What does UDH stands for in the name of an electrostatic accelerator 5UDH-2 Tandem accelerator? Does it is related to its working or just a trade mark?
That is a naming scheme of National Electrostatic Corp. (NEC), and is likely trademarked. The number has to do indirectly with the maximum terminal potential. Their website (pelletron.com) has more info. Not employed by NEC, long time user of their machines (and many others).