Study of ergodic divertor edge density regimes on the tokamaks Tore Supra and TEXTOR, and sensitivity of tunnel probe electron temperature measurements to a suprathermal electron component
Controlled thermonuclear fusion offers one possible option to meet our future energy needs in a sustainable way. Magnetic confinement in a so-called 'tokamak'-machine is a possible approach towards the achievement of a burning plasma. An important issue in this tokamak research is the transition of the plasma edge to the inner wall. A first topic that is addressed in this thesis, is the ergodic divertor (ED) configuration. An ED achieves the transition between the confined plasma and the wall in a layer where the flux lines have been ergodized by a proper resonant magnetic perturbation. The connection between up- and downstream plasma parameters during ED operation in the tokamaks Tore Supra en TEXTOR has been investigated experimentally by means of Langmuir probes. As an important first step in the theoretical interpretation of those experiments, a Hamiltonian field line mapping code, which had been previously developed for the TEXTOR dynamic ergodic divertor, has been adapted to the geometry of the Tore Supra ED. Subsequently, this adapted code has been used to study some of the properties of the Tore Supra ED magnetic field line structure, as well as to make a qualitative comparison of the sensitivity of the TEXTOR and Tore Supra ergodic divertor magnetic topology to changes in the central density. A second topic of this thesis concerns certain interpretation issues regarding the current-voltage characteristics obtained by a newly developed type of Langmuir probe for the investigation of edge plasmas. More in particular, the sensitivity of the TP to a small population of nonthermal electrons has been investigated in addition to the influence of suprathermal electrons on the scaling and structure of the Debye and the magnetic sheath at the inside of the tunnel probe.
Physics and Astronomy, Debye sheath, suprathermal electron population, tunnel probe, Langmuir probe, ripple, density regime, symplectic Hamiltonian magnetic field line mapping, TEXTOR, ergodic divertor, tokamak, Tore Supra, controlled thermonuclear fusion, magnetic confinement, magentic sheath, plasma-wall transition
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Ghent University. Faculty of Engineering