Experimental characterization of a density peak at low magnetic fields in a helicon plasma source

We investigated characteristics of a density peak observed in a magnetic field B-0 lower than 100 G in the case of using helicon plasma sources with particular wavelength. For B-0 > 30 G, the antenna launches an electromagnetic wave as a slow wave, the phase velocity of which becomes close to the electron thermal velocity under the density-peak condition for various gas pressures. The Landau damping frequency is higher than the electron-neutral and the electron-ion collision frequencies, which indicates that the wave produces the plasma via Landau damping at low B-0. The wavelengths estimated from the density ne and B-0 for the density peaks agree with those of the electromagnetic field generated by helicon antennas of various lengths. The measured density is found to vary under the condition of the agreement between wavelengths of the propagating wave and the antenna-excited field during the density increase. One of the causes of the density peak appearing as a function of B-0 is considered to be the wavelength variation and the corresponding change of phase velocity of the slow wave which is enabled to propagate in the plasma by the introduction of B-0.