Numerical experiment for focus of rotating and propagating LIB in Plasma I—Quasi-neutral approximation

Abstract

Focusing processes of a rotating and propagating light ion beam in the drift region are studied numerically by using a 2-dimensional hybrid (particle–fluid) code. An intense ion beam with the current density of 8 kA/cm2 and the total current of 2·5 MA, which is extracted from the diode with the applied voltage of 5·6 MV, is injected into the drift region filled with a low-density plasma. When a radial magnetic field is applied to the neighborhood of entrance, the beam ions start to rotate in the azimuthal direction owing to the Lorentz force. When the pressure of the background plasma is chosen such as the density of the beam becomes comparable with that of the background plasma in the vicinity of the focal spot, the current-neutralization fraction decreases and large self-magnetic fields are induced. The beam is confined by the fields within a small radius, even after passing the focal spot. Because the angular momentum of the beam is conserved, the beam rotation velocity increases up to the same order of the propagation one at a few mm radius. This rotation motion induces the azimuthal magnetic field and stabilizes the beam propagation. In the case where the plasma pressure was 3·0 Torr and the 0·2-Tesla radial magnetic field was applied over the distance of 2·0 cm near the entrance, the maximum beam intensity of 108TW/cm2 in the axial direction was obtained and the half width at half maximum (HWHM) of the focused profile was 3·5 mm.