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Abstract
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 socalled 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 twodimensional driftfluid model which includes the effects of dynamic electron and ion temperatures, and a threedimensional model derived from first principles, which excludes the effects of dynamic temperatures.
The results from the twodimensional 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 updown symmetry seen for cold ions blobs and causes blobs to propagate faster and remain more coherent.
The results from the threedimensional model reveals that dynamics along the magnetic field lines in a magnetic connement device also influences the propagation of blobs. A connection to the socalled 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 socalled scrapeoff layer power falloff 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.
In this study the propagation of coherent plasma laments is investigated by means of numerical simulations. The investigations are carried out using both a twodimensional driftfluid model which includes the effects of dynamic electron and ion temperatures, and a threedimensional model derived from first principles, which excludes the effects of dynamic temperatures.
The results from the twodimensional 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 updown symmetry seen for cold ions blobs and causes blobs to propagate faster and remain more coherent.
The results from the threedimensional model reveals that dynamics along the magnetic field lines in a magnetic connement device also influences the propagation of blobs. A connection to the socalled 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 socalled scrapeoff layer power falloff 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.
Original language  English 

Place of Publication  Lyngby, Denmark 

Publisher  Technical University of Denmark 
Number of pages  106 
Publication status  Published  2018 
Keywords
 Coherent filaments
 Plasma blobs
 Driftfluid equations
 Numerical Modelling
 Scrapeoff layer
 Power falloff length
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 1 Finished

Studies of Coherent Structures in Magnetically Confined Plasmas
Olsen, J. M. B., Madsen, J., Nielsen, A. H., Nielsen, S. K., Vera, C. H., Vianello, N. & Rasmussen, J. J.
01/07/2015 → 10/10/2018
Project: PhD