Mohamed Ezzat Publications Content


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Ascasíbar, E., et al., M. Ezzat, et al., J. M. García-Regaña, et al., and the TJ-II team Overview of recent TJ-II stellarator results Nuclear Fusion, 59/11, pp. 1-13, 2019. [Download PDF] [View Abstract]The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented.

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Ezzat, M. Advanced neoclassical impurity transport modelling with its experimental comparison for TJ-II , MSc Thesis, Ghent University, 51 pp., 2018. [View Abstract]The absence of the disruptive instabilities, increasing of the confinement time with the ECRH heating and the steady-state operation make stellarator concept as a competitive candidate for future fusion reactors as the tokamak. Impurity accumulation in the core though is one of the stellarator drawbacks because it dilutes the plasma and increases the radiation losses contributing to the plasma collapse. Neoclassical theory predicts a non-ambipolar transport of different species electrons and ions in stellarators due to magnetic field ripples that are produced by the three-dimensional coils structure. Non-ambipolar transport creates, depending on the collisionality of each species, inward (outward) an ambipolar radial electric field for ion (electron) root regimes. Ion root regime has been predicted for the future stellarator reactor scenarios, which imply very likely impurity accumulation. However, outward transport has been observed during an improved confinement regime so-called \textit{HDH mode} at W7-AS (\textit{K. McCormick 2002}) and the \textit{impurity hole} at LHD (\textit{K. Ida 2009}) but without without satisfactory theoritical explanation. Historically, neoclassical treatment considers only the radial component of the electric field, which is a good approximation for the bulk species, but not for the higher charge species like impurity. Recent approaches have considered the tangential component due to the variation of the electrostatic potential within a flux surface which is more important for high charge impurity (see \[\textit{J. M. {Garc\'ia-Rega\~na} 2013}\] and reference therein). Advanced modelling of this variational part has been done here for TJ-II plasma introducing a parameter which can be measured indirectly. Direct measurement of the variational part is non-trivial and had been carried only for plasma edge (\textit{M. A. Pedrosa 2015}). Here, the indirect measurements cover the whole cross section by constructing the radiation map at two toroidal planes in TJ-II that carried impurity density distribution and in sequence the variational electrostatic part. Linearized impurity-ion collision operator (in \textit{I. Calvo 2018-Arxiv}) had been employed for impurity simulation because it is higher collisional instead of the usual pitch angel scattering operator (in \textit{C. D. Beidler 2011}) for bulk species with low collisions.