Hiya,
Some comments from me.
This is a wavelength converter tube and not an image intensifier. As such it is very low gain compared to a night vision scope, even a Gen 1. The tube used in the Electroviewer 7215 is what is called a Gen 0.
It receives Near Infrared wavelengths at its front target and and drives a phosphor imaging screen at the rear which the user views. It thus converts NIR to Visible light. The p.d across the tube can be around 10kV so that sounds right to me. There were issues with soft X-Ray emissions on poorly designed units but you should be fine with one from Electrophysics.
Due to the low gain of the converter tube, you can operate this unit in normal daylight just like a Vidicon camera. The lens has an IRIS that can be adjusted to prevent overloading and you should not point the unit at intense sources of photon energy within its pass-band as that can burn the tube phosphor. There is little need for a pinhole type lens cap with these low gain units.
When testing the unit, make sure that the IRIS on the lens is open and that you can see through the lens. These units can sometimes be fitted with IR or ND filters on the lens front, rear or even in the units lens mount itself. Such would effectively blind the unit to all but the designed filter response. An IR TV remote control makes a good test source as I am sure you already know
Failure modes on these units are actually quite few. The HT power supply can fail so you get no anode acceleration voltage. You appear to be OK on that front as 11kV sounds healthy to me. The tube can crack (due to impact) or just leak at a sealing point, and lose vacuum. Without a vacuum, internal HV arcing can occur due to the high anode to cathode voltage differential and lack of insulating vacuum. This can cause brief flashes that sometimes appear on the phosphor screen as green flashes but are also visible as blue flashes between electrodes when viewing the naked tube in darkness.
These tubes are very simple in operation. You apply lots of volts to them across the anode and cathode and they just work !
The tube normally presents virtually no load to the HT supply and so the supply is not designed to drive a low impedance. If the tube or its shroud suffer an insulation failure that permits arcing or presents a low impedance to the HT supply, it can collapse the HV and so the tube fails to operate. Look for any signs of arcing at the anode and cathode areas and measure the potential coming from the HT supply whilst connected to the tube. I have seen RTV used as insulation around HT feeds that had broken down over time and arcing to surrounding conductive material followed.
As I expect you know, the >10kV may be from a high impedance source but both it and a charged tube will bite you if they get a chance !
Do exercise a little care around one of these tubes. They do have the potential to generate soft X-Ray.
Fraser