Study of density peaking in a diverging magnetic field helicon experiment

Abstract

Density peaking phenomena have been studied in different magnetic field configurations in a low field (< 100 Gauss) helicon discharge. The study has been carried out in the linear helicon plasma device (Barada et. al., Rev. Sci. Instrum. 83, 063501, 2012) using argon gas with m = +1 right helical antenna operating at 13.56 MHz by varying the magnetic field from 0 Gauss to 100 Gauss (G) with two different magnetic field geometry. The plasma density varies with varying the magnetic field at constant input power and gas pressure and reaches to its peak value at a critical magnetic field value (s). For a magnetic field of 88 G near the antenna the density rises at an axial location away from the antenna in the diffusion chamber having a diverging magnetic field. On the m=−1 propagation side of the antenna, the density peak vanishes around 30 G which is well evident on the m=+1 propagation side. The results are explained on the basis of resonance cone propagation of right circularly polarized helicon waves and the vanishing of density peak at 30 G along with observation of density peak at 5 G and 12 G at z=−18 cm (m=−1) propagation side are explained with polarization reversal mechanism of electromagnetic waves.