Features Of The Secondary Runaway Electron Flow Formed In An Elongated, Atmospheric Pressure Air Gap

We investigate the formation of a secondary flow of runaway electrons with energies on the order of 100keV in an air gap after the passage of a primary picosecond flow of similar fast particles, arising in the cathode region where the distribution of the electric field is very inhomogeneous. The dependence of the secondary flow onset delay on the gap length and the field strength is demonstrated. The reasons for the discrimination of the two flows in time are considered analytically. The occurrence of fast secondary electrons in an anode-located part of the gap is demonstrated under conditions when the ionization wave generated in the cathode region by the primary runaway electrons is cutoff. For this, an intermediate aluminum foil electrode under a floating potential has been used. This third electrode, which divides the gap into two sections, is partially transparent for the runaway electrons. It has been also shown that runaway electrons demonstrate the earliest arrival to the anode if they arise due to the photoionization of the gas by bremsstrahlung of the primary particles bombarding an intermediate electrode made of tantalum foil.