Formation of the Blue Core in Argon Helicon Plasma

Helicon plasmas are typically associated with a core, a radially localized central area of strong ion light emission. Here, we investigate the role of electrostatic instabilities that lead to the formation of the classic blue core. We show that helicon plasma can also occur without the distinct core. In these conditions, the plasma is dominated by low-frequency resistive drift wave (RDW) instabilities propagating in the electron diamagnetic drift direction. When the intense sharp core is present, a new global equilibrium state is achieved where three radially separated plasma instabilities exist simultaneously. The density gradient driven RDWs separate the plasma radially into an edge region and a core region. The edge is dominated by strong, turbulent, shear-driven instabilities, while the core shows very coherent high azimuthal mode number fluctuations propagating in the ion diamagnetic drift direction and associated with enhanced ion emission. The particle flux is directed outward for small radii and inward for large radii, thus forming a radial particle transport barrier. The radial extent of the inner mode and radial location of the particle transport barrier is the same as the radius of the blue core. This new equilibrium, with the three coexisting radially separated plasma instabilities, leads to the formation of a very strong enhanced blue core. For a range of operating parameters, just prior to the blue core formation, the system undergoes incomplete intermittent transitions between the two equilibrium states, leading to the visual perception of a broad less intense helicon core. This is the first time that the development of the helicon core is shown to be associated with changes in radial transport driven by inherent low-frequency plasma instabilities.