Reducing phase-dependent emittance growth with local flattopping

The efficiency of the proposed H^- 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 ν_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^th harmonic, λ/4 cavity installed in the TRIUMF cyclotron has been used for such a purpose. Results of both computer simulations and beam tests will be presented