<p>Laser-induced plasmas since their discovery in the
1960’s have found numerous applications in laboratories and industries. Their
uses range from soft ionization source in mass spectroscopy, development of
compact particle accelerator, and X-ray and deep UV radiation sources to
diagnostic techniques such as laser-induced breakdown spectroscopy and laser
electronic excitation tagging. In addition, the laser-induced plasma is important
for studying of various nonlinear effects at beam propagation, such as laser
pulse filamentation.</p>
<p>This
work deals with two challenging aspects associated with laser-induced plasmas.
First is the study of Multi-Photon Ionization (MPI) as
a fundamental first step in high-energy laser-matter interaction critical for
understanding of the mechanism of plasma formation. The
second is application of laser induced plasma for diagnostics of combustion
systems.</p>
<p>Numerous attempts to determine the basic
physical constants of MPI process in direct experiments, namely photoionization
rates and cross-sections of the MPI, were made; however, no reliable data was
available until now, and the spread in the literature values often reached 2–3
orders of magnitude. This work presents the use of microwave scattering in
quasi-Rayleigh regime off the electrons in the laser-induced plasma as method
to measure the total number of electrons created due to the photoionization
process and subsequently determine the cross-sections and rates of MPI.
Experiments were done in air,<i> O<sub>2</sub>, Xe, Ar,
N<sub>2</sub>, Kr</i>, and <i>CO</i> at room temperature and atmospheric pressure and femtosecond-laser pulse at 800 nm wavelength was utilized. Rayleigh microwave scattering (RMS) technique was used to
obtain temporally resolved measurements of the electron numbers created by
the laser. Numbers of electrons in the range 3 × 10<sup>8</sup>–3 × 10<sup>12</sup> were
produced by the laser pulse energies 100–700 <i>μ</i>J and corresponding
electron number densities down to about 10<sup>14</sup> cm<sup>-3</sup> in the
center of laser-induced spark were observed. After the laser pulse, plasma
decayed on the time scale from 1 to 40 ns depending on the gas type and
governed by two competing processes, namely, the creation of new electrons from
ionization of the metastable atoms and loss of the electrons due to
dissociative recombination and attachment to oxygen. </p>
<p>Diagnostics
of combustion at high pressures are challenging due to increased collisional
quenching and associated loss of acquired signal. In this work, resonance
enhanced multiphoton photon ionization (REMPI) in conjunction with measurement
of generated electrons by RMS technique were used to develop diagnostics method
for measuring concentration of a component in gaseous mixture at elected
pressure. Specifically, the REMPI-RMS diagnostics was developed and tested in
the measurements of number density of carbon monoxide (<i>CO</i>) in mixtures with nitrogen (<i>N<sub>2</sub></i>) at pressures up to 5 bars. Number
of REMPI-induced
electrons scaled linearly with <i>CO</i> number density up to about 5×10<sup>18</sup>
cm<sup>-3</sup> independently of buffer gas pressure up to
5 bar, and this linear scaling region can be
readily used for diagnostics purposes. Higher <i>CO</i> number densities were associated laser beam energy loss while travelling
through the gaseous mixture. Four (4) energy level model of <i>CO</i> molecule was developed and direct measurements
of the laser pulse energy absorbed in the two-photon process during the passage
through the <i>CO</i>/<i>N<sub>2</sub></i> mixture were conducted in order to analyze the
observed trends of number of REMPI-generated electrons with <i>CO</i> number density and laser energy.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12396584 |
| Date | 16 June 2020 |
| Creators | Animesh Sharma (8911772) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/MICROWAVE_SCATTERING_FOR_DIAGNOSTICS_OF_LASER-INDUCED_PLASMAS_AND_DENSITIES_OF_SPECIES_IN_COMBUSTION_MIXTURES/12396584 |
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