The
ion accumulator and buncher in the LEBIT project
has the task to accept the continuous ion beam from the gas cell and to convert
it into cooled low-energy pulsed beams. The basic ingredients here are an RFQ
ion guide and a linear Paul trap. Helium
is used as a buffer gas for slowing down and cooling of the ions. The ion guide
is operated at a pressure of about 5e-2 mbar in order to quickly
cool the incoming ion beam. The ions are then accumulated in the trap section at
a lower pressure of 2e-3 mbar before they are finally ejected as a
short, cooled pulse. The separation of the ion accumulator into a cooling and a
trapping stage at different pressures minimizes reheating of the ions during the
ejection process. The system can be operated at liquid-nitrogen temperature.
Besides an increase of the acceptance of the system and a decrease of the
cooling time this significantly reduces the emittance
of the resulting pulse compared to operation at room temperature.