Device and procedure for vacuum evaporation of a material, plasma arc device and utilisation of the procedure.

0001The present invention relates to a device for evaporating material in a vacuum by means of an arc with a target, which has at least a part of its surface a material to be evaporated, the arc discharge being operated in a region where a substantial part of the arc current is mostly flows through small spots on the target surface; with a device for igniting the vaporization arc; with a method according to the preamble of claim 8; with a method for igniting the arc; with a plasma arc arrangement for applying coatings to a substrate with a device and with an application of the method for the vapor deposition of substrates by means of a plasma arc.

0002Methods and systems for applying coatings made of relatively high-melting materials by depositing the material evaporated in vacuo with the aid of an electric arc are known and have been described in a variety of ways.

0003From DE-OS 21 36 532 a system is known in which an electric arc is generated between an envelope-shaped anode and a cathode mounted on a cooling bed, the surface of the cathode containing a metal to be evaporated. By attaching a screen to the cathode, the cathode spot generated on the cathode is to be prevented from jumping over laterally from the evaporation surface. The problem with this system is that due to the accidental unguided wandering of the cathode spot, the cathode surface is removed unevenly and, in addition, due to local overheating, there are always splashes that lead to faults on the coating to be produced or. lead in the coating. US Pat. No. 3,625,848 also describes an analog system, the cathode here being made from the coating material which is to be evaporated. The system basically has the same disadvantages as the system according to DE-OS 21 36 532.

0004In the system according to US Pat. No. 4,556,471, a magnetic field or by attaching a permanent magnet attempts to influence the arc so far in its "migration" that the cathode surface or the so-called target is removed largely uniformly. The cathode is additionally insulated from the vacuum chamber. Basically, however, the arc spot on the cathode continues to move unguided and random, which means that the disadvantages described are only partially eliminated and the risk of splashes remains. A system is described in US Pat. No. 4,620,913, where a special anode arrangement is to be used to prevent the spot from migrating or jumping over from the cathode. DE-OS 35 28 677 also aims to cause the arc spot to jump away by attaching arc limiting means, the spot being additionally intended to have a directed movement on the cathode by attaching a magnetic field source. Pulsed magnetic field generation causes the spot, depending on the polarization, to move in the direction of or against the magnetic field. It is only a "directional", but not an actual guiding of the stain, so that - probably somewhat reduced - the risk of local overheating and thus of splashes is still not completely eliminated. It has also not been possible to implement systems of this type satisfactorily.

0005In DE-OS 33 45 493 the migration of the spot is again prevented by arranging a delimitation ring on the cathode, but without influencing the random movement of the arc spot. DE-PS 33 45 442 also claims the attachment of a limiting ring made of a magnetically permeable material.

0006In DE-PS 31 52 131 a magnetic field is generated in the vacuum chamber by a so-called solenoid. This magnetic field causes the cathode ray spot to migrate. Furthermore, an ignition pulse generator is provided for the continuous ignition of cathode ray spots, which then migrate towards an end face as a result of the magnetic field. However, the spot is not actually controlled, but is only caused to wander from the random generator along the cathode surface to the end face.

0007CH-PS 657 242 describes how the resulting splashes or macro particles are excreted in the plasma. A so-called plasma conductor and a coaxially arranged electromagnet are arranged, which is connected in opposition to a focusing solenoid. This creates a special magnetic field that deflects the plasma beam in such a way that splashes or Macro particles are expelled from the plasma conductor. This redirection is associated with large material losses. In addition, the deflection mechanism, which is coated much more strongly than the substrates, has to be constantly cleaned, which is associated with great effort and considerable activity. The same is claimed in FR-A 2 524 254. DE-PS 32 34 100 in principle follows the content of the two aforementioned documents, with additional modifications being proposed to ensure a more uniform coating of a workpiece. However, these in no way relate to the management of the stain on the cathode surface.

0008In DE-OS 37 31 127 an arc is operated in a pulsed manner, ie the substance temperature is compared with an upper temperature limit, respectively. the arc is interrupted if this upper limit is exceeded. This is to prevent local overheating on the cathode surface. Splashes that occur nevertheless should be deflected by attaching a magnetic field / shielding plate, ie that pulsing is only a very inadequate solution. Shields with a magnetic field and screens are uneconomical because they require a lot of maintenance and severely affect the coating speed. The method described is particularly suitable for decorative coatings and less for coatings in the technical field.

