Modelling of turbulent particle transport in finite-beta and multiple ion species plasma in tokamaks

Recent experimental results carried out on Frascati Tokamak Upgrade (FTU) with the use of Liquid Lithium Limiter (LLL) show that the presence of lithium impurity can give rise to an improved particle confinement regime in which the main plasma constituents are transported towards the core whereas the impurity particles are driven outwards. The aim of our research was to further investigate this process using gyrokinetic simulations with the GKW code to calculate the particle flux in FTU-LLL discharges, and to provide a physical explanation of the above phenomena with a simplified multi-fluid description. The fluctuations in the FTU tokamak are dominantly electro-static (ES), magnetic perturbations are expected to be important in high beta tokamak plasmas, such as those in the Mega Amp`ere Spherical Tokamak (MAST). The effects of impurities on the electro-magnetic (EM) terms of turbulent particle transport are investigated in a typical MAST H-mode discharge. The first chapter of the thesis is dedicated to provide an understandable but thorough introduction to the gyrokinetic equation and the code GKW. It summarizes the concept of the Lie- transform perturbation method which forms the basis of the modern approach to gyrokinetics. The gyrokinetic Vlasov–Maxwell system of equations including the full electro-magnetic perturbation is derived in the Lagrangian formalism in a rotating frame of reference. The simulation code GKW is briefly introduced and the calculation of the particle fluxes is explained. In the second chapter the FTU-LLL and MAST experiments are introduced and the gy- rokinetic simulations of the two discharges are presented. It is shown that in an ES case the ITG driven electron transport is significantly reduced at high lithium concentration. This is accom- panied by an ion flow separation in order to maintain quasi-neutrality, and an inward deuterium pinch is obtained by a sufficiently high impurity density gradient. The EM terms are found to be negligible in the ion particle flux compared to the ExB contribution even at relatively high plasma beta. However, the EM effects drive a strong non-adiabatic electron response and thus prevent the ion flow separation in the analyzed cases. The third chapter provides a detailed description of a multi-fluid model that is used to gain insight into the diffusive, thermo-diffusive and pinch terms of the anomalous particle transport. It is based on the collisionless Weiland model, however, the trapped electron collisions are introduced (Nilsson & Weiland, NF 1994) in order to capture the micro-stability properties of the gyrokinetic simulations. The model is compared with analytical and numerical results in the two-fluid, adiabatic electron and large aspect ratio limits, showing good qualitative agreement. In the fourth chapter the fluid analysis of the FTU-LLL discharge is presented. It is shown that the inward deuterium pinch is achieved by a reduction of the diffusive term of the ITG driven main ion flux in presence of lithium impurities. The ITG mode responsible for the majority of the radial particle transport has been found to be the only unstable eigenmode rotating in the ion diamagnetic direction. Eigenmodes associated with the deuterium and lithium temperature gradients can be separately obtained when the Larmor-radius of the two ion species are more distinct, in which case the effect of lithium on the main ion transport is reduced and the inward deuterium flux is weaker

Horizontal scraped surface heat exchanger — Experimental measurements and numerical analysis

This paper deal with the overall heat transfer process and confrontation of experimental measurements and their numerical solutions on simplified model, inside the horizontal scraped surface heat exchanger. The experimental measurements were done on two horizontal scraped surface heat exchangers connected in series. As a product was thermally treated water, heated by vapor in the first stage and cooled by cold water in the second stage. Applied mass flow of the product: m=250, 500, 750, 1000 kgh−1 and rotary velocity of the shaft, scraper blades: rpm=20, 30, (45), 60, 90 min−1. For numerical analyses the simplified model was used, with taking only the area between the heat transfer tube, and the shaft with an aim to compare the result to experimental measurements and validate the obtained overall heat exchange, as justify the simplification. As a results from experimental measurements were obtained the correlations for Nusselt number in a form of Nu=f(Re,Pr,ηf/ηw). Based on the confrontation of results, it can be stated that the level of simplification used at numerical solutions, gives still an acceptable accuracy of overall heat transfer values. From numerical simulations were obtained further results as the velocity, temperature fields, which were used to make certain adaptation on proposed construction and their examination by additional numerical simulations. All these acquired results lead to better understanding the overall process inside the horizontal scraped surface heat exchangers and the proposed construction of mutators can increase the efficiency of heat transfer process for many products in a real processing

