Then I relocated the breakout from the side to the top, in hope that the rising hot air would pull the discharge out a bit longer.

Finally, to add insult to injury, I plugged it into a variac and cranked it up to 270V rather than 240. That brought the charging voltage up to 700V, which is about as much as I dare go, since the transzorbs start to clamp at 800V... and the IGBTs explode at 900. As a half-hearted protective measure I set up a large muffin fan to cool all the primary components.

Then I turned the controller up to the maximum 1200bps and let it rip. 13.5 inch white-hot arcs to ground, 10" streamers into air, and a terrible screaming noise, were the result. In the picture above, you can see both arcs and streamers. It's a 0.75 sec exposure due to the high arc brightness.

From here on, the way forward is a single turn Litz primary, double the tank capacity, and four IGBTs instead of two. That should see us to 19" unless the secondary goes on fire or something equally horrific.