The invention comprises a tandem accelerator method and apparatus, which is part of an ion beam injection system used in conjunction with multi-axis charged particle radiation therapy of cancerous tumors. The negative ion beam source includes an injection system vacuum system and a synchrotron vacuum system separated by a foil, where negative ions are converted to positive ions. The foil is sealed to the edges of the vacuum tube providing for a higher partial pressure in the injection system vacuum chamber and a lower pressure in the synchrotron vacuum system. Having the foil physically separating the vacuum chamber into two pressure regions allows for fewer and/or smaller pumps to maintain the lower pressure system in the synchrotron as the inlet hydrogen gas is extracted in a separate contained and isolated space by the injection partial vacuum system.
