US3363102A - High voltage pulse transmission system employing field emission diodes for successively pulsing a load - Google Patents

Description

2 Sheets-Sheet 1 W Q E32 v mm vw mm w r :33 m2; m?

P. HARBOUR mmamooum Jan. 9, 1968 J.

HIGH VOLTAGE PULSE TRANSMISSION SYSTEM EMPLOYING FIELD EMISSION DIODES FOR SUCCESSIVELY PULSING A LOAD Filed Jan. 25, 1965 JOHNR HARBOUR INVENTOR BY BUCKHORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS 9, 1968 J P. BARBOUR 3,363,102

HIGH VOLTAGE PULSE TRANSMISSION SYSTEM EMPLOYING FIELD EMISSION DIODES FOR SUCCESSIVELY PULSING A LOAD Filed Jan. 25, 1965 2 Sheets-Sheet 2 M BR mm N n M LO/II J N9 Y 5 m a I 1| g d 1 g I \\\\L BUCKHORN, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS Patented Jan. 9, 1968 3,363,102 HIGH VOLTAGE PULSE TRANSMISSION SYSTEM EIVIPLOYING FIELD EMISSION DIODES FOR UCCESSIVELY PULSING A LOAD John P. Barbour, McMinnville, Oreg., assignor to Field Emission Corporation, McMinnville, Oreg., a corporation of Oregon Filed Jan. 25, 1965, Set. N0. 427,653 8 Claims. (Cl. 250-102) ABSTRACT OF THE DISCLOSURE An electrical pulse transmission system of high frequency and power capability is described in which a plurality of sequentially triggered high voltage pulse generators are connected to a common load, such as a field emission X-ray tube, through a plurality of field emission diode switches which isolate the pulse generators from each other. The diode switches each have a cathode in the form of a plurality of sharp electron emitting ele ments and are capable of combined field emission and vacuum arc operation to transmit pulses of high current.

The subject matter of the present invention relates generally to the transmission of electrical pulses of high voltage and high current from a plurality of sources to a common load as a train of successive pulses, and specifically to a high voltage pulse transmission system including a plurality of field emission diode switches connected between the common load and the outputs of a plurality of energy discharge pulse generators in order to electrically isolate such pulse generators from each other because they are triggered at different times. Each diode switch has a plurality of field emission cathode elements to transmit a high voltage, high current pulse through such switch in a forward direction by a combination of field emission and vacuum arc operation and to block the transmission of pulses through the switch in the reverse direction.

The pulse transmission system of the present invention -is especially useful when employed to transmit electrical pulses to an X-ray tube for producing narrow, short rise time pulses of X-rays of high intensity. Such an X-ray system may employ a plurality of electrical pulse generators similar to that disclosed in co-pending US. patent application, Ser. No. 245,182, entitled X-ray Unit, filed by W. P. Dyke et al., Dec. 17, 1962, now US. Patent 3,256,439, whose outputs are connected through different field emission rectifier switches to a common load in the form of an X-ray tube of the field emission cathode type shown in the above-mentioned co-pending application. However, the pulse transmission system of the present invention may be used with other common loads requiring high energy electrical pulses of high voltage and high current such as various microwave devices, including magnetrons and klystrons.

The pulse transmission system of the present invention has several advantages over previous systems, including a faster repetition rate which is not limited by the charging time of the electrical pulse generators. As a result the present system can produce a train of electrical pulses having a frequency up to about 1 megacycle per second, which is the switching time of the field emission diode switches. In addition, the pulse transmission system herein described transmits pulses of higher current and voltage having lower rise times due to the use of such switches. For example, one commercial embodiment of the present system transmits rectangular output pulses of 150 kilovolts and 2000 amperes amplitude having a pulse width of about 70 nanoseconds and a rise time of about 14 nanoseconds with the spacing between pulses being about 30 to 50 microseconds.

When these electrical pulses are applied to a load in the form of an X-ray tube of the field emission, vacuum arc type, such tube emits a train of narrow X-ray pulses of high intensity and short rise time having high dosage rates. These X-ray pulses can be employed to record the images of rapidly moving objects or fast happening events directly on large sections of film or other recording device without electronic intensification to provide images of better resolution. Such an X-ray system is also simpler and less expensive than previous systems employing a continuous X-ray source of low intensity, and an image intensifier tube. In addition, no camera shutter mechanism is needed in the present system because of the high frequency of the pulsing X-ray source. This also allows the film to be moved continuously during exposure such as by rotating a strip of film on a circular drum to transport the film sections into position. In addition, such an X-ray system employs only a single X-ray tube, rather than a plurality of pulsed X-ray sources as employed in some previous systems, thereby eliminating any image distortion due to different exposure angles of such previous systems.

