Qu, Zhisong
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...[Show more] 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.
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