How is Bohm’s criterion satisfied in the neighborhoodof a negatively biased grid immersed in a low temperatureplasma, or is it?*

We present experimental studies 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 = 70 cm, diameter = 60 cm) at the University of San Diego, in which a Kr  DC, hot filament discharge was operated with a neutral pressure of 0.1-0.5mTorr, 1 ≤ ne ≤ 3 × 10⁹cm−3 and Te ∼ 1 − 3eV . Our grid is approximately 50% transparent stainless steel mesh, approximately 25 lines/cm. Prior measurements in a similar device indicated that 〈v〉 calculated from the ion velocity distribution function (ivdf) was 0.8*(kTe/M)^1/2±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 method in the limit of zero emission. 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 these results with measurements (ivdf, plasma potential, and density axial profiles) in the neighborhood of an opaque, solid, negatively biased plate free from counter streaming ions.