Simulation perspective on observing Alfven wave parametric decay in the laboratory*

The Alfven wave parametric decay instability (PDI) – where a pump Alfven wave decays into a backward Alfven wave and a forward ion acoustic wave – may contribute to several space phenomena, such as solar coronal heating and turbulent cascades, but only limited observational evidence of the instability exists. Here, we present simulation perspective on directly observing Alfven wave PDI on the Large Plasma Device (LAPD), using hybrid simulations (kinetic ions, neutralizing electron fluid) that adopt LAPD relevant wave/plasma parameters and configurations. First, using planar wave injection in quasi-1D simulations, it is found that PDI is triggered only when the instability growth overcomes Landau damping of the ion acoustic wave and when the instability has time to emerge before the daughter waves convect away. Second, using 3D rotating magnetic field injection, the finite wave source size is found to mainly affect PDI excitation through wave damping – strong damping associated with small source sizes (k_perp * di >> 1, as in LAPD) may decrease the pump wave amplitude to the point where PDI can be completely suppressed. Experimental and numerical investigations are therefore underway to seed the PDI process by launching both the pump Alfven wave and a small, backward-propagating Alfven wave. Detailed laboratory measurement of the basic plasma process will help benchmark existing PDI theories and clarify the role of PDI in spacecraft observations.