US2392380A - High-voltage apparatus - Google Patents

Description

R. H. VARIAN HIGH VOLTAGE APPARATUS Filed Dec. 7, 1942 2 Sheets-Sheet l FlG. l.

18! I Q &3!

I I I ll 1 26a Y 2 1 I60. l4 PHASE SHIFTING Z NETWORK g l Z PULSER 3 .r 84 5 PULSER LOW POWER U.H.F. 36

SOURCE m '3 x O 2 TIME WE V FIG. 3.

P P P PULSER lo INVENTOR Russsu. H. VARIAN ATTORNEY Jan. 8, 1946. R. H. VARIAN 2,392,380

HIGH VOLTAGE APPARATUS Filed Dec. '7, 1942 2 Sheets-Sheet 2 I ll V 1 PULSER FIG. 5.

-IL3O TRANSFORMER ".9 I I r 1/ INVENTOR RUSSELL H. VARIAN 30 ATTORNEY Patented Jan. 8, 1946 HIGH-VOLTAGE APPARATUS Russell H. Varian, Wantagh, N. Y., assignor to Sperry Gyroscope Company, Inc., Brooklyn,

N. Y., a corporation of New York Application December 7, 1942, Serial No. 468,306

32 Claims.

This invention relates to the production of high velocity electron streams and particularly to ultra high frequency apparatus and methods for producing high voltage electron streams.

As the preferred embodiment of the invention, apparatus for producing high voltage electron streams which collide with a target and produce penetrating X-rays will be described, but it will be understood that this disclosure is by way of example only and does not limit the scope of the invention which embraces the production of high velocity electrons for any purpose. In my preferred apparatus I obtain high voltage X-rays by passing an electron beam through a suitable high voltage ultra high frequency field produced by a space resonator as of the type shown in U. S. Letters Patent No. 2,259,690 in which I am co-inventor, and then causing the beam to strike against a metal target to produce X-rays.

Earlier patents relating to ultra high frequency apparatus employing space resonant devices of the general type to be considered relative to my specific disclosure herein, as, for example, Hansen No. 2,190,712 and Webster et al. No. 2,227,372, have suggested the possibilities of producing X- rays therefrom, and this invention may be essentially regarded as a development resulting from those suggestions.

In view of the above, it is a major object of the invention to provide novel methods and apparatus employing ultra high frequency oscillations for accelerating electrons to high velocities. A specific novel adaptation of this object is the production of high voltages for producing X- rays and the like.

A further object of the invention is to provide novel methods and apparatus for producing high speed electrons wherein pulses of electrons to be accelerated are passed through a recurrent oscillating field only when those oscillations have attained'a maximum level.

A further object of the invention is to provide novel methods and apparatus for producing high speed electrons wherein pulses of electrons are passed through an oscillating electromagnetic field in such phase with the oscillations of the field that the electrons are given very high and preferably maximum acceleration during passage through the field.

Another object of the invention is to provide novel methods and apparatus for producing high speed electrons wherein pulses of electrons are injected into a recurrent oscillating electromagnetic field substantially only when the oscillations have reached maximum level, and wherein the electrons are introduced into the field in predetermined phase to be accelerated by the high frequency oscillations of the field. Preferably the oscillating field is contained within a hollow conductive resonator.

A further object of the invention is to provide a. novel method and apparatus for producing high speed electrons wherein the electrons in a pulsating electron beam are velocity grouped prior to introduction into an accelerating high frequency oscillating electromagnetic field so as to be in prescribed phase with said field for maximum acceleration.

It is a further object of the invention to provide a novel method and apparatus for producing high speed electrons wherein a pulsating electron beam is swung through a selected path for introducing electrons into a high intensity oscillating electromagnetic field only in predetermined phase to be accelerated by the oscillations of the field.

It is a further object of the invention to provide novel methods and apparatus for producing high speed electrons wherein direct current pulses are supplied to periodically excite an oscillating electromagnetic field, and electron pulses for injection into the field are controlled proportionally to the rate of change of said exciting current pulses so as to be introduced into said field in predetermined phase with the oscillation periods. Preferably this control is accomplished by a novel transformer arrangement in series with the exciting current pulse supp y.

A further object of the invention is to provide a novel apparatus for producing high speed electrons wherein an electron beam adapted to be injected into a high intensity oscillating electromagnetic accelerating field is preliminarily subjected to the action of a related high intensity oscillating electromagnetic field for introducing the electrons into said accelerating field in proper phase for maximum acceleration. Preferably this object is attained at least in part by altering the potential of the apparatus producing said preliminary action as a unit with respect to the accelerating apparatus, so as to obtain a uniform electron beam.

A further object of the invention is to provide a mobile X-ray or like generator having a novel hollow resonator of relatively small physical dimensions and having relatively low shunt impedance to its exciting voltage.

A further object of the invention is to provide a novel hollow resonator structure having a single resonant circuit and shaped for maximum eniciency of oscillation.

A further object of the invention is to provide novel excitation arrangements for a hollow resonator adapted to be periodically excited to a relatively high oscillation voltage level by pulsating energy, wherein the resonator is excited to a predetermined intermediate level by synchronized pulses of energy introduced into the resonator sufliciently prior to the pulses producing high voltage oscillation. to build up the field to that intermediate level by the time said pulses producing high voltage oscillation are introduced, and thereby keep power consumption to a minimum. Preferably the pulses producing excitation to the intermediate level are of somewhat longer duration than those producing the high voltage oscillation level.

It is a further object of the invention to provide novel ultra high frequency apparatus embodying an oscillator which is excited to at least a substantial minimum level of excitation and then periodically raised to a relatively high level.

of oscillation, wherein the power required to attain a very high accelerating voltage applied to the the minimum level and impulsively produce the higher level is a minimum.

