Experimental studies of the Bohm criterion in a two-ion-species plasma using laser-induced fluorescence

The Bohm criterion is studied experimentally in the case of a two ion species plasma. Measurements are carried out in Ar and Ar + He plasmas (P-Ar I similar to 0.1 mtorr, 0 less than or equal to P-He/P-Ar less than or equal to 25, and 0 less than or equal to n(He)(+)/n(e) less than or equal to 0.5, T-e less than or equal to 2 eV) created in an unmagnetized dc hot filament discharge confined by surface multidipole magnetic fields. Laser-induced fluorescence (LIF) measurements of Ar II ion velocity distribution functions (ivdfs) within the presheath up to the sheath edge show that the ions reach the sheath edge traveling faster than their individual Bohm speed by more than 75%, approaching a speed equal to the ion sound speed of the system.