Aylwin Iantchenko

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A synthetic phase-contrast imaging diagnostic with spatial filtering for gyrokinetic simulations

Justin Richard Ball, Stephan Brunner, Stefano Coda, Aylwin Iantchenko, Gabriele Merlo

A Phase-contrast imaging (PCI) diagnostic provides measurements of line-integrated electron density fluctuations. Localisation along the laser beam path can be achieved with a spatial filter that selects the wave-vector directions of the fluctuations contributing to the PCI measurement and is a key feature of the PCI diagnostic installed on the TCV tokamak and also of a similar system planned for JT-60SA. We have developed a synthetic diagnostic that models measurements from PCI taking into account the effect of such a spatial filter. The synthetic tool is based on the principle of integrating over selected diagnostic volumes the electron density fluctuations generated by turbulence simulations, and applying an appropriate spatial filter in wave-vector space. We demonstrate the effect of the filter for a positive and a negative triangularity TCV discharge, and illustrate the potential of the synthetic diagnostic for better understanding the corresponding experimental results. We consider different types of filters and make first-principle estimates of the localisation of the measurement. Finally, using gyrokinetic simulations that include electromagnetic effects, collisions and four kinetic species, we make first predictions of the characteristics of the measurements using the planned set-up of PCI on JT-60SA.

IOP Publishing Ltd2023

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Turbulence driven by small-scale instabilities results in strong heat and particle transport, which significantly shortens the confinement time and prevents the formation of a self-sustained plasma reaction in magnetic confinement devices. Control and possible mitigation of this transport is critical to achieve controlled fusion, which requires a proper understanding of the underlying physics of turbulent processes.This thesis explores both computational and experimental methods to study turbulent fluctuations, focusing on modelling and the use of the Tangential Phase Contrast Imaging (TPCI) diagnostic that measures electron density fluctuations. On the experimental side of this work, the TPCI diagnostic is upgraded with new equipment to perform ion scale measurements. The diagnostic is used in an initial study of turbulence when the shape of the plasma changes from positive to negative triangularity in a diverted configuration, and to study the effects of an electron cyclotron beam on turbulence.However, the main part of the thesis is devoted to gyrokinetic GENE simulations of turbulence and modelling of TPCI signals. The simulated density fluctuations are post-processed with a synthetic diagnostic that has been developed to model measurements from TPCI, taking into account the effect of a spatial filter for selecting the measured wave-vector directions. Modelling, with GENE and the synthetic diagnostic, is performed for several TCV plasma scenarios with the aim of improving the interpretation of the TPCI signals. The synthetic diagnostic is used to establish a link between the TPCI signals and the underlying modes, and to find new ways to apply the TPCI diagnostic to measure Ion Temperature Gradient (ITG) and Trapped Electron Mode (TEM) turbulence. It is also demonstrated how computationally cheap linear simulations can be used to predict certain aspects of the TPCI signals.In the final part of this work a computational study of turbulent fluctuations in the JT-60SA machine is also carried out, motivated in part by the concomitant design of a TPCI diagnostic for that device. Turbulence in a representative, planned, high-performance JT-60SA plasma discharge, scenario 1, is modelled with local gyrokinetic flux-tube simulations. The discharge features a double-null separatrix, 41 MW of combined Neutral Beam Heating (NBH) and Electron Cyclotron Heating (ECH), and a high predicted ratio Î² of the plasma kinetic to magnetic pressure. Electromagnetic effects, in particular compressional magnetic field fluctuations, are essential in this scenario due to the large Î². A detailed linear and nonlinear study of the fluctuations is performed and conclusions are drawn on the capability of reduced transport models to predict profiles in reactor-relevant conditions and on the projected performance of the JT-60SA machine. Finally, predictions are provided of the characteristics of the TPCI measurement, using the results from the gyrokinetic study and the planned diagnostic setup on JT-60SA.

EPFL2023

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Study of correlations between LOC/SOC transition, intrinsic toroidal rotation reversal and TEM/ITG bifurcation with different working gases in TCV

Filippo Bagnato, Basil Duval, Aylwin Iantchenko

The effects of different working gases on the transition from linear ohmic confinement (LOC) regime to saturated ohmic confinement (SOC) regime and its relation to the intrinsic toroidal rotation reversal phenomenon were explored in the TCV tokamak. The energy confinement saturation was studied across D, H and He density ramps, and a range of ECRH injection power and through variations of ohmic plasma current. The occurrence of rotation reversal, concomitantly with the LOC-SOC transition, was observed only for certain cases, making us formally exclude a causal relation between the two phenomena. A strong correlation between the evolution of toroidal rotation profiles and electron density gradients was, however, observed, in agreement with previous works (Lebschy et al 2017 Nucl. Fusion 58 026013; Hornsby et al 2018 Nucl. Fusion 58 056008). Linear gyrokinetic simulations were performed to probe the turbulent regime of these discharges, showing a dominance of trapped electron mode (TEM) during the LOC phase and a mixture of TEM and ion temperature gradient (ITG) following the transition to SOC regime in D. Such a TEM/ITG bifurcation was less pronounced in H and He. MHD activity was monitored throughout the discharges and possible correlations between sawteeth instability activity, energy confinement time saturation and rotation reversal are highlighted.

IOP Publishing Ltd2023

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