EP2080254B1 - Radiofrequency plasma generation device - Google Patents

Abstract

A device including two plasma generation electrodes, a series resonator having a resonant frequency above 1 MHz and including a capacitor with two terminals, and an induction coil surrounded by a screen, the capacitor and the coil being placed in series, the electrodes being connected to the respective terminals of the capacitor. The ratio of the spark plug to the radius of the screen is equal to 0.56. The device can optimize the Q-factor of such a device by adjusting the radius of the coil to that of the screen.

Description

The present invention relates generally to the generation of plasma in a gas, and more particularly to inductively coupled plasma generation devices. Plasma generation is used in particular for the controlled ignition of internal combustion engines by the electrodes of a candle, but can also be used, for example, for sterilization in an air conditioning process or pollution control systems.

More specifically, the invention relates to a plasma generating device comprising two electrodes, a series resonator having a resonance frequency greater than 1 MHz and comprising a capacitor provided with two terminals, and an inductive winding surrounded by a shield, the capacitor and the coil being arranged in series, the electrodes being connected to the respective terminals of the capacitor.

Such a device is in particular described in the form of a candle in the document FR 2,859,830 . This type of spark plug has reduced internal stray capacitances and forms a series resonator with a high overvoltage coefficient. Although this device makes it possible to maintain a radiofrequency voltage between its electrodes for the generation of a plasma, its optimization has hitherto been problematic.

In this context, the object of the invention is to provide an even more efficient radiofrequency plasma generation device.

To this end, the device of the present invention, also in accordance with the definition given in the preamble above, is essentially characterized in that the ratio of the winding radius r _int to the shield radius r _ext is between 0.5 and 0.6 and preferably 0.56.

• La figure 1 représente une représentation schématique en coupe d'un exemple de bougie utilisable dans le système de génération de plasma ; et
• La figure 2 représente un graphique représentant une étude du coefficient de surtension (y) en fonction du rapport r_int/r_ext (x).
• La figure 1 illustre des détails de la structure d'un dispositif de génération plasma radiofréquence de l'art antérieur, sous forme d'une bougie à étincelle de surface, pour lequel l'application d'une excitation radiofréquence s'avère particulièrement avantageuse.

Other features and advantages of the invention will become clear from reading the following description which is given by way of non-limiting example and with reference to the figures.

• The figure 1 represents a diagrammatic representation in section of an example of a candle that can be used in the plasma generation system; and
• The figure 2 represents a graph representing a study of the overvoltage coefficient (y) as a function of the ratio r _int / r _ext (x).
• The figure 1 illustrates details of the structure of a radiofrequency plasma generation device of the prior art, in the form of a surface spark plug, for which the application of radiofrequency excitation is particularly advantageous.

The spark plug 110 may be fixed on the cylinder head 104 of an internal combustion engine 105 of a motor vehicle.

The surface effect spark plug 110 comprises a low voltage cylindrical electrode which serves as a metal base 103 intended to be screwed into a recess made in a motor cylinder head and opening inside the combustion chamber. The base 103 is intended to be electrically connected to ground. Thus, the base 103 surrounds a cylindrical high voltage electrode 106 arranged in a central position. The electrode 106 is isolated from the base 103 via an insulating sleeve 100. The insulating sleeve consists of a material whose relative permittivity is greater than 1, for example a ceramic. The spark plug has a gap 105 between the dielectric 100 and one end of the electrode 103.

For automotive ignition applications, those skilled in the art will utilize electrodes and insulation having materials and geometry adequate to initiate combustion in a mixture at a combustion density and to resist the plasma thus formed.

The figure 1 also shows a sectional view of a candle advantageously incorporating a series resonator as described in the document of the prior art mentioned above. The spark plug 110 has a connection terminal 131, connected to a first end of an inductive winding 112. The second end of the inductive winding 112 is connected to an inner end of the high voltage electrode 106. This end is also in contact with an insulating element 111 forming the capacitor.