A further object of the invention is to provide a novel ultra high frequency hollow resonator apparatus wherein a substantially annular beam is injected into the resonator to set up a high intensity oscillating field and a second electron beam, preferably central with the annular beam, is injected into the resonator field.

Further objects of the invention will presently appear as the description proceeds in connection with the appended claims and the annexed drawings, wherein Fig. 1 is a partially diagrammatic view including an axial section of a preferred apparatus embodying the principles of the invention and having a hollow resonator for accelerating the electrons to X-ray voltages.

Fig. 1A is a graphic representation of the timing of the auxiliary excitation and main power pulses preferred in the invention.

Figs. 2 and 3 are graphical representations illustrating operation of the apparatus of Fig. 1.

Fig. 4 is a diagrammatic representation of the manner in which the apparatus of Fig. 1 may be used.

Fig. 5 is a partially diagrammatic view of a further embodiment of the invention wherein loosely coupled transformer means is provided for synchronizing the pulses of electrons with the oscillation periods of the resonator.

Fig. 6 illustrates in like manner a further embodiment of the invention wherein a swinging electron beam is injected into the resonator only in proper phase relation to the high frequency oscillations in the resonator.

Fig. 7 illustrates a further form of resonator which may be used in any embodiment of the invention.

Research on space resonator devices leading to the above-identified patents established that the shunt impedance (Rs) of a space resonator, which is comparable to the resistance of a parallel LC circuit at resonance in ordinary radio computations, varies in magnitude directly as the square root of the resonant wavelength, or inversely as the square root of the oscillation frequency. This means that the oscillation voltages obtainable from a continuous driving current of specified magnitude supplied from the cathode through the resonator are also proportional to the square root of the resonant wavelength, since this oscillation voltage is proportional to the product of the driving current and the resonator shunt impedance. Hence, since the shunt impedance increases as the resonator size increases, it has heretofore been assumed that to obtain high oscillation voltages most economically required the use of space resonators of relatively large dimensions having high shunt impedances and relatively low oscillating frequencies.

My present invention includes the d o y electrons prior to striking the target to generate the X-rays. To maintain such a high voltage continuously in an oscillation generator including a hollow resonator would result in excessive power dissipation. It has been found advantageous to successively excite the hollow resonator at the required high voltage for very short time intervals separated by relatively long time intervals during which the voltage within'the resonator is very much smaller. In this manner. the required high oscillation voltag within the hollow resonator is obtained without the high power consumption which would be necessary to maintain the high voltage continuously.

Thus, according to the invention, I propose to produce intermittent but highly periodic X-ray emission by applying power to high frequency oscillator generating apparatus in direct current pulses of suitable duration. For minimum power consumption it is desirable that each power pulse'be of minimum duration for building up the oscillations to full operating voltage, but the pulses must not be shorter than the build-up time of the resonator. This difliculty can be overcome by employing an independent auxiliary oscillator to continuously introduce a small amount of energy at resonant frequency into the resonator so that the resonator is caused to maintain oscillation at a relatively low voltage level between pulses instead of reducing to zero. Under such conditions it is possible to use driving pulses of considerably shorter duration and therefore of appreciably less power than would be essential for reaching full oscillation on each driving pulse in the absence of the auxiliary oscillator. The power saving will exceed the power supplied from the auxiliary oscillator. In my invention I therefore supply power pulses at suitable intervals for speedily fully oscillating a partially excited oscillator as will be later explained in detail.

In general, with reference to the invention, substantially any power pulse duration including very short pulses, may be obtained by appropriate design in the pulser. Although such short power pulses make it possible to achieve the required high voltage in the resonator without high average power input, the instantaneous high power values during each pulse are, of course, not reduced. This would ordinarily require very large feeder lines, which is undesirable. However, in the invention I employ relatively small feeder lines continuously supplying power to a pulser circuit adapted to store the incoming power in a conveniently designed condenser, and then discharge the accumulated power in very short Deriodic bursts through the resonator.

Such condensers are apt to be the most expensive and heaviest part of the equipment, both of which considerations are essential in design of commercial apparatus. Design usually begins with the assumption that a certain amount of money only is available for the condenser and the rest of the apparatus is correlated for most efficient operation with that condenser. My investigations have shown that best results are obtained, and the highest voltages useful for accelerating electrons to form X-rays may be provided from a condenser of given size, through association of the condenser with a very high frequency oscillator having a small closed hollow resonator, as in the invention herein described. This can be shown by the following considerations.

Where a hollow resonator is powered by an electron current from a Pulse circuit having a storage condenser, the amount of charge required from the condenser during each pulse and hence the condenser size) is determined by the product of the current through the resonator and the time the current must be maintained to build up the amplitude of the oscillations in the resonator from the maintained low level to full value. This factor of course determines the size of the condenser as well as establishing a practical minimum pulse duration, since the peak current is limited by the nature of the cathode used.

For a particular resonator shunt impedance and cathode emission current, however, just above the value required to start oscillations in the resonator, and especially during the initial major portion of the buildup period, the amplitude of oscillation and hence the oscillation voltage in the resonator during each pulse will increase in the order of one power of e in seconds, or will be multiplied by e every seconds, where Q is 21r times the ratio of the energy stored to energy loss per cycle in the resonator, f is the frequency of oscillation at resonance and e is the base of the natural logarithms. Actually the exciting current is usually somewhat above the starting value, and so the rate of buildup is somewhat faster. Q varies substantially inversely proportionally to the square root of the frequency I, because of the increased losses at higher frequencies due to decreased skin depth. The buildup time T from low level to a predetermined high level oscillation is proportional to and, therefore, varies inversely as the 3/2 power of I. For a given high oscillation voltage, the beam current required is inversely proportional to the shunt impedance, and is thus proportional to the square root of f. The condenser size depends on the product of the current and the buildup time, and therefore is inversely proportional to the frequency f, for a given oscillation voltage. With higher frequencies, and a given condenser size, the oscillation voltage obtained durin a given pulse duration will therefore increase, so that under these conditions higher oscillation voltages are obtainable, as is desired. This is a distinctive feature of the invention, and is contrary to prior art teachings which used larger resonators for obtaining higher voltages, and hence used lower frequencies for such higher voltages.

