The transition scenario from stability to drift wave turbulence is experimentally investigated in a magnetized low-b plasma with cylindrical geometry. It is demonstrated that the temporal dynamics is determined by the interaction and destabilization of spatiotemporal patterns, in particular, traveling waves. The analysis of the temporal and the spatiotemporal data shows that the bifurcations sequence towards weakly developed turbulence follows the Ruelle-Takens scenario. [S0031-9007(97)04530-4] PACS numbers: 52.35. Kt, 05.45. + b, 52.35.Ra It is an essential feature of bounded plasmas to establish edge localized gradients in the density, the space charge potential, and the particle temperatures. The magnetized plasma is then subjected to a class of low-frequency electrostatic fluid drift instabilities, the collisional drift waves. The dynamics of collisional drift waves is based on the tight coupling of fluctuations caused by E 3 B and diamagnetic drifts perpendicular to the magnetic field and a resistive parallel electron response. Linear drift waves travel predominantly in the transverse direction with electron diamagnetic drift velocity, have a radial eigenmode structure, and tend to establish axially standing modes. Despite important recent progress in theory [1] and experiment [2], the nature of the drift wave turbulence is still far from being understood. In particular, little is known of the strongly nonlinear regime in between the linear instability onset and the fully developed turbulence. In this paper, we describe an experimental study of the transition from a stable state to weakly developed drift wave turbulence in a bounded cylindrical low-b plasma. When the control parameter is increased, the transition follows a well-defined scenario, analogously to the already classical observations in neutral fluids [3]. Of high general interest in spatially extended, dissipative systems is the relationship between the temporal dynamics and spatiotemporal patterns [4], for instance, traveling waves, and we thus devote special attention to this important subject.The drift wave experiment was performed in a triple plasma device with a magnetized central chamber [5]. In one chamber a thermionic argon discharge is operated as plasma source (gas pressure P 8 3 10 24 mbar). The weakly ionized plasma diffuses into the central section and forms a magnetized column (magnetic field B 70 mT) of length l 1.6 m with a Gaussian radial density profile n͑r͒ n 0 exp͑2r 2 ͞2r 2 0 ͒ of width r 0 2.0 cm. The plasma column is bounded on both ends by transparent grids separating it from the source chambers. In the center of the column the electron temperature is T e 1.2 eV and the electron density is n e 2 3 10 16 m 23 . From laser diagnostics in thermionic discharges an ion temperature close to gas temperature was inferred [6], i.e., T e ͞T i ഠ 40. The drift wave characteristic length scales are set by the reduced gyroradius r s 1.0 cm and the inverse density gradient length L 21 n d͑ln n͒͞dr 1͞r 0.5 cm 21 [7]. The time scale is ...

A probe array with 64 azimuthally arranged Langmuir probes is presented as a new diagnostic tool for the investigation of drift waves. A parallel data acquisition system provides full spatio-temporal data of azimuthally propagating waves. For both regular and turbulent states of current-driven drift waves, the information provided by such space-time patterns is compared with results obtained from conventional two-point correlation methods. The probe array allows one to directly estimate the time-averaged wave number spectrum. In a turbulent state, the spectrum yields to a power law of S(k)∝k−3.6±0.1.

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A statistical analysis of density fluctuations in a cylindrical non-fusion device is performed. The experimental setup is implemented in order to reach a turbulent behavior of the linear plasma column. Two different turbulent regimes are obtained corresponding to two selected sets of values for the discharge parameters. The first regime displays a rotating column characterized by the presence of a shear layer separating the plasma bulk from the tenuous plasma in the shadow of the limiter, the latter showing a strong intermittent behavior and superdiffusion. The second regime corresponds to a weakly rotating column in which coherence is lost in the plasma bulk and a standard diffusive process takes place in the shadow region. These findings are supported by the calculation of the Hurst's exponent using wavelet-analysis techniques. Furthermore the intermittent behavior is characterized and related to the diffusive process. Finally the shape of the probability distribution function of density fluctuations seems to be well described by an analytical form suggested on the basis of Tsallis generalized statistics.

The turbulent regime of a rotating magnetized plasma column has been studied. The detection and the spatiotemporal analysis of structures by means of conditional sampling techniques is performed. Because of the overall rotation and centrifugal effects, the structures inside the turbulence move on average along a spiral trajectory leading to a net radial convection of the charged particles to the walls. The development of a poloidal electric field inside the structures has been measured. It leads to the observed outwards radial E x B drift in agreement with the expectations of recent theoretical works.