US2924714A - Electron accelerator - Google Patents

Description

Feb. l 9, 1960 Filed Aug. 9, 1956 A. P. DAVISv Er AL ELECTRON ACCELERATOR v 5 Sheets-Sheet 1 Feb 9, 19604 Filed Aug. 9, 1956 A. P. DAVIS ETAL ELECTRON ACCELERATOR 5 Sheets-Sheet 2 Feb. 9, 1960 A. P. DAvls E'rAL 2,924,714

ELEcTRoN AccELERA'roR Filed Aug. 9, 195e y s sheets-sneer s Feb. 9, 1960 A. P. DAvls `ETAL ELEcTRoN AccELERAToR 5 Sheets-Sheet 4 Filed Aug. 9, 1956 Feb. 9, 1960 A. P. DAVIS vETAM. 2,924,714

ELEcTRoN ACCELERATOR Filed Aug. 9, 1956 5 Sheets-Sheet 5 United States `Patent d ELECTRON ACCELERATOR Arthur P. Davis and Adnan Waly, Brooklyn, N.Y., asslgnors to Electronized Chemicals Corporation, Brooklyn, N.Y., a corporation of Delaware Application August 9, 1956, Serial No. )603,102

claims. (c1. zsodtas)v The present invention relates to electron accelerators and more particularly to an electron accelerator having a very high power capacity which supplies high velocity electrons in bursts of extremely. short duration and at a high burst repetition rate.

Although an accelerator of the type which `forms the subject matter of this invention can be constructed 1in a rather wide range of power and voltage capacities, the particular embodiment herein illustrated and described, supplies electrons at a velocity of something over two million volts and at a continuous input power capacity of approximately sixty thousand watts. The repetition rate for the electron bursts may be conveniently at a rate of sixty per second although other rates may be desirable as will appear. The burst duration is about ive and one-half microseconds. The wattage or electron density during the bursts is therefore seen to be extremely high.

The high Voltage circuit for the accelerator which forms the subject matter of the present invention is a variation of the classic Marx impulse circuit which produces high voltages by charging a large number of capacitors in parallel to an intermediate voltage and then, discharging them in series across spark gaps which isolate the individual capacitors prior tothe discharge. The ultimate potential realized during the discharge is a function of the charging voltage times the number of capacitor and spark gap groups in the circuit.v j

One of the objects of the present invention isV to provide a practical high density electron accelerator of the above character which can be manufactured in quantity at reasonable costv and in which the circuit elements are largely standardized and easily replaceable.

An additional object is to provide a novel accelerator of the above character in which high voltage fields are rather precisely controlled so as to permitv voltages of the order necessary in a relatively low cost, very compact machine. I

Still another object is to provide an electron accelerator of the short duration pulse type in which the electron beam from the accelerating tube is almost monochromatic.

Yet another object is to provide an electron accelerator capable of supplying high velocity electrons in bursts of short duration and in which the character of the electron beam is particularly adapted for irradiating substances so that all portions of the substance will receive substantially the same electron dose.

Still another object is to provide improved components.`

for such an accelerator so as to greatly extend the service life and reduce maintenance problems and costs.

Other objects and advantages will become apparent ing equipment of the present invention;

- details;

from the following description of a preferred embodiment of our invention which is illustrated inthe accompanying drawings. f

In the`drawings, in whichsimilar characters of referf ence refer to similar parts throughout the several fviews;

Fig. 1 is a 'diagrammatic perspective kview illustrating a typical installed arrangement of the electron accelerat- Fig. 2 is a somewhat diagrammatic perspective `View drawn to larger scale of the top of Fig. 1 with portions of the structure broken away so as to reveal underlying Fig. 3 is a somewhat diagrammatic perspective view of a portion of the'mechanism with certain of the struc; ture broken away or removed so as to reveal features of the construction. r

Fig. 4 comprises a diagrammatic'electrical circuit for the principal portion of the apparatus together withan electro-mechanical diagram of the arrangement of the elements providing this circuit;

