Plasma Profiles In A Cylindrical Helicon Discharge With Converging Magnetic Source Field

Detailed Langmuir probe measurements of the plasma density, plasma potential, and electron temperature in the cylindrical helicon device VINETA [C. M. Franck et al., Phys. Plasmas 9(8), 2002] are presented. The probe measurements cover the entire radial-axial plane including the helicon antenna region. Two magnetic field configurations are compared: homogeneous magnetic field in the helicon antenna region and magnetic field gradient at the transition between the antenna and the discharge chamber. It is demonstrated that the converging magnetic field leads to the formation of steep plasma density and potential increase, which is strongly correlated with the position of the maximum field gradient. No electron heating detached from the helicon source is observed. The measurements strongly suggest that the plasma profiles are the direct result of volume reduction due to magnetic field convergence, which leads to the formation of a single sheath-like layer in front of the helicon source. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.