Long thins strip of air between aluminum electrodes is electrically excited to produce ultraviolet light, but because it's very long and thin, it starts lasing

Long thins strip of air between aluminum electrodes is electrically excited to produce ultraviolet light, but because it's very long and thin, it starts lasing action (photons travelling along the length of the channel induce air to emit more photons in phase and travelling in the same direction) which produces laser beam. It's essentially ultraviolet laser from projector transparency sheets, some tinfoil and high voltage source.