Double blue-core discharge characterization in an inhomogeneous Ar helicon plasma

The double blue-core phenomenon has been observed in an inhomogeneous Ar helicon plasma under the double-end magnetic field configuration (SNSN), in comparison with the single-down-end configuration (00SN) and the field-free configuration (0000). The discharge images, optical emission spectra, electron excitation temperatures and plasma ion densities as a function of applied power were diagnosed using a Nikon camera, an optical emission spectrometer, and a Langmuir probe, respectively. The experimental results were interpreted in terms of axial eigenmode, plasma impedance, and radial component Er of the electric field based on theoretical calculations and simulation methods via HELIC software. Positive feedback has been found between the magnetic field gradient and the helicon plasma discharge characteristics. The spectral line intensity, plasma density, axial eigenmode, and plasma impedance varied with the magnetic field gradient. The findings provide valuable insights for designing Ar helicon plasma sources required for different applications, such as materials processing or aerospace propulsion.