Abstract
A fast imaging camera is used to unveil the spatio-temporal properties of radially convective events in the CSDX linear plasma device [M. J. Burin et al., Phys. Plasmas, 12, 052320 (2005)]. The exposure time is set to 1μs and the time between frames to 10μs. The time series from a Langmuir probe and from a pixel in the 50000-frame movie are compared and cross-correlated. Excellent agreement between the two diagnostics is found for spatial scales greater than 2.5mm. The fluctuations inside the main plasma column are found to change between different poloidal mode numbers as a function of time. Accordingly, the power spectra determined in these linear devices reflect the sum over these modes. Outside the main plasma column, avaloids are observed to remain attached to the main plasma, hence their behavior does not become independent of the dynamics inside the main plasma column. Avaloid properties, assessed from imaging, agree with Langmuir probes done on various devices, except that the radial length is found to be much larger than previously determined because the blob-shape assumption is not valid. The link between fluctuations inside and outside the main plasma column indicates that the nonlinear evolution of the m=1 poloidal mode number is responsible for the creation of avaloids.
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