01" Fig. 3. The initial-section formedat the entrance end of the-sleeve121by endiiwall' 28 andlring 0| has a length somewhat lessthan one-half the centerto-centerdistance betweenarin'gsi 6 I, E I

Preferably, the. inner diameterof the tapered ringsli i, 6 IL: is selectedto. be progressively larger in the direction of the electron flow; In addition,.the:width"of. thetapered ringsel, 6| may be made progressively larger along the length of theguidell. As discussed pr viouslyin connection with the taperedrings 6!, 68, the inner edges .63 may be rounded to discourage field emission.

In'operation, a beam of' electronsis made to traverse the longitudinal "axis of the guide 2'5 in the directionindicated' bythe' arrow '04. Where relatively slowly injected electrons and where relatively low radio frequency excitation power are employed, the progressively larger longitudinal dimensions of the sections 54, 56, 51, 58, 59 permit the electrons to increase in velocity. To this end, the respective lengths of the sections 55, 57, 53, 59 are selected so that electrons pass through an individual section in a time during which the field is favorable for providing acceleration or gain of energy of the electrons. A transit of one of the sections 54, 55, 51, 58, 59, 62 having been completed, the electrons move into the adjoining section 55, 55, 5'3, 58, 59, 52, the electromagnetic field of which is suitable for imparting additional acceleration. In this manner an electron is carried along from one section to the next until it has attained a velocity approaching that of light.

Care must be taken to achieve the correct spacing of the successive sections 54, 55, 51, 58, 59, 62 as a function of the initial electron velocity and the imparted acceleration due to the excitation radio frequency fields. Otherwise, an electron may not enter a section 54, 55, 51, 51, 59, 62 at a time favorable for extracting energy from the field and, in fact, may do work on the field, i. e., deliver some of its energy to the electromagnetic field with a resulting loss of energy of the electron. In the ideal case, the electrons will substantially traverse each section in a time corresponding to the half period of the excitation energy.

A symmetrical coupling arrangement for introducing radio frequency excitation power into the apparatus is shown in Fig. 5, which is a modified form of the device of Fig. 2. The excitation energy is coupled to the device at the entrance terminal thereof, providing for the symmetrical coupling of radio frequency power into the apparatus at the end adjacent the electron source.

As described in connection with Fig. 2, a wave guide 2! having highly conductive internal surfaces is divided into sections by means of constrictions which are formed by conductive annular rings 29. With the exception of the end sections 33, the spacing between successive sections is substantially equal to the free-space half-wavelength of the excitation frequency. At the left end of the guide 27 an apertured disc 23 having an internal surface of highly conductive material is provided, thereby forming a freespace one-quarter wavelength section 33 intermediate the inner surface of the disc 28 and the adjacent annular ring 29.

A hollow cylindrical housing 56 coaxial with the guide 2? is connected in an air-tight manner to the disc 28 and forms an enclosure for the symmetrical radio frequency coupling device. For coupling energy to the guide 21, cylindrical coupling sleeves or antenna 5'! located coaxially with respect to the guide 2'! is arranged preferably extending partially into the adjacently located quarter-wavelength section 33. To 'energize the antenna Bl, a coaxial transmission line having an inner conductor 68 and outer conductor 59, the inner conductor being supported by a one-quarter wavelength shorted stub H, is arranged extending radially to the guide 27 and the coupling sleeve fi'l. One end of the outer conductor 59 is connected to the edges of a suitably formed aperture 12 in the housing 56, with the inner conductor 68 connected to and supporting the antenna 51. The other end of the concentric line 10 is connected to asuitable source of radio frequency energy as shown and. described in connection with Fig. 2.

While only one coaxial line 10 coupling device is shown, it is, of course, understood that several such lines might be employed. In such case, it is preferable to maintain a symmetrical disposition of the lines feeding the antenna 51, in order to preserve the appropriate phase relationships. However, if unbalance occurs in this respect, adjustable phase shifting devices (not shown) may be included in the coaxial lines.

