Laboratory-Based Study of Seeded Alfven Wave Parametric Decay Instability in Low-Beta Plasma*

We investigate seeded Alfven wave parametric decay instability (PDI) based on a low-beta laboratory plasma using 1D and 3D hybrid kinetic-fluid simulations. The PDI involves a large forward pump Alfven wave decaying into a backward Alfven wave and a forward ion acoustic wave, relevant to phenomena such as solar wind turbulent cascades and solar coronal heating. Laboratory investigation of PDI with a single pump Alfven wave has been challenging. This study explores a novel approach using a small-amplitude counter-propagating seed Alfven wave to initiate decay of a large-amplitude pump Alfven wave. We quantify PDI growth by measuring seed wave amplification, defined as the ratio of seed wave amplitude with the pump on to that with the pump off. Our simulations reveal spatial variations in seed amplification along the wave propagation axis, where seed amplitude ratios both above and below unity are observed and modulated with a period approximately half of the seed wave wavelength. This spatial modulation is corroborated by our experiments conducted at the Large Plasma Device at UCLA. Initial interpretation attributes this modulation to partial seed wave reflection at the pump wave injection location; a method of measuring PDI growth rates by leveraging the spatial modulation data is being developed. The close synergy between simulation and experiment is critical for demonstrating seeded PDI in laboratory settings, which can validate PDI theories and provide valuable insights for interpreting future spacecraft data.