A theory is developed for the current to a negatively biased probe in a moving weakly ionized plasma or a stationary ion sheath. It is found that at low ionization densities the current is no longer space-charge limited but is, in fact, limited by the electric field generated by the probe. For a cylindrical probe the current per unit length I=2πμn0eV[log(I/n0evfrp)−1/2] −1, where μ is the mobility of the ion, e is the electronic charge, V is the probe potential (negative), I is the probe current, vf is the plasma flow velocity, n0 is the plasma ionization density, and rp is the probe radius. For a spherical probe the current I=4πμn0erpV. Measurements with a cylindrical probe at densities down to 1012/m3 show good agreement with theory.

Measurements are described on a planar sheath established in a moving flame plasma seeded with potassium. By varying the quantity of seed the ionization density of the flame is altered so as to allow a range of sheath current-voltage characteristics which vary from saturation at low densities to a substantial voltage dependence at high densities. When the sheath is suddenly collapsed the ionization within what was the sheath region is observed to recover towards its equilibrium value. Measurements of current density, ionization density, and sheath thickness taken during these experiments show good agreement with a sheath model in which convection and/or ionization regeneration act as the supply of ions to the sheath.

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Probe measurements throughout a planar convection-sustained ion sheath reveal that upstream of the sheath: (a) no appreciable diminution of ionization density occurs, unlike the stationary continuum situation, and (b), there exists an electric field which is approximately equal to the ratio of the flow velocity to the electron mobility. Downstream of the sheath edge, measurements of the fall off of the ion and electron densities are reported. The above measurements are found to be in satisfactory agreement with the sheath convection theory of Smy.