Variation of ion density in a high-voltage laboratory during impulse-voltage testing

The ionisation density in the air of a closed laboratory has been measured before and during impulse-voltage testing up to 500 kV, both without and with a gamma-ray source or an X-ray source of additional radiation placed near the gap under test. It has been found that ions caused by corona formed at the d.c. generator and on the capacitors of the impulse-voltage generator, 8 m from the gap under test, increase the ion density at the gap by an order of magnitude above typical values of 3 × 108 ions/m3 background radiation. After generation of a single impulse, a time interval of 20¿30 s has been shown to be sufficient for the residual ionisation to fall to background level