Fig. 5 is a vertical transverse-sectional viewthrog'h" one of the spark gaps used in the present device;

Fig. 6 is a graphic'representation of the energy ab.- sorption within a substance bombarded by substantially: monochromatic high velocity electrons; and A Fig. 7 is a view primarily in vertical central `section showing a portion of the electron accelerator .of:1' `ig.` l:

Electronaccelerating apparatus of the'present typef is made up of several interconnected circuiti'and me-s chanical elements. In general there is an accelerating' tube having a cold cathode at one end andV an anode? and electron permeable window at the other. VThe space' within the tube is quite highly evacuated although some gas remains therein. On a large scale, such as is here? required, the low pressure within the tube is preferably-V in parallel to a lowerl potential,l in the present instancef by a direct current charger which operatesby stepping; up and Vrectifying the available alternating current poweri Means are also provided for periodically ring the tube.- after the capacitors have been charged. fifi Referring to Fig. l of thedrawings, in which a'rtypical installation of an electron accelerator of the type-form" ing the subject matter of this invention is illustratedsomewhat diagrammatically, it will be seen that the" equipment is housed within a building, a building oor' being indicated at 10. Charging current for the accel-' erator is supplied by a pair of chargers 12 and`13 whichy need no particular description since they may be ofr wel-1 known type and consist merely oftransformers and rectifiers such that their circuits supply relatively high* voltage direct current power. One of these chargesv is connected so as to supply power at a positivev potential-f of sixty-eight thousand volts with respect to ground; whereas the other charger supplies a negative potential- The potential difference as between the outputs of the two charges is thus, approximately one hundred thirty# six thousand volts, which in this instance is the charg ing voltage. v f

The mechanism in this machine which combines the; capacitors and spark gaps and some ancillary equipment `to be described presently, is indicated at k14 anclUre- L-Y Patented Feb. 9,

3 i :eives the output from the chargers 12 and 13 by way )f lines 16 and 18 which enter the assembly by way )f insulating bushings 20 and 22.

The electron accelerating tube is arranged at the center Lud coaxial with the condenser and spark gap mechmism and its lower end or anode 24 is shown as proecting downwardly through the floor so that the exit vindow of the tube is spaced a short distance above the upport Vfor an article or substance to be bombarded. 'n the `present instance this support is illustrated as a noving conveyor belt 26. The accelerating tube needs lo special description, since it may be conventional, for nstanceof the type described in Patent No. 2,043,733, ssued June 9, 1936, to Arno Brasch and Fritz Lange. ccelerating tubes of this character are preferably of a aminated construction. That is, the tube with the cathde at one end and the anode and window at the other, sformed of alternate rings o-f conducting and insulating naterial so as to limit the electrical iield gradient across ndividual sections of the insulating material to a relaively low value, thereby preventing the possibility of reak-down of the insulating material under the high 'oltage stress `to which electron accelerating tubes are ubjected. This principle is explained in the above menioned patent and is currently a well known construction echnique. For further details and variations if intersted -see the following United States patents: No. 2,099,- 27 issued to Arno Brasch and Fritz Lange; No. 2,429,- .l7 issued to Arno Brasch; No. 2,449,872 issued to Arno lrasch and Wolfgang Huber.

The vacuum pump for maintaining an appropriate subytmospheric pressure within the electron accelerating tube s indicated at 28, this pump being connected to the tube ty Way of a duct 30. The portion of the equipment .'hich comprises the accelerating tube (excepting the :wer or window end), the capacitors, the spark gaps and ome additional equipment is enclosed within a pressure 'essel consisting of a base plate 34 and bell tank 36 havug'its open end secured against the plate by bolts 38.

tank is shown as being made of steel, but some adantage in reducing the stray capacity of the system is tbtained if a glass reinforced plastic tank is used. The ost on the basis of present manufacturing methods of uch plastic tanks is higher, however.

The controls and monitoring portion of the apparatus re ,centered in an operators console at 40.