0009In US Pat. No. 4,673,477, the cathode spot is guided by means of a permanent magnet. The anode is ring-shaped and the cathode is displaced parallel to the ring surface as a disk. A magnet is arranged behind the cathode, by means of which the cathode spot is moved on the cathode surface. Although this arrangement reduces spatter, the construction must again be described as complicated.

0010Also in EP-A 0 284 145 the anode is ring-shaped and the cathode is arranged parallel to the ring plane as a cylinder, the latter being rotatable about its axis. In order to prevent indiscriminate migration of the cathode spot, a magnet arrangement is provided, which is slidably arranged within the cathode cylinder. By rotating the cylinder and longitudinally displacing the magnet, the cathode spot is guided on the surface to prevent spattering. The proposed arrangement comprises a very complicated construction, which cannot be described as very practical.

0011In the two cases described above, it has also been shown that the spatter reduction depends very much on the cathode material, although it is insufficient for the materials of interest in practice, such as titanium.

0012The object of the present invention is therefore still the requirement for a method or an arrangement by means of which a spark or. an arc spot is stabilized and controlled for the evaporation of a coating material to overcome the disadvantages of an uncontrolled non-guided spark or speckle, as extensively shown above. The design and the process should also be as simple as possible and, if necessary, even be able to be added to existing systems.

0013According to the invention, this is achieved by means of a device, preferably according to at least one of the claims, such as in particular according to claim 1, and by means of a method, preferably according to at least one of the claims, such as in particular according to claim 8.

0014What is proposed is a device for vaporizing material by means of an arc in a vacuum with a target, which has at least a part of its surface a material to be vaporized, which is connected as part of an arc discharge, the arc being operated in an area where an essential Part of the arc current mostly flows through small spots on the target surface, the device additionally including an electron beam gun or a laser for generating a local vapor cloud on the target surface, thereby stabilizing and guiding the base point of the arc.

0015The target is preferably the cathode of the spark or arc.

0016The laser or the electron beam is preferably constructed in such a way that the electron beam or laser beam can be guided on the target surface. It is possible, for example, to arrange the object lens of the laser so as to be movable or to further provide a mirror which is arranged to be movable for the moving of the focal spot. Simultaneously with the guiding of the electron beam or laser beam, the arc spot is also guided on the cathode surface by following the local vapor cloud generated on the target surface by the electron beam or laser beam. It is also possible to arrange the target movably.

0017Another big problem, which all the different, to the state of the art vacuum evaporation plants resp. The process is common, is the ignition of a spark resp. Arc. In DE-OS 34 13 728 it is proposed, for example, that the arc is ignited by moving a contact rod or an arc igniter, which on the one hand rests on the cathode surface and on the other hand is connected to the power source for generating the arc, away from the cathode surface. When the ignition rod engages the cathode surface, there is a closed circuit path. When the ignition rod is moved away, an arc jumps from the wire onto the cathode. This arc jumps immediately to the anode and is therefore ignited. The pneumatically controlled construction described in this DE-OS 34 13 728 for guiding an arc ignition wire to and from the cathode surface is complicated and obviously very susceptible to failure.

0018EP-A-0 211 413 describes a ring-shaped ignition device which is arranged around the cathode. There is a small distance between the inner edge of the ignition ring and the target surface, which is connected via a thin film, for example consisting of titanium nitride, for generating the closed circuit. During the ignition process, this thin film is partially evaporated and the ionized material enables the arc to jump from the anode to the cathode. In the subsequent coating process of a substrate, the film is restored.

0019This construction too must be described as complicated, and is also very susceptible to failure, in particular due to the constant evaporation and redeposition process of the thin film.

0020In DE-OS 35 16 598 the arc is ignited mechanically, using a lever which is mounted in a bellows. The construction, which is reminiscent of that in DE-OS 34 13 728, is also complicated and prone to failure.

0021In US Pat. No. 4,612,477, the arc is ignited electrically using a so-called trigger electrode which is connected to a spark bridging circuit in such a way that breakdown between the trigger electrode and the cathode takes place at predetermined time intervals. This respective voltage interruption leads to the ignition of the arc between the cathode and the anode. In particular, the design of the circuit is extremely complicated.

