FIELD OF THE INVENTION

The invention relates to a plasma source apparatus. Furthermore, the invention relates to a method of treating an exhaust gas from a combustion engine and to a method of treating a surface.

BACKGROUND OF THE INVENTION

Different prior art microwave operated plasma sources are known. For example, it is known to excite a plasma by a propagation of electromagnetic surface waves, i.e. a surface- wave-sustained discharge. A further example is the production of a plasma by superimposing a static magnetic field and a high-frequency electromagnetic field at an electron cyclotron resonance frequency. A further example is a microwave coaxial plasma source or a waveguide- based plasma source. Operations with noble gases, at low pressure as well as high pressure are known. Also, the range of applications of plasma sources is large, e.g. comprising surface cleaning, surface modification, surface activation, plasma vapor deposition, the semiconductor industry, liquid treatment, air purification, medicine, hygiene etc.

However, improved plasma apparatus are still required, particularly regarding specific applications - e.g. with respect to plasma sources used with ambient air at atmospheric temperature and pressure or with exhaust gases. Improvements are also still desired regarding process speeds and energy efficiency.

More specifically, there is still a need for an efficient plasma source apparatus for use with ambient air and/or exhaust gas, particularly for a plasma source apparatus which provides for an advantageous air purification, e.g. by removing unwanted or hazardous molecules from the ambient air and/or the exhaust gas. There is also still a need for an efficient, versatile and cost effective plasma apparatus for surface treatments, e.g. for cleaning and/or temporary functionalization of surfaces (ideally, without inflicting permanent damage on sensitive surfaces).

SUMMARY According to a first aspect, the application discloses a plasma source apparatus, comprising a first electrode, a second electrode, a voltage source, and a flow channel. The voltage source is coupled to at least one of the first electrode and the second electrode and configured to generate a potential difference between the first electrode and the second electrode. The flow channel is configured to accommodate a flow of gas therethrough. The second electrode at least partially surrounds the first electrode, wherein the flow of gas passes between the first electrode and the second electrode. The second electrode defines a helical shape, wherein the first electrode is at least partially disposed within the helical shape. Preferably, the first electrode is at least partially disposed within the flow channel. Preferably, the first electrode has a pin-like shape defining a longitudinal axis.

The plasma source apparatus may be configured to generate a low temperature plasma. As described in more detail hereinafter, it is believed that such low temperature plasma may be advantageously used for a treatment of temperature sensitive materials. The apparatus may be configured to be operated at atmospheric pressure in air or other gases.

The plasma source apparatus allows using distributed electrons in gas flows to dissociate unwanted trace gases. This is a fast process taking place in a millisecond range. The apparatus is suited to generate a homogeneous plasma distribution by a gentle, efficient activation process. The design of the apparatus allows for self-ignition of the plasma.

The helical shape may define a cylinder, for example a circular cylinder, where a symmetry axis of the cylinder coincides with the longitudinal axis defined by the pin-like shape of the first electrode. This allows for a particular efficient plasma generation. A main axis of the flow channel may as well coincide with the longitudinal axis.

The first electrode may have a diameter normal to the longitudinal axis which is at least 0.4 mm, or at least 1.0 mm. The diameter may be 3.5 mm or less, or 2.5 mm or less, or 2.0 mm or less.

The second electrode may comprise a conductor (in particular, a conductor forming the helical shape of the second electrode) having a width and/or thickness. The width and/or thickness may be at least 0.1 mm, or at least 0.2 mm. Alternatively or additionally, the width and/or thickness may be 0.5 mm or less, or 0.3 mm or less. The conductor may be a wire. A wire diameter may be at least 0.1 mm, or at least 0.2 mm. The wire diameter may be 0.5 mm or less, or 0.3 mm or less.

The second electrode may have a number of windings which is at least 2.5, or at least 3, or at least 3.5. The number of windings may be 5 or less, or 4.5 or less, or 4 or less. The helical shape of the second electrode may have a pitch distance which is at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm. The pitch distance may be 10 mm or less, or 9 mm or less, or 8 mm or less, or 7 mm or less.

The second electrode may be positioned inside the flow channel and/or at an inner surface of the flow channel. The second electrode may be fixed to an inner peripheral wall surface of the flow channel. An extension of the second electrode measured along the longitudinal axis may be greater, equal, or less than an extension of the first electrode along the longitudinal axis. It is preferable that the extension of the second electrode is greater than the first electrode.

The first electrode and/or the second electrode may be made of a material comprising tungsten and/or molybdenum; preferably, the first electrode and/or the second electrode are made of tungsten and/or molybdenum. These materials are advantageous, because they exhibit particularly suitable electrical conductivity and a high melting point. Alternatively, another material having a good electrical conductivity and a high melting point may be used.

The flow channel may comprise a glass tube, preferably a quartz glass tube. The flow channel may be a glass tube, preferably a quartz glass tube. Quartz glass is advantageous since it allows for a long-term stability of the flow channel, high temperature resistance and particularly good material quality.

The flow channel may have an inner diameter which is at least 3 mm or at least 5 mm. The inner diameter of the flow channel may be 10 cm or less or 5 cm or less. The flow channel may have a length measured along the longitudinal axis which is at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 50 mm. The length may be 80 mm or less, or 70 mm or less, or 60 mm or less.

The voltage source may comprise an alternating voltage generator. This is preferred since unlike direct current generating a plasma with the alternating current does not require large electric potentials. Thus, the alternating voltage is advantageous regarding both, efficiency and safety of the apparatus. The voltage source may be configured to generate an alternating voltage having a frequency of 2.45 GHz ± 500MHz, preferably ± 100 MHz and/or a peak power having a value of at least 30 W, or at least 35 W, or at least 40 W, or at least 45 W, or at least 50 W. The peak power may be 1100 W or less, 700 W or less, or 500 W or less.

Preferably, the frequency and/or the peak power of the voltage source is adjustable. For example, the plasma source apparatus may comprise a controller allowing a user to adjust the frequency and/or the peak power.

The voltage source may be electrically coupled to at least one of the first electrode and the second electrode. The coupling may include a mechanical coupling or an inductive coupling. For example, voltage source may be coupled (inductively or electrically contacted by a conductor) with the second electrode but not with the first electrode. In this case, the first electrode may be capacitively and/or inductively coupled with the second electrode without being otherwise coupled to the voltage source. Alternatively, both the first and the second electrode may be coupled to the voltage source (e.g. inductively or electrically contacted by a conductor).

In other words, one of the first and second electrodes may be configured as an electrically floating electrode. In particular, the second electrode may be configured to be an electrically floating electrode. When configured as such floating electrode, the electrode is not connected to ground and not connected to the voltage source.

The voltage source may be configured to operate in a continuous mode. Alternatively or additionally, the voltage source may be configured to operate in a pulsed mode. A pulsed operation is advantageous, because it allows for a quick activation of a surface material. A pulsed operation is advantageous also, because it allows for temperature control of the plasma source which is especially necessary for temperature sensitive material treatment. A pulsed mode is particularly suited for treating certain materials which are difficult to activate, such as for example polytetrafluorethylene (PTFE), often simply called Teflon. Moreover, a pulsed operation may also be advantageous for reducing toxic molecules in a gas, e.g. in an exhaust gas.

