Three PPPL scientists win competitive awards to conduct frontier plasma science work

World-class expertise in the study of plasma – the hot, charged state of matter composed of free electrons and atomic nuclei, or ions, that makes up 99 percent of the visible universe – has won frontier science projects for three physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). The three-year awards provide PPPL principal research physicists Sam Cohen, Erik Gilson and Yevgeny Raitses with $330,000 each per year to pursue projects ranging from the study of plasma particles thousands of times thinner than a human hair to the formation of celestial bodies. The PPPL projects represent three of the nine awarded by DOE.

These awards, based on competitive peer review, aim to expand key areas of plasma science. “Better understanding the properties of plasma has many important scientific and technological applications, including high-power lasers, advanced microelectronics, and astrophysics,” said James W. Van Dam, Associate Director of DOE’s Office of Science for Fusion Energy Sciences. “The research, led by DOE National Laboratory teams, funded under this announcement will help ensure continued American leadership in this critical field.”

Here is a quick look at the designated projects.

Exploring energetic electrons. Energetic electrons are unexpectedly very hot – in excess of 100 M °C – particles produced in the radio-frequency heating of magnetized plasmas. Such electrons were found by graduate student Peter Jandovitz and former graduate student Charles Swanson, now a PPPL physicist, in low-temperature low-power magnetic-mirror plasmas. “The question is with such low power, what can possibly cause the creation of energetic electrons,” said physicist Sam Cohen, who is investigating this phenomenon.

Results of the project could apply to plasma processing in the microelectronics field in which energetic electrons can bombard semiconductor materials and create defects, Cohen said.

He plans to study high-temperature electron creation in his Princeton Field-Reversed Configuration (PFRC) device, which uses magnetic mirrors to create the magnetic field that confines the plasma. The project will explore the basic physics of the process and will seek answers to questions such as where the electrons develop, how the shape of the magnetic fields, and why the plasma properties affects them. Cohen will use X-ray detectors to measure and analyze these electrons.

To explain the formation of very energetic cosmic rays, Physicist Enrico Fermi theorized 72 years ago that a charged particle increases energy when bouncing between moving magnetic interstellar clouds. Fermi held that a series of particle collisions with a magnetic field that is perpendicular to the particles like a moving wall would produce energetic cosmic rays.

However, the research by scientists on the PFRC found energetic particle behavior in which particles were moving parallel to the mirror configuration magnetic field.

Investigating star formation. Funding for the Magnetorotational Instability (MRI) experiment at PPPL will aid the search for a phenomenon that could explain the formation of stars and planets. Scientists conjecture that the MRI could explain the observed rates of dust and other material swirling in so-called accretion disks around cosmic objects like stars and black holes and how that material collapses inward and congeals into celestial bodies.

“Our recent experiments and accompanying simulations showed intriguing and unexpected signs of the instability; this funding will allow us to take our research on this instability to the next level,” Gilson said.

The MRI experiment aims to replicate the instabilities that are thought to cause the swirling clouds of dust to collapse into the growing bodies that they orbit. The experimental device consists of two nested cylinders with the space between them filled with a liquid-metal alloy. The cylinders rotate at different rates, mimicking the different rotational rates of material in accretion disks.

Gilson and colleagues will use the funding to alter the MRI experiment to allow more variation in the liquid metal conditions. The scientists will investigate whether the top and bottom caps and enclosing walls of the device might be causing the unexpected results.

Gilson noted that the project team includes Princeton University professor and PPPL Distinguished Researcher Hantao Ji, as well as Princeton University professor Jeremy Goodman, and that the funding will support Yin Wang, a postdoctoral researcher who will conduct most of the research. The funding will also support related computer simulations produced by Wang and PPPL theoretical physicist Fatima Ebrahimi. This new funding will also be able to support graduate students interested in working with the MRI experiment for a thesis project.

Measuring dusty plasmas. Dusty plasmas house charged non-plasma particles sized from a few nanometers to several microns. These particles are known to influence a wide range of plasma behavior. “Dusty plasma is a huge field,” said Raitses, the lead principal investigator who is sharing leadership with PPPL physicist Shurik Yatom and Mikhail Shneider, a senior research scientist at Princeton University. “Dusty plasma is relevant to fields ranging from space physics to microelectronics,” Raitses said.

The researchers aim to measure the charge of dust particles, which serve as a major influence on the properties and dynamics of the dusty plasma. The project will study low-temperature, or cold, plasmas that compare with the million-degree fusion plasmas that have been the hallmark of PPPL research. Determination of the charge is a key scientific and practical problem.

While a majority of dusty plasma measurements have been indirect, the new venture “will address the challenge of independent [on-site] and real-time measurements of the charge and the mass of particles,” said the PPPL proposal. Researchers plan to develop and combine two laser techniques in coordination with electron density measurements to meet the challenge.

These techniques are technically called “Laser-Stimulated Photo-Detachment (LSPD)” and “Laser-Induced Incandescence (LII).” The first is a method for removing electrons from the dust while the second will measure the size and density of the foreign particles in the plasma and a microwave probe will detect the changes in particle density.

Yatom developed the LII diagnostic for a previous project on nanosynthesis that the researchers are now leveraging. “All three diagnostics together will allow us to deduce the charge using a model to be developed by Shneider,” Raitses said.

The three-year project aims to integrate theoretical and experimental studies of LSPD electron detachment that can be applied to the measurement of particle charges in a broad range of dusty plasmas. The project will collaborate with a number of dusty plasma research groups from Auburn University and the University of Minnesota. Co-PIs also hope that the project will attract graduate students to conduct their research on the challenging problems of diagnosing dusty plasmas.

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