Fluctuation-induced Flux Research Articles

Avalanche statistics of fluctuation-induced fluxes from the SLPM and the W7-AS stellarator

Measurements of fluctuating floating potentials and ion saturation currents at different radial locations in the Santander Linear Plasma Machine (Castellanos et al 2005 Plasma Phys. Control. Fusion 47 2067) and at the edge of the W7-AS stellarator by means of radially movable Langmuir probes allow to infer the corresponding fluctuation-induced radial flux temporal series. Avalanche-like transport events are identified in the time series and statistically characterized in terms of avalanche size/duration/quiet-time distributions and size-duration scaling relations. Transport is diffusive in the inner and intermediate radial region of the SLPM r<rtr≈2.6 cm, undergoing a transition at rtr , becoming non-diffusive in the outermost region of the device, r>rtr . The results obtained at the edge of the W7-AS stellarator are similar to those found in SLPM for r>rtr , i.e. consistent with what would be expected for scale-free, self-similar plasma transport dynamics near a critical state.

Dynamo-driven plasmoid formation from a current-sheet instability

Axisymmetric current-carrying plasmoids are formed in the presence of nonaxisymmetric fluctuations during nonlinear three-dimensional resistive MHD simulations in a global toroidal geometry. We utilize the helicity injection technique to form an initial poloidal flux in the presence of a toroidal guide field. As helicity is injected, two types of current sheets are formed from (1) the oppositely directed field lines in the injector region (primary reconnecting current sheet), and (2) the poloidal flux compression near the plasma edge (edge current sheet). We first find that nonaxisymmetric fluctuations arising from the current-sheet instability isolated near the plasma edge have tearing parity but can nevertheless grow fast (on the poloidal Alfven time scale). These modes saturate by breaking up the current sheet. Second, for the first time, a dynamo poloidal flux amplification is observed at the reconnection site (in the region of the oppositely directed magnetic field). This fluctuation-induced flux amplification increases the local Lundquist number, which then triggers a plasmoid instability and breaks the primary current sheet at the reconnection site. The plasmoids formation driven by large-scale flux amplification, i.e., a large-scale dynamo, observed here has strong implications for astrophysical reconnection as well as fast reconnection events in laboratory plasmas.

On the Origin of Rotation in the Universe

The evolution of small perturbations produced by gravitational instability in ideal gases is examined in a Newtonian approach. As opposed to the classical theory, here effects associated with fluctuationinduced fluxes of matter are taken into account. It is shown that for sufficiently small wave numbers, both longitudinal and vortex perturbations can grow and this explains the origin of galactic rotation in an initially uniform, isotropic, irrotational universe.

Transport and drift-driven plasma flow components in the Alcator C-Mod boundary plasma

Boundary layer flows in the Alcator C-Mod tokamak are systematically examined as magnetic topology (upper versus lower-null) and plasma density are changed. Utilizing a unique set of scanning Langmuir–Mach probes, including one on the high-field side (HFS) midplane, the poloidal variation of plasma flow components in the parallel, diamagnetic and radial directions are resolved in detail. It is found that the plasma flow pattern can be decomposed into two principal parts: (1) a drift-driven component, which lies within a magnetic flux surface and is divergence-free and (2) a transport-driven component, which gives rise to near-sonic parallel flows on the HFS scrape-off layer (SOL). Toroidal rotation, Pfirsch–Schlüter and transport-driven contributions are unambiguously identified. Transport-driven parallel flows are found to dominate the HFS particle fluxes; the total poloidal-directed flow accounts for ∼1/3 to all of the ion flux arriving on the inner divertor. As a result, heat convection is found to be an important player in this region, consistent with the observation of divertor asymmetries that depend on the direction of B × ∇B relative to the active x-point. In contrast, the poloidal projection of parallel flow in the low-field SOL largely cancels with Er × B flow; toroidal rotation is the dominant plasma motion there. The magnitude of the transport-driven poloidal flow is found to be quantitatively consistent with fluctuation-induced radial particle fluxes on the low-field side (LFS), identifying this as the primary drive mechanism. Fluctuation-induced fluxes on the HFS are found to be essentially zero, excluding turbulent inward transport as the mechanism that closes the circulation loop in this region.