By referring to Fig. 2, in which the structure of the op of the tower unit 14 is shown in somewhat greater .etail, it will be seen that the accelerating tube 24 at he center of the tower is -surrounded by a sleeve 50 of msulating material which extends from top to bottom. mound the sleeve 50 are arranged the capacitors 52, he charging chokes 54, wave shaping inductances 56 nd the spark gaps 58. These elements, which extend enerally radially outwardly of the sleeve 50 in layers re surrounded by corona rings 60 at each layer. Each f the vtiers or layers represents a potential diiference 'ith respect to the adjacent layers, as will appear more [early presently, of about 68,000 volts. These layers re substantially identical and a description of the top nit will therefore serve to clarify the entire structure.

Vertical support pillars 62 extend from top to bottom f the tower and in general are spaced so as to provide :om therebetween for the individual electrical circuit wmponents. These pillars are formed of insulating ma- :rlal and act to support the individual capacitors, charglg chokes, discharge inductances, spark gaps, corona ngs and resistors as will appear.

'I 'he pillars and sleeve 50 may conveniently be formed E interlocked relatively short sections which are molded l appropriate shape and they may if desired be of lamiited construction by interspersing thin metal plates at ytervals between adjacent insulating sections. By referng particularly to Fig. 3 it will be seen that each of the tpacitor units, 52for instance, is enclosed within a metal 4 case having well rounded corners and edges. The avoidance of any sharp edges or projections is important. Note that each case on its inner end, that is, the end closest to the sleeve 50, is provided with a cylindrical projection or peg 64 formed of insulating material. This peg slides into a socket 66 formed in the wall of the insulating sleeve 50. Near their opposite ends, the cans for the capacitors 52'are provided on their sides with lingers 68 formed of insulating material, which slide into notches 70 formed in the adjacent vertical pillars 62.

Thus a condenser pack 52 can be removed from the assembled mechanism by simply pulling outwardly upon the can so as to slide the ngers 68 out of the notches 70 and the peg 64 from the socket 66. This plug-in arrangement greatly facilitates assembly and disassembly of the electron accelerator and subsequent servicing thereof. The chokes are similarly provided with pegs at their inner ends for insertion into complementary sockets in the insulating Sleeve 50 and with ngers such as at 72 for insertion into notches formed in the sides of the pillars 62.

Insulating brackets 74 secured to the outer faces of the pillars extend outwardly and are turned upwardly at their free ends to form hooks upon which the metal corona rings 60 are hung as is best seen in Fig. 3.

The spark gaps 58 are manufactured as separate enclosed units, as will be explained presently, and are plugged in at their ends tosockets 76 which are carried in some instances .by conducting brackets 78 attached to the corona rings and in other instances by brackets 80 which are secured to the sleeve 50. These brackets 7S and 80 form a portion of the electrical circuit as well as structural supports for the spark gaps.

Other electrical connections between the individual circuit components are provided by lengths of tubing as at 88 which snap into place. For instance, each of the capacitor packs is provided with contact balls 90 at each -side outwardly of the fingers 68 which are connected internally of the cans to the capacitors. Each piece of tubing 88 has cup-shaped ends which are spring loaded outwardly and snap over the appropriate balls thereby connecting together a set of two adjacent capacitor packs. Substantially this same arrangement is used for interconnecting the other circuit elements although it will be appreciated that some of these tubes 88 will be longer and have different conformation than others, as for instance the one to the left of Fig. 3 which interconnects two capacitors a considerable distance apart and has an intermediate plug-in support 92 to the bracket which supportsthe choke 54 from the adjacent pillars.

