Studies of coherent lamentary structures in magnetically conned plasmas

Thermonuclear fusion has been proposed as a sustainable, clean and safe energy source to meet the energy demands of the future. There are, however, still several challenges that need to be overcome in order to realise a viable fusion power plant. One of the challenges are so-called coherent laments or blobs. These laments constitute a signicant fraction of the transport of plasma towards the outer vessel walls of fusion relevant magnetic connement devices. It is thus crucial for the realisation of a fusion power plant to gain a better understanding of such coherent laments.

In this study the propagation of coherent plasma laments is investigated by means of numerical simulations. The investigations are carried out using both a two-dimensional drift-fluid model which includes the effects of dynamic electron and ion temperatures, and a three-dimensional model derived from first principles, which excludes the effects of dynamic temperatures.

The results from the two-dimensional model reveal that the inclusion of dynamic temperatures has a signicant impact on the propagation of blobs. Especially the inclusion of finite ion temperatures is found to strongly impact the blob propagation. Finite ion temperatures break the up-down symmetry seen for cold ions blobs and causes blobs to propagate faster and remain more coherent.

The results from the three-dimensional model reveals that dynamics along the magnetic field lines in a magnetic connement device also influences the propagation of blobs. A connection to the so-called sheath causes blobs to rotate due to Boltzmann spinning. It is also found that the width of the blob fronts parallel to the magnetic field does not strongly influence the blob propagation, however the parallel extend of the blobs does have an effect. The blobs with larger parallel extend propagate faster than the smaller blobs and they are found to break into smaller fragments at the later stages of propagation. An effect that is not observed for smaller blobs.

Finally we investigate how the so-called scrape-off layer power fall-off length, λ_q, scales with a range of parameters. It is found that coherent structures signicantly influence λ_q, and a simple scaling for how λ_q scales with the electron temperature and the safety factor is found. This scaling is found to compare well with experimental results.