It is therefore one object of the present invention to provide an improved system for transmitting electrical pulses of high voltage and high current to a common load from a plurality of different sources in the form of a train of electrical pulses of a high repetition rate.

Another object of the present invention is to provide an improved pulse transmission system employing field emission diode switches between a common load and a plurality of separate triggered electrical pulse generators to isolate such pulse generators from each other.

A further object of the present invention is to provide an improved pulse transmission system of simple and inexpensive construction for transmitting a plurality of narrow electrical pulses of high voltage and high current successively from a plurality of different pulse generators of the energy storage type to a common load in the form of a train of pulses having a high repetition rate which is not limited by the charging time of such pulse generators.

An additional object of the present invention is to provide an improved system for producing a series of narrow X-ray pulses of high intensity and high dosage rate at a fast repetition rate from a single source of Xrays.

Still another object of the present invention is to provide a system for producing a series of X-ray pulses of high intensity by transmitting a train of electrical pulses of high voltage and high current to an X-ray tube of the field emission, vacuum arc type from a plurality of triggered pulse generators through diode switches of the field emission, vacuum arc type.

A still further object of the invention is to provide an improved radiographic system producing a plurality of X-ray pulses of great intensity and high dosage rates to record the images of rapidly moving objects or fast happening events in a simple and inexpensive manner to produce high resolution images Without the use of electronic image intensification or camera shutters.

Additional objects and advantages of the present invention will be apparent from the following detailed descriptions of preferred embodiments thereof and from the attached drawings, of which:

FIG. 1 is a schematic diagram of an X-ray apparatus including an electrical pulse transmission system in accordance with the present invention;

FIG. 2 is a longitudinal section view of one embodiment of a field emission diode switch employed in the system of FIG. 1;

FIG. 3 is a longitudinal section view of another embodiment of a diode switch employed in the system of FIG. 1, shown on-an enlarged scale; and

FIG. 4 is a magnified partial vertical section View taken along the line 44 of FIG. 3.

One embodiment of the electrical pulse transmission system of the present invention is shown in FIG. 1 to include a plurality of triggered electrical pulse generators 10, 12 and 14, of the energy storage and discharge type. The outputs of each of the pulse generators 10, 12 and 14 are connected through coaxial cabletransmission lines 16, 18 and 19, respectively, to the input terminals of a plurality of field emission diode switches 20, 22 and 24, respectively. Each of these diode switches has a field emission cathode structure including a plurality of pointed needles orother cathode elements having emitter portions of small radius of curvature to enable the field emission of electrons from such elements. The structure of such diode switches is hereafter described in greater detail with reference to FIG. 2 or to FIGS. 3 and 4. The diode switches are mounted within X-ray shielded tube heads 28, 3G and 32, respectively, which each plug into a common junction box 34 so that the output terminals of the diode switches are connected together to a common output terminal on such junction box. The output terminal of the junction box 34 is connected througha coaxial cable transmission line 36 to an X-ray tube 38.

The X-ray tube 38 may have a field emission cathode structure including a plurality of sharp pointed needle elements 49 of tungsten supported in radially spaced relationship about a conical anode 42 of tungsten in the manner of co-pending U.S. patent application Ser. No. 245,182, now US. Patent 3,256,439. When an electrical pulse is transmitted to the X-ray tube from one of the pulse generators 16, 12 and 14 through one of the diode switches 20, 22 and 24, electrons are emitted from at least some of the cathode needles 40 due to the high electrical field surrounding the points of such needles and some of the needles are partially vaporized to produce a vacuum are between the anode and cathode. The electrons are accelerated and focused to bombard the conical anode 42 to emit X-rays 44 from such anode which are transmitted through a thin window in the end of the evacuated tube envelope, adjacent the apex of the conical anode. Short X-ray pulses of extremely high intensity are produced because "of the vacuum arc operation of the X-ray tube which greatly increases the current transmitted-between the cathode structure and the anode to a value of about 1,000 to 2,000 amperes. This extremely high current flows only for the short time that an electrical pulse is applied .to the X-ray tube and is terminated before any ions of vaporized anode metal are transmitted to the cathode elements to prevent such ions from distorting the needle shape of such elements. The high current of vacuum are operation is caused by positive ions of cathode metal neutralizing the negative space charge normally surrounding such needles to enable a much greater current to flow to the anode.

