WO1998026480A1

WO1998026480A1 - Controlled vacuum discharger

Info

Publication number: WO1998026480A1
Application number: PCT/IL1997/000383
Authority: WO
Inventors: Yuri Livshiz; Oren Gafri
Original assignee: Pulsar Welding Ltd.
Priority date: 1996-12-13
Filing date: 1997-11-24
Publication date: 1998-06-18
Also published as: IL119826A0; AU5132198A

Abstract

A controlled vacuum discharger for rigid switching of intense electric currents has a vacuum chamber holding air at a low pressure (P), two metal bodies functioning as main discharge electrodes and a plurality of ignition devices for initiating electric discharge between the two metal bodies. The two metal bodies having each a current discharge surface, the two discharge surfaces being disposed within a vacuum chamber and being parallel to one another with a distance (D) between them, electricity being discharged from one of the discharge surfaces to the other electrode at a breakdown potential (U). The ignition devices having each an ignition tip in proximity to one of the discharge surfaces so as to allow generation of an ignition electric spark at or in proximity to the said one of the discharge surfaces.

Description

CONTROLLED VACUUM DISCHARGER

FIELD OF THE INVENTION

The present invention concerns a vacuum discharger wherein electric current is discharged between two electrodes separated at a distance from one another. One application of the discharger is in switching of intense current, (e.g. 100-1,000 kA), high potential (several kN) electric pulses (1 kN = 1,000 Volts).

BACKGROUND OF THE INVENTION

Pulse magnetic forming (P F) is a process in which a metal workpiece or a portion thereof is deformed by an abrupt mechanical stress induced by pulsed magnetic force which causes the workpiece to deform.

Apparatuses and methods for PMF processes can be found in U.S. Patents

3,654,787 (Brower), 3,961,739 (Leftheris), 4,170,887 (Baranov), 4,531,393

(Weir), 4,807,351 (Cherian et al.) and 5,442,846 (Snaper). The PMF process utilizes a discharge capacitor or a bank of capacitors (capacitor battery) which is electrically connected to a forming coil. The PMF process requires an intense magnetic field which is created by a very rapid discharge of electric energy, stored in the capacitor battery, into the forming coil. A high performance PMF process utilizes a very intense current, typically above about 100 kA (kA = 1,000 Amperes). Rapid switching of such intense current for effective PMF process requires a switch which is capable of holding a high potential at rest which can pass such high current, and which has a very low internal inductance (typically below 50 nanohenry (nHn) and preferably even below 20 nHn). Furthermore, for industrial use, it is necessary that such switches should be capable of a prolonged life, e.g. of at least above about 100,000, preferably above about 1,000,000 pulses, and which will operate quietly.

One known switching device useful for rapid switching of high power high current pulses such as those required for a PMF process is the controlled vacuum discharger (CVD). Such dischargers comprise two electrodes having a discharging surface disposed within a vacuum chamber.

The theory of operation of CNDs is based on Paschen's Law:

U = f(P«D) (1) namely the breakdown voltage between the two electrodes (U) is a function of the product of the distance between the electrodes (D) and the pressure within the chamber (P) (see also Fig. 1).

Known CNDs work in the range defined by the left side of the curve (zone A). Accordingly, the pressure in the vacuum chamber is relatively low, namely very high vacuum (10^"4-10^~6 mmHg (torr)); achieving such a high vacuum requires sophisticated equipment and skilled personnel.

GENERAL DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a novel CVD for rapid switching of intense electrical currents, e.g. in the range of 100- 1,000 kA, particularly in the range of 200-500 kA.

In accordance with the invention there is ^■ provided a CVD comprising a vacuum chamber holding air at a low pressure P, two metal bodies functioning as main discharge electrodes and being connectable to a current discharge circuitry, and comprising a plurality of ignition devices for initiating electric discharge between the two metal bodies; the two metal bodies having each a current discharge surface, the two discharge surfaces being disposed within a vacuum chamber and being parallel to one another with a distance D between them, electricity being discharged from one of the discharge surfaces to the other electrode at a breakdown potential U; the ignition devices having each an ignition tip in proximity to one of the discharge surfaces so as to allow generation of an ignition electric spark at or in proximity to the said one of the discharge surfaces; the vacuum in the chamber being such that the parameters of the pressure P, expressed in mmHg, the potential U expressed in kV and of the D expressed in cm, obey the following relation (2): Z = lnU/ln(P«D) (2) wherein Z represents a value between about 0.1 to about 0.6.

