Pair of CO2 Laser tubes. Front a 7W 350mm long and back a 10W 400m long. Cathode (GND) and the water cooled output coupler is on the left. Anode (+kV) and gold film HR mirror is on the right. Notice the silicone tubing between the cavity water jacket and the O/C cooling. ID of tubing is 3.2mm
The cheapest watercooler I could find was a PC cooling kit. The Thermaltake BigWater 735. This gives you a pump, radiator, reservoir, tubing, ethylene glycol coolant and a spare CPU cooler block. The tubing is however 3/8 inch ID so some adaptation is required.
I found a surplus 11kV PSU and was able to test it on a 1 Meg Ohm dummy load I made. The load contains 21 x 47k 7W ceramic cased resistors. Each successive resistor is rotated 90 degrees from its neighbors, giving a + shaped spiral. A 1k resistor in the base allows load current to be monitored. A fan in the base allows prolonged operation.Tube with water cooling connected.
Now the HVPS is connected. 8 x 47k resistors in series provide a ballast, taking the PSU from 11kV (tube striking) to 8kV (tube running) at the rated tube current of 10mA