A device for generating an ion beam includes a support; a liquid metal ion source connected to the support at the lower end of the ion source and surrounded by a cryogenic trap which is capable of preventing volatile chemicals from reaching the ion source; an ion extraction means for extracting the ions emitted by the source through an opening disposed near the upper end of the ion source; and means for generating a magnetic field in the opening of the extraction means, the generated magnetic field capable of preventing charged particles from reaching the ion source.