The space within the `tank36 is pressurized with a relatively inert gas which helps to prevent electrical breakdown as between the various elements maintained at a high relative potential. `For this purpose a mixture of nitrogen and carbon dioxide at a pressure of about 25 to `30 atmospheres is satisfactory. Lower pressures and other gases may be used, depending upon individual design variations. This gas also acts as a coolant and is circulated by the mechanism indicated generally at 100. Thls circulating and cooling mechanism needs no particular description since it consists merely of one or more electrically driven blowers (such as shown at 300 in Fig. 7) which take the gas from a low point within the tank and pass it through a water cooled heat exchanger and thence by way `of any suitable duct to the bottom of the annular ,space between the insulating sleeve 50 and the discharge tube 24. The cool `gas passes upwardly toward the top of the tower and outwardly through a plurallty of holes in the sleeve 50 so arranged as to cause the cool gas to flow over the capacitors, chokes, and spark gaps Varranged around the sleeve. After cooling these circuit components the gas vpasses outwardly into the vspace between the tower and the tank wall 36 and thence downwardly Yso as to be recycled by the circulating and cooling unit.

A few of the cooling holes through thewall of the sleeve 50 are indicated at 102, their size and position belng best determined by the amount of heat rejected by the various circuit components. In general, the charging chokes will need the most cooling.

At the top of the tower the various circuit components are enclosed beneath a cover 106 formed of sheet metal which is shaped as half a torus. The outer periphery of this cover has the same diameter and is in alignment with the top corona ring 60, while the portion at the inner diameter thereof rests upon the upperend of the accelerating tube 24 and serves as the conductor between the tube cathode and the output of the circuit components just described. Optionally, the inner portion of the torus may rest upon the top edge of the sleeve 50 in which case a jumper 107 is used to connect the tube top or cathode to the cover. To prevent cooling air from being blown out of the annular space at the top, a simple cover 109 may be used to close this opening.

Ordinarily at the top of a high voltage structure of the general nature of that just described, a hemispherical cover is conventional for controlling and limiting the electrical eld. Customarily, therefore, the diameter of the sphere of which such a dome is a portion is determined by the diameter of the cylindrical structure to becovered. The height of the cover, therefore, is equal to approximately half the diameter of the tower structure. We have found, however, that the radius of curvature of a cover of this character from the electrical standpoint can be much less than that determined simply by the physical diameter of the tower structure. The torus arrangement shown has a radius of curvature which is more than electrically adequate and has a considerable advantage in that it reduces the overall height of the cover structure by about one-half. It is also beneficial in that it reduces the stray capacitance between the cover and tank 36.

The spark gaps are shown in Fig. 5 where it will be seen that the cathode at 110 and the anode 112 are sealed within an envelope 114 in the present instance glass, and are connected therethrough by supports 11S to terminal caps 116 and 118, respectively. Spark gap cat-bodes in the type of service to which these are subjected ordinarily have a relatively short life. In the present instance this problem has been largely solved. The cathode at 110 is formed of sintered powdered substantially pure iron and the lower end thereof dips into a small pool of mercury at 120. The mercury wets the iron and forms a thin coating thereover. The result is that the electrical discharge vaporizes the mercury rather than the iron and the mercury Vapor thus produced in turn condenses upon the cooler surface of the envelope and returns to the pool at 120, additional mercury being continuously absorbed from the pool to replace that vaporized.

So as to reduce the physical size of these spark gaps and extend their life, they are carefully heated, evacuated and washed with helium and hydrogen to remove the enclosed air and nally are refilled with hydrogen to a pressure of approximately two to three and one-half atmospheres.

This is accomplished by a connection to the outer end of the tubular anode support 11S which has an opening 117 in the wall thereof close to the back reentrant surface of the anode. The anode may be made of any of several materials, but molybdenum has proved to be highly satisfactory for this lpurpose.

The sockets for accepting the spark gaps have convexly curved shields as indicated at 122 and 124 to aid in controlling the electrical field and internally these shields are provided with rings 126 which form a spring loaded slip t with the cylindrical terminal caps 116 and 118.

