US3551677A

US3551677A - Field reversal type pulse generator having a shorting switch in the form of a plurality of parallel spark gaps

Info

Publication number: US3551677A
Application number: US606864A
Authority: US
Inventors: John L Brewster
Original assignee: Field Emission Corp
Current assignee: Field Emission Corp
Priority date: 1967-01-03
Filing date: 1967-01-03
Publication date: 1970-12-29
Anticipated expiration: 1987-12-29

Description

O United States Patent 1111 3,551,677

[72] lnventor John 1.. Brewster FOREIGN PATENTS [2]] App] xfiggr 975,911 11/1964 Great Britain. [22] Filed Jan. 3, 1967 0 OTHER ERENCES [45] Patented Dec.29, 1970 Fitch et a1. Novel Principle of Transient High Voltage [73] Assignee Field Emission Corporation Generation Proceedings of the IEE, Vol. 111, No. 4, Apr.

McMinnville, Oreg. 1964, pp. 849 855 a corporauo of Oregon Primary Examiner-Ralph G. Nilson Assistant Examiner-C. E. Church [54] FIELD REVERSAL TYPE PULSE GENERATOR AtromeyBuckhorn, Blore, Klarqulst and Sparkman HAVlNG A SHORTING SWITCH IN THE FORM OF A PLURALITY 0F PARALLEL SPARK GAPS 14 Claims, 3 Drawing Figs.

ABSTRACT: A field reversal-type pulse generator is [52] U.S.Cl. 250/93, described in which a plurality of parallel spark gaps are -50/102250/49'5' 307/108 ployed as the shorting switch at the end of one of a pair of [5] 1 llll. CI HOSg 1/20 coaxial transmission lines in such pulse generator The parallel [50] Fitld ofSearch 250/93, spark g p have g dielectrics and are caused to break down 33omnqulred 320/1; 108; simultaneously by being irradiated with ultraviolet light 250/49' 102 emitted from the ionized gas of a main spark gap when charg- 1561 1123;132:121: 5;; 11m: ZZZla?0ZilZiXlf UNITED STATES PATENTS generator or a strip transmission line pulser is employed as the 2,652,502 5 Melville et 7/ 1 charging pulser. In one embodiment the field reversal pulse ,8 7/ 1 958 h fiel 320/1 generator produced an output pulse of 600 kilovolts and 7000 J L 7/1964 Godlove 2 /1 amperes, having a width of 3 nanoseconds. The load con- 3,173,006 3/1965 Dyke et al.... 250/49.57 nected to the output of the pulse is an electron radiation tube 3,309,523 3/1967 Dyke et al.... 250/102 or an X-ray tube, both having field emission cathodes in the 3,322,976 5/1967 Blank 320/1 form ofa plurality ofspaced needle-shaped emitters.

FIELD REVERSAL TYPE PULSE GENERATOR HAVING A SIIORTING SWITCH IN THE FORM OF A PLURALITY F PARALLEL SPARK GAPS The subject matter of the present invention relates generally to electrical pulse generators capable of generating narrow, short rise time pulses of high voltage and high current, and in particular to a field reversal type of pulse generator including at least two transmission lines that are charged in parallel and are discharged in series through a common load which may be a field emission X-ray tube or a field emission electron radiation tube to produce an intense pulse of X-rays or beta rays. The transmission lines are discharged by causing a plurality of parallel spark gaps having a pressurized gas dielectric and connected as a shorting switch across the other end of one of the transmission lines opposite from the load to'break down, thereby producing an output voltage pulse across the load substantially equal to the charging voltage of each line and having a pulse'width approximately equal to twice the one-way transit time of one line. In order to enable the parallel spark gaps to break down at substantially the same time and thereby lower the total inductance of the shorting switch, such spark gaps are irradiated with intense visible light, ultraviolet light, X- rays, beta raysor other suitable ionizing radiation prior to over-voltage breakdown.

