Plasma source for generating mixed gases for sterilization and disinfection

Abstract

A plasma source for generating a sterilizing and / or disinfecting gas mixture, the plasma source comprising: an ionization chamber having a dielectric tubular portion, an inlet for supplying a gas or gas mixture to the ionization chamber, an outlet for discharging the sterilizing and / or disinfecting gas mixture from the ionization chamber, a first electrode disposed inside the dielectric tubular portion, and a second electrode disposed outside the dielectric tubular portion, the plasma source comprising a high voltage source having a high voltage output terminal, an electrical conductor connecting the output terminal to the first electrode or the second electrode, and the plasma source further comprising a forced gas cooling device for cooling the ionization chamber.

Description

[Technical field]

[0001] The present invention relates to the disinfection and / or sterilization of medical instruments, such as dental instruments, using plasma. More generally, the present invention relates to methods and apparatus for generating disinfectant and / or sterilizing gas mixtures. [Background technology]

[0002] A reusable medical instrument is an instrument that can be reused by a health care provider in diagnosing and / or treating multiple patients. Examples of reusable medical instruments include medical instruments used in dental procedures, such as scalpels, syringes, scopes, mirrors, drills, burrs, discs, handpieces, excavators, turbines, files, reamers, etc.

[0003] When such medical instruments are used on a patient, they become contaminated with blood, tissue, and other biological debris, such as microorganisms. To avoid the risk of infection from contaminated instruments, reusable instruments can be sterilized. Sterilization results in a medical instrument that can be safely used multiple times on the same patient or multiple patients. Proper sterilization of reusable medical instruments is essential to ensure patient safety.

[0004] Various sterilants may be used to sterilize medical instruments. Traditionally, steam or hydrogen peroxide has been used. More recently, plasma devices have been used to ionize a gas or a mixture of gases, and the ionized gas is used as the sterilant. Electrons in the plasma impact the gas molecules, causing dissociation and ionization of these molecules, producing a mixture of reactive species. It is known to expose the medical instruments directly to the plasma, or to expose the medical instruments to the (partially) recombined plasma, also called afterglow. See, for example, S. Moreau et al., "Inactivation of Bacillus subtilis spores using a plasma afterglow stream: influence of operating conditions," J. Appl. Phys. Vol. 88, No. 2, July 15, 2000.

[0005] Some attempts have been made to improve plasma sterilization: US Patent Application Publication No. 2011 / 0027125A1 discloses a system comprising a chamber and a plasma generator that produces free radicals in combination with the use of a hydrogen peroxide solution.

[0006] It is also known to use atmospheric and superatmospheric pressure plasma sources.

[0007] A disadvantage of plasma sources is that by producing an at least partially ionized gas mixture, the composition of the germicide and / or sterilant produced can vary significantly by varying the temperature and pressure of the plasma. Summary of the Invention

[0008] The present invention aims to provide an improved plasma source for producing germicidal and / or sterilizing gas mixtures.

[0009] Thus, according to a first aspect, a plasma source for generating a sterilizing and / or sterilizing gas mixture is provided. The plasma source includes an ionization chamber. The ionization chamber includes a dielectric tubular portion. The dielectric tubular portion may form a wall of the ionization chamber. The ionization chamber includes an inlet for feeding a gas or a gas mixture into the ionization chamber. In the ionization chamber, the gas or gas mixture is transformed into a sterilizing and / or sterilizing gas mixture. The ionization chamber includes an outlet for discharging the sterilizing and / or sterilizing gas mixture out of the ionization chamber. The inlet may be disposed at a first end of the tubular portion. The outlet may be disposed at an opposite second end of the tubular portion. Thus, the gas or gas mixture may pass through the tubular portion. The ionization chamber includes a first electrode disposed inside the dielectric tubular portion and a second electrode disposed outside the dielectric tubular portion. The first electrode may, for example, extend longitudinally within the tubular portion along the axis of the tubular portion. The second electrode can be formed on the outer surface of the tubular part. The second electrode can be a separate part, such as a metal sheet. The second electrode can also be a conductive layer deposited on the outer surface of the tubular part, for example a metal layer (plasma) evaporated on said surface. The plasma source comprises a high voltage source having a high voltage output terminal, with an electrical conductor connecting said output terminal to the first electrode or the second electrode. The high voltage terminal can be connected, for example, to the first electrode. The second electrode can be connected to a ground terminal. The electrical conductor is preferably less than 50 cm long. The plasma source comprises a forced gas cooling system for cooling the ionization chamber.

