Browsing by Author "Mahdizadeh, Navid"

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Open Access
Investigation of three-dimensional turbulent structures in the torsatron TJ-K
(2007) Mahdizadeh, Navid; Stroth, Ulrich (Prof. Dr.)
In fusion plasmas, the turbulent transport is responsible for the major fraction of particle and heat losses, which degrade the confinement quality in a fusion reactor. Therefore, the investigation of the turbulence dynamics is of great importance. In the core of fusion plasmas, instabilities of interchange-type drive the turbulence, whereas in the plasma edge, the driving mechanism of turbulence is not yet clear. However, numerical simulations of plasma turbulence show that the drift wave should be the dominant instability in this region. It is well known that the edge plasma influences strongly the global confinement properties. Furthermore, the turbulence in the edge sets up a boundary condition for core and scrape of layer transport, which has still not been understood. Hence, the understanding of the physics of the plasma edge is an important issue of fusion research. The key element of the drift wave is the parallel electron dynamics. It can couple the drift wave to the shear-Alfvén wave and determines the degree of instability and the level of transport. On the other hand, the turbulence dynamics parallel to the magnetic field is strongly coupled to the dynamics perpendicular to it. Hence, detailed understanding of drift-wave turbulence needs both information on the perpendicular and the parallel dynamics. For the experimental investigation of plasma turbulence, diagnostics with high temporal and spatial resolution are required. Langmuir probes are well suited for such measurements. In dense and hot fusion plasmas, however, the use of Langmuir probes is limited to the scrape-off layer, which is the region outside the last closed flux surface. The toroidal low-temperature plasma in TJ-K is dimensionally similar to the one in the edge of fusion plasmas. In contrast to fusion plasmas, the whole plasma volume in TJ-K is accessible to Langmuir probes. This allows the use of probe arrays with a large number of tips and high temporal and spatial resolution. Such highly resolved spatial and temporal measurements cannot be carried out in fusion plasmas. A further important advantage of TJ-K is that the comparison of experimental and simulated data is possible in the plasma confinement region. To this end, simulations for the parameters of the TJ-K plasma were carried out using the turbulence code GEM3. In this work, for the first time, the three-dimensional nature of drift waves has been verified experimentally inside the confinement region of the toroidal plasma in TJ-K. The perpendicular dynamics of turbulence has been studied with the focus on the poloidal wavenumber spectra and the scaling of the turbulent structure with the drift scale. To this end, a 64 tip Langmuir probe array has been used, which is poloidally positioned on a flux surface. For the first time, the parallel dynamics of turbulence has been investigated in the core of a toroidally confined plasma. In contrast to previous experiments, multi-probe measurements were carried out to get simultaneous information on the shape and the propagation direction of the turbulent structures. The results for the parallel wave number and the parallel propagation velocity have been compared with results from the simulation code GEM3. It is demonstrated that the propagation in the direction parallel to the magnetic field is affected by Alfvén dynamics. Together, these results strongly confirm previous investigations, which have demonstrated the importance of drift-wave turbulence in TJ-K and therefore also in fusion edge plasma.