Plasma-generating device, plasma surgical device and use of a plasma surgical device

Abstract

The present invention relates to a plasma-generating device, comprising an anode, a cathode and at least one intermediate electrode, said intermediate electrode being arranged at least partly between said anode and said cathode, and said intermediate electrode and said anode forming at least a part of a plasma channel which has an opening in said anode. Further, the plasma-generating device comprises at least one coolant channel which is arranged with at least one outlet opening which is positioned beyond, in the direction from the cathode to the anode, said at least one intermediate electrode, and the channel direction of said coolant channel at said outlet opening has a directional component which is the same as that of the channel direction of the plasma channel at the opening thereof. The invention also concerns a plasma surgical device and use of such a plasma surgical device.

Claims

1. A plasma surgical device having an operational end, the plasma surgical device comprising: an anode at the operational end of the plasma surgical device, the anode having a hole therethrough; a cathode having a tapered portion narrowing toward the anode; an intermediate electrode, having a hole therethrough, the intermediate electrode being arranged between the cathode and the anode; an end sleeve forming an outer casing of the operation end of the plasma surgical device, the end sleeve having an outer diameter of less than 5 mm; and an insulator sleeve extending along and surrounding only a portion of the cathode and having a distal end, wherein the hole through the intermediate electrode and the hole through the anode, at least in part, form a first channel for conducting plasma and discharging a plasma jet through an external outlet opening at the operational end of the plasma surgical device; wherein a gap between the end sleeve and the intermediate electrode and a gap between the end sleeve and the anode, at least in part, form a second channel for conducting a coolant for cooling the intermediate electrode and the anode for discharging the coolant through one or more external outlet openings at the operational end of the plasma surgical device; wherein the end sleeve of the plasma surgical device is replaceable so as to provide different external outlet openings of the first and second channels corresponding to different desired properties and performance characteristics of the plasma surgical device, wherein only a part of the tapered portion of the cathode projects beyond the distal end of the insulator sleeve, and wherein a distal end of the cathode is located some distance away from an inlet of the first channel.

2. The plasma surgical device of claim 1, in which the one or more external outlet openings of the second channel are arranged in the anode.

3. The plasma surgical device of claim 1, in which a substantial portion of the second channel is substantially parallel to the first channel.

4. The plasma surgical device of claim 1, in which angles between the second channel at the one or more external outlet openings of the second channel and the first channel at the external outlet opening of the first channel are between +30 and ?30 degrees.

5. The plasma surgical device of claim 4, in which the angles are zero.

6. The plasma surgical device of claim 4, in which the second channel at the one or more external outlet openings of the second channel angles toward the first channel.

7. The plasma surgical device of claim 4, in which the second channel at the one or more external outlet openings of the second channel angles away from the first channel.

8. The plasma surgical device of claim 1, wherein during operation a coolant in the second channel flows toward the one or more external outlet openings.

9. The plasma surgical device of claim 1, in which during operation a coolant in the second channel is in contact with the intermediate electrode.

10. The plasma surgical device of claim 1, wherein the outer sleeve forms an integral structure with the anode.

11. The plasma surgical device of claim 1, in which the second channel has two or more external outlet openings.

12. The plasma surgical device of claim 11, in which the two or more external outlet openings of the second channel are arranged around the external outlet opening of the first channel.

13. The plasma surgical device of claim 12, in which the second channel has four or more external outlet openings.

14. The plasma surgical device of claim 1, in which a cross-section of one of the at least one of the one or more external outlet openings of the second channel is elongated.

15. The plasma surgical device of claim 1, wherein the gap between the end sleeve and the intermediate electrode and the gap between the end sleeve and the anode, at least in part, form two or more second channels.

