Techniques to produce and accelerate radioactive ion beams

The production and acceleration of the Radioactive Ion Beams (RIB) continues the long line of nuclear investigations started in the XIXth century by Pierre and Marie Curie, Henri Becquerel and Ernest Rutherford. The contemporary applications of the RIBs span a wide range of physics fields: nuclear and atomic physics, solid-state physics, life sciences and material science. ISOLDE is a world-leading Isotope mass-Separation On-Line (ISOL) facility hosted at CERN in Geneva for more than 40 years, offering the largest variety of radioactive ion beams with, until now, more than 1000 isotopes of more than 72 elements (with Z ranging from 2 to 88), with half-lives down to milliseconds and intensities up to 1011 ions/s. The post acceleration of the full variety of beams allows reaching final energies between 0.8 and 3.0 MeV/u. This thesis describes the development of a new series of FEBIAD (“Forced Electron Beam Induced Arc Discharge”) ion sources at CERN-ISOLDE. The VADIS (“Versatile Arc Discharge Ion Source”) series allows reaching better performances through the optimization of the source design for specific ISOL requirements, through the adaptation of the nominal operation parameters or plasma properties for each specific application; the best performances cannot be achieved with a single fixed design. Several VADIS designs are proposed based on a new analytical model of the FEBIAD sources performances, inferred from extensive experimental investigations and refined through numerical simulation. Two of them have already been implemented, providing improved ionization efficiencies for the stable noble gas tracers and best yields for the investigated radioactive isotopes obtained so far (noble gases with half-lives from several milliseconds to several minutes). The ion source development at ISOLDE is motivated not only by present demands (increased ionization efficiency and element selectivity, lower ionization time), but also by the demands of future projects (HIE ISOLDE, EURISOL) which will have to deal with increased gas loads due to higher beam power impinging on the production target. The proposed VADIS series also provides optimized designs of arc discharge ion sources to fulfill any of the above requirements. All the stages of the development are described, including the justification of the choices followed during this study: - The review of the methods employed to produce RIBs, with emphasis on ISOL (Chap.1); - The review of the ion sources employed in ISOL facilities, with in particular the detailed description of the physical phenomena in the arc discharge ion sources (Chap.2); - The description of the employed tools (experimental, analytical and numerical) (Chap.3); - The extensive experimental investigations (Chap.5 and 6) that allowed the inference of a new theoretical model based on the different individual physical processes affecting the ionization of the neutral atoms (Chap.4), which can predict the source performance over a wide variation range of the operation parameters; - The analysis of the observed performance limitations (Chap.7); - The description of the VADIS concept, together with the possible applications (Chap.9); - The production and use of the first designs of the VADIS series at ISOLDE, based on models which correct the limitations of the previous MK5 and MK7 standards (Chap.8).