The rescaled range analysis techniques are used to investigate long-range dependence in plasma edge fluctuations [Mandelbrot and Wallis, Water Resources Res. 4, 909 (1969)]. This technology has been applied to data from several confinement devices such as tokamaks, stellarators, and reversed-field pinch. The results reveal the self-similar character of the electrostatic fluctuations at the plasma edge with self-similarity parameters ranging from 0.62 to 0.72. These results show that the tail of the autocorrelation function decays as a power law for time lags longer than the decorrelation time and as long as times of the order of the confinement time. In cold plasma devices (Te<1 eV at the core), there is no evidence of algebraic tails in the autocorrelation function. Some other characteristic features of the autocorrelation function and power spectrum have been investigated. All of these features are consistent with plasma transport as characterized by self-organized criticality.

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.

This paper describes recent experimental investigations of the nonlinear dynamics of collisional current-driven drift waves in a linear low-β discharge. It is shown that the bias of an injection grid leads to rigid-body rotation of the cylindrical plasma column that strongly destabilizes the drift waves, thus providing a control parameter for the drift-wave dynamics. In the nonlinear regime, when the control parameter is increased, the transition scenario from stability to weakly developed turbulence is studied. Two successive Hopf bifurcations, a modelocked state and its gradual destabilization to chaos and finally turbulence follow the classical Ruelle-Takens transition scenario known from neutral fluids. In addition to the temporal dynamics, the spatiotemporal evolution of drift waves is studied by means of circular Langmuir probe arrays with high spatial and temporal resolution. With each Hopf bifurcation, a drift-mode onset is associated and the bifurcation from quasi-periodicity to mode locking corresponds to the transition from non-resonant to resonant mode interaction. The mode-locked state forms a persistent spatiotemporal pattern that is destabilized by the occurrence of defects. In contrast, the turbulent state is a fully disordered, intermittent state.

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