Real-time observation of runaway-electron breakdown of air in the laboratory conditions

Summary form only given. By now, direct evidences on existence of runaway-electron breakdown (RB) phenomenon in atmospheric air (see, e.g., and the cited literature) were obtained in measurements of RF bursts and bremsstrahlung radiation time-correlated with propagation of atmospheric showers burned by high energetic cosmic particles and with atmospheric discharges (i.e., lightning). The problem to prove RB phenomenon in the direct experiment was in a large spatial scale of runaway-electron avalanche which is typical for a weak E-field of thunderstorm atmosphere (tenses of meters and units of kilovolts per centimeter, respectively). For a high E-fields (tenses and hundreds of kilovolts per centimeter) RB phenomenon becomes possible in macro-scales (centimeters), and at low threshold energy for electrons to run away (units of kiloelectronvolts). Besides, such RB appears in picosecond time scale. In the report we present experimental evidences of RB phenomenon where above-mentioned parameters were provided in the laboratory conditions. For the E-field strength of250 kV/cm, air breakdown was initiated by 50 ps-width electron beam. By the beam propagation in air filled electrode gap, a delayed current of RE electron avalanche has been observed in a real time. An upper limit of electrons energy and the avalanche width corresponded to that predicted by RB theory. The gap breakdown followed the injection of initiating e-beam with picosecond accuracy. With the absence of e-beam injection, no alternative slow mechanism of breakdown initiation was indicated.