Effects of wave damping and finite perpendicular scale on three-dimensional Alfv\'en wave parametric decay in low-beta plasmas

Shear Alfven wave parametric decay instability (PDI) provides a potential path toward significant wave dissipation and plasma heating. However, fundamental questions regarding how PDI is excited in a realistic three-dimensional (3D) open system and how critically the finite perpendicular wave scale -- as found in both the laboratory and space plasmas -- affects the excitation remain poorly understood. Here, we present the first 3D, open-boundary, hybrid kinetic-fluid simulations of kinetic Alfven wave PDI in low-beta plasmas. Key findings are that the PDI excitation is strongly limited by the wave damping present, including electron-ion collisional damping (represented by a constant resistivity) and geometrical attenuation associated with the finite-scale Alfven wave, and ion Landau damping of the child acoustic wave. The perpendicular wave scale alone, however, plays no discernible role, with different wave scales exhibiting similar instability growth. These findings are corroborated by theoretical analysis and estimates. The new understanding of 3D kinetic Alfven wave PDI physics is essential for laboratory study of the basic plasma process and may also help evaluate the relevance/role of PDI in low-beta space plasmas.