[0010] The plasma source is configured to generate a sterilizing and / or disinfecting gas mixture. It will be appreciated that in some circumstances it is sufficient to generate a sterilizing gas mixture suitable for sterilizing an object, where most, but not necessarily all, microorganisms are killed. In other circumstances it is preferred to generate a sterilizing gas mixture suitable for sterilizing an object, where substantially all, microorganisms are killed.

[0011] It has been found that cooling the ionization chamber, e.g., the tubular portion of the ionization chamber, with forced gas, such as forced air, provides a particularly good germicidal and / or sterilizing gas mixture, especially when combined with relatively short electrical conductors.

[0012] Forced gas cooling is believed to have a beneficial effect on the temperature stability of the plasma source and improve the quality of the sterilizing and / or disinfecting gas mixture. In this regard, gas cooling has been found to perform better than liquid cooling, such as water cooling. Liquid cooling is believed to be more corrosive to plasma source components than gas cooling, resulting in larger temperature fluctuations.

[0013] Shorter electrical conductors also appear to have a beneficial effect on the quality of the sterilizing and / or disinfecting gas mixture. Although not fully understood, shorter electrical conductors are believed to have a beneficial effect on electromagnetic compatibility (EMC) and / or reduce the electrical impedance of the system, resulting in beneficial outcomes. Reduced temperature fluctuations may contribute to a more consistent production of the desired disinfecting and / or sterilizing components in the gas mixture.

[0014] Optionally, the forced gas cooling system is arranged to force a flow of cooling gas through the dielectric tubular portion in a direction substantially perpendicular to the longitudinal axis of the tubular portion, e.g., perpendicular to the longitudinal axis of the tubular portion. Such a flow has been found to effectively enhance the quality of the sterilizing and / or disinfecting gas mixture. Alternatively, or in addition, the forced gas cooling system can be arranged to force a flow of cooling gas through the dielectric tubular portion in a direction substantially parallel to the longitudinal axis of the tubular portion.

[0015] Optionally, the forced gas cooling system includes a temperature control system for controlling the temperature of the plasma and / or the ionization chamber and / or the tubular portion. The temperature control system may include a temperature sensor and a controller.

[0016] Optionally, the forced gas cooling system includes a detector for detecting a malfunction of the cooling system and is configured to shut off or reduce power to the high voltage source when a malfunction is detected, thus avoiding overheating of the plasma source in the event of a malfunction of the cooling system.

[0017] Optionally, the electrical conductor has a length of less than 50cm, preferably less than 30cm, more preferably less than 20cm.

[0018] Optionally, the plasma source includes a first end cap, the first end cap including the inlet and closing the dielectric tubular portion at a first end. Optionally, the plasma source includes a second end cap, the second end cap including the outlet and closing the dielectric tubular portion at a second end opposite the first end. These end caps provide an effective and mechanically simple way to provide the inlet and / or outlet to the dielectric tubular portion. Preferably, the first and / or second end caps are made of an electrically insulating material.

[0019] Optionally, the dielectric tubular portion includes a wall formed of quartz or a glass, such as a borosilicate glass such as Pyrex® or Duran®.

[0020] Optionally, the plasma source includes a housing, for example made of metal. Optionally, the ionization chamber, the high voltage source, and at least a portion of the forced gas cooling system are disposed within the housing. This provides a plasma source that is easy to install and replace. Also, electromagnetic compatibility (EMC) is easily achieved when the housing is made of metal.

[0021] The gas or gas mixture provided to the inlet may be a humidified gas or gas mixture, for example humidified air. The gas or gas mixture may have a specific humidity, for example 2-25 grams of water vapor per kilogram of gas, for example about 10 grams of water vapor per kilogram of gas. The gas or gas mixture may be humidified, for example as described in co-pending patent application NL2025110, which is incorporated herein by reference.

[0022] According to a second aspect, there is provided a sterilisation device for sterilising medical instruments comprising a plasma source as described herein.

[0023] According to a third aspect, there is provided a method for generating a sterilizing gas mixture. The method includes supplying a gas or gas mixture through a dielectric tubular member having a first electrode disposed inside the dielectric tubular member and a second electrode disposed outside the dielectric tubular member. The method includes applying a high voltage difference between the first electrode and the second electrode. The method includes cooling the dielectric tubular member using a forced gas cooling system.

