Damping of waves in helicon-wave range of frequency in molecular- and rare-gas plasmas

Abstract

The damping of electromagnetic waves in helicon-wave range of frequency in molecular- and rare-gas plasmas is investigated. Test waves are excited by applying 144 MHz of rf to a single-loop antenna in H2 and He plasmas under the external magnetic field B0⩽1 kG. High-density H2 plasmas are produced at an optimum frequency for a fixed B0. Axial structures of the wave magnetic field are measured. The pressure dependence of k∥ and ki on B0 is investigated, where k∥ and ki are, respectively, the real and imaginary parts of wave number parallel to B0. The measured values of k∥ agree well with the ones calculated from the cold plasma theory in H2 and He plasmas. The effective collision frequency νeff=kiωce/k⊥ is evaluated from the measured ki, where k⊥ and ωce are the perpendicular component of wave vector and electron cyclotron frequency, respectively. As a result, whereas νeff in He plasmas is consistent with the calculated value, that in H2 is nearly a factor of 3 larger than the calculated one.