The Alfven wave is the fundamental low frequency normal mode of a magnetized plasma. Alfven waves are ubiquitous in laboratory plasmas such as the tokamak and near-earth space plasmas such as the solar wind and the earths magnetosphere. The nonlinear physics of Alfven waves plays a central role in many natural processes. From a weak turbulence point of view, interactions between Alfven waves are fundamental to the cascade of energy in magnetic turbulence. Field-aligned structures (e.g. density cavities) in the magnetosphere are thought to be created by ponderomotive forces or heating due to nonlinear Alfven waves. Electron acceleration by large amplitude Alfven waves is thought to be important in space plasmas, and may explain auroral electron acceleration. While the linear characteristics of these waves have been explored in detail, nonlinear effects associated with large amplitude Alfven waves have not been elucidated in laboratory experiments. The research proposed in this career plan will focus on phenomena associated with the propagation of large amplitude Alfven waves in a laboratory experiment. These phenomena include: (1) electron heating and acceleration, (2) formation of field-aligned density structure, (3) interaction of Alfven waves with self-generated filamentary structures and (4) beat-wave interactions between shear Alfven waves, including stimulated parametric decay. Large amplitude shear Alfven waves have been generated in the Large Plasma Device (LAPD) at UCLA using either a resonant cavity or simple loop antennas. At the largest amplitudes, strong electron heating is observed, localized to current channels associated with the wave. In addition, evidence for electron acceleration is observed in Langmuir probe measurements. The nature of this heating and particle acceleration will be studied, focusing on dependence on the wave frequency (!/!c,i), perpendicular structure, and background plasma parameters. Along with heating, density modifications are observed both in Langmuir probe and interferometer measurements. The role of ponderomotive forces, heating or other phenomena (e.g. ionization) in creating the density modifications will be investigated. The interaction between the Alfven waves and the self-generated field-aligned temperature and density structures will also be explored. Previous studies of large amplitude Alfven waves in LAPD have focused on beat-wave interactions, in particular a co-propagating modulational interaction. These beat-wave studies will be extended, focusing on counter-propagating interactions including stimulated parametric decay of shear Alfven waves into ion acoustic waves.
The educational development aspects of the proposed career plan are focused on two primary tasks: (1) a high school outreach program targeted at teachers and students at some of the most disadvantaged high schools in Los Angeles and (2) a significant upgrade of the facilities used for teaching laboratory plasma physics at UCLA. The high school outreach program will include a summer workshop for two teachers a year, targeted at providing resources for and assistance in developing inquiry-based approaches to learning physics. The plasma laboratory course facilities and curriculum will be upgraded, seeking to modernize the course and broaden the impact of the course by attracting students interested in many subfields of physics and other disciplines.
This CAREER Development Award supported studies of the properties of an important class of waves that exist in plasmas (ionized gases) where magnetic fields exist: Alfven waves (named for Nobel Prize winner Hannes Alfven). Alfven waves are analogous to the waves that propagate along a guitar string when it is plucked; magnetic field lines that thread through the plasma can vibrate like the guitar string. Our work focused in particular on interactions between these waves: as one wave travels along the magnetic field and encounters another wave, their collision can generate a third wave (a so-called three-wave nonlinear interaction). These wave collisions allow the waves to exchange energy with one another and can lead to a turbulent state (Alfvenic turbulence). The intellectual merit of this work stems from the importance of these nonlinear interactions and of Alfvenic turbulence to a wide range of natural and laboratory plasmas (e.g. the solar wind, the interstellar medium and laboratory plasmas for fusion energy research (tokamaks)). The primary outcome of our work was the first observation and documentation of a nonlinear interaction between Alfven waves in a laboratory plasma. We followed up this first observation with an application of nonlinear interactions to the control of instabilities that occur in laboratory plasmas: drift waves. Drift waves and drift wave turbulence occur spontaneously in laboratory plasma confinement devices like tokamaks. This turbulence leads to a faster-than-desired loss of heat and particles from these devices and represents a significant challenge to acheiving net energy production via nuclear fusion reactions in the laboratory. We were able to control the drift-wave instability through nonlinear interactions with Alfven waves, perhaps paving the way for active-control techniques to control the loss of heat and particles from fusion reactors. The broader impacts of this project were realized primarily through contributions to education and training. Two graduate students completed their PhD thesis through involvement with and support from this project. Eight different undergraduate students participated in research supported by this award. One of these undergraduates won an award for her presentation at a American Physical Society Division of Plasma Physics meeting. The project had a direct impact on undergraduate education at UCLA through supporting the revamping of an upper division laboratory course in plasma physics. A high school outreach effort was also a key part of the project. Three high school teachers, all from schools in disadvantaged areas in Los Angeles, participated in a workshop which allowed them to participate in research and also to develop materials for their classroom. The classroom materials included demonstrations as well as curriculum targeted at bringing elements of plasma physics into their classrooms (e.g. we built plasma discharge tubes and developed a lesson on lightning). Following that workshop, the relationship with these teachers continued through giving opportunities for their students to participate in laboratory research. Four different high school students had this opportunity over the course of the project.
