Method for the production of plasma

A device for supplying voltage to a plasma producer comprising a chamber having an outflow opening, and an anode and a cathode arranged in the chamber, the anode and cathode defining a gap therebetween, the voltage supplying device comprising a charging circuit, a capacitor battery having an input connected to the charging circuit and an output connected to the anode and the cathode, and an ignition set supplying HF signals connected to the anode and the cathode, a maximum voltage supplied by the capacitor battery being smaller than an arc-over voltage of the anode-to-cathode gap.

The invention relates to a device for supplying voltage to a plasma
 producer.
 In a known device of this kind, a plasma gas is blown substantially
 continuously through a chamber in which the anode-to-cathode gap is
 located. A fluctuating current flow is ensured by way of the arc gap
 through a control of the power supply. Usually, the current will fluctuate
 with a frequency of 1 to 10 Hz, with the maximum current usually being 7
 to 15 times the minimum current.
 The power supply is usually formed by a transformer with downstream
 rectifier. Furthermore, in the known methods the anode-to-cathode gap is
 charged with a voltage corresponding to the arc drop voltage of the arc,
 with a separate ignition pulse being provided for igniting the arc.
 The relevant aspect in the known method is that the arc will burn
 continuously, even though its output will fluctuate.
 This is problematic for various applications due to its continuous energy
 output.
 Thus, a plasma will emit UV radiation to a considerable extent which could
 be used for the sterilisation of objects, for example. However, the
 simultaneous radiation of a considerable quantity of heat constitutes a
 problem.
 It is the object of the present invention to avoid such disadvantages and
 to provide a a device for supplying voltage to a plasma producer, in
 which.
 The proposed measures will lead to the advantage that plasma pulses of only
 a very short duration can be produced. Such plasma pulses, despite their
 very high temperature, can be tolerated by even relatively sensitive
 materials without causing any damage as a result of their short duration
 because the energy introduced over a longer period into the material to be
 treated can be kept below a harmful level.
 It is not absolutely necessary to introduce a gas in the anode-to-cathode
 path. Metal vapours emerging from the surface of the anode and cathode
 occur as a result of the temperature of the arc, which metal vapours are
 ionised by the arc and form a plasma which flows out of an outlet opening
 of a chamber receiving the anode and the cathode.
 Holding the voltage pulses to 10.sup.-5 to 10.sup.-3 seconds, and
 preferably the pauses between the voltage pulses to 10 to 100 times the
 duration of the voltage pulses, allows keeping at a low level the energy
 introduced by a plasma produced in accordance with the invention into a
 subject charged with said plasma, so that even sensitive subjects can be
 processed with such a plasma whose individual pulses have a high energy
 density.
 The invention accomplishes these objects with a device for supplying
 voltage to a plasma producer comprising a chamber having an outflow
 opening, and an anode and a cathode arranged in the chamber, the anode and
 cathode defining a gap therebetween, the voltage supplying device
 comprising a charging circuit, a capacitor battery having an input
 connected to the charging circuit and an output connected to the anode and
 the cathode, and an ignition set supplying HF signals connected to the
 anode and the cathode, a maximum voltage supplied by the capacitor battery
 being smaller than an arc-over voltage of the anode-to-cathode gap.
 The proposed measures lead to a very simple arrangement, whereby the pulse
 times can be determined very easily by a dimensioning of the capacitors
 and the resistance of the circuit comprising the anode-to-cathode gap as
 well as the charging circuit for determining the respective time
 constants.
 Since a very rapid heating of the medium disposed in the interior of the
 chamber will occur during the ignition of the arc, it will expand very
 rapidly and will flow outwardly with a high kinetic energy through the
 outlet opening. In the following interpulse period, air can flow into the
 cooling chamber from the ambient environment, so that the same can be
 operated in a practically regenerative manner and no gas flow that
 permanently flows through the chamber needs to be forced.
 Since the individual plasma pulses exit with a high speed, there will not
 be any mixture with the ambient atmosphere during their emergence and thus
 there will not be any divergence of the plasma jet. In this context, tests
 have shown that the produced plasma pulses have a behaviour similar to
 that of globular lightning. This also ensures a very high energy density
 on the subject to be treated.
