Laser beam runs on the outside of the laser tube!

I was cutting some stuff on my 120W laser when it started loosing power mid cut until it didn't cut at all. The current gauge was still showing amp draw when the laser was firing. I stopped the machine and opened the back up to investigate. This is what is happening when I fire the laser:
https://www.youtube.com/watch?v=e0rf...0&spfreload=10

The weird thing is that the machine hasn't been moved or manhandled in any way. It's been cutting fine. The tube has run roughly 200-300 hours with a huge compressor chiller running all the time. Temperature of the tube has to my knowledge never been higher than 28°C

I'm going to see if there is anything wrong with either the cathode or anode wires, but I'm sceptical as it hasn't been touched. Has the tubed leaked?

Any ideas?

Cheers,
David

Its gone. It hasn't leaked but that coiled tube at the back has a crack and the arc of electricity that creates the beam can now get to the outer reservoir part of the tube. FYI, that is why that little tube is coiled, to have a longer distance to the reservoir section than straight down the inner tube and the electricity will follow the shorter path.