The X-ray tube 38 may be positioned adjacent the path of a moving object 46, such as a bullet or exploding wire, so that such object is positioned between the X-ray tube and a recording device 48 to expose the object to the X-rays and produce a separate image of the object on the recording devicefor each X-ray pulse. Any number of a plurality of different types of recording devices can be employed in the radiographic system of the present invention. For example, the recording device 48 can be merely a single fixed X-ray film to record the path of the object 46 by multiple exposure of such film to a plurality of consecutive X-ray pulses. Alternatively, the recording device can be in the form of a plurality of moving film sections mounted on a rotating drum or other carrier so that a different one of such film sections is exposed for each X-ray pulse. A faster recording device can be formed by employing an X-ray sensitive, fluorescent screen and a plurality of separate light sensitive films focused on such screen through different light paths, each including a voltage actuated light valve, such as a Kerr cell, which is opened at a time corresponding to the triggering of one of the pulse generators for a brief time sufiicient to expose the film associated with such valve to the X-ray pulse produced thereby. In addition, other conventional recording devices can be employed including 56, respectively. The outputs of the storage modules are connected in series through a plurality of spark gaps which are ionized by causing ultra-violet light to be emitted from the ionized gas in one spark gap to all of the remaining spark gaps substantially instantaneously when such one spark gap is caused to break down by the application of an electrical trigger pulse in a manner hereafter described. All of the pulse generators 10, 12 and 14 are connected to a common source 58 of pressurized nitrogen gas which is employed in the spark gaps because it emits a large quantity of ultra-violet light when ionized. It is believed that the ultra-violet light emitted from; the triggered spark gap causes photo-electrons to be emitted from the electrodes of the remaining spark gaps which together with the ionizing effect of the ultra-violet light produces the breakdown of the remaining gaps.

The trigger input terminal of each of the pulse generat-ors 10, 12 and 14 is connected to the secondary winding of voltage step up transformers 60, 62 and 64, respectively, whose primary winding is connected to the output of trigger amplifier and variable time delay circuits 66, 68 and 70, respectively. The input terminals of the amplifiers 66, 68 and 70 are connected to a common trigger input terminal 72 to which a trigger pulse is applied at a time corresponding to the movement of the object 46 past the X-ray tube. As a result of the difierent time delays of circuits 66, 6-8 and 70', the trigger pulse is transmitted to the pulse generators 10, 12 and 14 at dif- D ferent times corresponding to different positions of the object as it passes between the X-ray tube and the recording device. The transformers '60, 62 and 64 increase the voltage of such trigger pulses to about 10 kilovolts before applying them to such pulse generators. Each of the circuits 66, 68 and 70 has a variable resistor 74, 76 and 78, respectively, for varying the time delay of the trigger pulse transmitted through such circuit.

One embodiment of the diode switch employed for switches 26, 22 and 24 is shown in FIG. 2 to have a planar electrode structure and includes an anode S0 in the form of a fiat circular disc of metal attached to support pins 81 and 82 extending through a seal in the reentrant end of an evacuated glass envelope 84. A field emission cathode structure 86, including a plurality of spaced needle elements 88 of tungsten extending from the side of a common circular support disc 90 toward the anode, is attached at the opposite side of such support disc to a support rod 92. The support rod extends through the other end of the envelope and is sealed to such envelope by an annular metal flange 94 attached to such rod. A filament wire 96 coated with getter material may be provided within the tube envelope adjacent the rear surface of the anode 80 to flash the getter material onto the envelope wall only adjacent the reentrant end; One end of the getter wire 96 is connected to anode support pin 82 and its other end is connected to another support pin 97 to enable the transmission of heating current through such getter wire through leads 9S and 100 respectively connected to support pins 82 and 97. After the gettering operation, lead 100 is disconnected, and lead 98 is connected as the input signal conductor of the diode switch when positive voltage pulses are applied thereto. Of course, the cathode support rod 2 would serve as the input conductor of the switch if negative voltage pulses were applied thereto.

