Bohm presheaths

Summary form only given, as follows. Experimental measurements of presheath properties in cold laboratory plasmas indicate that many of the assumptions often made to derive the Bohm velocity and presheath properties are invalid. Questionable assumptions include: the assumption of quasi neutrality at the sheath/presheath boundary, the assumption that individual ion species in a ion species plasma all exit the plasma at the individual species Bohm velocities, the assumption of zero ion temperature, the neglect of charge exchange, and the assumption of a single fluid velocity. Data include emissive probe measurements of plasma potentials, LIF measurements of ion distribution functions, Langmuir probe measurements of electron density and Mach probe measurements of drift velocities.