Investigation of Ion Extraction Anomalies at TwinEBIS

Abstract: The electron beam ion source REXEBIS at CERN-ISOLDE delivers highly-charged ra- dioactive ions for post-acceleration in the HIE-ISOLDE LINAC. Research and develop- ment of components for electron beam ion sources is carried out at the TwinEBIS test stand, currently equipped with a Brillouin electron gun called MEDeGUN. Excessive ion extraction losses have been reported at TwinEBIS during commissioning tests of the gun, substantially reducing the efficiency of the device. As the beam time required for an experiment is directly related to the delivered intensity, maintaining a high extraction efficiency is therefore paramount. With this in mind, a detailed study was launched to investigate if these losses could be attributed to a misalignment of the EBIS components in the collector region. Simulations of a misaligned geometry were performed in CST Particle Studio with two test beams. Furthermore, a study on the formation of a virtual cathode in the suppressor region was carried out, aiming to evaluate if losses could be produced by over-focusing of the ion beam. Also, a measurement series of the effective electron current density was undertaken at REXEBIS to determine if an electron-ion two-stream instability is present, that could lead to significant ion heating. Simulations aimed to validate the MEDeGUN electron beam resulted in a perveance of 1.083 uA/V3/2, which is in good agreement with previously performed measurements(accuracy of <2%). Ion beam simulations in the collector region did not result in losses on the extractor for both test beams with an asymmetry of 0 mm and 1.75 mm. Only with a larger asymmetry of 3.5 mm, losses comparable to the measured values were found for the high neutralisation and extracted current test beam, where 43% of the injected current was lost on the extractor. Visual inspection of ’dust’ in the ion extractor confirmed ion bombardment, however, the deposition was found to be radially uniform. The losses observed in the extractor can also not be explained by the formation of a virtual cathode, since the operational regime of the suppressor is distant from the critical condition. An electron-ion two-stream instability was not confirmed by measurements carried out at REXEBIS, as the current density instead increased with the distance from the electron gun. This is contrary to the measurements at TwinEBIS, where the effective electron current density is reduced, consistent with an instability.