Why does lightning go so far? A 100-MV spark will only bridge 20 metres or so; yet a thunder cloud at 100 MV can launch lightning flashes over 1 km long. This is because lightning travels in steps. A ‘stepped leader’ stroke descends about 20 m from the charged cloud, and pauses. The current ionizes the air along its track; it becomes so conducting that in a few microseconds its sharp end is raised to the 100-MV potential of its cloudy origin. A sharp point at a high potential, of course, is an ideal initiator of electric breakdown. So the stroke proceeds another 20 m, and recreates another sharp conducting point lower down, which breaks down the local air in its turn. Once begun, a stepped flash can travel indefinitely.
So, says Daedalus, with a 100-MV source we could launch our own lightning through any distance of air. But how to aim it? An ultraviolet laser has been used to ionize a path through low-pressure gas, to guide an electron beam; but air would absorb such a beam. These days, however, gyratron valves can generate brief 100-MW pulses at 150 GHz. So DREADCO plasma physicists, with some trepidation, are launching a pulsed gyratron beam from a sharp-ended hollow waveguide which has been charged to 100 MV. The intense narrow beam will energize the air along its track, so that the first 20-m lightning step will follow the beam. The skin effect of the 150-GHz modulation will preferentially ionize the outside of the discharge, forming a hollow waveguide carrying the bulk of the radiation to the end of the step. Here the process will repeat. At the next gyratron pulse, the discharge will step another 20-m step in the beam direction, and so on. Indeed, even a 5-MV source should work; its lightning would advance along the guiding beam in 1-m steps.
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