In Fig. 4 the principal circuit for the device is illustrated in electrical circuit diagram form at the right hand side, with the general physical arrangement of the circuit elements being indicated directly to the left for purposes of easy comparison. In this diagram the positive 68,000

' 6 volt lead from the power supplyy is shown at 130, while the similar negative lead appears at 132. f

For convenience in comparison, the numbers used to indicate the circuit components where possible are the same as those used in Figs. 2and 3, excepting that the individual layers of the tower have been indicated generally by the letters A, B, C and D. In this diagram, A indicates the layer at the bottom of the tube, B the layer next thereabove, D the top layer, and C the layer next below the top. `Between layers B and C there are a multiplicity of additional layers which are substantially identical to either B or C. In this connection it should be noted that the circuit arrangements of layers B and C are substantially identical excepting for the alternate arrangement of the spark gaps which is merely for convenience in positioning these elements and making the connections. These' minor differences alternate from layer to layer so that successive layers have their mechanical placement and electrical connections in the order B- C-B-C-B-C, and so on.

By tracing the charging current it will be evident that there is a connection from the charging positive lead l through a'choke 54 to one side of the capacitor banks at 52 in the layer A. Similarly, the negative charging lead 132 is connected through a choke 54 and discharge inductances S6 to the other side of these condenser banks.

The positive potential is also applied through a charging choke 54 in the layer B to the condensers 52 in that layer, whereas the negative potential is applied tothe opposite side of these capacitors by Way of layer A charging choke S4, and layer B charging choke 54 andthe wave shaping inductances 56 in layer B. The positive side, that is, the left hand side of the capacitors in layer B is connected through two spark gaps 58 i1i` series to the negative side of the capacitors in the next lower bank by way of the wave shaping inductances 56 in this lower bank.

The two spark gaps 58 in series, since they are connected across the leads 130 and 132, support a potential difference of about 136,000 volts. The center point be# tween these two spark gaps is connected to the corona ring of the particular layer. The corona ring, therefore, is essentially at ground potential during charging, but

would probably have some tendency to oat one side or I the other of ground, depending upon how closely the individual circuit components are matched to each other'.

To avoid any problem from this source, the corona rings are connected together by high value resistors which are indicated at 150. These resistors, like the spark gaps, are not electrically strictly speaking in any particular layer, but rather serve as elements which interconnect adjacent layers. Physically the resistors may be arranged between the spark gap support brackets that are secured to the insulating column 50. They are conveniently connected into the circuit in this embodiment by means of ball and socket fittings as are the other elements.

The bottom layer A differs from the others somewhat in that the two spark gaps 152 and 154 in series, are connected between the positive charging line 130 and ground and therefore each of these supports halfthe positive potential of 68 kv. Each of these gaps, therefore, is designed to support 34 kv. and to break down at considerably less than twice this value. All other spark gaps support 68 kv. and break down at less than 136 kv.

The central point between the two 34 kv. spark gaps 152 and `154 is indicated at "160 and this point is connected to ground through the secondary 162 of a trigger transformer 164. This secondary is also shunted by a high value resistor 166 similar to the resistors 150. The primary 1618 of the trigger transformer 164 is connected to the output of a pulse circuit 170. This pulse circuit needs no special description since it periodically merely provides a short duration pulse in a well known manner. For this purpose we prefer to use a substantially con-` ventional thyratron type circuit which conveniently may apague Y26 may operate a simple control mechanism 27 which is 4connected to re the pulse circuit in synchronism with the conveyor, so that objects or material on the conveyor will receive a predetermined dose regardless of conveyor speed up to the maximum cycling rate of the circuit.

The layer at C is electrically a duplication of that of B and needs no special description. The top layer at D is also similar to those in the intermediate layers excepting that the negative potential side of this subcircuit is connected to the field control cap 106 and thence to the cathode of the tube 24.

