[en] The neoclassical transport is investigated in the torsatron TJ-K, which is operated with a low-temperature plasma. In the low-collisionality regime neoclassical losses are not intrinsically ambipolar, leading to the formation of a radial electric field which acts on both neoclassical and turbulent transport. This electric field is measured with a combination of Langmuir and emissive probes. The data are compared with the ambipolar electric field calculated with an analytic model. The experimental fields are positive and larger than the calculated ones. Direct losses of the fast electrons might explain this discrepancy. (orig.)

[en] Spontaneous transitions from low-confinement (L-mode) into high-confinement (H-mode) regimes are supposed to be triggered by turbulence generated poloidal ExB shear flows, so called zonal flows (for a review, see Ref. [1]). According to theoretical models, ExB shear flows act on turbulence and related radial turbulent transport through the shear decorrelation mechanism [2]. In particular, the reduction in transport could be due to the reduction of the radial correlation lengths [2] leading to a reduced radial step size of diffusive transport processes associated with turbulent structures [3] or even due to a deformation or tilt of these structures through changes in the phase relation between density fluctuations (en) and velocity fluctuations (evr = e E^mu⁼B) [4], where B and e E^mu^are the magnetic field strength and the fluctuations in the poloidal electric field, respectively. In this work, the microscopic mechanism is investigated, which leads to the decor- relation of turbulent structures and the reduction of radial turbulent transport due to shear flows. In TJ-K, indications for the presence of zonal flows have not been found, yet. Hence, external plasma biasing is applied in order to generate sheared poloidal E x B flows. Poloidal plasma rotation is driven by currents imposed by a biased electrode [5, 6]. Transitions into regimes with H-mode-like properties have been achieved [7] with biasing. Results of biasing experiments in TJ-K are reported. As a main result it turned out that the cross-phase between E x B velocity and density fluctuations plays the key-role in transport reduction. Spectral energy has been shifted from low frequencies to a coherent high-frequency mode where the cross-phase indicates inward transport. (Author)

[en] The spatio-temporal dynamics of large-scale structures in curved magnetic field of simple magnetized torus (SMT) are presented. The experiments are performed in the SMT BLAAMANN and employ the techniques of conditional averaging (CA) and cross-correlation (CCF) of Langmuir probe data. By means of a detailed comparison of radio-frequency and thermionic discharges, it is shown that general features of the observed large-scale structures are independent of the discharge mechanism. Further, the observed large scale structures have a pronounced spatio-temporal periodicity, which suggests a description as mode-like. The conjectured relationship between the observed coherent structures and the inverse energy cascade is therefore questioned again. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

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[en] Results are presented from recent experimental and numerical turbulence studies carried out on the low temperature plasma of the torsatron TJ-K. Fluctuation spectra are analysed by wavelet techniques indicating a large amount of intermittency in both numerical and experimental data. Since in both cases no critical gradient is present, the intermittency is not due to SOC. A 64-tip Langmuir probe array was used to measure poloidal wave number spectra. Density fluctuation spectra with a spectral index of -3 are found. The cross-phases between density and potential fluctuations are small on all scales. This is consistent with drift-wave and in variance with interchange turbulence. Experiments carried out with different gases show a spatial scaling of the turbulent events with ρ_s as predicted from theory. The dependence on ρ_s is confirmed by correlation measurements carried out between the tips of an 8 × 8 Langmuir probe matrix. (orig.)

[en] The discharge modes of a thermionic low pressure discharge (p<1Pa) are investigated with the one-dimensional particle-in-cell simulation codes PDP1 and XPDP1 [C. K. Birdsall, IEEE Trans. Plasma Sci. 19, 65 (1991)]. The simulation results provide a model approach for stable discharge modes, hysteresis, and for nonlinear relaxation-oscillations. During this potential-relaxation instability, nonlinear structures, e.g. electron holes and double layers, are observed. A Pierce--Buneman-mode is suggested as a trigger mechanism for the onset of the instability. The detailed oscillation process can be subdivided into three distinct phases: expansion phase, double layer phase, and relaxation phase. This allows one to explain the parameter dependencies of the oscillation frequency. For a periodically driven discharge, mode-locking in a period-2 state is found and explained by the model. The mode-locking phenomenon is studied systematically. The results of the simulations are well confirmed by experimental observations presented in Part II of this paper [T. Klinger et al., Phys. Plasmas 2, 1822 (1995)]. copyright 1995 American Institute of Physics

[en] Plasma confinement can be strongly improved by transport barriers. First in the ASDEX tokamak, spontaneous transitions from a low-confinement (L-mode) into a high-confinement regime (H-mode) have been observed. L-H transitions are accompanied by the formation of a transport barrier at the plasma edge. E x B shear flows have been considered as a candidate for trigger- ing the transport barrier. They are assumed to limit the radial correlation length of turbulent structures and, thus, reduce radial turbulent transport. Besides spontaneous L-H transitions, improved confinement regimes can also be achieved by externally induced electric fields. In the concept of plasma biasing, the plasma potential is locally modified by an inserted electrode. In this contribution, biasing is applied to the low-temperature plasma in the torsatron TJ-K in order to investigate the mechanism of transport reduction due to shear flows. The plasma is throughout accessible for probe diagnostics and the dimensionless parameters are similar to those at the edge of fusion plasmas. Turbulent structures are detected by means of an 8 x 8 Langmuir probe array in order to study the shear decorrelation mechanism. Different biasing schemes were tested to create sufficiently strong shear flows to have an impact on turbulent structures and radial transport. A clear effect was achieved with ring-like electrodes aligned on a flux surface. The plasma conditions and the fluctuations could strongly be influenced. Steepened density gradients and reduced fluctuation and transport levels were obtained when the shear was increased inside the confinement region. The direction of the poloidal propagation of turbulent structures changed from the electron-diamagnetic to the E x B-drift direction when strong radial electric fields were induced. The structures were found to be distorted, but a decrease of the radial correlation length was not observed. Transport reduction can be traced to enhanced stability reflected in the cross-phase between poloidal electric field and density fluctuations and also to shortened lifetimes. (author)

[en] The generation of a torus-shaped dust cloud in an anodic plasma is decribed. The confined dust particles perfom a rotational motion around the torus major axis. The structure of the cloud in dependence of the external parameters are observed and the rotation velocity of the particles was measured and compared with a simple estimate.

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