Transition of blue-core mode helicon discharge

This study explores the transitional characteristics of blue-core mode helicon discharge, which to our knowledge was not particularly focused before. Parameters are measured on a recently built advanced linear plasma device, i.e., a multiple plasma simulation linear device [Sun et al., Fusion Eng. Des. 162, 112074 (2021) and Wu et al., Plasma Sources Sci. Technol. 33, 085007 (2024)] by various diagnostics including the Langmuir probe, an optical emission spectrometer, and a standard high-speed camera. It is found that the jump direction of electron density (from low level to high level) is opposite to that of electron temperature (from a high level to a low level). Electron density increases significantly, and the radial profile becomes localized near the axis when the blue-core mode transition occurs. With increased field strength, electron density increases whereas electron temperature drops. The radial profile of electron temperature looks like a “W” shape, i.e., minimizing around the edge of the blue-core column. Electron density increases with background pressure, while electron temperature peaks around certain pressure value. High-speed videos show that the plasma column oscillates radially and experiences azimuthal instabilities with high rate once entered the blue-core mode. An electromagnetic solver based on Maxwell's equations and a cold-plasma dielectric tensor is also employed to compute the wave field and power absorption during blue-core mode transition, to provide more details that are valuable for understanding the transitional physics but not yet available in experiment. The results show that the wave field in both radial and axial directions changes significantly during the transition, its structure differs from antenna to downstream, and the power dependence of the wave magnetic field is overall opposite to that of a wave electric field. This work presents comprehensive characteristics of the transitional blue-core mode discharge and is important to both physics understanding and practical applications.