The term "parallel" used above and further below should not be understood in a euclidian sense, namely as representing only two straight lines parallel to one another, but rather meaning two surfaces which may be planar, may have a curvature, e.g. be cylindrical, etc., which are equidistant from one another throughout the entire surfaces; namely a line normal to one surface will also be normal to the other surface with all such lines being of the same distance.

According to one embodiment of the invention, the two metal bodies consist of a first body having a first cylindrical member and of a second body having a second cylindrical member, the first cylindrical member having a diameter larger than that of the second cylindrical member, the two cylindrical members being coaxial and having portions facing one another, the facing portions defining the current discharge surfaces of the two metal bodies. The ignition devices, in accordance with this embodiment, may be held in receptacles in one of the two (typically the first) cylindrical members, being electrically isolated from the member- which they are disposed. Typically, the CVD of this embodiment will comprise three or more, typically four to six ignition devices, disposed on one lateral line, equidistant from one another. Preferably, the ignition devices are arranged such that straight lines from one ignition device to an adjacent ignition device passes through empty space (i.e. space not containing a solid object). This means that a light irradiation emitted from one ignition device can directly reach an adjacent ignition device.

By one example of a CVD according to said one embodiment, said first metal body comprises a cup-like shaped structure consisting of an end wall and cylindrical side walls, and said second metal body comprises a circular planar member and a cylindrical member integral therewith and standing normal thereto. The diameter of the cylindrical member is smaller than that of the cylindrical side walls of the first electrodes and the two electrodes being arranged such that said cylindrical member of the second electrode is accommodated within the space defined by the cup-like shaped structure.

In accordance with another embodiment of the invention, the two metal bodies consist of a first body having a first planar member and of a second body having a second planar member, the first planar member being circular and the second planar member being annular with the center of the two planar members being on the same axis, and having portions which are opposite one another, the opposite portions defining the discharge surfaces of the two electrodes. The plurality of ignition devices, typically more than three, and preferably four to six, may be disposed within either one of the two planar members, with the plurality of ignition devices being preferably arranged such that they are all equally distanced from the center of the planar member in which they are disposed. The ignition devices, similarly as in the case of the aforementioned one embodiment, are electrically isolated from the planar member in which they are disposed and have an ignition tip which is proximal to a discharge surface, preferably that of the member in which they are disposed.

By one example of said another embodiment, the first metal body has a central integral stem normal to said planar member, and said second metal body comprises a cylindrical member, said stem being of a diameter smaller than said cylindrical member and being accommodated within the space defined by said cylindrical member, there being an electrically non- conductive substance disposed between them, the end of said stem and said - Z> -

cylindrical member distal from the planar members serving for electrical contact to an electric circuitry. A CVD in accordance with this embodiment may comprise also a casing made of an electrical non-conducting material and defining, either by itself or together with one or both of the planar members, the vacuum chamber, with the ends of the stem and the cylindrical members projecting outward from said casing.

The ignition device is preferably a monopole electrode device with a single electrode ending at the ignition tip, connected to one pole of an ignition circuitry, with the other pole of the ignition circuitry being connected to one of the main electrodes (that which comprises the discharge surface which is proximal to the ignition tip). In such a case, an ignition spark will be initiated between the ignition tip and the proximal discharge surface. It is however also possible for the ignition device to be a bipolar electrode device comprising two electrodes, each one being connected to one pole of the ignition circuitry, with an ignition spark being initiated between these two electrodes.

The vacuum in the vacuum chamber may be an a priori created vacuum maintained by means of an airtight seal; although typically, the vacuum chamber comprises an outlet being in communication with a vacuum pump which is continuously operative to generate a desired vacuum in the vacuum chamber. Preferably, in order to dampen possible pressure fluctuations which may occur inside the vacuum chamber during operation, the vacuum chamber may be communicating with an auxiliary, buffer vacuum chamber, and the larger affective volume will thus yield lower pressure fluctuations.

The present invention also provides a device for generating high current electric pulses through a load circuitry, comprising a power sourer and a switching device being a CVD according to the invention, and a pulse generating, ignition circuitry connected to the ignition device for providing an ignition spark.

