Device for generating an atmospheric plasma beam, and method for treating the surface of a workpiece

Abstract

A device for generating an atmospheric plasma beam for treating the surface of a workpiece includes a tubular housing with an axis, an inner electrode within the housing, and a nozzle assembly with a nozzle opening for discharging a plasma beam to be generated in the housing. The direction of the nozzle opening runs at an angle relative to the axis, and the nozzle assembly can be rotated about the axis. By the aforementioned device, disadvantages are at least partly eliminated and uniform treatment of the surface is achieved in that a shield surrounds the nozzle assembly, and the shield is designed to change the intensity of the interaction of the plasma beam to be generated with the surface of the workpiece depending on the rotational angle of the nozzle assembly relative to the axis. Also provided are a system and method for treating the surface of a workpiece.

Claims

1. A device for generating an atmospheric plasma beam for treating a surface of a workpiece, comprising: a tubular housing which has an axis; an inner electrode arranged within the housing; and a nozzle arrangement which has a nozzle opening for discharging a plasma beam to be generated in the housing, wherein a direction of the opening runs at an angle relative to the axis, wherein the nozzle arrangement is rotatable relatively about the axis, wherein a shield includes a body that laterally surrounds the nozzle arrangement such that the body is positioned to create an angle-dependent interaction with the nozzle arrangement, and wherein the body is configured to change an intensity of an interaction of the plasma beam to be generated with the surface of the workpiece depending on the angle of rotation of the nozzle arrangement relative to the axis.

2. The device according to claim 1, wherein the shield is formed over only a partial section in the azimuthal direction.

3. The device according to claim 1, wherein the shield is formed over two partial sections symmetrically to the axis in the azimuthal direction.

4. The device according to claim 1, wherein the axial length of the shield varies in the azimuthal direction.

5. The device according to claim 4, wherein the variation in the length of the shield occurs in steps or continuously.

6. The device according to claim 1, wherein an inner surface of the shield, at least in an area of a distal edge, has an azimuthally varying angle relative to the axis.

7. The device according to claim 1, wherein the shield is designed to be adjustable in its position relative to the nozzle arrangement in the direction of the axis, in the radial direction, or in the direction of the axis and in the radial direction.

8. The device according to claim 7, wherein the body of the shield has at least two shield elements which are designed to be adjustable independently of one another, wherein the shield elements are provided for changing the intensity of the interaction of the plasma beam to be generated with the surface of the workpiece depending on the angle of rotation of the nozzle arrangement relative to the axis.

9. The device according to claim 1, wherein a heating device is provided for heating the shield.

10. A method for treating a surface of a workpiece, comprising: generating a plasma beam rotating about an axis by means of a device generating an atmospheric plasma beam, the device having an axis and having a nozzle arrangement rotating relatively about the axis, moving the device with the rotating plasma beam along the surface to be treated, and changing an intensity of an interaction of the plasma beam with the surface of the workpiece depending on the angle of rotation of the nozzle arrangement relative to the axis by means of a shield.

11. The method according to claim 10, wherein the rotating plasma beam is shielded by the shield more strongly parallel to a direction of movement than transversely to the direction of movement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail below by means of exemplary embodiments with reference to the figures.

(2) FIGS. 1a-c show graphical illustrations for explaining the principle of operation in the prior art and according to the present invention,

(3) FIG. 2 shows a device known from the prior art for generating a plasma beam,

(4) FIGS. 3a-c show a first exemplary embodiment of a device according to the invention for generating a plasma beam,

(5) FIGS. 4a-c show a second exemplary embodiment of a device according to the invention for generating a plasma beam,

(6) FIGS. 5a-c show a third exemplary embodiment of a device according to the invention for generating a plasma beam,

(7) FIGS. 6a,b show a fourth exemplary embodiment of a device according to the invention for generating a plasma beam,

(8) FIGS. 7a,b show a fifth exemplary embodiment of a device according to the invention for generating a plasma beam,

(9) FIGS. 8a,b show a sixth exemplary embodiment of a device according to the invention for generating a plasma beam,

(10) FIGS. 9a,b show a first exemplary embodiment of an apparatus according to the invention for generating a plasma beam and

(11) FIGS. 10a,b show a first exemplary embodiment of an apparatus according to the invention for generating a plasma beam.

DESCRIPTION OF THE INVENTION

(12) In the following description of the different exemplary embodiments according to the invention, the same components are provided with the same reference symbols, even though the components in the different exemplary embodiments can have differences in size and shape.