This circuit operates in the following fashion:

The capacitor banks in the several layers are isolated from each other by the several spark gaps and are .charged so that in each bank there are two charged parallel branches or groups of capacitors with each of these branches consisting of several capacitors in series (in the present instance, each branch provides four capacitor packs 52 with two capacitors in series in a can comprising each pack) After all capacitors have been charged so that each series branch supports a potential of about 136 kv., the pulse circuit 170 energizes the trigger transformer primary 1\68. This energizes the transformer secondary 162 and momentarily raises the potential across the spark gap 154 sufficiently above 34,000 volts to cause this spark gap to break down. Discharge of this spark gap results in a potential difference across the spark gap 152 of about 68,000 volts, with the result that this spark gap in turn breaks down, and so on throughout the system. This discharge cycle takes place in a matter of only a few microseconds and in older type circuits results in a sharp rise and then a sharp drop in the potential difference between the tube cathode and the grounded anode. The maximum potential difference between the cathode and anode in a typical system might therefore be 2,000,000 volts, but the entire voltage range up to 2,000,000 would also be present in such quantity as to make the 2,000,000 volt portion represent only a small fraction of the discharge energy. This conventional type discharge thus produces electrons at a velocity of 2,000,000 volts mixed with a large quantity of electrons at low and intermediate voltages. Such activity results in a great loss in e'lciency and excessive radiation at the surface and at shallow depths as compared to the deep interior of the substance being treated.

Monochromatic radiation (radiation at a single voltage) gives an energy absorption substantially as indicated by the curves of Fig. 6. lf it is assumed, for instance that the radiation cornes from the left and that the surface is at M-N and that the deepest energy absorption is at R, the area lying beneath the curve P--Q-R represents the energy absorption at various depths. From this it is apparent that the greatest energy absorption is not at the surface, but at a level about one-third or so of the distance into the interior. By similarly radiating the substance from the opposite side, the energy absorption will be as represented by the curve S-Q-T. The etfect of radiating from both sides, therefore, is to given an absorbed energy distribution as represented by the composite curve P-V-S. This curve is of course one of a family, since by using a thicker or thinner layer of the substance being radiated, the degree of overlap of the curves P-Q-R and S-Q-T can be changed. ln any event, from these curves it will be apparent that almost, but not quite monochromatic radiation is desirable. Some but not much lower voltage radiation is desirable to raise the points P and S approximately to the level of V.

Monochromatic radiation requires that the high voltage pulse applied to the accelerating tube have the form of a square wave. We have found that excellent efficiency can be achieved with a substantially square Wavechaving a short rise time `so `that the total` discharge Vco'vers a period of about 5.5 microseconds lwithsthe comparatively flat top .of the wave occupying a period .of `about 4 microseconds. This is accomplished by theinteraction .of the capacitors and `the vwave shaping `inductances :56 .previously mentioned.

In Vthe circuit given Iand those of a related nature various values may of course be used and will depend Vto some extent upon the Vpower capacity and upon the distributed capacity and inductance of the system.

`ln the interest of completeness, however, the following values have been found to be satisfactory and may be considered typical in the embodiment described.

The storage capacity of thercapacitor packs 52 is about 4 watt seconds per pack at the voltage used. The wave shaping inductances '56 have a value of about 2 millihenrys with about 15% of this value as mutual inductance. The resistors have a value of about 3.5 megohms, and a suitable value for the isolation chokes 54 is about 300 millihenrys. Although it is not essential, we prefer to provide the inductances 56 as separate coils which can be shifted slightly with respect to each other so as to vary the mutual inductance, thereby providing a means for tuning or aligning the system. Note also that each of these inductance units 56 consists of one coil of about l millihenry in series With one of the condenser banks `52 and `this coil plus another, also of about l millihenry is in series with the other condenser bank in the same layer so that the discharge time of one of the capacitor banks is shaded slightly with respect to the other, thereby lling out or ybroadening the flat top of the discharge wave.

ln order to facilitate reference between the right and left portions of Fig. 4, numbers from 200 to 213 have been used to indicate points of equal potential, thus making it easier to trace the leads from one layer to the next.

ln Figures 2 and 3 which are somewhat diagrammatic, the charging chokes 54 and the wane shaping inductances 56 have been shown as open coils in order to distinguish them from the capacitors. 'Ihis is satisfactory, but preferably they should be potted in cans as are the capacitors. By enclosing the capacitors and the inductances of both types in cans which are smooth and have well rounded corners and edges the eld between the cans is so well controlled that the layers may be brought extremely close together without di'iculty. Furthermore, this closeness of the cans concentrates the fields into the region between the cans and subjects the insulating material to much less field stress.

