Analysis of energetic particle-driven alfvénic instabilities in tokamak and stellarator plasmas using three dimensional numerical tools
- Autores: Allah Rakha
- Directores de la Tesis: Mervi Mantsinen (dir. tes.), Jean-Marie Noterdaeme (codir. tes.), Lluís Batet (tut. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Shimpei Futatani (presid.), Manuel García Muñoz (secret.), Luis Raul Sánchez Fernández (voc.), Jonathan Graves (voc.), Filip Beunis (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Nuclear y de las Radiaciones Ionizantes por la Universidad Politécnica de Catalunya
- Materias:
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- Resumen
In this thesis, a detailed analysis of the experientially observed energetic particle-driven Alfvénic instabilities in tokamak and stellarator plasmas using three dimensional numerical tools based on the reduced MHD model has been carried out. In TJ-II stellarator plasmas, modelling of chirping and steady modes assesses their coexistence on the persistent appearance of the corresponding combination of the toroidal (n) and poloidal (m) mode numbers through the rotational transform sensitivity analysis both in iota lowering and iota rising dynamic plasmas. Modelling of the experimentally observed frequency sweeping phenomenon during the presence of Alfvénic activity with radially extended low-shear and radially localized strong-shear non-monotonic (NM) iota profiles provides an extended spectrum of Alfvénic modes in the wide frequency range as compared with normal (monotonic) iota profile results. Comparison of mode frequencies calculated using a standard dispersion relation and those modelled with the reduced MHD clustered frequency solver AE3D shows an agreement with the selection of iota minimum values, which supports MHD spectroscopy calculations.
Wave particle interaction (WPI) studies for the resonance function calculations developed using Monte Carlo transport model based on the 3D MHD equilibria for the TJ-II plasmas suggest for low bounce harmonics (p) the possibility of describing the non-linear evolution of the AEs in TJ-II by a sum of two ion populations with different weighting factors, one of which is dominated by drag and the other by diffusion. As the bounce harmonic increases, the resonance region starts to expand and can cover a significant area of the particle phase space until this resonance region vanishes at high bounce harmonics.
In ASDEX Upgrade tokamak plasmas a bifurcated MHD equilibrium is reconstructed with formation of 3D helical core as saturated magnetic axis and the remaining torus with an axisymmetric equilibrium. The formation of helical core is characterized as an 3D perturbation in an axisymmetric equilibrium state. Helically distorted MHD equilibria exit for the axisymmetric devices if q = 1 rational surfaces are present. Alfvén continuum calculations with the bifurcated equilibria lead to the frequency splitting between the highest frequency branch and the lowest frequency branch continua at the frequency accumulation point. Radially localised shifting of modes happens via coupling of the adjacent n-1 continuum around an accumulation point. Modelling including 3D effects correctly reproduces the phenomenon of continuum frequency splitting and provides a possible solution for the differences of few kHz in frequency splitting, which remained unexplained with the 2D kinetic calculations. The pressure scaling confirms the increase of helical excursion of the magnetic axis in equilibrium reconstruction and hence the range of continuum frequency splitting. The existence of low-frequency continua and its splitting around the frequency accumulation point are in agreement with the experimental observations for the low-frequency modes.
This dissertation provides an extensive comparison of the experimental and modelling results for the TJ-II stellarator plasmas along with the effect of the formation of bifurcated MHD equilibria on Alfvén continua in AUG tokamak plasmas.
