Formation of Hollow Density Profile in Magnetically Expanding Plasma Produced by Helicon Antenna

The radial density profile in the magnetic nozzle of the Helicon eXperimental (HeX) device is found to be modified from being centrally peaked to that of hollow nature as external applied magnetic field is increased. It occurs above a characteristic magnetic field value for which the ions become magnetized in the expansion chamber. The hollow density formation is independent of the location of geometric expansion and also on the presence of a radial electric field. The density profile in the source chamber behind the nozzle, however, remains peaked on-axis irrespective of the magnetic field. The electron temperature there, is observed to be hollow and this characteristics continues to the expansion chamber along the field lines. Rotation of the tail electrons in the azimuthal direction due to the gradient-B drift in the expansion chamber leads to an additional off-axis ionization and forms the hollow density profile there. It is shown that, if the ions are not magnetized, then off-axially produced additional plasma is not confined and the density profile retains the on-axis peak nature. The present work demonstrates how the knowledge of the ion magnetization together with tail electrons significantly contributes to the design of an efficient helicon plasma based thruster.