Ion saturation currents to spherical and cylindrical electrostatic probes in collisional plasmas

Simple analytical formulas are discussed relating ion saturation currents to cylindrical or spherical electrostatic probes and the charged particle concentration in an undisturbed plasma. It is shown that normalized values of saturation currents may be approximately considered as universal functions of the diffusive Peclet number (which characterizes the normalized velocity of the plasma flow past the probe), the form of these functions being independent of the species of neutral component of plasma, the species of ions, parameters of undisturbed plasma, etc. A comparison with a large set of various experimental data available in the literature is presented. It is shown that the above formulas describe most of these data with the accuracy of a factor of 3.