16. A method of using a plasma surgical device having a cathode including a tapered portion narrowing toward an anode, an insulator sleeve extending along and surrounding only a portion of the cathode and having a distal end, only a part of the tapered portion of the cathode projecting beyond the distal end of the insulator sleeve, a distal end of the cathode being located some distance away from an inlet of a first channel, and a second channel and a replaceable end sleeve forming an outer casing of the operational end of the plasma surgical device, said sleeve providing different external outlet openings of the first and second channels corresponding to different desired properties and performance characteristics of the plasma surgical device, the method comprising: discharging a plasma jet on a spot of a biological tissue from an outlet opening of the first channel in the end sleeve; cooling electrodes of the plasma surgical device by passing a coolant through the second channel; and discharging the coolant near the spot of the biological tissue through one or more outlet openings of the second channel in the end sleeve.

17. The method of claim 16 further comprising: restricting the discharged plasma jet radially and longitudinally with the discharging coolant at the spot of the biological tissue.

18. The plasma surgical device of claim 1, wherein the intermediate electrode extends partially along the cathode.

19. The plasma surgical device of claim 1 wherein the discharged coolant is operable to restrict the discharged plasma jet radially and longitudinally.

20. The plasma surgical device of claim 1 adapted for minimally invasive surgery.

21. The plasma surgical device of claim 1, wherein the end sleeve has an outer diameter of less than or equal to 3 mm.

22. A plasma-generating device comprising: a plasma chamber for generating plasma, a plasma channel extending longitudinally from the plasma chamber to a plasma outlet at an operational end of the of the plasma-generating device, the plasma channel defining a path for discharge of the plasma; an anode at the operational end of the plasma-generating device with the plasma channel passing therethrough; a cathode having a tapered portion narrowing toward the anode, only a part of the tapered portion projecting into the plasma chamber, a distal end of the cathode being located some distance away from an inlet of the plasma channel; an intermediate electrode, the intermediate electrode being arranged between the cathode and the anode with the plasma channel passing therethrough; and a coolant channel extending longitudinally in the plasma-generating device and having a coolant outlet at the operational end of the of the plasma-generating device, whereby coolant liquid flowing through the coolant channel cools a portion of the plasma-generating device proximate to the cooling channel and the coolant liquid discharges through the coolant outlet.

23. The plasma-generating device of claim 22, wherein the coolant outlet is arranged in the anode.

24. The plasma-generating device of claim 23, wherein the coolant outlet angles toward the plasma channel arranged in the anode.

25. The method of claim 16, wherein the coolant is discharged at a rate of between 1 and 5 ml/s.

26. The method of claim 17 further comprising: coagulating, vaporizing, or cutting of the biological tissue with the plasma jet.

27. The method of claim 26, wherein the biological tissue is a tissue of one or more of: heart, brain, liver, spleen, kidney, and skin.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in more detail with reference to the accompanying schematic drawings which by way of example illustrate currently preferred embodiments of the invention.

(2) FIG. 1a is a cross-sectional view of an embodiment of a plasma-generating device according to the invention;

(3) FIG. 1b is a partial enlargement of the embodiment according to FIG. 1a;

(4) FIG. 2a is a cross-sectional view of an alternative embodiment of the plasma-generating device;

(5) FIG. 2b is a front plan view of the plasma-generating device according to FIG. 2a;

(6) FIG. 2c is a front plan view of an alternative embodiment of the plasma-generating device according to FIG. 2a; and

(7) FIG. 3 is a cross-sectional view of another alternative embodiment of a plasma-generating device.

DESCRIPTION OF PREFERRED EMBODIMENTS

(8) FIG. 1a shows in cross-section an embodiment of a plasma-generating device 1 according to the invention. The cross-section in FIG. 1a is taken through the centre of the plasma-generating device 1 in its longitudinal direction. The device comprises an elongate end sleeve 3 which accommodates a plasma-generating system for generating plasma which is discharged at the end of the end sleeve 3. The generated plasma can be used, for instance, to stop bleedings in tissues, vaporise tissues, cut tissues etc.

