Downstream control of negative-ion plasma parameters in helicon discharge

Plasma parameters, such as electron temperature, plasma density and potenti al oscillation were controlled by applying a negative potential to a grid i n the downstream region of an electronegative plasma excited by an m = +/- 1 mode helical antenna. The fine mesh grid, with spacing comparable to the Debye length was found to quench the plasma characteristics in the source r egion whereas the coarse mesh grid did not. The fine mesh grid performed be tter cooling of the electron temperature and lowered the plasma oscillation . Oscillating amplitude was found to increase with an increase in the SF6 g as concentration, while the time-averaged space potential was found to decr ease, implying the effect of negative ions. The measurement of positive ion energy distribution functions predicted an ion beam travelling from the pl asma source region towards the diffused region because of the potential dif ference between the two regions separated by the grid bias.