In accordance with a specific embodiment of the invention, the load circuitry is a PMF coil assembly for forming of metal workpicces. The invention will now be illustrated in more detail in relation to some specific embodiments with occasional reference to the annexed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graphical representation of Paschen's Law which gives a relation of the product of the distance between the electrodes and the air pressure within the electrodes and the breakdown voltage;

Fig. 2 shows the longitudinal cross-section through a CVD according to an embodiment of the invention;

Fig. 3 shows a transverse cross-sectional view through lines III — III in Fig. 2;

Fig. 4 is an enlarged cross-sectional view of an ignition electrode;

Fig. 5 shows operational steps of an electric discharge of the CVD of Figs. 2 and 3:

Fig. 5A shows the initial ignition step, and Fig. 5B shows the progress of the electric's discharge;

Fig. 6 shows a longitudinal cross-section through a CVD according to another embodiment of the invention; Fig. 7 shows a view from the direction of arrow VII in Fig. 6; and

Fig. 8 shows an electrical circuitry of a PMF apparatus employing a CVD of the Figs. 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION Reference is first being made to Fig. 1 which is a graphical representation of Paschen's Law (on a logarithmic scale). The left zone of the curve represents the situation of a low P«D product, which in practice usually means a very high vacuum. The zone marked B represents the situation of a higher P»D product, which in practice usually means a situation of lower vacuum. The present invention is unique in that it works in the range of the parameters defined by relation 1 above (namely, wherein Z is within the range of about 0.1 to about 0.6).

Reference is now being made to Figs. 2 and 3 showing a controlled vacuum discharger (CND) according to one embodiment of the invention. The CND generally designated 12 comprises: a first metal body 14 serving as a first main electrode and having a general cup-like shape and having an end wall 16, cylindrical side walls 18 and an annular shoulder 20; a second metal body 22 serving as a second main electrode comprising a planar circular member 24 with an axial opening 26 and a off- axial opening 28 and having an integral cylindrical member 30 with its walls being normal to the planar member 24, and having an open end 32; and comprising a ring 34 made of an insulating material such as plastic, teflon, nylon, rubber, ceramic and others. The two main electrodes 14 and 22 are made of metal, e.g. steel or stainless steel. Planar member 24 of electrode 22, ring 34 and annular shoulders 20 of electrode 14 are clamped together in a sealing engagement by clamping means (not shown) and thus define together a closed vacuum chamber 36. In this specific embodiment, planar member 24 and electrode 14 define also the casing of the CND. It will be appreciated, that it is also possible, in accordance with other embodiments, to coat the external metal surfaces by an electrically insulating layer which may be important for personal safety. It is also possible for the entire structure to be encased in a larger sealed enclosure in which case ring 34 will serve mainly as a spacer between the electrodes and there will be no need for a sealing engagement between it and between planar member 24 and shoulders 20.

Cylindrical side walls 18 and cylindrical member 30 are coaxial. Cylindrical side walls 18 and cylindrical member 30 have respective portions facing one another which define respective discharge surfaces 38 and 40 of the two electrodes. Vacuum chamber 36 holds a vacuum created by a vacuum pump

(not shown) connected to opening 26. During operation, there are fluctuations in the pressure within chamber 36 and in order to dampen these fluctuations there is provided an auxiliary vacuum buffer chamber, shown schematically as box 42 communicating with the vacuum chamber 36 via opening 26. One of electrodes 14 and 22 is connected to a capacitor battery

(not shown) and the other to a load circuitry which connection, in this specific embodiment, is achieved by means of a coaxial cable 44, of which the internal leads are connected to connectors 46 of electrodes 22, and the external leads are connected to connector 48 of electrode 14. As will be appreciated, use of a coaxial cable is example only, and other cables may also be used.

Disposed in cylindrical walls 18, at about the midline of the discharged surface 38, are a plurality of ignition devices 50, six in the specific embodiment. A more detailed cross-sectional view of an embodi- ment of an ignition device can be seen in Fig. 4. The ignition device 50, according to this specific embodiment, comprises a metal electrode body 52, an insulating plug 53, a nut 54 screw engaged with metal bush 55, a rubber washer 56 and another insulating plug 57. Electrode body 52 which is insulated from the surrounding metal walls has an ignition tip 60 in proximity to the discharge surface 38. The ignition device in this specific embodiment is a monopole electrode where first metal body 14 constitutes the other pole. When the electrode body 52 is charged to a potential above a threshold potential, e.g. above about 4 kV, there is a discharge spark which is formed between tip 60 and surface 38 thus initiating a discharge of current between the main electrodes as will be explained further below. Furthermore, as can be seen in Fig. 3, each ignition device views adjacent devices, namely, a straight line between adjacent devices (represented by broken lines in Fig. 3) passes through empty space (except for gas contained therein). Consequently, when a spark is generated in one ignition tip, the UV light irradiated by the spark excites gas molecules which it encounters, including gas around the adjacent ignition tips; this excitation accelerates the conversion of the gas at adjacent ignition tips into a plasma, and as a result, there is an essentially synchronous spark in all the ignition devices 50.

