Etudes et conception d'un refroidisseur radiofréquence à gaz-tampon pour des faisceaux de hautes intensités

Abstract : The topic of this thesis is the study and the conception of a RFQ Cooler with buffer gas for high intensity radioactive beams. This project is in the frame of the next extension of GANIL, Spiral2, and the future low-energy facility DESIR (« Decay, Excitation and Storage of Radioactive ions »). The goal is to reduce the beams emittance of Spiral2 beams to allow their purification (ideally at isobaric level) with a high resolution separator. This cooler consists on a quadrupolar structure on which ions are confined by RF potential in opposite phase at an energy of 100 eV. A light buffer-gas, typically helium, is injected in the quadrupole and, after each collision, the ion lose a part of its energy and is finally cooled. The main problem on our project concerns the space charge. The existing devices are able to cool currents of few 10nA whereas we have to treat beam intensities around 1 μA which induce an increase of the Coulomb repulsion between ions. That needs to produce strong RF fields which induce high RF potentials (≈ 10 kVpp) and a low inner radius (r0 ≈ 3 à 5 mm). We have worked on a first prototype, SHIRaC-Phase1 (« Spiral2 High Intensity Radiofrequency Cooler »), with a 3 mm-inner radius, built at CSNSM-Orsay and moved at LPC-Caen at the end of 2007. The main R&D effort concerns the electronic part. A first RF system, based on a LC resonant circuit, has been developed and has provided up to 2500 Vpp between 4.5 and 6.3 MHz. In these conditions, we have checked that we didn't have strong limitations from electrical breakdown between our electrodes. With this device, we have reduced the beam emittance to a value around 2 π.mm.mrad at 60 keV and the longitudinal energy spread to 146 meV. The maximum transmission of Sodium 23Na+ and Rubidium 87Rb+ is 25% with an ionization source for which the beam quality is better than Spiral2. For this reason, we have conceived a new cooler with an acceptance of 80 π.mm.mrad at 60 keV. This second prototype has a larger inner radius (r0 ≈ 5 mm) and new sets of injection and extraction electrodes. The performances of the RF system have been improved to reach amplitudes up to 7 kVpp for frequencies between 5.9 MHz and 7.3 MHz. It takes account of the security and maintenance considerations for Spiral2.
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F. Duval. Etudes et conception d'un refroidisseur radiofréquence à gaz-tampon pour des faisceaux de hautes intensités. Physique Nucléaire Théorique [nucl-th]. Université de Caen, 2009. Français. ⟨tel-00435557v2⟩

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