High-pressure Langmuir probe in a weak flowing plasma or a plasma sheath

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=2πμn0eVlog(I/n0ev/subf/r/subp/)-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, v/subf/ is the plasma flow velocity, n0 is the plasma ionization density, and r/subp/ is the probe radius. For a spherical probe the current I=4πμn0er/subp/V. Measurements with a cylindrical probe at densities down to 1012/m3 show good agreement with theory