Resonant Directional Excitation of Helicon Waves by a Helical Antenna

High-density plasmas are produced by 13.56 MHz, 1–2 kW discharges in a magnetic field B 0 of 0.02 T, using a helical antenna of length d=12–24 cm and azimuthal rotation of π. When the direction of B 0 is reversed, the positions of the highest electron density and the highest wave amplitude move from one side of the antenna to the other side. In order to understand such behavior, a small-amplitude wave is excited in the helicon plasma using three types of antennas: right-hand (M=+1), left-hand (M=-1) and linear (M=±1) polarization antennas. Only right-hand polarized waves are observed irrespective of antenna type. In the case of the M=+1 antenna, large-amplitude helicon waves are resonantly launched toward one side of the antenna when the axial wavelength λ//∼d or 2d. Such antenna action is explained in terms of wave interference.