US3629605A

US3629605A - Apparatus for providing a steep voltage step across a load in electric high-voltage circuit

Info

Publication number: US3629605A
Application number: US835112A
Authority: US
Inventors: Nils Robert Nilsson
Original assignee: INSTRUMENT SCANDITRONIX AB
Current assignee: INSTRUMENT SCANDITRONIX AB
Priority date: 1968-06-24
Filing date: 1969-06-20
Publication date: 1971-12-21
Anticipated expiration: 1988-12-21
Also published as: GB1242655A; SE312376B; DE1931759A1; FR2011591A1

Abstract

An apparatus for providing a steep voltage step across a load has two sets of electric conductors with electrodes forming spark gaps, said sets of conductors being connected to be charged in parallel by high-voltage source and discharged in series by spark gaps in order to impress a multiple of the tension of said highvoltage source upon the load and said sets of conductors being shaped and arranged to form around such of said spark gaps a transmission conducting zone in which the impedance continuously varies in a direction away from the spark gap and is at its minimum at the outer boundary of the zone in order to make the voltage step steep by impedance transformer action for the high frequencies generated at breakthrough in the spark gap.

Description

United States Patent APPARATUS FOR PROVIDING A STEEP VOLTAGE STEP ACROSS A LOAD IN ELECTRIC HIGH- VOLTAGE CIRCUIT 4 Claims, 4 Drawing Figs.

US. Cl 307/106 Int. Cl ll03k 3/00 Field of Search 250/93;

References Cited Primary Examiner-Robert K. Schaefer Assistant Examiner-H. J. Hohauser Att0rneyJohn Lezdey ABSTRACT: An apparatus for providing a steep voltage step across a load has two sets of electric conductors with electrodes forming spark gaps, said sets of conductors being connected to be charged in parallel by high-voltage source and discharged in series by spark gaps in order to impress a multiple of the tension of said high-voltage source upon the load and said sets of conductors being shaped and arranged to form around such of said spark gaps a transmission conducting zone in which the impedance continuously varies in a direction away from the spark gap and is at its minimum at the outer boundary of the zone in order to make the voltage step steep by impedance transformer action for the high frequencies generated at breakthrough in the spark gap.

APPARATUS FOR PROVIDING A STEEP VOLTAGE STEP ACROSS A LOAD IN ELECTRIC HIGH-VOLTAGE CIRCUIT In high voltage technics it is often desired to have the possibility of providing extremely steep voltage steps across a load. One example is the X-ray flash technique, and for the sake of simplicity the following description will treat of this technique, though the present invention has many other uses.

In the X-ray flash technique there is used as energy source a pulse generator which can deliver a high instantaneous effect in the discharge tube. It is desired, int. al. that the generator pulse has as short a rise time as possible and a well defined pulse length and pulse amplitude, and that the generator has a low output impedance. It was tried up to now to satisfy these requirements substantially by one of two expedients. One expedient implies the use as energy storing unit of a coaxial cable which is charged by means of a high-voltage source and discharged by a gas discharge, for example in a high-pressure spark gap. When high initial voltages are required this method is complicated as one has to use a complicated high-voltage source, such as a Van de Graaff generator. The other expedient implies the use of so-called Marx generators which as energy storing unit utilize a number of capacitors which are charged in parallel connection by means of parallel connection resistances and/or inductances, but are discharged in series through spark gaps. Pulse-producing networks of capacitors and inductances may possibly be substituted for the capacitors. The problem associated with these devices when used together with, e.g., X-ray flash tubes, is int. al. difficulty of providing sufficiently low-inductive systems and sufficiently steep voltage steps.

This invention has for its object to overcome the drawbacks associated with the prior art devices of the type above accounted for. The invention thus relates to an apparatus for providing a steep voltage step across a load in an electric highvoltage circuit which comprises at least one spark gap formed by two electrode means and connected to a high-voltage source and to the load, said spark gap being adapted at breakthroughs to generate the voltage step across the load. Characteristic of this apparatus is that the electrode means are connected to or constitute part of two relatively electrically insulated electric conductor means which together form a transmission conducting zone which surrounds the spark gap and in which the impedance continuously varies, preferably monotonously sinks, in a direction away from the spark gap and is at its minimum at the outer boundary of the zone, said zone serving as energy storing capacitor along or together with further capacitor means connected to the conductor means. The apparatus according to theinvention provides steeper voltage steps across the load than do the prior art pulse generators, and permits generation of very short duration pulses whose voltage amplitude can readily be varied. The apparatus according to the invention also is of a simple construction allowing large pulse generators to be built by the assembly of mutually identical units.