The pulses may be described by a pulse duration (time between a start of a pulse and an end of the pulse), a repetition period of the pulses (or “pulse repetition period”, hereinafter also briefly “period”), the repetition rate of the pulses (which equals the reciprocal of the period), and the duty cycle of the pulses (which equals the ratio between the pulse duration and the period). During each pulse, a number of voltage changes (e.g., a voltage change having a sinusoidal wave, with a certain number of periods) may be applied. The number depends on the pulse duration and the frequency of the voltage changes provided by the voltage source. Between pulses, the voltage changes may be interrupted. Thereby, each pulse may be described by a power signal having a generally rectangular shape (e.g., a first approximation rectangular shape).

A pulse duration may be at least 10 ns, or at least 20 ns, or at least 50 ns, or at least 100 ns, or at least 1 ps, or at least 10 ps, or at least 100 ps. The pulse duration may be 1 s or less, or 100 ms or less, or 10 ms or less, or 1 ms or less, or 1 ps or less, or 0.1 ps or less. The plasma source apparatus may comprise a controller for repeatedly operating the voltage source at a determined pulse duration. Such controller may also allow the user to adjust the pulse duration. An adjustability of the pulse duration advantageously allows for a “fine tuning” of the plasma source apparatus.

As described hereinafter, without wanting to be bound by theory, the inventors have found that the use of an appropriate pulse duration, which may depend on the type of gas being treated and the specific configuration of the plasma source apparatus, is helpful for achieving an advantageous treatment of gas, in particular for reducing the concentration of one or more undesired molecules in the gas being treated (e.g., toxic and/or environmentally unfriendly molecules in an exhaust gas), while minimizing the creation of the same or other undesired molecules (e.g., the creation of the same or other toxic and/or environmentally unfriendly molecules in the exhaust gas).

A repetition rate of the pulses may be 2 MHz or less. The repetition rate of the pulses may be 200 kHz or more. Preferably, the repetition rate is adjustable. For example, the plasma source apparatus may comprise a controller allowing a user to adjust the repetition rate. This allows for a fine tuning of the apparatus with different repetition rates.

A duty cycle may be at least 0.5%, or at least 1%, or at least 5%, or at least 10%. The duty cycle may be 100% or less, or 90% or less, or 80% or less, or 70% or less, or 60% or less. Preferably, the duty cycle is adjustable. For example, the controller may allow the user to adjust the duty cycle.

A rise time of the voltage source may be at least 1 ns, or at least 2 ns, or at least 5 ns. The rise time may be 1 ms or less, or 100 ps or less, or 10 ps or less. Preferably, the rise time is adjustable. For example, the controller may allow the user to adjust the rise time. As described hereinafter, without wanting to be bound by theory, the inventors have found that the use of an appropriate rise time, which may depend on the type of gas being treated and the specific configuration of the plasma source apparatus, is helpful for achieving an advantageous treatment of gas, in particular for reducing the concentration of one or more undesired molecules in the gas being treated (e.g., toxic and/or environmentally unfriendly molecules in an exhaust gas), while minimizing the creation of the same or other undesired molecules (e.g., the creation of the same or other toxic and/or environmentally unfriendly molecules in the exhaust gas). As known in the art, the rise time is defined as the time required for a pulse to rise from 10 per cent to 90 per cent of its steady value.

The above parameters peak power, rise time, pulse width, period, and duty cycle preferably should be set in dependence of the intended use of the plasma source apparatus. For example, regarding an exhaust gas treatment, it is believed, without wanting to be bound theory, that a shorter rise time may be better for the destruction of toxic gases. Moreover, it is believed, again without wanting to be bound theory, that a shorter period may be better for the destruction of toxic gases. In this respect, a pulse duration of less than 500 ns, less than 250 ns, preferably less than 100 ns, and more preferably less than 50 ns is believed to be advantageous. Such pulses preferably have a rise time of less than 10 ns, less than 5 ns, or less than 2 ns. Furthermore, it is believed, again without wanting to be bound theory, that a higher peak power may provide a more efficient gas treatment. For the treatment of exhaust gas a peak power of at least 0,1 kW, at least 0,5 kW or at least 1 kW is preferred. However, the peak power could also be higher, such as at least 5 kW or at least 10 kW. The duty cycle for such gas treatment is preferably less than 50%, more preferably less than 40%, in particular when using a peak power of between 0,5 kW and 5 kW. For example, the duty cycle could be less than 40% or less than 30% when using a peak power of 0,5 kW to 1,5 kW. One example would be a peak power of 1.1 kW with a duty cycle of 10% to 30%, a rise time of less than 10 ns and a pulse duration of less than 100 ns, preferably less than 50 ns.

For example, regarding a surface treatment, it is believed, without wanting to be bound theory, that a peak power of at least 0,1 kW, at least 0,5 kW or at least 1 kW is preferred. However, the peak power could also be higher, such as at least 5 kW or at least 10 kW. The duty cycle for such gas treatment is preferably less than 50%, more preferably less than 40%, in particular when using a peak power of between 0,5 kW and 5 kW. For example, the duty cycle could be less than 40% or less than 30% when using a peak power of 0,5 kW to 1,5 kW. One example would be a peak power of 1.1 kW with a duty cycle of 10% to 30%, a rise time of less than 10 ns and a pulse duration of less than 100 ms, preferably less than 50 ms. Short pulse durations, such as in the range of less than 250 ns or less than 100 ns, may be employed when using higher peak powers, such as at least 0.1 MW, at least 0.5 MW or at least 1 MW.

As indicated above, the plasma source apparatus preferably comprises a controller configured to control the voltage source. The configuration may be such that the user can adjust via the controller the frequency and/or the peak power and/or the pulse duration and/or the repetition rate and/or the duty cycle and/or the rise time of the voltage source.

The plasma source apparatus may further comprise a coaxial adaptor. Such coaxial adaptor may be configured for a direct connection to a standard coax adaptor, for example a 7/16 type, a N-type, or TNC, depending on the running power of the electrode.

The first electrode may be electrically connected to a metal core of the coaxial adaptor. For this purpose, the plasma source apparatus may further comprise a metal adaptor configured to electrically connect said first electrode to said metal core of the coaxial adaptor. The metal adaptor may have an elongate opening, e.g. a bore extending along the longitudinal axis, which is configured to receive the first electrode in such a way that the first electrode is held by the metal adaptor. Preferably, the design is such that the electrode can be held by the metal adaptor at several different positions along the longitudinal axis. The metal adaptor may be rotationally symmetric with respect to the longitudinal axis.

The metal adaptor is advantageous since it allows for a suitable flexibility regarding a fine tuning of the position of the first electrode along the longitudinal axis. The position of the first electrode vis-a-vis the second electrode influences the impedance matching of the apparatus, the formation of the plasma and the initial ignition.

The metal adaptor may have an outer diameter of at least 2 mm, at least 3 mm, at least 4 mm, or at least 4.5 mm. The outer diameter may be 10 mm or less, or 5 mm or less. The first electrode may protrude from the metal adaptor by a section of the first electrode having a length measured along the longitudinal axis of at least 8 mm, or at least 9 mm, or at least 10 mm, or at least 12 mm, or at least 15 mm, or at least 18 mm. The length of the protruding section may be 50 mm or less, 30 mm or less, or 28 mm or less, or 25 mm or less, or 23 mm or less, or 20 mm or less.