As shown in British Pat. No. 988,778 of J. C. Martin, field reversal pulse generators, sometimes'called Blumlein circuits, may be made of strip transmission lines, coaxial transmission lines or artificial transmission lines of the lumped constant type. The minimum pulse width and rise time of the high voltage output pulses produced by prior field reversal pulse generators have been limited primarily by the inductance'of the shorting switch employed at one end of one of the transmission lines. In order to reduce this shorting switch inductance it has previously been proposed to provide a plurality of parallel shorting switches between which the signal current is divided so that the total switch inductance is greatly reduced. However this decrease of inductance is only possible .when all of the parallel switches are closed substantially simulswitch such switch must be replaced after each pulse or after a small number of pulses, which is extremely inconvenient and prevents rapid repulsing. If oil is employed in the multiple gap switches, the breakdown of the liquid dielectric produces decomposition impurities which vary the dielectric strength even after cleaning of the liquid and cause subsequent breakdowns to take place at different voltages.

The field reversal pulse generator of the present invention overcomes the above problems by employing a plurality of parallel spark gaps as the shorting switch, such spark gaps being filled with a gas dielectric whichis under high pressure to increase its dielectric strength. In order to enable these parallel spark gaps to break down substantially simultaneously such spark gaps are irradiated with ultraviolet light, X-rays or other suitable radiation before the electrical pulse applied thereto reaches overvoltage voltage breakdown. This irradiation of the parallel spark gaps of the shorting switch may be accomplished by employing a main spark gap connecting the field reversal pulse generator to a charging pulser, as the source of ultraviolet radiation. In this case the main spark gap and the parallel spark gaps are positioned within a gastight chamber containing air, nitrogen or some other suitable gas which emits ultraviolet light during breakdown. In addition, it has. been found advantageous to employ either a Marx surge generator of the type shown in US. Pat. No. 3,256,439 of W. P. Dyke et al. or a spiral strip transmission line type of pulser shown in British Pat. No. 975,91 1 of RA. Fitch et al. as the chargin nulser emoloved in the present field reversal pulse The field reversal pulse generator system of the present invention when used with a field emission electron radiation tube of 60 ohms as its load has produced an output pulseof 600 kilovolts and 7000 amperes having a pulse widthof 3 nanoseconds and a transmitted electron pulse of similar characteristics having an electron energy of 500 kiloelectron volts. The dose per'pulse is 2 megarads on the'beam axis at a distance of one-half inch from the exterior surface of the anode window and the energyper pulse' ot thefiransmitted electron beam was 10 joules, which is theequivalent of approximately 3000 megawatts of power. I I I It is therefore one object of the present inventio'rit'o provide an improved field reversal pulse generator systemwhich is capable of producing high voltage pulses of narrowerpulse width and shorter rise time. I I

Another object of the present invention is to provide an improved field reversal pulse generator system having a shorting switch in the form of a plurality of parallel spark gaps of gas which are irradiated to enable such'spark gaps to break down substantially simultaneously in order to reduce the inductance of such shorting switch.

A further object of the present invention is to provide an improved field reversal pulse generator system which is pulse charged from a Marx surge generator or a spiral strip transmission line pulser in order to produce at output pulse of extremely high voltage and high current.

Still another object of the present invention is to provide an I improvedfield reversal pulse generator system of compact and economical construction, which is capable of producing a large number of output pulses without replacement of -its shorting switch.

A still further object of the present invention is to provide an improved field reversal pulse generator system having a load in the form of a field emission X-ray tube or a field emission electron radiation tube in order to produce short pulses of X-rays or beta rays of extremely high intensity and dose rate.

Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which FIG. 1 is a schematic diagram of one embodiment of the field reversal pulse generator system of the present invention employing a Marx surge generator-charging pulser;

FIG. 2 is a vertical section view of the field reversal pulse generator of the present invention connected to a field emission electron radiation tube; and

FIG. 3 is a partially schematic view of another embodiment of the field reversal pulse generator system of the present invention employing a spiral strip transmission line charging pulser.

