Current Research

My research interests involve experiments to investigate the underlying physics relevant to magnetic confinement of fusion plasmas. Currently I am applying advanced turbulence diagnostics to plasma transport and turbulence research at the DIII-D tokamak. In particular, I collaborate with the Transport Model Validation (TMV) group at DIII-D to use multi-scale, multi-field turbulence measurements to test and validate gyrokinetic theories of plasma turbulence.

My plasma physics interests are quite broad, but at this point in time I am especially interested in microwave diagnostics (reflectometry, electron cyclotron emission (ECE) radiometers, and scattering), as well as beam emission spectroscopy (BES) diagnostics, imaging diagnostics, high-field and low-field side fluctuation measurements, magnetic fluctuation measurements, and experimental/theoretical comparisons between Thomson scattering and ECE electron temperature measurements in high temperature tokamak discharges. I am interested in higher order statistical analysis and wavelet analysis techniques, especially how these are are applied to experiments to test theories for the interactions of turbulence and plasma flows and nonlinear wave interactions. I would like to learn about experiments that test anomalous transport theories and neoclassical (classical) transport theory in tokamaks (basic plasma devices). Comparisons between stellarator and tokamak experiments, in particular the development of experiments to test the differences predicted for drift-wave instabilities in axisymmetric and non-axisymmetric geometries, are of great interest. I am also interested in learning about connections between laboratory plasmas and astrophysical plasmas.

Past Research Projects

DIII-D Tokamak
General Atomics, San Diego, CA.
January 2006-December 2008

My thesis project focused on the experimental investigation of core electron temperature turbulence using correlation radiometry of electron cyclotron emission (ECE) in the DIII-D tokamak. Theories of transport in tokamaks that are based on collisional effects (called neoclassical transport theories) cannot often account for the levels of transport that are observed in tokamak experiments. The deficit, called anomalous transport, is widely believed to be caused by plasma turbulence. The turbulence and associated heat and particle transport may be driven by radial gradients in electron and ion temperature and density. Measurements of multiple fluctuating fields simultaneously are crucial for understanding the basic physical nature of the plasma turbulence. Core measurements of small amplitude, broadband electron temperature fluctuations via correlation ECE (CECE) complement a suite of density fluctuation diagnostics at the DIII-D tokamak. I was fortunate to have worked closely with Dr. Schmitz, Dr. Peebles and the UCLA PDG to develop the CECE diagnostic for my thesis project. This allowed me to participate in all aspects of the commissioning of the CECE diagnostic at DIII-D; from initial design, construction and testing to installation, maintenance, day-to-day operation and data analysis. The temperature fluctuation measurements are made using standard correlation techniques: two radiometer signals from different frequency bands, but from within the correlation length of the turbulence, are correlated to determine the temperature fluctuation amplitude and spectrum. One goal of this study was to provide new data that could be used to make detailed comparisons between theory, computer simulation and experimental results in an effort to better understand tokamak turbulence and anomalous transport. In particular, the impact of ion-scale (ITG/TEM) electron temperature fluctuations on electron thermal transport has been explored in detail. This project produced the first observations that core electron temperature fluctuations are reduced in H-mode plasmas (Schmitz et al. PRL 2008) and the first comparison of the profiles of electron temperature and density fluctuations measured in the core of L-mode plasmas to nonlinear gyrokinetic simulations (White et al. POP 2008). These new experimental results and the comparison with advanced turbulence simulations have provided new evidence that long-wavelength electron temperature fluctuations can be transport relevant.

National Spherical Torus Experiment (NSTX)
Princeton Plasma Physics Lab (PPPL), Princeton, NJ.
July 2005 - December 2005

Zonal flows and shear flows are believed to play a crucial role in the regulation of drift-wave turbulence in plasmas. Zonal flows are believed to be excited via a nonlinear energy transfer from drift-waves and drift-wave turbulence at small scales to the larger, flow scales in the plasma. This project focused on the application of bispectral data analysis to data from the Gas Puff Imaging (GPI) diagnostic to search of the interaction between turbulent fluctuations and shear or zonal flows in the edge plasma during L-H transitions in NSTX. The bicoherence provides a measure of quadratic, nonlinear three-wave coupling. The bicoherence of turbulent potential and density fields measured with Langmuir probes has been observed to increase just prior to and during the L-H mode transition in DIII-D (Moyer et al, PRL 2001). By calculating the evolving three wave-coupling properties between turbulent and large scales with bispectral statistics, we can attempt to identify the formation of possible shear and/or zonal flows at the L-H transition using data from the Gas Puff Imaging (GPI) diagnostic at NSTX. The GPI experimental arrangement for these studies utilized a radial and poloidal array of 13 detectors to measure the HeI or D-alpha light emitted from the plasma. The array was used to measure profiles of the bicoherence before, during, and after an L-H transition. By calculating the bicoherence of the time series from the discrete fast GPI chords possible coupling between high frequency (high-k) and low frequency (low-k) modes, especially just before the L-H transition, was explored. The results of this NSTX study showed no increase in three-wave coupling in the GPI signals before the L-H transition (White et al. POP 2006). This work was done under the advisorship of Dr. Stewart Zweben at PPPL.

Electric Tokamak
University of California, Los Angeles (UCLA), Los Angeles, CA.
July 2003 - January 2006

The turbulence in tokamak edge plasmas is generally thought of as ballooning in nature, that is, the fluctuation levels will be largest on the low-field side of the tokamak, where the curvature is unfavourable and can drive many theoretically predicted modes unstable. Measurements of edge turbulence in tokamaks is generally limited to the outboard (low-field) side of the device due to issues of access for probes and microwave (mm-wave) measurements. Inboard (high-field) studies of edge turbulence are desirable in order to make accurate turbulent transport estimates and in order to understand the basic physics of edge turbulence in tokamaks. Due to its large aspect ratio (R/a =5), the Electric Tokamak (ET) at UCLA offered excellent inboard access for measurements of edge turbulence. Turbulence is observed on the inboard side of ET, with amplitude and spectrum comparable to the outboard edge, indicating that the turbulence drive may be poloidally symmetric. However, poloidal asymmetry in the turbulent correlation length is observed, with shorter correlation lengths on the inboard edge, indicating a better confinement in the higher field region. Detailed studies of these asymmetries were carried out using fixed Langmuir probe and magnetic probe arrays on the inboard and outboard edges measuring floating potential, ion saturation current and magnetic fluctuations.