Effect of inhomogeneous magnetic field on plasma generation in a low magnetic field helicon discharge

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