Global gyrokinetic simulations of electrostatic microturbulent transport in LHD stellarator and ADITYA-U tokamak

Abstract

Tokamak and stellarator are two leading contenders in the quest to achieve nuclear fusion from magnetically-confined plasmas. They differ in terms of magnetic field structure in the toroidal direction. Irrespective of the magnetic field configuration, both the tokamak and the stellarator are prone to microturbulence, which is believed to be the major cause of particle and heat loss from the device. Thus, their understanding and control are paramount for the viability of nuclear fusion.

In this thesis, first-principles-based global gyrokinetic simulation studies of the electrostatic microturbulence are presented in the ADITYA-U tokamak, and Large Helical Device (LHD) stellarator and the effects of impurities on the microturbulence are investigated in both machines.

In the first part of the thesis, the global gyrokinetic simulations of the ion temperature gradient (ITG) and trapped electron mode (TEM) in the LHD stellarator are carried out with kinetic electrons using the numerically generated monotonic smooth plasma profiles. ITG simulations show that kinetic electron effects increase the growth rate and turbulent transport levels compared with simulations using adiabatic electrons. Zonal flow dominates the saturation mechanism in the ITG turbulence. However, its effect is weak on TEM turbulence. Following this, a realistic experimental discharge is analyzed in the presence of boron impurities. Simulations show the co-existence of ITG and TEM turbulence, with the linear frequencies matching well with the experimental observations. Nonlinear simulations show that the reduction in nonlinear transport is a combined effect of the change in plasma profile and plasma dilution due to boron impurities.

In the second part of the thesis, the 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 with 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. Following this, the effect of argon impurities on turbulence and transport is investigated. Simulations show that the primary mechanism responsible for the reduction in transport is the change in plasma profile due to argon puffing.

Finally, a novel framework is presented in cylindrical coordinates to get rid of the difficulties of the null point (X-point), where the poloidal magnetic field vanishes, along with the singular behavior of the safety factor and Jacobian in Boozer coordinates that enables the whole volume gyrokinetic simulations of fusion plasma.