Measurement of the velocity of a rarefied gas flow using an ionic tag

Abstract

The article expounds a method for measuring the velocity in a flow close to free-molecular from the motion of an ion tag, formed using an electron beam. It examines the effect of the field of the beam and of the inherent volumetric charge of the tag on the results of the measurements; the conditions under which this effect can be neglected are calculated. The velocity In highly rarefied jets of nitrogen and argon was measured. The method of an ion tag, for example [1, 2], consists in the fact that the gas is ionized (tagged) using a narrow pulsed beam of electrons, and the velocity of the gas is determined from the time of flight of the tag from a known base. The tag consists in an ion cloud, since the electrons forming with ionization of the gas have an energy of several electron volts and leave the flow after a time t_e which is small in comparison with the time of the motion of the ions t_i (t_e<10⁻²t_i). The presence of the inherent volumetric charge of the ion cloud leads to washing-out of the ion tag and to its additional acceleration. With an increase in the degree of rarefaction, particularly in a free-molecular flow, where the ions move without collisions with the molecules of the flow, the effect of the inherent fields increases, and the velocity of the tag may differ considerably from the velocity of the neutral gas. Insufficient attention has been paid to this question in the literature.