US3360678A - Fast pulse generator utilizing an electron beam to cause an arc breakdown across thegap region of a coaxial line center conductor - Google Patents

Description

Dec. 26, 1967 Q. A. KERNS 3,360,678

FAST PULSE GENERATOR UTILIZING AN ELECTRON BEAM TO CAUSE AN ARC BREAKDOWN ACROSS THE GAP REGION OF A COAXIAL LINE CENTER CONDUCTOR Filed May 27, 1965 2 Sheets-Sheet 1 IOO KV POWER SUPPLY Fl LAMENT SUPPLY IINVENTOR.

QUENTIN A. KERNS ATTORNEY.

Dec. 26, 1967 Q. A. KERNS 3,360,678

FAST PULSE GENERATOR UTILIZING AN ELECTRON BEAM TO CAUSE AN ARC BREAKDOWN ACROSS THE GAP REGION OF A COAXIAL LINE CENTER CONDUCTOR Filed May 27, 1965 2 Sheets-Sheet 2 V 0 a t \1 i (1) m5 (9 2] r C? M Q F3 g3 03 1O I: g L ammy f INVENTOR. \1 QUENTIN A. KERNS ATTORNEY.

United States Patent FAST PULSE GENERATOR UTILIZING AN ELEC- TRON BEAM TO CAUSE AN ARC BREAKDOWN ACROSS THE GAP REGION OF A COAXIAL LINE CENTER CONDUCTOR Quentin A. Kerns, Orinda, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed May 27, 1965, Ser. No. 459,484 12 Claims. (Cl. 315-3) The present invention relates generally to an electrical pulse generator and more particularly to apparatus for producing short electrical pulses characterized by very rapid rise and decay times.

Very fast pulses with a length in the order of a single nanosecond (1 l() second) are difficult to produce, particularly if very rapid rise and decay times are necessary. Such pulses are useful for controllably simulating the type of pulse produced by nuclear charged particles and therefore are useful for predicting the response of nuclear particle detectors to charged particles. Fast pulses are also useful in high precision timing apparatus where it is further necessary that the time at which the pulse is produced be very accurately controllable, with minimum delay between the application of a trigger impulse and the production of the switching pulse.

In the present invention, pulses are generated by cre ating a steady-state electrical charge between the inner and outer conductors in a short section of a coaxial transmission line. When the charge in the short line section is suddenly discharged, a pulse is created. Thus, to generate the desired type of pulse, it is necessary to provide a switch in the coaxial line for removinng the charge on the center conductor. However, if high frequency signals or fast pulses are being carried in a coaxial line, the size and spacing of the inside and outside conductors should be constant, that is, the characteristic line impedance should be constant along the entire length of the line. Physical irregularities in a line as created by ordinary switches may create electrical perturbations which cause signal reflection with consequent loss of signal power and create deleterious spurious pulses. Therefore, when a switch is provided in a coaxial line, the switching mechanism should not affect the diameter of the central conductor nor the spacing between the central conductor and the outside shield conductor of the coaxial line. Unfortunately, the mechanism of conventional switches occupies considerable space within the cable and necessitates altering the cable dimensions. Furthermore, the operation of most conventional switches cannot be accurately timed in terms of a microsecond or less.

The present invention provides for a very rapid switch action at the central conductor of a coaxial line, the switching elements causing minimal perturbation of the line impedance. In the invention, a miniature electron gun disposed outside the outer conductor of the line directs an accelerated beam of electrons into the coaxial cable at a switch gap, one side of which is the charged section of the central conductor. The electron beam causes an arc breakdown across the gap and thus discharges the line. Ordinarily, such a switching means is impractical since the space between the inner and outer conductors of a coaxial line must contain air or some other gas since breakdown across the switch gap must occur through a gas. In any practical apparatus, a beam of electrons can be accelerated only in a vacuum: In the present invention, the electron beam is created in a vacuum tube and passed through a thin electron transparent vacuum sealing-window in the outer conductor and thus into the gaseous atmosphere around the switch p- It is an object of the present invention to provide an improved pulse generator producing very rapid pulses with fast rise and decay times.

It is an object of the present invention to provide an improved pulse generator by utilizing very fast action switching action in a coaxial transmission line.

It is another object of the present invention to provide an improved pulse generator having a coaxial line switch causing minimal change in the characteristic impedance of the line.

It is another object of the present invention to provide an improved pulse generator utilizing a coaxial line switch having no moving parts and capable of being remotely controlled.

It is another object of the present invention to provide a pulse generator capable of an output pulse immediately after receipt of an input trigger pulse.

