Nonlinear gyrokinetic PIC simulations in stellarators with the code EUTERPE

In this work, the first nonlinear particle-in-cell simulations carried out in a stellarator with the global gyrokinetic code EUTERPE using realistic plasma parameters are reported. Several studies are conducted with the aim of enabling reliable nonlinear simulations in stellarators with this code. First, EUTERPE is benchmarked against ORB5 in both linear and nonlinear settings in a tokamak configuration. Next, the use of noise control and stabilization tools, a Krook-type collision operator, markers weight smoothing and heating sources is investigated. It is studied in detail how these tools influence the linear growth rate of instabilities in both tokamak and stellarator geometries and their influence on the linear zonal flow evolution in a stellarator. Then, it is studied how these tools allow improving the quality of the results in a set of nonlinear simulations of electrostatic turbulence in a stellarator configuration. Finally, these tools are applied to a W7-X magnetic configuration using experimental plasma parameters.Comment: 24 pages, 19 figure

Turbulent transport of impurities in 3D devices

A large diffusive turbulent contribution to the radial impurity transport in Wendelstein 7-X (W7-X) plasmas has been experimentally inferred during the first campaigns and numerically confirmed by means of gyrokinetic simulations with the code stella. In general, the absence of strong impurity accumulation during the initial W7-X campaigns is attributed to this diffusive term. In the present work the diffusive contribution is also calculated in other stellarator plasmas. In particular, the diffusion (D) and convection (V) coefficients of carbon and iron impurities produced by ion-temperature-gradient (ITG) turbulence are obtained for W7-X, LHD, TJ-II and NCSX. The results show that, although the size of D and V can differ across the four devices, inward convection is found for all of them. For W7-X, TJ-II and NCSX the two coefficients are comparable and the turbulent peaking factor is surprisingly similar. In LHD, appreciably weaker diffusive and convective impurity transport and significantly larger turbulent peaking factor are predicted. All this suggests that ITG turbulence, although not strongly, would lead to negative impurity density gradients in stellarators. Then, considering mixed ITG/Trapped Electron Mode (TEM) turbulence for the specific case of W7-X, it has been quantitatively assessed to what degree pellet fueled reduced turbulence scenarios feature reduced turbulent transport of impurities as well. The results for trace iron impurities show that, although their turbulent transport is not entirely suppressed, a significant reduction of V and a stronger decrease of D are found. Although the diffusion is still above neoclassical levels, the neoclassical convection would gain under such conditions a greater specific weight on the dynamics of impurities in comparison with standard ECRH scenarios.Comment: 16 pages, 8 figure

Turbulent impurity transport simulations in Wendelstein 7-X plasmas

A study of turbulent impurity transport by means of quasilinear and nonlinear gyrokinetic simulations is presented for Wendelstein 7-X (W7-X). The calculations have been carried out with the recently developed gyrokinetic code stella. Different impurity species are considered in the presence of various types of background instabilities: ITG, TEM and ETG modes for the quasilinear part of the work; ITG and TEM for the nonlinear results. While the quasilinear approach allows one to draw qualitative conclusions about the sign or relative importance of the various contributions to the flux, the nonlinear simulations quantitatively determine the size of the turbulent flux and check the extent to which the quasilinear conclusions hold. Although the bulk of the nonlinear simulations are performed at trace impurity concentration, nonlinear simulations are also carried out at realistic effective charge values, in order to know to what degree the conclusions based on the simulations performed for trace impurities can be extrapolated to realistic impurity concentrations. The presented results conclude that the turbulent radial impurity transport in W7-X is mainly dominated by ordinary diffusion, which is close to that measured during the recent W7-X experimental campaigns. It is also confirmed that thermo-diffusion adds a weak inward flux contribution and that, in the absence of impurity temperature and density gradients, ITG- and TEM-driven turbulence push the impurities inwards and outwards, respectively.Comment: 19 pages, 10 figures, 2 table