(13) Before examining a first exemplary embodiment, a plasma nozzle arrangement forming the basis of the present invention should be explained with the aid of FIG. 2.

(14) The device 2 shown in FIG. 2 and known from EP 1 067 829 B1 for generating a plasma beam has a tubular housing 10 which in its upper area in the figure is widened in diameter and is mounted rotatably on a fixed supporting tube 14 by means of a bearing 12. A nozzle channel 16 is formed inside the housing 10 and leads from the open end of the supporting tube 14 to a nozzle opening 18.

(15) An electrically insulating ceramic tube 20 is inserted into the supporting tube 14. A working gas, for example air, is fed through the supporting tube 14 and the ceramic tube 20 into the nozzle channel 16. The working gas is swirled by a swirl device 22, which is inserted into the ceramic tube 20, such that it flows in a vortex-like manner through the nozzle channel 16 to the nozzle opening 18, as is symbolised in the figure by a screw-like arrow. Thus, a vortex core is formed in the nozzle channel 16 and runs longitudinally to the axis A of the housing 10.

(16) A pin-shaped inner electrode 24 is mounted on the swirl device 22, which protrudes coaxially into the nozzle channel 16 and to which a high-frequency high voltage is applied by means of a high-voltage generator 26. A high-frequency high voltage is typically understood as a voltage of 1 to 100 kV, in particular 1 to 50 kV, preferably 5 to 50 kV, at a frequency of 1 to 100 kHz, in particular 10 to 100 kHz, preferably 10 to 50 kHz. The high-frequency high voltage can be a high-frequency alternating voltage, but it can also be a pulsed direct voltage or an overlay of both voltage forms.

(17) The housing 10 consisting of metal is earthed via the bearing 12 and the supporting tube 14 and serves a counter electrode, so that an electric discharge can be produced between the inner electrode 24 and the housing 10.

(18) The inner electrode 24 arranged inside the housing 10 is preferably aligned parallel to the axis A, in particular the inner electrode 24 is arranged in the axis A.

(19) The nozzle opening 18 of the nozzle channel is formed by a nozzle arrangement 30 consisting of metal which is screwed into a threaded hole 32 of the housing 10 and in which a channel 34 is formed which is tapered and curved towards the nozzle opening 18 and runs obliquely in relation to the axis A. In this way, the plasma beam 28 emanating from the nozzle opening 18 forms an angle with the axis A of the housing, which in the example shown is approximately 45. This angle can be varied as required by changing the nozzle arrangement 30.

(20) The nozzle arrangement 30 is hence arranged at the end of the discharge path of the high-frequency arc discharge and is earthed via the metallic contact with the housing 10. The nozzle arrangement 30 thus channels the emanating gas and plasma beam, wherein the direction of the nozzle opening 18 runs at a prespecified angle relative to the axis A. The direction of the nozzle opening 18 can be defined parallel to the normal of the nozzle opening 18.

(21) Since the nozzle arrangement 30 is connected to the housing 10 in a torque-proof manner and since the housing 10 is, on the other hand, rotatably attached with respect to the supporting tube 14 via the bearing 12, the nozzle arrangement 30 can rotate relatively about the axis A. A toothed wheel 36 is arranged on the widened upper part of the housing 10 and is in drive connection with a motor (not shown) via a toothed belt or a pinion.

(22) During operation of the device 2 through the high-frequency high voltage an arc discharge is produced between the inner electrode 24 and the housing 10 due to the high frequency of the voltage. The electric arc of this high-frequency arc discharge is carried along by the swirled inflowing working gas and channelled in the core of the vortex-like gas flow, so that the electric arc then runs almost rectilinearly from the tip of the inner electrode 24 longitudinally to the axis A and only branches in the area of the lower end of the housing 10 or in the area of the channel 34 radially to the housing wall or to the wall of the nozzle arrangement 30. In this way, a plasma beam 28 is generated which discharges through the nozzle opening 18.

(23) The terms electric arc or arc discharge are in the present case used as a phenomenological description of the discharge, since the discharge occurs in the form of an electric arc. The term electric arc is otherwise also used as a discharge form in direct voltage discharges with essentially constant voltage values. In the present case, however, it is a high-voltage discharge in the form of an electric arc, i.e. a high-frequency arc discharge.

