Central Region Orbit Dynamics in the TRIUMF Cyclotron

Orbit dynamics in the central region have been studied with the aim of optimizing the design for beam quality and RF phase acceptance. Axial ion motion near the centre is dominated by the strong phase dependent RF electric forces at the dee gaps; the consequent restriction of the phase acceptance to ions lagging the RF voltage peak can be lifted both by using an asymmetric dee gap geometry and by the addition of third harmonic to the RF fundamental. A transition from six- to three-sector magnet geometry at the centre to increase the axial focusing was also tried, but was found to give rise to unacceptable ±9° phase oscillations if the dee gap were oriented to avoid the usual gap crossing resonance. Other effects producing coherent radial oscillations, such as first harmonic magnetic field components and dee-voltage asymmetries, have been assigned tight tolerances. The proposed design also makes possible near perfect centring for ions of all phases. Numerical orbit tracking has shown that a beam with the emittance of the external ion source can be accelerated to 20 MeV within 0.25 π in. mrad for an RF phase band at least 25° wide: in the absence of inflationary effects at higher energies this would yield a 500 MeV beam with ±600 keV energy resolution.