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Downstream Control of Negative-Ion Plasma Parameters in Helicon Discharge

Plasma parameters, such as electron temperature, plasma density and potential oscillation were controlled by applying a negative potential to a grid in 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 region whereas the coarse mesh grid did not. The fine mesh grid performed better cooling of the electron temperature and lowered the plasma oscillation. Oscillating amplitude was found to increase with an increase in the SF6 gas concentration, while the time-averaged space potential was found to decrease, implying the effect of negative ions. The measurement of positive ion energy distribution functions predicted an ion beam travelling from the plasma source region towards the diffused region because of the potential difference between the two regions separated by the grid bias.