| Popis: |
Helicon plasma sources are typically associated with a core, a radially localized central area of strong light emission. Here we describe new experimental results that clearly distinguish between the capacitive to helicon mode transition in an rf heated, argon plasma and the formation of the classic “blue” helicon core. We find that for certain source parameters, helicon plasma (discrete jump to high densities ∼ 1019 m−3 with increasing power and magnetic field, strong Ar - II dominated emission etc.) occur without the formation of the core. For such conditions, the plasma is dominated by resistive drift wave [RDW] instabilities driven by the radial density gradient rotating in the electron diamagnetic drift direction. The resulting particle flux is radially outwards for all radii. For controlled changes in the source parameters, we are able to trigger the formation of the core. A new global equilibrium state is achieved where we find the simultaneous existence of three radially separated plasma instabilities. The density gradient region, still dominated by RDWs, separates the plasma radially into the edge region and the core region. The edge region is dominated by strong, turbulent, shear driven Kelvin - Helmholtz [KH] instabilities, while the core region shows coherent Rayleigh - Taylor [RT] modes driven by azimuthal rotation. The RT modes rotate in the ion diamagnetic drift direction and are associated with enhanced light emission. The particle flux is directed outward for small radii and inward for large radii, thus forming a radial particle transport barrier which leads to a slight increase in the core plasma density. Simultaneously the Ar - II emission from the core region increases by an order of magnitude. The radial extent of the inner RT mode and radial location of the particle transport barrier is the same as the radius of the blue core. This new equilibrium with the RT - RDW - KH instabilities leads to the formation of a very strong enhanced blue core. For a range of operating parameters, just prior to this new global equilibrium state with the enhanced blue core, the system undergoes incomplete intermittent transitions between the two equilibrium states, leading to the visual perception of the “helicon core” in a time averaged sense. 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 plasma instabilities. |
