Spatial and temporal evolutions of blue-core helicon discharge driven by a planar antenna with concentric rings

We investigate the spatial and temporal dynamics of blue-core helicon discharge using a novel planar antenna with four concentric rings on the LEAD (Linear Experimental Advanced Device, JINST 15: P11002, 2020). As RF power increases, the discharge transitions through E, H, W, and blue-core modes at 1100 W, exhibiting a hollow density profile peaking off-axis. High-speed imaging reveals counterclockwise rotation in the blue-core mode (800 rad/s at 2000 W), contrasting with clockwise rotation in non-blue-core modes, driven by a radial electric field reversal. A two-fluid flowing plasma model predicts a centrifugal instability that is strongest at small axial wavenumbers, weakening as the wavenumber increases. The blue-core mode diminishes beyond thresholds of 2.0 Pa or 800 G. This study highlights unique density profiles and rotational dynamics, advancing the understanding of helicon plasma behavior.