0022Analogously, the sparking in US Pat. No. 4,673,477 is also carried out electrically by a high-voltage ignition unit. This arrangement is also complicated and further difficulties often arise in the separation between the two discharges.

0023It is therefore another object of the present invention to provide a device. to propose a method according to which the ignition of a spark or. Arc occurs in a simple and as trouble-free manner as possible.

0024According to the invention, this is achieved by means of a device according to claim 5.

0025It is proposed that the device according to the invention for igniting a spark described above. the evaporation arc by means of the conventional electron beam evaporator source. is used by means of a suitable laser beam. The electron or. Laser beam melted the surface of the target, which is connected as an electrode of a spark source, locally or over the entire surface. A local vapor cloud is formed over a small area of ​​the target surface and partial ionization of the vapor is achieved by impacting the evaporating material with the electrons of the electron or laser beam. This is sufficient to ignite the spark by applying the open circuit voltage between the two electrons of the spark, respectively. Arc.

0026Further features according to the invention are set out in claims 6 and 7.

0027Furthermore, a method for evaporating a material by means of an arc on a target in a vacuum is proposed, the target having at least on its surface a material to be evaporated, which is connected as part of an arc discharge, and wherein the arc discharge is operated in an area where a substantial part of the arc current flows through at least small spots on the target surface, in that a local vapor cloud is generated on the target surface by means of an electron beam or a laser, specifically in such a way that the base point of the arc or spark is stabilized in this vapor cloud and is guided with it. It is also possible to create a puddle under the steam cloud on the target surface.

0028The electron beam or the laser beam can preferably be guided on the target surface in order to avoid the arc. To lead spark base on the target surface. The electron beam or laser beam can of course be guided according to a predetermined pattern, according to which a uniform removal of the target surface by the guided arc or. Spark spot is guaranteed.

0029The method according to the invention is preferably operated in such a way that, on average over time, the operating power of the arc discharge exceeds the operating power of the electron beam or the laser, as a result of which the evaporation of the material on the target surface mostly takes place by means of the arc discharge.

0030The energy density of the electron beam or laser beam is regulated in particular in such a way that a discharge current of more than 30 amperes is made possible with a discharge voltage of only 10 to 15 volts.

0031It is further proposed that the evaporation chamber lying above the target maintains a dilute atmosphere of noble gas, oxygen, nitrogen, a gaseous carbon compound, a metal-organic gaseous or boron-containing gaseous compound.

0032If the target surface comprises a slightly melting material, the electron beam or laser beam is preferably defocused to such an extent that the puddle around the arc base of the arc discharge constantly forms a funnel, in the bottom of which liquid target material always runs, whereby the arc is stabilized without guidance.

0033It is further proposed that the movement of the electron beam or laser focal point takes place so rapidly that the intrinsic movement of the arc discharge base, which is preferably the cathode base, is thereby suppressed.

0034The inventive devices mentioned above are particularly suitable for use in a plasma arc arrangement for applying coatings to a substrate.

0035The methods according to the invention are also suitable for igniting the evaporation arc on the target, which is done by means of an electron beam or the laser. Corresponding methods according to the invention are characterized according to the wording according to one of claims 13 to 15.

0036The methods described are also suitable for vapor deposition of substrates by means of a plasma arc in a high vacuum.

0037The processes according to the invention are particularly suitable for the production of coatings consisting of oxides, nitrides, oxynitrides, borides, carbides and fluorides for optical applications by evaporating an element or a compound of the corresponding element from groups 2a, 3a, 3b, 4a, 4b, 5a , 5b, 6a in a corresponding reactive gas atmosphere.

0038The invention will now be explained in more detail, for example, with the aid of the attached figures and specific exemplary embodiments. The attached Fig. 1 shows a high vacuum system with a spark discharge arrangement and an electron gun. 2 shows a schematic diagram of a plant according to the invention, suitable for carrying out Examples 4 and 5.

0039In a high vacuum system 9 with the corresponding pump opening 10 for evacuating the vacuum system 9, a conventional electron gun 6 is mounted, which is cooled by means of water cooling 7, 8. The electron gun 6 is arranged in such a way that an electron beam emitted by the electron gun, guided through the pole pieces of an electromagnet 5, strikes the rotating target 1, which is mounted insulated from the system floor by an insulator 4 and is also cooled by water cooling 2, 3.