The plasma source apparatus may further comprise a shielding housing surrounding the first electrode and/or the second electrode. Preferably, the shielding housing is conductive. For example, the shielding housing may be made from a metal. The shielding housing may have an inner surface contacting an outer surface of the flow channel. The flow channel may have an open end which protrudes from the shielding housing along the longitudinal axis. The plasma source apparatus may further comprise a dielectric spacer disposed electrically between the first electrode and the shielding housing. The dielectric spacer may surround the metal adaptor circumferentially. The dielectric spacer may be made of a material comprising PTFE and/or a ceramic material, including 3D (additive manufacturing) printable ceramic materials, and/or quartz glass and/or a silicon based semi-conductive material. PTFE is advantageous, because it is cost effective and easy to handle. Ceramic allows for a particularly stable and long-term functionality. The dielectric spacer may have an outer diameter of at least 10 mm or at least 15 mm. The outer diameter may be 50 mm or less, or 20 mm or less.

Preferably, the plasma source apparatus is configured to electrically match 25 Ohm to 95 Ohm, more preferably 40 Ohm to 85 Ohm, e.g. 50 Ohm or 75 Ohm. This allows for a particularly effective transmission of a microwave energy by the electrodes. A good matching is particularly advantageous regarding the self-ignition of the plasma when using low power, e.g. 30 W to 50 W.

The plasma source apparatus may further comprise a gas inlet configured to allow the gas to flow into the flow channel. The gas may be or may include ambient air and/or nitrogen and/or oxygen and/or a noble gas and/or a CO2 based gas mixture. The gas may include impurities of NO and/or CO2 and/or O2 and/or ME and/or a hydrocarbon. The gas may be an exhaust gas from a combustion engine. Generally, using ambient air allows for a reduction of the running costs of the plasma source apparatus.

The plasma source apparatus may further comprise a gas source providing the gas. The configuration may be such that the flow of gas and/or a flow of the gas from the gas source into the flow channel is adjustable, preferably adjustable within a range having a lower boundary value of 1 L/min, or 2 L/min, or 5 L/min, or 10 L/min, or 20 L/min, or 30 L/min, or 50 L/min. An upper boundary value of the range may be 500 L/min, or 450 L/min, or 400 L/min, or 350 L/min, or 300 L/min, or 200 L/min.

The configuration may be such that an average speed of the flow of gas is adjustable within a range having a lower boundary value of 5 m/s, or 10 m/s, or 15 m/s, or 20 m/s, or 30 m/s. An upper boundary value of the range may be 350 m/s, or 300 m/s, or 250 m/s, or 200 m/s, or 150 m/s, or 100 m/s, or 50 m/s, or 20 m/s, or 15 m/s. For example, the plasma source apparatus described herein may be particularly advantageous for use at an average speed between 5 m/s and 15 m/s.

The plasma source apparatus may be configured for a temperature of the gas being within a range having a lower boundary value of -40°C, or -20°C, or 0°C, or 10°C, or 20°C, or 50°C. An upper boundary value of the temperature range may be 1100°C, or 900°C, or 800°C, or 500°C, or 300°C. The plasma source apparatus may be configured for a pressure of the gas being within a range between 80 kPa and 120 kPa.

The plasma source apparatus may be water cooled or air cooled. The plasma source apparatus may be used for surface sputtering when using a coating material as the first electrode.

The plasma source apparatus disclosed herein is also advantageous in that it may be scaled up or down (from a miniature electrode design for portable applications to a high power model for quick surface activation/modification).

According to a second aspect, the application discloses a plasma source apparatus, for example for the treatment of a gas (e.g. an exhaust gas, e.g. from a combustion engine), the apparatus comprising a first electrode, a second electrode, a voltage source, a controller controlling the voltage source, and a flow channel configured to accommodate a flow of gas therethrough, wherein said flow of gas passes between said first electrode and said second electrode. The voltage source is coupled to at least one of the first electrode and the second electrode. The controller preferably controls the voltage source to generate a plurality of potential difference pulses between the first electrode and the second electrode, wherein a pulse duration of the pulses preferably is less than 250 times, less than 150 times, less than 50 times, or less than 10 times the rise time of the respective pulse. The controller preferably controls the voltage source to generate a plurality of potential difference pulses between the first electrode and the second electrode, wherein the pulse duration is 500 ns or less, 250 ns or less, 100 ns or less, or 50 ns or less. As described hereinbelow, it is believed that short pulses may reduce the amount of toxic molecules generated by the plasma. The pulse duration preferably is at least 1.5 times the rise time, more preferably, at least 2.0 times the rise time.

Preferably, the rise time is less than 100 ns, less than 10 ns, or less than 5 ns.

The peak power of the pulses preferably is at least 30 W, or at least 35 W, or at least 40 W, or at least 45 W, or at least 50 W. As described hereinbelow, it is believed that such peak powers may be helpful for destroying toxic molecules contained in the gas. The controller preferably generates an alternating voltage having a frequency of 2.45

GHz ± 500MHz, preferably ± 100 MHz.

As will be appreciated by the skilled reader, the features described with reference to the first aspect of the disclosure above are equally relevant to the apparatus according to the second aspect. According to a further aspect, the application discloses a method of treating an exhaust gas from a combustion engine. The method comprises the following steps: (i) providing a combustion engine, producing an exhaust gas; (ii) providing a plasma source apparatus according to the present application; (iii) connecting the plasma source apparatus to the combustion engine; (iv) generating the flow of gas passing between the first electrode and the second electrode such that the flow of gas is at least partly a flow of the exhaust gas; and (v) generating the potential difference between the first electrode and the second electrode by activating the voltage source. According to a further aspect, the application discloses a method of treating a surface. The method comprises the following steps: (i) providing a plasma source apparatus according to the present application; (ii) generating the flow of gas passing between the first electrode and the second electrode; (iii) generating the potential difference between the first electrode and the second electrode by activating the voltage source; and (iv) directing the flow of gas leaving the flow channel to the surface.

Any of the two latter mentioned methods may further comprise the following step: operating the voltage source such that an electrical discharge between the first electrode and the second electrode is maintained within the flow channel for at least 1 s, or at least 5 s, or at least 10 s, or at least 30 s, or at least 1 min, or at least 5 min.

As will be appreciated by the skilled reader, the features described with reference to the first and second aspects of the disclosure above are equally relevant to these methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the application will be explained in more detail with reference to preferred examples which are illustrated in the attached drawings, in which:

Fig. l is a schematic cross-sectional view of a plasma source apparatus according to the present application.

Fig. 2 is a schematic perspective view of the apparatus.

Fig. 3 is a schematic illustration of a system for exhaust treatment, comprising a plasma source apparatus according to the application.

Fig. 4 is a further illustration of a plasma source apparatus according to the application.

Fig. 5 is a diagram illustrating an effect of a pulse width on a net reduction or production of NOx.

Fig. 6 is a diagram illustrating a surface activation of different materials.

Fig. 7 illustrates a surface activation of PTFE, regarding a water contact angle and ageing effects after treatment.

Fig. 8 illustrates a surface activation of PTFE, regarding a surface free energy and ageing effects after treatment.

Fig. 9 shows a picture of an oscilloscope, illustrating temporal dynamics of a forward power (lower line) and a reflected power (upper line) in case of a wide pulse width.