The invention will be more fully described in the following, reference being had to the accompanying drawings which illustrate some embodiments.

in the drawings:

FIG. 1 is a diagrammatic view of a simple apparatus according to the invention serving to elucidate the principles underlying the invention;

FIG. 2 is a diagrammatic view of a Marx type pulse generator realized by application of the present invention;

FIGS. 3, and

F IG. 4 show structural details in a pulse generator according to FIG. 2.

F16. 1 shows two electrode means I and 2 in the form of two facing, central, elevated portions on two circular coaxial plates 3 and 4 which are planar disregarding the electrode elevated portions which are insignificant both in point of height and diameter. The plates with the electrode means thereon are relatively electrically insulated, and a suitable dielectric, such as a gas or a gas mixture, is provided between them. The electrode means 1 has a central hole into which one end of a trigger electrode 5 penetrates in an electrically insulated manner. A trigger current source 8 is connected via lines 6 and 7 to the (DC electrode 5 and the plate 3. A high-voltage source (DC voltage) 9 is connected by means of the line 7 to the plate 3 and by means of a line 10 to one end of a load 11, such as an X-ray flash tube, and a resistance or inductance 12 connected parallel with the load. The other end of the load 11 and the component 12 is connected to the plate 4 by means of a line 13.

The plates 3 and 4 are charged to a high-voltage difference by means of the high-voltage source via lines 7, l0 and 13 and the resistance or inductance l2, whereupon the trigger current source 8 is caused to generate a breakthrough in the trigger spark gap between the electrode 1 and the trigger electrode 5, a breakthrough that initiates a breakthrough between the electrode means 1 and 2, the energy stored in the plates 3 and 4 being rapidly discharged through load 11 (little energy has time to escape through the resistance or inductance 12) in the form of a substantially rectangular pulse of extremely small rise time.

The electrode means I and 2 form a spark gap for positively bringing about a discharge of the plates 3 and 4 at their center axis, and the plates form a transmission conducting zone which surrounds the spark gap between the electrode means I and 2 and in which electric energy is stored in order then to be discharged by a spark at the center of the zone. Such a transmission conducting zone formed by circular planar plates 3 and 4 has int. al. the following properties:

I. The wave velocity of the electromagnetic waves which at the discharge at the center of the plates move in an outward direction towards the periphery of the plates and in an inward direction towards the center axis thereof, is dependent upon the distance of the waves from the center axis of the plates.

2. The impedance in the transmission conducting zone at any point of the zone is proportional to the distance between the plates 3 and 4 and inversely proportional to the distance of the point from the center axis of the plates.

lt follows from the said two properties that the transmission conducting zone will function as an impedance transformer for high frequencies. At the discharge in the center the highest frequencies will be amplified and give rise to extremely steep voltage and current steps across and in the load, respectively. As a result, the discharge at a suitable load assumes the shape of a rectangular pulse of very small rise time and a relatively small fall time which is dependent upon the load. The energy supplied to the load will thus become greater than at an ordinary sinusoidal discharge pulse. Since the impedance varies with the distance between the plates 3 and 4, more rapid and steeper pulses can be obtained by reducing said distance.

The use of a gas or a gas mixture between the plates 3 and 4 and their spark gap electrode means 1 and 2 has many advantages of which int. al. the following may be mentioned. The breakthrough voltage is readily varied by variation of the gas pressure. The gas is regenerated after each discharge and need not thus be renewed between discharges. The mechanical construction is the same for different gases. However, there is nothing to prevent the use of liquids or solid insulators between the plates and the spark gap electrodes. If the spark is allowed to traverse fluid or solid substances the length of the spark passage for a given voltage applied can be considerably reduced, which will also considerably improve the pulse properties. When particularly steep voltage steps are desired the use of a plastics foil of good breakthrough resistance is recommended as material between the plates. In such a case, of course, the construction must be so adapted that the foil is readily exchangeable between discharges.

