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GSI Scientific Report 2014 - GSI Report 2015-1

We summarize the current status of e+e--coincidence experiments performed by the EPOS and ORANGE collaborations at GSI. © 1995. 583 C 237 246 Cited By :18

The accelerator facility installed at the Institut fuer Kernphysik (IKF) combines a 14 GHz electron cyclotron resonance ion source (ECRIS) and a variable energy radio frequency quadrupole accelerator (ve-RFQ)[1,2]. The provided highly charged ions have an energy range between a few keV - using the beam delivered from the source - up to 200 keV/u by using the post acceleration of the ve-RFQ. The setup is designed to deliver a wide spectrum of ions in sufficiently high charged states for atomic physics and materials research. Besides this the ion source is used for studies of the production of highly charged ions with the intention to improve quality and intensity of ion beams. In addition to these activities there are some special topics which deal with the investigation of phenomena on the ECRIS plasma and the production of metal ions by laser ablation technique (see also contributions to this workshop S. Runkel et al. And V. Mironov et. al). The present status and further activities of the facility and a view of the different projects will be reported.

A Q-switched YAG:Nd3+ laser was used to evaporate a metal (Cd) target mounted close to a biased disk in the vacuum chamber of the 14 GHz Frankfurt ECRIS. The formation of the laser ablation plasma and the pulsed injection of neutral particles into ECR plasma resulting from the evaporation were studied in order to optimise the production of metal-ion beams and to perform diagnostics of the ECR plasma. The pulsed highly charged Cadmium ion beams were detected under the injection of neutral atoms of Cadmium into an Argon ECR discharge. The ionisation and confinements times for these ions were determined by using time-resolved spectroscopy of the charge state spectra. At the moment of the Cadmium injection, a pulsed increase of the Argon beam currents was detected, which is attributed to changes of the plasma potential. The amplitudes of these pulses are comparable with afterglow currents. Further investigations may provide a chance to use this effect for the injection of ion beams into the pulsed accelerators.

To investigate the basic production processes of highly charged ions and combined phenomena of an ECRIS plasma (e. g. influence of secondary electrons and plasma instabilities) time resolved experiments have been carried out at the Frankfurt 14 GHz ECRIS [1] (see also the contributions to this workshop by O. Hohn et al. and V. Mironov et al.). We report time resolved measurements of the extracted ion currents by pulsing the biased disk voltage [2]. The measurements have shown that the extracted ion currents respond too fast to explain the "biased disk effect" (i. e. the intensity increase of highly charged ions) by enhanced ion breeding. Furthermore the influence of the pulsed biased disk on plasma instabilities has been investigated. It has also been shown that this method can be used to extract pulsed ion beams from an ECRIS.

Metal-dielectric (MD) structures of pure (99.999%) aluminum foils were previously studied [1, 2] in the National Institute for Physics and Nuclear Engineering (NIPNE), Bucharest, Romania showing high secondary electron emission properties. Consequently, 26 mm diameter disks of such structures (Al-Al2O3) were tested in the ECR ion source of the Institut fuer Kernphysik (IKF) der J. W. Goethe Universitat, Frankfurt/Main, Germany, allowing to demonstrate their ability to significantly increase the ECRIS performances in what concerns the production of high charge state ions [3]. New experiments carried on in Bucharest on a special facility [2] stressed out the possibility to develop high emissive MD structures starting from lower purity (99%) aluminum foils. This result allowed us to make a special cylinder of 1 mm wall thickness electrolytically treated so that only the inner face had a MD structure layer while the external surface remained metallic. Such a cylinder introduced in the plasma chamber of an ECR ion source provides a high rate of secondary electrons that enhance the ECR plasma electron density while its metallic external surface provides a good electric and thermal contact with the plasma chamber. The tests performed with such a MD aluminum cylinder in the IKF 14 GHz ECR ion source, successfully demonstrated the possibility to shift the ECRIS output toward very high charge states (Ar16+) due to the strong secondary electron emission of the MD inner surface of the cylinder.