What kinds of things are done using high voltage? Some of them you already know about. They are still used for CRT visual displays, CRT displays for photo typesetting (imagine a CRT with 1200 lines per inch resolution), photomultiplier tubes, radar, X-ray generators, assassinating mosquitoes at your backyard barbecue party, and so forth. Sometimes though, the unusual need comes along.
There was this bread bakery some while ago that was using high voltage to lower their oven operating costs. They told us that when 40 thousand volts were applied from above and below in the oven chamber where loaves of bread were being passed through on a conveyer, the high voltage reduced the required natural gas usage by 20 percent.
Somebody was awfully clever about this. They had devised a high-voltage power supply using the innards of a 1950s vintage television set. There was a flyback that worked with a 6DQ6 horizontal driver tube and a 1B3GT high-voltage rectifier. How they had gotten it up to 40kV they didn't say, but since home television CRTs like the 21YP4, for example, only needed perhaps 20kV, I was very impressed.
Reliability was their issue. Although they could still produce loaves of bread without the high voltage when things broke down, the fuel savings had warranted frequent repairs. We made a "modern" 40kV supply for them and they were very happy.
Then there was this car company that made diesel engine-powered vehicles and they were trying to minimize the generation of soot. They needed to measure soot emissions so they put corotrons as soot sensors into the exhaust pipes. Simplistically, they looked sort of like this sketch, a thin wire within a conductive half-circle shroud.
The requirement for powering these things was given to us as 12kV at 1mA, so we designed the power supply to deliver just that, built a couple, and made the shipment. Shortly afterward, we heard back about the performance that had been achieved. The report was very brief.
Corotrons, as it happens, are required to be driven from a controlled current source, but we had followed the customer's specification in making a controlled voltage source.
The remedy was simple though. I studied the schematic and prepared some instructions. It was a simple modification, something like, "Remove R10 and CR4; connect a jumper from U2 pin 3 to the low end of R8..." and thereby the unit would be converted to a 1mA constant current power supply with a compliance voltage of 12kV.
It was with great pleasure that a few months later we received a lovely letter of appreciation for that product.