Formation of the Secondary Runaway Electron Flow in an Elongated Atmospheric Gap

We analyze features of the burst of fast electrons arriving to the anode of elongated, air-filled gap with a sharply inhomogeneous electric field with a delay relative to the primary picosecond runaway electron flow. Sufficiently high energy of the secondary particles qualifies them also as runaway electrons. The secondary flow appearance suggests impact ionization of the gas in the main part of the gap by the primary runaway electrons, as well as the presence of a residual electric field and its dynamic transformation due to propagation of the fast ionization wave toward the anode. Considering this concept, one can explain the sensitivity of the delay between two electron flows to the cathode-anode distance and to the regime of the ionization wave cut-off by an additional, floating-potential foil electrode partially transparent for primary electrons. The use of an ionization agent in the form of bremsstrahlung from a similar, tantalum foil electrode opaque to primary electrons leads to an earlier arrival of the secondary fast electrons to the anode.