Wave phenomena, hot electrons, and enhanced plasma production in a helicon discharge in a converging magnetic field

Plasma characteristics and wave structures are examined for an m=0 helicon discharge operated in a nonuniform magnetic field. With the increase of the magnetic field gradient in the antenna region, plasma production rapidly grows and the density maximum moves downstream, towards the stronger field. Probe and optical measurements reveal a narrow layer of hot electrons (with temperature up to 7 eV) that extends far from the antenna along converging magnetic lines. A radially small-scale wave structure with peaks of the order of a few skin lengths is detected within the hot layer by magnetic and capacitive probes. Computations based on a simple plane model show that the inclination of the magnetic field lines to the plasma surface underneath the antenna is a critical factor for the distribution of the rf power absorption and for the formation of the small-scale structure. If the inclination angle exceeds the resonance group velocity angle, the near antenna absorption falls and the power flux penetrates deep into plasma along the magnetic lines. The fine wave pattern is explained considering the propagation characteristics of the helicon waves. Directional and extended power deposition together with high electron conductivity along the magnetic lines are argued to cause generation of hot electrons and enhanced plasma production. (C) 2004 American Institute of Physics.