Nozzle assembly, device for generating an atmospheric plasma jet, use thereof, method for plasma treatment of a material, in particular of a fabric or film, plasma treated nonwoven fabric and use thereof

Abstract

A nozzle assembly for generating an atmospheric plasma jet includes an inlet, through which the jet can be introduced into the nozzle assembly, and a channel connected to the inlet so that the plasma jet introduced is conducted through the channel. Multiple nozzle openings are provided in the channel wall along the channel, through which a plasma jet can exit the assembly. The cross section of the channel in the region of a nozzle opening is shaped in such a way that a virtual medial plane runs between a virtual first tangent plane of the cross section through the nozzle opening and a virtual second tangent plane of the cross section opposite thereto and parallel to the first tangent plane divides the cross section into a first cross-sectional area at the nozzle opening. The cross-sectional surface of the first cross-sectional area differs from the cross-sectional surface of the second.

Claims

1. A device for generating an atmospheric plasma jet, comprising: a discharge space, wherein the device is configured to generate the atmospheric plasma jet in the discharge space, wherein a nozzle assembly is connected to the discharge space in such a way that the atmospheric plasma jet generated in the discharge space is introduced into the nozzle assembly via an inlet of the nozzle assembly, wherein the nozzle assembly comprises a channel which is connected to the inlet of the nozzle assembly such that the atmospheric plasma jet introduced into the inlet of the nozzle assembly is conducted through the channel, wherein multiple nozzle openings are provided in a channel wall along the channel, through which the atmospheric plasma jet which is conducted through the channel can exit the nozzle assembly, wherein a reference medial plane runs in a middle of a cross-section of the channel between a reference lowermost plane of the cross-section across one nozzle opening of the multiple nozzle openings and a reference uppermost plane of the cross-section on a side of the channel opposite to the one nozzle opening, wherein the reference medial plane, the reference lowermost plane, and the reference uppermost plane are parallel to each other, and wherein the cross-section of the channel in a region of the one nozzle opening is shaped in such a way that the reference medial plane divides the cross-section into a first cross-sectional area adjacent to the one nozzle opening and a second cross-sectional area on the side of the channel opposite to the one nozzle opening, and wherein a cross-sectional surface of the first cross-sectional area differs in size or shape from a cross-sectional surface of the second cross-sectional area.

2. The nozzle assembly according to claim 1, wherein the channel has a straight section, and the multiple nozzle openings are arranged in the channel wall in an extension direction of the channel.

3. The nozzle assembly according to claim 1, wherein the channel is connected on both sides to the inlet, such that the plasma jet introduced into the nozzle assembly through the inlet is conducted into the channel from both sides.

4. The nozzle assembly according to claim 1, wherein a diameter of the multiple nozzle openings in the channel walling is at most a quarter of a diameter of the channel.

5. The nozzle assembly according to claim 1, wherein the cross section of the channel widens as a distance from the inlet increases.

6. The nozzle assembly according to claim 1, wherein the nozzle assembly is formed in several parts with a nozzle element, which comprises the channel with the multiple nozzle openings, and with a distributor element, which comprises a distribution channel through which the plasma jet introduced through the inlet is conducted to the channel on one or both sides of the channel.

7. The nozzle assembly according to claim 1, wherein the cross-sectional surface of the second cross-sectional area is greater than the cross-sectional surface of the first cross-sectional area.

8. The nozzle assembly according to claim 1, wherein the nozzle assembly is formed in several parts with a first part, in a surface of which a first recess is introduced, and with a second part in a surface of which a second recess is introduced, wherein the first part and the second part adjoin each other such that the first recess and the second recess face each other and form the channel.

