Nanosecond images of OH LIF intensity at z = (a) 1 mm, (b) 3 mm, (c) 5 mm, (d) 7 mm, (e) 13 mm, (f) −20 mm (center of needle-to-ring spacing), the grounded aluminum foil and the tungsten wire electrode are also marked. The values of the color scale are normalized. Images are all captured at the time of 1 μs after the falling edge of the voltage pulse.