Previously it was mentioned that stray capacity between the corona cap 106 andthe tank 36 is reduced by using the arrangement shown. This situationcan be improved further by using a larger tank, a pear-shaped tank which is larger at the top -or a glass reinforced plastics tank. The importance of straycapacity is that it reduces the eiciency somewhat by giving a lower voltage back wave to the discharge. More important, this back wave may cause a reverse discharge of the spark gaps, which has a tendency over a period of time to erode the anodes. The embodiment shown is believed by us to be an excellent compromise of these several factors which affect the cost of the equipment and its size.

From the above description of a preferred embodiment of our invention shown as incorporated in aspecic structure, it will be apparent that modications and variations may be made without departing from the scope .or spirit of the invention and that therefore the scope of the invention is to be measured by the scope of the following claims. Y

Having described our invention, what we claim as new and useful and desire to secure by Letters Patent of the United States lis:

l. In an electron accelerator, means forming a tower, said tower comprising a centrally disposed electron discharge tube, means forming an insulating supporting structure surrounding said tube, a multiplicity of capacitors, charging chokes and spark gaps supported by said structure in a plurality of substantially circular layers, said layers being substantially similar to each other, circuit means interconnecting said capacitors, chokes and spark gaps in a Marx type circuit so that the maximum potential difference between adjacent layers during discharge is substantially constant from layer to layer, means forming a pressure vessel enclosing said tower, charging means outside said vessel for supplying a charging current at a desired charging voltage, and circuit means connecting said charging means to the rst said circuit means.

2. In an electron accelerator, means forming a tower, said tower comprising a centrally disposed electron discharge tube, means forming an insulating supporting structure surrounding said tube, a multiplicity of capacitors, charging chokes and spark gaps supported by said structure in plug in relationship thereto in a plurality of substantially circular layers, said layers being substantially similar to each other, circuit means interconnecting said capacitors, chokes and spark gaps in a Marx type circuit so that the maximum potential diierence between adjacent layers during discharge is substantially constant from layer to layer, means forming a pressure vessel enclosing said tower, charging means outside said vessel for supplying a charging current at a desired charging voltage, and circuit means connecting said charging means to the iirst said circuit means.

3. In an electron accelerator, means forming a tower, said tower comprising a centrally disposed electron discharge tube, means forming an insulating supporting structure surrounding said tube, a multiplicity of capacitors, charging chokes and spark gaps supported by said structure in a plurality of substantially circular layers, said layers being substantially similar to each other, circuit means interconnecting said capacitors, chokes and spark gaps in a Marx type circuit so that the maximum potential diierence between adjacent layers during discharge is substantially the maximum discharge potential divided by the number of layers, means forming a pressure Vessel enclosing said tower, charging means outside said vessel for supplying a charging current at a desired charging Voltage, and circuit means connecting said charging means to the first said circuit means.

4. In an electron accelerator, means forming a tower, said tower comprising a centrally disposed electron discharge tube, means forming an insulating supporting structure surrounding said tube, a multiplicity of capacitors, charging chokes and spark gaps supported by said structure in a plurality of substantially circular layers, said layers being substantially similar to each other, means interconnecting said capacitors, chokes and spark gaps in a Marx type circuit so that the maximum potential diterence between adjacent layers during discharge is substantially constant from layer to layer, said marx type circuit providing a charging circuit by way of said chokes for said capacitors in parallel and a discharge circuit by way of said spark gaps for said capacitors in series, said discharge circuit including inductance means for giving the output of said discharge circuit a substantially square wave impulse characteristic, means forming a pressure vessel enclosing said tower, said vessel being filled with an inert uid, an intermediate voltage circuit outside said vessel for supplying a desired charging voltage, and circuit means connecting said intermediate voltage circuit to said charging circuit.