The electrodes 103 and 106 are in this example separated by the dielectric material 100. The series resonator integrated in the spark plug 110 comprises the inductive winding 112 and the insulating element 100 also forming the capacitor between the electrodes 103 and 106. The capacitor and the inductive winding 112 are arranged in series. The serial capacitance of the series resonator is formed of the capacitor and internal parasitic capacitances of the candle. This capacitance is arranged in series with an inductor to form the series resonator. When the length of the connection between the inductor and the capacitor is reduced, we reduce the parasitic capacitances in the candle. The candle 110 is thus used to maintain the alternating voltage between the electrodes 103 and 106, in the desired frequency range, preferably from 1 Mhz to 20 Mhz.

The integrated series resonator in the candle preferably has a single inductive coil 112, facilitating the manufacture of such a candle.

A large number of turns in the single winding 112 is necessary to obtain an inductance of the order of 50 μH. However, a large number of turns generates parasitic capacitances. The single inductive winding 112 preferably has an axis (identified by the dashed line) and consists of a plurality of turns superimposed along its axis. It is thus understood that the projection of a turn is identical to the projection of all the turns along this axis. The parasitic capacitances are then limited by not superimposing turns radially.

The spark plug further advantageously comprises a shield 132 connected to a ground and surrounding the inductive winding 112. The field lines are thus closed inside the shielding 132. The shielding 132 thus reduces the parasitic electromagnetic emissions of the spark plug 110. coil 112 can indeed generate intense electromagnetic fields with the radiofrequency excitation that is intended to apply between the electrodes. These fields may notably disrupt embedded systems of a vehicle or exceed thresholds defined in emission standards. The shield 132 is preferably made of a non-ferrous material of high conductivity, such as copper or silver. We can in particular use a conductive loop as shielding 132.

The coil 112 and the shield 132 are preferably separated by an insulating sleeve 133 of a suitable dielectric material, having a dielectric coefficient of greater than 1, and preferably a good dielectric strength to further reduce the risk of breakdown or failure. effluvia, causing energy dissipation. Of course, the lower the energy dissipation, the greater the amplitude of the voltage applied between the electrodes and the longer the life of the candle is high. The dielectric material may for example be one of the silicone resins sold under the references Elastosil M4601, Elastosil RTV-2 or Elastosil RT622 (the latter having a breakdown voltage of 20 kV / mm and a dielectric constant of 2.8). It can be provided that the outer surface of the sleeve 133 is metallized to form the shielding 132 above.

In general, it will be preferred to wind the winding 112 around a solid element 134 made of insulating and / or non-magnetic material, preferably both. This further reduces the risk of breakdown and parasitic capacitances.

A plasma formed using such a device has many interests in the context of automotive ignition such as a significant decrease in the rate of misfires in a stratified lean mixture system, a reduction of wear of the electrodes or an adaptation of the ignition initiation volume as a function of density.

Radiofrequency excitation is also adapted to a plasma deposition application, in a gas having a density of between 10 ^-2 mol / L and 5 * 10 ^-2 mol / L. The gas used in this application can typically be nitrogen or air, especially ambient air.

The radiofrequency excitation is further adapted to an application for the depollution of a gas having a density of between 10 ^-2 mol / L and 5 * 10 ^-2 mol / L.

The radio frequency excitation is furthermore suitable for a lighting application using a gas having a molar density of between 0.2 mol / l and 1 mol / l.

According to the present invention, in order to optimize the overvoltage coefficient Q = Lw / R, it is necessary to determine L, representing the self, and R representing the resistance. For this, a long winding model with rectangular turns is adopted.

The current which passes the son of the coil 112 will be distributed between the inner surface and the outer surface of the son in the ratio of magnetic fields. Considering that the winding is long enough, and thanks to the shielding, the magnetic field is homogeneous in the winding support and in the space between the winding and the shielding. The flux in the space between the winding and the shielding is therefore substantially equal to the flux in the winding support, and the magnetic fields are therefore in the ratios of the sections, which gives: $${\mathrm{B}}_{\mathrm{ext}}\mathrm{=}{\mathrm{B}}_{\mathrm{int}}\mathrm{*}{{\mathrm{r}}^{\mathrm{2}}}_{\mathrm{int}}\mathrm{/}\left({{\mathrm{r}}^{\mathrm{2}}}_{\mathrm{ext}}\mathrm{-}{{\mathrm{r}}^{\mathrm{2}}}_{\mathrm{int}}\right)$$

where r _int is the winding radius, r _{ext is} the shield radius, B _{int is} the magnetic field in the winding, and B _{ext is} the magnetic field between the winding and the shield.