9. The device according to claim 1, wherein the device is configured to generate the atmospheric plasma jet by means of an arc-like discharge in a working gas, wherein the arc-like discharge can be generated by applying a high-frequency high voltage between electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 shows a device for generating an atmospheric plasma jet,

(3) FIG. 2 shows an exemplary embodiment of the nozzle assembly according to the invention and an exemplary embodiment of the device according to the invention for generating an atmospheric plasma jet, in an exploded view,

(4) FIG. 3 shows the exemplary embodiment of the nozzle assembly and the exemplary embodiment of the device from FIG. 2 in a sectional view,

(5) FIG. 4 shows an alternative exemplary embodiment of the nozzle assembly and the device in sectional view,

(6) FIG. 5 shows a further alternative exemplary embodiment of the nozzle assembly and the device in sectional view,

(7) FIG. 6 shows an exemplary embodiment of the use according to the invention and of the method according to the invention,

(8) FIG. 7 shows a photograph of an untreated nonwoven fabric,

(9) FIG. 8 shows a photograph of a plasma-treated nonwoven fabric as an exemplary embodiment of the plasma-treated nonwoven fabric according to the invention,

(10) FIGS. 9a-b show an exemplary embodiment of the sanitary product according to the invention,

(11) FIG. 10 shows a further exemplary embodiment of the nozzle assembly according to the invention and of the device according to the invention,

(12) FIG. 11 shows a further exemplary embodiment of the nozzle assembly according to the invention and of the device according to the invention,

(13) FIG. 12 shows a further exemplary embodiment of the nozzle assembly according to the invention and of the device according to the invention,

(14) FIGS. 13a-c show channel cross sections of further exemplary embodiments of the nozzle assembly according to the invention,

(15) FIGS. 14a-c show photographs of experiments on different nozzle assemblies and

(16) FIGS. 15a-c show channel cross sections of the nozzle assemblies from the experiments.

DESCRIPTION OF THE INVENTION

(17) In the following, the design and operation of a device for generating an atmospheric plasma jet will first be described.

(18) The device 2 comprises a tubular housing 4 in the form of a metal nozzle tube. The nozzle tube 4 has at one of its ends a conical taper 6, on which a replaceable nozzle head 8 is mounted, the outlet of which forms a nozzle opening 10, from which the plasma jet 12 emerges during operation.

(19) At the end opposite the nozzle opening 10, the nozzle tube 4 is connected to a working gas supply line 14. The working gas supply line 14 is connected to a pressurised working gas source (not shown) with variable flow rate. During operation, a working gas 16 is introduced from the working gas source through the working gas supply line 14 into the nozzle tube 4.

(20) In the nozzle tube 4, a swirl device 18 is further provided with a rim of bores 20, arranged obliquely in the circumferential direction, through which the working gas 16 introduced into the nozzle tube 4 is swirled during operation.

(21) The downstream part of the nozzle tube 4 is therefore perfused by the working gas 16 in the form of a vortex 22, whose core runs on the longitudinal axis of the nozzle tube 4.

(22) In the nozzle tube 4, an inner electrode 24 is additionally centrally arranged, which extends in the nozzle tube 4 coaxially in the direction of the nozzle opening 10. The inner electrode 24 is electrically connected to the swirl device 18. The swirl device 18 is electrically insulated from the nozzle tube 4 by a ceramic tube 26. Via a high-frequency line 28, a high-frequency high voltage is applied to the inner electrode 24, which is generated by a transformer 30. The nozzle tube 4 is earthed via an earth line 32. The applied voltage generates a high-frequency discharge in the form of an electric arc 34 between the inner electrode 24 and the nozzle tube 4. This area in the nozzle tube 4 thus represents a discharge space 36 of the device 2.

(23) The terms “arc”, “arc discharge “and “arc-like discharge” are used herein as phenomenological descriptions of the discharge, since the discharge occurs in the form of an electric arc. The term “electric arc” is otherwise used as a discharge form in DC voltage discharges with substantially constant voltage values. In the present case, however, it is a high-frequency discharge in the form of an electric arc, i.e. a high-frequency arc-like discharge.

