Measurements of negative ion density in O2/Ar electron cyclotron resonance plasma

An attempt to measure the negative ion density in an O2/Ar electron cyclotron resonance (ECR) plasma is made with two methods. The first one is to estimate the density ratio of negative ions to positive ions, α, from the reduction rate of the ion and electron saturation current obtained with the Langmuir probe. The second one is to calculate α from the measured phase velocity of the fast mode of ion acoustic waves (IAWs). The IAWs are launched along magnetic fields using a wire-antenna where positive pulse voltages are applied, and are detected as a fluctuation of ion saturation current. As a result, α<0.3 is found. It is also found that α obtained with both methods agree qualitatively. Furthermore, the spatial distribution and the gas mixture ratio dependence of α are investigated. It is found that α has the optimum value at the gas mixture ratio of 15%, and that α near the wall is larger than that in the center region.