The invention will be better understood by reference to the accompanyin drawing of which:

FIGURE 1 is a broken-out view of a fast pulse generator including a coaxial line with which an electron gun is provided for efiecting switching, and FIGURE 2 is a broken-out view of another embodiment of the fast pulse generator.

Referring now to FIGURE 1, there is shown a coaxial transmission line 11 having a round outer shield conductor 12 and an inner center conductor 13 disposed along the axis of the shield. Typically, the center conductor 13 is supported by a plurality of insulative cones 14 disposed at intervals inside the shield 12, each cone having a central aperture in which the center conductor is supported.

First and second sections 15 and 20 of center conductor 13 are electrically separated by a switch gap 16 provided in the center conductor. The switch gap 16 is between spaced apart overlapping ends 17 and 18 of the conductor 13, the ends being flattened on each side of the gap. The ends 17 and 18 are thus electrodes of a switch gap 16 across which an arc is created to obtain the switch function. End 18 is multiapertured so that an electron beam can be directed through apertures 19 and across the gap 16.

A power supply 21 has a negative terminal connected to the outer shield 12 while a positive terminal is connected through a current limiting resistor 22 to the first section 15 of center conductor 13. The output end of second section 20, that is, the end of section 20 opposite end 18, is terminated in the characteristic impedance of the coaxial line 11. Such impedance is indicated in FIGURE 1 as a resistor 23 connected from the shield 12 to the center conductor 13. When the switch is open, that is, there is no arc across the gap 16, the full potential of power supply 21 is present across the gap. Thus the voltage potential of supply 21 must be sulficiently low that self-triggering breakdown does not occur across the gap 16.

Controlled triggering of an arc across the gap 16 is obtained by injecting a beam of electrons through the apertures 19 to ionize the gas in the gap so that a low resistivity current path is created from end 18 to end 17. Such electron beam is created in a special vacuum tube 29 in which there is an insulative cylindrical evacuated tube envelope 31 having at one end a conventional control grid 32, cathode 33, and heater structure 34, providing a beam of electrons directed toward a flat circular anode 36 which closes the opposite end of the envelope. Anode 36 is typically a one mil thick beryllium or nickel diaphragm.

A plurality of annular electron accelerating electrodes 37 are coaxially mounted in the wall of tube 29, between the grid 32 and the anode 36, a potential difference being provided between each electrode 37 to accelerate electrons emitted from the cathode 33 toward the anode 36. To prevent damage to the envelope 31 and to prevent deposits from being formed on the envelope as a result of sputtering or arcing between the electrodes 37, each electrode has a flared configuration to shield the envelope, the smaller diameter ends of the electrodes being directed towards anode 36. For similar reasons, the anode 36, which is thin and fragile, is protected from being ruptured by arcs by a cylindrical shield ring 38, which is disposed coaxially with the anode at the inner surface thereof so that any arcs will strike such ring in preference to the anode 36. A many apertured circular outer shield 39 is disposed at the end of tube 29 against anode 36 for protecting the anode from external physical and electrical damage. A similar apertured shield 41 is disposed against the inside surface of the anode 36 to prevent air pressure from bulging the anode inwardly. The apertures of inside shield 41, outer shield 39, and end 18 of the center conductor 13 are aligned to facilitate the passage of the electrons therethrough.

The electrical potentials for the electrodes 37 of tube 29 are obtained from a high voltage power supply 46 having a potential of approximately 100 kilovolts. A voltage divider 47 comrpised of a plurality of series connected resistors 48 is connected across the power supply 46. Each of the electrodes 37 is electrically connected at an intermediate point along the divider so that the electrical potential of the electrodes is progressively more negative going from the anode 36 toward the control grid 32. A filament power supply 49 provides heating current to the filament 34 for heating the cathode 33. A bias voltage supply 51 has a negative terminal connected to the negative terminal of the high voltage power supply 46. The cathode 33 is connected to the positive terminal of bias supply 51 through a cathode resistor 53 so that a cut-off bias is normally present between cathode 33 and grid 32, that is, normally no electrons pass through the control grid.

If a negative voltage pulse is applied to a control terminal '2, connected to the cathode 33, the negative grid bias potential is overcome and electrons from the cathode 33 pass through the control grid 32 and are accelerated toward anode 36 by electric fields between the electrodes 37. In operation, some of the electrons are lost by collision with the apertured shield 41, but most of the electrons pass through the apertures, striking the anode 36 with sufficient velocity to pass through, and emerging from the apertures in shield ring 39. Since the conductor section 20 is at the same potential as the outer conductor 12 but is negative relative to the conductor section 15, electrons from the tube 29 are accelerated through the apertures 19 to the end 17 and ionize the gas between the ends 17 and 18, causing breakdown across the gap 16. Because of the low impedance of the are, the effect is similar to closing a switch across gap 16.

