Student Research

My research combines the direct measurement of electromagnetic torque with extensive three dimensional modeling to gain fundamental understanding of the neoclassical toroidal viscous torque that results from nonaxisymmetric fields. The first challenge is to help design an upgrade of the DIII-D magnetic sensors for measurements of the plasma response to three dimensional fields. My efforts focus on modelling the sensor sensitivity to islands or lack-thereof using the Ideal Perturbed Equilibrium Code (IPEC), and in ensuring that the upgrade provides the capability to measure the Maxwell stress tensor at the wall for n>1. In the future, devoted modelling and experimental studies of the NTV and total electromagnetic torque will help map dependencies on islands, rotation, collisionality, and/or beta. Comparison between analytic theory, numerical simulation, and experimental measurements will provide fundamental insight into these torques and the dominant mechanisms behind them.

Advisors: Jonathan Menard (Primary), Jong-Kyu Park, and Edward Strait (General Atomics)

The search for suitable materials for the first wall is an important area of research in fusion science. Recently, attention has been focused on liquid lithium as an option. One of the best features of the use of liquid lithium for a first wall is that it leads to large reductions in recycling, the process by which plasma ions that diffuse out of the plasma become neutralized by interaction with the first wall, and then re-enter the plasma as neutrals. While recycling helps maintain the plasma's particle inventory, these neutrals usually have very low energies; thus their reintroduction to the plasma edge has a cooling effect. The edge temperature is brought down, increasing the thermal gradient, which can drive instabilities and degrade confinement.

The Lithium Tokamak Experiment (LTX) is a spherical tokamak designed to study the low-recycling regime through the use of a liquid-lithium coated shell conformal to the last closed flux surface. A low recycling rate is expected to flatten core electron temperature profiles, raise edge temperatures, and strongly affect electron density profiles.

My research is centered around the effects low-recycling walls have on the electron temperature and density profiles in LTX. These measurements can be taken using a Thomson scattering diagnostic, comprised of a high-power pulsed ruby laser, collection optics, a spectrometer, and an intensifed CCD. The goals of my thesis are to determine the electron temperature and its profile as a function of recycling, correlate the electron temperature profile peaking and the confinement, correlate the recycling and the edge temperature using a combination of Thomson scattering and Langmuir probe data, and determine if the edge temperature correlates with the recycling predicted computational or analytic models.

Advisors: Richard Majeski and Ben LeBlanc

The use of liquid lithium (Li) plasma-facing components (PFCs) represents a promising path toward enhancing the confinement and stability properties of a tokamak plasma. The exact nature of the lithium-plasma interaction is largely unknown, however, due to the complexity of the tokamak environment. My research involves experimental isolation some of the physical and chemical mechanisms at work at the plasma-wall boundary, including impurity sputtering, Li passivation, erosion, recycling, power handling, and neutron loading. These issues are particularly important in the high heat flux region at the bottom of the machine known as the divertor. Studies are conducted through the use of linear plasma devices such as the MAGNUM-PSI experiment in the Netherlands and PISCES-B in San Diego, California, as well as an on-site diagnostic neutral beam (DNB). Li-coated molybdenum and carbon targets are bombarded by a high-energy plasma (or neutral) source. The responses of the edge plasma (density, temperature, recycling rate, etc.) and Li surface (composition, erosion rate, thermal conductivity, etc.) are carefully studied using an extensive suite of diagnostics. Results from these experiments will provide valuable information on the optimal Li-coated PFC for the NSTX-Upgrade experiment, scheduled to come online at Princeton Plasma Physics Laboratory in 2014.

Turbulence is considered to be one of the major sources of anomalous particle transport in fusion devices. A turbulent plasma does not confine particles as well as a laminar plasma. As such, the understanding and mitigation of tokamak turbulence is crucial for magnetic confinement fusion. One possible method of suppressing turbulence is through sheared flows. If the plasma spins rapidly, the localized shearing in the flow prevents turbulent eddies from growing tolarge scales. This is particularly the case with spherical tokamaks such as NSTX. A combination of a low aspect-ratio and powerful neutral beam injection cause the NSTX plasma to rotate quickly and set up sheared flows. Thus, it is highly probable that certain turbulent modes are suppressed in NSTX. I am currently using the GYRO gyrokinetic code to run non-linear simulations of NSTX discharges to study the effects of shearing (particularly from ExB forces) on turbulent transport. GYRO is a massively-parallel non-linear tokamak microturbulence simulation tool developed by Drs. Jeff Candy and Ron Waltz at General Atomics in San Diego, California.

Advisor: Greg Hammett

Fusion is a promising clean energy source. One idea for a fusion reactor is a stellarator: a twisted donut-shaped device that uses magnetic fields to contain plasma. To make fusion, we need to keep the plasma very hot and dense for a long enough time. However, turbulence can cool the plasma and break confinement by transporting heat and particles out to the edge near the stellarator walls. Mathematically, turbulence can be described by gyrokinetics. For my thesis research, I am computationally studying turbulence in stellarator plasmas, through improvements to the gyrokinetic code GS2 to make it more flexible and able to handle stellarator geometry. I developed a new grid generator to create the geometry input file for GS2 and benchmarked GS2's linear stability results against three other gyrokinetic codes for a couple of stellarator geometries. For the remainder of my thesis, I am using GS2 to systematically study the effect of several plasma parameters on linear instabilities in current and under-construction stellarators.

Advisors: Greg Hammett and David Mikkelsen