[Ion sources][1] are devices that allow creating ion beams (e.g. argon ions) and to project them outside the device, for example to be further processed by a particle accelerator, or to irradiate materials or biological tissues etc. 

The ions are usually created by a plasma inside a cavity filled with a noble gas maintained at low pressure inside the cavity.

So, the cavity cannot have a hole to let the ions get out, otherwise the vacuum would be destroyed. So, by what mean do the ion get out of the cavity?


  [1]: https://en.wikipedia.org/wiki/Ion_source