Strong Non-linear Alfvén Wave Interactions in a Laboratory Experiment

Alfvénic turbulence is pervasive in the solar wind and thought to be common in space and astrophysical plasmas. The solar wind has allowed us to learn much about this turbulence, however, there are many open questions about how it works and shapes these systems. Lab experiments provide a controlled environment to test the basic physics of such turbulence. Here, we present an experiment on the Large Plasma Device at UCLA, in which interacting low-frequency Alfvén waves at small k_⊥ρ_s are studied. We show that both counter-propagating and co-propagating waves result in a spectrum of new modes generated by their non-linear interaction. Estimated strength parameters, the wave frequencies involved, and their amplitude dependence on the initial waves, indicate the interaction to be in the strong regime - that applicable to space/astrophysical systems. The co-propagating waves produce a comparable spectrum to the counter-propagating ones, indicating a non-MHD interaction. We discuss a new nonlinearity, that scales with k_⊥d_i, which may be responsible, and may also be important for understanding unexplained features of the solar wind. We test this with 3D hybrid simulations, showing a good match to the theory. We also demonstrate energy transfer to higher k_⊥ structure through the non-linear interactions, indicative the processes that may be involved in the Alfvénic turbulence in the solar wind. This series of experiments also allows other aspects of strong Alfvénic turbulence to be studied, such as residual energy, and kinetic range interactions, which will also be discussed.