Before the arc across gap 16 is created, an electric field is provided between the first section 15 of the center conductor of the coaxial line and the outer conductor 12 by the power supply 21. With the formation of an arc across the gap 16, the energy in the electric field is suddenly applied to the remainder of the coaxial line and a very sudden steep wavefront pulse is propagated down the line 11, appearing across the terminating resistor 23. The relatively high impedance of resistor 22, compared to the impedance of the arc, effectively decouples the power supply 21 from the conductor section 15 during the pulse forming process. Both the rise and decay time of a pulse are very short while the duration of the pulse is equal to the time it takes an electromagnetic wave to traverse the coaxial line from the gap 16 to the open end of first coaxial line section 15 and back.

The physical arrangement of certain elements of the invention may be varied, another configuration being shown in FIGURE 2. The beam accelerating vacuum tube 29 in FIGURE 2 is identical to that described with regard to FIGURE 1 and is therefore not shown in detail, but the construction of the coaxial line 61 differs in that the outer shield conductor 62 has apertures 59 in an end plate 63 through which the electron beam from tube 29 is directed. The central conductor 64 of the coaxial line 61 has an end section 66 of reduced diameter adjacent the end plate 63, the section 66 being covered with insulation 67 over which a metal sleeve 68 is disposed. The sleeve 68 has the same diameter as the central conductor 64 and forms one plate of a capacitor, the other plate being the section 66. The end 69 of sleeve 68 faces the apertures 59, the end 69 and plate 63 being electrodes of a spark gap 71. A power supply 72 has a positive terminal coupled through an isolating resistor 73 to the sleeve 68 while the negative terminal is connected to the outer shield 62. The coaxial line 61 is terminated in the characteristic impedance of the line, as represented by a resistor 74.

In operation, an electrostatic field is established between the sleeve 68 and shield 62 by the power supply 72. A trigger pulse applied to the tube 29 causes a beam of electrons to pass through the apertures 59 and strike the end 69, ionizing the gas therebetween. An are immediately forms between the end 69 and the end 63, the low resistance of the are effectively connecting the sleeve 63 to the end 63. The capacitance between the sleeve 68 and the center conductor section 66 causes a pulse to be applied to the center conductor, the length of the pulse being twice the time it takes an electromagnetic wave to travel the length of the sleeve 68, the wave going from the end plate 69 to the opposite end of the sleeve 68 and returning.

Such fast pulses are useful in the evaluation of fast pulse circuits, particularly for controllably simulating the type of pulse created by nuclear radiation in detection instruments. When the pulse generator is utilized in a timing device it is important that the pulse be formed immediately upon application of a trigger pulse to the control terminal 52. In the present invention the delay between the input and output pulses is typically five nanoseconds (5-10- seconds) or less.

While the invention has been disclosed with respect to particular embodiments, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a fast pulse generator, the combination comprising a coaxial transmission line having an outer and an inner conductor, a first portion of said inner conductor being spaced from the outer conductor and from the remainder of the inner conductor of said line, means producing an electrostatic field between said first portion of said inner conductor and said outer conductor of said coaxial line, and a pulsed electron beam generator directing energized electrons between said first portion of said inner conductor and an adjacent one of said conductors to effect discharge of said electrostatic field.

2. In a fast pulse generator, the combination comprising a coaxial transmission line having an inner conductor and an outer shield conductor, said inner conductor being electrically separated into first and second portions, a direct current power supply providing a charge on said first portion of inner conductor relative to said shield conductor, an electron gun of the class having an electron emitting cathode and a control electrode and an anode which is adapted to pass electrons therethrough, said electron gun having electron accelerating means disposed between said anode and said control'electrode, said anode being positioned to pass said electrons into a region within said shield conductor and proximal to said first portion of said inner conductor to initiate discharge thereof.

3. The fast pulse generator as described in claim 2, wherein said electron gun is further comprised of a plurality of progressively more positive coaxial annuli spaced between said control electrode and said anode and each having a center aperture for said electron beam.