As shown in FIG. 1, the field reversal pulse generator system of the present invention includes two transmission lines L, and L formed by two pairs of conductors l0, l2, and l4, 16. These transmission lines are of the same length and have substantially the same uniform characteristic impedance Z,,. The first line L, and the second line-L, are connected together at the ends of conductors 10 and 14 and in series with a load 18, which may be a field emission electron irradiation tube having a conducting resistance approximately equal to twice the characteristic impedance of lines L, and L The load 18 may be connected to the first and second transmission lines by a third transmission line L, formed by conductors 20 and 22, which are connected, respectively, to conductors l2and I6 of the first and second lines in a similar manner to the pulse generator of British Pat. No. 988,778 referred to above. If the third transmission line L, is employed it should be provided with a" characteristic impedance 22,, equal to twice the impedance of the first andsecond transmission lines L, and L so that load 18 terminates the third transmission line in its characteristic impedance.

In order to charge the first and second transmission lines lines L, and L their conductors l0 and 14 are connected in and a charging inductance 28. The charging pulser may be a Marx surge generator -of the type shown in U.S. Pat. No. 3,256,439 mentioned previously, in which the energy storage elements are lumped constant transmission lines which are charged in parallel from a source of DC charging voltage connected to terminal 30 and discharged in series through a plurality of spark gaps connecting the ends of such transmission lines together. Discharge of the Marx surge generator 24 may be accomplished by a trigger pulse applied to one of such spark gaps at input terminal 32 which causes such Marx surge generator to produce an output pulse of, for example, 700 kilovolts and 5000 amperes having a pulse width of about 30 nanoseconds. This output pulse is transmitted through the charging inductance 28 to the main spark gap 26, causing such spark gap to break down and charges each of the transmission lines L, and L, in parallel to a voltage approximately equal to its 700 kilovolts. For charging purposes conductor 12 of the first transmission line L, may be grounded and the conductor 16 of the second transmission line L: connected to such ground through a charging inductance 34 in parallel with the load 18. Also for most efficient charging the parallel capacitance of the two transmission lines L and L should be equal to the output capacitance of the Marx surge generator. The total series charging inductance of

coils

28 and 34 is made of a value to provide an LC charging time of about times the width of the output pulse produced by the field reversal pulse generator across load 18, for isolation purposes. It should be noted that for low voltage applications the transmission lines 1.. and L, may be charged slowly from a conventional DC voltage source, in which case the Marx surge generator 24 and the main spark gap 26 may be eliminated.

However, for high voltage applications the transmission lines are'pulse charged by the Marx surge generator or the spiral strip transmission line pulser of FIG. 3 to reduce the high voltage insulation problems which occur when a very high voltage C 99w i p 9yqdzsv ha a a is Graatts r- In order to discharge the transmission lines L and L and cause the field reversal pulse generator of FIG. 1 to produce an output ,pulse, a shorting switch formed by a plurality ofparallel gas dielectric spark gaps 36 connected across the end of the first transmission line L, opposite from the end connected to the load 18, is closed by overvoltage breakdown of such spark gaps. As in conventional Blumlein circuits or field reversal pulse generators, an open circuit termination 38 is provided at the end of the second transmission line L opposite from the end connected to load 18. When the charging voltage on the first transmission line L exceeds the breakdown voltage of the parallel spark gaps 36, such spark gaps break down and produce a short circuit termination at the corresponding end of the first transmission line causing such line to discharge. In order to enable the parallel spark gaps 36 to break down substantially simultaneously, ultraviolet light is transmitted thropgh such parallel spark gaps as indicated by dashed line 40, from the main spark gap 2 6. Thus the main spark gap 26 contains air, nitrogen or some other suitable gas under a high pressure of approximately 250 lbs. per square inch to provide a high dielectric strength and to enable such gas to emit ultraviolet light to the parallel spark gaps 36 upon the breakdown of the main spark gap 26. It should be noted that X-rays, beta rays, or other ionizing radiation, such as intense visible light emitted from a laser, may be employed in place of ultraviolet by using an external source of radiation which must be employed when DC charging is adopted. A laser has previously been used to switch a field reversal pulse generator by irradiating a single-shorting switch gap, as shown in Iat. No. 3,432,664 of]. B. Robison.