(24) Due to the swirling flow of the working gas, this electric arc 34 is channelled in the vortex core in the region of the axis of the nozzle tube 4, so that it branches only in the region of the taper 6 to the wall of the nozzle tube 4.

(25) The working gas 16, which rotates with high flow velocity in the region of the vortex core and thus in the immediate vicinity of the electric arc 34, comes into intimate contact with the electric arc 34 and is thereby partially transferred to the plasma state, so that an atmospheric plasma jet 12 emerges from the device 2 through the nozzle opening 10.

(26) FIG. 2 shows an exemplary embodiment of the nozzle assembly according to the invention and an exemplary embodiment of the device according to the invention for generating an atmospheric plasma jet, in an exploded view. FIG. 3 shows the nozzle head and the device in a sectional view.

(27) The device 40 comprises the nozzle assembly 42 and the device 2 from FIG. 1, wherein, instead of the exchangeable nozzle head 8, a connecting piece 44 of the nozzle assembly 42 is connected to the nozzle tube 4. The connecting piece 44 has a tapered inner channel 46, which forms the lower part of the discharge space 36 of the device 2. During operation, the plasma jet 12 emerges from the lower opening 48 of the connecting piece 44 and enters the further components of the nozzle assembly 42. Accordingly, the lower opening 48 may be considered as an inlet of the nozzle assembly 42.

(28) The nozzle assembly 42 furthermore comprises a distributor element 50 composed of two parts 50a-b and a nozzle element 52. A groove 54 is introduced into the nozzle element 52, which forms a channel 56 having a first end 58 and a second end 60 in the assembled state of the nozzle assembly 42, as shown in FIG. 3. In the channel walling of the channel 56 multiple nozzle openings 62 are introduced along the channel side by side.

(29) The parts 50a-b of the distributor element 50 have respective grooves 64a-b which in the assembled state form a distribution channel 66. The distribution channel has a branch 68 and connects the inlet 48 to both the first end 58 and the second end 60 of the channel 56.

(30) When a plasma jet 12 is generated with the device 2 during operation, it passes through the inlet 48 at the connecting piece 44 into the distribution channel 66 and is thus conducted to both ends 58, 60 of the channel 56 and through the channel 56, so that it emerges from the nozzle assembly 42 in the form of a plurality of partial jets 70 from the nozzle openings 62. In this way, a curtain is generated of a plurality of partial jets 70 adjacent to one another, wherein the individual partial jets 70 have a reduced intensity in relation to the plasma jet 12, so that, for example, a nonwoven fabric 72 can be transported past the nozzle openings 62 for plasma treatment, without being damaged.

(31) The fact that the plasma jet 12 is introduced via the distribution channel 66 into the channel 56 on both sides, causes the individual partial jets 70 to have a relatively similar intensity. Optionally, the intensity of the individual partial jets 70 can be further evened out by forming the channel with a cross section that widens slightly from both ends 58, 60 to the centre of the channel, thereby counteracting an excessive pressure drop in the case of longer distances to the inlet 48.

(32) The nozzle assembly 42 also has an aluminium heat sink 74 with cooling fins 76 surrounding the other components, through which the heat load introduced into the nozzle assembly 42 by the plasma jet 12 can be dissipated.

(33) FIG. 4 shows an alternative exemplary embodiment of the nozzle assembly and the device in a sectional view. The device 40′ and the nozzle assembly 42′ are substantially structurally identical to the device 40 and the nozzle assembly 42, respectively. Identical parts are respectively provided with the same reference numerals.

(34) The nozzle assembly 42′ differs from the nozzle assembly 42 only in that the channel 56 is connected to the inlet 48 such that the plasma jet is directed into the channel 56 from one side. For this purpose, the distributor element 50′ and the nozzle element 52′ are formed as shown in FIG. 4.

