Electrostatic instabilities, turbulence and fast ion interactions in the TORPEX device

Alice Burckel, Ambrogio Fasoli, Lucia Isabel Federspiel, Ivo Furno, Kyle Gustafson, Davoud Iraji, Benoît Labit, Joaquim Loizu Cisquella, Stefan Müller, Gennady Plyushchev, Mario Podesta, Francesca Maria Poli, Paolo Ricci, Christian Gabriel Theiler
Institute of Physics, 2010
Journal paper

Abstract

Electrostatic turbulence, related structures and their effect on particle, heat and toroidal momentum transport are investigated in TORPEX simple magnetized plasmas using high-resolution diagnostics, control parameters, linear fluid models and nonlinear numerical simulations. The nature of the dominant instabilities is controlled by the value of the vertical magnetic field, Bv, relative to that of the toroidal field, BT. For Bv/BT > 3%, only ideal interchange instabilities are observed. A critical pressure gradient to drive the interchange instability is experimentally identified. Interchange modes give rise to blobs, radially propagating filaments of enhanced plasma pressure. Blob velocities and sizes are obtained from electrostatic probe measurements using pattern recognition methods. The observed values span a wide range and are described by a single analytical expression, from the small blob size regime in which the blob velocity is limited by cross-field ion polarization currents, to the large blob size regime in which the limitation to the blob velocity comes from parallel currents to the sheath. As a first attempt at controlling the blob dynamical properties, limiter configurations with varying angles between field lines and the conducting surface of the limiter are explored. Mach probe measurements clearly demonstrate a link between toroidal flows and blobs. To complement probe data, a fast framing camera and a movable gas puffing system are installed. Density and light fluctuations show similar signatures of interchange activity. Further developments of optical diagnostics, including an image intensifier and laser-induced fluorescence, are under way. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector in scenarios for which the development from linear waves into blobs is fully characterized. A theory validation project is conducted in parallel with TORPEX experiments, based on quantitative comparisons of observables that are defined in the same way in the data and in the output of numerical codes, including 2D and 3D local and global simulations.

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https://infoscience.epfl.ch/record/163264?ln=en

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Controlled nuclear fusion could provide our society with a clean, safe, and virtually inexhaustible source of electric power production. The tokamak has proven to be capable of producing large amounts of fusion reactions by confining magnetically the fusion fuel at sufficiently high density and temperature, thus in the plasma state. Because of turbulence, however, high temperature plasma reaches the outermost region of the tokamak, the Scrape-Off Layer (SOL), which features open magnetic field lines that channel particles and heat into a dedicated region of the vacuum vessel. The plasma dynamics in the SOL is crucial in determining the performance of tokamak devices, and constitutes one of the greatest uncertainties in the success of the fusion program. In the last few years, the development of numerical codes based on reduced fluid models has provided a tool to study turbulence in open field line configurations. In particular, the GBS (Global Braginskii Solver) code has been developed at CRPP and is used to perform global, three-dimensional, full-n, flux-driven simulations of plasma turbulence in open field lines. Reaching predictive capabilities is an outstanding challenge that involves a proper treatment of the plasma-wall interactions at the end of the field lines, to well describe the particle and energy losses. This involves the study of plasma sheaths, namely the layers forming at the interface between plasmas and solid surfaces, where the drift and quasineutrality approximations break down. This is an investigation of general interest, as sheaths are present in all laboratory plasmas. This thesis presents progress in the understanding of plasma sheaths and their coupling with the turbulence in the main plasma. A kinetic code is developed to study the magnetized plasma-wall transition region and derive a complete set of analytical boundary conditions that supply the sheath physics to fluid codes. These boundary conditions are implemented in the GBS code and simulations of SOL turbulence are carried out to investigate the importance of the sheath in determining the equilibrium electric fields, intrinsic toroidal rotation, and SOL width, in different limited configurations. For each study carried out in this thesis, simple analytical models are developed to interpret the simulation results and reveal the fundamental mechanisms underlying the system dynamics. The electrostatic potential appears to be determined by a combined effect of sheath physics and electron adiabaticity. Intrinsic flows are driven by the sheath, while turbulence provides the mechanism for radial momentum transport. The position of the limiter can modify the turbulence properties in the SOL, thus playing an important role in setting the SOL width.

