Mechanism of branching in negative ionization fronts

When a strong electric field is applied to nonconducting matter, narrow channels of plasma called streamers may form. Branchlike patterns of streamers have been observed in anode directed discharges. We explain a mechanism for branching as the result of a balance between the destabilizing effect of impact ionization and the stabilizing effect of electron diffusion on ionization fronts. The dispersion relation for transversal perturbation of a planar negative front is obtained analytically when the ratio D between the electron diffusion coefficient and the intensity of the externally imposed electric field is small. We estimate the spacing lambda between streamers and deduce a scaling law lambda approximately D(1/3).     

Comment on "Dynamics of wetting fronts in porous media"

A new method to model unsaturated flow in porous media was presented in Phys. Rev. E 58, R5245 (1998). We analyze the proposed approach and illustrate some significant shortcomings.     

Reply to "Comment on 'Dynamics of wetting fronts in porous media' "

In our work [Phys. Rev. E 58, R5245 (1998)] we introduced a dynamic phenomenological approach to model propagation of localized wetting fronts in porous media. Gray and Miller in their Comment [Phys. Rev. E 61, 2150 (2000)] criticize our approach on several issues. The main criticism addresses the problem of mass conservation in our model. In this Reply we argue that their criticism is incorrect.     

Measurements of electric-field strengths in ionization fronts during breakdown

Using laser-induced fluorescence-dip Stark spectroscopy, we performed time-resolved, direct measurements of electric-field strengths during the breakdown phase of a low-pressure, pulsed discharge in xenon. With this experimental technique we could for the first time quantitatively measure the time evolution of the driving force of the plasma breakdown process: the electric field. Moving ionization fronts were measured with submicrosecond resolution. These ionization fronts were sustained by a spatially narrow, rapidly moving region of strong electric field.