Improvements to the Beam Properties of the TRIUMF Cyclotron

The behaviour of the internal H
^-
and external proton beams has been considerably improved during the past year. Better steering near the centre has resulted in the internal vertical emittance being reduced to 1π mm-mrad, while the external beam emittances are now 3π mm-mrad vertically and 3π mm-mrad horizontally, for 90% of the beam. Digitization of probe data together with computer-aided trim coil tuning has enabled the beam to be centred vertically to within ±6 mm; this has been important in simplifying the simultaneous extraction of two beams at independently variable energies (183 to 520 MeV) and intensities (split-ratios from 1/1 to 1/5000). Beam losses in the cyclotron are <20%; direct evidence is presented for gas and electromagnetic stripping, and also for a loss of a few per cent by resonant processes. New techniques have been developed to measure the phase, and have enabled the phase excursions (Δsinϕ) to be reduced from ± 0.7 to <± 0.2 below 400 MeV. At high energies the phase excursions reach ±0.4, as anticipated from the magnetic field survey. However, a method is proposed by which separated turns could still be achieved and the energy spread reduced to 0.1 MeV, just as in a perfectly isochronous field.