Candidat finaliste prix EPFL de doctorats 2007 - Stefan Müller

Turbulence in basic toroidal plasmas.

This thesis work contributes to the improvement of the experimental assessment of turbulence in fusion-relevant toroidal plasmas via the construction and dedicated exploitation of the TORPEX basic toroidal plasma experiment. Several contributions to the construction phase of TORPEX between November 2001 and March 2003 are presented, including design studies of the magnetic-field configurations and the concept and realization of the IT system. For the studies of fluctuations and turbulence, a toroidal magnetic field complemented by a small vertical component is used in rf-driven discharges. An analytical model of the particle confinement mechanism in this topology is developed and experimentally verified. The basic plasma properties are characterized as a function of the available control parameters, namely the toroidal and vertical magnetic field, the injected microwave power, and the neutral gas type and pressure. The fluctuations and turbulence in TORPEX are investigated via the specifically developed plasma-imaging probe HEXTIP, which employs a hexagonal disposition of 86 Langmuir probes, covering the entire poloidal cross section. Established one- and two-point Fourier-space analysis techniques are generalized to be suitable for such a 2D diagnostic. A novel real-space analysis method for fluctuating spatiotemporal structures is developed based on a pattern-recognition approach combined with a massive statistical analysis of measurements of many realizations of turbulent structures, permitting to assess the probabilistic properties of the turbulent fluctuations directly with convincing detail and statistical accuracy. The combined force of the generalized Fourier-space and the probabilistic real-space analysis techniques, applied automatically to a wide variety of discharges, is used to characterize the fluctuations in the TORPEX plasma in detail. Structures are observed to occur mostly on the low-field side, in patterns expected for drift-interchange type instabilities. The magnitude of the fluctuations is generally comparable to the time-average density level and the areas occupied by the structures extend over a significant fraction of the poloidal cross-section. The development of a 3D fluid simulation code, ESELTPX, is initiated in the framework of this thesis, aiming at performing global nonlinear simulations of TORPEX plasma discharges for detailed experiment-theory comparison. The present state of the project is summarized and future development strategies are outlined.