This is intended to serve as a brief description of my Ph.D. thesis project. I hope to have the full text posted here as well, but that might be a heavy read. This is a work in progress, so new sections will be posted as they are written.

Tentative Title

Plasma Flows and Electron Heat Transport Driven by Filamentary Pressure Structures

One Sentence Summation

For my thesis in the field of experimental plasma physics I will use an electron beam to generate a hot filament of plasma in the background of a cooler plasma and then study how the energy moves around.

(a) A solid in which the particles are fixed in some particular order. (b) A liquid in which the particles have no particular order and fill the bottom of their container. (c) A gas fills the entire volume of its container.

Plasma

Plasma is sometimes referred to as the fourth state of matter. This is an acceptable definition. Consider a solid material, a block of ice. Heat this ice enough and it will melt into the liquid state. Continue heating this water and it will evaporate into steam, a gas. If we keep heating the gas, then the individual atoms will ionize. This means that the electrons will separate from the nuclei. This resulting mixture of negatively charged electrons and positively charged ions (nuclei) is a plasma.

Expanded view of a single atom and a collection of particles that compose a plasma.

Stars are made out of plasma, including the Sun. That's a good idea of how hot a gas must get before it ionizes. There are simpler examples of plasma, including lightning bolts and fluorescent lights (when they are turned on).

LAPD Device

My thesis work is being conducted at the Large Plasma Device (LAPD), which is part of the Basic Plasma Science Facility at the University of California, Los Angeles. The best pictures and descriptions of this device are found at their website.

The LAPD creates a cylindrically shaped plasma that is approximately 17 meters (m) long and 70 centimeters (cm) in diameter. The plasma is created by firing electrons from one end of the chamber to the other. As the electrons speed through the chamber they strike neutral atoms and knock electrons away from their nuclei. A magnetic field is generated through the plasma by outer current loops (purple and yellow in the image below). The magnetic field can be varied in both magnitude and direction, though in this project it is kept straight along the axis of the cylinder and at a uniform magnitude for each data run. A range in magnetic field strength from 500 Gauss (G) to 1800 G is studied. To put this in perspective, consider that the magnetic field of the Earth, near Los Angeles, and near the surface, is approximately 0.5 G. The Earth's magnetic field is a complex topic in its own right, but consider the strength of this field to be very weak.

The Large Plasma Device at UCLA. It is difficult to get a good picture that displays the entire 20 meter length of the machine.

The LAPD generates a plasma once every second. These plasmas are so similar that multiple shots can be averaged together to determine generic features of the plasma. This allows users to map out large areas of the plasma. Probes are placed in fixed positions for any single plasma discharge (i.e. shot), and then moved to a new location for later shots. In an experiment it is common to acquire twenty shots at each fixed position of the measurement device.

The main plasma lasts for approximately 10 milliseconds (ms). The afterglow, which is the plasma used in my experiment, can last for over 100 ms. As the plot below shows, the afterglow plasma is an environment in which the discharge is off and there is no additional heating from the cathode source. During this stage the plasma may be considered the "leftover" from the main discharge. The afterglow is considerably cooler than the main plasma and its density decreases in time as the atoms recombine.

The plasma density (ne) and machine discharge current (Idis) plotted as a function of time. The blue shaded region is the primary discharge and the green shaded area is the afterglow plasma in which my experiment is conducted.

Why Use the LAPD?

As later sections will describe, this thesis project is centered on energy transport in areas of relevance to astrophysical and fusion plasmas. The LAPD is a brilliant device for studying astrophysical plasmas because its length allows for the process of energy transport along the magnetic field lines to be examined. Plasma properties generally move incredibly quickly parallel to magnetic field lines and are therefore difficult to study. Satellites in space can study these phenomena because the systems are huge, as in the solar wind which can be studied along a path extending between the Sun and the Earth. Such studies in space are difficult and can be expensive. The LAPD can be used to study some of the same physical processes because it has such a long shape. In effect, the LAPD reproduces space (from a plasma point of view). The machine is long so it takes the energy just enough time to travel the length that it can be studied in depth.

The geometry of the LAPD also enables the study of energy transport perpendicular to the background magnetic field lines. This is of particular interest to the field of fusion plasma physics because the loss of energy out of fusion devices is a major problem in the development of viable fusion power reactors. The LAPD allows for extensive measurements to be taken along this direction.

To be written...

This section will be written once I have completed my thesis work. The reason for this is that the physics story cannot be told until I know the plot. In this case, the plot is what I am searching for through my work.

Tags: graduate school, thesis project, experimental plasma physics