  Measurement of radial density profile in both the source and expansion
chambers of a helicon plasma device have revealed that it is always centrally
peaked in the source chamber, whereas in the expansion chamber near the
diverging magnetic field it becomes hollow above a critical value of the
magnetic field. This value corresponds to that above which both electrons and
ions become magnetized. The temperature profile is always peaked off- axis and
tail electrons are found at the peak location in both the source and expansion
chambers. Rotation of the tail electrons in the azimuthal direction in the
expansion chamber due to gradient-B drift produces more ionization off-axis and
creates a hollow density profile; however, if the ions are not magnetized, the
additional ionization does not cause hollowness.