en posant $$\mathrm{I}={\mathrm{I}}_{\mathrm{int}}+{\mathrm{I}}_{\mathrm{ext}}\mathrm{et}\mathrm{x}\mathrm{=}{\mathrm{r}}_{\mathrm{int}}/{\mathrm{r}}_{\mathrm{ext}}$$

on déduit : $${\mathrm{I}}_{\mathrm{int}}\mathrm{/}\mathrm{I}\mathrm{=}\mathrm{1}\mathrm{-}{\mathrm{x}}^{\mathrm{2}}\mathrm{et}{\mathrm{I}}_{\mathrm{ext}}\mathrm{/}\mathrm{I}\mathrm{=}{\mathrm{x}}^{\mathrm{2}}$$

où I représente le courant électrique, I_ext le courant électrique dans le blindage et I_int le courant électrique dans le bobinage.Assuming that the distribution of the current is entirely surface, the application of Navier-Stockes at μ ₀ B on a square circuit of width equal to the pitch crossing the surface gives: $${\mathrm{I}}_{\mathrm{ext}}\mathrm{=}{\mathrm{B}}_{\mathrm{ext}}\mathrm{/}\left({\mathrm{\mu }}_{\mathrm{0}}\mathrm{*}\mathrm{not}\right)\mathrm{and}{\mathrm{I}}_{\mathrm{int}}\mathrm{=}{\mathrm{B}}_{\mathrm{int}}\mathrm{/}\left({\mathrm{\mu }}_{\mathrm{0}}\mathrm{*}\mathrm{not}\right)$$

by asking $$\mathrm{I}={\mathrm{I}}_{\mathrm{int}}+{\mathrm{I}}_{\mathrm{ext}}\mathrm{and}\mathrm{x}\mathrm{=}{\mathrm{r}}_{\mathrm{int}}/{\mathrm{r}}_{\mathrm{ext}}$$

we deduce : $${\mathrm{I}}_{\mathrm{int}}\mathrm{/}\mathrm{I}\mathrm{=}\mathrm{1}\mathrm{-}{\mathrm{x}}^{\mathrm{2}}\mathrm{and}{\mathrm{I}}_{\mathrm{ext}}\mathrm{/}\mathrm{I}\mathrm{=}{\mathrm{x}}^{\mathrm{2}}$$

where I represents the electric current, I _ext the electrical current in the shield and I _int the electric current in the coil.

The variable x which represents the ratio of the winding radius to the radius of the shielding is thus expressed, it is now necessary to express R and L as a function of x so as to find a value of x maximizing Q = Lw / R.

Soit : $$R=\rho \frac{n2\pi }{\delta \cdot \mathit{pas}}{r}_{\mathit{ext}}\left(2x^4+x^3-2x^2+1\right)$$

The energy balance of the losses gives: $${\mathit{RI}}^{\mathit{ed}}=\rho \frac{\mathrm{not}2\pi }{\delta \cdot \mathit{not}}\left(r_{\mathrm{int}}\left(I_{\mathit{ext}}^2+I_{\mathrm{int}}^2\right)+r_{\mathit{ext}}{I}_{\mathit{ext}}^2\right)$$

Is : $$R=\rho \frac{\mathrm{not}2\pi }{\delta \cdot \mathit{not}}{r}_{\mathit{ext}}\left(2x^4+x^3-2x^2+1\right)$$

Ainsi le coefficient de qualité est égal à : $$Q=\frac{\mathit{Lw}}{\mathit{R}}\mathit{=}\frac{{\mathit{\mu }}_0\mathrm{\delta \omega }}{2\rho }{r}_{\mathit{ext}}\frac{x\left(1-x^2\right)}{\left(2x^4+x^3-2x^2+1\right)}$$

Sachant que $\delta =\sqrt{\frac{2\rho }{{\mu }_0\omega }},$

on en déduit que : $$Q=\frac{\mathit{Lw}}{\mathit{R}}\mathit{=}\frac{{\mathit{r}}_{\mathit{ext}}}{\delta }\frac{x\left(1-x^2\right)}{\left(2x^4+x^3-2x^2+1\right)}$$

