Global gyrokinetic simulations of electrostatic microturbulent transport in lhd stellarator and aditya-u tokamak
- Bangalore : Indian Institute of Science, 2023
- 174p. ill. col. e-Thesis 37.91 Mb
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.