Equally important to note that this system will NOT work in a vacuum, but will start to work again if normal pressure is restored - further support for the ionised gas theory.
Alan Smith, it is very important to verify if a current can be forced in vacuum by applying an external voltage (in one direction AND in the opposite). If no current is detected, or a very weak current compared to when the gas is present, we can safely conclude that the "ionising radiation" is electromagnetic in nature (e.g. soft X-rays) and not made by charged particles. If a current is detected, it would probably be unidirectional (as in a diode), meaning that charged particle are emitted by the WE. Well, there are for sure more accurate interpretations, but this is a good starting point to understand what's going on.