The measured emission spectra of the OH radical subsequent to laser-induced optical breakdown in air are analyzed to infer spectroscopic temperature and species number density. Emissions from the UV A2sigma+ --> X2IIi transition dominate the spectra in the wavelength range of 306-322 nm and for time delays from the optical breakdown of 30-300 micros. Contributions from other species to the recorded OH emission spectra were also investigated for spectroscopic temperature measurements in the range of 2000-6000 K and for OH number densities in the range of 10(14) - 2 x 10(16) cm(-3). Monte Carlo simulations are applied to estimate errors in the analysis of the hydroxyl spectra.
We report measurements of laser-induced plasma spectra subsequent to infrared Nd:YAG laser-induced breakdown in air. Spectroscopic temperature and species number densities are inferred by use of the nonequilibrium air radiation (NEQAIR) code. For selected diatomic species that dominate the recombination emission spectrum in the plasma decay, accurate diatomic line strengths are used to determine the temperature. The number density is estimated from the calculated plasma emission spectra for the experimental conditions. The application of the NEQAIR code in analyses of optical breakdown micro-plasma is critically evaluated. We use appropriately adapted line-strength files to extend the capability of the program. The inclusion of Lambda doubling for the computation of diatomic molecular spectra is elaborated. Of particular interest are high temperature molecular spectra of typically 6000 Kelvin that are measured at a delay time of nominally 30 micro-second. We discuss least-square data reduction results including initial Monte-Carlo simulations of recorded emission spectra.
IntroductionOptical breakdown plasma is generated by focused Nd:YAG infrared laser radiation of 1 to 100 TW/cm2 intensity.Typically rich emission spectra result. The decaying laser-induced plasma is characterized by the application of time-resolved spectroscopy techniques. Initial spectroscopic signatures comprise contributions from free-electron radiation and atomic species emissions. Atomic line broadening and line shifts can be measured to characterize the initial plasma state.1.2 The spectroscopic features of the plasma decay include the occurrences of primarily recombination spectra following optical breakdown.'-' It is of interest to obtain temperature and species density information from measured spectra.
A fluid description of the presheath of a magnetized plasma is used to model a divergent electron cyclotron resonance (ECR) plasma source. The fluid equations are moments of the time-independent Boltzman equation when cross-field particle motion occurs only through a static E×B drift. Closure is obtained by neglecting third-order moments. The electrons are assumed to have constant temperature along the magnetic field, to obey a Maxwell–Boltzmann potential-density relationship, and to be warmer than the ions. Interactions between plasma and neutral gas are included by specifying the profile of the gas density along the magnetic field and collision cross sections. A form of the equations is derived that can be used to study ions with anisotropic temperatures. The model is used to estimate the axial profiles of the density, ion flow, and ion temperature of an ECR plasma source. The calculated global relationships between (1) the electron temperature and the equilibrium neutral gas density, and (2) the absorbed microwave power and the equilibrium plasma density are comparable with experimental measurements. Furthermore, the calculated ion temperature is comparable to recently reported measurements [Appl. Phys. Lett. 57, 661 (1990) and Appl. Phys. Lett. 58, 458 (1991)].
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