Sputtering effects in a helicon plasma with an additional immersed antenna

A plasma source excited by a double saddle helicon antenna outside the glass vacuum vessel has been modified by inserting a second copper antenna in contact with the plasma through the glass end plate. Both have the same frequency of 13.56 MHz but have a different phase. The immersed antenna is electrically floating, allowing a negative self-bias to form, leading to ion bombardment and sputtering of the copper onto the inner walls of the source tube. Dramatic changes in all plasma parameters (plasma density and potential, electron temperature, self-bias) are measured as the copper film increases in thickness, effectively shielding the power coupling of the helicon antenna. For low helicon powers the density decreases with time, but for high powers a copper free path is left in the glass adjacent to the helicon antenna due to re-sputtering of the deposited copper and no change in the plasma density is observed. This opens the possibility of having a 'negative' helicon antenna made of a copper cylinder with the antenna being the 'cut out' portion, opposite to the normal construction of helicon systems