(24) In operation the housing 10 rotates with a high speed of rotation about the axis A, so that the plasma beam 28 describes a lateral surface of a cone which brushes over the surface of a workpiece (not shown) to be processed. If then the device 2 is moved along on the surface of the workpiece or inversely the workpiece is moved along on the device 2, then a relatively uniform treatment of the surface of the workpiece is obtained on a strip, the width of which corresponds to the diameter of the cone on the workpiece surface described by the plasma beam 28. By varying the distance between the mouthpiece 30 and the workpiece, the width of the area of the pre-treated area can be influenced. By means of the plasma beam 28, which for its part is swirled, striking the workpiece surface obliquely, an intensive effect on the workpiece surface is achieved by the plasma. The swirl direction of the plasma beam can be in the same direction or in the opposite direction to the rotational direction of the housing 10.

(25) The intensity of the plasma treatment by the rotating plasma beam 28 is dependent on the distance of the nozzle opening 18 from the surface, on the one hand, and on the angle of impact of the plasma beam 28 on the surface to be treated, on the other hand.

(26) FIGS. 3a to 3c show a first exemplary embodiment of a device 4 according to the invention having a device 2 which has the same design as what was previously described with the aid of FIG. 1. According to the invention, a shield 40 is provided which surrounds the nozzle arrangement 30. The form of the shield 40 has a cylindrical inner surface 42 in the section projecting downwards beyond the lower edge of the nozzle arrangement 30, this cylindrical inner surface 42 in sections having steps 44. Thus, the shield 40 in the azimuthal direction forms sections 46 with a greater axial length and sections 48 with a smaller axial length. Hence, the shield 40 changes the intensity of the interaction of the plasma beam 28 with the surface of the workpiece depending on the angle of rotation of the nozzle arrangement 30 relative to the axis A.

(27) As shown in FIG. 3a, the plasma beam 28 strikes one of the longer sections 46 of the shield 40, so that the plasma beam 28 is deflected or reflected inwards, respectively. FIG. 3b shows the lower section of the device 4 according to the invention in a position rotated by 90 compared to the one illustrated in FIG. 3a. Here, the plasma beam 28 is directed onto one of the shortened sections 48 and can almost emanate from the nozzle arrangement 30 without any interaction with the shield. The shield 40 or the arrangement of the sections 46 and 48 is formed symmetrically to the axis A in the azimuthal direction.

(28) The design of the shield can also be recognised in FIG. 3c in a view of the device 2 from below. The different illustrated forms of the plasma beam 28 are intended to make clear that the plasma beam 28 depending on the angle of the inner surface 42 is influenced more strongly in the area of the longer section 46 than is the case in the area of the shorter section 48. Hence, the result is an intensity of the interaction of the plasma beam 28 with the surface of the workpiece which varies in the azimuthal direction.

(29) As illustrated in FIGS. 3a to 3c, the shield 40 is formed such that it surrounds the nozzle arrangement 30 over the entire circumference, wherein two shorter sections 46 and two longer sections 48 are provided in each case. An embodiment in which the shield is only formed over one section or two sections in the azimuthal direction is not illustrated in FIG. 3.

(30) FIGS. 4a to 4c show a further exemplary embodiment of a device 6 according to the invention with a device 2. In contrast to the exemplary embodiments illustrated in FIGS. 2 and 3, the nozzle arrangement 30 can be rotated relative to a stationary housing 10. Here, the housing 10 is conically tapered at its discharge end and forms an axial/radial bearing for a conically widened upstream part of the nozzle arrangement 30. In the example shown, the bearing is formed as a magnetic bearing 38. The nozzle arrangement 30 is pressed against the conical bearing surface of the housing 10 by the dynamic pressure of the outflowing air, but is held contact-free in the housing by the magnetic bearing 38 such that on its entire circumference it forms a narrow gap having a width of only approximately 0.1 to 0.2 mm with the housing. The earthing of the mouthpiece 30 is effected by spark discharge across this gap.

(31) The nozzle opening 18 functions as a rotary drive for the nozzle arrangement 30 and is not aligned in the exact radial direction, but has a tangential component, so that an aerodynamic drive is formed by the partly tangentially emanating air together with the plasma beam 28. Alternatively to this, the aerodynamic drive can also be effected by means of blades or fins (not illustrated) arranged inside the nozzle arrangement 30 which are impinged by the air flowing in a swirling manner through the channel 34.

(32) This embodiment of the bearing arrangement and of the drive has the advantage that the rotary drive is simplified in terms of design and the moment of inertia of the rotating masses is limited to a minimum.