An electron gun 34, similar to that previously discussed in connection with Fig. 2 is supported from a glass bell or press 13 by a lead-in strut 37.

To provide for the maintenance of an internal vacuum, the glass bell 13 is connected by means of an air-tight seal 14 to the end of the housing 66 and forms with a portion of the latter an enclosure for the electron gun 34. Intermediate the antenna 61 and the electron gun 35 an elec tromagnetic field terminating diaphragm "I5 is located normally of the tube 66.

For certain applications, it is important to operate the apparatus without the excitation of certain modes or types of electromagnetic field patterns in the guide 21. A particular advantage of the symmetrical coupling arrangement described above is that certain undesired modes are not excited, that is, those modes which do not have axial symmetry of the electric field.

In operation, a beam of electrons emanating from the electron source 34 is periodically projected through the aperture of field terminating diaphragm 16 and through the coupling sleeve 51. Upon entering the guide 2'! the beam of electrons is subjected to the radio frequency fields and is accelerated or energized in its transit therethrough in the manner previously described in connection with Fig. 2. The diaphragm, while permitting the passage of electrons from the electron gun 34 to the resonator 21, minimizes the loss of the excitation energy endwise of the guide 2'! in the direction of the press 73.

A further modification of the device of Fig. 2 in which the excitation energy is introduced at the terminal end of the Wave guide 2'! is shown in Fig. 6. This arrangement has the advantage of the symmetrical coupling of radio frequency energy into the guide 2'1 described in connection with Fig. 5 and at the same time provides a somewhat simpler and more economical construction. As before, the internodal spacings oi the field in the guide 21 are made substantially equal to free-space half-wavelength at the operating frequency, An apertured disc 25' together with an adjacent annular ring 29 forms a one-quarter wavelength section 53 at the exit end of the guide 21. For introducing the excitation energy a coaxial transmission line TI having an inner conductor 78 and an outer conductor I9 is provided, with collar 85, which is rigidly attached to disc 28', supporting line H. It is understood that a suitable seal will be employed between the collar 83 and outer conductor l9 of the coaxial line 11, and between inner conductor 18 and outer conductor 79, in order to preserve the internal vacuum of the apparatus. A probe antenna 8! which is an extension of the inner conductor 73 projects into the quarter wavelength section 33 at the exit end of the guide 27 and is concentric therewith. To support the inner conductor 18 a dielectric ring 82, having suitable dimensions to prevent spurious reflections of energy, may be provided. Moreover, as an al- 15 ternative to the. dielectric rings; 8240.1: toprovide additional supportrioi'. the innerconductor iiia one-quarter wave shqrt-circuited stubv (not shown) may be employed.

To offer a negligible obstruction: to the high energy electrons the antenna 8! and concentric line il perferably may, be made of some'light metal, suchas magnesium, aluminum, beryllium, etc. Similarly, if, desired, for reasons of manufacturing simplicity; or. otherwise, the disc 28 may be made of these, materials.

If the electrons emanating from the terminal end of the resonator 21 are to-be employed for the production of X-rays, a target of suitable material may, be positioned along the longitu dinal axis of the guide 2? beyond the disc 28.

The above described symmetrical coupling may be employed in, conjunction with the coupling arrangement, shown in Fig. 5, where an additional amount of excitation energy is required. In such a. caseprecautionary measures must be taken to obtain the proper phase relationship between the energy supplied both at the input andoutput end'of the guide Z'l. This proposed arrangementhas all the advantages of the symmetrical coupling and atthe same time provides for introduction into the apparatus of an increased amount ofexcitation energy.

The operation of the apparatus shown in Fig. 6 is substantially similar to that previously described inconnection with Fig. 2.

In Fig. '7 there is shownavv further modification of the present invention in which the excitation energy is coupled to'the apparatus at a plurality of points. As previously discussed, the internodal'spacing of the electromagnetic energy contained within the wave guidefl is reduced by the use, of suitably disposed rings 29 which preferably havetheir inner edges 3i rounded. For uniformly. distributed excitation, a coaxial transmission line 8 1 having an inner conductor 85 and an, outer conductor 81 isarranged adjacent to the guide 27 alongits length. In order to support the inner conductor 86 of the coaxial line 84, quarter wavelength shorted stubs (not shown) or other similar means may be employed.

