Experimental Investigation of an Improved Isochronous Mode for the ESR

The Isochronous Mass Spectrometry (IMS) is an experimental technique for direct mass measurements of shortlived exotic nuclei which has been developed at the storage ring ESR of GSI [1]. A bottleneck for the present IMS experiments is the low transmission from the fragment separator FRS to the ESR, which is mainly due to large negative dispersion in the straight sections of the ESR (see Fig. 1). In order to facilitate the transmission and to improve the isochronicity of the ring, the present isochronous optics of the ESR has been modified [2]. In comparison with the present isochronous optics the new improved optics was calculated using 10 quadrupole families, since the 20 ESR quadrupoles are connected to the power supplies in pairs (instead of 5 quadrupole families for the present mode). In the new isochronous optics the ESR is achromatic with respect to the cooler and has a smaller negative dispersion at the target position (D≈-5 m) (see Fig. 1). The achromatism of the ESR increases the injection efficiency and the isochronicity of the ESR shall be improved [2]. This calculated setting has been tested in a dedicated machine experiment. In the experiment we used a U beam with an energy of 390 MeV/u (γ = 1.42) in the ESR. The injection scheme of the new optics is completely different from the present one. In addition a new setting of horizontal correctors was required for the injection. Therefore, the desired isochronous optics (γt = 1.42) was reached iteratively, starting from the well known standard ESR operation mode (γt ≈ 2.4) and stepwise changing of all quadrupole values by about 10%. After each step the beam was injected, stored and cooled and the γt was deduced from revolution frequency measurements. The final isochronous setting