[PAST EVENT] Xin Wang, Physics - Oral Exam for the Ph.D.

June 6, 2016
Small Hall, Room 122
300 Ukrop Way
Williamsburg, VA 23185Map this location
Particle transport is an important topic in plasma physics. It determines the density profile of a burning plasma within a tokamak ? a magnetic confinement device. Microscopic turbulent particle transport is two orders of magnitude larger than other transport mechanisms for electrons and small ions. In order to confine a plasma in a tokamak with a core density that exceeds the fusion criteria [1], it is essential to study turbulent particle transport. This thesis investigates how different plasma parameters such as the toroidal rotation and microscopic instabilities affect turbulent particle transport in the DIII-D tokamak. First, we show how toroidal rotation can indirectly affect particle transport, through its contribution to the radial electric field and thus the E ?B shearing rate. The plasma discharge which has best confinement is the one whose E ? B shearing rate is larger than or at least similar to the growth rates that drive turbulent transport at the plasma edge. Second, for the first time on DIII-D, we observe a correlation between electron density gradient and instability mode frequency in the plasma core. We find that, when the turbulence is driven by the ion temperature gradient (ITG), the local density gradient increases as the absolute frequency of the dominant unstable mode decreases. Once the dominant unstable mode switches over to the trapped electron mode (TEM) regime, the local density gradient decreases again. As a result the density gradient reaches a maximum when the mode has zero frequency, which is corresponds to the cross over from ITG to TEM. This correlation opens a new opportunity for future large burning plasma devices such as ITER to increase the core density by controlling the turbulence regime. Finally, we show that, in low density regime, a reduction in core density is observed when electron cyclotron heating (ECH) is applied. This reduction is not the result of a change in turbulence regime nor the result of a change in the density gradient in the core. Through detailed time-dependent experimental analysis, linear gyro-kinetic simulations, and comparison to turbulence measurements we show that this reduction in core density is the result of an increase in turbulence drive at the plasma edge.

Xin Wang grew up in Changzhou, China. He graduated from the University of Science and Technology of China in 2012 with a Bachelor of Science degree in applied physics. That same year he began his graduate studies at William & Mary, where he started working with Dr. Saskia Mordijck. His research focuses on studying how different plasma conditions such as toroidal rotation or instability types can affect the turbulent particle transport in magnetic confined burning plasmas. After graduating, Xin will begin working as a Quantitative Associate at Wells Fargo.