(9) The plasma-generating device 1 according to FIG. 1a comprises a cathode 5, an anode 7 and a number of electrodes 9, 9, 9 arranged between the anode and the cathode, in this text referred to as intermediate electrodes. The intermediate electrodes 9, 9, 9 are annular and form part of a plasma channel 11 which extends from a position in front of the cathode 5 and further towards and through the anode 7. The inlet end of the plasma channel 11 is the end closest to the cathode 5; the plasma channel extends through the anode 7 where its outlet opening is arranged. A plasma is intended to be heated in the plasma channel 11 so as to finally flow out through the opening of the plasma channel in the anode 7. The intermediate electrodes 9, 9, 9 are insulated and spaced from each other by an annular insulator means 13, 13, 13. The shape of the intermediate electrodes 9, 9, 9 and the dimensions of the plasma channel 11 can be adjusted to any desired purposes. The number of intermediate electrodes 9, 9, 9 can also be optionally varied. The embodiment shown in FIG. 1a is provided with three intermediate electrodes 9, 9, 9.

(10) In the embodiment shown in FIG. 1a, the cathode 5 is formed as an elongate cylindrical element. Preferably the cathode 5 is made of tungsten with optional additives, such as lanthanum. Such additives can be used, for instance, to lower the temperature occurring at the end of the cathode 5.

(11) Moreover the end 15 of the cathode 5 which is directed to the anode 7 has a tapering end portion. This tapering portion 15 suitably forms a tip positioned at the end of the cathode as shown in FIG. 1a. The cathode tip 15 is suitably conical in shape. The cathode tip 15 can also consist of a part of a cone or have alternative shapes with a tapering geometry towards the anode 7.

(12) The other end of the cathode 5 which is directed away from the anode 7 is connected to an electrical conductor to be connected to an electric energy source. The conductor is suitably surrounded by an insulator. (The conductor is not shown in FIG. 1a.)

(13) Connected to the inlet end of the plasma channel 11, a plasma chamber 17 is arranged, which has a cross-sectional surface, transversely to the longitudinal direction of the plasma channel 11, which exceeds the cross-sectional surface of the plasma channel 11 at the inlet end thereof. The plasma chamber 17 which is shown in FIG. 1a is circular in cross-section, transversely to the longitudinal direction of the plasma channel 11, and has an extent L.sub.ch in the longitudinal direction of the plasma channel 11 which corresponds approximately to the diameter D.sub.ch of the plasma chamber 17. The plasma chamber 17 and the plasma channel 11 are substantially concentrically arranged relative to each other. The cathode 5 extends into the plasma chamber 17 at least half the length L.sub.ch thereof and the cathode 5 is arranged substantially concentrically with the plasma chamber 17. The plasma chamber 17 consists of a recess formed by the first intermediate electrode 9 which is positioned next to the cathode 5.

(14) FIG. 1a also shows an insulator element 19 which extends along and around parts of the cathode 5. The insulator element 19 is suitably formed as an elongate cylindrical sleeve and the cathode 5 is partly positioned in a circular hole extending through the tubular insulator element 19. The cathode 5 is substantially centred in the through hole of the insulator element 19. Moreover the inner diameter of the insulator element 19 slightly exceeds the outer diameter of the cathode 5, thereby forming a distance between the outer circumferential surface of the cathode 5 and the inner surface of the circular hole of the insulator element 19.

(15) Preferably the insulator element 19 is made of a temperature-resistant material, such as ceramic material, temperature-resistant plastic material or the like. The insulator element 19 intends to protect adjoining parts of the plasma-generating device from high temperatures which can occur, for instance, around the cathode 5, in particular around the tip 15 of the cathode.

(16) The insulator element 19 and the cathode 5 are arranged relative to each other so that the end 15 of the cathode 5 which is directed to the anode projects beyond an end face 21, which is directed to the anode 7, of the insulator element 19. In the embodiment shown in FIG. 1a, approximately half the tapering tip 15 of the cathode 5 projects beyond the end face 21 of the insulator element 19.