To introduce radio frequency power from the coaxial line to-the resonator, coupling loops 88, 89, 99, 9I are provided which extend through apertures common to walls of the outer conductor t1 and the guide 21'. Theends of each coupling loop are connected to the inner surface of the, wall of the outerconductor 8i and the inner surface of the wall of the guide'2'i', respectively. The sizes of the coupling loops 88-91 are preferably increased along the length of the guide 27 from its entrance terminal to its exit terminal, to minimize the reflection of energy in coaxial line ti l, as discussed more fully below. Similarly, the apertures-associated with the coupling loops 88- ill may be progressively enlarged to accommodate their respective loops.

It will be noted, as shown more clearly in Fig. 8, that a portion of the walls of the outer conductor 81' and the guide 2"! overlap along their lengths in the vicinity of the coupling loops lit-9i.

In operation, coaxial line 84 is connected at one end to a source of radio frequency energy, such as the magnetron oscillator 52 shown in Fig. 2. With impedance matching existing between the oscillator 42 and the coaxial line a l power losses in the line 84 are'negligible and an extremely low standing wave ratio is present therein. The coupling loops 83-9l are made progressively larger throu houtlthe axial? extent of the trans.- mission; line fi t thegsize-oi each; loop being selectedso thatit extracts a predetermined amount of power from the line 84; More specifically, the the, dimensions of the individual loops are chosen so that a constantly increasing percentage of the power remaining inv the transmission line til is introducedinto-theguide 27 by each successive loop $34M. In this manner; a substantially equal amount of power is introduced into the guide 21" by each of the loops 88 through ti and, moreover, all of the, radio frequency energy is extracted from the, line 84- prior to its terminal end. Asa result, there issubstantially no energy which maybe reflected backthrough the transmission line 84 in the direction of the electromagnetic energy source and, hence, standing Waves are substantially avoidedin the line a l.

Apartfrom the specific arrangement for introducing the radio frequency excitation energy into the guide 21, the operation of the device is similar to, thatdescribediin.connection with Fig.

Of course, it is understood that the coaxial line 84 does not necesarily. haveto extend-for the full lengthof theguide 21', but may at any convenient point be made to extend directly to the electromagnetic source.- Such proposed lines may merely be effective for energizing a single or but a few coupling loops, such as the loops 38, 89, 9B. Other, transmission lines arranged in the manner of linet tmaybe employed to excite distinct groups of coupling loops. As a result, a plurality of coaxial line; feeding sections such as that shownin Fig. 7' may be employed with a single or a plurality ofihig-h frequency generators. In all of the foregoing versions suitable phase shifting apparatus may be advantageously em ployed in the respectively feeding lines to obtain the proper phase relationshipamong the-input energy.

The distributed input coupling arrangement of Fig. 7 may. also be modified by employing a plurality of coaxial lines arranged longitudinally with respect to the guide 2? shown in Fig. '7 and spaced uniformly. around the periphery of the guide 27. This arrangement provides not only for the uniformly distributed introduction of, excitation power into. the guide 2?, but in leltddition. achieves a symmetricalexcitation as We In- Fig. 9- there isshown a modification of the arrangement of Fig. 7 which also achieves a distributed coupling of the excitation energy to the guide 21'. This embodiment diiTers from the showing of Fig. 7 essentially in'the omission of the-coupling loops'and the, utilization of coupling slots 92, 93, 94 having progressively larger dimensions in the direction of energy propagation along line 84 for efiecting a transfer of excitation energy to the guide 27'. The walls of the guide 21' and the outer conductor 8'! of the coaxial line 84-aremadeto overlap in the vicinity of the slots; 92, 93, 94 as shownmore clearly in Fig. 10.