Fig. 10 shows a corresponding picture in case of a shorter pulse width. Fig. 11 shows a corresponding picture in case of a very short pulse width.

Fig. 12 shows a schematic diagram, illustrating some pulse parameters.

DETAILED DESCRIPTION OF EXAMPLES

Examples according to the present application will be described with reference to the drawings in which identical or similar reference signs designate identical or similar elements. The features of the examples may be combined with each other, unless specifically noted otherwise.

Fig. 1 shows a schematic cross section of a plasma source apparatus according to the application. The plasma source apparatus is suitable e.g. for a treatment of an exhaust gas or for a surface treatment.

The apparatus comprises a first electrode 3, a second electrode 4, and a voltage source 10 which is coupled to one of the electrodes 3, 4, here exemplarily to the first electrode 3 and to the second electrode 4. The voltage source 10 is configured to generate a potential difference between the first electrode 3 and the second electrode 4, such that a plasma is generated between the electrodes 3, 4. Thus, a plasma between the electrodes 3, 4 can be generated by the plasma source apparatus via an activation of the voltage source 10. The design of the apparatus is such that the plasma can be generated by self-ignition.

The first electrode 3 has a pin-like shape defining a longitudinal axis L. The second electrode 4 defines a helical shape, wherein the first electrode 3 is at least partially disposed within the helical shape. The helical shape defines a circular cylinder, where the longitudinal axis L coincides with a longitudinal axis of the cylinder. The electrodes 3, 4 preferably are made from a tungsten-based or molybdenum -based material.

The first electrode 3 preferably has a diameter measured perpendicularly to the longitudinal axis L between 0.5 mm and 3.0 mm. The second electrode 4 preferably is a wire having a thickness between 0.1 mm and 0.5 mm. A pitch distance p4 of the helical shape of the second electrode 4 is between 3 mm and 10 mm. A number of windings of the helical shape of the second electrode 4 preferably is between 2.5 and 5.

Further, the plasma source apparatus comprises a flow channel 2 configured to accommodate a flow of gas therethrough. The flow channel 2 is designed to confine the plasma and/or to support a jet formation of the plasma. The first electrode 3 is at least partially disposed within the flow channel 2 and the second electrode 4 at least partially surrounds the first electrode 3. The flow channel 2 preferably is a tube having a circular cylindrical shape. An inner surface of the flow channel 2 preferably has a circular cylindrical shape. Preferably, the circular cylindrical shape of the inner surface of the flow channel 2 generally corresponds to the cylinder defined by the helical shape of the second electrode 4. An inner diameter d2 of the flow channel 2 preferably is between 3 mm and 15 mm. In the example shown, the flow channel 2 is a quartz glass tube. This material is particularly suited regarding long term stability and high temperature resistance.

In the example shown, the second electrode 4 is fixed to the inside surface of the flow channel 2 along a section thereof defining the helical shape.

A main axis of the flow channel 2 coincides with the longitudinal axis L. A length l2 of the flow channel 2 measured along the longitudinal axis L may be freely chosen, however, preferably it is greater than a length of the second electrode 4 measured along the longitudinal axis L.

The flow of gas passes between the first electrode 3 and the second electrode 4. The gas, here also called working gas, can be for example ambient air, Nitrogen, Oxygen, a noble gas or a gas mixture with impurities of NO, CO2, O2, NH ₃ and/or one or more hydrocarbons. The gas mixture may be a CO2 based gas mixture including impurities of NO, NH ₃ and/or one or more hydrocarbons, e.g. when an exhaust gas from a combustion engine is processed.

The voltage source 10 is configured to generate the plasma under an atmospheric pressure at a central frequency of 2.45 GHz ± 100 MHz with a peak power in a range of 30 W to 1100 W. The voltage source 10 is designed to be operated in a continuous mode (CW) and/or in a pulsed mode.

The voltage source 10 preferably comprises a controller (not shown) configured to allow a user to adjust the central frequency, and/or the peak power, and/or a pulse duration, and/or a repetition rate, and/or a duty cycle, and/or a rise time.

Further, the plasma source apparatus comprises a shielding housing 1, e.g. in the form of a tube for electrically shielding the electrodes 3, 4. The shielding housing 1 acts also as a cavity which confines the high frequency electromagnetic wave inside the tube or the flow channel 2. The shielding housing 1 surrounds the electrodes 3, 4 when seen in a cross section normal to the longitudinal axis L. The shielding housing 1 may have a shape which is at least to a first approximation cylindrical. The flow channel 2 preferably protrudes with an open end 9 from the shielding housing 1 along the longitudinal axis L. The configuration may be such that the gas can flow out of the flow channel 2 via the open end 9. The gas may be released into the ambient at the open end 9 or may be transferred away (e.g., via a hose or pipe connected to the open end 9) for further processing.

Further, the plasma source apparatus comprises a coaxial adaptor 8 having a metal core, wherein the first electrode 3 is electrically connected to the metal core. The coaxial adaptor 8 can be selected individually as long as the power rate is sufficient for the electrode system.

Further, the plasma source apparatus comprises a metal adaptor 6 configured to electrically connect the first electrode 3 to the metal core of the coaxial adaptor 8. The metal adaptor 6 may have a diameter measured perpendicular to the longitudinal axis L of 5 mm ± 1 mm. The metal adaptor 6 has an elongate opening or bore formed along the longitudinal axis L and designed for receiving a portion of the first electrode 3 such that the first electrode 3 is held by the metal adaptor 6. Preferably, the configuration is such that a position of the first electrode 3 along the longitudinal axis L in which the first electrode 3 is held by the metal adaptor 6 can be adjusted (e.g., stepless), preferably within a range along the longitudinal axis L of at least 10 mm or at least 20 mm. This provides flexibility for fine tuning the position of the first electrode 3 with respect to the second electrode 4. Adjusting the position of the first electrode 3 allows for an impedance matching of the plasma source apparatus, and may influence jet formation as well as initial ignition. For example, the first electrode 3 may protrude from the metal adaptor 6 along the longitudinal axis L by a section of the first electrode 3 having a length of between 15 mm and 30 mm.

The metal adaptor 6 has an outer shape defining a circular cylinder having a symmetry axis coinciding with the longitudinal axis L. Preferably, an outer diameter of the metal adaptor 6 is about 5 mm, e.g. 5 mm ± 1 mm. However, it will be appreciated by the skilled reader that also other shapes and dimensions may be employed.

Further, the plasma source apparatus comprises a dielectric spacer 5 surrounding the metal adaptor 6 circumferentially. Preferably, the configuration is such that the metal adaptor 6 directly contacts an elongate opening or inner bore of the dielectric spacer 5 extending along the longitudinal axis L. The dielectric spacer 5 is configured to maintain a space between the metal shielding housing 1 and the metal adaptor 6. The dielectric spacer 5 can be made, e.g., of PTFE. This allows for an inexpensive manufacturing. For a more stable and long-term solution, a ceramic-like material is preferred. The dielectric spacer 5 can be also made of quartz glass or a silicon based semi -conductive material. The dielectric spacer 5 may have an outer diameter of 16 mm ± 2 mm. The dielectric spacer 5 may contact directly an inner surface of the shielding housing 1.

As exemplarily illustrated in Fig. 1, the dielectric spacer 5 and the flow channel 2 may be arranged one behind the other along the longitudinal axis L, e.g. in a non-overlapping manner.