(33) In contrast to FIG. 3, the exemplary embodiment according to FIG. 4 is designed in such a way that the variation in the length of the shield 40 does not occur in steps, but constantly at least in sections in a curved form, in particular in the form of a sine function. As a result, there are continuous and hence smoother transitions between the longer sections 46 and the shorter sections 48 and therefore a more uniform variation in the intensity of the plasma beam 28 on the surface to be treated.

(34) In addition, it can be recognised in FIG. 4a that in the area of the longer sections 46 the inner surface 42 is oriented inwards in the area of the lower edge 50. The effect of the reflection and deflection of the plasma beam 28 is increased by means of this additional measure which is independent of the formation of the sections 46 and 48 in stepped or continuously varying form.

(35) In FIG. 4a, the device is illustrated with an angle of rotation of the nozzle arrangement 30, in which the plasma beam 28 strikes one of the longer sections 46 and hence is reflected and deflected. As a consequence, the intensity of the plasma beam 28 is more strongly spread to the inner space surrounded by the shield 40.

(36) FIG. 4b shows the device with a nozzle arrangement 30 rotated by 90 compared to the position illustrated in FIG. 4a. In this position the plasma beam 28 is directed in the direction of one of the shorter sections 48 and is therefore not or only marginally influenced by the shield 40.

(37) FIG. 4c shows the device 2 in a view from below, from which the symmetrical design of the shield emerges. The different illustrated forms of the plasma beam 28 are intended to make clear that the plasma beam 28 depending on the angle of the inner surface 42 is influenced more strongly in the area of the longer section 46 than is the case in the area of the shorter section 48. Hence, the result again is an intensity of the interaction of the plasma beam 28 with the surface of the workpiece which varies in the azimuthal direction.

(38) FIGS. 5a to 5c show a further preferred exemplary embodiment of a device 8 according to the invention for generating an atmospheric plasma beam for treating the surface of a workpiece which also has a device 2 and a shield 40.

(39) According to FIG. 5a, the inner surface 42 of the shield 40 in the area of the distal edge 52 has an azimuthally varying angle relative to the axis A, wherein the emanating plasma beam 28 strikes the section 52 which essentially has an inner surface 42 running parallel to the axis A. In this way, the plasma beam, as has already previously been described for the other exemplary embodiments, is reflected and deflected, so that the intensity of the plasma beam 28 is more strongly directed towards the interior of the shield 40.

(40) FIG. 5b shows the device 8 in an angular position of the nozzle arrangement 30 which is rotated by 90 compared to the position illustrated in FIG. 5a, so that the inner surface 42 is directed outwards in the area 52. The shield 40 therefore widens the interior of the shield in this angular position. In the illustrated position, the plasma beam 28 emanating from the nozzle arrangement 30 only strikes the area 52 of the shield 40 to a minor degree and therefore remains almost unaffected.

(41) FIG. 5c shows the previously described device 8 in a view from below, in which the two different angular positions of FIGS. 5a and 5b are illustrated. The different illustrated forms of the plasma beam 28 are intended to make clear that the plasma beam 28 depending on the angle of the inner surface 42 is influenced varyingly strongly in the area of the of the lower area 52. Hence, the result is an intensity of the interaction of the plasma beam 28 with the surface of the workpiece which varies in the azimuthal direction.

(42) Previously, exemplary embodiments were explained with shields 40 in which either sections 46 and 48 of different lengths or sections of the inner surface 42 are formed at different angles relative to the axis A. However, it is also possible, within the scope of the invention, to have exemplary embodiments, in which sections of different lengths are combined with inner surfaces at different angles relative to the axis A.

(43) The previously explained exemplary embodiments of the devices 4, 6 and 8 according to the invention produce an intensity profile of the plasma treatment of a surface which is changed or is changeable in the azimuthal direction. This intensity profile can be applied in the stationary state, i.e. when the device 4, 6 or 8 is not being moved with respect to the surface to be treated, at certain positions on the surface depending on the application. If, for example, a limited, for example cross-shaped, surface section of the surface is to be treated with plasma, then it is possible within the scope of the invention to design the shield 40 in the previously described way such that there is a corresponding pattern of the plasma treatment below the shield 40 when the nozzle arrangement 30 rotates about the axis 40.

(44) With each of the previously described embodiments of the device 4, 6 or 8 according to FIGS. 3 to 5 a method according to the invention for treating the surface of a workpiece can also be carried out as follows. A plasma beam 28 rotating about the axis A is generated by means of a device 4,6 or 8 generating an atmospheric plasma beam, the device having an axis A and a nozzle arrangement 30 rotating relatively about the axis A. The device 4, 6 or 8 with the rotating plasma beam 28 is moved along the surface to be treated and, by means of a shield 40 having sections 46 and 48 or 50 or 52, the intensity of the interaction of the plasma beam 28 with the surface of the workpiece is changed depending on the angle of rotation of the nozzle arrangement relative to the axis A.

