This CAREER project is focuses on creating and studying confined non-neutral and electron-positron plasmas in the Columbia Non-neutral Torus (CNT), and to use this research program to interact with and educate graduate students, undergraduate students, high school teachers and students, and the general public. CNT is a unique and simple toroidal magnetic confinement device that is specifically designed for the studies proposed here. It will, for the first time, allow a systematic study of plasmas with arbitrary degree of neutrality, from pure electron to quasi-neutral, a systematic study of toroidal magnetic confinement in the presence of extreme electric fields and strong flows, and the creation of the first ever laboratory electron-positron plasma. Electron-positron plasmas are unique in that the mass ratio between the two species is one and the charge ratio is one. Hence, these plasmas are highly symmetric and very different from regular ion-electron plasmas. As a result, electron-positron plasmas have unique properties, such as not supporting acoustic waves and electrostatic drift waves. At the same time, they can be analyzed numerically and analytically more easily than ion-electron plasmas, because both species evolve on the same spatial and temporal scales. Electron-positron plasmas may be considered the hydrogen atom of plasma physics, the simplest possible plasmas.
CNT is a state of the art University experiment, with a large degree of involvement of graduate and undergraduate students. It is an ideal training ground for future experimental scientists. It also facilitates interaction and collaboration with high school science teachers and students, a collaboration that will be further expanded in the future.
This grant supported experimental research on the Columbia Non-neutral Torus (CNT), a plasma physics experiment at Columbia University. CNT is a stellarator, a magnetic configuration capable of confining charged particles and therefore plasma. Plasma is the fourth state of matter, usually reached at very high temperatures, in which a gas of free charged particles interact collectively, primarily through electromagnetic forces. The stellarator plasma confinement concept is more than 60 years old and interest in stellarators has recently enjoyed a revival, in particular for the confinement of very hot plasmas, plasmas that can be used to create fusion energy, the same source of energy that powers the sun. The research on CNT, however, did not directly focus on fusion energy. Rather, unusual and unique relatively low temperature plasmas were confined and studied. Normally, plasma exists as a mixture of free electrons and ions – in amounts where the negative charge of the electrons almost exactly equals the positive charge of the ions, a so-called quasineutral plasma. By contrast, non-neutral plasmas were confined and studied in CNT. At the one extreme, pure electron plasmas were created and studied, and their properties were measured and compared to theoretical expectations. Some of the results from these experiments actually have relevance to fusion energy, whereas others help pave the way to an electron-positron plasma, a unique plasma not yet created on Earth, but such plasmas are believed to exist around neutron stars and black holes. Overall the results help expand our base of knowledge of plasmas confined by magnetic fields. In CNT, it was possible to vary the degree of neutralization from the aforementioned extreme regime of pure electron plasma, to the other much more familiar regime of quasineutral plasma. For the first time, experiments were done where the degree of neutralization could be controlled to be at any point in between the two regimes, exploring the physics of "partially neutralized plasma" for the first time. In addition to the unique physics, the experiment served a very useful and much needed purpose as a training ground for young scientists. Seven students, five of whom directly received support from this grant, received their PhD’s in plasma physics working on CNT or theory related to CNT. More than a dozen undergraduate students were also involved with the research on CNT, many of whom went on to pursue graduate studies in science – mostly in physics, with plasma physics being the dominant major chosen.
