Propagation and damping of m=+1 and m=−1 helicon modes in an inhomogeneous plasma column

The effect of the density gradient on the dispersion of the m=+1 and m=−1 helicon modes is investigated both theoretically as well as numerically. In particular, attention is focussed on the helicon wave damping that is closely related to the rf power absorption and the Poynting flux. It is shown that the propagation characteristics of the m=−1 mode changes drastically when the density gradient is sufficiently strong. This mode exhibits a cutoff that depends on the gradient scale length and the plasma parameters. In the regime where both helicon modes propagate, the excitation of the m=−1 mode is unlikely as it is strongly damped. The results give evidence that the density gradient effect is decisive for the formation of helicon discharges, which are predominantly sustained by the m=+1 mode.