However, there is an upper limit on the frequencies which may be used in devices of the present type. Thus, the amount of cathod current required during each pulse to attain a given steady oscillation voltage is proportional to the square root of the frequency of oscillation. The cathode area available for passage of this current through a resonator of particular shape is inversely proportional to the square of the oscillation frequency since the linear dimensions of the resonator vary in inverse proportion to frequency. There is therefore a maximum frequency determined by the obtainable cathode current density. This restriction on the available maximum oscillation frequency is also a practical limitation on the oscillation voltage and condenser size.

The duration of the shortest pulse it is practical to generate also forms a limitation on the maximum frequency. Since the oscillation voltage must build up to maximum value during the pulse, a minimum value of Q for this shortest pulse is determined. This value of Q depends on frequency, and fixes the upper practical frequency limit.

We now consider the necessary relations between the low level or moderate oscillations and the high level or maximum oscillation voltage. If the low level oscillations are too low, excessive high level power is required from the pulsed beam to build up oscillations to the maximum level. If the low level oscillations are too high, more power is expended in these low level oscillations than is saved by pulsing the high power excitation. The optimum condition is found when the power input sustaining the low level oscillations is equal to the pulsed high level power. If, for example. each pulse lasts Vouc of the total time between pulses, then FL) V0 should be of the order of /1000, where V1 is the moderate oscillation-sustaining voltage between pulses, and V0 is the maximum oscillation voltage during each pulse. If

is very much less than /1000, more power from the low level source will be dissipated in the resonator between pulses than is consumed during the high level pulse, whereas if I seconds, the build-up time is approximatel I which is about as short an oscillation build-up time as has been found to be practical under actual conditions. Thus, the duration of each pulse is preferably of the order of seconds.

Still greater economy in power may be had if the sustaining power is also pulsed, but with a longer duration and lower amplitude than the driving pulse to achieve the high voltage, aswill be explained in detail,

Since a large amount of power must be transferred into the resonator in a very short time and it is desirable to keep the cathode driving voltage as reasonably low as possible, a very large exciting current mustbe put into the resonator. This requires that the grid area for the drivin beam must be relatively large and this larg grid area makes the shunt impedance that the resonator has toward the driving beam relatively low. As will appear, I preferably employ a space resonator of such dimensions as to obtain maximum grid area for a particular resonant frequency.

The fact that the resonator used in the apparatus of the invention has low shunt impedance toward the driving beam is further advantageous in that it increases the ratio of available driving power to energy loss. Thus, assuming the resonator Q to remain unchanged, the energy dissipated or lost because of the existence of the capacity of the grids is proportional to AVGZ, where A is the projected grid area and Vo is the voltage between the grids. Although this might indicate that higher oscillation voltages would be obtained by resonators having high shunt impedance toward the driving voltage, as a practical matter emission current density from the cathode has a fixed maximum after which the power deliverable to the resonator is proportional to AVG.

The efficiency or ratio of driving power to loss is, therefore, inversely proportional to Va. The smaller Va is, the greater efliciency is therefore obtained. For a given driving power, this requires A to be increased as Vc is decreased, so

' that a low shunt impedance resonator is required.

In actual practice, Q does not remain absolutely constant and tends to decrease with lower shunt impedance, thus diminishing the above advantage slightly, but this is outweighed by the other considerations explained. The low shunt impedance resonator is thus further advantageous because it permits a relatively high current without requiring high current density. The high input current which thus may be used reduces the required input voltage for a desired power input, reduces slightly the time required for building up the oscillation voltage during each pulse, and operates adequately with a low shunt impedance resonator.

As aresult of the above analysis, it is clear that according to the invention an efficient high frequency X-ray or similar apparatus requiring high speed electrons may be designed employing a cavity resonator of small physical dimensions having low shunt impedance toward the driving beam and alarge projected grid area. This resonator operates very efliciently with high current input which means that the cathode driving voltage is maintained at a minimum for given power input. I have discovered thata condenser capable of delivering high current at relatively low voltage may be employed in the pulse circuit for delivering the required power pulses for intermittent excitation of the electric field for producing sufficient acceleration of an electron beam to produce X-rays.

I have further discovered that the high voltages incident to X-ray operation may be handled by an ultra, high frequency resonator device of low shunt impedance for the driving beam and high impedance for the electrons that produce X-rays, and that because of the inherent limiting factors above discussed it is neither essential-nor efficient to use large resonators to cluding a working head In of convenient size.

A hollow closed generally cylindrical metal resonator body ll, formed with concaved reentrant central end portions l2, has an upstanding'substantially cylindrical extension wa1l l3 at one end on which is sealed 2. glass cover I through a gas-tight sealing rim designated at [5.

Cover l4, extension wall l3'and the side and bottom walls of resonator N form a gas-tight enclosure which is evacuated to a'high vacuum! An annular cathode I6 is suitably supported within the enclosure in alignment with an an-" nular perforated grid l1 formed or seated in the upper end of resonator ll. Within resonator II a partition l8 parallel to the resonator ends supports a concentric annular tubular form l9 provided at opposite ends with annular perforated grids 2| and 22- axially aligned with grid l1. Be-' low grid 22 is a reticulated annular closed grid 23 carried by the bottom wall of resonator II, This grid 23 is used to suppress secondary electron emission from the bottom wall.

