No abstract

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 ...

Based on a thorough experimental study of the equilibrium plasma state and the description of coherent drift modes in the framework of a linear, nonlocal model, the response of drift modes to different external driver signal is reported. Purely temporal perturbations as well as corotating and counter-rotating electric fields of various mode numbers are applied to the system. Drift mode synchronization and periodic pulling, typical for driven nonlinear oscillator systems, are investigated in detail. The full spatio-temporal behavior of periodic pulling is observed with multiprobe arrays.

Experimental measurements of time series and frequency spectra characterize a periodic nonlinear interaction between pairs of self-excited, propagating, ionization waves simultaneously present in the positive column of a neon glow discharge. No periodically varying external driving force is applied. The interaction is the spatio-temporal extension of the previously established temporal periodic pulling process, the incomplete entrainment of a driven nonlinear oscillator. The particular mode playing the role of the driving perturbation in the interaction can be selected and is identified by inspection of the asymmetric, multisideband spectrum of light fluctuations, which reflect the system’s dynamics. A comparison between the spatio-temporal and temporal periodic pulling shows that the former is associated with a relatively strong driving force.