The diode switch shown in FIG. 2 has a vacuum arc operation similar to that described above with regard to the X-ray tube 38 except that the material of the anode is a metal having a high atomic number to prevent the emission of X-rays. Any X-rays which are emitted from the rectifier tube are absorbed by tubular lead shielding 102 surrounding such tube which is provided within the tube heads 28, 30' and 32 of FIG. 1. When a positive voltage pulse is applied to anode 80 or a negative Voltage pulse is applied to the cathode structure 85, a strong electrical field is produced adjacent the points of the cathode needles 88 to cause the field emission of electrons from the points of at least some of such needles. It should be noted that each of the needle points has a radius of curvature on the order of centimeters or less and is spaced about 3.75 millimeters from the anode. Portions of at least some of such needles are melted to produce positive ions of metal vapor between the cathode needles and the anode. These positive ions neutralize the negative space charge usually surrounding the cathode needles and enable a much greater flow of current of the order of 1000 to 2000 amperes between the cathode structure and anode of the switch tube. Since the points of certain of the cathode needles will be positioned slightly closer to the anode than others or will have smaller radii of curvature, these needles will emit electrons for a given applied electrical pulse while the other needles will not. In this manner, the lifetime of the tube is extended to several thousand pulses.

When the positive pulses of 150 kilovolts are applied to the cathode of the diode switches, no current flows in the reverse direction from the output to the input terminals of such switches because of the lower intensity electrical field produced adjacent the anode 80 which is below 5 l0+ volts per centimeter. This is substantially less than the high electrical field produced adjacent the cathode needles which is greater than S X10+ volts per centimeter when such positive pulse is applied to the anode of the switches. Thus, a high voltage pulse produced by pulse generator 10 and transmitted through switch tube 20 in the forward direction is not passed in the reverse direction through switch tubes 22 and 24 but is transmitted to the X-ray tube 38 instead. In this manner, the diode switches isolate the pulse generators from each other.

Another embodiment of the rectifier switch is shown in FIGS. 3 and 4 and is similar to the switch of FIG. 2 but has a coaxial electrode structure. The anode 80' is in the form of a circular cylinder, and the cathode structure 86 includes a plurality of separate support blocks 90' each having needle elements 88' extending therefrom. The cathode blocks 90 are uniformly spaced about the anode 80' with the cathode needles directed radially inward toward such anode. Each of the cathode blocks 90' is attached by welding within a longitudinal fold 104 in a common support tube 106 of metal which is secured in a similar manner to a circular end plate 108 of metal forming a portion of the evacuated tube envelope. The end plate 108 is welded to one end of a metal tube 110 which is secured at its other end to the glass envelope 84' by a conventional glass-to-metal seal. A tubular shield member 112 is attached to support 106 around the anode in such a position to prevent any metal vapor emitted from the cathode needles 88' or such anode from striking the glass envelope 84 in the region immediately adjacent the glass-to-metal seal between such envelope portion and the metal end portion 110 to prevent short circuiting the anode to the cathode. The construction of the cathode structure including the support blocks 90' and the needle elements 88 may be similar to that disclosed in co-pending U.S. patent application, Ser. No. 114,125, filed June 1, 1961 by W. P. Dyke et al., now U.S. Patent 3,174,043. It should be noted that the metal vapor deposited on the inner surface of the glass envelope portion 84 due to melting of the cathode needles or anode acts as a gettering material to maintain the vacuum within such tube at a high level.

The coaxial construction of the switch tube of FIGS. 3 and 4 enables the forward conduction impedance of such tube to more nearly match the characteristic impedance of the coaxial cables to which it is connected to minimize signal reflections. Thus, in one embodiment the output impedance of each of the pulsers 10, 12 and 14 and the characteristic impedance of each of the coaxial cables 16, 18 and 19 is ohms as well as the forward conduction impedance of the switches 20, 22 and 24. In addition the reverse impedance of the switch is about 1,000 times that of the forward impedance of such switch or about 75,000 ohms in order to provide good isolation between the pulse generators.

It is obvious that many changes may be made in the details of the above-described preferred embodiments of the present invention without departing from the spirit of the invention. For example, other configurations of rectifier switches can be employed instead of those shown in FIGS. 2 to 4, such as a tube having a hollow cylindrical anode surrounding a coaxially mounted cathode structure or a tube employing a single flat disc having a razor edge type emitting portion as its cathode structure surrounding or being surrounded by the anode. Therefore the scope of the present invention should only be determined by the following claims.

