A RING-SHAPED CENTER CONDUCTOR GEOMETRY FOR A HALF- WAVE RESONATOR*

Half-wave resonators (HWR) are used and being proposed for the acceleration of high-intensity proton and heavy-ion beams in the 0.1 < β < 0.5 velocity range. The highest performing half-wave resonator geometries use a center conductor with a race-track shaped cross section in the high-electric field region; a feature shared with spoke cavities which are also being proposed for the same velocity regime. We here propose a ring-shaped center conductor instead of the race-track shape. Preliminary studies show that the ring geometry has a similar peak surface electric field as the race-track one, and several other advantages. In particular, the ring-shaped geometry has: a lower peak surface magnetic field for a given accelerating voltage, a 36% higher shunt impedance for the same peak fields, and no quadrupole electric field asymmetry which was observed in the race-track geometry. For solenoid-based symmetric focusing, the quadrupole component may lead to unnecessary emittance growth which is not acceptable in highintensity ion linacs. We will present a detailed comparison and a discussion of the two geometries.