Further, the plasma source apparatus may comprise an o-ring 7 for a gas-tight seal between the shielding housing 1 and the coaxial adaptor 8. An operation of the plasma source apparatus without a flow of gas or with an unsuited gas mixture may significantly harm or even damage the electrodes 3, 4 or may produce a toxic gas. The o-ring 7 contributes to suited conditions for holding a certain requested or desired rate of the flow of gas and composition of the gas.

The apparatus may further comprise a preferably non-conductive housing 12 surrounding the shielding housing 1. Fig. 2 shows a schematic perspective view of the apparatus with such housing 12. Fig. 4 shows a plasma source apparatus providing a plasma jet 52 for treating a surface 53. As can be seen, the apparatus is fed with a working gas through a hose 51. The plasma jet 52 exits the flow channel 2 via the end 9.

Returning to Fig. 1, the plasma source apparatus comprises a gas inlet 14 configured to allow the gas to flow into the flow channel 2. The gas inlet 14 preferably is provided such that the gas flows therethrough in a direction perpendicular or oblique to the longitudinal axis L. Preferably, the gas inlet 14 is disposed within a plane parallel to the longitudinal axis L. The gas inlet 14 may lead through the dielectric spacer 5, the shielding housing 1, and/or the housing 12

The plasma source apparatus may further comprise a gas source 16 for providing the gas. For example, a combustion engine may constitute the gas source 16. As schematically illustrated in Fig. 3, a system may comprise the plasma source apparatus - here indicated by reference sign 104 - connected to a combustion engine 100 such that an exhaust gas of the combustion engine 100 is directed via the gas inlet 14 into the flow channel 2 of the plasma source apparatus 104 to generate the flow of gas. The system may further comprise a tubing 106, 108 for guiding the exhaust gas to the plasma source apparatus 104, e.g. via a catalytic convertor 102.

The configuration may be such that adequate generation of plasma is possible when the flow of gas and/or a flow of the gas from the gas source 16 into the flow channel 2 is within a range having a lower boundary value of 1 L/min, or 2 L/min, or 5 L/min, or 10 L/min, or 20 L/min, or 30 L/min, or 50 L/min. An upper boundary value may be 500 L/min, or 450 L/min, or 400 L/min, or 350 L/min, or 300 L/min, or 200 L/min. Depending on the configuration and use of the plasma source, the flow of gas may be adjustable.

Further, the configuration may be such that adequate generation of plasma is possible when an average speed of the flow of gas is within a range having a lower boundary value of 5 m/s, or 10 m/s, or 15 m/s, or 20 m/s, or 30 m/s. The range of the average flow speed may have an upper boundary value of 350 m/s, or 300 m/s, or 250 m/s, or 200 m/s, or 150 m/s, or 100 m/s, or 50 m/s, or 20 m/s, or 15 m/s. Depending on the configuration and use of the plasma source, the average speed may be adjustable.

The plasma source may be configured for a pressure of the gas being about atmospheric pressure. But the plasma source apparatus can also be configured for treatment of compressed gas. For example, the pressure of the gas at the gas inlet 14 may be between 1 bar and 150 bar. Preferably, the pressure of the gas at the gas inlet 14 is between 3 bar and 50 bar, more preferably between 5 bar and 50 bar, even more preferably between 5 bar and 10 bar.

The plasma source apparatus is preferably electrically tuned to match 50 Ohm or alternatively 75 Ohm. This allows for an efficient transmission of microwave energy into the electrode system. A good matching enables an self-ignition ignition of the plasma, particularly in case of a low power operation, i.e. for example at 30 W to 50 W. The principle to ignite the discharge is that the microwave energy is delivered into a space confined by the first and second electrode 3, 4. First ignition happens either at a free end of the first electrode 3 or at a specified point along the second electrode 4 depending on the electrical field distribution, which can be locally enhanced by a careful mechanical design of the second electrode 4 and the first electrode 3.

Keeping the length of a transmission line from the voltage source 10 to the first electrode 3 and/or to the second electrode 4 short, avoids a loss of microwave energy during transmission to the first and/or second electrode 3, 4.

The diameter of the metal adaptor 6 and the diameter and the material of the dielectric spacer 5 should be designed to match 50 Ohm (or 75 Ohm). Exemplary values regarding PTFE (having a relative dielectric constant of 2.1) are

• Diameter of the metal adaptor 6: 5mm

• Outside diameter of the dielectric spacer 5: 16 mm

• Inside diameter of the dielectric spacer 5 fits to the outside diameter of the metal adaptor 6

• Length of the section of the first electrode 3 protruding from the metal adaptor 6: 8 mm to 30 mm, preferably 15 mm to 25 mm

• Inside diameter d2 of the flow channel 2: 3 mm to 15 mm

• Pitch distance p4 of the second electrode 4: 3 mm to 10 mm

• Number of windings of the second electrode 4: 2.5 to 5

• Thickness of the second electrode 4: 0.1 mm to 0.5 mm

• Material of the first electrode 3 and/or second electrode 4: tungsten or similar with high melting temperature

• Diameter of the first electrode 3: 0.5 mm to 3 mm

The electrode system can be driven by a large range of voltage configurations, for example as follows:

• Peak power: 30 W to 1100 W

• Duty cycle: 0.5 % to 100%

• Pulse duration: 10 ns to Is (or continuous mode)

• Rise time: 5 ns to 1 ms

For an efficient treatment of target samples, for example surfaces of materials, the formation (length and perpendicular extension) of the plasma jet, a residual time of the plasma treatment, and the temperature of the plasma can be adjusted according to the material(s) to be treated.

The parameter settings may differ significantly depending on the gas mixture used and/or the voltage source is used. Next, two examples of parameter settings regarding the voltage source 10 are presented, a first example with a peak power of 1 kW and a second example with peak power of 400 W. The test gas was ambient air from an air compressor. A test showed that different sets of test parameters may give similar results on material surface activation/modification:

First example (1 kW version):

• Peak power: 1 kW

• Duty cycle: 20%

• Period: 100 ms

• Gas flow rate: 15 L/m

• Quartz glass tube inside diameter: 7 mm

Second example (400 W version):

• Peak power: 400 W

• Duty cycle: 30%

• Period: 1 s

• Gas flow rate: 10 L/m

• Quartz glass tube inside diameter: 5 mm

Next, aspects regarding a fine tuning of parameters for chemical reactions are presented.

It is possible to monitor a forward microwave (MW) power from the voltage source 10 to the first electrode 3 and a reflected power by using a high-speed diode. This allows analyzing temporal dynamics of the discharge and link them to chemical test results.

Fig. 9 shows an image of an oscilloscope, illustrating temporal dynamics of a forward power (lower line) and a reflected power (upper line), scaled to voltage, for a wide pulse which consists of three phases of the discharge: Phase 1, a fast evolution of the voltage in a time scale of 100 ns; phase 2, a transition phase with a slow voltage slope at a time scale of a few seconds; and phase 3, a stable glow discharge phase with a generally constant voltage lasting for the rest of the pulse time.

Fig. 10 shows a corresponding image, illustrating temporal dynamics of the forward and reflected power in case of a shorter pulse width. Here, the pulse only comprises phase 1 and phase 2. Fig. 11 shows a corresponding image, illustrating temporal dynamics of the forward and reflected power with a very short pulse. The pulse shows nearly only phase 1.

