UC San Diego

Laser-produced plasmas have been extensively utilized in applications such as EUV lithography, laser-induced breakdown spectroscopy, inertial confinement fusion, and nanoparticle generation. Optimizing laser and plasma parameters in these applications is crucial for maximizing attributes like conversion efficiency, line emission, and nanoparticle generation. Achieving this optimization requires a detailed understanding of plasma property evolution during laser-material interaction and plasma expansion, which must be assessed using various plasma diagnostic tools. In this work, electrostatic probes, emission and absorption spectroscopy, interferometry, simulations, and analytical models were employed to investigate plasma property evolution under various conditions. Each method had unique advantages and limitations, and offered different insights into the various stages of the plasma lifecycle.The initial focus of this work was on understanding the appearance of different ion peaks observed in Faraday cup studies of LPP expansion into vacuum, categorized as the prompt, ultrafast, fast, and thermal ion peaks. Although previous studies had reported on the existence of these peaks, their physical origin was not well understood. In order to better understand their physical origin, a parametric study was conducted, exploring the variation in the peak properties as a function of distance, angle, material, and laser intensity. By comparing the results with different analytical models of plasma expansion, it was possible to attribute the properties of the ion peaks to different physical processes occurring during the laser-plasma interaction. More detailed investigations into the early evolution of the plasma properties during the laser-plasma interaction were conducted by using a combination of 2D FLASH simulations and Nomarski interferometry. The FLASH simulations provided the 2D spatial and temporal evolution of the electron density, temperature, and charge state of the plasma for times up to 20 ns after the arrival of the laser pulse. Comparison of the simulation results with measurements of electron density obtained using Nomarski interferometry showed excellent consistency, with slight deviations being attributed to experimental errors and uncertainty in equation of state parameters. Investigations of the dependence of the plasma properties on laser wavelength were also shown to be in partial agreement with analytical models, though it was found that the analytical models overpredicted the variation in temperature due to not taking into account radiation losses. Subsequent studies looked at the effects of ambient environments on the plasma dynamics, where the plasma became confined. In this case, interferometry and electrostatic probes could no longer be used due to the presence of shocks in the interferograms and due to ambient gas preventing the ions from reaching the electrostatic probe. Instead, measurements of plasma properties were performed using a combination of emission and absorption spectroscopy. Comparisons of the two methods indicated that small deviations were present in the measured temperatures. However, by modeling simulated spectra, it was shown that these deviations could be reduced by accounting for self-absorption effects in the plasma. The study also showed that confinement of the plasma plume resulted in the plasma reaching a state of local thermal equilibrium that persisted over long periods, even when the electron densities in the plasma should have fallen well below the McWhirter criterion. However, the exact mechanisms that allowed the plasma to remain in a state of local thermal equilibrium were not well understood. Limitations in the range of plasma properties that could be measured from emission and absorption spectroscopy alone indicated a need for an alternative diagnostic method. One such method was saturated absorption spectroscopy, with the main advantage of this method being its ability to provide Doppler-free linewidth measurements. However, very limited work had been performed on the application of saturated absorption spectroscopy to laser-produced plasma due to difficulties arising from poor signal to noise ratios caused by rapidly changing plasma conditions. In this study, it was shown that saturated absorption spectroscopy could be successfully applied to laser-produced plasma through careful optimization of the probe parameters, ambient conditions, and spectral line selection. Measurements of Doppler-free linewidths were used to obtain information on power, pressure, and natural line broadening present in the plasma. It was shown that such measurements can be used for obtaining information on various plasma properties, crowded spectral features, spectroscopic constants, and isotopic shifts.