Studies of the nonlinear physics of Alfvén waves in the large plasma device

The LArge Plasma Device (LAPD) at UCLA is a 17 m long, 60 cm diameter magnetized plasma column with typical plasma parameters n_e ~ 1 × 10¹²cm^-3, T_e ~ 10eV, and B ~ 1kG. The linear and nonlinear physics of Alfvén waves have been the focus of recent research using LAPD ¹. In this presentation, two recent studies of the nonlinear physics of Alfvén waves will be discussed. First, the nonlinear three-wave interaction process at the heart of the parametric decay instability is studied by launching counter-propagating Alfvén waves from antennas placed at either end of LAPD. The two waves generate a nonlinear beat-wave response in the plasma which can be tuned in frequency by changing the frequency of the pump Alfvén waves. As the beat frequency is varied, a resonance in the the beat wave response produced by the two Alfvén waves is observed and is identified as a damped ion acoustic mode ²; this identification is based on the measured dispersion relation. Other properties of the interaction including the spatial profile of the beat mode and propagation direction are also consistent with theoretical predictions for a three-wave interaction between the two pump Alfvén waves. A second study of the nonlinear physics of Alfvén waves explores the three-wave interaction relevant to the cascade of energy in magnetohydrodynamic turbulence ³. In this set of experiments, the interaction between counter-propagating Alfvén waves is studied, focusing on the interaction between a low-frequency, large amplitude wave (k_|| ~ 0) and a higher frequency wave. The observed daughter wave is consistent with theoretical predictions for this important interaction.