Plasma dynamic on a target irradiated by intense electron beams

Theoretical consideration shows, that under the influence of an intense electron beam first an ion flux and then a plasma is formed on the surface of a target as a result of gas desorption and evaporation of target material. The target plasma is created when the density of the ion flux achieves a certain upperlimit. The time necessary for plasma formation depends on the beam current density and the efficiency of gas desorption and ionization. This time becomes a few microseconds under typical vacuum conditions of about 5.0·10⁻⁵ mbar, electron beam current density in the range of 10 A/cm² and kinetic energy of electrons in the range of 100 keV. When the density of the ion flux reaches a limiting value the beam potential decreases to a level, which is half of the initial one. A transient layer is formed between the plasma boundary and the electron beam. For conditions mentioned above its length is a few centimeters. The target plasma expands into the beam drift region, with a velocity that increases in time. The expansion of the target plasma is the main reason for neutralization of the electron beam space charge.