Global gyrokinetic simulations of electrostatic microturbulent transport in lhd stellarator and aditya-u tokamak

includes bibliographical references and index

PhD; 2023; Department of Physics

Fusion plasma could act as a viable source of clean and unlimited energy,
replacing fossil fuels and fission-based reactors. In this direction, inertial confinement fusion (ICF) and magnetic confinement fusion (MCF) are two major
technological alternatives to achieve the nuclear fusion from the burning plasmas. ICF uses powerful lasers, such as one at Lawrence Livermore National
Laboratory, to fuse the fusion reactants. Whereas MCF uses strong magnets that
produce the magnetic field in a torus-shaped geometry to confine the fusion
fuel. MCF, which is the focus of this thesis work, has gained considerable attention over the past few years for the commercialization of fusion reactors and
thus inspired the world’s largest fusion reactor, ITER (International Thermonuclear Experimental Reactor), to demonstrate the efficient replication of fusion
reactions on Earth and hence to show the viability of nuclear fusion as a clean
energy source. In this quest, the tokamak and the stellarator are two leading
contenders for achieving nuclear fusion from magnetically confined plasmas.
They differ in terms of magnetic field configuration in the toroidal direction.
A tokamak is an axisymmetric device, and a stellarator is a non-axisymmetric
device. Both of these concepts have inherent advantages and disadvantages in
achieving nuclear fusion from toroidally burning plasmas. In particular, for
the viability of fusion, the plasma confinement time should be sufficiently long
to achieve a net energy balance. However, the measured particle and energy
losses in fusion plasma are higher than those of collisional processes. This socalled anomalous transport is due to the small-scale instabilities, called microinstabilities, that are a major cause of particle and heat loss from the device.
Irrespective of the magnetic field structure, both the tokamak and the stellarator are prone to microturbulence, and thus their understanding and control are
of paramount importance.
In this thesis, first-principles-based global gyrokinetic simulation studies
of the electrostatic microturbulence are presented in the Large Helical Device
(LHD) stellarator and ADITYA-U tokamak, using the state-of-the-art code GTC
(Gyrokinetic Toroidal Code), and the effects of impurities on the microturbulence are investigated in both machines. (ADITYA-U is the first Indian tokamak, and LHD is one of the world’s largest superconducting stellarators in
Japan.)
In the first part of the thesis, global gyrokinetic simulations of the ion temperature gradient (ITG) and trapped electron mode (TEM) driven turbulence
in the LHD stellarator are carried out with kinetic electrons using the monotonic smooth numerical plasma profiles. ITG simulations show that kinetic
electron effects increase the growth rate by more than 50% and more than double the turbulent transport levels compared with simulations using adiabatic
electrons. Zonal flow and microturbulence are ubiquitous in nature. Zonal
flow dominates the saturation mechanism in the ITG turbulence. Nonlinear
simulations of the TEM turbulence show that the main saturation mechanism
is not the zonal flow but the inverse cascade of high to low toroidal harmonics.
Further nonlinear simulations with various pressure profiles indicate that the
ITG turbulence is more effective in driving heat conductivity, whereas the TEM
turbulence is more effective for particle diffusivity.
In the second part of the thesis, global gyrokinetic simulations of electrostatic microturbulent transport for the experimental discharge of the LHD stellarator are carried out in the presence of boron impurity using GTC. The simulations show the co-existence of the ITG turbulence and TEM before and during
boron powder injection. ITG turbulence dominates in the core, whereas TEM
dominates near the edge, consistent with the experimental observations. Linear
TEM frequency increases from ∼ 80 kHz to ∼ 100 kHz during boron injection,
and the ITG linear frequency decreases from ∼ 20 kHz to ∼ 13 kHz, consistent with the experiments. The poloidal wave number spectrum is broad for
both ITG: 0 − 0.5 mm−1 and TEM: 0 − 0.25 mm−1
. The nonlinear simulations
with boron impurity show a reduction in the turbulent transport compared to
the case without boron. The comparison of the nonlinear transport shows that
the ion heat transport is substantially reduced in the region where the TEM
is dominant. However, the average electron heat transport throughout the radial domain and the average ion heat transport in the region where the ITG
is dominant are similar. The simulations with boron show the effective heat
conductivity values qualitatively agree with the estimate obtained from the experiment.
In the third part of the thesis, global gyrokinetic simulations of the electrostatic microturbulence driven by the pressure gradients of thermal ions and
electrons are carried out for the ADITYA-U tokamak geometry using its experimental plasma profiles and collisional effects. The dominant instability is TEM,
based on the linear eigenmode structure and its propagation in the electron diamagnetic direction. Collisional effects suppress turbulence and transport to a
certain extent. Simulations by artificially suppressing the zonal flow show that the zonal flow is not playing a critical role in the TEM saturation, which is
dominated by the inverse cascade. The frequency spectrum of the electrostatic
fluctuations is in broad agreement with the experimentally recorded spectrum
in the ADITYA-U, with a bandwidth ranging from ∼ 0 to 50 kHz.
In the fourth part of the thesis, the global gyrokinetic simulations of the
electrostatic microturbulent transport in the ADITYA-U tokamak are performed
in the presence of argon impurity and radial electric field determined from
the toroidal rotation. The dominant instability shares the features of ITG turbulence and TEM based on the direction of propagation and its response to
the zonal flow. The radial electric field itself suppresses the turbulence and
transport by changing the E~ × B~ shear, in agreement with the experimental observations. However, due to their low concentration, including argon ions in
the gyrokinetic simulations does not affect the transport. A comparison of the
simulations before and after argon puffing shows that the primary mechanism
responsible for the reduction in transport is due to the change in plasma profile after argon puffing, which changes the linear instability drive due to the
change in the profile gradient. Further simulation studies would be necessary
to decipher the underlying mechanism for the change in plasma profile after
argon puffing.
Finally, a novel framework is presented in the cylindrical coordinates to get
rid of the difficulties of the null point (X-point), where the poloidal magnetic
field vanishes, along with the singular behaviour of the safety factor and Jacobian in Boozer coordinates. This framework allows cross-separatrix coupling,
which makes it feasible to carry out whole-volume gyrokinetic simulations of
fusion plasmas.
To summarize, this thesis presents first-principles-based global gyrokinetic
simulations of electrostatic microturbulence and the effects of impurities on
the microturbulence in the LHD stellarator and ADITYA-U tokamak. These
microinstabilities act as one of the dominant channels for the transport of particles and heat flux in fusion plasma, so their understanding and control are
crucial for the viability of nuclear fusion.