CAREER: Laboratory studies of large amplitude Alfven waves

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

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.