Analysis of the Minimum Duration of the Runaway Electron Flow in an Air Electrode Gap

Properties of the runaway electron flow (REF) generated in an air-filled high-voltage interelectrode gap with a drastically inhomogeneous electric field distribution are presented. We have shown that REF initiated near the tip of a conical cathode involves high-energy electron fraction with ultrashort length of about 10 ps. Analytical and numerical considerations show that the REF generation and cessation is determined by a combination of ionization processes in the gas in the vicinity of the cathode, resulting in multiplication of free electrons and expansion of the cathode plasma layer, and dynamic processes that switch on/off a threshold electric field at the anode side plasma border which is sufficiently high for electrons to become runaway. The characteristic time of REF generation and termination can be estimated from the rate of ionization at a near-threshold field as ~2 ps. This elementary time determines the pulse length for the runaway electron current that was calculated to be ~6 ps, which is close to its experimental value.