The operation or this device, .aswell as possible modifications of thestructure, is similar, to those described, inconnectionwith Figs. 7 and 8.

lf'he distributedcoupling disclosed in connection with Figs. '7 through 10 has at least three important advantages: (1) The starting transient at the beginning; of each. pulsedoes not have a long duration compared with that of the single coupling. arrangements. (2') Undesired modes of the electromagnetic field are suppressed to an even. reat r extentv th n with the coup i compartments H13, 4434,

1(3) I he ma 92 throng-11 94 is substantially less than with fewer :coupling units, which is 'an important 'fac- 1 itor a'trhigh power levels.

Theiprefer red embodiment of the present in "vention, 'vvhich attains maximum "electron -e'n- *ergy with." great power economy through the use 'of a symmetricail-distribution couplingiof theira- .110

-'-dio frequency into the'wave-guide, tapered loading and synchronous :pulsing :of the excitation energy and the electron source, is iilustrated in --Fi'g. 11. in this'arrangement the advantages of both the symmetrical and distributive apparatus in introducing the excitation :power into the resonator are achieved. Moreover, --1oading ='feature previously 'described in connec- 'tion with Fig-ki is advantageously utili'z'ed'so-that the '-tapered electrons having initial velocities-substantially less than the velocity of light may beemployed as well as -iowerexcitation energy. Furthermore; provision is made for pulsing both the input energy to the resonator and the electron source,

"such pulsing being particularly effective for ob-i --taining a highenergy charged particle beam.

A wave (guide .96 is provided with constrictions which reduce the internodal spacing of T the-electromagnetic field, as previously discussed.

However, the guide 95 is constituted of a plurality of energy-directing sleeves 91, 98, 99,1111,

M32 which divide the guide into a plurality of I95. Each compartment 493, Hi l, titis-constituted of an energy- -directing 'sieeve 91-98, 99, -i-fii,wl-02 with an en- .larged diameter portion 181, 198,1139, Ill, H2,

respectively, which extends over the sleeve '01? the :adjacent compartment without making physical contact therewith, to provide radially and axially extending annular slots H3, H4, H6, H1,

418. Theseslo-ts H3, 1 I L'HS, I ll, I [-8 are preferably made .to have progressively :lower capacitances in the direction ofelectromagnetic energy propagation so that progressively larger percentages-of the power remaining in the enlarged coaxial supply line i It may be coupled to the wave guide 9 5. "These annular capacitances may be made progressively smaller by progressively increasing the spacing or progressively decreasing the efiective areas in the annular capacitance regions, or both. n v

To support eachcempartrnent its, 1&4, -li.6.,-a plurality of radially extending rods 12'! made of .anappropriate material, such has of dielectric composition are arranged intermediate the outer .wall of thesleeves 9198,99, lfll, IE2, and the' inner wah o'ft'he enlarged outerconduCtorJl-ZZ.

the wall of the outer conduetorlf'z toaccommodate screws 52d iorsecuring one end of the rods i2i3'th'ere'to. "The "other endl'of the rod's-l'23 are fflared with their end surface suitably-raced for Toooperating with the outer Wall of the sleeves 93,,"394 $81., $552.. A Wave-directing end-Sleeve 12-6 "is connected with the inner conductor .IfZTof the enlarged outer conductor 1 22 of the fenlarged coaxial supply line H9 with the constricted outer conductor :53! of the constricted coaxialsupply line; 8., tin-cuter conductorconic'al. portion I 32 "Suitably located apertures 123 are provided in "by the wave guide 96. For'i-nterconnectin'g the,

