ABSTRACT
68Recent studies in the Controlled Shear Decorrelation experiment (CSDX) reported a sharp non-monotonic global transition (at a critical magnetic field of B = 140 mT, with all other source parameters kept constant) in the plasma dynamics during the route to fully developed broadband turbu- lence. For B < 140 mT, the plasma is dominated by density gradient driven resistive drift wave (RDW) instabilities, propagating in the electron dia- magnetic drift direction. The resulting particle flux is radially outwards. For B > 140 mT, a new global equilibrium is achieved where we observe the simultaneous existence of three radially separated plasma instabili- ties. 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 at the core have very high azimuthal mode number, propagate in the ion diamagnetic drift direction and are associated with intense argon ion (Ar-II) emission. In this regime, the radial particle flux is directed outward for small radii and inward for large radii, thus forming a radial particle transport bar- rier leading to stiff profiles, decreased turbulence levels and increased core plasma density. Simultaneously, the Ar-II light emission from the core region increases by an order of magnitude leading to the formation of a very bright blue core. Blue cores have been previously observed in helicon plasma though its origin is hotly debated in the helicon source community. The radial extent of the inner RT mode and radial location of the particle transport barrier coincides with the radial extent of the inner blue core. Simultaneously, we find enhanced axial and azimuthal plasma flows in the core plasma, further helping in keeping the core and the edge distinctly separated. This new global equilibrium with simultaneous RT-DW-KH instabilities shows very interesting and rich plasma dynam- ics including intermittency, formation and propagation of blobs, forma- tion of a radial particle transport barrier, inward particle flux going up against density gradients, etc. This transition also suggests that changes in the cross-field radial particle transport due to low frequency instabili- ties are crucial to helicon core formation.
