  The use of Z-pinch facilities makes it possible to carry out well-controlled
and diagnosable laboratory experiments to study laboratory jets with scaling
parameters close to those of the jets from young stars. This makes it possible
to observe processes that are inaccessible to astronomical observations. Such
experiments are carried out at the PF-3 facility ("plasma focus", Kurchatov
Institute), in which the emitted plasma emission propagates along the drift
chamber through the environment at a distance of one meter. The paper presents
the results of experiments with helium, in which a successive release of two
ejections was observed. An analysis of these results suggests that after the
passage of the first supersonic ejection, a region with a low concentration is
formed behind it, the so-called vacuum trace, due to which the subsequent
ejection practically does not experience environmental resistance and
propagates being collimated. The numerical modeling of the propagation of two
ejections presented in the paper confirms this point of view. Using scaling
laws and appropriate numerical simulations of astrophysical ejections, it is
shown that this effect can also be significant for the jets of young stars.
