The big 'uns will use 200mm wafer-size thyristors, banked in series and in parallel... the only important 400kV 'power' items are long-line power trunks. Maybe a few Marx generators, but the last one of those I'm aware of was disassembled years ago.
For a small power output, just generate some corona in a bottle, and surround the bottle with solar cells... neon fill will generate mainly red, that's good for silicon cells.