X‐ray observations were made during fourteen 1.5 to 2.0 m high‐voltage discharges in air produced by a 1.5 MV Marx circuit. All 14 discharges generated x‐rays in the ∼30 to 150 keV range. The x‐rays, which arrived in discrete bursts, less than 0.5 microseconds in duration, occurred from both positive and negative polarity rod‐to‐plane discharges as well as from small, 5–10 cm series spark gaps within the Marx generator. The x‐ray bursts usually occurred when either the voltages across the gaps were the largest or were in the process of collapsing. The bursts are remarkably similar to the x‐ray bursts previously observed from lightning. These results should allow for the detailed laboratory study of runaway breakdown, a mechanism that may play a role in thunderstorm electrification, lightning initiation and propagation, and terrestrial gamma‐ray flashes (TGFs).
