We present and analyze characteristics of the runaway electron flow in a high-voltage (the voltage rise rate of up to 1.5 MV/ns) air-filled electrode gap with a strongly nonuniform electric field. It is demonstrated that such a flow contains a high-energy electron component of duration not more than 10 ps. According to numerical simulations, runaway electron generation/termination is governed by impact ionization of the gas near the cathode and switching on/off a critical (sufficient for electrons to run away) electric field at the boundary of the expanding cathode plasma. The corresponding characteristic time estimated to be 2–3 ps is defined by the ionization rate at a critical field.

The conditions under which runaway electrons are generated in a gas diode with a strongly nonuniform electric field created by electrodes of specific geometry have been investigated. For an edge cathode, the equation of motion for electrons has been solved analytically. According to the solutions, for electrons to run away at the periphery, in the low field region, it is necessary that the applied potential difference be greater than a certain threshold determined by the electrode gap spacing and by the parameters of the gas. This condition supplements the classical electron runaway condition according to which the field strength near the emitting edge of the cathode should exceed some value depending only on gas parameters. It turned out that for a sharp-edged cathode, the new condition imposes more stringent requirements on the field strength compared with the classical one. Our calculations are supported by experiments in which electron runaway conditions were determined for a set of cathodes with different edge radii.

Fulfillment of the condition that the voltage rise time across an air gap is comparable with the time of electron acceleration from a cathode to an anode allows a flow of runaway electrons (REs) to be formed with relativistic energies approaching that determined by the amplitude of the voltage pulse. In the experiment described here, an RE energy of 1.4 MeV was observed by applying a negative travelling voltage pulse of 860-kV with a maximum rise rate of 10 MV/ns and a rise time of 100-ps. The voltage pulse amplitude was doubled at the cathode of the 2-cm-long air gap due to the delay of conventional pulsed breakdown. The above-mentioned record-breaking voltage pulse of ∼120 ps duration with a peak power of 15 GW was produced by an all-solid-state pulsed power source utilising pulse compression/sharpening in a multistage gyromagnetic nonlinear transmission line.

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We investigate the formation of a secondary flow of runaway electrons with energies on the order of 100 keV 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.