(17) A gas supply part (not shown in FIG. 1a) is connected to the plasma-generating part. The gas supplied to the plasma-generating device 1 advantageously consists of the same type of gases that are used as plasma-generating gas in prior art instruments, for instance inert gases, such as argon, neon, xenon, helium etc. The plasma-generating gas is allowed to flow through the gas supply part and into the space arranged between the cathode 5 and the insulator element 19. Consequently the plasma-generating gas flows along the cathode 5 inside the insulator element 19 towards the anode 7. As the plasma-generating gas passes the end 21 of the insulator element 19, the gas is passed on to the plasma chamber 17.

(18) The plasma-generating device 1 further comprises one or more coolant channels 23 which open into the elongate end sleeve 3. The coolant channels 23 are suitably partly made in one piece with a housing (not shown) which is connected to the end sleeve 3. The end sleeve 3 and the housing can, for instance, be interconnected by a threaded joint, but also other connecting methods, such as welding, soldering etc, are conceivable. Moreover the end sleeve suitably has an outer dimension which is less than 10 mm, preferably less than 5 mm, in particular between 3 mm and 5 mm. At least a housing portion positioned next to the end sleeve suitably has an outer shape and dimension which substantially corresponds to the outer dimension of the end sleeve. In the embodiment of the plasma-generating device shown in FIG. 1a, the end sleeve is circular in cross-section transversely to its longitudinal direction.

(19) The coolant channels 23 suitably consist of through-flow channels which extend through the device and open into or in the vicinity of the anode 7. Moreover parts of such coolant channels 23 can be made, for instance, by extrusion of the housing or mechanical working of the housing. However, it will be appreciated that parts of the coolant channel 23 can also be formed by one or more parts which are separate from the housing and arranged inside the housing.

(20) The plasma-generating device 1 can be provided with a coolant channel 23 which is provided with one or more outlet openings 25. Alternatively, the plasma-generating device 1 can be provided with a plurality of coolant channels 23, which each can be provided with one or more outlet openings 25. Each coolant channel 23 can also be divided into a plurality of channel portions which are combined in a common channel portion, which common channel portion can be provided with one or more outlet openings 25. It is also possible to use all or some of the channels 23 for other purposes. For example, three channels 23 can be arranged, two being used to be passed by coolant and one to suck liquids, or the like, from a surgical area etc.

(21) In the embodiment shown in FIG. 1a, a part of the coolant channel 23 extends through the end sleeve 3 and around the intermediate electrodes 9, 9, 9. The coolant channel 23 according to FIG. 1a is provided with a plurality of outlet openings 25.

(22) Moreover the outlet openings 25 of the coolant channel 23 are arranged beyond, in the direction from the cathode 5 to the anode 7, the intermediate electrodes 9, 9, 9. In the embodiment shown in FIG. 1a, the coolant channel 23 extends through the end sleeve 3 and the anode 7. Moreover the channel direction of the coolant channel 23 at the outlet openings 25 has a directional component which is the same as that of the channel direction of the plasma channel 11 at the opening thereof. According to FIG. 1a, two such outlet openings 25 are shown. Preferably the plasma-generating device 1 is provided with four or more outlet openings 25.

(23) Coolant channels 23 can partly be used to cool the plasma-generating device 1 in operation. As coolant, use is preferably made of water, although other types of fluids are conceivable. To provide cooling, a portion of the coolant channel 23 is arranged so that the coolant is supplied to the end sleeve 3 and flows between the intermediate electrodes 9, 9, 9 and the inner wall of the end sleeve 3. In operation of the device, it is preferred to let a flow amount of 1-5 ml/s flow through the plasma-generating device 1. The flow amount of coolant may, however, be optionally varied depending on factors such as operating temperature, desired operating properties, field of application etc. In surgical applications, the coolant flow rate is typically between 1 and 3 ml/s and the temperature of the coolant flowing out through the outlet opening 25 is typically between 25 and 40? C.

