I've searched high and low for data regarding this from scientific papers to books and I cannot find anything in regards to the approximate impedance to excite a vacuum vessel to plasma state. In particular I want to built a RF Plasma cleaning chamber, however, I am not sure how to design the matching network for the field. I have tried using low power ~2W approximately 20-50ohms impedance, however, I was unable to create a plasma. I have been reading that the required RF power to excite a gas to plasma state decreases quite a large amount as it approaches a vacuum and then shoots up exponentially as the vacuum level approaches a perfect vacuum. Anyway, what sort of impedance should I design my RF amplifier with in order to excite some plasma? Does anyone happen to have data on this? Typically I would determine this experimentally, but I lack a tunable matching network and since I plan on designing this amplifier from the ground up it would be easier to get an idea of what impedance to expect!