Recurrence quantification analysis of turbulent fluctuations in the plasma edge of Tokamak Chauffage Alfvén Brésilien tokamak (original) (raw)

Recurrence Analysis of Turbulent Fluctuations in Magnetically Confined Plasmas

2016

Recurrence plots and their quantification became a modern tool in nonlinear data analysis, currently being used in a myriad of scientific disciplines, a diversity that characterizes the Recurrence Plot Workshop since its beginning. In this work we review some applications of recurrence quantification analysis to data analysis in a fusion plasma, namely turbulent fluctuations in the plasma edge of Tokamak Chauffage Alfven Bresilien tokamak, which is a magnetic confinement plasma machine. A similar analysis was performed in a low-density plasma device called Texas Helimak (University of Texas at Austin). Our results point out that the deterministic content of the fluctuations tends to increase as we approach the plasma edge, indicating a concentration of the recurrences therein. This favours the use of fluid models to describe the physics of the plasma edge turbulence.

Recurrence in plasma edge turbulence

Physics of Plasmas, 2001

Common statistics of turbulent electrostatic fluctuations observed at the plasma edge and scrape-off layer are analyzed in three tokamak devices that have different configurations. The statistics of experimental data collected using fixed sampling time is the same than the statistics of the time for which the oscillation return to a specified reference interval of values. This observation, in addition to the finding of power-scaling laws for some average quantities with respect to either the sampling time or the size of the reference interval, suggests that turbulence on tokamaks have recurrent characteristics, typical of a recurrent chaotic low-dimensional system. Furthermore, the first Poincaré recurrence time and other dynamical tools are used to simulate the mentioned fluctuation statistical properties.

The Characterization of Edge Plasma Intermittency in T-10 and TCABR Tokamaks

2007

Turbulence is the natural state of plasma in fusion devices . Its statistical properties are essential for the understanding of the confinement in tokamak. The fluctuations observed in tokamaks, stellarators and linear machines (see, e.g., ) are self-similar, suggesting the universality of self-similarity properties at the edge of magnetized plasmas. The self-similarity properties of the edge turbulence are responsible for the memory effect and large-scale correlation in space and time due to intermittent structures. Intermittent transport resulting from coherent structures such as vortices, zonal flows, streamers, and blobs, leads to substantial losses above the ones predicted by classic diffusive scaling . Experimental investigations of plasma turbulence have highlighted deviation (due to the strong intermittency) from Kolmogorov's K41 model prediction . There are numerous experimental observations of magnetized plasma turbulence that share a lot of features of neutral fluid turbulence (see ) including many scales, cascades, strong mixing, anomalous scaling and so on. Despite the large amount of experimental data that has been obtained in fusion devices, our understanding of the turbulence and diffusive transport process in magnetized plasmas is still rather limited. In this work, we focus on quantitative estimate of self-similarity and intermittency of edge plasma turbulence in T-10 and in TCABR tokamaks.

The MDF technique for the analysis of tokamak edge plasma fluctuations

Journal of Plasma Physics, 2013

Tokamak edge plasma was analyzed by applying the multifractal detrend fluctuation analysis (MF-DFA) technique. This method has found wide application in the analysis of correlations and characterization of scaling behavior of the time-series data in physiology, finance, and natural sciences. The time evolution of the ion saturation current (Is), the floating potential fluctuation (Vf), the poloidal electric field (Ep), and the radial particle flux (Γr) has been measured by using a set of Langmuir probes consisting of four tips on the probe head. The generalized Hurst exponents (h(q)), local fluctuation function (Fq(s)), the Rényi exponents (τ(q)) as well as the multifractal spectrum f(αh) have been calculated by applying the MF-DFA method to Is, Vf, and the magnetohydrodynamic (MHD) fluctuation signal. Furthermore, we perform the shuffling and the phase randomization techniques to detect the sources of multifractality. The nonlinearity shape of τ(q) reveals a multifractal behavior o...

Density, temperature and potential fluctuations in the edge plasma of the FTU tokamak

Nuclear Fusion, 1998

Density, temperature and plasma potential are measured simultaneously at the same spatial point in the SOL plasma of FTU at a rate faster than the typical turbulence frequency. This is done through a harmonic analysis of the current drawn on a single Langmuir probe swept in voltage at high rate (≈0.9 MHz). The current signal is detected with a down frequency conversion by means of a homodyne technique. The fluctuation levels are around 30% for all three quantities, but the autocorrelation times of temperature and potential are shorter than that of density: <3 µs against ≈13 µs. The magnitude and mutual coherence of the fluctuations are affected by the connection length Lc of the flux tube connected to the probe, offering a possible explanation of the variation of both the particle and the energy transport coefficient with Lc found experimentally.

