An RFQ Mass Filter for SHIPTRAP

At SHIPTRAP, exotic nuclei produced in fusion reactions and separated from the primary beam in the velocity filter SHIP are stopped in a gas-filled stopping cell. The ions are extracted, bunched and transferred to a double Penning trap system, where their masses can be measured. Contaminant ions created in the gas-filled stopping cell and abundantly produced reaction products different from the nuclei of interest can significantly reduce the performance of SHIPTRAP. In order to remove the contaminant ions and thus increase selectivity and efficiency of the experiment, a radio-frequency quadrupole (RFQ) mass filter is being developed. It will also allow for diagnosis and identification of the ions delivered from the stopping cell. Design Goals. The challenge of constructing a mass filter is to reach a maximum in transmission at a given resolution. At SHIPTRAP a high transmission is of particular importance, in order to avoid losses of the rare exotic ions. Since transmission losses are caused particularly at the entrance and the exit of the mass filter, the mass filter is developed in a matched combination with an RFQ cooler and an RFQ buncher located in front of, and behind the mass filter, respectively. The following causes for transmission losses were examined in detail with help of the simulation programs COMSOL, SIMION [1] and ITSIM [2, 3]: (i) Collisional losses: In order to cool the ions the RFQ cooler and buncher are operated with buffer gas. This leads to a significant pressure also in the mass filter. Collisions with the rest gas cause a change in the ions’ oscillation amplitudes and therewith to transmission losses and deteriorated peak shapes. Computer studies have been used to determine a tolerable upper limit for the residual gas pressure in the mass filter and a lower limit for the buffer gas pressure in the RFQ cooler and buncher and hence the diameter of the apertures between the RFQs. (ii) Ion entrance: The use of an entrance diaphragm results in field distortions and thus to an expansion of the beam and larger oscillation amplitudes. This causes ion losses on the quadrupole rods and the exit diaphragm. These losses can be minimized by reducing the field distortions. For this purpose a fourfold segmented electrode diaphragm operated with RF voltage was developed. In addition, simulations show that the effect of the field distortions can be reduced by the use of Brubaker lenses [4]. (iii) Ion exit: It is possible to reduce ion losses on the exit diaphragm by operating the instrument in a focusing mode [5]. Ions that are travelling with the same velocity have nodes in their oscillations at the same position (Fig. 1, left). Through an appropriate choice of the DC potentials,