I claim:

1. A system for transmitting pulses of high voltage and current successively from a plurality of different sources a common load, comprisirw:

a plurality of electrical pulse generators each producing a pulse of high voltage and high current when said pulse generators are triggered and having a recovery time before the pulse generator is capable of producing another pulse;

trigger means for selectively triggering said pulse generators to cause the production of said pulses at different times so that said pulses are spaced apart and the time between successive pulses can be less than the recovery time of one pulse generator;

a common load; and

a plurality of rectifier switches having their output terminals connected in common to said load and their input terminals connected to diiferent ones of said pulse generators to transmit said pulses to said load and to isolate said pulse generators from each other, each of said switches including an anode and a field emission cathode structure.

2. A system for transmitting pulses of high voltage and current successively from a plurality of diiferent sources to a common load, comprising:

a plurality of electrical pulse generators each producing a pulse of high voltage and high current when said pulse generators are triggered to discharge the electrical energy stored in said pulse generators; charge means for charging said pulse generators to a predetermined DC. voltage in a given charging time;

trigger means for selectively triggering said pulse generators to cause the production of said pulses at different times so that said pulses are spaced apart and the time between successive pulses can be less than the charging time of one pulse generator;

a common load; and

a plurality of rectifier switches having their output terminals connected in common to said load and their input terminals connected to diiferent ones of said pulse generators, each of said switches including an anode and a field emission cathode structure includa 7 ing a plurality of electron emitting elementssof small radius of curvature directed toward said anode.

3. A system for transmitting pulses of high voltage and current successively from a plurality of different sources to a common load, comprising: V

a plurality of electrical pulse generators each producing a pulse of high voltage and high current when said pulse generators are triggered and having a recovery time before the pulse generator is capable of producing another pulse; v trigger-means for selectively triggering said pulse generators to cause the production of saidpulses at different times so that said pulses are spaced apart and the time between successive pulses can be less than the recovery time of one pulse generator;

a common load; and

a plurality ofrectifier switch tubes having their output terminals connected in common to said load and their input terminals connected to different ones of said pulse generators, each of said switch tubes including an anode and a field emission cathode a structure mounted within an evacuated envelope, said cathode structure including a plurality of electron emitting elements of small radius of curvature supported in proper spaced relationship'withurespect to said anode and said pulses being of sufiicient amplitude to cause the field emission of electrons from at least some of the cathode elements to said anode and to produce a vacuum are between said anode and'said cathode elements when one of said pulses is applied to its associated switch tube by the pulse generator connected to its input terminal so that said one pulse is transmitted through said associated switch tube to said load but to prevent the pulses produced by the other pulse generators from being transmitted through said associated switch tube away from said load.

4. A system for transmitting pulses of high voltage and current successively from a plurality of different sources to a common load, comprising:

a plurality of electrical pulse generators each producing a pulse of substantially the same high voltage and high current when said pulse generators are triggered to discharge the electrical energy stored in said pulse generators;

charge means for charging said pulse generators to the same predetermined DC. voltage in a given charging time; a

trigger means for selectively triggering said pulse generators to cause the production of said pulses at different times so that said pulses are spaced apart and the time between successive pulses can be less than the charging time of one pulse generator;

a common load; and

a plurality of rectifier switch tubes having their output terminals connected in common to said load and having their input terminals connected to diiferent ones of said pulse generators, each of said switch tubes including an anode and a field emission cathode structure mounted within an evacuated envelope, said cathode structure including a plurality of needle elements with points of small radius of curvature supported in proper spaced relationship with respect to said anode and said pulses being of sufficient amplitude to cause the field emission of electrons from at least some of the needle elements to the anode and to produce a vacuum are between said anode and said needle elements when one of said pulses is applied to its associated switch tube by the pulse generator connected to its input terminal so that said one pulse is transmitted in a forward direction through said associated switch tube to said load but to prevent the pulses produced by the other pulse generators from being transmitted in a reverse directo produce a plurality of short X-ray pulses of high intensity and fast repetition rate, comprising:

a plurality of electrical pulse generators each producing a pulse of high voltage and high current when said pulse generators are triggered to discharge the electrical energy stored in said pulse generators;

charge means for charging said pulse generators to a predetermined DC voltage in a given charging time;

trigger means for selectively triggering said pulse generators successively to cause the production of said pulses at difierent times so that the time between V successive pulses is less than the charging time of one ulse generator;

an X-ray tube having an anode and a field emission cathode structure; and r a plurality of rectifier switch tubes having their output terminals connected in common to said X-ray tube and their input terminals connected to different ones of said pulse generators, each of said switch tubes including an anode and a field emission cathode structure, said cathode structure having a plurality of spaced emitting elements of small radius of curvature directed toward said anode.