Characterizing electrostatic turbulence in tokamak plasmas with high MHD activity

Journal of Physics: Conference Series, 2010

One of the challenges in obtaining long lasting magnetic confinement of fusion plasmas in tokamaks is to control electrostatic turbulence near the vessel wall. A necessary step towards achieving this goal is to characterize the turbulence level and so as to quantify its effect on the transport of energy and particles of the plasma. In this paper we present experimental results on the characterization of electrostatic turbulence in Tokamak Chauffage Alfvén Brésilien (TCABR), operating in the Institute of Physics of University of São Paulo, Brazil. In particular, we investigate the effect of certain magnetic field fluctuations, due to magnetohydrodynamical (MHD) instabilities activity, on the spectral properties of electrostatic turbulence at plasma edge. In some TCABR discharges we observe that this MHD activity may increase spontaneously, following changes in the edge safety factor, or after changes in the radial electric field achieved by electrode biasing. During the high MHD activity, the magnetic oscillations and the plasma edge electrostatic turbulence present several common linear spectral features with a noticeable dominant peak in the same frequency. In this article, dynamical analyzes were applied to find other alterations on turbulence characteristics due to the MHD activity and turbulence enhancement. A recurrence quantification analysis shows that the turbulence determinism radial profile is substantially changed, becoming more radially uniform, during the high MHD activity. Moreover, the bicoherence spectra of these two kinds of fluctuations are similar and present high bicoherence levels associated with the MHD frequency. In contrast with the bicoherence spectral changes, that are radially localized at the plasma edge, the turbulence recurrence is broadly altered at the plasma edge and the scrape-off layer.

Emissive probe measurements of plasma potential fluctuations in the edge plasma regions of tokamaks

Review of Scientific Instruments, 2003

The plasma potential ⌽ pl and its fluctuations ⌽ pl were measured by electron emissive probes in the edge plasma regions of two fusion experiments: the Instituto Superior Técnico Tokamak ͑ISTTOK͒ ͑Lisbon, Portugal͒, and the Czech Academy of Sciences Torus ͑CASTOR͒ tokamak ͑Prague, Czech Republic͒. Into ISTTOK, three emissive probes were inserted outside the last closed flux surface ͑LCFS͒ on different minor radii. In CASTOR, two emissive probes, poloidally separated, and two cold cylindrical probes, mounted on the same shaft, were used, which could be radially shifted outside and inside the LCFS. The advantages of a sufficiently emissive probe are that in principle ⌽ pl and ⌽ pl can be measured directly, without being affected by electron temperature fluctuations or drifting electrons.

Validating simulations of core tokamak turbulence: current status and future directions

Journal of Physics: Conference Series, 2008

Validating predictive models of turbulent transport in magnetically confined plasmas requires comparisons of detailed fluctuation statistics, in addition to net energy flows. Using measurements from new and improved diagnostics on the DIII-D tokamak Nucl. Fusion 42 614], we have performed a series of comparisons against predictions from the GYRO code [Candy J and Waltz R E 2003 J. Comp. Phys. 186 545]. The development and application of synthetic diagnostics that model the spatial sensitivities of a given experimental fluctuation diagnostic is essential for these comparisons. At € r /a = 0.56, we find very good agreement between the predicted and measured energy fluxes and fluctuation power spectra. However, at € r /a = 0.8 the simulations underpredict the energy flows by a factor of seven and fluctuation amplitudes by a factor of three, but successfully reproduce the shapes of the experimentally measured fluctuation power spectra. At both locations significant attenuation in the synthetic power spectra and fluctuation levels is observed relative to "unfiltered" levels. Additional results contrasting local and nonlocal simulation results and convergence in toroidal mode number spacing are presented.

Measurements of ion temperature fluctuations in the Tokamak Fusion Test Reactor

Nuclear Fusion, 1998

First of a kind measurements of high-frequency ion temperature microturbulence in fusion-grade plasmas have been made in TFTR. The ion temperature fluctuations and carbon density fluctuations were found to have spectra similar to those of the ion density fluctuations across the plasma radius. The ratio of the relative fluctuation levels, (T /T)/(ñ/n), is 2 ± 0.5 from r/a = 0.59 to r/a = 0.99. The fact that this ratio is greater than unity is consistent with the general expectations of ion temperature gradient driven turbulence theory and suggests that ion drift modes dominate trapped electron modes in the turbulent spectrum. The temperature fluctuation spectra were found to exhibit a narrow transition region between distinctive edge and core turbulent modes, as has been seen with ion density fluctuations. The ratio of the relative fluctuation levels is greater than unity across this transition, which suggests that, despite the different modes present, the underlying instability is driven by the ion temperature gradient.