The fast diodes measure the forward and reflect power through the coaxial adaptor 8 connected between the voltage source 10 and the first electrode 3. The voltage presented on the oscilloscope as shown in Figures 9 to 11 is directly correlated with the power level (forward or reflected). When the applied pulse is long enough, it shows three phases of the ignition/discharge dynamics. If the pulse is shorter, only phase 1 and phase 2 are measured and for very short pulses only phase 1 of the discharge dynamics is observed. As discussed hereinafter, a varying length of phases 1 to 3 is believed to influence the composition of the gas.

Without wanting to be bound by theory, it is believed that phase 1 is an initial phase which ignites the plasma and where a dramatic drop of the voltage happens in a time scale of a few nanoseconds. In this phase, the primary electrons and most of the high energy electrons are generated so that it will mainly determine the electron energy distribution in the plasma. The hypothesis is that in this phase, most of the destruction, mainly by electron impact dissociation (EID) or by dissociative attachment (DA) of the target gas molecules will occur. The efficiency for EID and DA is determined by the energy input into the system during this phase and how fast the energy is absorbed during this phase. According to this hypothesis, the peak voltage/power of the voltage source is the main factor determining the efficiency of the apparatus for generating plasma. Moreover, the rise time may also influence the efficiency. The shorter the rise time, the faster can the energy coupled into the plasma source during the phase 1 for high electron production.

Without wanting to be bound by theory, it is believed that phase 2 is a transition phase before going into a stable discharge mode. In this phase, the discharge is trying to match the whole system and find a balanced or stable condition. One can see slight changes of the voltage. The inventors, without wanting to be bound by theory, hypothesize that in this phase the EID/DA processes may play a less important role, although one can still expect some destruction of the target gas molecules.

Without wanting to be bound by theory, it is believed that phase 3 is a stable discharge phase where one observes a stable discharge glow (volume discharge, plasma jet). Here the voltage remains nearly constant until the end of the pulse and the plasma system essentially acts like a current source. Generally speaking, a lot of heat will be produced. The inventors, without wanting to be bound by theory, hypothesize that in this phase unnecessary production of toxic gas will occur. When the plasma source apparatus is used to destroy unwanted molecules (in particular, when treating an exhaust gas), such production of toxic gas may counteract the desired effect of unwanted molecule destruction.

Generally speaking, it is frequently desired that the EID/DA processes should be as efficient as possible. Moreover, it is desired in certain applications (e.g., treatment of exhaust gas but also others) that the production of toxic molecules should be as low as possible. Based on the hypothesis that the energetic electrons are produced in phase 1 (during the ignition process) and hence the destruction process is particularly efficient, it is believed that short high intensity pulses may provide for an improved efficiency and/or reduction of toxic molecules in the gas.

The peak voltages and/or the peak powers disclosed hereinabove, in combination with the indicated pulse durations and/or in combination with the indicated rise times are thus believed to be advantageous for providing an improved efficiency and/or an improved reduction of toxic molecules. As the skilled person will appreciate, these operating parameters may also be advantageous with other electrode configurations (e.g., irrespective of whether the first electrode is configured as a pin and/or the second electrode forms a helical shape).

Next, possible applications of the plasma source apparatus are described.

Automotive treatment of exhaust

Taking as an example, a synthetic gas mixture close to a cold start gasoline exhaust, which is composed of Nitrogen with 1% Oxygen, 12% carbon dioxide, 1.5% of water vapor added, and using different raw emission concentrations of NO (in the range of 400 ppm to 4000 ppm, which should be reduced in the exhaust by fast EID/DA processes initiated by plasma electrons), the following was assumed by the inventors (without wanting to be bound by theory):

Most of the dissociation of NO should happen in phase 1 where the peak power provided by the voltage source 10, the rise time of the pulse signal and the repetition rate of the pulse will determine the absolute destruction rate assuming a high quality matching between the voltage source 10 and the electrodes is achieved.

When phase 2 appears, the destruction rate may slightly increase, but production of NO will also increase. It was thus assumed that a balance between production and destruction of NO may occur.

In phase 3, the heat production is the major feature and it will contribute mainly to the production of NO. For a better overall NO net destruction, short pulses should be used and phase 3 should be minimized or avoided.

The hypothesis outlined above was tested in a gasoline-like mixture with different flow rates through a flow channel 2 in form of a 5 mm inner diameter glass tube. Results are presented in Fig. 5. The results clearly show that in order to reach a net reduction of NOx (not only considering NO), the pulse width should be close to the time scale of phase 1, i.e. less than 100 ns (less than 0.1 ps). Furthermore, Fig. 5 shows that a shorter pulse width achieves a higher net reduction.

The absolute net reduction is then limited mainly by the peak power and repetition rate of the voltage source 10.

In addition, to achieve a good heat exchange efficiency and impact probability of the electrons to target gas molecules, here NO or NOx, the volume flow and diameter of the flow channel, i.e. the flow speed, are correlated for achieving an improved net destruction of NOx. Further, Fig. 5 shows that at a relatively long pulse of 50 ns (limited by the voltage source) at 25 L/min flow resulted in a worse NO destruction than a flow rate of 40 L/min or more. This presumably is due to a better cooling effect by a higher gas flow.

To obtain consistent reductions in NOx at higher flow speeds is of interest, since the volume flow of the exhaust gases will vary considerably depending on a driving mode.

Surface treatment

For a surface treatment the classical measurements envisage the parameters “water contact angle” and “surface free energy”. These parameters, including the longevity of the surface modification/activation, determine whether processes such as coating, bonding, optical and/or other features are possible for different materials.

A series of tests have been conducted to determine the efficacy of the plasma source apparatus and to optimize the functionality (see Figures 6 to 8). Fig. 6 shows prior art values regarding a surface activation of different materials as comparison values. Fig. 7 illustrates a surface activation of PTFE, regarding a water contact angle in dependence of a plasma treatment time, and ageing effects after treatment. Fig. 8 illustrates a surface activation of PTFE, regarding a surface free energy in dependence of a plasma treatment time and ageing effects after treatment.

The performance of the present microwave plasma source apparatus shows comparable or even better results than those published for prior art plasma sources, as can be seen from the Figures 6 to 8. This is true for the treatment time, the achieved surface activation levels and the ageing response. One significant advantage of the plasma source apparatus according to the present application lies in its low manufacturing costs and its low running cost.

Fig. 12 shows a schematic diagram illustrating pulses of voltage changes in dependence of time (e.g., at a microwave frequency of 2.4 GHz). During each pulse, voltages changes in accordance with several periods of a sinusoidal wave defined by the microwave frequency are delivered. The graph indicates the rise time, the pulse duration, the pulse repetition period, and a power signal (over time) having a generally rectangular shape for each pulse (one complete pulse being shown).