If lines L and L, are both charged to a voltage V, when the shorting switch spark gaps 36 break down a negative voltage pulse V is transmitted from the left to the right in line L until such voltage pulse reaches the junction with lines L and L Since line L is terminated in a matched load, the pulse is transmitted into line L; as a voltage V. However, the pulse is transmitted into line L as a voltage, which is reflected back from the open circuit termination 38 as a pulse discharging line L, until it again reaches line L;,. At the same time a portion of the pulse transmitted to the right in line L, is reflected at the open circuit end of such line which is transmitted back to the short as a pulse pulse is transmitted back to the junction of L; where it meets the such junction so such pulses cancel each other. This causes the voltage on line L; to go to zero and terminates the V output pulse applied to the cathode of tube 18. It should be noted that the width of this output pulse is equal to twice the one way transit time of each of the first and second transmission lines L and L assuming such lines are the same length.

As shown in FIG. 2, one embodiment of the field reversal pulse generator of the present invention is formed by three

coaxial conductors

42, 44 and 46, with the intermediate conductor 44 corresponding to the common conductors l0 and 14 of FIG. 1, the inner conductor 42 corresponding to the conductor 16 of FIG. 1 and the outer conductor 46 corpulse transmitted to the left in line L, at

Y responding to the conductor 12 of FIG. 1. The closed left end of the intermediate conductor 44 is threaded into a spark gap electrode 48 so that an insulator cone 50 is clamped between such members. The insulator cone 50 and the outer conductor 46 are fastened by bolts 52 around the periphery of their right ends to a mounting plate 54. A plurality of secondary spark gap electrodes 56 are threaded through the side of the outer conductor 46 adjacent the left end of such outer conductor and positioned inside such outer conductor spaced about the outer edge of spark gap electrode 48 in order to provide six parallel spark gaps 36 which form the shorting switch between

conductors

44 and 46. The main spark gap 26 is formed between a main electrode 58 threaded on one end of a charging conductor 60 and a hemispherical tip portion 62 threaded on electrode 48. An insulator sleeve 64 of plastic is molded about the charging conductor 60 and cooperates with the main gap electrode 58 to clamp the right end of an insulator cone 66 therebetween.

A gastight chamber 68 containing the main spark gap 26 and the secondary spark gaps 36 is formed by a tubular metal housing 70 having an inner flange fastened to the left end insulator cone 66 by bolts 72 and an outer flange fastened to support plate 54 by bolts 74. Suitable seals, such as rubber 0- rings are provided between insulator cone 66 and housing 70 as well as between such insulator cone and main spark gap electrode 58 to seal one end of the gas chamber. Similar sealing rings are provided between housing 70 and support plate 54 as well as between insulator cone 50 and support plate 54 and between such insulator cone and electrode 48 to provide a gas tight seal at the other end of the chamber. Air, nitrogen or other suitable gas dielectric is pumped into the gastight chamber 68 through a gas inlet passage 76 provided in one side of the support plate 54 in order to fill such chamber with gas under high pressure of approximately 250 lbs. per square inch.

Insulator cones

50 and 66 may be made of fiberglass filled plastic for greater strength to stand such high pressure.

The charging conductor 60 and spark gap electrode 58 attached thereto are supported in axial alignment with the intermediate coaxial conductor 44 and the electrode 48 secured thereto, so that the main spark gap 26 is positioned in a common light path with all of the secondary spark gaps 36. Thus when the main spark gap breaks down it emits ultraviolet light to all of the secondary spark gaps to prime" such secondary spark gaps and enable them to break down substantially simultaneously at a later time due to an overvoltage breakdown.