(35) To counteract an excessive pressure drop in the channel 56 and to equalise the intensities of the partial jets 70, the cross section of the channel 56 may optionally slightly expand as the distance from the inlet 48 increases (i.e. from left to right in FIG. 4).

(36) FIG. 5 shows an alternative exemplary embodiment of the nozzle assembly and the device in a sectional view. The device 40″ and the nozzle assembly 42″ are substantially structurally identical to the device 40′ and the nozzle assembly 42′. Identical parts are respectively provided with the same reference numerals.

(37) The nozzle assembly 42″ differs from the nozzle assembly 42′ only in that an additional gas feed 57 is provided, through which a gas 59 can be introduced into the channel 56 separately from the plasma jet. For this purpose, the groove 54″ extends as shown in FIG. 5 to the edge of the nozzle element 52″ and an opening is provided in the heat sink 74″ for introducing the gas 59 into the channel 56. By introducing the gas 59, in particular nitrogen, the plasma jet can additionally be cooled in the channel 56, so that the partial jets 70 emerging from the nozzle openings 62 enable a very gentle treatment of nonwoven fabrics.

(38) FIG. 6 shows an exemplary embodiment of the use according to the invention and of the method according to the invention. In particular, the device 40 can be used to treat delicate nonwoven fabrics with plasma.

(39) For this purpose, the web-type nonwoven fabric 72 may be transported past the nozzle openings of the device 40 (or alternatively also 40′ or 40″) as shown in FIGS. 3-5, in order to treat the nonwoven fabric 72 over its entire length. The nozzle openings are preferably arranged transversely to the transport direction of the nonwoven web 72, as illustrated in FIG. 4, so that the nonwoven fabric 72 can be treated with the device 40 over a certain width, optionally over the entire width or a partial width of the nonwoven web 72.

(40) In order to further reduce the load on the nonwoven web 72 during the plasma treatment, the nonwoven web 72 is transported over rollers 78a-b respectively in front of and behind the treatment region 77 with the device 40, such that the rollers rotate at the same speed. In this way, tensile forces are reduced on the nonwoven web 72 in the treatment region 77. To further reduce the tensile forces, a treatment table 79 in the form of an aluminium plate is provided, over which the nonwoven web 72 is transported in the treatment region 77. In the transport direction behind the treatment region 77 suction openings 80 are provided in the treatment table 79, through which the ozone or nitrogen oxides can be sucked, which arise in the case of the preferred use of nitrogen as a working gas for the device 2 and 40 respectively.

(41) Since the device 40 allows a damage-free treatment of delicate fabrics such as the nonwoven web 72 even under atmospheric pressure, the device can be operated as shown in FIG. 6 without a vacuum chamber. In particular, inline operation, in particular within a continuous process line, is possible because no input and output operations are required.

(42) FIG. 7 shows a photograph of an untreated nonwoven fabric from the side. The nonwoven fabric comprises individual intertwined fibres, in particular plastic fibres, which produce a relatively compact fabric. The illustrated nonwoven fabric has a thickness of approx. 1 mm.

(43) FIG. 8 shows a photograph of the nonwoven fabric of FIG. 7 after being plasma treated with the device 40 shown in FIG. 3. FIG. 8 thus shows an exemplary embodiment of the plasma-treated nonwoven fabric according to the invention. After the plasma treatment, the nonwoven fabric has a greatly increased thickness of approx. 5 mm and correspondingly a less compact structure with a lower density. It has been found that this leads to an improvement in the capillarity of the nonwoven fabric, so that liquids pass through the fabric more effectively. Furthermore, the plasma treatment achieves a hydrophilisation of the fibres, so that the fabric can absorb liquids faster.

(44) FIG. 9a-b now shows an exemplary embodiment of a sanitary product according to the invention for absorbing liquids, in plan view (FIG. 9a) and in cross section (FIG. 9b) along the sectional plane designated by “IXb” in FIG. 9a. In the present case, the sanitary product 82 is a sanitary napkin, but a corresponding design is also possible with a diaper or a pad.

