Formation of annular plasma downstream by magnetic aperture in the helicon experimental device

Abstract

In the Helicon eXperimental (HeX) device, the geometric aperture is fixed, but the position of the magnetic aperture can be varied. Working with Argon gas in the pressure range of 1 − 10 × 10 − 4 mbar, an annular plasma (density ∼ 10 16 m − 3) is formed downstream, always in front of the magnetic aperture. This occurs irrespective of the relative position of the geometric aperture or the presence of a radial electric field. This is in contrary to the earlier proposition made by others that a radial electric field is necessary to produce a hollow plasma profile. Instead, the ionization of neutrals in the radially outer region by the tail electrons, rotating fast due to gradient-B drift in the azimuthal direction, seems to account for the observed off-axis density peaking in the present experiment. This also explains the variation of the plasma annulus diameter seen here by changing the input radio frequency power ( 100 − 800 W ).