The coaxial field reversal pulse generator of FIG. 2 is provided within a liquid dielectric filled chamber 78 formed by a metal housing member 80 secured at one end to support plate 54 by bolts 82 and secured at its opposite end to a mounting ring 84 by bolts 86. The tubular metal housing 80 also functions as a coaxial conductor for the third transmission line L; of the field reversal pulse generator. Also the left end portion of such housing conductor acts as an extension of the outer conductor 46 of the first transmission line L,, thereby corresponding to conductor 12 of FIG. 1. The right end portion of housing 80 forms the outer conductor of the third transmission line L thereby corresponding to conductor 20 of FIG. 1. The mounting ring 84 is are welded to one end of an inductor coil 88,- the other end of which is are welded to the funnel shaped open right end of the inner conductor 42. A support tube 90 of plastic insulator material such as methylmethacrylate is provided inside the inductor coil 88 between the end of the inner conductor 42 and a metal flange 92 forming part of a field emission diode electron radiation tube 18 and which is secured to the mounting ring 84 by bolts 94. Support tube 90 maintains the inner conductor 42 spaced the proper distance from mounting ring 84 to correctly position such inner conductor with respect to the intermediate conductor 44.

The electron radiation tube 18 includes an envelope portion 96 of glass which is sealed to a cathode support rod 98 extending through one end of such envelope and is also sealed to the metal flange 92 at the opposite end of the envelope. An electron permeable anode window 100 of metal foil is secured to flange 92 to provide an evacuated envelope for the tube 18. The field emission cathode of such tube is formed by a plurality of needle-shaped emitting elements 102 attached to the inner end of the cathode support rod 98 so that the tips of such emitting elements are spaced substantially uniformly from the anode window 100. The cathode support rod 98 is electrically connected to the inner conductor 42 by a plurality of leaf spring contacts 104 provided on a connector cup secured to the exterior end of the support rod 98 with such spring contacts engaging the inner surface of conductor 42. Thus the cathode support rod 98 also functions as the inner conductor of the third transmission line L thereby corresponding to conductor 22 in FIG. 1.

In order to provide a liquid tight chamber 78, sealing rings of rubber or plastic are provided between the support plate 54 and housing 80, between housing 80 and mounting ring 84 as well as between such mounting ring and flange 92. An oil inlet 106 is provided through the bottom of the housing 80 adjacent the mounting plate 54 and conventional means (not shown) are employed to connect such inlet to an oil reservoir and associated pump. In addition an oil outlet passageway 108 is provided in the top of the mounting plate 54 in communication with the liquid chamber 78 and the oil reservoir to enable such chamber to be filled with transformer oil or other liquid dielectric. Also the space between the plastic support tube 90 and the glass envelope 96 is filled with the oil or other liquid dielectric provided in chamber 78 because holes are provided through such support sleeve to enable the liquid in such chamber to circulate therethrough.

A ground connection 109 is provided on the mounting plate 54 in order to ground the

outer conductors

46 and 80 of the transmission line as well as the anode window 100 of the electron radiation tube. A monitor signal output terminal l may be provided on the housing 80 with the signal conductor of such monitor terminal connected to a capacitor ring 112 provided on the inner surface of the housing 80 and electrically insulated therefrom by a thin layer of plastic or other insulating material. The capacitor ring 112 forms a voltage divider with the coaxial conductors of the transmission lines and transmits a small portion of the voltage produced on the intermediate electrode 44 to a suitable monitor device such as a cathode ray oscilloscope.

The electron radiation tube 18 may be of the field emission vacuum arc type, in which a portion of the cathode emitter elements 102 is vaporized by each output pulse to produce positive ions of cathode material which neutralize the negative space charge ordinarily surrounding such cathode in order to greatly increase the amount of current which flows between the cathode and anode. The current of the electron pulse transmitted through anode window is in the neighborhood of 7000 amperes per pulse. The electrons 114 transmitted through the anode window may be employed to irradiate the object under investigation which may I be positioned immediately adjacent the anode window. However, it is also possible by employing an external X-ray target 116 of tungsten or other suitable material positioned outside of the electron tube in a position to be bombarded by the electrons 114 to produce X-rays 118. Thus the apparatus of FIG. 2 is capable of producing short, high intensity pulses of electrons (sometimes called beta rays) and/or X-rays if so desired.

