Ghaneshwar Gautam

Hydrogen Alpha Self-Absorption Effects in Laser-Induced Air Plasma

Time-resolved spectroscopy measurements of the hydrogen alpha Balmer series line following laser-induced optical breakdown in laboratory air are designed to investigate in detail the determination of electron density from Stark-broadened spectral line shapes. Comparisons of results obtained from Hβ and Hγ lines indicate higher electron density inferred from Hα early in the plasma decay, suggesting self-absorption occurs. However, detailed comparisons for time delays of 300 and 400 ns after optical breakdown reveal the minute extent of self-absorption in air breakdown experiments from (i) differences of electron density determined from the N+ lines and the Hα line, and/or from (ii) differences in recorded data sets with/without the mirror for the various time delays in the experiments.

Hydrogen Balmer Series Measurements in Laser-Induced Air Plasma

Time-resolved spectroscopy is employed to analyze micro plasma generated in laboratory air. Stark-broadened emission profiles for hydrogen alpha and beta allow us to determine plasma characteristics for specific time delays after plasma generation. Stark shift, asymmetry, and full width half maximum measurements are used to infer electron density. The measurements of hydrogen alpha and beta Balmer series line shapes are analyzed using various theory results. Our laser-induced breakdown spectroscopy arrangement uses a Q- switched Nd:YAG laser operating at the fundamental wavelength of 1064 nm that is focused for plasma generation. The hydrogen alpha and beta lines emerge from the free electron background radiation for time delays larger than 0.3 ps and 1.4 ps, respectively. Neutral and ionized nitrogen emission lines allow us to infer electron density for time delays from 0.1 to 10 μs. The electron density values are compared with results obtained from hydrogen Balmer series line shapes.

Emission spectroscopy of expanding laser-induced gaseous hydrogen–nitrogen plasma

Microplasma is generated in an ultra-high-pure H2 and N2 gas mixture with a Nd:YAG laser device that is operated at the fundamental wavelength of 1064 nm. The gas mixture ratio of H2 and N2 is 9 to 1 at a pressure of 1.21 ± 0.03 105 Pa inside a chamber. A Czerny-Turner-type spectrometer and an intensified charge-coupled device are utilized for the recording of plasma emission spectra. The line-of-sight measurements are Abel inverted to determine the radial distributions of electron number density and temperature. Recently derived empirical formulas are utilized for the extraction of values for electron density. The Boltzmann plot and line-to-continuum methods are implemented for the diagnostic of electron excitation temperature. The expansion speed of the plasma kernel maximum electron temperature amounts to 1  km/s at a time delay of 300 ns. The microplasma, initiated by focusing 14 ns, 140 mJ pulses, can be described by an isentropic expansion model.

Time-Resolved Emission Spectroscopy of Atomic and Molecular Species in Laser-Induced Plasma

This work examines atomic and molecular signatures in laser-induced plasma in standard ambient temperature and pressure environments, including background contributions to the spectra that depend on the laser pulse-width. Investigations include solids, gases, and nano-particles. Abel inversions of measured line-of-sight data reveal insight into the radial plasma distribution. For nominal 6 nanosecond laser pulses and for pulse-energies in the range of 100 to 800 milli-Joules, expansion dynamics and turbulence due to shock phenomena are elucidated to address local equilibrium details that are frequently assumed in spatially averaged emission spectroscopy. Chemical equilibrium computations reveal temperature dependence of selected plasma species. Specific interests include atomic hydrogen (H) and cyanide (CN). The atomic H spectra, collected following optical breakdown in ultra-high-pure hydrogen and 9:1 mixtures of ultra-pure hydrogen and nitrogen gases, indicate spherical shell structures and isentropic expansion of the plasma kernel over and above the usual shockwave. The recombination radiation of CN emanates within the first 100 nanoseconds for laser-induced breakdown in a 1:1 CO2:N2 gas mixture when using nanosecond laser pulses to create the micro-plasma. The micro-plasma is generated using 1064 nm, 150 mJ, 6 ns Q-switched Nd:YAG laser radiation. Measurements of the optical emission spectra utilize a 0.64 m Czerny-Turner type spectrometer and an intensified charge-coupled device.