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How eigenmode self-interaction affects zonal flows and convergence of tokamak core turbulence with toroidal system size

Justin Richard Ball, Stephan Brunner, Ajay Chandrarajan Jayalekshmi, Julien Stanislas Pierre Dominski, Gabriele Merlo

Self-interaction is the process by which a microinstability eigenmode that is extended along the direction parallel to the magnetic field interacts non-linearly with itself. This effect is particularly significant in gyrokinetic simulations accounting for kinetic passing electron dynamics and is known to generate stationary E×B zonal flow shear layers at radial locations near low-order mode rational surfaces (Weikl et al. Phys. Plasmas, vol. 25, 2018, 072305). We find that self-interaction, in fact, plays a very significant role in also generating fluctuating zonal flows, which is critical to regulating turbulent transport throughout the radial extent. Unlike the usual picture of zonal flow drive in which microinstability eigenmodes coherently amplify the flow via modulational instabilities, the self-interaction drive of zonal flows from these eigenmodes are uncorrelated with each other. It is shown that the associated shearing rate of the fluctuating zonal flows therefore reduces as more toroidal modes are resolved in the simulation. In simulations accounting for the full toroidal domain, such an increase in the density of toroidal modes corresponds to an increase in the toroidal system size, leading to a finite system size effect that is distinct from the well-known profile shearing effect.

2020

Studying the effect of non-adiabatic passing electron dynamics on microturbulence self-interaction in fusion plasmas using gyrokinetic simulations

Ajay Chandrarajan Jayalekshmi

Microturbulence driven by plasma instabilities is in most cases the dominant cause of heat and particle loss from the core of magnetic confinement fusion devices and therefore presents a major challenge in achieving burning plasma conditions. The role of passing electron dynamics in turbulent transport driven by ion-scale microinstabilities, in particular Ion Temperature Gradient (ITG) and Trapped Electron Mode (TEM) instabilities, has been given relatively little attention. In first approximation, these particles, which are highly mobile along the confining magnetic field, are assumed to respond adiabatically to the low frequency ion-scale modes. However, near mode rational surfaces (MRSs), the non-adiabatic response of passing electrons becomes important and can no longer be neglected. This non-adiabatic electron response actually has a destabilising effect and leads to generation of fine-structures located at the MRSs of each eigenmode. This thesis focuses on the effects of non-adiabatic response of passing electrons in tokamak core turbulence. One such effect of non-adiabatic passing electrons that is of particular interest to this work is the self-interaction mechanism. It is essentially a process by which a microinstability eigenmode that is extended along the direction parallel to the magnetic field interacts non-linearly with itself, in turn generating E x B zonal flows. Unlike the usual picture of zonal flow drive in which microinstability eigenmodes coherently amplify the flow via modulational instabilities, the self-interaction drive of zonal flows from these eigenmodes are uncorrelated with each other. In the case of ITG driven turbulence, using novel statistical diagnostic methods, it is shown that the associated shearing rate of the fluctuating zonal flows therefore reduces as more toroidal modes are resolved in the simulation. In simulations accounting for the full toroidal domain, such an increase in the density of toroidal modes corresponds in fact to an increase in the system size, leading to a finite system size effect that is distinct from the other better known system size effects such as profile shearing or finite radial extend of the unstable region. The study of non-adiabatic passing electron dynamics is pursued further to include more reactor relevant conditions such as collisions and background shear flow. It is found that, with increasing collisionality, electrons behave more adiabatic-like, especially the trapped electrons away from MRSs, thereby leading to a decrease in the growth rate of ITG eigenmodes. Furthermore, the shortened electron mean free path in presence of collisions leads to a radial broadening of the fine-structures at the MRS of corresponding eigenmodes. In nonlinear simulations, the turbulent flux levels decrease with increasing collisionality, as a result of the reduced drive from the less unstable ITG eigenmodes. The radial width of the fine structures at MRSs is found to reduce with increasing collisionality as a result of reduced nonlinear modification of the eigenmodes in turbulence simulations. A study of the effect of collisions on the self-interaction mechanism reveals that for physically relevant values of collisionality, the effect of self-interaction is still significant. A preliminary study of the effect of background E x B flow shear shows that the fine-structures associated with the non-adiabatic passing electron response persist even with finite background flow.

EPFL2020

Effects of collisionality and T e /T i on fluctuations in positive and negative δ tokamak plasmas

Stephan Brunner, Ajay Chandrarajan Jayalekshmi, Stefano Coda, Ambrogio Fasoli, Matteo Fontana, Gabriele Merlo, Laurie Porte, Olivier Sauter

The effects of negative triangularity () on confinement and fluctuations in plasmas covering a large range of parameters were investigated on the tokamak à configuration variable (TCV). The conditions explored in this paper include discharges where neutral beam (NB) heating was employed to obtain an electron-ion temperature ratio across a large fraction of the plasma profile. This significantly extended the range of negative plasmas studied on TCV towards conditions more relevant to future reactor-like tokamaks. Negative triangularity was found to improve confinement over the full range of collisionality studied () and (). The amplitude of radiative temperature fluctuations, measured using a correlation electron cyclotron emission diagnostic over the range , was found to be reduced, in negative with respect to positive plasmas, for all combinations of parameters explored. This was, in particular, verified for a pair of positive and negative plasmas with comparable density and under different conditions of NB heating. Linear gyrokinetic simulations found the dominant turbulence regime, in the strongly NB heated discharges, to be a mixture of trapped electron modes (TEMs) and ion temperature gradient driven modes. This is in contrast to ohmic or electron cyclotron heated discharges, for which the dominant turbulence regime was found to be pure TEM. Negative triangularity was found to lead to partial stabilization of the most unstable modes for low wavenumbers in both turbulence regimes. These findings demonstrate that negative triangularity could provide significant confinement improvement over a large range of parameters, that include conditions closer to future reactor-like machines (, low collisionality).

2019