Numerical simulations of the ion capture process for laser-generated plasmas interacting with electron cyclotron resonance ion sources

The most effective devices able to feed modern accelerators with heavy ions (either in terms of reliability or performance) are electron cyclotron resonance ion sources (ECRIS), which are able to generate intense currents of multiply charged ions. One of the disadvantages of the ECRIS consists of the limitation in the production of metallic ions (usually obtained by means of ovens and sputtering techniques), especially in the case of refractory elements. Laser ablation is an effective method for such elements. The coupling between a laser ion source (LIS) and an ECRIS has been studied at some facilities (INFN-LNS, RIKEN, University of Frankfurt, ANL): the experiments were successful but an adequate modeling and additional experimental activities are mandatory. Preliminary calculations have shown that the ion energy from the LIS should be lower than a few hundreds of eV in order to permit an effective coupling of LIS ions with the ECRIS plasma. A detailed investigation of the ion capture by the ECRIS plasma has now been carried out by means of numerical simulations based on the Monte Carlo method. The energy distribution function of the LIS plasma has been taken into account, as well as the density and temperature distribution of the ECRIS plasmas and the pattern of the magnetic field in the resonance region. The captured fraction of LIS ions strongly depends on the temperature (the lower energy component of the ion energy distribution function is easily absorbed by the ECRIS plasma). The effect of negatively biased targets has been also investigated: simulations demonstrate that for negative potentials of the order of 0.4 kV the captured fraction abruptly increases, and in some particular cases it exceeds 50% of the total ion current.