Infoscience

Thesis

Propagation and absorption of low frequency waves in two and three dimensional warm plasmas

In order to model a broader range of phenomena taking place in three-dimensional plasmas, the LEMan code has been extended to a warm formulation. As the wave propagation is strongly influenced by the parallel wave vector, special attention has been paid for its consistent computation. The choice to concentrate on this quantity has limited the complexity of the problem to 0th order in the Finite Larmor Radius expansion. Two methods have been implemented in LEMan. The first technique implies the inversion of a polynomial matrix and requires incorporating the dielectric tensor as a convolution in the linear system. It has been tested successfully and applied to a wide range of cases up to three-dimensional geometries in the Alfvén domain. The frequency and damping of global modes that appear in the gaps formed by the breaking of symmetry can then be determined. Strong variations are observed depending on the presence, for example, of a mode conversion to the Kinetic Alfvén Wave. This method has however displayed numerical limitations in the Ion-Cyclotron Range of Frequencies (ICRF). A new technique based on the iterative evaluation of the parallel wave vector has then been developed. After having tested its agreement with the convolution method, computations in the ICRF domain have been undertaken in two and three-dimensional configurations including the Large Helical Device where the power deposition has been determined. In order to model the contribution of fast ions, a bi-Maxwellian distribution function has been added to the warm dielectric tensor. The fast ion population absorbs, in this case, an important fraction of the total power in the ICRF domain. In addition, a displacement of the Toroidicity-induced Alfvén Eigenmode frequency has been observed. The code has been finally optimised and parallelised permitting to perform computations in complicated geometry such as stellarators in the ICRF domain. Furthermore, the approach used here yields a much shorter computation time than for most of the other codes dealing in this domain of frequency.

Fulltext

Related material