Simulations for two colliding plasma bubbles embedded into an external magnetic field

Abstract

A commercial magnetohydrodynamic (MHD) simulation package USIM is used to simulate two colliding plasma bubbles, which are not moving in the same horizontal line along the X direction. One similar experiment is performed on Shenguang II laser facility, in which four laser beams each with a wavelength of 0.351 m, total energy of 1.0 kJ, pulse duration of 1ns, are irradiated on an Al target with a thickness of 50 m. Every two beams constitute one 150-m-diameter focal spot with an intensity of 1015 W/cm2. The X-ray radiation results show the asymmetric and peach-like plasma bubbles, which are different from the results obtained before. Here we report the possible reason for the asymmetric and peach-like structure in experiment. External magnetic field on the order of 1 T is chosen to perform the simulations, which could be a possible applied B field in future experiments performing on the Shenguang II laser facility. In the simulations, different cases, especially the effects of different directional external magnetic fields, are considered. When the reversal directional magnetic fields are embedded in the Y direction, the magnetic field lines are frozen in the plasma bubbles, moving and approaching to each other gradually with the magnetic field lines. The change of the direction of magnetic field lines in the interaction region indicates that the magnetic reconnection has been happened. The outflows between two plasma bubbles in the experimental result could be explained by magnetic reconnection, which can efficiently convert stored magnetic energy into kinetic energy and thermal energy by accelerating and heating plasma particles. The density jump at the position of the bow structure indicates the generation of shock waves, where the velocity of flow v is also larger than the sound speed vs. When the same directional attractive magnetic fields are embedded in the Y direction, magnetic field lines are piled up in the central part, where the magnetic field density is high, which indicates that the magnetic repulsion has been happened. Magnetic repulsion also delays the colliding between two plasma bubbles. The shock waves each with a width of 4 m are also found in this case. The X-ray images in experiment and the density images in simulations show the similar peach-like structures, where the density results could be used to explain the X-ray radiation result for, I(v,Te)（2）/（Te） e（-（hv）/（kTe）, I is the radiation intense, v is the plasma velocity, Te is the electron temperature, is the plasma density.Magnetic reconnection is the possible reason for the asymmetrical and peach-like structure in the experiment by comparing all kinds of simulation cases. The present simulation results will be of benefit to the future designing of experimental setup on the Shenguang II laser facility, although a two-fluids model is needed to build a spontaneous magnetic field for the real plasma bubbles.