0040The target 1 also serves as the cathode for the spark discharge. This spark discharge comprises a water-cooled anode 12 with a water cooling 14, which is insulated from the system via an insulator 13. In an example embodiment according to the invention, the anode 12 has the dimensions 30 × 10 cm and is mounted at a distance of approximately 10 cm from the target 1.

0041An intermediate floor 11 serves as a pressure stage in the event that reactive or noble gases are admitted via a gas inlet 16, so that the pressure in the charging chamber 21 rises above the maximum permissible pressure for the electron gun.

0042The circuit 15 symbolizes the power supply of the spark, which can be, for example, a high current generator. The objects to be coated, such as optical substrates, are fastened on the rotating substrate holder 17, which is insulated from the system via insulations 18. The rotary union 19 is water-cooled. The substrate holder can be placed at a potential that is negative with respect to the cathode by means of a voltage source 20.

0043When the high vacuum system 9 is now put into operation, an electron beam is guided onto the surface of the target 1 by means of the electron gun and the pole pieces of the electromagnet 5. As a result, a point-like patch of the material to be evaporated is melted on the target surface and this material is partially evaporated. The ionization of the vapor is achieved by the ionization of the evaporating material with the electrons of the electron beam. This ionization is sufficient to ignite the spark by applying the open circuit voltage between the two electrodes of the spark, respectively. between targets 1 and anode 12. The skipping spark resp. The arc jumps on the pre-melted, spot-like spots on the target surface.

0044The electron beam gun 6 is preferably arranged in the high vacuum system 9 such that the electron beam can be guided on the surface of the target 1 as desired. By means of this movement, the arc can now also be guided in any manner on the target surface, as a result of which the target material can be evaporated evenly and without spatter. If the arc is interrupted, the point surface of the target surface automatically melts a new spark and the arc is maintained. A substrate, which is arranged on the substrate holder 17, is coated in accordance with generally known process techniques.

0045Instead of an electron gun 6, a laser can of course also be arranged in the high vacuum system, as a result of which the surface of the target 1 is locally melted by a corresponding laser beam. The spark is again ignited by the necessary ionization of the vaporized material or of the residual gas atmosphere in the feed chamber 21. Even if a laser is attached instead of an electron beam gun, it is possible to guide the corresponding spot on the surface of the target after the arc has been ignited, in order to ensure a uniform and spatter-free removal of the target surface.

0046The use of an electron beam gun shown in Fig. 1 is suitable for firing or. Carrying a spark in any high vacuum system.

Examples:

Example 1:

0047A type BAI 640 K cubic steaming system was equipped as follows: In its bottom, decentred, a water-cooled crucible was installed. Next to the crucible in the bottom was an electron beam gun with an axial magnetic field, the cathode chamber of which was pumped out differentially. The distance from the exit of the electron beam to the center of the cathode was 100 mm. The electron beam gun had a maximum output of 8 kW. The crucible was made of copper, water-cooled and had a diameter of 80mm. It was electrically isolated from both the chamber floor and the mass of the cannon. It could be rotated with an electric motor. The crucible was connected to the negative pole of a DC voltage supply device of the type of a welding transformer (max. 250 A) with copper cables of 10 mm using a cable which had been inserted into the system in an insulated manner. The positive pole of the power supply was connected to a water-cooled auxiliary anode with a separate, electrically insulated and water-cooled bushing. This was rectangular, 250 x 100mm, and stood like a pennant at a distance of 60mm from the crucible. Opposite the floor was a turntable, the axis of which led through the plant center, on which test specimens were attached. The crucible was charged with 350g titanium and the system was closed and pumped out. The test specimens were heated using a method customary for tool coating and their surface was cleaned in an argon plasma. The electron beam gun was then switched on and its power increased to 700 watts. The tension between filament and crucible was 10.6 kV. The crucible was rotated at a frequency of 2 revolutions / minute. The focal spot with a diameter of approximately 1 mm was focused on the sector of the crucible closer to the exit opening of the electron beam. The pressure in the chamber was less than 0.002 Pa. The welding transformer was then switched on. Its open-circuit voltage of 130 V was enough to end a discharge. The current was regulated to 200 A. The operating voltage was 41 V. A circular trench formed in the melt, the bottom of which corresponded to the base of the electron beam. A powerful plasma was formed over this melt. The substrates were connected to a voltage of -80 V relative to the crucible potential by means of an additional supply device. The ion current on the substrate carrier was 8 A. Without a welding transformer, the substrate current was too low to be able to be measured. After 15 minutes, the power supplies were switched off and the system was flooded. There was a fine-crystalline layer of 4 µm titanium on the substrates. The surface roughness of the test specimens remained unchanged and corresponded to an average roughness depth of Ra, 0.04 µm.