This annular cathode and grid construction may be of the form disclosed in Fig. 5 of my Patent No. 2,242,275, issued May 20, 1941, or in Fig. 12 of U. S. Letters Patent No. 2,259,690, issued October 21, 1941, to which reference is hereby made for details of structure and operation.

Centrally of upper dished wall portion I2 is seated a perforated grid 24 for admitting the electron beam from cathode 25 into the interior of resonator ll. Cathode 25 is provided with a conical focusing electrode 26 which focuses electrons emitted from cathode 25 through apreliminary perforated focusing grid 21 and into the resonator.

Heater filaments 28 and 29 for the resonator and X-ray cathodes l6 and 25, respectively, are energized by suitable transformer coupling to a supply line at 3| as indicated. The leads supplying the cathodes are sealed in suitable gas tight joints where they pass through the wall of cover M,

In the exposed end wall of resonator I I, I preferably provide a water-cooled target 32 of suitable design for emission of X-rays produced by impact of the high voltage beam. A suitable annular cooling attachment for the resonator is indicated at 30.

The power circuit for the resonator includes a pulse generator of suitable design 33 embodying a condenser 34 for periodically storing continuous current energy and dischargin at intervals for intermittently producing high voltage pulses of required length and power. As shown at [6a in Fig. 1, the pulse circuit is connected directly to cathode iii. A transformer 35 in series with the condenser 34 is connected at 25a to cathode 25 so that electron pulses for producing X- rays are injected into the resonator at the timethat the oscillating electromagnetic field within the resonator has built to full oscillation voltage. A rectifier 30 is connected in shunt with the transformer 35 for a purpose to be described.

An auxiliary independent source of ultra high frequency energy 36 is coupled as by a concentric line 31 to resonator l I, within the upper chamber aseaeeo which it terminates in the usual energy couping loop 38. This is to provide moderate exc tation of the field between grids l1 and 2| so that the oscillations in resonator II will not have to build up from zero voltage during each pulse from pulser 33. The frequency of this auxiliary source 36 is equal to the resonant frequency of resonator II. The oscillation voltage in resonator ll maintained by oscillator 36 may be about $6 of the peak voltage. The voltage driving cathode l6 may be in the neighborhood of 50,000 volts.

If desired, oscillator 36 may deliver energy continuously to resonator H so that it tends to continuously sustain the moderate voltage level at all times, both during and between the periods that main pulses are received from cathode I6.

I have. however, discovered that a considerable further saving in power may be effected if the power supplied to source 36 is also delivered in pulses related to the driving pulses supplied to cathode l6 as shown diagrammatically in Fig, 1A. This may be accomplished by providing for oscillator 36 any suitable energy source 84, such as a known multivibratcr assembly, having a. pulsating square topped wave output, such as at 80 in Fig. 1A. The output of pulser 33 to cathode I6 is connected by lead 8| to pulser source 64 through a suitable phase shifting or dela network 32 adapted to control the output of pulser 84 so that each pulse therefrom is related in time to the main power pulse 83 from cathode IS in the manner disclosed in Fig. 1A.

Oscillator 36 is thereby periodically excited by pulser 84, and transmission line 31 feeds corresponding pulses of ultra high frequency energy to provide excitation of resonator II to moderate level by the time that the main pulse is injected by cathode I 6. The invention is of suflicient scope to cover any suitable arrangement for obtaining this timing of the periodic energy pulses.

The dotted line in Fig. 1A represents an optimum condition for relation of the pulser which may be obtained by apparatus of suitable design, but the square topped wave arrangement shown in the solid lines represents a good and readily obtained practical embodiment of this phase of the invention.

Where the moderate exciting voltage is pulsed. it may be considerably higher than 6 of the peak voltage if desired.

As indicated by Fig. 1A, pulse 80 is preferably of considerably longer duration than main pulse 83 and is not continuous throughout the entire time between pulses 83. The reason that pulse 80 is of longer duration than pulse 83 is that ordinarily the time required to build up the oscillation voltage in the resonator from zero to the moderate excitation level is longer than the time required to build it up further to peak voltage, because the voltage rise from zero to moderate excitation requires passage through more powers of e and thus occurs at a slower rate, this rate depending on the increase in excitation to be obtained.

It is essential only that the field produced by pulse 80 be built up to the required moderate excitation level by the time that the main excitation pulse from cathode I6 is started. This requirement means that pulse 80 must be initiated and maintained only during the period required to provide that moderate excitation level in resonator I I at the proper time, Consequently there is a period between pulses when no energy, either from pulser 83 or source 36, is supplied to resonator II.

In substance therefore I have provided for periodically exciting resonator II to moderate excitation level so timed that wnn the main pulse is injected from cathode IS, the field within resonator ll need be built up only from that moderate level to peak value. Any suitable apparatus for so timing the periodic moderate excitation of the resonator with the main pulses maybe used without departing from the spirit of the invention.

Resonator II is preferably of annular form as illustrated in order to provide the large grid areas essential to handling large currents during high power operation at minimum driving voltages. This shape gives much increased grid area with attendant low shunt impedance. In a practical embodiment, I use a ten or twelve square centimeter annular cathode area in a resonator having a resonant wavelength of about ten centimeters. The driving power pulse duration is preferably about f seconds as above explained. Preferably, I use an oxide-coated cathode which has an emission current density of several amperes per square centimeter. Thus, each pulse could be made to have a peak current of 30-40 amperes which would represent a required power input of 1500 to 2000 kilowatts with 50,000 volts applied to the cathode. However, since the pulses last only 5 of the total time between pulses, the required average power input is only about 1.5 or 2 kilowatts.

