Self-consistent dynamics of impurities in magnetically confined plasmas: turbulence intermittency and nondiffusive transport

Self-consistent turbulent transport of high-concentration impurities in magnetically confined fusion plasmas is studied using a three-dimensional nonlinear fluid global turbulence model which includes ion-temperature gradient and trapped electron mode instabilities. It is shown that the impurity concentration can have a dramatic feedback in the turbulence and, as a result, it can significantly change the transport properties of the plasma. High concentration impurities can trigger strong intermittency that manifests in non-Gaussian heavy tails of the probability density functions of the E × B fluctuations and of the ion-temperature flux fluctuations. At the heart of this self-consistent coupling is the existence of inward propagating ion-temperature fronts with a sharp gradient at the leading edge that give rise to instabilities and avalanchelike bursty transport. Numerical evidence of time nonlocality (i.e., history dependence) in the delayed response of the flux to the gradient is presented.Postprint (published version

A reduced MHD model for ITG-NTM interplay

International audienceA 6-field reduced-MHD model is derived for plasma dynamics. The new model describes coherently both Ion Temperature Gradient mode and Tearing mode, and includes neoclassical effects. The model allows the construction of an energy-like quantity with a linear pressure contribution that is conserved except for dissipative, finite Larmor radius and neoclassical terms. This model may be used to study the nonlinear interaction between ITG microturbulence and neoclassical tearing mode, which is responsible for large-scale magnetic islands in tokamaks, and opens the way to a coherent description of turbulent impurity transport in magnetic islands

Dynamics of Magnetic Islands driven by Ballooning Turbulence

International audienceMagnetic island generation by remote ballooning turbulence close to the plasma edge is investigated through ux-driven 3D Reduced-MHD simulations. The various coupling mechanisms are investigated : mono-helicity nonlinear coupling, multi-helicity nonlinear coupling and linear toroidal coupling. The dominant process depends on the imposed heat ux driving the turbulence. The remote drive happens in two successive phases corresponding to dierent coupling paths. While an island is remotely generated in both phases, it is dominated by dierent mode numbers, or harmonics, and therefore has a dierent shape. The size of the generated island is found to be proportional to the imposed heat ux, without threshold. The shape of the island in the saturated regime also depends on the imposed heat ux, with a more distorted shape at low power levels

Nonlinear dynamics of turbulence driven magnetic islands. II. Numerical simulations

International audienceThe nonlinear dynamics of a turbulence driven magnetic island (TDMI) is investigated numerically in a reduced magnetohydrodynamic fluid model. The significance of identifying a characteristic signature of a TDMI for its experimental observation is discussed. The principal focus of our simulations is on the nature of the pressure profile flattening inside a TDMI, and we show that, in agreement with analytical predictions, a partial flattening occurs when the island size exceeds a critical value that is a function of the small scale interchange dynamics. We also present a model and test it numerically, which links explicitly the interchange turbulence and the island pressure flattening. Published by AIP Publishing

Nonlinear dynamics of turbulence driven magnetic islands. I. Theoretical aspects

International audienceThe nonlinear properties of a turbulence driven magnetic island (TDMI) are investigated. Starting from a minimal magnetohydrodynamic fluid model that provides for the generation of a TDMI and using scale separation arguments along with numerical simulation findings, we elucidate the links between the nonlinear transport properties of such magnetic islands and the characteristic features of the small scale turbulence. We also explain the phenomenon of partial pressure flattening inside the TDMI

Influence of azimuthal instabilities on electron motion in a Hall Effect Thruster

Since the beginning of Hall Effect thrusters (HET) research, anomalous electron conductivity has been an outstanding question, preventing the development of predictive thruster models. The increasing interest in this technology for space exploration encourages further attempts to clarify this aspect of the thruster physics. The possible contribution of "near-wall conductivity" to the overall electron conductivity has been long discussed, but it now seems clear that anomalous conductivity due to plasma turbulence must be present to explain the experimental results. Recent PIC simulations have shown that azimuthal instabilities can lead to axial electron transport. In this paper, a simplified study of the influence of an azimuthal instability on electron transport is carried out in the case of a PPS1350 thruster. Adding a fluctuating azimuthal electric field to the time averaged field in the (r, z) plane given by a hybrid model, we have studied single electron trajectories from the cathode to the anode and deduced macroscopic electron transport properties. Our results show how electrons can be driven by the instability from the cathode to the anode and provide an estimate of the mean axial speed of the electrons in the discharge channel. Different amplitudes and wave numbers of the monomode azimuthal instability have been investigated. = electron speed at t x t = electron position at t τ = electron transit time from cathode to anode Nomenclatur

Generation of a magnetic island by edge turbulence in tokamak plasmas

International audienceWe investigate, through extensive 3D magneto-hydro-dynamics numerical ă simulations, the nonlinear excitation of a large scale magnetic island ă and its dynamical properties due to the presence of small-scale ă turbulence. Turbulence is induced by a steep pressure gradient in the ă edge region [B. D. Scott, Plasma Phys. Controlled Fusion 49, S25 ă (2007)], close to the separatrix in tokamaks where there is an X-point ă magnetic configuration. We find that quasi-resonant localized ă interchange modes at the plasma edge can beat together and produce ă extended modes that transfer energy to the lowest order resonant surface ă in an inner stable zone and induce a seed magnetic island. The island ă width displays high frequency fluctuations that are associated with the ă fluctuating nature of the energy transfer process from the turbulence, ă while its mean size is controlled by the magnetic energy content of the ă turbulence. (C) 2015 AIP Publishing LLC

Study of high frequency MHD modes from ECE radiometer in Tore Supra

Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz. To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible. With such an improvement, fast particle driven modes with frequencies up to 200 kHz were detected. A 4-channel correlation ECE system using YIG filters with tuneable frequency was also installed. It allows fine radial scans of MHD modes and correlation length measurements. For the two kinds of YIG filter in use, the minimum frequency separation between two ECE channels that could be achieved was established measuring the correlation coefficient between the respective radiation noises. Finally, by modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, thus giving a better determination of the phase radial structure of ECE oscillations. The poloidal structure of MHD modes can then be identified from ECE data and, for off axis ECE lines of sight, the direction of the plasma rotation can also be determined. This method allows identifying the occurrence of an inverse cascade of electron fishbone modes ranging from m/n=4/4 to 1/1 (m and n are the poloidal and toroidal mode numbers, respectively) which appears in lower hybrid current drive plasmas

Study of high frequency MHD modes from ECE radiometer in Tore Supra

Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz. To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible. With such an improvement, fast particle driven modes with frequencies up to 200 kHz were detected. A 4-channel correlation ECE system using YIG filters with tuneable frequency was also installed. It allows fine radial scans of MHD modes and correlation length measurements. For the two kinds of YIG filter in use, the minimum frequency separation between two ECE channels that could be achieved was established measuring the correlation coefficient between the respective radiation noises. Finally, by modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, thus giving a better determination of the phase radial structure of ECE oscillations. The poloidal structure of MHD modes can then be identified from ECE data and, for off axis ECE lines of sight, the direction of the plasma rotation can also be determined. This method allows identifying the occurrence of an inverse cascade of electron fishbone modes ranging from m/n=4/4 to 1/1 (m and n are the poloidal and toroidal mode numbers, respectively) which appears in lower hybrid current drive plasmas