The ion accumulator and buncher in the LEBIT project is a two-stage linear Paul trap system designed to fully accept the 5keV-DC beam from the gas cell and convert it into low-energy pulsed beams occupying the least phase-space possible (="cold beam").
The system mainly consists of a cooler and a trap section, linked by a "micro"-RFQ. The cooler section is to dissipate the ions’ transverse energy and most of their axial energy. It is operated at a pressure of pHe=5e-2 mbar and long enough to cool ions to thermal equilibrium. The trap section is where ions are accumulated in a lower pressure environment (pHe=2e-3 mbar) and from where they are ejected as ion pulses.
The separation of the ion accumulator into a cooling and a trapping stage is to avoid reheating of the ions during the ejection process. The cooler and the trap sections are connected by a micro-RFQ-guide which allows differential pumping.
Both the cooler section and the m-RFQ-guide make use of wedge-type cylindrical DC electrodes that provide an approximately constant axial drift field (see insert). The trap section is made of seven ring electrodes encompassing the RFQ rods and two acceleration electrodes that allow to shape proper trapping and ejection potentials.
Both the cooler section and the trap section will be built with the option to run them at LN2-temperature. Besides an increase of the acceptance of the system and a decrease of the cooling time this will significantly reduce the emittance of the resulting pulse compared to an operation at room temperature.
Only two DC voltages DC1 and DC2 in addition to the RFQ voltages are needed to generate the field needed to guide the ions through the cooling and micro-RFQ sections with the wedge-type electrodes shown.