A linear accelerator is now in use which increases the energy of protons from a 4‐Mev Van de Graaff injector to a final energy of 31.5 Mev. The accelerator consists of a cavity 40 feet long and 39 inches in diameter, excited at resonance in a longitudinal electric mode with a radio‐frequency power of about 2.1×106watts peak at 202.5 Mc. Acceleration is made possible by the introduction of 46 axial ``drift tubes'' into the cavity, which is designed so that the particles traverse the distance between the centers of successive tubes in one cycle of the rf power. The proton bunches are longitudinally stable as in the synchrotron, and are stabilized transversely by the action of converging fields produced by focusing grids. The electrical cavity is constructed like an inverted airplane fuselage and is supported in a vacuum tank. Power is supplied by 9 high‐powered oscillators fed from a pulse generator of the artificial transmission line type. Output currents are 3×10−7ampere, average, and 60 &mgr;a, peak. The beam has a diameter of 1 cm and an angular divergence of 10−3radian.