In normal operation of the apparatus shown in Fig. l, resonator ll becomes a self-excited oscillator when the cathode I6 is properly energized. An electron stream passes from energized annular cathode l6 through the space between grids H and 2|. Normally the electric field between grids I1 and 2| would become oscillatory to at least some degree due to thermal agitation of the electrons or other non-uniform electron velocity or density conditions, even in the absence of oscillations from source 36, and this oscillatory field would produce b anching of electrons in the stream passing through the drift space within form 19 and arriving between grids 22 and 23. A resonance would then immediately automatically build up in resonator ll if the flight time through the drift space is correct and become effective to increase the small oscillatory field between grids I1 and 2 1, thus further increasing electron bunching in the stream. This cyclic process would continue until the oscillations in resonator ll would have built up to a steady high value. For further details as to this manner of self-excited oscillation, reference is made to my said Patent No. 2,242,275.

In the apparatus of the invention I am not forced to rely on such minute oscillations as thermal agitation of the electrons for initial oscillations, as oscillator 36 supplies suficient energy to start the oscillations at a relatively higher moderate level as above described. Once the oscillations start building up upon application of the pulse from cathode l6, however, the cyclic buildup process proceeds as described during the pulse until full oscillation amplitude or level is reached.

In Fig. 2, the solid line illustrates the nature of the voltage pulses delivered from pulser 33, which is also the nature of the pulses delivered by cathode l6. Fig. 3 illustrates the nature of the voltage output from the secondary winding of transformer 35. There are two induced voltage pulses in the transformer secondary during each rimary pulse, one when the primary pulse is building up, and one when it subsides, but they are oppositely directed.

The pulse P that makes cathode 25 more positive with respect to grounded grid 21 will of course cause no electron emission therefrom, but the second pulse N which is in the reverse direction making cathode 25 more negative causes the required high increase in electron emission therefrom. As shown by the dotted lines in Fig, 2, the impedance of the transformer may cause a lag in the pulse shut-off. This is relatively unimportant, but it is generally important to prevent this impedance from causing a similar delayed rise when the pulse is initiated. This is accomplished by shunting out the transformer primary at the start of the pulse N, as by rectifier 30', but not at the close of the pulse, as then the current would be in the blocking direction of the rectifier.

Each relatively short delayed electron pulse from cathode 25, corresponding to a pulse N in Fig. 3, is timed to be injected into the resonator whenthe electromagnetic held within the resonator has been built up by the associated exciting power pulse from cathode l to full voltage and maximum oscillation.

Thus, with each pulse from condenser 34, a large amount of power is transferred from cathode |6 into resonator II which automatically builds up the electromagnetic field therein to oscillate at ultra high frequency. Provision of auxiliary oscillator 36 insures that resonator I is very quickly built up to full oscillation at resonant frequency during each pulse.

The high voltage available in the oscillating electric field within resonator II is employed to accelerate the electrons emitted by cathode 25 to velocities suitable for producing very penetrating X-rays. The flight time of the electrons between wall portions I: of the resonator is preferably equal to less than one-half cycle. In this manner, for power inputs in the range of 1.5 to 2 kilowatts as above described, I may obtain electron velocities of hundreds of thousands of electron volts. Because of the penetrating character of the X-rays generated when the high voltage electron beam strikes target 32, the X-rays are capable of penetrating the target and appear outside head III as indicated.

Working head |0 of the X-ray generator, because of the low impedance requirements for the space resonator, may comprise an enclosure not more than six inches across its largest dimension. In actual use it can be connected by a suitable flexible shielded cable 39 to the relatively bulky pulser 33 which may be mounted on a convenient work bench, as shown in Fig. 4. Since all of the high voltage and other lines leading to head "I are within cable 39, the head may be safely and conveniently moved about at will for exploring a casting 40 or like industrial use, and because of its small compact nature may be introduced into cavities not accessible to previously known bulky X-ray generators. The wires within cable 39 need be insulated for only about 50,000 volts and should be so arranged that the cable has but small reactance, to avoid undue complications arising from transmission of the sharp pulses carried thereby. Although the excitation in the generator is some hundreds of thousands of volts, no insulation therefor is needed as that high voltage exists only inside the resonator.

Fig. 5 illustrates another embodiment of the invention having special cathode arrangements and circuits for insuring that the electrons to be accelerated are injected into the resonator not only in pulses timed in proper phase with each pulse from cathode |6 as shown in Figs. 2 and 3, but also in electron groups arranged in proper phase with the high frequency oscillations, for accelerating the electrons to maximum speed.

Resonator II is preferably an annular figure of revolution similar to resonator II in Fig. 1, with an annular grid I! being provided in its inner end wall in alignment with annular cathode I6. Wall 40 is provided on its axis with a medial depressed portion 4| having a central entrance grid 42 for resonator Grid 42 is in axial alignment with cathode 25 which supplies the X-ray producing electrons. A small cylindrical metal resonator shell 43 is coaxial with resonator I A hollow tube 44 projects from the interior of shell 43 in close proximity to wall 40. An entrance grid 45 is provided in resonator 43 to receive the electrons projected by cathode 25, and grids 46 and 41 are provided at the ends of tube 44. A focusing grid 48 is mounted/on wall 40 in the path of the electron stream between grids. Cathode 25 and grids 45, 46, 41, 48, and 42 are all aligned with resonators II and 43.