The following values were obtained in operating the field reversal pulse generator apparatus of FIG. 2 with a Marx surge generator. With the first and second transmission lines L and L each having a characteristic impedance of 30 ohms and the third transmission line L, and the electron radiation tube 18 each having an impedance of 60 ohms, an output voltage pulse of 600 kilovolts and 7000 amperes having a pulse width of approximately 3 nanoseconds and a transmitted electron pulse of about 520,000 electron volts were produced.

In order to provide a simpler and more compact system, the field reversal pulse generator of FIG. 2 may be pulse charged from a second field reversal pulse generator 24 which may be of a spiral strip transmission line pulser, as shown in FIG. 3. This spiral strip transmission line pulser 24' is similar to that described in British Pat. No. 975,91 l by R. A. Fitch et al. Thus the charging pulser 24 of FIG. 3 is formed by a first conductor 120 and a second conductor 122 of .001 inch thick aluminum foil which are separated by a first insulator sheet 124 and a second insulator sheet 126 .004 inch thick of polyethylene tetraphthalate plastic film. These interweaved sheets of alternating metal foil and plastic film are wound in a multiple turn roll to provide a pair of spiral strip transmission lines whose inner ends are interconnected by one turn of conductor 120. Thus one end of each line is positioned in the center of the spiral and the other end of each line is positioned on the outside of the spiral while

insulator sheets

124 and 126 form the dielectrics of the two lines. The outer end of the second conductor foil 122 is connected through a charging inductance 128 to a source of DC charging voltage 130 in order to charge such transmission lines in parallel to the voltage of such source.

A normally open-shorting switch 132 is connected between the output ends of

conductors

120 and 122 across the line having dielectric 124. This shorting switch 132 is closed to cause the discharge of the spiral strip transmission line pulser 24' in a similar manner to that described above with respect to the field reversal pulse generator of FIG. 1. Thus the open circuit termination 134 of the second transmission line is provided by the gap between the outer end of the second conductor 122 and the second turn of the first conductor 120. The first conductor 120 may be grounded and the output terminal of the charging pulser 24' taken from the inner end of the first conductor 120 to transmit an output pulse through charging inductor 28 and main spark gap 26 to the field reversal pulse generator of FIG. 2, which is illustrated by rectangular block 136 in FIG. 3.

The output pulse of the spiral strip line charging pulser 24 is of a generally triangular shape whose peak voltage V, is approximately V 2nV where n is the number of turns of

conductors

120 and 122 and V is the charging voltage. The output capacitance C of the spiral strip line charging pulser is approximately C C where C is the individual capacitance of one conventional strip line. As in the case of the coaxial transmission line field reversal pulse generator of FIG. 2. the inductance of the shorting switch 132 reduces the rise time of the output pulse. Therefore a plurality of parallel spark gaps may be employed for the shorting switch 132 of FIG. 3, in which case an external source of ultraviolet light will be necessary for irradiating such spark gaps if the DC voltage supply .130 is employed as the charging voltage. Resistive losses in the conductors also degrade the wave shape of the of the output pulse of charging pulser 24'. A third loss factor is a result of the transformer coupling between the two strip lines. These loss factors limit the number of turns for a given core diameter and therefore the voltage magnification of the output pulse of the spiral transmission line pulser, such output pulse being of a triangular shape.

