Large area, atmospheric pressure plasma for downstream processing
An arcless, atmospheric-pressure plasma generating apparatus capable of producing a large-area, temperature-controlled, stable discharge at power densities between about 0.1 W/cm 3 and about 200 W/cm 3 , while having an operating gas temperature of less than 50° C., for processing materials outside of the discharge, is described. The apparatus produces active chemical species, including gaseous metastables and radicals which may be used for polymerization (either free radical-induced or through dehydrogenation-based polymerization), surface cleaning and modification, etching, adhesion promotion, and sterilization, as examples. The invention may include either a cooled rf-driven electrode or a cooled ground electrode, or two cooled electrodes, wherein active components of the plasma may be directed out of the plasma and onto an external workpiece without simultaneously exposing a material to the electrical influence or ionic components of the plasma.
1. An atmospheric-pressure plasma discharge apparatus comprising in combination:
a first electrode having a cylindrical inner surface and a first axis of symmetry;
a plurality of parallel tubes forming a second cylindrical electrode interior to said first electrode and having a second axis of symmetry co-axial with the first axis of symmetry, each tube of said plurality of tubes having an outer surface spaced apart a first chosen distance from the inner cylindrical surface of said first electrode, the outer surface of each tube of said plurality of tubes further being spaced apart a second chosen distance from the outer surface of an adjacent tube thereto;
an rf power supply in electrical contact with either of said first electrode or each tube of said plurality of tubes for providing rf power to said first electrode or said second electrode, the unpowered electrode of said first electrode or each tube of said plurality of tubes being placed in electrical contact with a ground electrical potential;
means for cooling each tube of said plurality of tubes to a chosen temperature; and
means for flowing a gas into the annular space between said first electrode and said second electrode and out of the annular space between said first electrode and said second electrode between the spaced-apart outer surfaces of each tube of said plurality of tubes;
whereby an atmospheric pressure plasma is generated between said first electrode and said second electrode.
2. The plasma discharge apparatus of claim 1 , wherein a hollow cathode effect is produced between the spaced-apart outer surfaces of adjacent tubes of said plurality of tubes.
3. The plasma discharge apparatus of claim 1 , wherein each tube of said plurality of tubes is placed in electrical contact with a ground electrical potential.
4. The plasma discharge apparatus of claim 1 , wherein each tube of said plurality of tubes has a circular or oval cross section.
5. The plasma discharge apparatus of claim 1 , wherein the gas flows through the surface of said first electrode before entering the space between said first electrode and said second electrode.
6. The plasma discharge apparatus of claim 5 , wherein at least one open channel is provided in the surface of said first electrode out of which the gas exits the inner surface of said first electrode, dimensions of the at least one open channel being chosen such that no electric discharge takes place in the region of the at least one open channel and said second electrode.
7. The plasma discharge apparatus of claim 1 , wherein means are provided for cooling said first electrode to a chosen temperature such that the gas temperature is less than 50° C.
8. The plasma discharge apparatus of claim 1 , wherein the gas comprises between approximately 85% and approximately 100% of helium.
9. The plasma discharge apparatus of claim 1 , further comprising means for moving a material at a third chosen distance from and concentric with said second electrode on the opposite side thereto from the inner surface of said first electrode.