Alfvén waves, a fundamental mode of magnetized plasmas, are ubiquitous in both space and laboratory plasmas. In particular, non-linear Alfvén wave interactions and instabilities may play a key role in important problems including the heating of the solar corona, the transfer of energy to small spacial scales in the solar wind, and Alfvén eigenmodes in tokamaks. At the Large Plasma Device (LAPD) at UCLA we are studying several non-linear Alfvénic processes through to be vital to these important problems. The LAPD allows for detailed spacial measurements in a relatively quiescent plasma, enabling experiments not possible in settings such as the turbulent solar wind. Recent results include 1) the first laboratory observation of the Alfvén-acoustic mode coupling at the heart of the Parametric Decay Instability [1] and 2) laboratory observations consistent with a decay instability in which a Kinetic Alfvén Wave (KAW) decays into co-propagating KAWs. The first study is conducted by launching counterpropagating Alfvén waves from antennas placed at either end of the LAPD. A resonance in the beat wave response produced by the two launched Alfvén waves is observed and is identified as a damped ion acoustic mode based on the measured dispersion relation. In a second experiment, a single high-frequency ω/ωci∼0.7 Alfvén wave is launched, resulting in daughter modes with frequencies and parallel wave numbers that suggest co-propagating KAWs produced by decay of the pump wave. The observed process is parametric in nature, with the frequency of the daughter modes varying as a function of pump amplitude. Efforts are underway to fully characterize the second set of experiments and explore other important processes; in particular, a new plasma source recently installed on the LAPD enables the study of non-linear Alfvén wave physics at high plasma beta approaching 1, a parameter regime of particular interest to the solar wind.
[1] S. Dorfman and T. Carter, Phys. Rev. Lett. 110, 195001 (2013).