(45) The sanitary product 82 has a shaping outer layer 83, a superabsorbent layer 84 (‘absorbent core’), a distribution layer (ADL/AQL) 86 made of plasma-treated nonwoven fabric, for example the nonwoven fabric 72 from FIG. 4, an absorption layer 88 made of nonwoven fabric treated in sections and a cotton layer 90 as a cover layer. The superabsorbent layer 84 may comprise, for example, liquid-absorbing powder, in particular superabsorbent polymers.

(46) When used as intended, the cotton layer is in contact with the skin surface and ensures a pleasant skin sensation. The absorbent nonwoven fabric 88 arranged underneath is plasma-treated only in the middle 92, while the edges 94 are untreated. In this way, the absorbent nonwoven fabric 88 has hydrophilic properties in the centre 92, so that liquid is conducted effectively into the underlying distribution layer 86. On the edges 94, however, the absorbent nonwoven fabric 88 has hydrophobic properties, thereby preventing liquid from leaking at the edges of the sanitary product 82. The targeted plasma treatment in the centre 92 of the absorbent nonwoven fabric 88 can in particular replace the hydrophilisation used in the prior art, which is more complex in terms of process technology and because of the application of surfactants.

(47) The distribution layer 86 arranged below the absorbent nonwoven fabric 88 distributes the liquid in the surface, so that the liquid then reaches the underlying absorbent core 84 having been distributed over a larger area. The plasma treatment of the absorbent nonwoven fabric 88 allows the liquid to be absorbed more quickly by the distribution layer 86.

(48) Through the use of the plasma-treated nonwoven fabric 72 for the absorbent nonwoven fabric 88 and/or the distribution layer 86, the production costs of the sanitary product 82 can be reduced, since it is possible to achieve absorbing or distribution layers with a short strike-through time even with more cost-effective nonwoven fabrics 72.

(49) FIGS. 10 and 11 show further exemplary embodiments and possible uses of the device described above.

(50) The device 100 shown in FIG. 10 has a similar configuration as the device 40 of FIG. 2, wherein the device 2 and the connecting piece 44, however, are positioned centrally to the nozzle assembly 42 and the distributor element 50 of the nozzle assembly 42 has a correspondingly configured course for the distribution channel 66. Alternatively, the device 100 may also be similar to the device 40′ of FIG. 4 or the device 40″ of FIG. 5.

(51) The nozzle assembly 42 is rotatable by means of a rotary actuator 102 about an axis perpendicular to the extension direction of the channel 56. In this way, with the partial jets 70 emerging from the nozzle openings 62, a larger surface area can be treated, so that the device 100 can be used for the large-area plasma treatment 100. In particular, the device 100 can be used for the plasma treatment of a fabric, in particular a nonwoven fabric, or a plastic film.

(52) FIG. 11 shows an alternative device 110, again of similar configuration to the device 40 of FIG. 2, but with the device 2 and connecting piece 44 positioned laterally on the nozzle assembly 42 and the distributor element 50 of the nozzle assembly 42 having a course of the distribution channel 66 correspondingly adapted. Alternatively, the device 110 may also be similar to the device 40′ of FIG. 4 or the device 40″ of FIG. 5.

(53) The nozzle assembly 42 is rotatable about an axis parallel to the extension direction of the channel 56 by means of a rotary actuator 112. The device 110 can likewise be used for the plasma treatment of a fabric, in particular a nonwoven fabric, or a plastic film.

(54) Furthermore, the device 110 may also be used for other purposes. In particular, a tubular component can be impinged from the inside with plasma, using the partial jets 70 projecting from the nozzle openings 62, for example, to treat a pipe inner wall with plasma.