EUROfusion Integrated Modelling (EU-IM) capabilities and selected physics applications

International audienceRecent developments and achievements of the EUROfusion Code Development for Integrated Modelling project (WPCD), which aim is to provide a validated integrated modelling suite for the simulation and prediction of complete plasma discharges in any tokamak, are presented. WPCD develops generic complex integrated simulations, workflows, for physics applications, using the standardized European Integrated Modelling (EU-IM) framework. Selected physics applications of EU-IM workflows are illustrated in this paper

Agressivité dans les sports de combat

Différence entre agressivité et combativité dans les sports de combat, à travers une étude menée sur des escrimeurs, des judokas et des lutteurs de haut niveau

Linear microstability analysis of a low-Z impurity doped tokamak plasma

Improved electron and deuterium energy and particle confinement in the presence of low-Z impurities have been observed in many tokamaks under various experimental conditions. Peaked electron density profiles have been obtained in the Frascati Tokamak Upgrade (FTU) ohmic plasmas where a high concentration of lithium has been detected following the installation of a Liquid Lithium Limiter (LLL). This paper presents the results of a gyrokinetic study on the effects of lithium and other low-Z impurities on the linear stability of deuterium and electron temperature driven modes and their associated fluxes for plasma parameters such as those found in the core of LLL-FTU plasmas. Simulations show that a lithium concentration in excess of n(Li)/n(e) = 15%, as estimated in the initial phase of a reference FTU discharge, is found to have a strong stabilizing effect on the TEM and high-frequency ETG modes. A significant stabilization of the electron driven modes can still be observed when the lithium concentration is reduced to 3%. In the presence of a significant impurity concentration (n(Li)/n(e) = 3-15%) the long wavelength ITG modes drive an inward electron and deuterium flux and outward lithium flux. This process may lead eventually to an increased electron and deuterium density peaking and a reduced Z(eff) (lithium density below nLi/ne = 1%)

Anomalous parallel momentum transport due to E x B flow shear in a tokamak plasma

Nondiffusive anomalous momentum transport in toroidal plasmas occurs through symmetry breaking mechanisms. In this paper the contribution of sheared E X B flows to parallel momentum transport [R. R. Dominguez and G. M. Staebler, Phys Fluids B 5, 3876 (1993)] is investigated with nonlinear gyrokinetic simulations in toroidal geometry. The background perpendicular shear is treated independently from the parallel velocity shear to isolate a nondiffusive, nonpinch contribution to the parallel momentum flux. It is found that the size of the term depends strongly on the magnetic shear, with the sign reversing for negative magnetic shear. Perpendicular shear flows are responsible for both symmetry breaking and suppression of turbulence, resulting in a shearing rate at which there is a maximum contribution to the momentum transport. The E X B momentum transport is shown to be quenched by increasing flow shear more strongly than the standard linear quench rule for turbulent heat diffusivity. (C) 2009 American Institute of Physics. [doi:10.1063/1.3227650

The nonlinear gyro-kinetic flux tube code GKW

A new nonlinear gyro-kinetic flux tube code (GKW) for the simulation of micro instabilities and turbulence in magnetic confinement plasmas is presented in this paper. The code incorporates all physics effects that can be expected from a state of the art gyro-kinetic simulation code in the local limit: kinetic electrons, electromagnetic effects. collisions. full general geometry with a coupling to a MHD equilibrium code, and E x B shearing. In addition the physics of plasma rotation has been implemented through a formulation of the gyro-kinetic equation in the co-moving system. The gyro-kinetic model is fivedimensional and requires a massive parallel approach. GKW has been parallelised using MPI and scales well up to 8192+ cores. The paper presents the set of equations solved, the numerical methods, the code structure, and the essential benchmarks. Program summary Program title: GKW Catalogue identifier: AEES_vI_0 Program summary URL: http://cpc.cs.qLib.ac.uk/summaries/AEES-vl-O.html Program obtainablefrom: CK Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU GPL v3 No. of lines in distributed program, including test data, etc.: 29998 No. of bytes in distributed program, including test data, etc.: 206943 Distribution format. tar.gz Programming language: Fortran 95 Computer: Not computer specific Operating system: Any for which a Fortran 95 compiler is available Has the code been vectorised or parallelised?: Yes. The program can efficiently utilise 8192+ processors, depending on problem and available computer. 128 processors is reasonable for a typical nonlinear kinetic run on the latest x86-64 machines. RAM: similar to 128 MB-1 GB for a linear run; 25 GB for typical nonlinear kinetic run (30 million grid points) Classification: 19.8, 19.9, 19.11 External routines: None required, although the functionality of the program is somewhat limited without a MPI implementation (preferably MPI-2) and the FFTW3 library. Nature of problem: Five-dimensional gyro-kinetic Vlasov equation in general flux tube tokamak geometry with kinetic electrons, electro-magnetic effects and collisions Solution method: Pseudo-spectral and finite difference with explicit time integration Additional comments: The MHD equilibrium code CHEASE [1] is used for the general geometry calculations. This code has been developed in CRPP Lausanne and is not distributed together with GKW, but can be downloaded separately. The geometry module of GKW is based on the version 7.1 of CHEASE, which includes the output for Hamada coordinates. Runningtime: (On recent x86-64 hardware) -10 minutes for a short linear problem; 48 hours for typical nonlinear kinetic run. Reference: [1] H. Lutjens, A. Bondeson, O. Sauter, Comput. Phys. Comm. 97 (1996) 219, http://cpc.cs.qub.ac.uk/ surnrnaries/ADDH_v1_0.html Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved

