Experimental study on hard x-rays emitted from metre-scale negative discharges in air

Abstract

We investigate the development of metre long negative discharges and focus on their x-ray
\nemissions. We describe appearance, timing and spatial distribution of the x-rays. They
\nappear in bursts of nanosecond duration mostly in the cathode area. The spectrum can be
\ncharacterized by an exponential function with 200 keV characteristic photon energy. With
\nnanosecond-fast photography we took detailed images of the pre-breakdown phenomena
\nduring the time when x-rays were registered. We found bipolar discharge structures, also
\ncalled ‘pilot systems’, in the vicinity of the cathode. As in our previous study of x-rays from
\npositive discharges, we correlate the x-ray emission with encounters between positive and
\nnegative streamers. We suggest that a similar process is responsible for x-rays generated by
\nlightning leaders.