Electron beam disruption due to ion release from targets-experimental observations

Abstract

Summary form only given, as follows. A major concern for the new generation radiography facilities like DARHT and AIRIX is that ions or ionized neutrals released from solid surfaces by beam impact can be accelerated and trapped by the beam potential. This time-dependent positive-charge distribution, inside the beam channel, can disrupt the beam. Possible release mechanisms include electron induced desorption of neutrals or ions, thermal desorption due to beam-target heating, and melting or vaporization of the solid target. To study this, we have performed experiments on the DARHT first axis (1.7 kA, 20. MeV, 60ns). Here, the beam, focused to a range of diameters, is transmitted through a thin target made of various materials. The time evolution of the beam radial profile is measured downstream of the target. For low current density, the downstream-beam radial profile is time invariant as expected for a pure electron beam. At higher current density, the downstream beam radial profile is clearly disrupted during the pulse including a large transverse instability. This is indicative of ion release at the target foil. Analysis of the data indicates that the ions are formed from thermally desorbed neutrals ionized by the beam. This is buttressed by observations of substantial neutral release at beam densities above the beam disruption threshold. Measurements to identify the desorbed species are underway and the results will be presented. In addition, the results of experiments to measure the properties of the accelerated ions will be presented. The implications of these findings for the DARHT second axis will be discussed.