FTU results with a liquid lithium limiter

Since the end of 2005 most of the plasma-wall interaction experiments on FTU have been focused on the possible use of liquid lithium as the plasma facing material. Liquid lithium limiter is an active method to deposit, during the plasma discharge, a lithium film on the walls with prolonged beneficial effects. Reliable operation with very clean plasmas, very low wall particle recycling, spontaneous peaking of the density profile for line-averaged density values (n) over bar (e) > 1.0 x 10(20) m(-3) have been obtained. These results have allowed us to extend the density limit to the highest value so far obtained ((n) over bar (e) = 4.0 x 10(20) m(-3) at I(p) = 0.7MA and B(T) = 7.1T, q(a) = 5.0, by gas puffing only) and to increase the energy confinement time by almost 50% with respect to the average value of 50 ms of the old ohmic FTU database. An accurate analysis of these plasmas has been carried out by means of a gyrokinetic code to establish the role of collisionality and density gradients on the observed phenomenology

Integrated equilibrium reconstruction and MHD stability analysis of tokamak plasmas in the EU-IM platform

International audienceIn the framework of the EUROfusion Work Package on Code Development for Integrated Modelling, a scientific Kepler workflow for the reconstructionof Tokamak plasma equilibrium was developed. It includes consolidated reconstruction codes such as EQUAL, CLISTE, EQUINOX and post-processing error bar estimator SDSS, all using the same physics and machine data ontology and methods for accessing the data used in the European Integrated Modelling (EU-IM) framework [6]. Presently implemented modules (actors) are interfaced to “data bundles” e.g. magnetic sensors, Thomson scattering diagnostics as well as poloidal field coil data, are packed into a “machine bundle”, to facilitate the data exchange in the workflow through selfconsistent datasets. The reconstruction codes feature polynomial or spline (natural or Bspline) representation for the profiles and non-uniform spatially distributed knots for the equilibrium regularisations are implemented. Equilibrium reconstructions relying on magnetics data only (magnetic diagnostic, PF/TF coils and iron core) or with added internal data (motional Stark effect, polarimetry or pressure) may be performed. For pedestal top/edge pressure profile assisted reconstructions, pre-processing of the experimental density and temperature data presently includes a median filter and time average around the time of interest, mapped to the flux coordinates obtained for that time in the previous (magnetics only) reconstruction. Ion density is assumed to be proportional to electron density and fast particle density is assumed negligibl

Experimental evidence of momentum transport induced by an up-down asymmetric magnetic equilibrium in toroidal plasmas

The first experimental evidence of parallel momentum transport generated by the up-down asymmetry of a toroidal plasma is reported. The experiments, conducted in the Tokamak a Configuration Variable, were motivated by the recent theoretical discovery of ion-scale turbulent momentum transport induced by an up-down asymmetry in the magnetic equilibrium. The toroidal rotation gradient is observed to depend on the asymmetry in the outer part of the plasma leading to a variation of the central rotation by a factor of 1.5-2. The direction of the effect and its magnitude are in agreement with theoretical predictions for the eight possible combinations of plasma asymmetry, current, and magnetic field