The five-centimetre-long plasma plume is generated when a stream of helium gas containing a trace of oxygen passes between two high-voltage copper electrodes. Helium is very difficult to ionize, but the plume’s oxygen molecules break into two highly reactive oxygen atoms, which then attack the bacteria. Previous plasma plumes have come with all sorts of difficulties: they have been just a few millimetres long, at least tens of degrees warmer than room temperature, and sometimes carried the risk of electrical arcing.