The invention relates to a method and apparatus of control and/or determination of energy and/or velocity of a beam of a charged particles extracted from a synchrotron, the synchrotron using a radio frequency field to redirect the charged particles through an extraction material. The method and apparatus for extracting a group of charged particles at a known and controlled energy is optionally used in conjunction with any apparatus and/or technique coupled to a synchrotron, such as a charged particle cancer therapy system. For example, the knowledge/control of the charged particle beam delivery energy is used in conjunction with a multi-axes and/or a multi-field tumor irradiation system in combination with a tumor treatment plan to accurately ablate or irradiate a tumor.
