Investigation of space charge effect in TRIUMF injection beamline

The TRIUMF cyclotron operates routinely at 200 μA extracted at 500 MeV. Because of growing demands for beam, up to 400 μA is envisaged: this would require ∼ 1 mA from the H^- ion source and injection beamline. The phase acceptance of the cyclotron is roughly 36°, so the local peak beam current just before injection would be higher than 4 mA. This leads to large space charge effects on the beam transverse envelope and longitudinal bunching efficiency. The beam profiles and the bunching efficiency were measured for different currents up to 575 μA . These were used in space charge transport calculations to determine the beam optic properties and the space charge neutralization level. Extending the calculations to higher intensities, it is found that with the present double drift double harmonic bunching system, the bunching efficiency decreases dramatically above a dc current of 600 μA . To enable reaching the envisaged 400 μA from the cyclotron, it requires either raising the cyclotron phase acceptance from the present ∼ 36° to 50° by for example increasing the energy gain per turn at injection, or by adding another fundamental harmonic buncher.