Example 2

:

0049The same structure as in Example 1 was used. The crucible was charged with 330g titanium. After that, the procedure was initially as in Example 1. The power of the electron beam gun was regulated at 7.4 kW. The crucible rotation was not switched on. The electron beam was focused on the center of the crucible. The diameter of his focal spot was about 7mm. The electron beam was wobbled with an automatic at a frequency of 30 Hertz. Then argon, 40 standard cc / min was admitted. The welding transformer was then switched on and its current regulated up to 110 A. The operating voltage was 10 V. A bright plasma again formed above the melt, which followed the wobble movement. Nitrogen was approved, namely 420 standard cm³ / min. The substrates were placed on a DC voltage of -20 V. After an hour, the electricity and gas supplies were switched off and the system opened. 8 µm stoichiometric gold-colored titanium nitride was deposited on the test disks. The hardness of the layer was 2300 HV. It had excellent resistance to erosion. The process temperature had not exceeded 220 ° C.

Example 3

:

0050A type BAI 640 K cubic steaming system was equipped as follows: In its bottom, decentred, a water-cooled crucible was installed. Next to the crucible in the bottom was an electron beam gun with an axial magnetic field, the cathode chamber of which was pumped out differentially. The distance from the exit of the electron beam to the center of the cathode was 160 mm. The electron beam gun had a maximum output of 8 kW. The crucible was made of copper, water-cooled and had a diameter of 80mm. It was electrically isolated from both the chamber floor and the mass of the cannon. It could be rotated with an electric motor. The crucible was connected to the negative pole of a DC voltage supply device of the type of a welding transformer (max. 250 A) with copper cables of 10 mm using a cable which had been inserted into the system in an insulated manner. The positive pole of the power supply was connected to a water-cooled auxiliary anode with a separate, electrically insulated and water-cooled bushing. This was rectangular, 250 x 100mm, and stood like a pennant at a distance of 60mm from the crucible. Opposite the floor was a calotte with glass panes. The crucible was charged with 60g silicon and the system was closed and pumped out. Oxygen was then admitted into the chamber, the flow of which was regulated so that the pressure in the system did not exceed 0.09 Pa. The electron beam gun was then switched on and its power increased to 600W. Around the same time, the crucible was set in rotation and the welding transformer switched on. Its current was ramped up to 140 amps. The power supply was switched off after 30 minutes. When the system was opened, a transparent layer of silicon oxide was deposited on the glass panes.

Example 4

:

0051As shown in Fig. 2 as a schematic diagram, in a cubic system BAI 760 K, 200mm from the system center, a water-cooled, circular cathode holder 101 was installed (diameter 120mm, thickness 10mm) so that the angle between the surface normal and the system axis was 70 ° amounted to. A 3mm thick, round titanium target 102 (diameter 48mm), which was soldered onto a round (60mm diameter), 12mm thick copper block, was screwed onto this cathode holder in such a way that good electrical contact could be established between the titanium target and the cathode holder. At a distance of 35 mm from the cathode, a water-cooled 108 ring-shaped (inner diameter 70 mm) anode 103 with a circular cross-section (diameter 12 mm), which was insulated and passed through the system base 106, was mounted parallel to the cathode. The cathode was grounded and connected to the negative pole 110 of a welding transformer (max. 250 A), the anode 103 was connected to the positive pole 111 of this current source. The laser beam 113 of a pulsed Nd: YAG laser (500 W) was guided into the system through a window 114 (diameter 40 mm), which was coated on both sides with an antireflection layer, and focused through the anode onto the cathode surface. The distance between the window and the cathode was 280mm. Outside the system, the laser beam was deflected by 90 ° by means of a rotating dielectric mirror, the surface of which formed an angle of 0.5 ° with the axis of rotation. This rotation made it possible to rotate the focal spot (diameter 0.7 mm) of the laser beam on the titanium target. The speed of rotation was adjustable between 0 and 6000 rpm. The focus was achieved using a biconvex, anti-reflective coated lens on both sides with a focal length of 500mm in front of the rotating mirror.

