Edilberto Sánchez

Collisionless damping of flows in the TJ-II stellarator

The results of global linear gyrokinetic simulations of residual flows carried out with the code EUTERPE in the TJ-II three-dimensional geometry are reported. The linear response of the plasma to potential perturbations homogeneous in a magnetic surface shows several oscillation frequencies: a Geodesic-acoustic-mode-like frequency, in qualitative agreement with the formula given by Sugama and Watanabe (2006 Plasma Phys. 72 825), and a much lower frequency oscillation in agreement with the predictions of Mishchenko et al (2008 Phys. Plasmas 15 072309) and Helander et al (2011 Plasma Phys. Control. Fusion 53 054006) for stellarators. The dependence of both oscillations on ion and electron temperatures and the magnetic configuration is studied. The low-frequency oscillations are in the frequency range supporting the long-range correlations between potential signals experimentally observed in TJ-II.

Improvements of the particle-in-cell code EUTERPE for petascaling machines

Abstract In the present work we report some performance measures and computational improvements recently carried out using the gyrokinetic code EUTERPE (Jost, 2000 [1] and Jost et al., 1999 [2] ), which is based on the general particle-in-cell (PIC) method. The scalability of the code has been studied for up to sixty thousand processing elements and some steps towards a complete hybridization of the code were made. As a numerical example, non-linear simulations of Ion Temperature Gradient (ITG) instabilities have been carried out in screw-pinch geometry and the results are compared with earlier works. A parametric study of the influence of variables (step size of the time integrator, number of markers, grid size) on the quality of the simulation is presented.

Linear and Nonlinear Simulations Using the EUTERPE Gyrokinetic Code

In this paper, we report on simulations that have recently been carried out using the EUTERPE gyrokinetic code. The scaling of the code has been studied up to 20 000 processing elements. Linear and nonlinear simulations of ion temperature-gradient instabilities have been carried out in screw-pinch geometry, and the results are compared with those previously obtained using the TORB code, finding a good agreement. The influence of a finite β on the growth rates of instabilities and on the zonal flows in a screw-pinch has also been studied. The results are compared with previous ones.

Results of the remote participation on TJ-II neutral particle analyzer

Neutral Particle Analyzer diagnostic has been operated remotely from Universitat Politecnica de Catalunya in Barcelona, separated by more than 600 km from Laboratorion Nacional de Fusion in Madrid. Remote participation tools are used to control the diagnostic, to visualize the experimental TJ-II data, and to participate in scientific meetings on TJ-II experiments and models. The results of such participation are shown in this work, namely the charge-exchange neutral spectra, the ion temperature profiles in TJ-II, and the results of transport analysis. Neutral spectra have two slopes, one of them attributed to the core neutrals and the other one to the neutrals coming from the edge. The ion temperature is obtained in a series of reproducible discharges, at every point from regression analysis of those spectra. The ion temperature profile happens to be pretty flat. Ion heat transport analysis is performed with Proctr and PRETOR-STELLARATOR codes and the results are compared.

Residual zonal flows in tokamaks and stellarators at arbitrary wavelengths

In the linear collisionless limit, a zonal potential perturbation in a toroidal plasma relaxes, in general, to a non-zero residual value. Expressions for the residual value in tokamak and stellarator geometries, and for arbitrary wavelengths, are derived. These expressions involve averages over the lowest order particle trajectories, that typically cannot be evaluated analytically. In this work, an efficient numerical method for the evaluation of such expressions is reported. It is shown that this method is faster than direct gyrokinetic simulations performed with the GENE and EUTERPE codes. Calculations of the residual value in stellarators are provided for much shorter wavelengths than previously available in the literature. Electrons must be treated kinetically in stellarators because, unlike in tokamaks, kinetic electrons modify the residual value even at long wavelengths. This effect, that had already been predicted theoretically, is confirmed by gyrokinetic simulations.

Particle-in-Cell Algorithms for Plasma Simulations on Heterogeneous Architectures

Within the fusion community, a large number of codes are in use to simulate various aspects of the plasma behaviour. Many codes have been written by physicists with a big emphasis on the physics without using the latest technologies available in computer science. For improving this situation, a project with acronym EUFORIA was created. The main target of this project is to increase the performance of key existing codes, either parallelizing the sequential codes or improving their parallelization. However, in general these codes were not thought for new supercomputers based on muticore architectures and only take advantage of the task level parallelism, basically using Message Passing Interface (MPI). In this paper we discuss several possible ways to apply hybrid parallelization techniques in fusion codes for exploiting new multiprocessor supercomputers under development.

On-surface potential and radial electric field variations in electron root stellarator plasmas

In the present work we report recent radial electric field measurements carried out with the Doppler reflectometry system in the TJ-II stellarator. The study focuses on the fact that, under some conditions, the radial electric field measured at different points over the same flux surface shows significantly different values. A numerical analysis is carried out considering the contribution arising from the radial dependence of

Observation of Oscillatory Radial Electric Field Relaxation in a Helical Plasma

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First experience with particle-in-cell plasma physics code on ARM-based HPC systems

as a possible correction term to the total radial electric field. Here

Semianalytical calculation of the zonal-flow oscillation frequency in stellarators

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