Plasma production by helicon waves with single mode number in low magnetic fields
Radio-frequency discharges are performed in low magnetic fields (0-10 mT) using three types of helicon-wave exciting antennas with the azimuthal mode number of $|m|$ = 1. The most pronounced peak of plasma density is generated in the case of phased helical antenna at only a few mT, where the helicon wave with $|m| = 1$ is purely excited and propagates. An analysis based on the dispersion relation well explains the density-peak phenomenon in terms of the correspondence between the antenna one-wavelength and the helicon wavelength. The $m=+1$ helicon wave propagates even in high magnetic fields where the density peaks are not observed, but the $m=-1$ helicon wave disappers. It is expected theoretically that the $m=-1$ helicon wave shows cutoff behavior in a low density region, [M. Kramer, Phys. Plasmas 6, 1052 (1999)], and the cutoff of $m=-1$ helicon wave experimentally observed coincides with the calculated one.