(45) Hence, a certain intensity profile can be set with the plasma treatment of the surface, so that, for example, either an intensity profile is obtained which is as homogeneous as possible or a profile which is known in the prior art, in particular a strip profile in which the intensity of the plasma treatment is increased.

(46) Preferably, the previously described method is carried out in such a way that the rotating plasma beam 28 is shielded by the shield 40 more strongly longitudinally to the direction of movement than transverse to the direction of movement, in particular is reflected or deflected inwards, respectively. Relating to the above described exemplary embodiments, this means that the direction of movement in FIGS. 3a, 4a and 5a is aligned upwards or downwards perpendicular to the plane of projection. In FIGS. 3c, 4c and 5c this direction runs horizontally to the right or left.

(47) In the areas in which otherwise an uninfluenced plasma beam 28 would strike the surface a less intensive treatment of the surface is obtained by means of this method. This is because the plasma beam 28 is reflected and deflected by the shield 40 and thereby distributed within the volume surrounded by the shield 40, whereby the intensity of the plasma beam 28 per surface unit is overall reduced. On the other hand, the plasma beam 28 strikes the surface almost unimpeded in the direction of movement in each case and can achieve a higher intensity of the pre-treatment per surface unit. In this way, an intensity distribution according to FIG. 1c can be obtained.

(48) In addition, FIGS. 5a and 5b show that a heating device 60 for heating the shield 40 is provided. In the present case, the heating device 60 is formed as an electrically heated cylinder which heats up the shield by means of intrinsic temperature and heat radiation. Hence, a loss of energy from the plasma beam 28 striking the shield is reduced or even minimised. In the most general sense, the heating element can also, independent of an azimuthally varying shield, be used for rotationally symmetrical shields.

(49) FIG. 6 shows an exemplary embodiment of a device 2 according to the invention for generating an atmospheric plasma beam for treating the surface of a workpiece, as was described, for example, in connection with FIG. 3. The illustrated shield 40 therefore has an azimuthal design which enables the intensity of the interaction of the plasma beam 28 with the surface of the workpiece to be changed depending on the angle of rotation of the nozzle arrangement 30 relative to the axis A by means of a varying length.

(50) In the exemplary embodiment illustrated in FIGS. 6a and b, the shield 40 is designed to be adjustable in its position relative to the nozzle arrangement 30 in the direction of the axis A. FIG. 6a shows an arrangement of the shield 40 with an axially advanced position, i.e. with a greater distance between the lower edge of the shield 40 and the nozzle arrangement 30 than is shown in FIG. 6b. The shield in FIG. 6b is arranged retracted relative to the lower edge of the nozzle arrangement 30 and therefore influences the emanating plasma beam 28 to a lesser extent than in the position according to FIG. 6a.

(51) FIGS. 7a and 7b show a further exemplary embodiment of a device 2 according to the invention for generating an atmospheric plasma beam for treating the surface of a workpiece, as was described, for example, in connection with FIG. 3. The illustrated shield 40 has several, but at least two, shield elements 40a, 40b at the lower end and these are designed to be adjustable independently of one another. The shield elements 40a and 40b can be both axially and radially adjusted along a direction running at an angle relative to the axis A. For this purpose, the shield elements 40a and 40b are arranged in guides (not illustrated) and can be fixed in one of a plurality of positions. A specific azimuthal distribution of the influencing of the plasma beam 28 can therefore be set by the plurality of peripheral shield elements 40a, 40b.

(52) FIG. 8a shows a shield 40 of a further exemplary embodiment of a device 2 according to the invention for generating an atmospheric plasma beam for treating the surface of a workpiece, as was principally described in connection with FIG. 5. In this embodiment, the lower edge of the shield 40 is provided with a plurality of individual recesses 52a of the distal edge 52.

(53) FIG. 8b shows a partial cross-section of the device 2, wherein the lower edge 52 with the recesses 52a forms an azimuthally circumferential pattern of sections with a stronger or weaker influence on the plasma beam 28. By appropriately choosing the individual angles and heights h of the recesses 52a, a specific angular distribution of the intensity of the plasma treatment of the surface can be achieved.