Specific exemplary parameters of the above illustrated CVD include: • Distance between the two main electrodes: 1-5 cm, preferably about 1.5-3 cm, typically about 2 cm;

Length of the discharge surfaces (i.e. the overlapping surfaces of the cylindrical side walls of the first electrode and cylindrical member of the second electrode): 15-45 cm, preferably about 20-30 cm, typically about 20 cm;

• Air pressure in the vacuum chamber: 0.01-100 mmHg, preferably 10- 80 mmHg, typically about 40-50 mHg.

A CVD with such parameters has a breakdown voltage in the range of about 0.5 to 25 kN, typically about 10 kV; and such a CVD can pass current up to about 500 or even 1000 kA, and will typically be used for the transfer of current in the range of 100-300 kA.

Reference is now being made to Fig. 5, showing the sequence of events occurring during discharge of electric current between the two main electrodes. At first as shown in Fig. 5A, an electric spark forms between the ignition tip 60 and discharge surface 38. This then gives rise next to a current discharge between discharge surface 38 and surface 40 of the other metal body, as can be seen in Fig. 5B. During this main electric discharge, the discharge zone moves away from the center towards the edges of the cylindrical member 30, as represented by arrows 72. Given the fact that discharge occurs simultaneously in several regions, the current distributes throughout the entire circumference of the discharge surfaces of the main electrodes. This translates into a relatively low current density in any given single zone which means relatively little erosion and low operational noise.

Reference is now being made to Figs. 6 and 7 showing another embodiment of a CVD in accordance with the invention. The CVD of this embodiment, generally designated 80, comprises: a first main metal body 81 serving as a first main electrode and comprising a planar circular member 82 and an integral stem 83; a second metal body 85 serving as the second main electrode and having a cylindrical portion 86 coaxial with stem 83 and an integral planar member 87 having the shape of an annulus; an insulating substance 88 is disposed within stem 83 and cylindrical portion 86; and a casing 89 made of an electrically non-conducting substance which is in sealing engagement both with member 82 and with cylindrical portion 86 thus defining a vacuum chamber 90. As can be seen in Fig. 6, members 81 and 87 are parallel to one another and are facing one another defining respective discharge surfaces 91 and 92. The structure 93 formed by stem 83, cylindrical portion 86 and insulating substance 88 is in fact a coaxial lead and is connectable, e.g. by an appropriate coaxial connector (not shown) to an electric circuitry 94, comprising a capacitor battery 95, with an associated power supply 95', and a load circuitry, e.g. pulse magnetic forming coil 96 for electromagnetic forming of metal objects. Stem 83 has a short extension 99 projecting out from the external surface of planar member 81, having a longitudinal bore 100 communicating with vacuum chamber 90 through transverse bores 101. Projection 99 serves as communication port with a vacuum pump (not shown) for creating a vacuum in vacuum chamber 90. Disposed within member 81 are a plurality of ignition devices 102, six in this specific embodiment (as can be seen in Fig. 7), which are similar to ignition devices 50 of the embodiment shown in Figs. 2-4.

The sequence of events occurring of the CVD according to this embodiment is similar to that described in Figs. 5A and 5B. Briefly, a spark initiated by the ignition devices 102 gives rise to a discharge of current between discharge surfaces 98 and 100 with the discharge zone moving in the direction of arrows 104.

An electric switching circuitry employing a CVD of the invention is shown in Fig. 8. The example here shows a CVD 114 according to the embodiment shown in Figs. 2 and 3. The circuitry comprises a capacitor battery 110, a power supply 112, the CVD 114, an electric load circuit- ry 116, which in this specific example is a coil for use in pulse magnetic forming of a metal workpiece, and a pulse generating ignition circuitry 118. Capacitor battery 110 which is charged by power supply 112, has one terminal 120 connected to a first main electrode 122 of CVD 114 and has another terminal 124 connected to one terminal 126 of the load circuitry 116. The other terminal 128 of the load circuitry, is connected to the second main electrode 130 of CVD 114. Ignition circuitry 118 has a high voltage power source 132, a capacitor battery 134, a trigatron switching mechanism 136 and resistor 138. Triggering to switching mechanism 136 is provided by a pulse transformer 139 the operation of which is initiated by switch 140. Coaxial cables 140 connect ignition circuitry 118 to CVD 114 such that the internal lead thereof is connected to the electrode ignition devices 142 and the external lead is connected to electrode 130. When switch 140 is closed, electricity stored in the capacitor battery 134 is discharged which brings to development of potential resistor 138, and this potential leads to generation of a spark by ignition device 142. This eventually leads to a discharge of current between the two main electrodes 122 and 130, in the manner explained above.

