Phillip's 6 inch at Nottingham 2012
The gap had recently been switched to a ARSG, prior to this it was a fan cooled static using 0.5 inch diameter x 1 inch long Tungsten slugs. The opposing flat end surfaces being the spark surface, with the body of the slugs fitted into heavy brass holders to act as heat sinks.
The slugs were in fact spring loaded within their brass holders and each was secured with a grub screw. Upon release of the grub screw the slugs would spring outwards. Setting the gap simply meant undoing the grub screws so each slug sprang out to meet its opposing partner on the other side. You then placed a feeler gauge in between the surfaces forcing them back a small distance, before tightening their respective grub screws.
On hitting the floor they almost seem to disperse like water droplets spilt on a hard surface.
Despite being a spun toroid at 34 inch diameter, the slightest imperfection can still causes breakout to occur. Evidenced by several small bright points with a thin streamer emitting from them.