While the invention has been described in detail in the drawings and forgoing description, such description is to be considered illustrative or exemplary and not restrictive. Variations to the disclosed examples can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain elements or steps are recited in distinct claims does not indicate that a combination of these elements or steps cannot be used to advantage, specifically, in addition to the actual claim dependency, any further meaningful claim combination shall be considered disclosed. The invention relates, for example, to the following aspects:

1. A plasma source apparatus, comprising: a first electrode; a second electrode; a voltage source coupled to at least one of the first electrode and the second electrode and configured to generate a potential difference between the first electrode and the second electrode; and a flow channel configured to accommodate a flow of gas therethrough, wherein the first electrode is at least partially disposed within the flow channel and the second electrode at least partially surrounds said first electrode, wherein said flow of gas passes between said first electrode and said second electrode; wherein the first electrode has a pin-like shape defining a longitudinal axis and the second electrode defines a helical shape, wherein the first electrode is at least partially disposed within the helical shape.

2. The plasma source apparatus of aspect 1, wherein the helical shape defines a cylinder and a symmetry axis of the cylinder coincides with the longitudinal axis.

3. The plasma source apparatus of aspect 1 or 2, wherein a main axis of the flow channel coincides with the longitudinal axis.

4. The plasma source apparatus of any of the preceding aspects, wherein the first electrode has a diameter normal to the longitudinal axis which is

• at least 0.5 mm, or at least 1.0 mm, and/or

• 3.5 mm or less, or 2.5 mm or less, or 2.0 mm or less.

5. The plasma source apparatus of any of the preceding aspects, wherein the second electrode comprises a conductor (e.g. a wire) having a width and/or thickness (e.g. a wire diameter) which is

• at least 0.1 mm, or at least 0.2 mm, and/or

• 0.5 mm or less, or 0.3 mm or less.

6. The plasma source apparatus of any of the preceding aspects, wherein the second electrode has a number of windings which is

• at least 2.5, or at least 3, or at least 3.5, and/or

• 5 or less, or 4.5 or less, or 4 or less.

7. The plasma source apparatus of any of the preceding aspects, wherein the helical shape of the second electrode has a pitch distance which is

• at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm, and/or

• 10 mm or less, or 9 mm or less, or 8 mm or less, or 7 mm or less. 8. The plasma source apparatus of any of the preceding aspects, wherein the second electrode (4) is positioned at an inner surface of the flow channel.

9. The plasma source apparatus of aspect 8, wherein the second electrode is fixed to an inner peripheral wall surface of the flow channel or wherein the second electrode is configured to be a floating electrode.

10. The plasma source apparatus of any of the preceding aspects, wherein an extension of the second electrode along the longitudinal axis is greater or less than an extension of the first electrode along the longitudinal axis.

11. The plasma source apparatus of any of the preceding aspects, wherein the first electrode and/or the second electrode are made of a material comprising tungsten and/or molybdenum, preferably, wherein the first electrode and/or the second electrode are made of tungsten and/or molybdenum.

12. The plasma source apparatus of any of the preceding aspects, wherein the flow channel comprises or is a glass tube, preferably a quartz glass tube.

13. The plasma source apparatus of any of the preceding aspects, wherein the flow channel has an inner diameter which is

• at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 7 mm, or at least 9 mm, and/or

• 15 mm or less, or 14 mm or less, or 13 mm or less, or 11 mm or less.

14. The plasma source apparatus of any of the preceding aspects, wherein the flow channel has a length which is

• at least 30 mm, or at least 35 mm, or at least 40 mm, or at least 50 mm, and/or

• 80 mm or less, or 70 mm or less, or 60 mm or less.

15. The plasma source apparatus of any of the preceding aspects, wherein the voltage source comprises an alternating voltage generator configured to generate an alternating voltage having a frequency of 2.45 GHz ± 100 MHz and/or a peak power having a value of

• at least 30 W, or at least 35 W, or at least 40 W, or at least 45 W, or at least 50 W, and/or

• 1100 W or less, or 1050 W or less, or 1000 W or less, or 700 W or less, or 500 W or less.

16. The plasma source apparatus of aspect 15, wherein the frequency and/or the peak power of the voltage source is adjustable, preferably wherein the plasma source apparatus comprises a controller allowing a user to adjust the frequency and/or the peak power.

17. The plasma source apparatus of aspect 15 or 16, wherein the voltage source is configured to operate in a continuous mode. 18. The plasma source apparatus of any of aspects 15 to 17, wherein the voltage source is configured to operate in a pulsed mode.

19. The plasma source apparatus of any of aspects 15 to 18, wherein a pulse duration is

• at least 10 ns, or at least 20 ns, or at least 50 ns, or at least 100 ns, or at least 1 ps, or at least 10 ps, or at least 100 ps, and/or

• 1 s or less, or 100 ms or less, or 10 ms or less, or 1 ms or less, or 100 ps or less, or 10 ps or less, or 1 ps or less. preferably wherein the pulse duration is adjustable, more preferably wherein the plasma source apparatus comprises a controller allowing a user to adjust the pulse duration.

20. The plasma source apparatus of any of aspect 15 to 19, wherein a repetition rate is 2 MHz or less, preferably wherein the repetition rate is adjustable, more preferably wherein the plasma source apparatus comprises a controller allowing a user to adjust the repetition rate.

21. The plasma source apparatus of any of aspects 15 to 20, wherein a duty cycle is

• at least 0.5%, or at least 1%, or at least 5%, or at least 10%, and/or

• 100% or less, or 90% or less, or 80% or less, or 70% or less, or 60% or less, or 40% or less, or 30% or less, wherein preferably the duty cycle is adjustable, more preferably wherein the plasma source apparatus comprises a controller allowing a user to adjust the duty cycle.

22. The plasma source apparatus of any of aspects 15 to 21, wherein a rise time is

• at least 1 ns, at least 2 ns, at least 5 ns, at least 10 ns, or at least 15 ns, or at least 20 ns, or at least 50 ns, and/or

• 1 ms or less, or 100 ps or less, or 10 ps or less, or 1 ps or less, or 0.1 ps or less, preferably wherein the rise time is adjustable, more preferably wherein the plasma source apparatus comprises a controller allowing a user to adjust the rise time.

23. The plasma source apparatus of any of the preceding aspects, further comprising a controller configured to control the voltage source.

24. The plasma source apparatus of any of the preceding aspects, further comprising a coaxial adaptor, wherein the first electrode is electrically connected to a metal core of the coaxial adaptor.

25. The plasma source apparatus of aspect 24, further comprising a metal adaptor configured to electrically connect the first electrode to the metal core of the coaxial adaptor.

26. The plasma source apparatus of aspect 25, wherein the metal adaptor has an outer diameter of

• at least 3 mm, or at least 4 mm, or at least 4.5 mm, and/or • 5 mm or less, or 4.5 mm or less.

27. The plasma source apparatus of aspect 25 or 26, wherein the first electrode protrudes from the metal adaptor by a section of the first electrode having a length of

• at least 8 mm, or at least 9 mm, or at least 10 mm, or at least 12 mm, or at least 15 mm, or at least 18 mm, and/or

• 30 mm or less, or 28 mm or less, or 25 mm or less, or 23 mm or less, or 20 mm or less.

28. The plasma source apparatus of any of the preceding aspects, further comprising a shielding housing surrounding the first electrode and the second electrode, wherein the shielding housing is conductive, preferably wherein the shielding housing is made from metal.

29. The plasma source apparatus of aspect 28, wherein the flow channel has an open end which protrudes from the shielding housing.

30. The plasma source apparatus of aspect 28 or 29, further comprising a dielectric spacer disposed electrically between the first electrode and the shielding housing.

