Production of high-density hydrogen plasmas by helicon waves in a simple torus

High-density plasma production by helicon waves in a simple torus has been investigated. The measured plasma density np versus external magnetic field B0 peaks at a condition close to the lower-hybrid resonance in H2, D2, and He. Three density regions are observed in the variation of rf power Prf. In the low-density region, helicon waves are not excited and plasmas are produced by an antenna induction field. In the medium- and high-density (H) regions, helicon waves with the m=+1 azimuthal mode are excited and plasmas are produced by the waves. In H2 plasmas, the second-radial mode is dominant over the first-radial mode at Prf>10 kW in the H region. The measured kz−np relation (kz is the wave number parallel to B0) of helicon waves in the H region shows deviation from that of the m=+1 mode derived using the uniform-plasma assumption. The kz−np relation is calculated for various radial-np profiles; parabolic, modified-parabolic, and Gaussian profiles. The difference in the kz−np relation of the m=+1 mode between the uniform- and nonuniform-plasma models is explained by adding a vacuum region between a uniform plasma and the radial boundary.