Measurement of rarefied gas flow velocity by ionic marking method

The paper discusses a method for measuring the gas velocity in a nearly free-molecular flow, which involves the determination of the velocity of motion of an ion label produced with the help of an electron beam. The influence of the field of the beam and of the intrinsic space charge of the label upon the results of the measurements has been examined. The conditions have been found under which the above effects may be neglected. The velocity measurements have been made in highly rarefied streams of nitrogen and argon. The ion label technique is as follows: A gas is ionized (labelled) using a narrow pulsed beam of electrons. The gas velocity is determined by measuring the time in which the label travels along a known base. The label represents an ionic cloud, for electrons produced by gas ionization have energies of several electron-volts and leave the flow during the time tsub(e), which is short in comparison with the time of the ion motion (tsub(e)<10-2tsub(i)). The intrinsic space charge of the ion cloud gives rise to the cloud diffusion and to its additional acceleration. As the degree of rarefaction grows higher, especially in the free-molecular flow, where ions move without collisions with the molecules of the flow, the influence of the intrinsic fields enhances and the label velocity may be significantly different from the neutral gas velocity. This problem has not yet received sufficient attention in the literature