 The HF ignition set allows a very precise determination of the ignition of
 the arc while ensuring that the end of the voltage pulse or the arc
 duration is determined by the discharge of the capacitor battery to a
 voltage below the arc drop voltage. This ensures even in the case of the
 ignition of the arc by means of a separate striking voltage source that
 the arc will extinguish between the individual pulses and no static
 current will flow through the anode-to-cathode gap.
 The HF ignition set also allows triggering the ignition of the arc even
 before reaching the arc-over voltage of the anode-to-cathode gap, as a
 result of which the duration of the arc and thus the duration of the
 plasma pulse can be kept extremely short without having to make any
 special efforts to ensure a particularly low-impedance arrangement of the
 discharge circuit of the capacitor battery.
 In principal, it also possible to use a technical A.C. system or a voltage
 source supplying a high-frequency alternating current in connection with a
 phase-angle control unit instead of the capacitor battery as a power
 supply means. In the case of electrodes made from different materials, it
 must be ensured that similarly polarised half-waves are partly connected
 through only so that voltage pulses with the same polarity are always
 applied to the different electrodes and substantially the same ratios as
 in the supply of the plasma torch with d.c. voltage pulses, like from a
 capacitor battery for example, are obtained.
 In electrodes made from the same materials, pulses with different polarity
 can be applied to each of the two electrodes.
 As electrodes which are made of different materials for the purpose of
 achieving a longer service life are usually charged with the same polarity
 in plasma torches, the anode and cathode are referred to generally in the
 description and the claims.
 In order to ensure the short pulses which are provided for according to the
 method of the invention, it will usually be appropriate to provide a
 through connection by means of the phase-angle control only in the falling
 branch of the respective half-wave, which also depends on the rigidity of
 the supplying voltage source. It can also be provided to block the
 phase-angle control after each through connection for a certain number of
 periods in order to reduce the repeat frequency of the plasma pulses to a
 desired level.
 Preferably, the diameter of the outflow opening of the plasma producer is
 10 .mu.m to 100 .mu.m. This has the advantage that the individual plasma
 pulses exit from the outlet opening of the chamber with a very high speed
 and impinge on the subject to be treated with a very high kinetic energy.
 Outgoing speeds of 1000 to 2000 m per second could be determined in
 experimental set-ups. In this way it is possible to manufacture very small
 bores in thin sheet metal or even weld points.
 The application of a plasma produced in accordance with the invention is
 also provided in accordance with the invention for sterilising objects, in
 particular interior spaces of hollow objects or conduits.
 In this process, any bacteria or viruses are killed rapidly and
 effectively, despite the short exposure time, by the high temperature of
 the individual plasma pulses, which are approx. 20,000 to 50,000.degree.
 C., and are simultaneously removed from the surface of the object to be
 sterilised by the kinetic energy of the plasma pulses so that no "bacteria
 carcasses" remain.
 As a result of the continued production of very short plasma pulses as
 provided for in accordance with the invention, they can also be used for
 surgical and dental purposes, e.g. instead of laser scalpels.
 Plasma torches with a relatively small output, e.g. from 0.5 kW to 10 kW,
 can be used in both cases.
 Furthermore, the plasma produced in accordance with the invention can also
 be used very favourably for spot welding or the production of seams made
 of weld spots.
 A behaviour similar to that of flow plasma can be obtained in the
 production of plasma pulses with a frequency of approx. 7 Hz without
 causing any relevant withdrawal of energy from the plasma jet as a result
 of the mixture of the border zones of the plasma jet with the ambient
 atmosphere, which would lead to an undesirable heating of the ambient
 environment and to an undesirable heating of the subject outside of the
 actual area of machining.
 As a result, considerably less energy as compared with the previously used
 flow plasma is required for welding with a plasma produced in accordance
 with the invention. Moreover, there is an overall lower heating of the
 subject and thus also lower thermal stresses and deformations of the
 subject. Furthermore, the solidification of the individual weld points
 occurs more rapidly than when welding with flow plasma as a result of the
 very small melting bath volumes. This allows achieving a favourable
 welding quality in every welding position, i.e. even in overhead
 positions.
 It is understood that the plasma torches required for the production of the
 plasma pulses must have a respective output, e.g. 20 kW up to 150 kW and
 more, depending on the parts to be welded. A spot weld of thin sheet can
 be produced with merely one plasma pulse with a short duration of only
 10.sup.-3 to 10.sup.-5 seconds for example.
