Development of an RF ion guide for trapping energetic radioactive nuclear ions

摘要

Although the projectile fragment separator (RIPS) at the RIKEN accelerator facility (RARF) provides wide variety of energetic radioactive beams, it is not adequate for low energy beam experiments such as precision spectroscopy of trapped ions. Aiming for an efficient deceleration and cooling of energetic radioactive ions, the development of an rf ion guide system which comprises a large He gas cell and an rf funnel structure in the cell is under progress. A proof of the principle (pop) machine has been successfully tested on-line for collection of 70-MeV/u~8Li ions. A compact gas cell of 70 cm in length and 10 cm in diameter with a He gas pressure of 30 Torr was placed after a variable wedge shaped energy degrader. The ions were stopped in the gas and were transported by a superposition of dc and rf electric field toward a small exit hole. The overall efficiency was 10~(―4) which can be separated into a gas stopping efficiency of 0.43 % and an ion-guide efficiency of 2.4 %. The former is limited by the volume of the cell and the pressure of the He gas. The latter is due to the present rf voltage limitation of 40 V. Both efficiency could be increased up to 10 % range by larger cell, higher pressure, and higher rf voltage, which would yield an expected overall efficiency of 1 %.