Single-shot emittance measurements at HLI and HITRAP

HITRAP is an ion trap facility which is designed for deceleration and cooling of highly charged ions produced by the GSI-accelerator facilities. The ions are decelerated in the ESR and then ejected at 4 MeV/u. Subsequently they are further decelerated by the HITRAP linac [1]. Since the HITRAP decelerator gets an ion bunch from the ESR every 50-60 seconds, it is necessary during linac commissioning to measure the transverse emittance within a single shot. The pepperpot method is the ideal choice for this task, because the horizontal and vertical emittance are measured at the same time. The GSI pepperpot emittance meter was built during the late nineties for high current emittance measurements at the UNILAC [2]. The ion beam is shot through a plate with a regular matrix of apertures. The beam spots of the transmitted ions are detected via scintillation on a phosphor screen, which is caught by an cooled CCD camera. Assuming that the aperture holes are points and by knowledge of the distribution of the scintillation on the phosphor one can calculate the transverse emittances only from geometry. Measurements at the UNILAC revealed problems in the emittance evaluation from pepperpot measurements. The emittance values were always larger than the values determined with the slit grid method. In order to proof the reliability of the pepperpot device, a comparison with the “nondestructive evaluationmethod”was done at HLI prior to the HITRAP measurements. Measurements were taken with both methods under the same beam conditions. Data evaluation showed that the pepperpot measurements showed approx. 50%–100% bigger emittances than the other method no matter how the settings of the pepperpotmeasurements were changed. Single-shot emittance results of HLI were 14.3mm·mrad in horizontal and 13.0mm·mrad in vertical direction compared to the non-destructive evaluation with horizontal 5.9mm·mrad and vertical 10.4mm·mrad. The data analysis of the correct emittance from pepperpot measurements is done by the following scheme. The background is cut in 1% steps of the maximum intensity and the emittance was calculated for each cut. Therewith one gets a curve shown in fig.1 that is linear only in a region, where the real beam is cut. By extrapolation to the 0% cut the real emittance of the ion beam is determined. The pepperpot system was transfered to the reinjection channel for the first two commissioning beam times of the HITRAP Double Drift Bunchers (DDB). First HITRAP measurements were done with 64Ni28+. It was found that the resolution of the current pepperpot aperture plate was not sufficient due to ion beam diameter restrictions. For