Drift-wave turbulence

Plasma turbulence is one of the most challenging topics in the physics of toroidally confined plasmas. It shows up as fluctuations in plasma potential and density (see figure). The highly complex phenomenon is of interest for fundamental research and for the economics of a future fusion reactor. Although numerical simulation of turbulence has progressed much in the past years, detailed experimental tests of the models are difficult to carry out. Due to advanced plasma parameters of fusion plasmas, detailed probe measurements inside the confinement region are not possible. The experiment TJ-K is a unique device for turbulence studies.

The interchange mechanism is responsible for turbulence in the core of fusion plasmas and the drift-wave instability is a candidate to drive turbulence in the plasma edge. Drift waves rely on a density gradient parallel to the magnetic field. The electrons respond to the parallel pressure forces and create a potential structure in phase with the density. The resulting ExB drift leads to a propagation of the perturbation in the electron-diamagnetic drift direction.

With Langmuir-probe arrays the drift-wave characteristics of the turbulence in the TJ-K plasma have been identified. They are:

• The elongated structure of the fluctuations
• The small density-potential cross-phase (figure)
• The electromagnetic component of the electron response
• The finite parallel wavenumber
• The propagation into the electron-diamagnetic drift direction