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Influence of Nonuniform Magnetic Field on the Helicon Discharge Excited by Various Antennas

The influence of a nonuniform magnetic field, which increases with the distance from an inductive antenna, on a helicon discharge has been studied. The discharge was excited in the azimuthally symmetric mode of helicon waves, m = 0, making use of antennas of two different types. It is shown that if the discharge is produced by a loop antenna, which supplies the RF energy through the side boundary of plasma and perpendicularly to the external magnetic field, then the ionization is concentrated at the discharge periphery. Under those conditions, the imposing of a nonuniform magnetic field reduces the loss of ionizing electrons at the wall and enhances the plasma generation. If the discharge is excited with a planar antenna along the magnetic field, then the main ionization occurs in the inner plasma region around the axis. In this case, an increase in the plasma density, if any, may be a result of the plasma contraction in the magnetic field with force lines convergent to the axis.