(24) The coolant which is intended to flow through the coolant channels 25 can also be used to screen the plasma jet and restrict the range of the plasma jet which is emitted through the outlet of the plasma channel 11 in the anode 7. The coolant can also be used to cool areas adjacent to a region, affected by the plasma jet, of an object.

(25) In the embodiment shown in FIG. 1a, the channel direction of the coolant channel 23 at the outlet openings 25 is directed at an angle ? towards the centre of the longitudinal direction of the plasma channel 11.

(26) The directed outlet portions allow that the plasma jet generated in operation can be screened in its longitudinal direction by the coolant flowing through the outlet openings 25 of the coolant channel 23. As a result, an operator who operates the device can obtain an essentially distinct position where the plasma jet will be active. In front of this position, suitably little effect from the plasma jet occurs. Consequently this enables good accuracy, for instance, in surgery and other precision-requiring fields of application. At the same time the coolant discharged through the outlet opening 25 of a coolant channel 23 can provide a screening effect in the lateral direction radially outside the centre of the plasma jet. Owing to such screening, a limited surface can be affected by heat locally, and cooled areas of the treated object, outside the area affected by the heat of the plasma, are affected to a relatively small extent by the plasma jet.

(27) FIGS. 2a-3 illustrate alternative embodiments of a plasma-generating device 1. Important differences between these embodiments and the embodiment according to FIG. 1a will be described below.

(28) In the embodiment shown in FIG. 2a, the channel direction of the coolant channel 123 at the outlet openings 125 is arranged substantially parallel to the longitudinal direction of the plasma channel 111. In this case, mainly screening of the plasma jet in the radial direction relative to the centre line of the plasma channel 111 is obtained.

(29) FIG. 3 shows another alternative embodiment of a plasma-generating device 201. In the embodiment shown in FIG. 3, the channel direction of the coolant channel 223 at the outlet openings 225 is directed at an angle ? away from the centre of the longitudinal direction of the plasma channel 211. This results in screening which increases in distance, relative to the centre line of the plasma channel 211, with an increased distance from the anode 207 and, thus, the outlet of the plasma channel 211.

(30) It will be appreciated that the embodiments according to FIGS. 1-3 can be combined to form additional embodiments. For example, different outlets can be directed and angled differently in relation to the longitudinal direction of the plasma channel 23; 123; 223. For example, it is possible to provide a plasma-generating device 1; 101; 201 with two outlet portions which are directed parallel to the plasma channel 11; 111; 211 and two outlet portions which are directed inwards to the centre of the longitudinal direction of the plasma channel 11; 111; 211. The variations, with regard to angle and direction of the channel direction of the coolant channel 23; 123; 223 at the outlet openings 25; 125; 225, can be optionally combined depending on the desired properties of the plasma-generating device 1; 101; 201.

(31) It is also possible to vary the angle of the channel direction at the outlet portions 25; 125; 225 in relation to the longitudinal direction of the plasma channel 11; 111; 211. Preferably, the outlet portions are arranged at an angle ?, ? of ?30 degrees in relation to the longitudinal direction of the plasma channel 11; 111; 211. In the embodiment shown in FIG. 1a the outlet portions are arranged at an angle ? of +10 degrees in relation to the longitudinal direction of the plasma channel 11; 111; 211. For the plasma-generating device shown in FIG. 1a, an angle ? of 10? means that coolant flowing out through the opening of the coolant channel will intersect the centre of the longitudinal direction of the plasma channel about 8-10 mm in front of the outlet of the plasma channel in the anode.

(32) In the embodiment shown in FIG. 3, the outlet portions are arranged at an angle ? of ?10 degrees in relation to the longitudinal direction of the plasma channel 11; 111; 211.