The pulse duration and the energy stored can be varied by variation of the dielectricity constant for the dielectric between the plates 3 and 4, or by changing the dimensions of the plates. It is also possible to connect special energy storing capacitors to the plates forming the transmission conducting zone.

In the above reasoning the discussion was conducted starting from the fact that the plates 3 and 4 forming the transmission conducting zone surrounding the spark gap are planar and parallel. The same reasoning, however, applies even if the plates are of another shape, e.g., conical with facing apices. What is essential is that the conduct means collectively constituting the transmission conducting zone surrounding the spark gap are so designed and arranged that the impedance in the transmission conducting zone continuously varies, preferably monotonously sinks, in a direction away from the spark gap and is at its minimum at the outer boundary of the zone. Thus in a direction away from the spark gap there must not occur in the transmission conducting zone any such discontinuous impedance changes as would imply reflexion of electromagnetic waves moving through the zone in a direction towards or away from the central spark gap.

When higher voltages are desired a plurality of transmission conducting zone units according to FIG. 1 can be built together to form a Marx-type pulse generator. An example of this is diagrammatically shown in FIG. 2. The generator according to FIG. 2 is enclosed is a pressure vessel 14 having two manhole covers 15 which in a manner not shown serve as fastenings for gas and electricity lead-in bushings and as fastenings for a load 11 built into the vessel, such as an X-ray flash tube (or for a leadout bushing for a cable drawn to an outer load). Erected on the bottom of the vessel are electricity insulating supports 16 which carry an electrically insulating plate 17. Placed on the plate 17 is an electrically conductive capacitor plate 18 and above said plate there are carried on electrically insulating supports 19 upstanding from the plate 17, several, in the present instance three, pairs of plates 3 and 4 forming transmission conducting zones and associated spark gap-forming electrode means 1 and 2. These plates 3 and 4 with the associated electrode means 1 and 2 are of the construction shown in FIG. 1 except that the uppermost plate 3 only is provided with a trigger electrode 5, while the other plates 3 have an electrode means 1 without any hole for a trigger electrode, and except that the uppermost plate 3 is extended past the supports 19 and is electrically connected to the wall of the pressure vessel. Extending between the uppermost plate 3 and the bottom of the vessel is an electrically insulating cylindrical wall 20 separating the cylindrical sidewall of the vessel and the

supports

16, 19 to increase the electric breakthrough resistance of the space between the sidewall of the vessel and the electrically conductive means carried by the supports beneath the uppermost plate 3. In the case selected, pressure gas is used in the vessel as a dielectric in the space 21 between the two plates 3 and 4 of the same pair, which form a transmission conducting zone between them. The plates 4 and 3 following upon each other and belonging to different pairs constitute energy storing capacitors, and use is preferably made between them of a solid or fluid substance as a dielectric 22. The same applies for the capacitor formed by the lowermost plate 4 and the plate 18.

The plates 4 are series connected in sequence by means of charging resistances or inductances 23, and the uppermost plate is connected by means of a similar resistance or inductance 23 to a conductor 24 which is connected toone terminal of a source of high direct voltage, provided outside the vessel. The other terminal of the DC voltage source is connected via a line 25 to the uppermost plate 3 and as a consequence to the electrically conductive wall of the vessel 14, to which also one terminal of the X-ray flash tube 11 is connected. The capacitor plate 18 and the plates 3 are series connected in sequence over charging resistances or inductances 26, and the plate 18 besides is connected via a central line 27 to the other terminal of the X-ray flash tube 11. The trigger electrode 5 and the uppermost plate 3 are connected to an outer trigger circuit via

lines

28,29.

The relatively slow charging of the alternating transmission conducting zones and capacitor zones between the plates 3, 4 and 18 by means of the high-voltage source takes place in parallel connection over the resistances or

inductances

23, 26

in such a way that the plates 3 and 18 assume the potential for one terminal of the high-voltage source and the plates 4 assume the potential for the other terminal of the high-voltage source, while the rapid discharge by ignition of the spark gaps between the electrodes 1 and 2 in the transmission conducting zones with the aid of the trigger electrode 5 takes place according to the Marx generator principle in series connection of the energy storing units so that the pulse through the load 11 will have a considerably higher voltage amplitude than what corresponds to the output voltage of the charging highvoltage source.

