Three-dimensional electromagnetic strong turbulence. II. Wave packet collapse and structure of wave packets during strong turbulence

摘要

Large-scale simulations of wave packet collapse are performed by numerically solving the three-dimensional (3D) electromagnetic Zakharov equations, focusing on individual wave packet collapses and on wave packets that form in continuously driven strong turbulence. The collapse threshold is shown to decrease as the electron thermal speed ν_e/c increases and as the temperature ratio T_iT_e of ions to electrons decreases. Energy lost during wave packet collapse and dissipation is shown to depend on _ec. The dynamics of density perturbations after collapse are studied in 3D electromagnetic strong turbulence for a range of T_iT _e. The structures of the Langmuir, transverse, and total electric field components of wave packets during strong turbulence are investigated over a range of ν_ec. For ν_e/c ? 0.17, strong turbulence is approximately electrostatic and wave packets have very similar structure to purely electrostatic wave packets. For ν_e/c ? 0.17, transverse modes become trapped in density wells and contribute significantly to the structure of the total electric field. At all ν_e/c, the Langmuir energy density contours of wave packets are predominantly oblate (pancake shaped). The transverse energy density contours of wave packets are predominantly prolate (sausage shaped), with the major axis being perpendicular to the major axes of the Langmuir component. This results in the wave packet becoming more nearly spherical as ν_e/c increases, and in turn generates more spherical density wells during collapse. The results obtained are compared with previous 3D electrostatic results and 2D electromagnetic results.