Claims

CLAIMS:

1. A controlled vacuum discharger comprising a vacuum chamber holding air at a low pressure P, two metal bodies functioning as main discharge electrodes and a plurality of ignition devices for initiating electric discharge between the two metal bodies; the two metal bodies having each a current discharge surface, the two discharge surfaces being disposed within a vacuum chamber and being parallel to one another with a distance D between them, electricity being discharged from one of the discharge surfaces to the other electrode at a breakdown potential U; the ignition devices having each an ignition tip in proximity to one of the discharge surfaces so as to allow generation of an ignition electric spark at or in proximity to the said one of the discharge surfaces; the vacuum in the chamber being such that the parameters of the pressure P, expressed in mmHg, the potential U expressed in kV and of the D expressed in cm, obey the following relation (2):

Z = lnU/ln(P«D) (2) wherein Z represents a value between about 0.1 to about 0.6.

2. A controlled vacuum discharger according to Claim 1, wherein the two metal bodies consist of a first metal body having a first cylindrical member and of a second metal body having a second cylindrical member, the first cylindrical member having a diameter larger than that of the second cylindrical member, the two cylindrical members being coaxial and having portions facing one another, the facing portions defining the discharge surfaces of the two electrodes.

3. A controlled vacuum discharger according to Claim 2, wherein the ignition devices are held in receptacles within one of the two cylindrical members, in which it is disposed.

4. A controlled vacuum discharger according to Claim 3, comprising three or more ignition devices disposed on one lateral line, equally distanced from one another.

5. A controlled vacuum discharger according to Claim 4, comprising six ignition devices.

6. A controlled vacuum discharger according to Claim 4 or 5, wherein each ignition device has at least two adjacent ignition devices within a line of site.

7. A controlled vacuum discharger according to any one of Claims 2-6, wherein: said first metal body comprises a cup-shaped structure consisting of an end wall and cylindrical side walls; said second metal body comprises a circular planar member and a cylindrical member integral therewith and being essentially normal thereto, the cylindrical member having a diameter smaller than that of the cylindrical side walls of the first electrode; the two metal bodies being arranged such that said cylindrical member of said second electrode is accommodated within a space defined by said cup-like shaped structure.

8. A controlled vacuum discharger according to Claim 7, said cuplike structure is fitted with an annular rim and said first electrode and said second electrode are fitted together with an intermediary ring made of an electrically non-conducting material being disposed between said planar member and said annular shoulders and forming an airtight attachment therewith, the planar member, the intermediate ring and the cup-like structure defining together said vacuum chamber.

9. A controlled vacuum discharger according to Claim 1, wherein the two metal bodies consist of a first body having a first planar member and of a second body having a second planar member, the first planar member being circular and the second planar member being annular with the center of the two planar members being on the same axis, and having portions facing one another, the facing portions defining the discharge surfaces of the two electrodes.

10. A controlled vacuum discharger according to Claim 9, comprising a plurality of ignition devices disposed on the first planar member.

11. A controlled vacuum discharger according to Claim 10, comprising three or more ignition devices all equally distanced from the center of said member.

12. A controlled vacuum discharger according to Claim 11, compris- ing six ignition devices.

13. A controlled vacuum discharger according to any one of Claims 9-12, wherein the first metal body has a central integral stem normal to said planar member, and said second metal body comprises a cylindrical member, said stem being of a diameter smaller than said cylindrical member and being accommodated within the space defined by said cylindrical member, there being an electrically non-conductive substance disposed between them, the end of said stem and said cylindrical member distal from the planar members serving for electrical contact to an electric circuitry.

14. A controlled vacuum discharger according to any one of the preceding claims, wherein the ignition device comprises a monopole electrode.

15. A controlled vacuum discharger according to any one of the preceding claims, for use in switching of large current or high potential electric pulses.

16. A pulse magnetic forming apparatus, comprising a switching device according to Claim 15.

17. A device for generating high current electric pulses through a load circuitry, comprising a power source and a switching device being a controlled vacuum discharger according to any one of Claims 1-15, and a pulse generating, ignition circuitry connected to the ignition device for providing an ignition spark.