Hybrid Ion Sources for the Production of Highly Charged Ion Beams from Metals

The ECLISSE experiment proved in 2002 the possibility to couple a Laser Ion Source (LIS) with an Electron Cyclotron Resonance Ion Source (ECRIS) in order to obtain ion beams from refractory metals with a cw beam structure. The use of a LIS inside the ECRIS plasma chamber may permit to increase the beam current and the ion charge state. The use of 3 generation ECRIS may extend the capabilities of this method, as the denser plasma may catch the ions extracted from the LIS with energies up to about 1 keV per charge state, much higher than it is possible for sources operating at lower frequency, as it was verified with the SERSE source. The perspectives opened by the availability of the MS-ECRIS source operating at 28 GHz will be described along with some details on the proposed experiments, to be carried out in 2008. THE ECLISSE EXPERIMENT Accelerators for nuclear physics demand higher current and charge states as well as the extension of the variety of species, especially to metal ion beams and heaviest species. The low cross sections of interesting reactions for nuclear and particle physics make the availability of mA beams mandatory. The improvements of ion sources have permitted up to now a significant increase of the available current, that is particularly important for the research programmes with radioactive ion beams, and of the available charge states, that increase the final beam energy (linearly for linac and quadratically for cyclotrons), thus making possible to extend the programmes with stable beams [1]. In particular, the demand of metal ion beams have increased and it exceeds often the ability of ECR ion sources that are the typical injector for cyclotrons and linear accelerators in nuclear research facilities. Moreover the oven that are used for the production of vapours inside the ECRIS fails with refractory elements and the sputtering method is not adequate for the production of beams with intensity above few tens of μA. For all these reasons a novel method for the production of intense beams of highly charged metal ions was proposed in 1998 at INFN-LNS. The concept of the hybrid ion source is the following: the first stage consists of a Laser Ion Source (LIS) which gives intense currents of electrons and multiply charged ions (q/m = 1/10 or lower) by means of metal laser ablation, then the plasma of an ECR ion source acts as a charge state multiplier [2]. The ECLISSE experiment (ECR ion source Coupled to a Laser Ion Source for charge State Enhancement) aimed to obtain intense beams in dc and pulsed mode, mainly from refractory elements. The SERSE source was preferred to other ECRIS because of its high plasma density, which guarantees a good trapping of LIS-generated ions. The negative bias of the laser-irradiated target can be used to decrease significantly the ion velocity. This feature is particularly relevant as the coupling efficiency between LIS-generated beams and the ECR plasma is rapidly decreasing with the increase of the energy of the incoming beam. The major points to be investigated appeared to be the coupling efficiency of the ion beam produced by the LIS to the ECR plasma, as well as the possibility to enhance the available charge state by an ECRIS. A pulsed Nd:YAg laser has been used at INFNLNS for the tests and its main features are described in Tab.1. Table 1: Main features of the laser used for ECLISSE.