Relating Gyrokinetic Electron Turbulence to Plasma Confinement in the National Spherical Torus Experiment

Abstract

Unraveling the relationship between plasma turbulence and transport is an important step in the development of magnetic fusion energy. In a magnetic fusion experiment, naturally occurring gradients in temperature and density profiles drive plasma turbulence. This turbulence can effectively transport heat and particles throughout the machine, thereby modifying the plasma's profiles. The result is a feedback system, with turbulence driving transport and transport altering the turbulent drive. Understanding this system is as important to development of fusion energy as it is challenging.

The relationship between gyrokinetic electron temperature gradient (ETG) driven turbulence and thermal confinement is particularly poorly understood. Empirical evidence suggests that ETG turbulence can not only exist in the National Spherical Torus Experiment (NSTX) but that it can also sometimes drive enough thermal transport to limit machine performance. Electron-scale (high-k) density fluctuations increase when the ETG mode is predicted to be active. Reversing the device's magnetic shear not only suppresses these fluctuations, but also triggers high electron confinement modes in NSTX known as electron internal transport barriers (e-ITBs). Controlling ETG turbulence with magnetic shear significantly enhances plasma confinement.

This dissertation supplements these experimental observations with numerical simulations. Applying the nonlinear gyrokinetic code GYRO to NSTX confirms the possibility of strong ETG-driven turbulent transport within the experiment. The associated thermal flux can indeed be high, accounting for at least one half of the inferred experimental level; however, the link between highk fluctuations and ETG turbulence is less firm. Additionally, the first nonlinear simulations of an NSTX e-ITB confirm that magnetic shear suppresses ETG turbulence and establishes transport barriers. While ETG turbulence can be strong in NSTX, its detrimental effects can be controlled with magnetic shear. How turbulence governs the performance of fusion experiments cannot be determined without an understanding of plasma transport. To that end, this work develops new algorithms for solving the steady-state plasma transport problem, integrating them into and testing them with the TGYRO code. An application of the new algorithms to a proposed ETG experiment on NSTX shows that adding impurities to radio-frequency heated plasmas can reduce turbulent transport and improve plasma performance.

This work is supported by the Princeton Plasma Physics Laboratory, which is operated by Princeton University for the U.S. Department of Energy under Contract No. DE-AC02-09CH11466, and the SciDAC Center for the Study of Plasma Microturbulence, and used the computational resources of both the Oak Ridge Leadership Computing Facility, located in the National Center for Computational Sciences at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy under Contract No. DE-AC05-00OR22725, and the National Energy Research Scientific Computing Center, which is supported by the Office of Science under Contract No. DE-AC02-05CH11231.