"of theendsl'eeve F26,'the'conical1portion i32 being fcon'nected 'to an intermediate outer "conductor I portion [35 .WhiCh iSWGflXifil with the hollow inner conductor spor'ition iil ifin order to supply excitation energy to the device, the :coaxial supply line =i-2sis connected to an excitationenergy source, sueh as a magnetron oscillator *42, th'e output of which is conitr'o'iled by a pulse generator i3. To provide a "source rdf e'i eetrons, which are to 'be accelerated, iian fe'lectromsource E i is arranged 'ext'erior to both othe enlarged ic'caxial supply line H9, which houses theiwave guidett, and the reduced diameter-supply line IE8. The electron source, which "contains suitable electron-emitting surface i-vlith I appropriate potential-fixing elements (not shown)-, is aligned to insure that the electrons 'arezlprejected successively through an aperture 13B the constri'cte'douter conductor [3 1, the hollow inner conductor portion and thence along the axis'of the wave guide In-add'ition to controlling 'the'outp'ut of the magnetronoscillato'r 12, 'the pulse generator 43 regulates the introduction of electrons int-o the waveguide 396. It is, of course, understood that electrons maybe introduced into the apparatus from the opposite end. However, with "this arrangement the form of 'the co'nstriction-s will be somewhat different,

as discussed below.

"In order to reduce the internodal spacing of --the electromagnetic wave contained within waveenergy guide -96, constrictions in the form of -annular rings are arranged throughout the tength of :guide "96. The compartment formed 0f the end sleeve [-25 and compartments "I33 and Add, :arecprovided-withannular rings 6|, 61", 433 in a manner similar to that previously described in connection with Fig. 4. Thus, the rings fil, 64 i'33 :have "increasedcenter to-centerspacings, --aperture diameters and'widths in the direction of electroma-gnetic energy propagation. To accommodatethe variation indimensions of the -rings -6l,,-6-l' I33, the-associated sleeves 126, '91, -;98, respectively, have appropriate longitudinal --dimens-ions. in the device of Fig. 11, there -has-been -sh-own'a plurality of compartments 1413,4434,suitahleforobtaining the tapered loading teaturewhich wasde'scribed more fully in connection with Fig.4. =Itwil1 be, of course, understood that where desired, a "greater number of such compartments W3, H14 may be utilized. Where it is-desired to inject electrons from the 'uppositeend ofthe guide -96, the tapered loading rings 61-, 51.2;133 will be arranged in the opposite order iso that the increasedcenter to-center spacing-of the rings 51, '51, I33 will occur in the direction of the path of the electron beam. With such an arrangement, the electron source 34 will be positioned at the opposite end of the guide '96. Conrpartrr-rents wt-are formed 'with annular ring id, which asirniiar-ly-reduce the internodal spacing;-of;the;electromagnetic energy contained .within theguide -96. Assuming that the electrons travelling longitud-inally through the guide '96 have obtained a velocity substantially equal to the velocity-oblig-ht, the Fannula'r rings '29 are arranged in such a manner-as to provide inter- 330K185. spacing substantially equal to the freespace half-wavelength eat the operatin'g frequency. in-this manner, the desired highlyen'er- :g-ized beam-oi charge'd particle's is obtained.

To "insure ":that the internal "vacuum or the apparatus is maintained; a- 'gla's's' be1114'lsurrounds the enlarged outer conductor I22, a pernects the outer conductor I3I and the glass bell I4I. Additional seals (not shown) may be used to provide a vacuum-tight connection between the wires I44 and the glass bell I4 I.

Conventional devices, such as short circuited quarter-wavelength line or stubs (not shown).

may be arranged radially to the constricted diameter coaxial supply line I23 to provide support for the inner conductor I27. If desired, one or more supports or struts similar to those used in connection with sleeves 91, 98, 99, Ifil, I02, may be;

employed for providing additional support for the end sleeve E28. Furthermore, suppressors of undesired nodes (not shown) may be included in the enlarged coaxial supply line H9. An annular dielectric ring I34, I36, I31, I38, I39 of suitable composition is shown included in the annular slots H3, H4, H6, H1, H8, respectively, to increase the power handling capacity of the coupling units and provide additional support for the sleeves 97, 98, 99, IOI, I62. desired, the dielectric rings I34, I36, I31, I38, I39, may be omitted.