Electrostatic turbulence and transport in a simple magnetized plasma

Gradient driven electrostatic instabilities are investigated in TORPEX [A. Fasoli, B. Labit, M. McGrath, S. H. Müller, M. Podestà, and F. M. Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device (R=1m, a=0.2m) in which plasmas are produced by microwaves (P⩽20kW) with frf=2.45GHz, in the electron cyclotron frequency range. Typical density and temperature are ne⩽1017m−3 and Te≃5eV, respectively. The magnetic field is mainly toroidal (⩽0.1T), with a small vertical component (⩽4mT). Instabilities that can be generally identified as drift-interchange waves are observed and characterized for different levels of collisionality with neutrals. The frequency spectrum and the spatial profile of the fluctuation-induced flux are measured. An 86-tip probe is used to reconstruct the spatio-temporal evolution of density structures across the plasma cross section. The measured structures are characterized statistically, and related quantitative observables are constructed.

Arrangement of emissive and cold probes for fluctuation and Reynolds stress measurements

An arrangement of three emissive probes and one cold probe was used to simultaneously determine the Reynolds stress and the fluctuation-induced flux in the edge region of the tokamak ISTTOK. The emissive probes are arranged in a 90º triangle and inserted into the tokamak so that two of the probes are separated radially and two are separated in the poloidal direction, with one probe being used for both directions. Since emissive probes deliver a better measure of the plasma potential than cold probes, with this arrangement the radial and the poloidal electric field and their fluctuations can be measured simultaneously, so that the Reynolds stress Re can be derived from the data. This setup is radially movable so that also the radial gradient of Re can be determined. In addition, a cold probe is inserted close to one of the outer emissive probes. The fluctuation-induced flux can be determined from a simultaneous measurement of the poloidal field fluctuations and those of the ion saturation current.

Self-Organized Criticality paradigm

According to the paradigm of the Self-Organized Criticality (SOC), the anomalous transport in tokamaks is caused by fast transient processes avalanches. One of the manifestations of these phenomena should be 1/f decay of electrostatic fluctuations power spectra in a certain frequency range. In this paper, the frequency spectra of floating potential, density and fluctuation-induced flux, measured by poloidal and radial arrays of Langmuir probes on the CASTOR tokamak, are presented. The floating potential and the fluctuation-induced flux decay from 30 kHz up to 100 kHz asf −1. The plasma density decays asf −1 in a more narrow band, 20–40 kHz. The possible limitation of SOC behavior for frequencies higher than 100 kHz due to intermittency is stressed. For this reasons the Probability Distribution Functions (PDFs) of floating potential fluctuations were computed at different time scales using wavelet transform. A clear departure of the computed PDFs from Gaussianity, which is a classical signature of intermittency, is observed at time scales under 10 μs (100 kHz).

Edge Turbulence Analysis by Means of Electrical Probes on Tokamak CASTOR

A poloidally oriented electrical probe array immersed into the plasma edge providing spatially and temporally resolved information of the plasma density and potential is used to study plasma edge turbulence at the CASTOR tokamak (R = 0.4 m, a = 0.085m). A model of ‘eddies’ formulated in [1] is experimentally proved by means of the mutual correlation between fluctuations of the plasma density and potential. Scans over principal plasma parameters (plasma current Ip, plasma density ne and toroidal magnetic field Bt) were performed and three different modes (three qualitatively different structures of fluctuations) were observed. The statistics of the fluctuation-induced flux were studied. A parameter virtually independent of the discharge conditions was found, its nature has not been explained yet.

Measurement of fluctuation-induced particle fluxes in the shadow of the diaphragm of the FT-2 tokamak

Movable multielectrode probes are used in an experimental study of fluctuations in particle density and electric field, their spectral and correlation characteristics, and the poloidal-radial distributions of fluctuation-induced drift fluxes in the shadow of the diaphragm of the FT-2 tokamak. The spatial regions in which these fluxes are particularly intensive at different stages of the discharge are determined. The part played by fluctuation-induced fluxes in the peripheral radial particle transport is estimated and found to be substantial.