The setting of the spark gaps between the electrodes 1 and 2 in the generator is such that the spark gaps as far as possible have the same breakthrough voltage. Practically, this is realized in that the sparking distance is made equally long in all gaps. This will also make it possible, when a gas or gas mixture is used as a dielectric in the spark gaps, to readily vary the breakthrough properties by variation of the gas pressure in the vessel.

Of essential importance for the function of the generator as an electric energy source of short rise time is that the ignition of the spark gap 1, 2 having the trigger electrode 5 rapidly results in ignition of also the other spark gaps l, 2. The construction according to FIG. 2 satisfies this requirement. During breakthrough in the spark gap 1, 2 having the trigger electrode 5 there occur in the associated transmission conducting zone 21 rapid variations of the potential distribution on the conductor means 3 and 4 of this zone. The strong capacitive connection between said spark gap and the following spark gaps 1, 2 via the intermediate capacitor zone results in that the variations of potential distribution in the spark gap 1, 2 having the trigger electrode are transmitted also to the following spark gaps l, 2. This will also force said latter spark gaps to be ignited. This circumstance plays a decisive role for the design of the ignition spark gaps in those cases when two or more such gaps are incorporated with the construction, and for the formation of a modulus system so that the generator can be built together by series connection of several units each comprising a number of transmission conducting zones having spark gaps and alternating capacitor zones.

FIGS. 3 and 4 show suitable detail embodiments of a generator according to FIG. 2.

In FIG. 3, the plate 18 is shown attached to the conductor 27 (connected to the load) by means of a screw 28 whose head fits in the outward bulge of the adjacent plate 4 constituting the electrode means 2. Between the plates 18 and 4, in the energy storing capacitor zone formed by them, there is a solid dielectric 29, and the peripheral portions of the plates are embedded in a distance ring 30 of electrically insulating material, such as epoxy resin. Correspondingly, the supetjacent plates 3, 4 constituting a capacitor zone between them are embedded in pairs at their peripheries in electrically insulating distance rings 30 and have between them a solid dielectric 29. When the electrode means 1, 2 are constituted by bulges on the plates 3 and 4 proper instead of by means mounted thereon, the dielectric 29 should be provided with electrically conductive foils 31 within the area of the bulges, said foils being in electric contact with the respective adjacent plate 3, 4. To ensure a constant distance between the plates 3, 4 defining transmission conducting zones, in the case of large diameter plates, electrically insulating distance rings 32 are provided on said plates also near the centers thereof. With the use of a gas or a gas mixture as a dielectric in the transmission conducting zones the distance rings 30, 32 are provided with gas penetration holes (not shown). To the outer distance rings 30 are connected two contact strips 33 to each of the plates 3, 4 and 4, 18, respectively, in the ring. The two contact strips in each ring are diametrically opposed and encased in insulating material 34. These contact strips are offset peripherally from distance ring to distance ring so that only two of the contact strips are visible in FIG. 3. These contact strips serve for the connection of the charging impedances (

details

23 and 26 in FIG. 2). Through the contact strips being offset as described,

the charging impedances will have an extension substantially parallel with the planes of the plates 3, 4, I8 and a length suitable with respect to the charging voltage. To prevent unfavorable effect of dust, if any, on the plates 3, 4, for instance in the form of unintentional breakthroughs in the transmission conducting zone, a disk 35 of insulating material can be inserted in the transmission conducting zones between the distance rings 30, 32, as has been shown for the sake of simplicity only for one of the transmission conducting zones in FIG. 3. By varying the thickness of the disk 35 also the length of the spark gap, that is the distance between the electrodes 1 and 2, can be finely adjusted.

In FIG. 3, the dielectric 29 in the capacitor zones and the insulating disk 35 have been shown as homogeneous layers. Same as at the manufacture of ordinary capacitors it is often advantageous to use instead of a single layer, several layers for increasing the breakthrough resistance. At voltages over about 50 kv. it is also possible to insert metallic layers or sheets between the insulating layers in the capacitor zones and in the transmission conducting zones. In the latter case a good optical and ionizing contact must be ensured between the different dielectric layer through holes at the centers of the metal sheets.

