| Abstract |
A major concern for the DARHT second axis (2kA, 18.6 MeV, 2000 ns) is that ions or ionized neutrals released from solid surfaces (e.g., apertures, septum, dumps, and targets) by beam impact can be accelerated and trapped by the beam potential. This positive charge will be electrically attracted to the beam and could disrupt it. Possible release mechanisms include electron induced desorption of neutrals or ions, thermal desorption due to target heating and melting or vaporization of the solid target. To study this, experiments were performed on the DARHT first axis. Here, the beam, focused to a range of diameters, is transmitted through thin foils made of various materials. The time-dependent beam radial profile is measured downstream of the target. For low current density (depending on the material used), the downstream-beam profile is time invariant as expected. At higher current density, the downstream-beam radius changes during the pulse followed by transverse instability. Data, particle-in-cell simulations, and comparisons are presented. |