Pulser 33' is connected to the primary 49 of a transformer having five secondary windings 5|, 52, 53, 54 and 55. Windings 5|, 54 and 55 are loosely coupled to primary 49, while windings 52 and 53 are closely coupled to primary 49 for reasons which will appear later. Cathode filaments 28 and 29 are connected across a battery 56 through windings 52-55, and a lead 51 connects resonator 43 through winding 5| to the positive terminal of a battery 59 in series with battery 56.

The interior of resonator 43 is electrically coupled with the interior of resonator II' by a high frequency transmission line such as the usual concentric line and loop connections indicated at 59, 6|, so that high frequency energy from resonator is transmitted to the interior of resonator 43. An isolating insulating member 60 which functions similarly to a blocking condenser insulates line 59 from line 6| so that they may be at different unidirectional potentials, but passes the high frequency energy from resonator to resonator 43. It is not shown in detail as it is not a part of this invention.

In operation of the apparatus of Fig. 5, pulser 33 delivers high power high voltage direct current pulses as in Fig. 1, but here the pulses are impressed across primary 49.

Cathode l6 is excited by the closely coupled secondaries 52 and 53 and receives a broad pulse such as shown at in Fig. 2. Cathode 25 is excited by the loosely coupled secondaries 54 and 55 and delivers electrons to grid 45 only near the close of each broad pulse in brief pulses timed corresponding to N in Fig. 3.

The simultaneous identical pulses initiated in each of the loosely coupled secondary windings 5|, 54 and 55 insure that structures 25, 43, 44 and 41 all change potential simultaneously by the same amount, thus insuring that a constant stream of electrons exits through grid 4'! to enter resonator during each pulse corresponding to N in Fig. 3. Since there is no voltage between grids 41 and 48 when there is no pulse from pulser 33, no current will enter resonator I except on the pulse N shown in Fig. 3. Resonator 43 together with drift tube 44 causes velocity grouping of the electrons passing therethrough during pulse N in the usual manner and these groups arrive in resonator H during each pulse N in proper phase to receive maximum acceleration from the field within the resonator. The accelerated electrons strike target 32 and produce X-rays as in Fig. 1.

Summarizing the above, current pulses pass through resonator 43 into resonator ll' only at the close of each powerpulse delivered by circuit 33, and are thus injected into resonator I! only when the oscillations in resonator II have reached a maximum. When this pulsating current does flow from resonator 43, the electrons are velocitygrouped in each pulse due to the high frequency electric field between grids 45 and 46 7 and their subsequent passage through drift space 44, and the velocity grouped electrons are given maximum acceleration when passing through resonator l l. The proper phase of these grouped electrons with respect to the field in resonator 43' is assured by connection 59.

Electrons in large numbers therefore enter resonator I i when the oscillation voltage is such as to impart maximum acceleration to them and only a few electrons flow into resonator I I when the oscillation voltage would give them less acceleration. The remaining conditions for accelerating the electrons entering resonator H to high voltage are as described for Fig. 1.

The provision of a plurality of loosely coupled secondaries puts all the sources of bias voltages at ground potential to avoid capacity effects in the apparatus. The same result may be obtained in other ways, as for example by supplying the pulses directly to the respective parts through leads from a single secondary and using radio frequency choke coils in the supply leads to keep the high frequency components from reaching back to the power sources.

Fig. 6 illustrates a further embodiment of the invention wherein a laterally swinging, lateralvelocity modulated electron beam is employed to introduce electrons into the X-ray generator resonator in such phase with the high frequency oscillations therein that the electrons pass through the resonator in largest numbers when the oscillation voltage is highest.

Cathode is arranged to discharge electrons in pulses. similarly to the manner described in Fig. 5, through an entrance grid 62 into the interior of a hollow metal resonator 63 having onposed reentrant lateral poles 64 and 65 and an exit grid 66. Similar to Fig. 5, the smaller resonator 63 is coupled by concentric line and loop arrangements 59, iii to the interior of the large resonator 61, and this arrangement provides that the electric field between poles B4 and 65, which is transverse to the direction of flow of the electrons between grids 62 and 66, imparts alternate lateral velocities to the'electrons and thus causes the beam to swing laterally in accord with the oscillating frequency of the field.

Resonator 63 is of the cylindrical shape obtainable by revolving the cross-section of the resonator 63 about an axis through the center A-A of the reentrant poles 64 and B5. The electron beam passes therethrough at right angles to this axis or transversely to the electric field instead of parallel thereto as in Fig. 5.

The swinging electron beam cyclically laterally traverses the top wall of resonator 61 as indicated and an entrance grid 50 is so located that electrons are injected into the resonator only at one extreme of lateral travel of the beam. An inclined tube 53' having a focusing grid 66' opposite undue absorption of the X-rays.

grid 80 aids in attaining this operation. This extreme is of course chosen so that the electrons enter resonator 61 only when the oscillation voltage in resonator 61 is in such phase as to accelerate the electrons. Timing of the electron pulses with the recurrence of the oscillating field is the same as in Fig. 5.

The operation of resonator 61 in accelerating electrons to high voltage is the same as previously described for resonator II. The principles of lateral beam swinging employed in this embodiment of the invention in resonator 63 are preferably the same as described in U. S. Letters Patent No. 2,272,165, issued February3, 1942, to Varian et al., to which reference is made for further detail.

Fig. 7 illustrates a further resonator construction 58 which may be employed instead of resonators H, I l or 63 in the invention but which is shown by way of example in the assembly of Fig. l. Resonator 68 is a substantially bell-shaped hollow container having a top wall formed with a central depressed portion 69 surrounded by an annular entrance grid section Ill. Smoother and focusing grid II is provided between entrance grid 12 and central cathode 25, and annular cathode I6 is disposed within the evacuated enclosure I3' above the grid H.

