Is Bohm's criterion satisfied near a negatively biased grid in a single ion species Krypton Plasma?*

We performed the first test of Bohm's Criterion for sheath formation in a single species Kr⁺ ion plasma in the vicinity of a negatively biased grid (-100V, with respect to chamber ground),  in a hot filament, DC discharge, multi-dipole device (length = 64 cm, diameter = 32 cm) at the University of San Diego,  in which a Kr discharge was operated with a neutral pressure of 0.1mTorr,  n_e~ 3 x 10⁹ cm^-3 and T_e  ~ 3.5eV. We found that the first moment of the ion velocity distribution function was 0.8 √(T_e/M)  ± 6% at the sheath edge, as determined by diode laser based laser-induced fluorescence (LIF) together with emissive probe measurements analyzed by the inflection point measurement in the limit of zero emission. This result raises questions about assumptions. It appears that counter ion flow through the grid complicates the results.  We also present results of ion flow to a solid, negatively biased boundary plate to test Bohm's Criterion in a low temperature weakly collisional pure Kr plasma, free of counter streaming ion flow for comparison.