Session FT4: Advanced Laser Beam Diagnostics

Laser scattering for temporal and spatial diagnostic of low temperature plasmas

Many recent industrial and technological applications like surface etching, inorganic films deposition, polymerization of surfaces or sterilization are developed within the field of low temperature plasmas. To study, monitor and model plasma processes is of great importance to have diagnostic tools that can provide reliable information on different plasma parameters. In general, laser scattering techniques provide a direct and accurate method for plasma diagnostic with spatial and temporal resolution. Laser Thomson scattering is used for the diagnostic of electron density and temperature, two of the most important parameters in low temperature discharges. With a similar setup Rayleigh and Raman scattering techniques are used for the diagnostic of gas density and temperature. In this contribution we deal with the different technical and theoretical aspects that are required for the application of these laser scattering techniques. Of special importance are the detection limit, laser stray light rejection and laser perturbations of the plasma. The present study is performed on different low temperature microwave discharges, both at low and atmospheric pressure. The laser scattering techniques provide information on the spatial distribution of the mentioned plasma parameters over different discharge conditions, including small micro-plasmas. Similarly, the temporal evolution of pulsed plasmas is studied, unraveling the features of the switching on and off phases of the discharges.   

Transient characteristics of a pulsed helium positive column as measured with laser-collision induced fluorescence

The two dimensional laser-collision induced fluorescence (2D LCIF) diagnostic technique is extended to modest pressures (0.1 Torr to 10 Torr) and is then utilized to examine the evolution of helium positive column in response to a pulsed current (up to 1.5 A). Temporally and spatially resolved measurements of the species in the pulsed column such as excited state distributions, electron densities and ``effective electron temperatures'' are obtained using the LCIF technique. It is observed, that during the initial response of the column to the applied pulse, the radial dependence of excited state species (23P state of helium) tracks that of the electron densities. On the other hand, significant deviation between radial profiles of the excited species and the radial profile of the electron densities is observed as the column evolves in time. While the electron densities remain radially peaked on the axis of the discharge, the excited state distribution flattens out at lower pressures ($<$ 2 Torr) and becomes peaked off axis at the higher pressure bound studied ($>$3 Torr). Global measurements of discharge current and line-integrated densities obtained with microwave interferometry are used both to calibrate the laser measurements as well as to ascertain reduced electric fields (E/N) and electron temperature. Trends observed in spatially resolved measurements are discussed and compared to simulation results.   

Development of a Time Synchronized CW-Laser Induced Fluorescence Measurement for Quasi-Periodic Oscillatory Plasma Discharges

An advanced CW laser induced fluorescence diagnostic technique, capable of correlating high frequency current fluctuations to the resulting fluorescence excitation lineshapes, has been developed. This presentation describes this so-called ``Sample-Hold'' method of time-synchronization, and provides the steps taken to validate this technique, including simulations and experimental measurements on a 60 Hz Xe lamp discharge. Initial results for time-synchronized velocity measurements on the quasi-periodic oscillatory mode of a magnetic cusped plasma accelerator are also presented. These results show that the positions of the ionization and peak acceleration regions in the device vary over the course of a discharge current oscillation.   

Non-linear optical diagnostic studies of high pressure non-equilibrium plasmas

Picosecond Coherent Anti-Stokes Raman Spectroscopy (CARS) is used for study of vibrational energy loading and relaxation kinetics in high pressure nitrogen and air nsec pulsed non-equilibrium plasmas in a pin-to-pin geometry. It is found that $\sim $33{\%} of total discharge energy in a single pulse in air at 100 torr couples directly to nitrogen vibration by electron impact, in good agreement with master equation modeling predictions. However in the afterglow the total quanta in vibrational levels 0 -- 9 is found to increase by a factor of approximately 2 and 4 in nitrogen and air, respectively, a result in direct contrast to modeling results which predict the total number of quanta to be essentially constant. More detailed comparison between experiment and model show that the VDF predicted by the model during, and directly after, the discharge pulse is in good agreement with that determined experimentally, however for time delays exceeding $\sim $10 $\mu $sec the experimental and predicted VDFs diverge rapidly, particularly for levels v = 2 and greater. Specifically modeling predicts a rapid drop in population of high levels due to net downward V-V energy transfer whereas the experiment shows an increase in population in levels 2 and 3 and approximately constant population for higher levels. It is concluded that a collisional process is feeding high vibrational levels at a rate which is comparable to the rate at which population of the high levels is lost due to net downward V-V. A likely candidate for the source of additional vibrational quanta is the quenching of metastable electronic states of nitrogen to highly excited vibrational levels of the ground electronic state. Recent progress in the development and application of psec coherent Raman electric field and spontaneous Thomson scattering diagnostics for study of high pressure nsec pulsed plasmas will also be presented.   

Cavity ringdown measurements of OH radicals in microwave induced argon plasma assisted combustion of methane/air mixtures

In order to study the mechanism of plasma assisted combustion, we have developed a system that injects a nonthermal low temperature atmospheric argon plasma into the burning flame of lean methane/air mixtures. The experimental results demonstrated the flammability enhancement of plasma assisted combustion in the lean flame of a fuel equivalence ratio as low as 0.2. In the argon plasma assisted combustion flame, we observed three different zones which were pure argon plasma zone, plasma-flame interacting zone, and pure flame zone. Optical emission studies showed distinct spectroscopic fingerprints of each zone. The emission intensities of OH radicals increased dramatically moving from pure plasma zone to plasma-flame interacting zone, and dropped severely from plasma-flame interacting zone to pure flame zone. In addition to the optical emission spectroscopy study, cavity ringdown spectroscopy (CRDS) was also applied in the measurements of absolute ground state OH radical number densities in the plasma assisted combustion flame. Results showed that the ground state OH radical number densities in the pure flame zone are on the order of 10$^{15}$ molecule/cm$^{3}$, and increasing within the range of first few millimeters from the combustor nozzle.   

Development of novel atomic absorption spectroscopic method using optical frequency comb

For progress of atomic absorption spectroscopy including plasma diagnostics, we proposed a novel system of laser spectroscopic method for single electronic transitions using a frequency-comb laser source. The optical frequency comb is one of the newly developed coherent light sources and has potentials to be applied not only to accurate measurement of laser frequencies but also to various infrared spectroscopic measurements. In the novel method, the frequency-comb laser beam passes through tested plasma and then the frequency-comb laser beam is merged with an additional single-wavelength laser beam to generate beat signals by the interference. A power spectrum of the beat signals in the merged laser beam is recorded with a bandwidth of several tens of GHz which is sufficient to analyze whole spectrum of a single electronic transition. This method, named a frequency-comb interference spectroscopy (FCIS), enables us to measure single-transition absorption spectra without using a large-scale spectrometer or scanning the laser-beam frequency. We have evaluated the FCIS method in absorption profile measurements of argon metastable atoms in a small RF discharge.