The buffer-gas cell and the extraction RFQ for SHIPTRAP

SHIPTRAP is a new ion trap system for high-precision mass measurements of transuranium recoil-ions from the SHIP facility at GSI. The system consists of a buffer-gas cell to thermalise the incoming ions, an extraction system to separate the ions from the buffer gas, an RFQ buncher to cool and accumulate the ions and a tandem Penning-trap system for isobaric purification and high-precision mass measurements. The buffer-gas cell and the extraction RFQ were developed, assembled and tested within this thesis at the MLL (Maier-Leibnitz Laboratory) in Garching and at the GSI. The main requirements given for the gas-cell/extraction-RFQ system are a fast and effective extraction of the stopped recoil ions. Therefore besides the purity of the system preventing recombination and molecule formation, ensured by the use of only non-organic material inside the cell and a highly efficient gas purification, especially the interaction between guiding electric fields and the gas flow inside the system has been optimised. The design optimisation of the different parts of the system was achieved using simulation programs like the VARJET code (gas flow), the SIMION code (ion trajectories in electric fields), a combination of both and the SRIM code (stopping ranges of ions in matter). In the actual setup of buffer-gas cell/extraction-RFQ the ions enter the cell through a thin metallic entrance foil (~4 µm thick, Ø 60 mm) and are stopped in the helium buffer gas (~50 – 200 mbar). The thermalised ions are then accelerated by electric fields (~10 V/cm) created by a segmented DC electrode inside the gas cell towards a funnel-shaped ring electrode structure. This funnel guides the ions by the applied RF and DC fields towards the extraction nozzle (Ø 0.6 mm) which acts as an interface between the stopping chamber and the extraction chamber. At the nozzle the buffer gas expands into the extraction chamber (10-1 – 10-3 mbar) creating a supersonic gas jet and dragging the extracted ions. Inside the extraction-RFQ structure the ions are separated from the buffer gas and are cooled during the transport. In order to guarantee an optimum purity, the setup is bakeable and a hydrocarbon-free pumping equipment is used. For characterisation measurements of the setup both stable and radioactive ions were used. For off-line measurements stable Er ions were created inside the buffer-gas cell by using a focused pulsed laser beam. These ions were used for field optimisations and the determination of the extraction time …