Experimental Research Projects
Plasma Rotation in the Presence of Torques from Nonaxisymmetric Fields
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)
Electron Temperature Profile Dependence on the Recycling Rate in the Lithium Tokamak Experiment
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
Experimental Investigation of Lithium PFCs for NSTX-U using Divertor Plasma Simulators
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.
Advisor: Robert Kaita
MHD Effects and Heat Transfer in Liquid Metals
In addition to being found in astrophysical systems such as stellar and planetary cores, liquid metals have been proposed as a plasma facing component in next generation fusion devices where current solid materials cannot survive the high incident heat flux (upwards of 100 MW/m2). One idea that has been proposed is a flowing liquid metal divertor. Ideally, the liquid metal would not only withstand the heat flux at steady-state, but the flow would also exhaust some of the power created within the plasma. However, due to their conductivity, liquid metals are strongly affected by external magnetic fields. For example, at high magnetic fields, currents induced within the fluid as it passes through the field can create an effective pressure that restricts the flow. Even at low to moderate field strengths, fluctuations in particular directions relative to the field are strongly damped, which has direct implications for the convective heat transfer within the fluid. In order to test the feasibility of a flowing divertor, experiments are being conducted to investigate how vortical structures and convective heat transfer change as a function of magnetic field.
Advisor: Hantao Ji
Study of Coronal Mass Ejection Trigger Mechanisms Using Laboratory Magnetic Flux Rope Plasmas
An emerging sub-field of plasma physics is Laboratory Plasma Astrophysics, which is the application of knowledge gained from laboratory plasmas to help understand solar and astrophysical phenomena. My thesis research involves studying line-tied magnetic flux ropes in the Magnetic Reconnection Experiment (MRX) with the goal of understanding how similar flux ropes in the solar corona may lead to energetic eruptions called Coronal Mass Ejections (CMEs). To create solar-relevant magnetic flux ropes in MRX, we form an arc discharge between two electrodes that are immersed in an externally applied magnetic field. As the current slowly rises in these flux ropes, they expand outward through a series of equilibria with the possibility of undergoing a sudden eruption. Such an eruption is triggered by a breakdown in the global force balance that governs the flux rope evolution. This type of event is consistent with observations of eruptions in the solar corona. Studies are ongoing to determine the exact conditions required to achieve an eruption and to understand the physical mechanisms that trigger and drive these dynamic events.
Advisors: Masaaki Yamada and Hantao Ji
Study of Nonlinear Dynamics of Intense Charged Particle Beams in the Paul Trap Simulator Experiment (PTSX)
The Paul Trap Simulator Experiment (PTSX) is a compact laboratory device that simulates the nonlinear dynamics of intense charged particle beams propagating over a large distance in an alternating-gradient magnetic transport system. The radial quadrupole electric field forces on the charged particles in the Paul Trap are analogous to the radial forces on the charged particles in the quadrupole magnetic transport system. The amplitude of oscillating voltage applied to the cylindrical electrodes in PTSX is equivalent to the quadrupole magnetic field gradient in accelerators. The temporal periodicity in PTSX corresponds to the spatial periodicity in magnetic transport system. My thesis research focus on the investigation of collective mode excitations, the properties of high-intensity beam with significant space-charge effect, random-noise-induced beam degradation, and the development of a laser-induced-fluorescence diagnostic.
Advisors: Ron Davidson and Erik Gilson