Ion acceleration in Ar-Xe and Ar-He plasmas. II Ion velocity distribution functions

Ion velocity distribution functions (ivdfs) are investigated by laser induced fluorescence in Ar-Xe and Ar-He expanding helicon plasmas as a function of gas composition In the case of Ar-Xe plasma, it was found that in the helicon source, both the Ar+ and Xe+ vdfs are unimodal Their parallel speeds are subsonic and unaffected by changes in gas composition At the end of the source, the argon ivdf shows a bimodal structure indicative of an electric double layer upstream of the measurement location The fast argon ion component parallel velocity increases with Xe fraction from 6 7 to 8 km/s as the Xe fraction increases from 0% to 4% In the expansion region, the bimodal character of Ar ivdf is maintained with a supersonic fast component reaching parallel speeds of 10 5 km/s For all the studied plasma conditions and different spatial locations, the Xe+ vdf exhibits a unimodal structure with a maximum parallel flow velocity of 2 2 km/s at the end of the source For Ar-He plasma, the Ar-ivdf is bimodal with the fast ion component parallel velocity increasing from 5 2 to 7 8 km/s as the He fraction increases from 0% to 30% For the same He fraction range, the slow argon ion population distribution changes from a single Gaussian to a wide distribution extending all the way from the speed of the fast population to 0 m/s (C) 2010 American Institute of Physics [doi 10 1063/1 3505823]