 The invention is now explained in closer detail by reference to the
 accompanying drawing, wherein:
 FIG. 1 schematically shows a sectional view through a holder with an
 inserted plasma producer;
 FIG. 2 schematically shows a sectional view on an enlarged scale through a
 plasma producer in accordance with FIG. 1;
 FIG. 3 schematically shows the electric circuit of a device in accordance
 with the invention.

In the embodiment in accordance with FIGS. 1 and 2 a holder 1 is provided
 which is made from an electrically insulating material such as ceramic, is
 substantially hollow-cylindrical and where an insert 2, which is also made
 of an insulating material, is pressed into one of its end zones.
 Said insert 2 is penetrated by a central tube forming a gas supply line 3
 and ending at the face side of the insert 2 projecting over the face side
 of holder 1. Insert 2 is further provided with two bores 4 which are
 disposed in a diametrical plane and in which press-fit parts 7 are held
 which are used as abutments and are penetrated by the cores 5 of
 connecting lines 6 with play.
 These connecting lines 6 are connected with a voltage supply which is shown
 in FIG. 3 and supplies voltage pulses at a predetermined frequency.
 Pressure springs 8 rest on said press-fit parts 7 and press outwardly the
 contact pins 9 which are soldered together with the cores 5. Contact pins
 9 are provided at their free end with a face-sided nose 10 which
 co-operates with a contact surface of a plasma producer 11 which is held
 in a fastening device 12 which is arranged on the face side of holder 1,
 said fastening device 12 is formed as a clip made from an electrically
 insulating material and in which the plasma producer 11 is inserted from
 above.
 Said plasma producer 11 is provided with a connecting element 13 which is
 made from an electrically insulating material such as ceramic, is arranged
 in its lower zone in a conically tapering manner and is provided at its
 lower face side with an opening 14.
 Said opening 14 is penetrated by an annular anode 15 which in the usual way
 is made from an electrically conducting and thermally heavy-duty material
 and is provided in its orifice zone with a nozzle opening 16.
 Anode 15 is provided with an upwardly conically expanding region which
 rests inwardly on the connecting part 13 and verges into a cylindrical
 zone.
 An intermediate part 17 rests on the upper face side of anode 15 which is
 provided with an annular shape and is made from an electrically insulating
 material such as ceramic.
 A holding part 18, which is made of an electrically well-conducting
 material such as copper, rests on the upper face side of the intermediate
 part 17. A cathode 19 is pressed into said holding part which is made from
 an electrically conducting and thermally highly resistant material such as
 a tungsten-cerium oxide alloy and is provided in its end zone close to the
 nozzle opening 16 of anode 15 with a conical arrangement.
 Anode 15 as well as the holding part 18 are appropriately pressed into the
 connecting part 13 for the purpose of determining the mutual position of
 the cathode 10 and the nozzle opening 16 of the anode.
 The anode 15, the intermediate part 17 and the holding part 18 with the
 pressed-in cathode 19 form jointly with the connecting part 13, a module
 of the device which can easily be built into the holder and can be removed
 again from the same.
 A pressure part 20 made of an insulating material rests on the upper face
 side of the holding part 18, which pressure part is provided with a bore
 21 for receiving the cathode 19 with play and projects beyond the face
 side of the connecting part 13.
 Said pressure part 20 co-operates with a lid 22 which is screwed onto an
 outside thread 23 arranged in a zone close to the upper face side of the
 connecting part 13.
 The connecting part 13 is provided with three radial bores 24, 25 which are
 arranged along a surface line, of which bores 24 allow the passage of the
 noses 10 of the contact pins 9 and lie in the zone of the holding part 18
 or anode 15. Bore 25 is arranged in the zone of the intermediate part 17
 and is flush with a radially extending inlet 26 of the intermediate part
 which leads to chamber 27 which is limited by the inner wall of the
 intermediate part 17 and is penetrated by the cathode 19.
 When the plasma producer, which is arranged as a module, is inserted in the
 holder 1, the bore 25 is also flush with the gas supply line 3 provided in
 the holder 1.
 For installing the plasma producer 11 which is arranged as a module, it is
 sufficient to withdraw the connecting lines 6 whose insulating sheaths 28
 are guided with play in the bores 4 of the insert 2 of the holder 1 and to
 insert the plasma producer 11 from above in clip 12. Thereafter one can
 release the connecting lines 6 and the contact pins 9 will snap into the
 bores 24 of the connecting part 13 and will secure the position of the
 plasma producer 11 in the holder 1. At the same time they are pressed with
 their face sides against the holding part 8 or anode 15 by means of
 springs 8 and thus a favourable electric contact is produced.