FIG. 4 shows a suitable embodiment when several trigger spark gaps are required for instance for meeting requirements for insignificant time jitter of the discharge. The uppermost plate 3 has attached to it a specific electrode means la which has a central hole for the trigger electrode 5. The uppermost plate 4 is designed in the manner described in connection with FIG. 3 and thus has a bulge acting s an electrode means 2, and it is separated from the subjacent plate 3a by means of an insulating layer 29, as already described. The plate 3a has no central bulge and carries instead at its lower side a metallic distance device 36 which has attached to it at the underside a metal plate 37 having a central hole for accommodating a spark gap electrode means 38 projecting from the underside of the plate 37. said electrode means 38 has a central hole into which penetrates a trigger electrode 39 which extends through a passage which leads through the distance device 36 and is filled with insulating material 40. The trigger electrode 39 and the distance device 36 are connected by mean of a coaxial cable 41 which extends through the field-free space between the

plates

3a and 37 out of the generator between the distance rings 30, to an outer trigger circuit (not shown) to produce a triggering spark between the trigger electrode 39 and the electrode 38 simultaneously as a triggering spark is produced between the trigger electrode 5 and the electrode la.All the plates 3 and 4 situated beneath the plate 37 are designed in the manner described in connection with FIG. 3. Should further trigger spark gaps be necessary they can be given the construction described in connection with the

plates

3a and 37.

The construction of trigger spark gaps described in connection with the

plates

3a and 37 provides a good capacitive connection between the various stages despite the arrangement of an additional trigger spark gap. Of course, it must be seen to it at the construction of the trigger circuits that breakthroughs in the various trigger spark gaps are'obtained as simultaneously as possible and that the trigger circuits for the additional trigger spark gaps are given a design that permits the main electrode associated with the trigger spark gap, e.g., electrode 38 in FIG. 4, to assume a high potential in the course of the discharge.

What I claim and desired to secure by Letters Patent is:

I. An apparatus for providing a steep voltage step across a load in an electric high-voltage circuit, comprising a high-voltage source, a plurality of pairs of electrode means, each pair forming a spark gap, two sets of electric conductor means, means electrically insulating one set of said conductor means for the other set, said conductor means being arranged in an array with alternate conductor means belonging to the same set, impedance means DC connecting said connector means of each set separately, means connecting said conductor means to said voltage source and the load, at least some of said conductor means being provided with said electrode means and being shaped and arranged to form around each pair of said electrode means and the spark gap formed thereby a transmission conducting zone which surrounds the spark gap and in which the impedance continuously varies in a direction away from the spark gap and is at its minimum at the outer boundary of the zone.

2. An apparatus according to claim 1 in which said conductor means provided with said electrode means are shaped and arranged to form around each pair of electrode means a transmission conducting zone in which the impedance monotonously sinks in a direction away from the spark gap.

3. An apparatus for providing a steep voltage step across a load in an electric high-voltage circuit, comprising a high-voltage source, a plurality of pairs of electrode means, each pair forming a spark gap, two sets of circular electric conductor plates, means electrically insulating one set of said plates from the other set, said plates being arranged coaxially in an array with alternate plates belonging to the same set, impedance means DC connecting said plates of each set separately, means connecting said plates to said voltage source and the load, at least some of said plates being provided at their center axis with said electrode means and being shaped and arranged to form around each pair of said electrode a. and the spark gap formed thereby a transmission conducting zone which surrounds the spark gap and in which the impedance continuously sinks in a direction away from the spark gap and the center axis of said plates.

4. An apparatus according to claim 3, in which said plates are substantially planar and parallel.

Claims

1. An apparatus for providing a steep voltage step across a load in an electric high-voltage circuit, comprising a high-voltage source, a plurality of pairs of electrode means, each pair forming a spark gap, two sets of electric conductor means, means electrically insulating one set of said conductor means for the other set, said conductor means being arranged in an array with alternate conductor means belonging to the same set, impedance means DC connecting said connector means of each set separately, means connecting said conductor means to said voltage source and the load, at least some of said conductor means being provided with said electrode means and being shaped and arranged to form around each pair of said electrode means and the spark gap formed thereby a transmission conducting zone which surrounds the spark gap and in which the impedance continuously varies in a direction away from the spark gap and is at its minimum at the outer boundary of the zone.