(55) FIG. 12 shows a further exemplary embodiment of the nozzle assembly according to the invention and of the device according to the invention. The device 40′″ and the nozzle assembly 42′″ are substantially structurally identical to the device 40′ or the nozzle assembly 42′ from FIG. 4. Identical parts are respectively provided with the same reference numerals.

(56) The nozzle assembly 42″ differs from the nozzle assembly 42′ in that the nozzle element 52′ has a first channel-shaped recess 120 and the distributor element 50′″ has a second channel-shaped recess 122, wherein the distributor member 50′″ and the nozzle element 52″ adjoin each other such that the first and second channel-shaped recesses 120 and 122 face each other and form the channel 56′″. By this configuration, various cross-sectional shapes of the channel 56′″ can be easily produced by shaping the recesses 120 and 122 accordingly. The nozzle openings 62 emanate from the first recess 120.

(57) For example, each of the first and second channel-shaped recesses 120, 122 may have a semicircular cross section of the same radius, so that the channel 56′″ has a circular cross section. The radius of the two semicircular cross sections of the first and second recesses 120, 122 may, for example, decrease continuously in the extension direction of the channel 56″, so that a channel 56′″ with a decreasing cross section results. Such a cross section of the channel 56′″ can be much more cost-effective and easier to produce with the two recesses 120, 122 than in a channel made of solid material.

(58) FIGS. 13a-c show three further possible cross sections 124′, 124″ and 124′″ of the channel 56′″ for further exemplary embodiments of the nozzle assembly according to the invention. For the sake of clarity, the figures show only the sectional plane without representing the edges located behind it. The nozzle assemblies correspond in each case to the nozzle assembly 42′″ from FIG. 12, wherein the first recess and the second recess and the channel 56′″ formed thereby have in each case one of the cross sections 124′, 124″ or 124′″ illustrated in FIGS. 13a-c. The schematic cross-sectional representations in FIGS. 13a-c correspond in each case to the sectional plane designated “XIII” in FIG. 12.

(59) FIG. 13a shows a first recess 120′ in the nozzle element 52′″ and a second recess 122′ in the distributor element 50′″, each having a semicircular cross section, wherein the semicircle diameter of the second recess 122′ is greater than the semicircle diameter of the first recess 120′. This results in a cross section 124′ of the channel of two semicircular discs opposite one another.

(60) FIG. 13a further shows the virtual first tangent plane 130 of the cross section 124′ through the nozzle opening 62 and the virtual second tangent plane 132 opposite thereto and running parallel thereto. The first tangent plane 132 passes through the mouth of the nozzle opening 62 into the channel and runs tangentially to the recess 124 or to the cross section 124′. Tangential here means that the first tangent plane 124 touches the channel cross section 124′ but does not intersect it.

(61) In the middle between the virtual first and second tangent plane 130 and 132, the virtual medial plane 134 is shown, which divides the cross section 124′ into a first cross-sectional area 126′ at the nozzle opening 62 and into a second cross-sectional area 128′ opposite the nozzle opening 62. Due to the different semicircular radii of the two recesses 120′ and 122′, the cross-sectional surface in the second cross-sectional area 128′ is greater than the cross-sectional surface in the first cross-sectional area 126′.

(62) FIG. 13b likewise shows a first recess 120″ in the nozzle element 52′″ and a second recess 122″ in the distributor element 50′, each having a semicircular cross section, but in this exemplary embodiment the semicircle diameter of the first recess 120″ is greater than the semicircle diameter of the second recess 122″. Furthermore, the virtual first and second tangent planes 130 and 132 are also shown in FIG. 13b, as well as the virtual medial plane 134 which divides the cross section 124″ into a first cross-sectional area 126″ at the nozzle opening 62 and into a second cross-sectional area 128″ opposite the nozzle opening 62. Due to the different semicircular radii of the two recesses 120″ and 122″, the cross-sectional surface in the second cross-sectional area 128″ is smaller than the cross-sectional surface in the first cross-sectional area 128″.

