In-flight Capture of an Ion Beam in a Paul Trap

Abstract

A system for collecting 60 keV ion beams was set up at the ISOLDE-3 radionuclide mass-separator facility at CERN. Electric field retardation was used to bring the ions to 20 eV for injection into a Paul trap. Tests were made with ¹³²Xe⁺ and ⁷⁹Br^? ions. A trap potential well depth of about 30 V could be created. At this well depth a collection efficiency of about 0·2% was obtained for a xenon beam of emittance about 2π mm-mrad and energy spread estimated to be of a few electronvolts. The efficiency for bromine ions was less, but still appreciable. The efficiencies achieved indicate that a sizeable fraction of the ions arriving at the correct rf window for acceptance by the trap were being collected. To the limit of the apparatus, the efficiency was increasing in proportion to the third power of the rf amplitude applied to the trap, indicating that it was proportional to the phase space volume of the trap. These results are now being used to design a larger Paul trap collection system for installation at the ISOLDE-BOOSTER facility. By including a pre-bunching system to fit an incoming dc beam to the rf acceptance, it is expected that almost complete collection of weak mass-separated high-velocity beams can be achieved.