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.

Popular Abstract

Particle accelerators can be found in many disciplines and play a vital role in our daily lives. They can be found in hospitals to image and treat cancers, in museums to study art and ancient relics, as implantation devices to create faster and more efficient electronic devices and might in the future even reduce environmental impact and long term hazards that are associated with nuclear waste. Accelerators are also used to produce high energy particle beams for experiments, such as at CERN-ISOLDE.
At this facility, researchers study the properties of exotic nuclei, otherwise not found on earth, by accelerating them and letting them collide with other nuclei. The particles are unstable, also know as radioactive, and will eventually transform into stable atoms. To facilitate the acceleration, the charge of these radioactive isotopes is increased by
removing electrons. Increasing the charge is like adding more people to a team pushing a car, the more people you have, the more force you can deliver and the easier it is to accelerate the car. Instead of sending people to ISOLDE to increase the charge, the REXEBIS electron beam ion source (EBIS) is used for this objective. Since the EBIS is dealing with radioactive ions that are rare, it can not afford to lose ions and therefore, the efficiency is of utmost importance.
At the TwinEBIS test bench, where new EBIS concepts are evaluated, excessive particle
losses were measured. A detailed study was launched to investigate if these losses could be attributed to a misalignment of the components in the TwinEBIS, or if they were caused by some other mechanism. Computer simulations were performed with two different
particle beams for several misalignment cases. A high fraction of ion losses occurred in
only one of the simulated cases. In the rest of the simulations, no losses were observed.
Measurement on the operational REXEBIS was also performed in order to understand
more about machine instabilities, that may provoke excessive ion losses.

• author: van Woudenberg, Niek
• supervisor: Fredrik John Carl Wenander; Joakim Cederkäll
• organization: Particle and nuclear physics; Department of Physics
• course: FYSM60 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: CERN, ISOLDE, Ion Source, EBIS, REXEBIS, TwinEBIS, Ion losses, Charge Breeding, Ion Extraction, CST Studio Suite
• language: English
• id: 9126819
• date added to LUP: 2023-06-19 15:01:42
• date last changed: 2023-06-19 15:01:42