Infoscience

Thesis

Plasma production and transport in a simple magnetised toroidal plasma

This Thesis addresses questions related to transport phenomena and the plasma production mechanisms by injection of microwaves in the electron-cyclotron frequency range in the simple magnetised toroidal plasma TORPEX. The second subject is investigated in detail in Part II. The mechanisms of the interaction between the injected microwaves and the plasma are identified. The experimental results highlight the different roles played by the electron-cyclotron and upper-hybrid plasma resonances in the absoprtion of the microwave power by the plasma. The effects of the plasma-wave interaction on the electron distribution function are investigated, confirming that the high-energy electrons that are able to ionise the neutral gas mainly come from interactions at the upper-hybrid resonance. Based on the experimental results, an expression is derived for the particle source term, which can be used in numerical codes simulating the plasma dynamics on TORPEX. The plasma production mechanisms are then related to the properties of the time-averaged plasma profiles. A set of control parameters, including the injected microwave power and the vertical magnetic field, are identified. These allow one to vary in a systematic way the plasma profiles, as needed for a detailed study of plasma instabilities and related transport. The study of particle and heat transport is undertaken in Part III. A number of experimental and analysis techniques, including a method based on the combination of "conditional-average sampling" and "boxcar-averaging", are applied to identify and quantify specific contributions to the total fluxes. The two-dimensional temporal behaviour of density, electron temperature and plasma potential is simultaneously reconstructed, thus contributing significantly to the characterisation of transport mechanisms at play in TORPEX plasmas. Two clearly distinct mechanisms are mainly responsible for the transport across the magnetic field. They are respectively associated to unstable low-frequency electrostatic modes, identified as drift waves and interchange modes, and to intermittent high-density plasma structures (or blobs). It is shown that the blobs originate from the intermittent radial expansion of the unstable modes in a region of strongly sheared E × B flow.

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