In operation, the pulse generator 43 regulates the supply of electromagnetic energy, which emanates from oscillator 42, to the enlarged coaxial.

supply line II 8. At the same time, the pulse generator 43 is effective for controlling the introduction of electrons projected from the electron source 34 into the wave energy guide 95. By reason of the delay line 44 interposed between the pulse generator t3 and the electron source at ample opportunity is provided for the electro magnetic energy within the guide 96 to reach the required intensity prior to the introduction of electrons. The electromagnetic excitation energy, which is propagated through the enlarged coaxial supply line H9 is coupled to the wave guide 96 through the annular slots H3, H4, H6, H1, H8. By employing progressively larger slot in the direction of energy propagation an increasing percentage of the power remaining within the enlarged supp-1y line H9 is introduced into the guide 96. With the varied size annular ring 65, SI, I33, electrons introduced with velocities somewhat lower than that of the velocity of light are permitted to absorb sufiicient energy from the electromagnetic waves contained Within guide 96 to attain a velocity approaching that of light. This action takes place within the end sleeve I26 and the compartments 03, I04. Once the electrons have approached the desired velocity, the constrictions 29' are uniformly arranged, providing an internodal spacing substantially equal to the free-space quarter wave-length at the operating frequency.

Thus, the arrangement shown in Fig. 11 represents the preferred embodiment of the invention and is particularly useful for obtaining electrons which have the desired magnitude of energy. By employing the desired loading, it is possible not only to employ electrons which have velocities substantially lower than that of the velocity of of charged particles permits the construction of an energizer capable of delivering charged particles having energy greatly in excess of anything However, if

heretofore known. The novel symmetrical distributive coupling arrangement is particularly effective for reducing the starting transient at the beginning of each pulse, suppressing undesired modes of electromagnetic propagations and imposing a greatly reduced power load upon each coupling unit. The latter point is particularly significant in apparatus operating at high power levels where voltage breakdown and undesirable heating must be avoided. Thus, the device of Fig. 11 is particularly effective for achieving the major objects of the invention.

Since many changes could be made in the above construction and many apparently Widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electron accelerator comprising a source of radio frequency excitation energy, a coaxial transmission line connected to said source of energy, said coaxial line having an enlarged portion, the enlarged inner conductor portion being hollow and comprising a wave guide for maintaining a cyclically-varying high frequency field, said guide having an entrance end, a source of electrons located adjacent said entrance end of said guide, a plurality of annular rings located within and concentric with said guide, and means for projecting said electrons into the entrance end of said guide, the Width and the axial spacing of said rings increasing along the length of the guide and the diameters of said rings increasing along the length of said guide, said spacings of said rings approaching the free-space halfwavelength of said high frequency field.

2. In an electron accelerator, a wave guide having internal conductive surfaces and having an entrance end, a source of electrons located adjacent said entrance end of said guide, a symmetrical coupling means for introducing excitation energy into said guide, said coupling means including a coaxial transmission line section having a tubular inner conductor located intermediate said source of electrons and said guide and concentric therewith for the passage of electrons therethrough, said guide having annular rings with center-to-center spacings corresponding to the free-space half-wavelength at the operating frequency whereby the propagation of undesired modes of electromagnetic energy is substantially eliminated.

3. An electron accelerator comprising a source of electrons which are to be accelerated, a wave guide having one end located adjacent said electron source, means for projecting said electrons into said guide, a source of high frequency excitation power, a coaxial line extending along the length of said guide and having a portion of one of its conductors common with the wall of said guide, saidcoaxial line being connected to said power source, said common portions of said conductor and said wall having a plurality of longitudinally spaced apertures for distributedly introducing excitation energy into said guide.

4. An electron accelerator comprising a wave guide having an internal conductive surface and constrictions for reducing the internodal spacing and uniformly'distributed round the periphery of said guide and further having portions of its conductor walls common with the wall of said guide, said coaxial line means being connected to said power source, said common portions of said walls having a plurality of longitudinally spaced apertures, whereby a symmetrical and distributed coupling of said excitation energy into said guide is accomplished.

