Fluid descriptions of externally heated tokamak plasmas
Creators: name: Qu, Zhisong
Publisher: name: The Australian National University
Year: 2016
Subjects: Tokamak
Plasma
Pressure Anisotropy
EGAM

description: External heating methods such as neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) generate a large amount of fast ions in tokamak plasmas. The widely implemented MHD single fluid theory with isotropic pressure is no longer sufficient to capture the physics of such plasmas. Despite the shortcoming of a fluid theory, such as the fluid closure problem and the lack of wave-particle interactions, the use of a fluid description in a tokamak with external heated fast ions is possible and has proved fruitful due to its simple and intuitive nature, as shown in this thesis. Due the presence of the fast ions, the total plasma pressure becomes anisotropic. In other words, the pressure parallel to the magnetic field differs from its perpendicular counterpart. We have upgraded the fast ion driven instability tool chain HELENA-MISHKA-HAGIS to new versions with pressure anisotropy, taking the simplification that the whole plasma (electrons, fast and thermal ions) is a bi-Maxwellian fluid. Based on this new tool chain and analytical analysis, we have identified the impact of pressure anisotropy induced by externally heated fast ions on the plasma equilibrium, waves and instabilities. It has been found that if an isotropic model is used to describe an anisotropic plasma, a range of problems will emerge depending on the inverse aspect ratio and the magnitude of anisotropy. These problems include the inconsistency of the poloidal (diamagnetic) current, the constant pressure surface shifting away from the flux surfaces, and finally a distortion of the current and q profile. Two MAST experimental discharges are analysed, while in one of them, #29221@190ms, all three problems are presented, confirming the prediction. The equilibrium reconstructions for this discharge with/without anisotropy give different q profiles. This difference in the q profile leads to different continua, different n=1 TAE mode structures, and finally, different growth rates and saturation levels. The tool chain has also been used to carry on other physics studies such as an investigation of the dependency of the continuous spectra on different fluid closures and level of anisotropy. In addition to the waves that are supported by the thermal plasma, and modified and driven unstable by the fast ions, there are a family of waves, the energetic particle modes (EPMs), whose existence and property are determined by the fast ions, such as the energetic geodesic acoustic modes (EGAMs). The EGAMs are m=n=0 bursting and chirping modes first observed in DIII-D counter beam experiments. By considering the fast ions as a fluid with a collective flow along the field lines, we have reached a dispersion relationship that gives an unstable branch at half of the thermal GAM frequency. We have also found that when the beam is cold, there is a good agreement between our fluid theory and the existing kinetic theories. However, since the fluid theory does not capture the physics of inverse Landau damping, the source of the instability must be reactive, in contrast to the previous understandings. Furthermore, a smooth transition between the reactive EGAMs and the wave-particle interaction driven EGAMs is found when the beam temperature gradually increases, resembling the transition between the two-stream instability and the bump-on-tail instability in a beam-plasma system. This local fluid model is then extended to a global one to capture the physics of EGAM radial mode structure in the regime where fast ion drift orbit width is smaller than the mode width. The dependency of the mode structure on the equilibrium q profiles and the beam injection direction is investigated. By demonstrating the above two applications of the fluid theory and the corresponding physics discoveries, we have proved the usefulness of a fluid treatment in tokamak plasmas with external heating, serving to understanding some of the basic fast ions physics and acting as a powerful and indispensable complement to its kinetic counterpart.