An apparatus for injecting a charged particle beam into an accelerator of an irradiation device, said irradiation device irradiating a tumor during use, said apparatus comprising: a negative ion source, said negative ion source configured to produce a negative ion beam, the negative ion beam comprising a cross-sectional area; a vacuum tube an ion beam focusing system configured to produce electric field lines running, in said vacuum tube, through the negative ion beam path, wherein the electric field lines focus the negative ion beam, said ion beam focusing system. further comprising: a first focusing electrode circumferentially surrounding the negative ion beam; and a second focusing electrode comprising metal conductive paths configured to at least partially block the negative ion beam; wherein first electric field lines run between said first focusing electrode and said second focusing electrode, wherein the negative ions encounter first force vectors (571,575) running up the first electric field lines that focus the negative ion beam, wherein said metal conductive paths comprise any of: a series of conductive lines running substantially in parallel across the negative ion beam path; a conductive mesh crossing the negative ion beam path; and a focusing foil crossing the negative ion beam path, said focusing foil having holes, said holes comprising a combined cross-sectional area of at least ninety percent of the cross-sectional area of the negative ion beam; and a converting foil, said converting foil configured to convert the negative ion beam into the charged particle beam. a negative ion source, said negative ion source configured to produce a negative ion beam, the negative ion beam comprising a cross-sectional area; a vacuum tube an ion beam focusing system configured to produce electric field lines running, in said vacuum tube, through the negative ion beam path, wherein the electric field lines focus the negative ion beam, said ion beam focusing system. further comprising: a first focusing electrode circumferentially surrounding the negative ion beam; and a second focusing electrode comprising metal conductive paths configured to at least partially block the negative ion beam; wherein first electric field lines run between said first focusing electrode and said second focusing electrode, wherein the negative ions encounter first force vectors (571,575) running up the first electric field lines that focus the negative ion beam, wherein said metal conductive paths comprise any of: a series of conductive lines running substantially in parallel across the negative ion beam path; a conductive mesh crossing the negative ion beam path; and a focusing foil crossing the negative ion beam path, said focusing foil having holes, said holes comprising a combined cross-sectional area of at least ninety percent of the cross-sectional area of the negative ion beam; and a converting foil, said converting foil configured to convert the negative ion beam into the charged particle beam. a first focusing electrode circumferentially surrounding the negative ion beam; and a second focusing electrode comprising metal conductive paths configured to at least partially block the negative ion beam; wherein first electric field lines run between said first focusing electrode and said second focusing electrode, wherein the negative ions encounter first force vectors (571,575) running up the first electric field lines that focus the negative ion beam, wherein said metal conductive paths comprise any of: a series of conductive lines running substantially in parallel across the negative ion beam path; a conductive mesh crossing the negative ion beam path; and a focusing foil crossing the negative ion beam path, said focusing foil having holes, said holes comprising a combined cross-sectional area of at least ninety percent of the cross-sectional area of the negative ion beam; and a converting foil, said converting foil configured to convert the negative ion beam into the charged particle beam. a series of conductive lines running substantially in parallel across the negative ion beam path; a conductive mesh crossing the negative ion beam path; and a focusing foil crossing the negative ion beam path, said focusing foil having holes, said holes comprising a combined cross-sectional area of at least ninety percent of the cross-sectional area of the negative ion beam; and a converting foil, said converting foil configured to convert the negative ion beam into the charged particle beam.