Analysis of the plasma generated in a Laser Ion Source

摘要

We present the experimental results concerning the study and the development of a Laser Ion Source (LIS). By means of an excimer laser we irradiated a metal target at high power density, realizing an efficient source of multiple charged ions. The analysis of the generated plasma plume was performed for three different laser spot sizes determining the threshold conditions of the ablation process. A diagnostic system with a Faraday cup was developed in order to detect the ion current along the propagation tube. Time-of-flight (TOF) measurements were performed, also inserting in front of the cup an adjustable voltage electrostatic barrier in order to get quantitative information about the ion flux and the kinetic energy of the produced ions. To study the plasma characteristics we evaluated the total etched material per pulse, 0.25 μg, and the fractional ionisation, 12%. A modified Maxwell-Boltzmann distribution was applied to provide a consistent description of the velocity distributions in the plume. The ablated material was spatially monitored by optical transmission analysis of a deposited film. Applying a high voltage to the LIS extraction gap, an ion beam containing Cu+1 (0.44mA), Cu+2 (0.34mA), Cu+3 (0.09mA), and Cu+4 (0.01mA) ions was obtained.