6, An X-ray system employing a single X-ray source to produce a plurality of short X-ray pulses of high intensity and fast repetition rate, comprising:

a plurality of electrical pulse generators each producing a pulse of high voltage and high current when said pulse generators are triggered to discharge the electrical energy stored in said pulse generators;

charge means for charging said pulse generators to a predetermined DC. voltage in a given charging time;

trigger means for selectively triggering said pulse generators successively to cause the production of said pulses at diiferent times so that the time between successive pulses can be less than the charging time of one pulse generator;

an X-ray tube having an anode and a field emission cathode structure including a plurality of spaced electron emitting elements;

a plurality of rectifier switch tubes having their output terminals connected in common to said X-ray tube and their input terminals connected to different ones of said pulse generators, each of said switch tubes including an anode and a field emission cathode structure mounted within an evacuated envelope, said cathode structure having a plurality of spaced emitting elements of small radius ofcurvature; and

means for applying said electrical pulses to the input terminals of each of said switch tubes to cause the field emission of electrons from at least some of said emitting elements to said anode in such switch tube and to vaporize portions of said elements and produce ions of cathode material which form a vacuum are between the anode and cathode structure that greatly increases the current of the electrical pulse transmitted in a forward conduction direction through said switch tube, and for transmitting said electrical pulses from the output terminals of said switch tubes to said X-ray tube to produce an X-ray pulse for each electrical pulse, while preventing said electrical pulses from being transmitted in the reverse conduction direction through other switch tubes.

7. A Cineradiographic system employing a single X-ray source to produce a plurality of short X-ray pulses of high intensity and fast repetition rate, comprising:

trigger means for selectively triggering said pulse generators successively to cause the production of said pulses at different times so that the time between successive pulses can be less than the charging time of one pulse generator;

an X-ray tube having an anode and a field emission cathode structure including a plurality of spaced electron elements, which emits a pulse of X-rays when one of said voltage pulses is applied to said tube;

a plurality of rectifier switch tubes having their output terminals connected in common to said X-ray tube and their input terminals connected to different ones of said pulse generators and each of said switch tubes including an anode and a field emission cathode structure within an evacuated envelope, said cathode structure having a plurality of emitting elements directed toward said anode; and

means mounted adjacent said X-ray tube for receiving said X-ray pulses after they have passed through a moving object in order to record the movement of said object.

8. An X-ray system employing a single X-ray source to produce a plurality of short X-ray pulses of high intensity and fast repetition rate, comprising:

.a plurality of electrical pulse generators each producing a pulse of high voltage and high current when said pulse generators are triggered to discharge the electrical energy stored in said pulse generators;

charge means for charging said pulse generators to a predetermined DC. voltage in a given charging time;

trigger means for selectively triggering said pulse generators successively to cause the production of said pulses at different times so that the time between successive pulses can be less than the charging time of one pulse generator;

an X-ray tube having an anode and a field emission cathode structure including a plurality of spaced electron emitting needles;

a plurality of rectifier switch tubes having their output terminals connected in common to said X-ray tube and their input terminals connected to ditferent ones of said pulse generators, each of said switch tubes including an anode of a circular cylinder shape and a field emission cathode structure mounted coaxially about said anode within an evacuated envelope, said cathode structure having a plurality of emitting elements of needle shape directed inward toward said anode; and

means for applying said electrical pulses to the input terminals of each of said switch tubes to cause the field emission of electrons from at least some of said emitting elements to said anode in such switch tube and to vaporize portions of said elements to produce a vacuum are between the anode and the cathode structure which greatly increases the current of the electrical pulse transmitted in a forward conduction direction through said switch tube, and for transmitting said electrical pulses from the output terminals of said switch tubes to said X-ray tube to produce an X-ray pulse for each electrical pulse, while preventing said electrical pulses from being transmitted in the reverse conduction direction through other switch tubes.

References Cited UNITED STATES PATENTS 2,923,818 2/1960 Wilson et al. 32861 2,942,190 6/ 1960 Fischman 32861 3,051,906 8/1962 Haynes 32861 3,171,030 2/1965 Foster et al. 250-- WILLIAM F. LINDQUIST, Primary Examiner.

RALPH G. NILSON, Examiner.

A. L. BIRCH, Assistant Examiner.

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