Reducing phase-dependent emittance growth with local flattopping

Abstract

The efficiency of the proposed H/sup -/ extraction in the TRIUMF cyclotron is improved by inducing a precessional component to the radius gain per turn by exciting a coherent radial betatron oscillation at /spl nusub r/=3/2. Beam test and computer simulation results show that accompanying this improvement is a growth in the transverse emittance of the extracted beam. The growth is, in part, due to phase-dependent mixing of the perturbed beam. This phase-dependence can be greatly reduced by flattopping the local energy gain per turn with the addition of a higher harmonic accelerating field in phase opposition to the fundamental. A 92 MHz, 4/sup th/ harmonic, /spl lambda4 cavity installed in the TRIUMF cyclotron has been used for such a purpose. Results of both computer simulations and beam tests will be presented. >