On the expansion of a collisionless plasma in vacuum

The process of ion acceleration is experimentally studied at the expansion of a collisionless plasma into vacuum. The experiments were made in a cylindrical vacuum volume with the length of 200 cm and diameter of 60 cm. The pressure of residual gas was approximately 5x10-6 mm Torr. A plasma with a steep front of the 1-5 cm width in the space was produced by a pulse source. An argon plasma was used. Measurements of electron and ion densities, electron temperature, motion velocity of the plasma front were made by a Langmuir cylindrical probe. It is shown that ions may be accelerated up to velocities of the order of thermal velocity at plasma expanding into vacuum. So, due to the development of instability, the electron heat flux from the source to the front which provides the feeding by the energy of electrons accelerating ions, turns out to be approximately 105-106 times smaller than it follows from the theory of binary collisions. In experimental conditions the maximum ion velocity amounted (15-20)Csub(s0) at the distance of 150 cm from the source, which corresponds to the energy E approximately equal to (100-200) Tsub(e0) (Csub(s0) is the velocity of ion sound, Tsub(e0) electron temperature). It is noted that the effect of ion acceleration up to velocities of U>>Csub(s0) and anomalously low (turbulent) electron heat conduction which are observed at plasma expansion into vacuum should play a significant role in processes taking place in solar wind, at charged particle acceleration in laboratory and cosmic plasma, in laser thermonuclear fusion