Expansion of a collisionless plasma in vacuum

Expansion of a collisionless plasma into vacuum which occurs with a front width Δ<10 rsub(D) or Δ> r approximately equal to 10 rsub(D) is investigated experimentally (rsub(D)=Tsub(e)/4PInesup(2)sup(1/2) is the local Debye radius which depends on the plasma parameters at the front vertex; n approximately equal to 107-1010 cm-3, Tsub(e) is approximately equal to 2-10 eV). It is shown that on moving away from the source the ions are continuously accelerated up to velocities which greatly exceed the ion sound velocity, the acceleration being due to the force of the electron pressure gradient at the front. The electron heat flow which supplies the ion-accelerating electrons with energy, turns out to be much smaller in the case of a collisionless plasma. The physical pattern of the processes is studied in detail