[0024] Optionally, the method includes supplying a high voltage to the first or second electrode via a relatively short electrical conductor.

[0025] It will be understood that any of the aspects, features and options described in terms of the plasma source apply equally to the sterilization apparatus and method, and vice versa. It will also be apparent that combinations of any one or more of the above aspects, features and options are possible. [Brief description of the drawings]

[0026] Embodiments of the present invention will now be described with reference to the accompanying drawings. [Figure 1] FIG. 2 is a schematic diagram of a plasma source. [Figure 2A] FIG. 2 is a schematic diagram of a plasma source. [Figure 2B] FIG. 1 is a cross-sectional view of a forced gas cooling system. [Diagram 3] FIG. 1 is a schematic diagram of a sterilization apparatus. [Figure 4] FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] 1 is a schematic diagram of a plasma source 1 for generating a sterilizing and / or disinfecting gas mixture. The plasma source 1 includes an ionization chamber 2, which is bounded by a number of walls. A first wall is formed by a dielectric tubular portion 4. The dielectric tubular portion may include or be formed by, for example, a glass tube made of quartz or glass, for example a borosilicate glass such as Pyrex® or Duran®.

[0028] In this example, the second wall is formed by a first end cap 6 closing the dielectric tubular portion 4 at a first end. In this example, the third wall is formed by a second end cap 8 closing the dielectric tubular portion 4 at a second end opposite the first end. Here, the end caps 6, 8 are hermetically connected to the tubular portion 4. Here, a seal 10, such as an O-ring, is provided between the end caps 6, 8 and the tubular portion 4.

[0029] The ionization chamber 2 includes an inlet 12 for supplying a gas or a gas mixture to the ionization chamber 2, where the inlet is located at a first end of the tubular portion. In this example, the inlet port 12 forms part of the first end cap 6. The ionization chamber 2 includes an outlet 14 for discharging the sterilizing gas out of the ionization chamber 2, where the outlet port 14 is located at a second end of the tubular portion 4. In this example, the outlet port 14 forms part of the second end cap 8.

[0030] The ionization chamber 2 includes a first electrode 16. The first electrode 16 is disposed inside the dielectric tubular portion 2. Here, the first electrode extends longitudinally within the tubular portion 2, here along the axis of the tubular portion 2. The first electrode 16 in this example has an elongated shape, such as a rod. Here, the first electrode 16 has a larger rod diameter at the portion where the plasma is generated. In this example, the ionization chamber 2 includes an electrical feedthrough 18 that forms an electrical connection from the outside of the ionization chamber to the first electrode 16 in the ionization chamber 2. Here, the feedthrough is disposed in the first end cap 6. In FIG. 1, a gap is depicted between the feedthrough 18 and the first end cap 6 for clarity. It will be understood that in practice the feedthrough forms an airtight electrical connection from the outside of the ionization chamber 2 to the inside of the ionization chamber 2. In this example, both the first end cap 6 and the second end cap 8 include a retainer to hold the first electrode in its position.

[0031] The ionization chamber 2 includes a second electrode 20. The second electrode 20 is arranged on the outside of the dielectric tubular portion 4. The second electrode 20 can be formed on the outer surface of the tubular portion 4. The second electrode can be a separate part, such as a metal sheet, arranged on the outer surface of the tubular portion 4 in intimate contact with the outer surface. In this example, the second electrode 20 is formed as a conductive layer deposited on the outer surface of the tubular portion 4, for example a metal layer (plasma) evaporated on said surface. In FIG. 1, a gap is depicted between the second electrode 20 and the tubular portion 4 for clarity. It will be understood that in practice the second electrode 20 and the tubular portion are in contact with each other.

[0032] The plasma source 1 comprises a high voltage source 22. The high voltage source 22 is arranged to provide a high voltage difference between two output terminals 24, 26. In this example, the first output terminal 24 is a high voltage output terminal and the second output terminal 26 is connected or connectable to a ground terminal. The high voltage provided to the first output terminal 24 can be a positive high voltage or a negative high voltage. In this example, a first electrical conductor 28 connects the first output terminal 24 to the first electrode 16. Here, a second electrical conductor 30 connects the second output terminal 26 to the second electrode 20. It will be understood that it is also possible for the second output terminal 26 and the second electrode 20 to both be connected to a ground terminal. In that case, the dedicated second electrical conductor 30 in the form of a lead can be omitted.

