Acceleration of Intense Positive Ion Beams at Megavolt Potentials

The long range objective of the ion accelerator development at RDI is to achieve a proton beam current of 5 to 10 mA at 3 to 5 MeV energy. Present performance is limited to 3 mA of mixed hydrogen ions at 3 MeV and 2 mA at 4 MeV despite the fact that both ion source and high voltage generators are capable of 10 mA service. Symptoms of excess ion beam loading are; rapidly increasing vacuum, X-ray production, leakage current, and beam halo. These observations can all be explained through the mechanism of scattering of the beam by residual gas in the acceleration tube. This effect is regenerative because scattered particles release more gas from the tube electrodes. The problem has been substantially reduced by a titanium getter-ion pump at the high voltage terminal. In addition to the pump, a crossed-field mass analyzer has been operated in the terminal, Comparative data on machine performance with analyzed beams has shown that the loading effects are more severe with molecular ions. The data suggest that the goal of 5 to 10 mA of protons can be achieved through the proper use of vacuum pumping and mass analysis prior to acceleration.