  The ionization efficiency of helicon plasma discharge is explored by changing
the low axial magnetic field gradients near the helicon antenna. The highest
plasma density is found for a most possible diverging field near the antenna by
keeping the other operating condition constant. Measurement of axial wave
number together with estimated radial wavenumber suggests the oblique mode
propagation of helicon wave along the resonance cone boundary. Propagation of
helicon wave near the resonance cone angle boundary can excite electrostatic
fluctuations which subsequently can deposit energy in the plasma. This process
has been shown to be responsible for peaking in density in low field helicon
discharges, where the helicon wave propagates at an angle with respect to the
applied uniform magnetic field. The increased efficiency can be explained on
the basis of multiple resonances for multimode excitation by the helicon
antenna due to the availability of a broad range of magnetic field values in
the near field of the antenna when a diverging magnetic field is applied in the
source.