[0033] The plasma source 1 includes a forced gas cooling system 32. The forced gas cooling system 32 of FIG. 1 includes a fan 34. The forced gas cooling system 32 of FIG. 1 further includes a guide 36 for directing the cooling gas towards the ionization chamber 2. The guide 36 can include, for example, a funnel. FIG. 2A is an exemplary three-dimensional view of the plasma source 1. FIG. 2B is a cross-sectional view of the forced gas cooling system 32 acting on the ionization chamber 2. In FIGS. 2A and 2B, the cross-section of the guide 36 tapers towards the ionization chamber 2. As shown in FIG. 2A, the guide can be formed by an elongated guide or funnel that extends along the length of the tubular portion 4, for example between the end caps 6 and 8. As shown, the length of this elongated guide or funnel can be similar or the same as the length of the tubular portion 4, for example within ±20%. In this way, cooling can be provided over substantially the entire length. For example, the forced gas cooling system may comprise an elongated funnel 36 having two side walls 36a, 36b tapering toward the dielectric tubular portion 4 and extending along a longitudinal axis. As shown, the funnel may be disposed between the fan 34 and the dielectric tubular portion 4. Also, multiple fans (not shown) may be disposed along the wide end of the funnel, e.g., side-by-side along the longitudinal axis. For example, the cooling system may be configured such that the fan forces a cooling gas flow into the wide end of the funnel 36, where the gas flow exits the narrow end of the funnel and impinges on one side of the tubular portion in a direction perpendicular to the longitudinal axis. For example, the wide end of the funnel 36 may be at least 1.2 times (20%), 1.5 times (50%), 2 times (2x), or more wider than the narrow end. Thus, a relatively strong gas flow may be readily generated at or near the tubular portion 4. For example, a gas flow can be induced from said one side of the tubular portion to the other (e.g., as a split gas flow) around the tube to extract heat from the ionization chamber 2. Also, instead of or in addition to the gas flows shown, other or further gas flows can be generated, for example, by a fan that drives the gas flow in a direction away from the ionization chamber 2. For example, one or more fans can be positioned to force a gas flow into and / or out of the housing 42.

[0034] In the example of Figure 2A, the plasma source 1 includes a housing 42, for example a metallic housing that is shielded from electromagnetic radiation. In this example, the ionization chamber 2, the high voltage source 22, and at least a portion of the forced gas cooling system 32 are disposed within the housing 42. In this particular example, the fan 34 forms part of the housing 42. In Figure 2A, the housing 42 is shown as transparent for clarity.

[0035] The plasma source 1 described so far can be used in a method 200 for generating a sterilizing and / or sterilizing gas mixture as follows. See also FIG. 4. A gas or gas mixture, such as air, e.g. air with a specific humidity, is fed into the ionization chamber 2 via the inlet 12 (step 202). In the ionization chamber 2, a plasma is generated by applying a high voltage difference between the first electrode 16 and the second electrode 20 (step 204). As a result, the gas or gas mixture flowing through the ionization chamber 2 is at least partially ionized, forming a sterilizing and / or sterilizing gas mixture. The sterilizing and / or sterilizing gas mixture leaves the ionization chamber via the outlet 14 (step 208).

[0036] During ionization, i.e., plasma generation, the ionization chamber is cooled (step 206) using a forced gas cooling system 32. In this example, a fan 34 generates an air flow that blows towards the ionization chamber 2. Here, the air flow is directed by a guide 36 towards the ionization chamber, for example towards the tubular portion 4. Here, the air flow is directed in a direction substantially perpendicular to the longitudinal axis of the tubular portion 4, here perpendicular to the longitudinal axis of the tubular portion 4. In principle, the forced gas cooling system can also be arranged to force a cooling gas flow through the dielectric tubular portion in a direction substantially parallel to the longitudinal axis of the tubular portion. However, blowing gas perpendicular to the tubular portion can dissipate heat faster and more uniformly. For example, a heated gas flowing in a perpendicular direction can leave the vicinity of the tubular portion more quickly than if the heated gas flowed along the length of the tubular portion. For example, a gas flowing in a perpendicular direction can have a substantially uniform temperature along the length of the tubular portion compared to a gas that heats up as it flows along the length of the tubular portion. Forced gas cooling can be used to maintain a substantially constant temperature of the ionization chamber and the gas or gas mixture therein, which has been found to have a beneficial effect on the quality of the sterilizing and / or disinfecting gas mixture.