It will be obvious to those having ordinary skill in the art that many changes may be made in the above described preferred embodiments of the present invention without departing from the spirit of the invention. Therefore the scope of the invention should only be determined by the following claims.

lclaim:

1. Pulse generator apparatus capable of producing narrow output pulses of high voltage and high current, comprising:

a plurality of energy storage transmission lines connected together to form a field reversal pulse generator including at least two lines of the same characteristic impedance having their output ends connected to a load matched to the output impedance of said field reversal pulse generator with the other ends of said two lines being provided, respectively, with an open circuit termination and a short circuit termination during discharge, and having a charging input connected in common with said transmission lines;

shorting switch means including a plurality of spark gap electrodes connected together to form a plurality of parallel spark gaps connected in parallel with each other and across the end of at least one of said transmission lines remote from said load;

means for providing a gas dielectric in said parallel spark gaps;

charging means for applying a voltage to said charging input to charge said transmission lines in parallel until said parallel spark gaps break down to short circuit said one end of said one line to discharge said transmission lines in series and cause said field reversal pulse generator to produce an output pulse; and

radiation means for irradiating said parallel spark gaps simultaneously with ionizing radiation from the same pulsed source of radiation to cause said parallel spark gaps to all break down at substantially the same time so that said output pulse is a narrow pulse of short rise time and successive output pulses are substantially uniform.

2. Pulse generator apparatus in accordance with claim 1 in which the charging means is a charging pulser having its output connected to the charging input of the field reversal pulse generator by a main spark gap formed by a pair of main spark gap electrodes separated by the gas dielectric.

3. A pulse generator in accordance with claim 2 in which the one main spark gap electrode connected to said charging input also forms one of the electrodes of each of the parallel spark gaps with the other electrodes of said parallel spark gaps being radially spaced uniformly from said one main spark gap electrode.

4. A pulse generator apparatus in accordance with claim 2 in which the main spark gap acts as the radiation means and emits ionizing light as a result of its breakdown.

5. A pulse generator apparatus in accordance with claim 4 in which all the spark gaps are provided in a gas tight chamber containing the gas dielectric at greater than atmospheric pressure and the emitted light is ultraviolet light.

6. A pulse generator apparatus in accordance with claim 5 which also includes a liquid dielectric filled chamber containing the transmission lines of the field reversal pulse generator.

7. A pulse generator in accordance with claim 6 in which the transmission lines are formed by a plurality of coaxial conductors.

8. A pulse generator in accordance with claim 2 in which the charging pulser is a Marx surge generator.

9. A pulse generator in accordance with claim 2 in which the charging pulser is a second field reversal pulse generator.

10. A pulse generator in accordance with claim 9 in which the second field reversal pulse generator includes a plurality of spiral strip line type of transmission lines.

11. A pulse generator in accordance with claim 4 in which the coaxial conductors include an inner conductor, an outer conductor and an mtennediate conductor positioned between said inner and outer conductors which form two transmission lines, said intermediate conductor having one end connected to the one electrode of the main spark gap to provide a common charging conductor for said transmission lines, said outer conductor having one end connected to said parallel spark gap electrodes which are radially spaced substantially uniformly from said one main spark gap electrode, said inner conductor having one end connected to the other end of said outer conductor through said load and a charging inductor in parallel with load, and the other ends of said intermediate and inner conductors being unconnected to provide open circuit term ir at ions at the corresponding ends of the transmission lines. 12. A pulse generator in accordance with claim 11 in which the load is an electron tube having a field emission cathode which is energized by the output pulse of the field reversal pulse generator.

' 13. A pulse generator in accordance with claim 12 in which the load is an electron radiation tube having a field emission cathode with a plurality of spaced emitter needles and an electron transparent anode window formed as a portion of its envelope.

14. A pulse generator in accordance with claim 13 which also includes an external X-ray target positioned outside of the tube adjacent the window so that electrons transmitted out of said tube through said window strike the target and cause X- rays to be emitted by said target.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION- Patent No. 3,551,677 Dated December 29, 1970 Inventor(s) John WSter It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 62, "overvoltage voltage" should read over voltage line 75, should read charging pulser employed in the present fi'fld reversal pulse generator system when pulse charging is employed Column 2, line 26, "at" should read an line 75, should read L and L their conductors l0 and 14 are connected in common to a charging pulser 24 through a main spark gap 26 Column 4, line 14, "'discharge" should'read -'discharges line 55, 3'0 should read --0" Column 7 lines 9 and 10 "of the of the" should read of the Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. I WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents