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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Turbulence and transport at the boundary of fusion plasmas

Alternative title: Turbulens og transport i randen av fusjonsplasma

Awarded: NOK 7.6 mill.

During the last 60 years there has been an ongoing international research effort aimed at utilizing the energy released in fusion of hydrogen nuclei - the power of stars - for clean production of electric energy. The most promising approach for a controlled thermonuclear fusion process involves magnetic confinement of high-temperature gases which are in the plasma state. A major obstacle towards this goal is significant losses of particles and energy perpendicular to the magnetic field and onto the reactor walls due to turbulent flows. This causes detrimental cooling of the plasma, increased erosion of the reactor walls and release of impurities from the walls into the plasma. Most of this transport is caused by motion of coherent, filamentary structures with excess plasma and heat, leading to strong intermittency and large fluctuations in boundary region of fusion experiments. The goal of this project was to investigate the physical mechanisms that causes the turbulence-driven transport of particles and energy in magnetized plasmas, identify the statistical properties of the fluctuations, and exploring possibilities for controlling the transport. The project comprises collaboration with the Massachusetts Institute of Technology, the National Fusion Research Institute in South Korea, the Swiss Plasma Center, the Technical University of Denmark, the University of Innsbruck, and the University of Oslo. The project has involved stochastic modeling, numerical simulations and analysis of measurement data from the Alcator C-Mod experiment at MIT, KSTAR at NFRI and TCV at the Swiss Plasma Center. Advanced statistical methods has been exploited to derive the distributions of amplitudes, waiting times, sizes and velocities of filamentary structures. Analytical theory has been developed and numerical simulations performed to address their dynamical evolution and transport properties. A new stochastic model has been developed, which gives a complete description of the fluctuations in the boundary of fusion plasmas. The model describes the strength of the fluctuations and their distribution. Predictions of the stochastic model also includes how frequently the plasma parameters cross a given threshold level and for how long they are above this threshold value. These properties are fundamental for describing interactions between the plasma and material walls in a reactor. Both the assumptions of the stochastic model and its predictions are confirmed by measurement data from the fusion experiments. The same statistical properties govern alle experimental control parameters investigated and all devices included in the study. This gives strong evidence of universality properties of turbulent motions. The results from the project has been published in leading scientific journals and has been presented in the most prominent conferences in the research field.

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Controlled thermonuclear fusion holds the potential as an unlimited source of clean electrical power. One major obstacle towards this goal is the interaction of hot plasma with material surfaces in the boundary region. With a team of excellent experts, this project will focus on turbulent motions and transport of particles and heat in the edge region of magnetically confined plasmas. The research will provide important knowledge for experiment design and will be published in highly ranked journals. The statistical properties of plasma fluctuations and transport will be derived from dedicated experiments on the Alcator C-Mod tokamak at MIT. Data time series of unprecedented duration will be obtained using sophisticated diagnostics techniques. Advanced statistical methods will be exploited to derive the distributions of amplitudes, waiting times, sizes and velocities of filamentary structures. Measurements at various locations in the boundary region will reveal the spatial asymmetries in turbulence structures and the role of particle motion along the magnetic field to the plasma-facing components. Stochastic modeling of plasma fluctuations will be made and plasma-wall interactions in the boundary region predicted. Analytical theory will be developed and numerical simulations performed to address the dynamical evolution of blob-like structures, which are found to dominate cross-field transport of particles and heat in the plasma edge region. The role of filament size, sheared flows, electromagnetic perturbations and resistivity will be clarified through a systematic parameter scan. Advanced turbulence simulations will be used to investigate the formation of filamentary structures, reveal their shape along the magnetic field, and predict the radial particle density and temperature profiles. Results from this project will advance our knowledge about the boundary of magnetized plasmas and help optimize the design and operation of future fusion experiments and reactors.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek