Plasma generator

1. A plasma generator comprising:

a plasma generating chamber for generating a plasma;

a propagation chamber having an entrance region for receiving an electromagnetic radiation, and a passage region for outputting said electromagnetic radiation towards said plasma generating chamber;

a plate-like separation element to separate said propagation chamber from said generating chamber;

said propagation chamber comprising an annular path along which said electromagnetic radiation propagates

said passage region being continuous along said annular path and having a defined length that increases in width from an upstream location close to the entrance region to a downstream location far from said entrance region, said passage region being defined in said plate-like element.

2. A generator as claimed in claim 1 , wherein said passage region has a width increasing with continuity on moving away from said entrance region.

3. A generator as claimed in claim 1 , wherein the conformation of said passage region is defined by two circular outlines that are not concentric with each other.

4. A generator as claimed in claim 1 , wherein the conformation of said passage region has a logarithmic spiral course.

5. A generator as claimed in claim 1 , wherein said passage region has an increasing width in a direction transverse to the direction away from the entrance region.

6. A generator as claimed in claim 1 , wherein said passage region is defined by at least one opening in said plate-like element.

7. A generator as claimed in claim 1 , wherein said passage region is defined by at least one portion of said plate-like element pervious to passage of said electromagnetic radiation.

8. A generator as claimed in claim 1 , wherein said entrance region for insertion of the electromagnetic radiation is defined at a peripheral position of said propagation chamber.

9. A generator as claimed in claim 1 , further comprising a waveguide connected to said propagation chamber for insertion of said electromagnetic radiation in a direction transverse to an equatorial plane of said path.

10. A generator as claimed in claim 1 , wherein said generating chamber has a plurality of through holes to enable insertion of a gas to be ionised in said generating chamber.

11. A generator as claimed in claim 10 , wherein said gas to be ionised is inserted in said generating chamber in one or more directions substantially transverse to a propagation direction of said electromagnetic radiation in said generating chamber.

12. A generator as claimed in claim 1 , wherein a propagation direction of said electromagnetic radiation in said path is transverse to a propagation direction of said electromagnetic radiation through said passage region.

13. A generator as claimed in claim 1 , further comprising filtering means associated with said generating chamber to enable the plasma generated by said electromagnetic radiation to come out of said generating chamber and to prevent escape of said electromagnetic radiation from said generating chamber.

14. An apparatus for surface treatment of materials, comprising a plasma generator as claimed in claim 1 .

15. A plasma generator comprising:

a propagation portion to propagate an electromagnetic radiation, said propagation portion having an entrance for receiving said electromagnetic radiation;

a plasma generating portion for generating a plasma with said electromagnetic radiation

a passage portion provided between said propagation portion and said generation portion, for allowing the electromagnetic radiation to enter the generating portion from the propagation portion

a plate-like separation element to separate said propagation portion from said plasma generating portion;

said propagation portion comprising an annular path along which said electromagnetic radiation propagates

wherein said passage portion is continuous along said annular path and has a window having an increasing width on moving away from said entrance, said window enabling at least partial passage of said electromagnetic radiation from said propagation portion towards said plasma generating portion,

said window being defined in said plate-like separation element.