 During the operation of the plasma producer 11, a gas such as helium,
 CO.sub.2 and the like can be introduced through the gas supply line 3 into
 the chamber 27 which is also limited, among other things, by an anode 15
 defining a nozzle opening 16, which gas flows around cathode 19 and
 simultaneously cools the same in operation.
 If a voltage pulse is applied whose voltage is over the arc-over voltage of
 the gap between anode 15 and cathode 19, an arc will be formed which
 produces a plasma that emerges from the nozzle opening 16 and can be used
 for producing a weld seam or for cutting materials for example. If the
 voltage applied to cathode 19 and anode 15 drops below the arc drop
 voltage, the same will go out and the current flow over the
 anode-to-cathode gap will be interrupted.
 Notice should be taken principally that an introduction of gas into chamber
 27 is not ultimately necessary and that the same also need not have any
 bore 25. In such a case the chamber 27, after the ejection of the plasma
 pulse, will suck in air from the ambient environment after the arc goes
 out. During the following ignition of a new arc, as a result of applying a
 further voltage pulse, the air is ionised by the arc and rapidly heated,
 as a result of which it expands in a respectively rapid manner and flows
 out from the nozzle opening 16 with a high speed.
 A voltage supply for a plasma producer according to FIGS. 1 and 2 is shown
 in FIG. 3.
 A capacitor battery 30 is connected by way of a charging resistor 31 with
 the connections X1 of a controllable DC voltage source 32. The capacitor
 battery 30 is provided with a fixedly connected capacitor 1C1 and a
 capacitor 1C2 which can be connected in parallel with the same through a
 switch 1S1. Groups of capacitors can be concerned in both cases.
 This capacitor battery 30 is connected by way of connecting lines 33, 34
 with the cathode and anode, of plasma producer 11.
 An RC module is switched in parallel to the capacitor battery 30 which is
 formed by a capacitor 1C3 and a resistor 1R1. This RC module forms a
 rejection circuit in conjunction with a choke 1L1 switched in the
 connecting line 34, which choke is provided for the protection of the
 capacitor battery 30 against HF signals.
 The outputs of an ignition set 35 are further connected to the connecting
 lines 33, 34. Said ignition set 35 is connected on the input side with an
 AC voltage source X2 and provided with a trigger switch 1S2 by which an
 ignition pulse can be initiated when actuated.
 During operation, the capacitor batteries 30 are charged according to the
 set voltage of the DC voltage source 32 which is adjustable between 50V
 and 300V and the time constant which is co-determined by the line
 resistances and the charging resistance.
 Once the capacitor battery reaches a voltage which corresponds to the
 arc-over voltage of the anode-to-cathode gap 15, 19 of the plasma producer
 11, an ignition of an arc between anode 15 and cathode 19 (FIG. 2) and
 thus the formation of plasma in chamber 27 of the plasma producer 11 will
 occur.
 At the same time the capacitor battery 30 will discharge according to the
 time constant given by its capacity, the line resistances and the
 resistance of the arc. If as a result of this discharge the voltage of the
 capacitor battery 30 drops below the arc drop voltage, the same goes out
 and the capacitor battery 30 charges up again, as a result of which the
 described process is repeated and a frequency is obtained which is
 determined by the charging and discharging time constants. The operation
 of the ignition set is not required.
 For certain applications it can be desirable to determine the ignition time
 of the arc precisely or to initiate such a one prior to reaching the
 arc-over voltage of the anode-to-cathode gap.
 In this case an ignition pulse is initiated by actuating the trigger switch
 1S2 which leads to the ignition of an arc between the anode 15 and the
 cathode 19 of the plasma producer 11 without the capacitor battery having
 reached a voltage corresponding to the arc-over voltage of this gap. In
 this way the pulse-duty factor, which can be selected between 1:10 and
 1:100 and even beyond this figure, can be changed respectively and the
 ratio between the arc duration and its pause during a cycle can be changed
 in the sense of an extension of the arc pause, since the energy of the
 high-frequency ignition pulses of the ignition set is sufficient for
 igniting the arc, but not for maintaining the same when the voltage of the
 capacitor battery drops below the arc drop voltage.