Beam Plasma Interaction in Multipulse Radiography

In multiple pulse radiography, electron beam pulses will encounter plasma consisting of low atomic weight elements such as water vapor and hydrocarbons, and even vaporized target material, generated by previous pulses. The beam will collisionlessly penetrate the intervening plasma, rather than pushing a hole through the plasma as would happen in the fluid approximation. The particle in cell code MERLIN 2D is used to simulate the interaction. When the beam density is much higher than the background density, the space charge field of the impinging beam rapidly blows away the low density electron background and a steady state results in which the beam undergoes a betatron oscillation along its axial length. When the beam density is much higher than the background density, the background electrons are instead pushed out along the axis. (The space charge of the ions from the high background density retain the bulk of the background electrons.) The system settles into an approximate equilibrium qualitatively similar to the previous one. When the beam impinges on a background with a density ramp the low density part of the background is blown away to large radii, just as in the first case, and the high density region behaves as in the second case for the same reason. Thus, the intervening plasma does not appear to disrupt the beam, at least over several betatron wavelengths.