[0037] It has also been found that a relatively short length of the first electrical conductor 28 providing the high voltage appears to beneficially affect the quality of the sterilizing and / or sterilizing gas mixture. Although not fully understood, it is believed that a relatively short electrical conductor reduces fluctuations in the supply voltage and thus reduces temperature fluctuations. The reduced temperature fluctuations may contribute to a more stable production of the desired sterilizing and sterilizing components in the gas mixture. Here, a relatively short length is a length of 50 cm or less, preferably 30 cm or less, and more preferably 20 cm or less. It will be understood that the first output terminal 24 may be directly connected to the first electrode 16. In this case, the length of the first electrical conductor 28 is zero. The second output terminal 26 may also be directly connected to the second electrode 20. In this case, the length of the second electrical conductor 30 is zero.

[0038] With the plasma generated, a forced gas cooling system 32 cools the ionization chamber 2, for example by cooling the outside of the tubular portion 4, thereby creating a temperature gradient within the ionization chamber 2.

[0039] The forced gas cooling system 32 may include a temperature control system 37 for controlling the temperature of the plasma and / or the ionization chamber 2 and / or the tubular portion 4. The temperature control system 37 may include a temperature sensor 39. The sensor 39 may be mounted, for example, inside the ionization chamber 2, on the inner or outer surface of the tubular portion, or outside the ionization chamber 2 near the tubular portion 4. Alternatively or additionally, the temperature sensor 39 may be located in the gas flow output from the ionization chamber 2. Controlling the temperature of the plasma, for example by controlling the temperature of the ionization chamber 2 or the tubular portion 4, may provide two advantages. Controlling the temperature of the plasma has a favorable effect on the temperature stability of the plasma source. Also, by adjusting the set point of the controlled temperature of the plasma, the quality of the sterilization and / or sterilization gas mixture, for example the composition of the sterilization and / or sterilization gas mixture, may be selected.

[0040] The forced gas cooling system 32 may include a detection system 38 for detecting a malfunction of the cooling system. The detection system 38 may be configured to shut off or reduce the power of the high voltage source 22 if a malfunction of the cooling system 32 is detected. In this way, overheating of the plasma source may be avoided in case of a malfunction of the cooling system 32. The detection system may include a detector 40 for detecting a malfunction of the cooling system 32. The detector 40 may include a gas flow sensor for monitoring the flow of the cooling gas. The detector 40 may include a current sensor for detecting the motor current of the fan 34. The detector 40 may include a temperature sensor, e.g., a sensor 39, for detecting the temperature of the plasma source 1, e.g., the temperature of the ionization chamber 2, the tubular portion 4 and / or the housing 42. The detector 40 may be, for example, a thermal switch.

[0041] Figure 3 shows an example of a sterilization apparatus 100 for sterilizing medical instruments 102, such as dental instruments. The sterilization apparatus 100 includes a plasma source 1 as described with reference to Figures 1 and 2A-2B. The sterilization apparatus 100 includes a sterilization chamber 104. A germicidal and / or sterilizing gas mixture flows from the plasma source 1 into the sterilization chamber 104 and is directed toward instruments 102 placed within the sterilization chamber 104.

[0042] The present invention is described herein with reference to specific examples of each embodiment of the present invention. However, it will be apparent that various modifications and variations are possible in the present invention without departing from the essence of the present invention. For purposes of clarity and brevity of description, features are described herein as part of the same or separate embodiments, but alternative embodiments having all or some combination of the features described in these separate embodiments are also envisioned.

[0043] In the example of Fig. 1, the first electrode is connected to a high voltage and the second electrode is connected to a ground terminal. It will be understood that it is also possible for the first electrode to be connected to a ground terminal and the second electrode to be connected to a high voltage. It is also possible for both the first electrode and the second electrode to be connected to a high voltage, for example one to a positive high voltage and the other to a negative high voltage. Preferably, the electrical conductor supplying the high voltage to the first electrode or the second electrode has a relatively short length of 50 cm or less, preferably 30 cm or less, more preferably 20 cm or less.

[0044] Preferably, the high voltage source described herein is configured to generate high voltage and / or high current within a relatively short time span, e.g., less than 50 milliseconds, preferably less than 20 milliseconds, after start-up (e.g., start-up at zero volts / amperes). More preferably, the voltage and / or current is ramped up after start-up, with an initial overshoot, e.g., at least 10% above the nominal operating voltage, to initiate plasma generation. For example, the high voltage source is powered by a power supply adapted to allow such rapid start-up. The inventors have found that these settings may result in a more stable and / or reliable plasma.

