Ionisers are basically high voltage generators of negative ions. The theory being basically that these ions attach themselves to unwanted particles in the air and generally removing smells and odours such as smoke.
This version is very basic and runs from a 12volt batter nut demonstrates some interesting principles about transformers and diode pumps.
Just to be absolutely clear, this project can be dangerous. I accidentally made a contact between the battery and the HV output and it really hurt!!