Radially propagating turbulent structures in a linear helicon plasma device

The propagation properties of plasma density structures in a turbulent linear cylindrical helicon plasma are investigated. The governing instability is of the drift-wave type. An azimuthal limiter is used to steepen the radial density gradient to achieve a weakly developed turbulent state. The spatiotemporal evolution of turbulent plasma density fluctuations is investigated by Langmuir probe array measurements. In the strong plasma density gradient region the probability distribution function (PDF) of density fluctuations is rather symmetric. Here, the relict of a single m = 1 drift mode dominate the correlation of the fluctuations. Radially further outwards the fluctuations are dominated by large positive density bursts, associated with strongly asymmetric PDFs. The spatiotemporal measurements reveal that these intermittent bursts can be ascribed to radially propagating turbulent structures, which originate in the strong plasma gradient region. Their radial velocity reaches 5% of the local ion sound speed.