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Characteristics of laser-induced plasmas and defocusing effects on target materials
Abstract
Elastically scattered incident radiation (ESIR) from a copper wire illuminated by a KrF laser pulse at $\lambda$ = 248 nm, shows a distinct two-peak structure which is dependent on the incident energy. The time required to reach the critical electron density (n$\sb{c}\approx 1.8$ $\times$ $10\sp{22}$ electrons/cm$\sp3$) can be estimated from experimental results. ESIR characteristics for water have been previously reported by Alexander et al. Initiation of the broadband emission for copper plasma started at 6.5 $\pm$ 1.45 ns after the arrival of the laser pulse, as compared to 11 $\pm$ 0.36 ns for water. Experiments were performed to determine the plasma propagation velocities resulting from KrF laser irradiation of copper wires (75 $\mu$m dia.) and water droplets (75 $\mu$m dia.) at irradiance levels ranging from 25-150 GW/cm$\sp2$. At irradiance of 100 GW/cm$\sp2$, the air plasma has two weak-velocity components which propagate toward and away from the incident laser while a strong-velocity component propagates away from the laser beam as detonation waves. Comparison of the measured breakdown velocities (in the range of 2.22-2.27 $\times$ 10$\sp5$ m/s) of air and the value calculated by the non-linear breakdown wave theory at irradiance of 100 GW/cm$\sp2$ showed a quantitative agreement within approximately 50% while the linear theory and Gaussian pulse for breakdown velocity failed. The detonation wave velocities of plasma from water droplets and copper wires for different focused cases were also measured and analyzed with the detonation wave theory. Time-resolved plasma temperatures of defocusing effects of target materials are measured and calculated with a multi-pair method. Due to plasma complexity, this method is useful for obtaining the average temperature of the heat which may contribute to target evaporation of ablation when spectra are not strictly gray. Results of time-resolved spectroscopic measurement show that the life time of the continuum spectrum from an absorbing material, copper, for the in-focus case was twice as long as that of the other cases. Subsequently, the temperature from the copper target started at 42,386 $\sp\circ$K, then rapidly increased to 100,655 $\sp\circ$K at 16 ns after the plasma initiated. At 30 ns, the temperature rapidly decreased to 47,949 $\sp\circ$K. The copper temperature for the other cases remained almost constant within that time. The highest temperature generated was for the in-focus case, which contributed the melting of the material, and subsequently the high pressure generated by the shock wave removed some of the melted area.
Subject Area
Mechanical engineering|Optics|Fluid dynamics|Gases
Recommended Citation
Song, Kyo-Dong, "Characteristics of laser-induced plasmas and defocusing effects on target materials" (1992). ETD collection for University of Nebraska-Lincoln. AAI9237676.
https://digitalcommons.unl.edu/dissertations/AAI9237676