Ion time-of-flight signals from nanosecond laser ablation plasmas excited in constricted cavities

Ion time-of-flight spectra from nanosecond pulse laser ablation at 1064 nm wavelength of a graphite target under repeated 10 Hz pulse repetition are investigated. By injecting focused laser beams at 12° from the surface normal, the ablated target formed a vertical cavity as the target is rotated continuously. For increasing number of pulses, decrease in positive signals for positive ion extraction while increase in negative signals for both 5 and 20 GW/cm2 laser power densities at −3 kV extraction voltage were observed. Increase in voltage onto a positive ion retardation plate installed in front of the Faraday cup array showed distinct negative ion peaks with decrease in positive ion signal intensity. Time-of- flight signals under increased number of pulses with the positively biased retardation plate showed less pronounced negative ion peaks for 5 GW/cm2 at -3 kV extraction, whereas the peaks persisted for 20 GW/cm2 laser power density. The influence of target cavity development on the negative ion formation is discussed.