4. In a fast pulse generator the combination comprising a coaxial transmission line of the class having a cylindrical outer conductor and a spaced apart inner conductor extending along the axis thereof, said inner conductor being divided into first and second axially spaced sections with said first section being separated from an adjacent one of said conductors by a discharge gap, said outer conductor having an aperture therein facing said discharge gap, a source of electrical potential connected across said outer conductor and said first section of inner conductor, and an electron accelerating tube disposed adjacent said outer conductor and having the electron emitting end thereof directed into said aperture and towards said discharge gap, said electron accelerating tube having a cathode opposite said first end and an electron transmitting anode at said first end.

5. A fast pulse generator as described in claim 4, wherein said anode of said electron accelerating tube is comprised of a thin foil disposed across said first end of said tube, and at least one relatively thick stiffener disposed across said first end of said tube against said foil, said stiffener having a plurality of distributed transverse passages therethrough for transmitting electrons to said foil.

6. In a fast pulse generator, the combination comprising a coaxial transmission line having an outer shield conductor and a central conductor, said central conductor of said transmission line having a gap therein, a power supply coupled across said gap and creating a potential differential thereacross and having a voltage less than a spontaneous spark breakdown value, an opening in said outer conductor adjacent said gap, and an electron gun disposed outside said coaxial line and having an electron discharging end directed through said opening and across said gap.

7. In a fast pulse generator the combination comprising a coaxial transmission line having an outer shield conductor and an inner conductor, said inner conductor being divided into a first and a second section each having first and second ends, said first end of said first section being spaced from said second end of said second section and providing a spark gap therebetween, said second end of said second section having apertures therethrough, a direct current power supply having a positive terminal connected to said first section of said inner conductor and having a negative terminal connected to said shield conductor, and an electron gun having an electron discharge end directed into said gap through said apertures in said second end of said second section of inner conductor.

8. A fast pulse generator as described in claim 7, wherein said first end of said first section of inner conductor overlaps said second end of said second section thereof along the axis of said coaxial line, said overlapping ends being flattened and being spaced apart in the transverse direction with respect to said axis.

9. In a fast pulse generator, the combination comprising a coaxial transmission line having an inner conductor and an outer shield conductor, an apertured conductive end plate disposed across an end of said shield conductor, a tubular sleeve disposed coaxially within said outer shield and adjacent said end plate, said sleeve being colinear with said inner conductor, means capacitively coupling said sleeve to said inner conductor, an electron beam generator directed through said apertured end plate to said sleeve, and a power supply having a positive terminal coupled to said sleeve and a negative terminal coupled to said shield conductor.

10. A fast pulse generator as described in claim 9, wherein one end section of said inner conductor has a reduced diameter and extends coaxially into said sleeve to form said capacitive coupling therebetween, a dielectric element being provided between said end section and said sleeve.

11. A fast pulse generator as described in claim 9, wherein a resistor is connected between said sleeve and the positive terminal of said power supply.

12. A fast pulse generator as described in claim 9, wherein said electron beam generator is comprised of an insulative cylindrical envelope, an electron emitting cathode disposed at a first end of said envelope, an electron accelerating means in which a plurality of spaced apart annular electrodes are disposed coaxially in said envelope at intervals along the path of said electron beam between said cathode and a second end of said envelope, a power supply providing progressively higher positive electrical potentials to said electrodes from said cathode to said second end of said envelope, a thin diaphragm disposed across said second end of said envelope, an apertured disc disposed against the outwardly facing surface of said diaphragm, and an annular protective electrode disposed coaxially within said envelope and projecting from said diaphragm toward said cathode.

References Cited UNITED STATES PATENTS 2,709,229 5/1955 Linder 315-3 X 3,319,114 5/1967 Larson 33397 X HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, Assistant Examiner.

Claims (1)

1. IN A FAST PULSE GENERATOR, THE COMBINATION COMPRISING A COAXIAL TRANSMISSION LINE HAVING AN OUTER AND AN INNER CONDUCTOR, A FIRST PORTION OF SAID INNER CONDUCTOR BEING SPACED FROM THE OUTER CONDUCTOR AND FROM THE REMAINDER OF THE INNER CONDUCTOR OF SAID LINE, MEANS PRODUCING AN ELECTROSTATIC FIELD BETWEEN SAID FIRST PORTION OF SAID INNER CONDUCTOR AND SAID OUTER CONDUCTOR OF SAID COAXIAL LINE, AND A PULSED ELECTRON BEAM GENERATOR DIRECTING ENERGIZED ELECTRONS BETWEEN SAID FIRST PORTION OF SAID INNER CONDUCTOR AND AN ADJACENT ONE OF SAID CONNDUCTORS TO EFFECT DISCHARGE OF SAID ELECTROSTATIC FIELD.

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