Introduction Different turbulence simulations show that the drift wave is a possible candidate to explain the turbulence in the edge of fusion plasmas. The important properties of drift-wave turbulence are a cross phase between density and potential fluctuations smaller than π/4 and a finite parallel wavenumber k‖. Furthermore, the key element of the drift wave is the parallel electron dynamics. It can couple the drift wave to the shear-Alfven 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 perpendicular dynamics. Therefore, a detailed understanding of drift waves requires fully three-dimensional investigations, i.e. of the dynamics perpendicular and parallel to the magnetic field. The toroidal low-temperature plasma in the torsatron TJ-K is dimensionally similar to the one in the edge of fusion plasmas [1]. 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. A further advantage of the device is that its plasma can be simulated by turbulence codes such as GEM3 [2]. In this paper, the perpendicular dynamics of turbulence is studied with the focus on the poloidal wavenumber spectra and the turbulent transport. For the first time, the parallel dynamics of turbulence has been investigated in the core of a toroidally confined plasma. The results of the parallel wavenumber and the parallel propagation velocity are compared with results from the simulation code GEM3.

Using the example of the sawtooth chain, we argue that the t−J model shares important features with the Hubbard model on highly frustrated lattices. The lowest single-fermion band is completely flat (for a specific choice of the hopping parameters ti,j in the case of the sawtooth chain), giving rise to single-particle excitations which can be localized in real space. These localized excitations do not interact for sufficient spatial separations such that exact many-electron states can also be constructed. Furthermore, all these excitations acquire zero energy for a suitable choice of the chemical potential µ. This leads to: (i) a jump in the particle density at zero temperature, (ii) a finite zero-temperature entropy, (iii) a ferromagnetic ground state with a charge gap when the flat band is fully occupied and (iv) unusually large temperature variations when µ is varied adiabatically at finite temperature.