Effect of non-uniform magnetic field on plasma generation in a low magnetic field helicon discharge

Efficient plasma production by helicon wave in a non-uniform magnetic field near the antenna center is being reported here. Experiments are carried out with different non-uniformity of magnetic field near the antenna keeping the magnetic field at the center of the antenna <100 G. Coupling efficiencies are studied by measuring antenna current with and without plasma. Plasma production efficiencies are also estimated in all the different magnetic field topologies. It has been observed that coupling efficiency increases with magnetic field non-uniformity. Observation of beat wave in the axial variation of axial wave magnetic field suggests the presence of different radial wave mode. Measurements of axial wave number along with the estimation of radial wave number suggest wave propagation near resonance cone causing more absorption. It is found that the density obtain by introducing non-uniform magnetic field results in higher density than conventional helicon. Finally, wavelength is measured for nonuniform magnetic field near the antenna when the magnetic field is kept at 25 G and 50 G at the antenna center. For the 25 G case, measured axial wavelength is found to be twice the length of the antenna. This suggests that half wavelength antenna excites full wavelength helicon wave. However, in the 50 G case, the measured wavelength is shown to be approximately equal to the antenna length. (author)