The X-ray target 13 preferably comprises a thin coating of gold or some other heavy metal on the fiat internal bottom wall 14 of resonator 68. This coating provides for maximum production of X-rays when struck by the. accelerated electrons, and is of suflicient thinness to avoid The X-rays freely penetrate and emerge from the copper bottom wall of the resonator as indicated. An annular cooling passage member 15 is secured upon the bottom wall of resonator 68. Coating 13 may be used instead of target 32 in Figs. 1, 5 and 6.

\ Resonator 68 is a special development of the type of space resonator disclosed in United States Letters Patent No. 2,269,456, issued January 13, 1942, to William W. Hansen and myself, which employs only a single enclosed resonant circuit dimensioned to maintain an oscillating field within the resonator and to effect electron grouping in an electron beam passing through the field parallel to electrostatic vector of the field. The distance between entrance grid Ill and bottom wall 14 is such that the electrons from cathode l8 entering at different points along the cycle of high frequency oscillations of the field become grouped during transit and deliver energy to the field at the resonant frequency of the field before reaching wall I4. The distance from grid 10 to wall 14 in an operative embodiment was found to be such that the flight time of the electrons between said grid 10 and said wall 14 is about two and one-quarter cycles, at the operating frequency.

Since the theory of operation of this resonator is fully developed and explained in detail in said U. S. Patent No. 2,269,456, further discussion thereof here is unnecessary and reference is made to this patent for further explanation. In the present invention it is essentially important only that resonator 68, instead of resonator H of Fig. 1, may be employed to accelerate electrons X-ray producing velocities.

The considerations of design discussed in th early part of this specification apply as well t resonator 68, so that it is desirable to employ a resonator having low shunt impedance to the driving voltage and large grid area for handling high entrance currents. This is accomplished by resonator 68 which is essentially a figure of revolution made by revolving thecross-section shown about an axis a--a parallel to its electric vector so as to provide large annular grid 10.

As in Fig. 1, cathodes l8 and inject electrons into the resonator in synchronized pulses, having definite phase and duration relations and the field cyclically maintained by the pulses from cathode l6 serves to accelerate the electrons injected by cathode 25 to high velocity for striking target 13. Except for the actual functioning of resonator 88 in producing electron grouping, the operation and uses of the apparatus of Fig. '7 are the same as for Fig. 1.

The shape of resonator 68 is new and 01K s'uilicient advantage to warrant further attention. As above explained, it is a generally bell-shaped fig-- ure of revolution about axis a-a having a continuous annular entrance grid surrounding a central depression in the top wall. The bottom wall is fiat, completing a hollow metal structure shaped according to the above design characteristics.

In the invention, any of the cathode arrangements may be used as desired with any of the resonator structures shown, as the resultant combinations may perform the intended function of the invention.

As 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 t .erecf, 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. In apparatus for producing high velocity electrons, means providing a high frequency oscillating electromagnetic field recurring periodically at spaced intervals, and means passing an electron stream through said field only in such phrase with the recurrence of said field that the electrons in .said stream are accelerated to substantially maximum velocity.

2. In apparatus for accelerating electrons to high velocity, means producing an intermittent electromagnetic field oscillating at high frequency, and means producing pulses of electrons and passing them through said field substantially only when the electrons are accelerated during passage through said field.

3. In ultra high frequency apparatus, means providing a high frequency oscillating field recurrin periodically at spaced intervals, and means injecting pulses of electrons into said field substantially only when the oscillations of said field are at maximum level.

4. In apparatus for accelerating electrons to high velocity, hollow resonator means, means for periodically generating high frequency oscillations within said resonator means, and means for periodically injecting pulses of electrons into said resonator substantially only when said oscillations are at maximum level.

5. In high frequency apparatus, a, resonator, means injecting pulses of electrons into said resonator to establish a recurrent high frequency oscillating field therein, and means injecting further pulses of electrons into said resonator substantially only during the latter portions of said first pulses so that said field is given suflicient time to build up to a, prescribed level before introduction of said further pulses.

6. In a method of accelerating electrons to high velocity, the steps of providing an electromagnetic field oscillating at high frequency and recurring at spaced intervals, and injecting pulses of electrons into said field substantially only in such phase therewith as to receive acceleration therefrom.

7. In a method of accelerating electrons to high velocity, the steps of providing an electromagnetic field oscillating' at high frequency and recurring at spaced time intervals, and injecting pulses of electrons into said field only when the oscillations thereof have reached substantially maximum level.

8. In apparatus for accelerating electrons to high velocity, means providing an electromagnetic field oscillating at high frequencyand recurring at spaced intervals, means passing pulses of electrons through said field in predetermined phase with the recurrence of said field, and means for introducing said electrons into said field in such relation as to receive maximum acceleration from said field.

9. In a 'method of accelerating electrons to high velocity, the steps of providing an electromagnetic field oscillating at high frequency and recurring at spaced time intervals, providing pulses of electrons for injection into said field in predetermined phase with the recurrence of said field, and introducing the electrons of said pulses into said field in predetermined phase for acceleration by said field.

10. In a method of accelerating electrons to high velocity, the steps of providing an electromagnetic field oscillating at high frequency and recurring at spaced intervals, providing pulses of electrons for injection into said field only when said field is at'substantially maximum level,

and introducing said electrons into said field in proper phase to be given maximum acceleration by said field.

11. In apparatus for accelerating electrons to high velocity, means providing an electromagnetic field oscillating at high frequency and recurring at spaced intervals, means passing pulses of electrons through said field in predetermined phase with the recurrence of said field, and means in said last-named means efi'ecting velocity grouping of the electrons in each pulse prior to entering said field.

