Realization and simulations of the new SPES Beam Cooler

Abstract

One of the aim of the new project Selective Production of Exotic Species (SPES) is to produce and select new, neutron reach, isotopes [1,2,3,4]. The technique adopted to produce these species is the Isotope Selection On Line (ISOL) that postpones the separation of the isotopes after the beam production, using a High-Resolution Mass Spectrometer (HRMS).In order to allow a good separation of isotopes , the transverse emittance and the energy spread of the beam should have very low values, for this reason the Beam Cooler (BC) is located between the ISOL target, i.e. the beam source, and the HRMS.In the SPES project, where heavy isotopes are expected (mass number >100), the spectrometer resolution must be preferably higher than dm/m = 5 × 10-5 and thus the features of the beam at the entrance of the HRMS should be at least εn 95% < 8.3 × 10-3 π mm mrad and σE < 1.5 eV.BC devices are devoted to improve the beam features in terms of emittance and energy spread. A new BC has been designed and realized by the Laboratoire de Physique Corpuscolaire (LPC) at Caen, France, for the SPES facility at Laboratori Nazionali di Legnaro (LNL), near Padova, Italy.BCs cool down the beam thanks to a dissipative process in which the thermal energy passes from the beam ions to another medium whose constituent is typically much lighter, Helium gas in our case. This process takes place inside a confinement system that on the one hand to limit the spread of the cooling medium, and on the other to allow the beam to continue along the required trajectory, in the presented device it is a row of radio frequency quadrupoles in an almost closed chamber.Some specifications of the BC described in this document: the RFQ is 723 mm long, its internal radius is 5 mm; the RFQ is included in the gas chamber which is 730 mm×280 mm×220 mm. Before it there is the injection part composed by, following the beam trajectory, a grounded pipe (388 mm long) and three focusing electrodes. On the opposite side there is the extraction part with two focusing electrodes.This document presents the main features of the new BC together with the results of a preliminary study where the beam dynamic has been simulated. The analysis embeds also the investigation of the gas distribution inside and outside the BC.The beam dynamic simulations are based on the Simion code [5] while the estimation of the gas distribution is computed with MolFlow+ [6].Simulations show that accurately setting the BC leads to a large improvement of the emittance while the energy spread still needs to be improved. Limiting the gas pressure in the acceleration zone seems to allow the required final boost.