Hydro-Magneto-PIC hybrid model for description of debris motion in high altitude nuclear explosions

A Hydro-Magneto-PIC (particle-in-cell) hybrid model is proposed to describe the motion of the fission debris in high altitude nuclear explosions (HANEs). Compared with the state-of-art numerical models, our model is able to stably compute the motion of the fission debris in a broader spatial region. In a real HANE, the physical process contains many spatial scales. The upward moving debris particles manifest kinetic properties due to the fact that the dilute ambient atmosphere and the downward expanding particles manifest a fluid-like pattern and can be approximated by the usual hydro-dynamical models. Meanwhile, the debris particles receive electromagnetic forces from both the geomagnetic fields and the charged particles at all frequencies. This broad scale of frequencies can induce large- and small-scale instabilities, which cannot be solved by the usual hydrodynamic equations. Considering the motions of the debris and the different properties of the high temperature ions, the low temperature ions and the neutral atmosphere, we consistently combine three models for completely describing the debris expansion. The high temperature ions are described by the PIC model for their intrinsic kinetic behaviors, the low temperature ions are described by the magneto-hydrodynamic model for their fluid property, and fluid equations are applied to the neutral particles with no electromagnetic force. The corresponding interactions among the three components are added into the equations as the source terms. With the combination of the three models, our algorithm can stably calculate the regions that are a few thousand kilometers in altitude. Our proposed model contains both the kinetic and fluid properties, and is stable in numerical implementations. Finally, we calculate the debris motion in the Starfish experiment. The results confirm a consistency of our proposed model with the observations. The spatial scale of our simulation results is consistent with the result in the Starfish experiment. In addition, we also plot the distribution of the debris with different projection angles at various snapshots. These results give us an intuition to understand the influence of the various factors, such as the friction of atmosphere, the magnetic pressure, flute instability and the Hall currents. Our model provides a tool for implementing the HANE simulation in a broader scheme, and can also be utilized in other plasma systems.