5. In an electron accelerator, means forming a tower, said tower comprising a centrally disposed electron discharge tube, means forming an insulating supporting structure surrounding said tube, a multiplicity of capacitors, charging chokes and spark gaps supported by said structure in `'a -plurality-of. substantially circular layers, said layers being substantially similar to each other, circuit means interconnecting said capacitors, chokes and spark gaps in a Marx type circuit so that the maximum potential difference between adjacent layers during discharge is substantially constant from layer to layer, means forming` a corona ring surrounding each of said layers, aihigh potential eld controlling cap shaped as substantially half a torus having a major diameter substantially equal to that of the corona rings at one end of said tower, means forming a pressure vessel enclosing said tower, charging means outside said vessel for supplying a charging current at a desired charging voltage, and circuit means connecting said charging means to the rst said circuit means.

6. In an electron accelerator, means forming a tower,

said tower comprising a centrally disposed discharge tube,

means forming an insulating sleeve surrounding said tube, -a supporting structure surrounding said sleeve, a multiplicity of capacitors, charging chokes and spark gaps supported by said sleeve and structure in a plurality of substantially circular layers, said layers being substantially similar to each other, circuit means interconnecting said capacitors, chokes and spark gaps in a Marx type circuit so that the maximum potential difference between adjacent layers during discharge is substantially constant from layer to layer, means forming a pressure vessel enclosing said tower, said vessel being lled with an inert gas to a pressure of several atmospheres, a heat exchanger, blower means for circulating gas through said heat exchanger, through said sleeve and outwardly thereof over said chokes, spark gaps and capacitors, charging means outside said vessel for supplying a charging current at a desired charging voltage, and circuit means connecting said charging means to the first said circuit means.

7. A high voltage spark gap comprising means forming a cathode of porous iron, means forming an anode spaced from said cathode, an envelope enclosing said anode and cathode, a pool of mercury in said envelope in Contact with said cathode, said envelope being lled to a pressure above atmospheric with an inert gas, a pair of spaced terminals, and conducting means connecting said terminals through said envelope to said cathode and anode.

8. In a high voltage impulse circuit of the type providing a multiplicity of capacitor banks, chokes and spark gaps, a charging circuit connecting said capacitor banks in parallel by way of said chokes, and a discharge circuit connecting said capacitor banks in series by way of said spark gaps, the improvement which comprises, providing said discharge circuit with a pair of spark gaps in series between each of said capacitor banks, a plurality of high value resistors, said resistors interconnecting the common points of all of said pairs of spark gaps.

9. Ina high voltage impulse type electron accelerator system, an electron accelerating tube having an electron window, a movable conveyor in front of said window, means for repetitiously supplying high voltage impulses to said tube, yand means for synchronizing the repetition rate of said impulses with the rate of movement of said conveyor.

10. In an electron accelerator, means providing an electron discharge tube and a high voltage impulse circuit therefor, said impulse circuit comprising a multiplicity of series-connected units, each unit comprising at least one spark gap and in series therewith a network of capacitors and wave shaping inductances, the relative values of said capacitors and inductances being such that the discharge of said capacitors are shaded slightly in time with respect to each other, so as to produce as square a pulse as possible, each of said units being bypassed by a charging choke, and a low voltage charging circuit connected in parallel with said units, whereby said capacitors are charged to a low voltage by said low voltago Ycharging circuit by way of said charging chokes und are discharged in series across said spark .gaps -and through the discharge tube.

References Cited in the le of this patent UNITED STATES PATENTS Allibone Nov. 20, 1934 Lloyd Jan. 31, 1939 12 Rompe v.. Apr. 4, 1939 McKibbe'n May 23, 1950 Van De y*Graai et 'a1 Aug. 1, 1950 Donath Sept. 25, 1951 Turner Dec. 18, 1951 Burrill June 8, 1954 Brujning Dec. 13, 1955 Trump Mar. 12, 1957

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