Is Bohm's criterion for sheath formation satisfied in the neighborhood of a negatively biased grid immersed in a low temperature plasma?

We present experimental studies of Bohm's Criterion for sheath formation in single species Kr⁺ and Ar⁺ ion plasmas in the vicinity of a negatively biased grid (-100V, with respect to chamber ground), in a hot filament, DC discharge, multi-dipole device (length = 70 cm, diameter = 60 cm) at the University of San Diego, in plasma discharges operated with a neutral pressure of 0.1-0.5mTorr, 1 ≤ n_e ≤ 3 X 10^9 cm^-3 and T_e ∼ 1-3 eV. Our grid is a stainless steel mesh, over 70% transparent with approximately 4 lines/mm. Prior measurements in a similar device indicated that 〈v〉 calculated from the ion velocity distribution function (ivdf) was 0.8 √kT_e/M) ± 6% at the sheath edge, as determined by diode laser based laser-induced fluorescence (LIF). The sheath edge was also located with the aid of emissive probe measurements analyzed by the inflection point method in the limit of zero emission. Because of isotope shifts, the Kr LIF signal needs to be deconvolved to obtain the ivdf, but this is not the case for Ar. Our concern is whether ion flow counter to that of ions falling through the presheath of the grid, toward the grid, alters sheath formation in some way. We plan to calibrate our results by performing the same measurements (ivdf, plasma potential, and density axial profiles) in an Ar plasma with both a grid and a solid boundary electrode.