Gyrokinetic simulations in stellarators using different computational domains

In this work, we compare gyrokinetic simulations in stellarators using different computational domains, namely, flux tube, full-flux-surface, and radially global domains. Two problems are studied: the linear relaxation of zonal flows and the linear stability of ion temperature gradient (ITG) modes. Simulations are carried out with the codes EUTERPE, GENE, GENE-3D, and stella in magnetic configurations of LHD and W7-X using adiabatic electrons. The zonal flow relaxation properties obtained in different flux tubes are found to differ with each other and with the radially global result, except for sufficiently long flux tubes, in general. The flux tube length required for convergence is configuration-dependent. Similarly, for ITG instabilities, different flux tubes provide different results, but the discrepancy between them diminishes with increasing flux tube length. Full-flux-surface and flux tube simulations show good agreement in the calculation of the growth rate and frequency of the most unstable modes in LHD, while for W7-X differences in the growth rates are found between the flux tube and the full-flux-surface domains. Radially global simulations provide results close to the full-flux-surface ones. The radial scale of unstable ITG modes is studied in global and flux tube simulations finding that in W7-X, the radial scale of the most unstable modes depends on the binormal wavenumber, while in LHD no clear dependency is found.Comment: submitted to Nuclear Fusio

Experiencias de Innovación docente en los Estudios Jurídicos: una visión práctica

Esta publicación se enmarca dentro de las actividades del Grupo de Investigación de la Universidad de Extremadura Fiscalitas & Iuris.Este trabajo surge con el objetivo principal de dar visibilidad y publicidad a las nuevas técnicas docentes en el seno de la Facultad de Derecho de la UEx. Como se sabe, se ha producido un innegable y significativo avance en el uso de nuevas técnicas docentes y también de las TICs aplicadas a la docencia en la Facultad de Derecho, no obstante, aún es necesario profundizar en el uso de las mismas y extenderlas entre todos los miembros del claustro de profesores, y fundamentalmente entre aquellos que llevan más años ejerciendo la docencia a través de la colaboración y la coordinación con los profesores noveles, que son quienes principalmente se sirven en mayor medida de tales instrumentos docentes. De otra parte, también era necesario que los docentes más experimentados pudieran encontrar un foro en el que transmitir y compartir con los noveles cuales son las técnicas e instrumentos docentes que ellos han venido utilizando durante el ejercicio de su magisterio, de modo que, en el marco de una relación sinalagmática, se produjera una interacción entre uno u otro grupo de docentes, a fin de fomentar el necesario debate y el intercambio de experiencias e instrumentos docentes, y en su caso el desarrollo y perfeccionamiento de los mismos; algo que hemos pretendido realizar con este trabajo, y que en buena medida hemos logrado. Las finalidades y objetivos concretos que perseguíamos, en atención a la situación expuesta eran fundamentalmente tres: • En primer lugar, la implementación de un proyecto de innovación docente integrado por una diversidad de actividades coordinadas, cada uno de ellas bajo la directa coordinación de un profesor o profesora de la UEx, aplicado a una o varias asignaturas impartidas en la Facultad de Derecho. • En segundo lugar, el establecimiento en la Facultad de Derecho de un foro de coordinación e intercambio de buenas prácticas docentes sobre la base de cada uno de las actividades coordinadas, en el que pudieran participar profesores noveles y veteranos. Para ello se desarrolló espacio virtual de innovación docente en estudios jurídicos, a través del Campus Virtual de la UEx, en el que los Profesores noveles y veteranos pudieron y puede compartir recursos e informaciones sobre prácticas de innovación. • Y, en tercer lugar, la difusión y consolidación de instrumentos de innovación docente directamente aplicadas a la docencia de los estudios jurídicos, mediante la transferencia de los resultados y la publicación de los mismos; a fin de que esta transferencia sirva de base a futuras profundizaciones en el campo de la innovación docente en los estudios jurídicos.Proyecto “Desarrollo, profundización e intercambio de buenas prácticas de innovación docente en la Facultad de Derecho” (UEx 2015-2016

Calculation of the bootstrap current profile for the TJ-II stellarator

Calculations of the bootstrap current for the TJ-II stellarator are presented. DKES and NEO-MC codes are employed; the latter has allowed, for the first time, the precise computation of the bootstrap transport coefficient in the long mean free path regime of this device. The low error bars allow a precise convolution of the monoenergetic coefficients, which is confirmed by error analysis. The radial profile of the bootstrap current is presented for the first time for the 100_44_64 configuration of TJ-II for three different collisionality regimes. The bootstrap coefficient is then compared to that of other configurations of TJ-II regularly operated. The results show qualitative agreement with toroidal current measurements; precise comparison with real discharges is ongoing

Overview of recent TJ-II stellarator results

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. ISSN:0029-5515 ISSN:1741-432