Experimental verification of Boltzmann equilibrium for negative ions in   weakly collisional electronegative plasmas
  Weakly collisional Ar-O2 electronegative plasmas are investigated in a dc multidipole chamber. An electronegative core and an electropositive halo are observed. The density ratio of negative ions to electrons ({\alpha}) in the nondrifting bulk is found to be 0.43. The profile of {\alpha} is found using both the phase velocity of ion acoustic waves and the drift velocity of positive ions determined by laser induced fluorescence. The experiment shows that negative ions are in Boltzmann equilibrium with a temperature of 0.06 \pm 0.02 eV. Double layers are not found separating the electronegative core and the electropositive halo. 
Y.-C. Ghim and N. Hershkowitz
physics.plasm-ph