(33) FIGS. 2b-2c are front views of different embodiments of the plasma-generating device 101 in FIG. 2a. FIG. 2b shows a design where the outlet openings 125 of the outlet portions are positioned beside and spaced from the outlet of the plasma channel 111 in the anode. In the embodiment shown in FIG. 2b, the outlet openings 125 are formed as eight circular lead-ins which communicate with the coolant channel 123. It is possible to optionally arrange more or fewer than eight circular lead-ins depending on desirable properties and performance of the plasma-generating device 101. It is also possible to vary the size of the circular lead-ins.

(34) FIG. 2c shows an alternative design of the outlet openings 125 of the coolant channel 123. FIG. 2c is a front view of the plasma-generating device 101 in FIG. 2a. In the embodiment shown in FIG. 2c, the outlet openings 125 are formed as four arched lead-ins which communicate with the coolant channel.

(35) It will be appreciated that the outlet openings 125 of the cooling channel 123 optionally can be designed with a number of alternative geometries and sizes. The cross-sectional surface of the outlet openings can typically be between 0.50 and 2.0 mm.sup.2, preferably 1 to 1.5 mm.sup.2.

(36) It is obvious that these different designs of the outlet openings 25; 125; 225 can also be used for the embodiments of the plasma-generating device as shown in FIGS. 1a-b and 3.

(37) The following description refers to FIGS. 1a-b. The conditions and dimensions stated are, however, also relevant as exemplary embodiments of the embodiments of the plasma-generating device shown in FIGS. 2a-3.

(38) The intermediate electrodes 9, 9, 9 shown in FIG. 1a are arranged inside the end sleeve 3 of the plasma-generating device 1 and are positioned substantially concentrically with the end sleeve 3. The intermediate electrodes 9, 9, 9 have an outer diameter which in relation to the inner diameter of the end sleeve 3 forms an interspace between the outer surface of the intermediate electrodes 9, 9, 9 and the inner wall of the end sleeve 3. It is in this space between the intermediate electrodes 9, 9, 9 and the end sleeve 3 where the coolant flows to be discharged through the outlet openings 125 of the coolant channel 23.

(39) In the embodiment shown in FIG. 1a, three intermediate electrodes 9, 9, 9, spaced by insulator means 13, 13, 13, are arranged between the cathode 5 and the anode 7. The first intermediate electrode 9, the first insulating 13 and the second intermediate electrode 9 are suitably press-fitted to each other. Similarly, the second intermediate electrode 9, the second insulator 13 and the third intermediate electrode 9 are suitably press-fitted to each other. However, it will be appreciated that the number of intermediate electrodes 9, 9, 9 can be optionally selected depending on the desired purpose.

(40) The intermediate electrode 9 which is positioned furthest away from the cathode 5 is in contact with an annular insulator means 13 which is arranged against the anode 7.

(41) The anode 7 is connected to the elongate end sleeve 3. In the embodiment shown in FIG. 1a, the anode 7 and the end sleeve 3 are integrally formed with each other. In alternative embodiments, the anode 7 can be designed as a separate element which is joined to the end sleeve 3 by a threaded joint between the anode and the end sleeve, by welding or by soldering. The connection between the anode 7 and the end sleeve 3 is suitably such as to provide electrical contact between the two.

(42) Suitable geometric relationships between parts included in the plasma-generating device 1, 101, 201 will be described below with reference to FIGS. 1a-b. It should be noted that the dimensions stated below merely constitute exemplary embodiments of the plasma-generating device 1, 101, 201 and can be varied depending on the field of application and the desired properties. It should also be noted that the examples described in FIGS. 1a-b can also be applied to the embodiments in FIGS. 2a-3.

(43) The inner diameter d.sub.i of the insulator element 19 is only slightly greater than the outer diameter d.sub.c of the cathode 5. In one embodiment, the difference in cross-section, in a common cross-section, between the cathode 5 and the inner diameter d.sub.i of the insulator element 19 is suitably equal to or greater than a minimum cross-section of the plasma channel 11. Such a cross-section of the plasma channel 11 can be positioned anywhere along the extent of the plasma channel 11.

