Heating mechanisms in a low pressure magnetized argon radio-frequency neutral loop discharge (NLD) are investigated. Results obtained in the NLD are compared with results obtained in an azimuthally isotropic m = 0 helicon discharge operated in the same setup. B-dot probe measurements are carried out showing that helicon waves are excited in the NLD. Phase resolved optical emission spectroscopic measurements demonstrate that electrons oscillate effectively not in the neutral loop (NL) but in a toroidal cusp located above the NL. In a simple transformer model we consider a diamagnetic current induced from the antenna windings into this cusp. The resulting calculated fields reproduce qualitatively well the measured helicon-NLD field. Furthermore, standing wave and beat patterns emerge in the axial direction. Depending on the dc magnetic field strength and plasma density different damping of the axial wave takes place. At strong damping, measured and calculated wave phase velocities approach the electron thermal velocity. Observed optical emission patterns corroborate the hypothesis of electron Landau damping being the main heating mechanism in both discharges.
Resonance wave discharge and collisional energy absorption in helicon plasma source