In addition, the self L can be calculated as follows: $$\mathit{LI}=\mathrm{not}{B}_{\mathrm{int}}\pi r_{\mathrm{int}}^2={\mu }_0\mathrm{not}\frac{I_{\mathrm{int}}}{\mathit{not}}\pi r_{\mathrm{int}}^2={\mu }_0\mathrm{not}\frac{I\left(1-x^2\right)}{\mathit{not}}\pi r_{\mathrm{int}}^2$$

Thus the quality coefficient is equal to: $$Q=\frac{\mathit{lw}}{\mathit{R}}\mathit{=}\frac{{\mathit{\mu }}_0\mathrm{\delta \omega }}{2\rho }{r}_{\mathit{ext}}\frac{x\left(1-x^2\right)}{\left(2x^4+x^3-2x^2+1\right)}$$

Knowing that $\delta =\sqrt{\frac{2\rho }{{\mu }_0\omega }},$

it follows that: $$Q=\frac{\mathit{lw}}{\mathit{R}}\mathit{=}\frac{{\mathit{r}}_{\mathit{ext}}}{\delta }\frac{x\left(1-x^2\right)}{\left(2x^4+x^3-2x^2+1\right)}$$

l'étude de cette fonction donne le graphique représenté à la figure 2 et permet d'établir que le maximum de la fraction polynomiale se situe à y=0,516 pour x = 0,56.So, by posing $\mathrm{there}=\frac{x\left(1-x^2\right)}{2x^4+x^3-2x^2+1\text{'}}$

the study of this function gives the graph represented at the figure 2 and establishes that the maximum of the polynomial fraction is y = 0.516 for x = 0.56.

Thus in conclusion, it follows from this calculation that the ratio of winding radius to shielding radius must be 0.56 to have a maximum overvoltage coefficient.

However, after having performed tests and as the curve shows, it would seem that a shielding radius winding radius ratio in a range of 0.5 to 0.6 gives very satisfactory results, allowing considerable improvement. the overvoltage coefficient.

This parameter thus makes it possible for any type of radiofrequency plasma generating device, for example an engine spark plug, to optimize their overvoltage coefficient.

It is important to note that the application of such a ratio range between the diameter of a winding and a shield is, according to a preferred embodiment applicable to a motor spark plug, but can also be applied to any radio frequency plasma generation device.

Claims (11)

1. Plasma generating device (110) comprising two electrodes (103, 106), a series resonator with a resonant frequency higher than 1 MHz and comprising a capacitor (111) equipped with two terminals and an inductive coil (112) surrounded by a shield (132), the capacitor and the coil being arranged in series, the electrodes being connected to the respective terminals of the capacitor, the device being characterized in that the ratio of the radius of the coil (r_int) to the radius of the shield (r_ext) is between 0.5 and 0.6 and preferably equal to 0.56.
2. Device according to Claim 1, characterized in that the series resonator comprises a single inductive coil (112).
3. Device according to Claim 2, characterized in that the series resonator has a resonant frequency in the range from 1 MHz to 20 MHz.
4. Device according to any one of the preceding claims, characterized in that the radiofrequency plasma generating device is an engine spark plug.
5. Device according to any one of the preceding claims, characterized in that the shield (132) and the inductive coil (112) are separated by an insulating sleeve (133) made of a material that has a dielectric coefficient greater than 1.
6. Device according to Claim 5, characterized in that the exterior surface (132) of the insulating sleeve is metalized and constitutes the shield.
7. Device according to any one of the preceding claims, characterized in that the shield comprises a conductive loop.
8. Device according to any one of the preceding claims, characterized in that the inductive coil (112) is wound around a solid element (134) made of a nonmagnetic material.
9. Device according to Claim 6 or Claim 8, characterized in that one of said insulating materials has a withstand voltage higher than 20 kV/mm.
10. Use of a device according to any one of the preceding claims for igniting combustion in an internal combustion engine motor vehicle.
11. Use of a device according to any one of the preceding claims for sterilization in the context of an air-conditioning method.

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