A design study is presented for a proton synchrotron (to be called a ``Cosmotron'') to accelerate protons to energies of the order of three billion volts. Protons are injected from a four‐million volt Van de Graaff machine. They are deflected electrostatically into a racetrack orbit including four quadrants of thirty‐foot radius of curvature, and four straight sections ten feet long. The energy gain per revolution supplied by the radiofrequency system will be about 1000 ev. Acceleration will continue to 1.0 seconds to a final energy of about three Bev. Construction of this machine is now in progress.