We present a detailed investigation of X‐ray emission from long laboratory sparks in air at atmospheric pressure. We studied 231 sparks of both polarities using a 1‐MV Marx generator with gap lengths ranging from 10 to 140 cm. The X rays generated by the discharges were measured using five NaI/PMT detectors plus one plastic scintillator/PMT detector, all enclosed in 0.32‐cm‐thick aluminum boxes. X‐ray emission was observed to accompany about 70% of negative polarity sparks and about 10% of positive polarity sparks. For the negative sparks, X‐ray emission was observed to occur at two distinct times during the discharge: (1) near the peak voltage, specifically, about 1 μs before the voltage across the gap collapsed, and (2) near the time of the peak current through the gap, during the gap voltage collapse. Using collimators we determined that the former emission emanated from the gap, while the latter appeared to originate from above the gap in the space over the high‐voltage components. During individual sparks, the total energy of the X rays that was deposited in a single detector sometimes exceeded 50 MeV, and the maximum energy of individual photons in some cases exceeded 300 keV. X‐ray emission near the peak voltage was observed for a wide range of electrode geometries, including 12‐cm‐diameter spherical electrodes, a result suggesting that the X‐ray emission was the result of processes occurring within the air gap and not just due to high electric fields at the electrode.
