Search

International audience; We 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.

This work presents a comparative study of the experimental observation of MHD instabilities identified as electron fishbone-like modes that appear in plasmas with lower hybrid current drive in FTU and Tore Supra tokamaks. Initially, the mode-induced electronic temperature oscillations measured by electron cyclotron emission were used to study the evolutions of the frequency and position of these modes. In FTU, where fishbones with and without bursting behaviour are observed, it was found that the evolutions of the mode position and frequency follow opposite trend in the two regimes. In Tore Supra, where abrupt changes between modes with different mode structures are often observed, it was found that the mode position evolves continuously and the wavenumbers follow an inverse cascade starting from an m/n = 4/4 mode and finishing in an 1/1 mode. In a second step, the energy of resonant electrons was estimated from the resonant condition of the precession drift frequency. It was found that in Tore Supra the resonant condition does not change during the frequency jumps. The relevance of the correction due to the pitch angle of the resonant electrons and the possible role of the energetic passing particles in the drive of these modes are discussed from the results obtained in both machines.

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.

International audience; A 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.