Embedding an electromagnetic region in a hybrid particle simulation

We propose a new computational approach to simulating a vacuum or neutral electromagnetic (EM) region embedded or adjacent to a plasma region modeled using the hybrid equations.

Our approach uses the fact that the hybrid model solves the fully EM form of the induction equation to advance the magnetic field in time. Hence, the magnetic field is solved globally on the same grid for both regions. This gives del.B = 0 to machine precision when using Yee’s algorithm for advancing the magnetic field in time.

The electric fields in the hybrid and EM regions are joined together smoothly based on an electron density cutoff. The boundary between hybrid and EM regions is fully dynamic and evolves in time based on the evolution of the electron density.

Because the Courant condition is more restrictive for our explicit finite difference time domain (FDTD) EM solver than for the hybrid solver, we subcycle the FDTD solver. This is not a limitation, however, as running with a ratio of 100 FDTD subcycles per hybrid timestep or more does not significantly impact the speed of the computation.