In magnetic confinement, the separatrix separates plasma with toroidally closed from that with open magnetic field lines. The separatrix is of special importance for fusion research. The transition from open to closed field lines leads to a discontinuity in the radial electric field and to a natural ExB-flow shear layer which is related to the generation of the important H-mode transport barrier. The region of open field lines, called Scrape-Off Layer (SOL), guides particles and energy from the core plasma into the divertor. The width of the SOL is crucial for the power density on the divertor plates - a critical issue for ITER. The SOL width is determined by turbulent transport that is powered by turbulence from inside the separatrix.
Experiments are carried out to study
- Turbulence in the plasma core
- Impact of the separatrix on turbulent characteristics
- Turbulent transport in the SOL
- Influence of shear flows on turbulence
Electrostatic fluctuations were measured in the scrape-off layer of the tokamak ASDEX Upgrade. A linear probe array was designed and built. The radial dependence of statistical properties and the poloidal flow velocities of the turbulent fluctuations as well as the turbulent transport have been measured. The direction of propagation changes from the core to the scrape-off layer (figure below) creating a shear layer.
For comparison, similar investigations were carried out on limited TJ-K plasmas. The results from the two devices show similarities which can be taken as a support of the concept dimensional similarity. Across the entire radial sweep a potential-density cross phase close to zero was measured, which is consistent with drift-wave turbulence.