EPFL2013

Simulation of plasma turbulence in the periphery of diverted tokamaks

Paola Paruta

The turbulent plasma dynamics in the periphery of a fusion device plays a key role in determining its overall performance. In fact, the periphery controls the heat load on the vessel walls, the plasma confinement, the level of impurities in the core, the plasma fuelling and the removal of fusion ashes. Hence, understanding and predicting the plasma turbulence in this region is of crucial importance for the success of the fusion program. The GBS code has been developed in past years to simulate plasma turbulence in the periphery of limited tokamaks. The goal of the present thesis is to extend GBS to the treatment of diverted scenarios. Such configurations are of interest for present state-of-the-art experiments and future fusion reactors. For the implementation of this geometry, we express the model in toroidal coordinates, abandoning the flux coordinates previously used in limited configuration, and overcoming the singularity that this coordinate system presents at the X-point of diverted configurations. The accuracy of the numerical scheme is improved by upgrading the second order finite differences scheme to fourth order on staggered grids. The resulting version of GBS is carefully verified through a series of tests (i.e., a benchmark with the previous version of GBS in limited configuration, a rigorous check of the correctness of the code implementation with the method of manufactured solutions, and a convergence study on a relatively simple diverted configuration). The results of a GBS simulation is then used to investigate the dynamics of coherent turbulent structures, called blobs, that characterise plasma turbulence in the periphery of fusion devices. A diverted double-null configuration is considered, and the blob motion is studied using a pattern recognition algorithm. The velocity of the blobs in the presence of an X-point matches the analytical scaling that we derived by considering the different blob properties in the divertor and main SOL regions, retaining the correction terms that account for blob density and ellipticity. In addition, we show that the blob current pattern observed in the simulation results match the theoretical expectations. Finally, the new version of GBS is run with a realistic diverted magnetic equilibrium, taken from an experiment carried out on the TCV tokamak. First insights of the turbulence properties are in good agreement with the current physical understanding of plasma dynamics in the periphery of diverted tokamaks.

EPFL2018

Turbulence and flows in the plasma boundary of snowflake magnetic configurations

Maurizio Giacomin, Paolo Ricci, Louis Nicolas Stenger

The power exhaust through the scrape-off layer (SOL) in fusion reactors is expected to be significantly higher than in ITER, thus questioning the extrapolation of the ITER exhaust solution to these devices. The snowflake (SF) magnetic configuration is one of the alternative exhaust configurations being considered to mitigate the heat vessel loads in fusion reactors. The SF configuration features a second-order null of the poloidal magnetic field, i.e. a point where the poloidal magnetic field and its spatial derivatives vanish. As a consequence, the null-point is connected to the wall through four legs, which define four strike points. The presence of the four strike points allows for a heat flux distribution on a larger area compared to standard divertor configurations that feature two strike points. SF configurations are obtained experimentally by generating two first-order X-points close to each other. When the two X-points coincide, a second-order null point is obtained. However, in practice, the two X-points never coincide perfectly and, according to their relative position, we distinguish between the snowflake plus (SF+) and snowflake minus (SF-). The configuration with the two X-points coinciding is usually referred to as ideal SF. All these configurations have been experimentally investigated in the TCV, NSTX, EAST, and DIII-D tokamaks and are considered for the DTT tokamak. Numerical simulations of SF configurations, carried out by means of the EMC3-Eirene and the SOLPS codes, are unable to reproduce the heat flux distribution observed experimentally, calling for detailed investigations of the turbulence and flows in the SF plasma boundary. We present the first global turbulent simulations of the plasma dynamics in SF configurations, including the ideal SF, the SF+ and SF- configurations [1]. These simulations carried out by using the GBS code [2], evolve self-consistently the fluctuating and equilibrium quantities, as they result from the interplay of a heat and particle source in the core, turbulent transport, and parallel losses to the vessel wall. The simulations allow us to disentangle the mechanisms behind the heat flux distribution among the different strike points. As pointed out by our simulations, the heat flux can be reduced by a factor of two in the SF configurations, with respect to single-null configurations. The activation of the unconnected strike points in the ideal SF and in the SF+ configurations, also observed in the experiments, is due to the presence of a steady ExB equilibrium flow in the null region that provides a cross-field transport mechanism towards the private flux region. The origin, the properties and the effects of this steady ExB equilibrium flow are carefully analyzed and described as well as its dependence on the direction of the toroidal magnetic field and on the distance between the two X-points.

2020