Lang Cui

Multi-instability plasma dynamics during the route to fully developed turbulence in a helicon plasma

We report experimental studies demonstrating a controlled transition to fully developed broadband turbulence in an argon helicon plasma in a linear plasma device. We show the detailed dynamics during the transition from nonlinearly coupled but distinct eigenmodes at low magnetic fields to fully developed broadband turbulence at larger magnetic fields. As the magnetic field (B) is increased from B ~ 40 mT, initially we observe slow smooth changes in the dynamics of the system (to B ~ 140 mT), followed by a sharp transition (within ~10 mT) to centrally peaked narrow density profiles, strong edge potential gradients and a pronounced bright, well-defined plasma core. At low magnetic fields, the plasma is dominated by drift waves. As the magnetic field is increased, a strong potential gradient at the edge introduces an E × B shear-driven instability. At the transition, another mode with signatures of a rotation-induced Rayleigh–Taylor instability appears at the central plasma region. Concurrently we also find large axial velocities in the plasma core. For larger magnetic fields, all the instabilities co-exist, leading to rich plasma dynamics and fully developed broadband turbulence at B ~ 240 mT.

Spontaneous profile self-organization in a simple realization of drift-wave turbulence

We report the observation of a transport bifurcation that occurs by spontaneous self-organization of a drift-wave and shear flow system in a linear plasma device. As we increase the magnetic field above a threshold ( BCr = 1200 G), a global transition occurs, with steepening of mean density and ion pressure profiles, onset of strong E×B shearing, a reduction of turbulence, and improved turbulent radial particle transport. An abrupt transition appears in the graph of turbulent particle flux versus density gradient. Hysteresis in the density gradient further confirms this transport bifurcation. The total Reynolds work on the flow sharply increases above threshold. This correlates with the increase of density steepness, which suggests the Reynolds stress-driven flow that plays an essential role in density steepening and transport bifurcation. A change in turbulence feature from drift waves (DWs) to a mix of DWs and ion temperature gradients also coincides with the transport bifurcation. Interesting phenomena...

Up-gradient particle flux in a drift wave-zonal flow system

We report a net inward, up-gradient turbulent particle flux in a cylindrical plasma when collisional drift waves generate a sufficiently strong sheared azimuthal flow that drives positive (negative) density fluctuations up (down) the background density gradient, resulting in a steepening of the mean density gradient. The results show the existence of a saturation mechanism for drift-turbulence driven sheared flows that can cause up-gradient particle transport and density profile steepening.

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.

Dynamics of formation of the blue core mode in argon helicon plasma

Summary form only given. 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.