Electron beam action on dust structures in plasma

Abstract

Summary form only given. The action of an electron beam with an 25 keV electron energy and a beam current of 1-20 mA on ordered dust structures of MgO particles (5-20 /spl mu/m) in low-pressure air (0.1 Torr) RF discharges (13.56 MHz) was studied experimentally. The beam with 2 mm diameter was emitted horizontally into the discharge large-volume chamber. An ordered dust structure was created over plane RF-electrodes 10 cm in diameter. The interaction depended on the beam current and distance d between the axis of electron beam and the plane of the ordered plasma-dust structure. Electron beam with current I<5 mA repulsed dust structure if d <1.5 cm and attracted it if d=2-5 cm. Attraction of structure outside of RF discharge area can be explained by action of the radial ambipolar electrical field which arised due to ambipolar diffusion in plasma produced by electron beam. Repulsion caused by ion drag in ambipolar field. Model takes into account the kinetic of electron losses and productions in radial direction, radial electric field and ion drag. The basic energy losses of the beam electrons were bounded with the ionization of gas. The internal beam charge was compensated. The calculated value of beam plasma concentration is two orders higher than concentration of RF plasma. Beam with I/spl ges/10 mA deformed the whole dust structure, displaced it in the horizontal direction, and carried it away from the discharge zone. The dust structure was carried away not only from the zone of beam action determined by the initial diameter of the beam but behaved similarly to an elastic body, which caved in under the beam action and then moved as a whole. The destruction of the structure was primarily caused by the dynamic action of beam electrons on dust particles. Dust particles acquire a large additional charge, because the energy of beam electrons and the energy of electrons beat out from the surface of particles (/spl sim/100 eV) substantially exceed the energy of plasma electrons. This caused intense scattering of beam and secondary electrons by dust particles, because the cross section of Coulomb scattering increased proportionally to the squared charge of particles and became commensurate with the interparticle distance. The effect of the dynamic action of an electron beam on a dust cloud can be used to operate or to locally remove particles from a plasma.