0052After reaching 2 * 10⁻³ Pa, a voltage of 100 V was applied between the cathode and anode by switching on the power supply. The spark was then ignited using a laser pulse lasting 6 milliseconds and an energy of 30.5 joules. Then the unguided spark ran at approx. 20 V and 90 A. It was found that the ignition condition (threshold energy of the laser pulse) strongly depends on the surface condition of the target (reflectivity) and on the speed of rotation of the laser beam.

0053The time interval between pulses can be selected so that the bottom discharge burns continuously. This allows the spark to be carried on the cathode surface. Instead of the mirror, other optical elements, such as concave mirrors or gratings, can also be used. Instead of the Nd: YAG laser, gas or semiconductor lasers can also be used.

Example 5

:

0054In a BAI 760 K cubic system, a water-cooled, circular cathode holder (diameter 120 mm, thickness 10 mm) was mounted 200 mm from the system center so that the angle between the surface normal and the system axis was 70 °. A 5 mm thick, rectangular tin target, which was soldered onto a copper plate with a thickness of 5 mm, was screwed onto this cathode holder in such a way that good electrical contact could be established between the tin target and the cathode holder. At a distance of 35mm from the cathode, an insulated water-cooled, ring-shaped (inner diameter 70mm) anode with a circular cross-section (diameter 12mm) was installed parallel to the cathode.

0055The cathode (and thus the tin target) was grounded and connected to the negative pole of a welding transformer (max. 250 A). The anode was connected to the positive pole of this power source. The laser beam from a pulsed Nd: YAG laser (500 W) was guided into the system through a window (diameter 40 mm), which was coated on both sides with an antireflection layer, and focused through the anode onto the cathode surface. The distance between the window and the cathode was 280mm. The laser beam was focused using a lens on both sides with an anti-reflective coating (focal length 500mm), which was rigidly connected to the laser. The movement of the focused laser beam within an area of ​​50 mm in diameter on the target was accomplished by mounting the laser outside of the system on a solid table that could be moved in two independent directions. After reaching 2 * 10⁻³ Pa, argon was admitted into the vacuum chamber until a pressure of 2 * 10⁻¹ Pa was reached. By switching on the power supply, a voltage of 100 V was applied between the cathode and the anode. The spark could be ignited using a laser pulse lasting 6 milliseconds and an energy of 2 J and ran on the tin target.

0056The spark then ran at approx. 20 V and 90 A. It was found that the ignition condition (threshold energy of the laser pulse) strongly depends on the surface condition of the target (reflectivity) and, evidently, on the speed of rotation of the laser beam.

Example 6

:

0057In a BAI 760 K cubic system, a water-cooled, circular cathode holder (diameter 120 mm, thickness 10 mm) was mounted 200 mm from the system center so that the angle between the surface normal and the system axis was 70 °. A 5mm thick, round tungsten target (diameter 30mm) was clamped onto this cathode holder, so that good electrical contact could be established between the tungsten target and the cathode holder. At a distance of 35mm from the cathode, an insulated, water-cooled, ring-shaped (inner diameter 70mm) anode with a circular cross-section (diameter 12mm) was passed parallel to the cathode.

0058The cathode was grounded and connected to the negative pole of a welding transformer (max. 250 A). The anode was connected to the positive pole of this power source. The laser beam from a pulsed Nd: YAG laser (500 W) was guided into the system through a window (diameter 40 mm), which was coated on both sides with an antireflection layer, and focused through the anode onto the cathode surface. The distance between the window and the cathode was 280mm. The focusing of the laser beam (diameter of the laser beam on the target: 0.65mm) was carried out by a biconvex lens (focal length 500mm) mounted on both sides outside the system, which could be moved so that the focal point of the laser beam can be moved anywhere on the W target could. The movement was controlled by computer. After reaching 2 * 10⁻³ Pa, a voltage of 100 V was applied between the cathode and the anode by switching on the power supply. The spark was then ignited by means of a staircase-shaped laser pulse (see sketch) with a total duration of 3 milliseconds and an energy of 7.5 J. The spark was recorded using a storage cathode ray oscilloscope.<img file="EP0444538A2_D0001.tif" />

0059The spark could be conducted continuously by a procedure similar to that in example 4 or 5.