12. In apparatus for producing highly penetrating X-rays, a source of intermittent power pulses, means energized by said source for periodically building up a high frequency oscillating electromagnetic field, means energized by said source for periodically injecting pulses of electrons into said field in such association that said electrons are accelerated during passage through said field, and target means in the path of said accelerated electrons adapted to produce penetrating X-ray when impacted by said accelerated electrons.

13. In apparatus for producing high velocity electrons, a source of intermittent power pulses, means energized by said source for periodically building up a high frequency oscillating electromagnetic field, and means also energized by said source for periodically injecting pulses of electrons into said field in such association that said electrons are accelerated during passage through said field.

14. In apparatus for producing high speed electrons, an electrical converter for transforming direct current pulses into high frequency oscillating currents establishing a recurrent electromagnetic field, means for supplying energizing current pulses to said converter, means energized from said supply means producing an output proportional to the rate of change of the current in said pulses, a source of electrons to be injected into and accelerated by said field, and means coupling said source to be energized by said output.

'15. In apparatus for accelerating electrons to high velocity, electron accelerating means pro-- viding an oscillatory electromagnetic field, means producing a stream of velocity modulated electrons for introduction into said field, and means altering the potential of said last-named means as a unit with respect to said electron accelerating means. I

16. In apparatus for accelerating electrons to high velocity comprising a hollow resonator, a substantially annular hollow metal body within said resonator, means producing and passing a substantially annular beam of electrons through said resonator and said body, and means producing and passing a second stream of electrons through the space surrounded by said hollow annular body, said space containing the electromagnetic field set up within said resonator.

17. In apparatus for accelerating electrons to high velocity, a hollow resonator, means producing and passing a substantially annular beam of electrons through said resonator for establishing an oscillating electromagnetic field therein, and means for producing and passing a second beam of electrons through said resonator field in such association that said second electron beam is subjected to large velocity changes during passage through said field.

18. In apparatus for producing high velocity electrons, a hollow resonator of electrically conductive material having a substantially continuous electron permeable wall section surrounding an intermediate wall portion having a further electron permeable section spaced from said first section, and means producing and passing separate and distinct synchronized electron streams through said sections.

19. In the apparatus defined in claim 18, said intermediate wall portion being depressed.

20. The method of producing high velocity electrons which comprises passing an electron beam through the oscillating electromagnetic field of a hollow resonator, while subjecting said field to short excitation pulses of a duration equal to substantially is approximately equal to the ratio of the duration 01' each power pulse to the interval between pulses, where V1 is the moderate excitation voltage and V0 is the maximum oscillation voltage during each pulse.

22. A resonator for high frequency apparatus comprising a hollow generally bell-shaped electrically conductive container having a wall closing the smaller end of said container formed with a substantially continuous electron permeable section surrounding an intermediate area, a spaced and distinct electron permeable section in said intermediate area, and a wall across the larger end of said container facing said electron permeable sections.

23. In high frequency apparatus, a hollow resonator, pulsing means for exciting a high frequency oscillating field within said resonator to a moderate oscillation level, further pulsing means for exciting said field to peak value, and means correlating said individual pulsing means so that said means for exciting the field to peak value becomes effective only when said moderate oscillation level has been attained.

24. In a method of exciting a hollow resonator, the steps of periodically exciting said resonator to a moderate oscillation level, and then periodically exciting said resonator to peak value only when said moderate oscillation level has been attained.

25. In high frequency apparatus, a hollow resonator, means periodically producing an oscillating field of moderate value within said resonator, and means periodically exciting said field to peak value.

26. In high frequency apparatus, a hollow resonator, means providing power in pulses for periodically establishing an oscillating field of moderate value within said resonator, and means impressing further pulses of power on said resonator to raise said field to maximum oscillation, said first pulses being of appreciably longer duration than said further pulses.

27. Apparatus for highly exciting an oscillator for short periods comprising means for applying high power pulses to said oscillator for recurrent periods of short duration as compared to the interval between pulses, and means for applying lower power pulses to said oscillator immediately preceding said high power pulses.

28. In the apparatus defined in claim 27, said lower power pulses being of appreciably longer duration than said high power pulses.

29. In apparatus for accelerating electrons to high velocity, means providing a recurrent electromagnetic field oscillating at high frequency, means passing pulses of electrons through said field in predetermined phase with the recurrence of said field, and means for introducing said electrons into said field in such relation as to receive maximum acceleration from said field, said last-named means comprising a hollow resonator oscillating at the same frequency as said field for velocity grouping the electrons in each pulse prior to introduction into said field.

30. In apparatus for accelerating electrons to high velocity, means providing a recurrent electromagnetic field oscillating at high frequency, means passing pulses of electrons through said field in predetermined phase with the recurrence of said field, and means for introducing said electrons into said field in such relation as to receive maximum acceleration from said field, said lastnamed means comprising a hollow conductive resonator enclosing an oscillation space and feedback means coupling said space with the recurrent field.

31. In apparatus for accelerating electrons to high velocity, means providing a recurrent electromagnetic field oscillatingat high frequency, means passing pulses of electrons in a stream through said field in predetermined phase with the recurrence of said field, and means for introducing saidelectrons into said field in such relation as to receive maximum acceleration from said field, said last-named means comprising means eflecting lateral swinging of the electron low resonator providing a recurrent electromagnetic field oscillating at high frequency, means passing pulses of electrons through said field in predetermined phase with the recurrence of said field, and meansvfor introducing said electro into said field in such relation as to receive in I mum acceleration from said field, said last-named means comprising a second hollow resonator in coaxial alignment with said first resonator and oscillating at the same frequency as said field for velocity grouping the electrons in each pulse prior to introduction into said field.

RUSSELL H. VARIAN.

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