Studies of blob formation, propagation and transport mechanisms in basic experimental plasmas (TORPEX and CSDX)
The findings of previous blob studies in the interchange-dominated regime of TORPEX helium (Muller et al 2007 Phys. Plasmas 14 110704) and hydrogen plasmas (Furno et al 2008 Phys. Rev. Lett. 100 055004) are summarized and compared. The onset of blobs is studied as a function of the vertical magnetic field B-z, proving the existence of blobs also in the drift-interchange-dominated regime characterized by B-z < 1mT. These blobs, despite being inherently three-dimensional and sheath-disconnected, exhibit statistical properties similar to the blobs in the interchange regime. Using conditionally averaged density and potential measurements, the entire time line of an interchange instability leading to the formation of wave-like structure patterns and blobs could be experimentally observed. These results show that a background E x B shear flow is not essential for the generation of blobs and that the phase shift between density and potential during the generation of blobs is pi/2 in all studied cases, demonstrating the interchange nature of blobs in TORPEX. Fast-camera and Langmuir probe measurements of blobs in the linear device CSDX (Tynan et al 2004 Phys. Plasmas 11 5195) support the hypothesis that an interchange instability is also responsible for the generation of blobs in the linear geometry, where the necessary effective gravity is provided by centrifugal forces.