5. A high power electron accelerator comprising a guide formed of a plurality of coaxial sleeves having internal conductive surfaces, each of said sleeves having an enlarged outer diameter portion at one end for overlapping the adjoining one of said sleeves to provide annular coupling slots, an outer conductor surrounding said guide and coaxial therewith, said outer conductor and the outer wall of said guide forming an enlarged coaxial supply line for supplying electromagnetic energy to said guide, said sleeves having internal annular rings coaxial therewith for reducing the internodal spacing of the electromagnetic energy therein, a portion of said annular rings having increased center-to-center spacings, widths and aperture diameters along the longitudinal axis of said guide in a portion'of said sleeves, the remainder of said rings having uniform dimensions and spacings in the remainder of said sleeves, a source of electrons positioned adjacent one end of said guide, an oscillator connected to said enlarged coaxial supply line for supplying thereto said electromagnetic energy, and a pulse generator connected to said oscillator for regulating the introduction of energy into said coaxial line, said pulse generator further being connected to said electron source through a delay line to insure the excitation energy attaining sufficient proportions prior to the introduction of electrons, whereby a high-energy beam of electrons is obtained.

6. An electron accelerator comprising a hollow wave guide having a longitudinal axis, means for producing a stream of electrons and projecting said stream within said guide along said axis, an outer conductor surrounding said wave guide and cooperating with the outer surface thereof to form a coaxial transmission line, said transmission line being adapted to be connected to a source of high frequency electromagnetic wave energy, and means including apertures in the Wall or said wave guide for transferring electromagnetic wave energy from said coaxial line to said wave guide, said apertures being radially symmetrical about said axis.

7. An electron accelerator comprising a hollow wave guide having a longitudinal axis, an electron gun adjacent one end of said guide and aligned on said axis to project a stream of electrons along said axis through said guide, a plurality of annular rings located within and concentric with said guide and spaced along said axis, consecutive rings along the initial portion of the length of said guide being of increasing inner diameters, coupling means including a tubular conductive member coaxial With said guide and located between said electron gun and said end of said guide, a source of high frequency alternating current energy connected to said coupling means, and a pulse generator connected to said alternating current source and to said electron gun to energize said source and said gun periodically.

8. An electron accelerator comprising a hollow wave guide having a longitudinal axis, an electron gun adjacent one end of said guide and aligned on said axis to project a stream of elec trons along said axis through said guide, coupling means including a tubular conductive member coaxial with said guide and located between said electron gun and said end of said guide, a source of high frequency alternating current energy connected to said coupling means, a pulse generator connected to said alternating current source and said electron gun to energize said source and said gun periodically, and a delay line included in the connection between said pulse generator and said electron gun, the delay of said line being approximately equal to the time required for an electromagnetic field to build up in said wave guide to substantially the full intensity to be produced therein by said alternating current source.

JOHN R. WOODYARD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,009,457 Sloan July 30, 1935 2,122,538 Potter July 5, 1938 2,147,454 Morton Feb. 14, 1939 2,220,839 Hahn Nov. 5, 1940 2,222,901 Hahn Nov. 26, 1940 2,245,670 Hollmann June 17, 1941 2,247,338 Ramo June 24, 1941 2,276,247 Hahn Mar. 10, 1942 2,280,824 Hansen Apr. 28, 1942 2,281,550 Barrow May 5, 1942 2,297,305 Kerst Sept. 29, 1942 2,367,295 Llewellyn Jan. 16, 1945 2,398,162 Sloan Apr. 9, 1946 2,423,390 Korman July 1, 1947 2,432,093 Fox Dec. 9, 1947 2,434,334 Shepard Jan. 13, 1948 2,439,401 Smith Apr. 13, 1948 2,463,267 Hahn Mar. 1, 1949 2,516,944 Barnett Aug. 1, 1950 2,540,488 Mumford Feb. 6, 1951 2,566,386 Varian Sept. 4, 1951 2,568,090 Riblet Sept. 18, 1951 OTHER REFERENCES Wideroe: Ein neues Prinzi zur Herstellung hoher Spannungen, Barhines fiir Electrot, volume 21, 1928, page 391.

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