Abstract: In a high frequency resonator for accelerating a beam of heavy ions in an accelerator, which resonator includes a cylindrical resonator housing having a beam inlet end via which the beam enters the housing, a beam outlet end via which the beam leaves the housing and a longitudinal axis defining a path for travel of the beam between the ends, two opposed end walls each at a respective end of the housing, and an even number of elongate, conductive current collectors disposed within the housing, extending longitudinally thereof, and spaced apart equidistantly about the beam axis, with every other collector being connected to one end wall and the intervening collectors being connected to the other end wall, the number of such collectors being at least four, each end wall is provided with a central opening surrounding the longitudinal axis of the housing, each current collector extends the full length of the housing, and each collector has two longitudinal ends one of which extends into the opening of that end wal