(44) In the embodiment shown in FIG. 1b, the outer diameter d.sub.c of the cathode 5 is about 0.50 mm and the inner diameter d.sub.i of the insulator element about 0.80 mm.

(45) In one embodiment, the cathode 5 is arranged so that a partial length of the cathode tip 15 projects beyond a boundary surface 21 of the insulator element 19. The tip 15 of the cathode 5 is in FIG. 1b positioned so that about half the length L.sub.c of the tip 15 projects beyond the boundary surface 21 of the insulator element 19. In the embodiment shown in FIG. 1b, this projection l.sub.c corresponds to approximately the diameter d.sub.c of the cathode 5.

(46) The total length L.sub.c of the cathode tip 15 is suitably greater than 1.5 times the diameter d.sub.c of the cathode 5 at the base of the cathode tip 15. Preferably the total length L.sub.c of the cathode tip 15 is about 1.5-3 times the diameter d.sub.c of the cathode 5 at the base of the cathode tip 15. In the embodiment shown in FIG. 1b, the length L.sub.c of the cathode tip 15 corresponds to about 2 times the diameter d.sub.c of the cathode 5 at the base of the cathode tip 15.

(47) In one embodiment, the diameter d.sub.c of the cathode 5 is about 0.3-0.6 mm at the base of the cathode tip 15. In the embodiment shown in FIG. 1b, the diameter d.sub.c of the cathode 5 is about 0.50 mm at the base of the cathode tip 15. Preferably the cathode has a substantially identical diameter d.sub.c between the base of the cathode tip 15 and the end of the cathode 5 opposite the cathode tip 15.

(48) However, it will be appreciated that it is possible to vary this diameter d.sub.c along the extent of the cathode 5. In one embodiment, the plasma chamber 17 has a diameter D.sub.ch which corresponds to approximately 2-2.5 times the diameter d.sub.c of the cathode 5 at the base of the cathode tip 15. In the embodiment shown in FIG. 1b, the plasma chamber 17 has a diameter D.sub.ch which corresponds to approximately 2 times the diameter d.sub.c of the cathode 5.

(49) The extent L.sub.ch of the plasma chamber 17 in the longitudinal direction of the plasma-generating device 1 corresponds to approximately 2-2.5 times the diameter d.sub.c of the cathode 5 at the base of the cathode tip 15. In the embodiment shown in FIG. 1b, the length L.sub.ch of the plasma chamber 17 corresponds to approximately the diameter D.sub.ch of the plasma chamber 17.

(50) In one embodiment the tip 15 of the cathode 5 extends over half the length L.sub.ch of the plasma chamber 17 or more than said length. In an alternative embodiment, the tip 15 of the cathode 5 extends over ? to ? of the length L.sub.ch of the plasma chamber 17. In the embodiment shown in FIG. 1b, the cathode tip 15 extends approximately over half the length L.sub.ch of the plasma chamber 17.

(51) In the embodiment shown in FIG. 1b, the cathode 5 extending into the plasma chamber 17 is positioned at a distance from the end of the plasma chamber 17 closest to the anode 7 which corresponds to approximately the diameter d.sub.c of the cathode 5 at the base thereof.

(52) In the embodiment shown in FIG. 1b, the plasma chamber 17 is in fluid communication with the plasma channel 11. The plasma channel 11 suitably has a diameter d.sub.ch which is about 0.2-0.5 mm. In the embodiment shown in FIG. 1b, the diameter d.sub.ch of the plasma channel 11 is about 0.40 mm. However, it will be appreciated that the diameter d.sub.ch of the plasma channel 11 can be varied in different ways along the extent of the plasma channel 11 to provide different desirable properties.

