VACUUM AND MAGNETIC-FIELD CONSTRAINTS IN A H- LIGHT-ION SYNCHROTRON

Acceleration of H- ions in a synchrotron imposes severe restrictions o n the level of residual pressure in the vacuum chamber and the maximum magnetic field in the magnets of the ring. Significant vacuum require ments are also imposed by the acceleration of ions. This paper discuss es these two aspects of the design of a combined H-/light ion synchrot ron for radiation therapy. The fractional loss of the accelerated beam induced by the two processes is evaluated on the basis of a general t reatment of the physics of these phenomena. The values of the vacuum a nd magnetic field necessary for normal operation of the machine are sp ecified and a discussion is given of the behaviour of the above quanti ties as a function of several parameters such as beam energy, composit ion and pressure of the residual gas in the vacuum chamber and beam ex traction time.