31. The plasma source apparatus of aspect 30, comprising the features of aspect 25, wherein the dielectric spacer surrounds the metal adaptor circumferentially.

32. The plasma source apparatus of aspect 30 or 31, wherein the dielectric spacer is made of a material comprising Polytetrafluoroethylene and/or a ceramic material and/or quartz glass and/or a silicon based semi -conductive material.

33. The plasma source apparatus of any of aspects 30 to 32, wherein the dielectric spacer has an outer diameter of (preferably when using a 7/16 coax adaptor)

• at least 10 mm, or at least 15 mm, or at least 15.5 mm, and/or

• 30 mm or less, or 16 mm or less, or 15.5 mm or less.

34. The plasma source apparatus of any of the preceding aspects, wherein the plasma source apparatus is configured to electrically match 50 Ohm or 75 Ohm.

35. The plasma source apparatus of any of the preceding aspects, further comprising a gas inlet configured to allow the gas to flow into the flow channel.

36. The plasma source apparatus of any of the preceding aspects, wherein the gas is or includes ambient air and/or nitrogen and/or oxygen and/or a noble gas and/or a CO2 based gas mixture.

37. The plasma source apparatus of any of the preceding aspects, wherein the gas includes impurities of NO and/or CO2 and/or O2 and/or NET and/or a hydrocarbon and/or water vapor. 38. The plasma source apparatus of any of the preceding aspects, wherein the gas is an exhaust gas from a combustion engine.

39. The plasma source apparatus of any of the preceding aspects, further comprising a gas source providing the gas.

40. The plasma source apparatus of any of the preceding aspects, wherein the configuration is such that the flow of gas and/or a flow of the gas from the gas source into the flow channel is adjustable, preferably adjustable within a range having

• a lower boundary value of 1 L/min, or 2 L/min, or 5 L/min, or 10 L/min, or 20 L/min, or 30 L/min, or 50 L/min, and/or having

• an upper boundary value of 500 L/min, or 450 L/min, or 400 L/min, or 350 L/min, or 300 L/min, or 200 L/min.

41. The plasma source apparatus of any of the preceding aspects, wherein the configuration is such that an average speed of the flow of gas is adjustable within a range having

• a lower boundary value of 5 m/s, or 10 m/s, or 15 m/s, or 20 m/s, or 30 m/s, and/or having

• an upper boundary value of 350 m/s, or 300 m/s, or 250 m/s, or 200 m/s, or 150 m/s, or 100 m/s, or 50 m/s, or 20 m/s, or 15 m/s.

42. The plasma source apparatus of any of the preceding aspects, wherein the plasma source apparatus is configured for a temperature of the gas being within a range having

• a lower boundary value of -40°C, or -20°C, or 0°C, or 10°C, or 20°C, or 50°C, and/or having

• an upper boundary value of 1100°C, or 900°C, or 800°C, or 500°C, or 300°C.

43. The plasma source apparatus of any of the preceding aspects, wherein the plasma source apparatus is configured for a pressure of the gas at the gas inlet of

• at least 5 bar, or at least 10 bar, or at least 50 bar; and/or

• 150 bar or less, or 100 bar or less, or 50 bar or less.

44. Method of treating an exhaust gas from a combustion engine, comprising the following steps:

(i) providing a combustion engine, producing an exhaust gas;

(ii) providing a plasma source apparatus of any of the preceding aspects;

(iii) connecting the plasma source apparatus to the combustion engine such that the flow of gas is at least partly a flow of the exhaust gas;

(iv) generating the flow of gas passing between the first electrode and the second electrode;

(v) generating the potential difference between the first electrode and the second electrode by activating the voltage source.

44. Method of treating a surface, comprising the following steps: (i) providing a plasma source apparatus of any of aspects 1 to 43;

(ii) generating the flow of gas passing between the first electrode and the second electrode;

(iii) generating the potential difference between the first electrode and the second electrode by activating the voltage source;

(iv) directing the flow of gas leaving the flow channel to the surface.

45. Method of aspect 44 or 45, further comprising the following step: operating the voltage source such that an electrical discharge between the first electrode and the second electrode is maintained within the flow channel for at least 1 s, or at least 5 s, or at least 10 s, or at least 30 s, or at least 1 min, or at least 5 min.

46. A plasma source apparatus, for example for the treatment of exhaust gas (e.g. from a combustion engine), the apparatus comprising a first electrode; a second electrode; a voltage source coupled to at least one of the first electrode and the second electrode; a controller controlling the voltage source to generate a plurality of potential difference pulses between the first electrode and the second electrode, wherein a pulse duration of the pulses is less than 250 times the rise time of the respective pulse and/or wherein a pulse duration of the pulses is 500 ns or less; and a flow channel configured to accommodate a flow of gas therethrough, wherein said flow of gas passes between said first electrode and said second electrode.

47. The apparatus of aspect 46, wherein the pulse duration of the pulses is less than 150 times the rise time of the respective pulse.

48. The apparatus of aspect 46, wherein the pulse duration of the pulses is less than 50 times the rise time of the respective pulse.

49. The apparatus of aspect 46, wherein the pulse duration of the pulses is less than 10 times the rise time of the respective pulse.

50. The apparatus of any of aspects 46 to 49, wherein the pulse duration of the pulses is 250 ns or less.

51. The apparatus of any of aspects 46 to 49, wherein the pulse duration of the pulses is 100 ns or less.

52. The apparatus of any of aspects 46 to 49, wherein the pulse duration of the pulses is 50 ns or less.

53. The apparatus of any of aspects 46 to 49, wherein the pulse duration of the pulses is 10 ns or less. 54. The apparatus of any of aspects 46 to 53, wherein the peak power of the pulses is at least 0.1 kW.

55. The apparatus of any of aspects 46 to 53, wherein the peak power of the pulses is at least 0.3 kW.

56. The apparatus of any of aspects 46 to 53, wherein the peak power of the pulses is at least 0.5 kW.

57. The apparatus of any of aspects 46 to 53, wherein the peak power of the pulses is at least 1 kW.

58. The apparatus of any of aspects 46 to 53, wherein the peak power of the pulses is at least 5 kW.

59. The apparatus of any of aspects 46 to 58, wherein the controller generates an alternating voltage having a frequency of 2.45 GHz ± 500MHz.

60. The apparatus of any of aspects 46 to 58, wherein the controller generates an alternating voltage having a frequency of 2.45 GHz ± 100 MHz.

61. The apparatus of any of aspects 46 to 60, further comprising the features according to any of aspects 1 to 43.

62. A method of treating a gas, preferably an exhaust gas, more preferably an exhaust gas from a combustion engine, the method comprising the following steps:

(i) producing the gas by a gas source, preferably by a combustion engine;

(ii) providing a plasma source apparatus of any of aspects 1 to 43 or 46 o 61;

(iii) connecting the plasma source apparatus to the gas source such that the flow of gas is at least partly a flow of the gas to be treated;

(iv) generating the flow of gas passing between the first electrode and the second electrode;

(v) controlling a controller to generate a plurality of potential difference pulses between the first electrode and the second electrode, wherein a pulse duration of the pulses is less than 250 times, less than 150 times, less than 50 times, or less than 10 times the rise time of the respective pulse and/or wherein a pulse duration of the pulses is 500 ns or less, 250 ns or less, 100 ns or less, or 50 ns or less.