Plasma Dynamic on a Target Irradiated by Intense Electron Beams
V. Engelko1 , G. Mueller2
1- Efremov Institute of Electrophysical Apparatus, 189631, St. Petersburg, Russia
2- Forschungszentrum Karlsruhe, Institut für Hochleistungsimpuls- und Mikrowellentechnik, Postfach 3640, D-76021 Karlsruhe, Germany
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 limiting value. The time necessary for plasma formation depends on the beam current density and the efficiency of gas desorption and ionization. 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 this time becomes a few microseconds.
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. Between the plasma boundary and the electron beam a transition layer is formed. For conditions mentioned above its length is a few centimeters.
The target plasma expands into the beam drift region. The velocity of the target plasma expansion increases with time. The expansion of the target plasma is the main reason for neutralization of the electron beam space charge. Ionization of the residual gas in the drift space by beam electrons and target ions is of less importance.