Do double layers separate an electronegative core and an electropositive halo in weakly collisional negative ion plasmas?*

Summary form only given. Weakly collisional electronegative plasmas were investigated in a dc multidipole chamber. The plasmas were generated by primary electrons emitted from hot thoriated tungsten filaments. The working gases consisted of combinations of Ar and neutral feedgases that easily form negative ions. Measurements were made near a negatively biased stainless steel plate located in the center of the chamber. A planar Langmuir probe indicated the electron density ne and the effective electron temperature Te. Laser-induced fluorescence (LIF), using a tunable diode laser (668.614 nm), was used to determine the profile of Ar+ drift velocity. An emissive probe using the inflection point method in the limit of zero emission measured the plasma potential. In an Ar-O₂ plasma with Ar neutral pressure = 0.10 mTorr and 02 at 0.30, ne ~ 3.8 X 109 cm"3 and the Te ~ 0.69 eV, an electronegative core and an electropositive halo were observed. The density ratio of negative ions to electrons (a) in the non-drifting bulk was found to be 0.43 from the phase velocity of an ion acoustic wave (IAW) using CW excitation at 100 kHz. The profile of a was found using both the phase velocity of the IAW and the drift velocity of positive ions determined by LIF. These measurements showed that the negative ions were in Boltzmann equilibrium with a temperature of 0.06 Acircplusmn 0.02 eV. Consistent with the small value of a, double layers were not found separating the electronegative core and the electropositive halo in this plasma. Measurements with other feed gases, which should have larger values of a, are now underway and will be reported.