Particle and fluid models for streamers: comparison and spatial coupling

To understand ionization fronts and the growth of streamer channels, both fluid and particle models have been developed. While fluid models are computationally efficient in regions with large particle densities like the interior of a streamer finger, particle models represent the full microscopic scattering physics and are appropriate for regions with low densities and for particles with high energies like in the "pulled" ionization front ahead of the streamer finger. Furthermore, the statistics of few single particles in the front can create fluctuations of velocity and ionization rate and might trigger inherent instabilities. The goal of the project is therefore to develop a computation scheme that is hybrid in space. Here, we compare planar ionization fronts in the fluid or the particle model. The ionization density behind the particle front is higher while front velocities are similar. We analyze and explain this discrepancy. We also sketch the concept of hybrid model coupling of different spatial regions.