(63) FIG. 13c shows a first recess 120′″ in the nozzle element 52′″ with a triangular cross section and a second recess 122′″ in the distributor element 50′″ with a semicircular cross section, so that the cross section 124′ shown in FIG. 13c results. Furthermore, the virtual first and second tangent planes 130 and 123 are also shown in FIG. 13c, as well as and the virtual medial plane 134 which divides the cross section 124″ into a first cross-sectional area 126′″ at the nozzle opening 62 and into a second cross-sectional area 128″ opposite the nozzle opening 62. In the cross section 124′″, the cross-sectional surface of the second cross-sectional area 126′″ is greater than the cross-sectional surface of the first cross-sectional area 128′″.

(64) The position of the virtual medial plane 134 is in principle independent of the contact surface between nozzle element 52′″ and distributor element 50′″. Thus, the medial plane 134 may coincide with the contact surface (see FIG. 13c), but does not have to (see FIG. 13a-b).

(65) Experiments have shown that a more uniform distribution of the plasma power to the partial jets emerging from the individual nozzle openings 62 can be achieved by an asymmetrical cross section of the channel 56′″, as shown for example in FIGS. 13a-c. Particularly good results were achieved when the cross-sectional surface of the second cross-sectional area was larger with respect to the nozzle opening 62 than the cross-sectional surface of the first cross-sectional area. Thus, the exemplary embodiments shown in FIGS. 13a and 13c are particularly preferred.

(66) Experiments have been performed which show the advantages of an asymmetrical channel cross section. For this purpose, in each case a device was operated which corresponded to the device 40′″ from FIG. 12 with different cross sections of the channel 56′″. FIG. 14a-c show photographs of the partial jets emerging from the nozzle openings 62 of the respective nozzle assembly. FIG. 15a-c shows the associated channel cross sections 140, 142, 144 of the nozzle assemblies used in each case for the experiments.

(67) The nozzle assemblies are respectively arranged at the top in FIG. 14a-c; the flow direction of the partial jets thus runs from top to bottom. The position of the plasma nozzle is as in FIG. 12 on the left side. For better visibility, the photographs were inverted. Thus, FIG. 14a-c actually show the photographic negatives, so that the actually luminous partial jets are dark, and the dark surroundings are bright.

(68) FIG. 14a shows the photograph of the partial jets of a nozzle assembly with a circular channel cross section 140 corresponding to FIG. 15a. The first and the second recess respectively have a corresponding semicircular shape with a semicircular radius r.sub.1, r.sub.2 of 2 mm in each case.

(69) FIG. 14b shows the photograph of the partial jets from a nozzle assembly with an asymmetrical channel cross section 142 corresponding to FIG. 15b. The first and second recesses each have semicircular shapes, but with a different semi-circle radius, wherein r.sub.1=1.5 mm and r.sub.2=2.55 mm.

(70) FIG. 14c shows the photograph of the partial jets from a nozzle assembly with an asymmetrical channel cross section 144 corresponding to FIG. 15c. The first and second recesses each have semicircular shapes, wherein r.sub.1=2.55 mm and r.sub.2=2 mm.

(71) A comparison of the photographs in FIG. 14a-c shows that the intensity of the plasma jet in the asymmetrical channel cross sections 142 and 144 (see FIG. 14b-c) is better distributed to the partial jets emerging from the nozzle openings 62 than in the symmetrical channel cross section 140 (see FIG. 14a). This is demonstrated in particular in the lengths of the visible luminous regions of the partial jets (dark in FIGS. 14a-c), which is quite different in FIG. 14a. Thus, the visible regions of the partial jets in FIG. 14a are significantly shorter on the left side (i.e., near the nozzle) than on the right side.

(72) A particularly uniform distribution of the plasma jet to the partial jets was achieved with the channel cross section 142 (see FIG. 14b), in which the second cross-sectional area has a larger cross-sectional surface than the first cross-sectional area.