[0045] Although for clarity and concise description, features are described herein as part of the same or separate embodiments, the scope of the invention may include embodiments having all or a combination of any of the described features.

[0046] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other features or steps than those listed in the claim. Moreover, the words "a" and "an" shall not be construed as being limited to "only one" but are used to mean "at least one" and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. 1. A plasma source for generating a sterilizing and / or disinfecting gas mixture, comprising: an ionization chamber, the ionization chamber comprising: a dielectric tubular portion; an inlet for supplying a gas or mixture of gases into the ionization chamber; an outlet for discharging the sterilizing and / or disinfecting gas mixture outside the ionization chamber; a first electrode disposed inside the dielectric tubular portion; a second electrode disposed outside the dielectric tubular portion; having The plasma source further comprises: a high voltage source having a high voltage output terminal, the high voltage source having an electrical conductor connecting the output terminal to the first electrode or the second electrode; a forced gas cooling system for cooling the ionization chamber; Equipped with the forced gas cooling system is positioned to force a flow of cooling gas through the dielectric tubular portion in a direction substantially perpendicular to a longitudinal axis of the tubular portion; the forced gas cooling system comprises an elongated funnel having two side walls extending along the longitudinal axis and tapering toward the dielectric tubular portion; the funnel is disposed between at least one fan and the dielectric tubular portion and configured to force the cooling gas flow by the fan into the funnel toward a narrow end of the funnel where the two side walls approach each other; the narrow end is disposed adjacent the dielectric tubular portion and has an elongated exit opening extending along the longitudinal axis; the outlet opening is disposed facing the dielectric tubular portion and configured to force the cooling gas flow exiting the outlet opening to flow to one side of the tubular portion in a direction substantially perpendicular to the longitudinal axis. Plasma source.

2. 10. The plasma source of claim 1, wherein the forced gas cooling system includes a detector for detecting a malfunction of the forced gas cooling system and configured to shut off or reduce power to the high voltage supply when a malfunction is detected.

3. 3. A plasma source according to claim 1 or 2, wherein the length of the electrical conductor is less than 50 cm, preferably less than 30 cm, more preferably less than 20 cm.

4. The plasma source of claim 1 , wherein the second electrode is a conductive layer deposited on an outer surface of the dielectric tubular portion.

5. The plasma source of claim 1 , wherein the second electrode is connected to ground.

6. a first end cap including the inlet and closing the dielectric tubular portion at a first end; a second end cap including the outlet port and closing the dielectric tubular portion at a second end opposite the first end; 6. The plasma source of claim 1 , comprising:

7. The plasma source of claim 6 , wherein the first end cap and / or the second end cap are constructed of an electrically insulating material.

8. The plasma source of claim 1 , wherein the dielectric tubular portion comprises a wall of quartz or glass, such as borosilicate glass.

9. 9. The plasma source of claim 1, further comprising a housing, the ionization chamber, the high voltage source, and at least a portion of the forced gas cooling system being disposed within the housing.

10. the high voltage source is configured to generate a high voltage and / or a high current within a relatively short time span of less than 50 milliseconds, preferably less than 20 milliseconds, after activation; the high voltage source is configured to ramp up a high voltage and / or a high current with an initial overshoot at start-up to initiate plasma generation, and then ramp down to a nominal operating value to maintain plasma generation; The plasma source of claim 1 , wherein the initial overshoot is at least 10% higher than the nominal operating value.

11. A sterilisation device for sterilising medical instruments comprising a plasma source according to any one of claims 1 to 10.

12. 1. A method for producing a sterilizing and / or disinfecting gas mixture comprising: supplying a gas or gas mixture through a dielectric tubular portion having a first electrode disposed inside the dielectric tubular portion and a second electrode disposed outside the dielectric tubular portion; applying a high voltage difference between the first electrode and the second electrode; cooling the dielectric tubular portion using a forced gas cooling system; Equipped with the forced gas cooling system comprises an elongated funnel having two side walls tapering toward the dielectric tubular portion and extending along a longitudinal axis of the dielectric tubular portion; at least one fan directs a flow of cooling gas into the wide end of the funnel such that a concentrated and / or accelerating flow of cooling gas exits the narrow end of the funnel and impinges on one side of the tubular portion in a direction perpendicular to the longitudinal axis, the impinging gas then flows around the tubular portion to another side of the tubular portion opposite the one side, and then leaves the tubular portion in a perpendicular direction.

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