Electric Breakdown in Long Discharge Tubes at Low Pressure

The review is devoted to studies of the processes and mechanisms of ignition of a glow discharge in tubes whose length significantly exceeds their diameter (long discharge tubes) at low pressures (similar to 10 Torr and lower) and moderate voltage rise rates (similar to 1 kV/mu s and lower). The electric field in such tubes before a breakdown is substantially nonuniform. Therefore, a breakdown occurs after an ionization wave (or waves) passes through the discharge gap at a speed of similar to 10(5)-10(7)cm/s. This makes the characteristics of the breakdown in long tubes significantly different from the breakdown between large and closely spaced electrodes, where the electric field is uniform before the breakdown and where the Townsend or, under strong overvoltage, streamer mechanism is realized. On the other hand, the nature of these processes is very different from those occurring in nanosecond discharges, which arise at voltages with a steepness of similar to 1 kV/ns and higher and are associated with high-speed (similar to 10(9)cm/s) ionization waves. The review is based on the materials of experimental and computational works published from 1938 to 2020. Breakdown processes, optical and electrical characteristics of the discharge gap during breakdown, and the influence of the external circuit parameters and external actions (shielding and illumination by external sources of visible radiation) are analyzed.