We present a study of a helicon plasma with SF6 gas, operating at 13.56 MHz and powers between 0 and 2000 W, pressure between 0.3 and 7.5 mTorr, and flow rates between 4 and 40 sccm. All the measurements of plasma and gas parameters were made in the downstream diffusion chamber, below the helicon source. Two main regimes have been observed : at low power (typically <200 W), the plasma is not luminous in the source and ionization takes place in the diffusion chamber where there is an electric field. At high power, the plasma is strongly dissociated and the ionization occurs in the source for pressure above 0.9 mTorr and in the diffusion chamber when the pressure is below. The fluorine atom concentration increases with power until a saturation level [F](sat) is reached. The gas flow rate determines the power required to reach the saturation whereas [F](sat) is proportional to the pressure. The power required for the dissociation can be an important part of the power injected into the reactor leaving less power for ionization. This leads to a decrease of the plasma density when the flow rate increases at constant pressure.