(54) An apparatus 100 according to the invention for treating a surface with atmospheric plasma is illustrated in FIGS. 9a and 9b. The apparatus 100 has two schematically illustrated devices 2 and 2 for generating an atmospheric plasma beam 28 and 28, as are known for example from the prior art and were explained above with the aid of FIG. 2.

(55) Each of the two devices 2, 2 has a tubular housing 10, 10 with an axis A or A, an inner electrode (not illustrated) arranged inside the housing 10, 10 and a nozzle arrangement 30, 30 having a nozzle opening 18, 18 for discharging a plasma beam 28, 28 to be generated in the housing 10, 10. Both devices 2, 2 are connected together rotatable about a common axis B by means of a frame 102, wherein in the frame a drive (not illustrated) is provided for generating a rotational movement of the devices 2, 2 about the axis B. The compressed-air connections and voltage connections are arranged in the frame 102 and are not illustrated in detail.

(56) The direction of the nozzle openings 18, 18 in each case runs at an angle , relative to the axis A. A, wherein the nozzle arrangement 30, 30 can be rotated relatively about the axis A, A. A drive (not illustrated), as was explained with the aid of FIG. 2, is provided in each case for generating a rotational movement of the nozzle arrangements 30, 30 about the respective axis A, A.

(57) In addition, the two devices 2, 2 are aligned at an angle , relative to the axis B, as FIGS. 9a and 9b show. The drive for generating a rotational movement of the devices 2, 2 and the drives for generating a rotational movement of the nozzle arrangements 30, 30 are synchronised together in such a way that during one rotation of the devices 2, 2 about the common axis B each of the nozzle arrangements 30, 30 performs two rotations about the respective axis A, A.

(58) It is preferred and illustrated in FIGS. 9a and 9b if the angle , of the nozzle openings relative to the respective axis A or A essentially coincides with the angle , of the devices 2, 2 relative to the axis B. An angular arrangement is thereby obtained, in which in two azimuthally opposing angular positions of the devices 2, 2 the plasma beams 28, 28 are aligned essentially perpendicular to the surface (see FIG. 9a), while in two angular positions rotated about 90 and 270 respectively thereto the plasma beams 28, 28 are essentially aligned at an angle of 2*, 2* relative to the surface, i.e. flatter (see FIG. 9b). The intensity of the plasma treatment of the surface thus varies twofold between a maximum and a minimum intensity during a rotation of the devices 2, 2 about the common axis B.

(59) A possibility of synchronising the rotational movement of the apparatus together consists in transferring the rotational movement of the nozzle arrangements 30, 30 via a planetary gear, which is arranged in the frame 102 and not illustrated in more detail, by the rotational movement of the devices 2, 2 about the axis B. A further possibility consists in electronically synchronising the respective drives together. In this case, the mechanical effort of a planetary gear is avoided.

(60) A further method for treating the surface of a workpiece can be carried out by a previously described apparatus, in which two rotating plasma beams are generated, in which the apparatus with the rotating plasma beams is moved along the surface to be treated, and in which the plasma beams are directed in two first angular positions 0, 180 of the rotational movement about the axis B at a steep, preferably perpendicular, angle onto the surface of the workpiece (see FIG. 9a), and in which the plasma beams are directed in two second angular positions 90, 270 of the rotational movement about the axis B at a flat angle, preferably at an angle which is double the angle of the nozzle openings relative to the axes A, A, onto the surface of the workpiece (see FIG. 9b).

(61) The previously explained method can be carried out statically in that only one partial area of the surface is treated with the plasma beams 28, 28.

(62) In a further embodiment of the invention, the apparatus is essentially moved in the direction of one of the two first angular positions 0, 180 of the rotational movement about the axis B along the surface. Hence, seen in the direction of movement, when the two plasma beams 28, 28 have an alignment which is essentially in the direction of movement, the surface is more intensively treated with plasma than in the angular positions which are adopted transverse to the direction of movement. Hence, an intensity distribution can be achieved according to FIG. 1c by the described method and the described apparatus.

(63) FIG. 10 now shows an exemplary embodiment with only one device 2, in which the axis B essentially runs close to the centre of gravity of the device 2. During the rotation about the axis B, the device 2 performs a wobbling motion which is produced by a drive (not shown). The alignment of the single plasma beam 28 then performs a similar azimuthal directional distribution, as has been previously explained with the aid of FIGS. 6a and 6b for the devices 2 and 2. In contrast to the embodiment according to FIG. 6, the diameter of the area treated with plasma by the apparatus is smaller.