(53) A transition portion 27 is arranged between the plasma chamber 17 and the plasma channel 11 and constitutes a tapering transition, in the direction from the cathode 5 to the anode 7, between the diameter D.sub.ch of the plasma chamber 17 and the diameter d.sub.ch of the plasma channel 11. The transition portion 27 can be formed in a number of alternative ways. In the embodiment shown in FIG. 1b, the transition portion 27 is formed as a bevelled edge which forms a transition between the inner diameter D.sub.ch of the plasma chamber 17 and the inner diameter d.sub.ch of the plasma channel 11. However, it should be noted that the plasma chamber 17 and the plasma channel 11 can be arranged in direct contact with each other without a transition portion 27 arranged between the two. The use of a transition portion 27 as shown in FIG. 1b allows advantageous heat extraction to cool structures adjacent to the plasma chamber 17 and the plasma channel 1.

(54) The plasma channel 11 is formed by the anode 7 and the intermediate electrodes 9, 9, 9 arranged between the cathode 5 and the anode 7. The length of the plasma channel 11 between the opening of the plasma channel closest to the cathode and up to the anode corresponds suitably to about 4-10 times the diameter d.sub.ch of the plasma channel 11. In the embodiment shown in FIG. 1a, the length of the plasma channel 11 between the opening of the plasma channel closest to the cathode and the anode is about 1.6 mm.

(55) That part of the plasma channel which extends through the anode is about 3-4 times the diameter d.sub.ch of the plasma channel 11. For the embodiment shown in FIG. 1a, that part of the plasma channel which extends through the anode has a length of about 2 mm.

(56) The plasma-generating device 1 can advantageously be provided as a part of a disposable instrument. For example, a complete device with the plasma-generating device 1, outer shell, tubes, coupling terminals etc. can be sold as a disposable instrument. Alternatively, only the plasma-generating device 1 can be disposable and connected to multiple-use devices.

(57) Other embodiments and variants are conceivable within the scope of the present invention. For example, the number and shape of the electrodes 9, 9, 9 can be varied according to which type of plasma-generating gas is used and which properties of the generated plasma are desired.

(58) In use the plasma-generating gas, such as argon, which is supplied through the gas supply part, is introduced into the space between the cathode 5 and the insulator element 19 as described above. The supplied plasma-generating gas is passed on through the plasma chamber 17 and the plasma channel 11 to be discharged through the opening of the plasma channel 11 in the anode 7. Having established the gas supply, a voltage system is switched on, which initiates a discharge process in the plasma channel 11 and establishes an electric arc between the cathode 5 and the anode 7. Before establishing the electric arc, it is suitable to supply coolant to the plasma-generating device 1 through the coolant channel 23, as described above. Having established the electric arc, a gas plasma is generated in the plasma chamber 17, which during heating is passed on through the plasma channel 11 to the opening thereof in the anode 7.

(59) A suitable operating current for the plasma-generating devices 1, 101, 201 according to FIGS. 1-3 is 4-10 ampere, preferably 4-6 ampere. The operating voltage of the plasma-generating device 1, 101, 201 is, inter alia, dependent on the number of intermediate electrodes and the length thereof. A relatively small diameter of the plasma channel allows relatively low consumption of energy and relatively low operating current in use of the plasma-generating device 1, 101, 201.

(60) In the electric arc established between the cathode and anode, there prevails in the centre thereof, along the centre axis of the plasma channel, a temperature T which is proportional to the relationship between the discharge current I and the diameter d.sub.ch of the plasma channel (T=k*i/d.sub.ch). To provide, at a relatively low current level, a high temperature of the plasma, for instance 10,000 to 15,000? C., at the outlet of the plasma channel in the anode, the cross-section of the plasma channel and, thus, the cross-section of the electric arc which heats the gas should be small, for instance 0.2-0.5 mm. With a small cross-section of the electric arc, the electric field strength in the channel has a high value.