Optimization of laser-target parameters for the production of stable lithium beam

A laser ion source coupled with a radio frequency quadrupole linac accelerator is being proposed as a suitable system for the production of a low energy, high-current stable lithium beam. In order to maximize the lithium yield, plasmas generated by laser ablation of different materials based on lithium (Li, LiOH, and LiNbO₃) have been characterized by using a Faraday cup and an electrostatic ion analyzer in the time of flight configuration. A wide range of laser power density has been investigated (10⁹-10¹² W/cm²) using two Nd:YAG lasers operating at different wavelengths (1064 nm and 532 nm), pulse durations (6 ns and 17 ns), and maximum energies (1400 mJ and 210 mJ). This paper outlines the pros and cons of the investigated materials by studying how the ion energy, yields, and charge state distributions are modified when the laser power density is changed. Considerable attention has been paid to the higher charge states of oxygen, which may occur with the same mass-to-charge ratio of Li³⁺. The analysis has evidenced that LiNbO₃ represents a valid target since it allows minimizing the O⁶⁺/⁷Li³⁺ ratio down to 2.5% by using a laser power density of 1.8 × 10¹⁰ W/cm². For such a condition, a Li³⁺ current of 1.4 mA/cm² has been measured.