B. Pokrzywka

Investigation of a cathode region of an electric arc

Investigations of the near-cathode region of the atmospheric pressure, 200 A arc burning in pure argon have been performed using spectroscopic techniques. Several slices at different distances from the cathode tip have been observed side-on. In order to perform diagnostic measurements of the plasma, the profiles of the Ar I and Ar II lines have been recorded. Phenomena inexplicable in terms of the model based on assumption of a local thermodynamic equilibrium state and constant pressure in the plasma have been observed and are shown on an Olsen-Richter diagram.

Stark width of - Ar I transition (696.543 nm)

The full width at half maximum (FWHM) of the 696.5 nm Ar I spectral line profiles was measured in the plasma temperature range from 13 500 K to 24 000 K and free electron density from 1.2 to . Results were obtained for this line emitted from the near-cathode region of an atmospheric pressure, 200 A arc burning in pure argon. Our results are compared with data existing up to now. In the electron density range of our experiment, the 696.543 nm Ar I line Stark FWHM is linearly proportional to the electron density. The observed dependence of the FWHM on the plasma temperature and electron density is in qualitative agreement with Griems theory; however, the ratio of calculated and measured FWHM was equal to 1.55, for the dependence on electron density and temperature. The dependence of FWHM on electron density, obtained by weighted least squares fitting of all our data with the constraint that the curve passes through the origin, is well represented by the following equation: , with in nm and in , for the temperature .

Spectroscopic investigation of the equilibrium state in the electric arc cathode region

Several thin layers of an arc plasma column were observed side-on at different distances from the cathode. The arc was operated in argon at atmospheric pressure with the arc current equal to 200 A. As the basic diagnostic method we used the Olsen - Richter diagrams. For determination of temperature the Larenz - Fowler - Milne and Boltzmann plot methods were applied. Electron density was measured from the Stark width of the ArII line and continuum intensity. The plasma near the cathode tip shows axial and radial dependence of its physical equilibrium state. In the hot core, the plasma is in LTE. Toward outer zones, the plasma state deviates from LTE. For plasma layers closer to the cathode, this deviation begins at smaller distances from the axis and for higher temperatures it vanishes for h > 3 mm. It is caused after all by the ArI ground state overpopulation. Theoretical estimations of the ArI ground state overpopulation give results consistent with experiment. Calculations show that this overpopulation is caused by inward transport of ground state atoms into the plasma column.

Thomson scattering from laser induced plasma in air

The laser induced plasma in air produced by 6 ns, 532 nm Nd:YAG pulses with 25 mJ energy was studied using the Thomson scattering method and plasma imaging techniques. Plasma images and Thomson scattered spectra were registered at delay times ranging from 150 ns to 1 μs after the breakdown pulses. The electron density and temperature, as determined in the core of the plasma plume, were found to decrease from 7.4 × 1017 cm−3 to about 1.03 × 1017 cm−3 and from 100 900 K to 22 700 K. The highly elevated electron temperatures are the result of plasma heating by the second, probe pulse in the Thomson scattering experiments.

Investigations of the cathode region of an argon arc plasma by degenerate four-wave mixing laser spectroscopy and optical emission spectroscopy

Degenerate four-wave mixing (DFWM) laser spectroscopy was used in local studies of atmospheric pressure argon plasma generated in a free-burning arc. The results of plasma diagnostics using the DFWM method were compared to the results obtained with optical emission measurements. In the cathode region of the arc the maxima of both the DFWM signal and the emission coefficient for the 696.5 nm Ar I line depend on the distance from the cathode tip. This effect proves the departure of the plasma state from local thermal equilibrium (LTE) as it has been reported by many authors. On the other hand the Stark shifts of the 696.5 nm Ar I line determined by the DFWM method in relation to plasma diagnostic results show no deviations from LTE on the arc axis down to 1.0 mm from the cathode tip.

Degenerate four-wave mixing in equilibrium argon arc plasma

The non-intrusive degenerate four-wave mixing (DFWM) method was used to study the local thermal equilibrium atmospheric-pressure argon arc plasma. The laser wavelength was in resonance with the - ArI transition, corresponding to the 696.5 nm emission line. The Abrams - Lind theory was verified and proved to be valid under the conditions of our plasma. In the high-laser-intensity limit, the DFWM signals were shown to be exclusively dependent on the population difference between the relevant argon states. Well resolved axial and radial profiles of the plasma temperature and the electron density were determined.

Laser spectroscopy of thermal plasma

Thermal plasma, due to its applications, is a research field of great importance, but reliable diagnostics of such plasma remains a challenging task. Spatially resolved methods, which provide local values of plasma parameters, are crucial for understanding the underlying physics. This can be achieved using pump–probe techniques. Two methods applicable and useful for thermal plasma diagnostics—four-wave mixing and scattering of laser beams—are discussed in this paper. Experimental examples of their application, namely four-wave mixing in argon arc plasma and scattering of laser light by laser-induced plasma, are presented.

Application of degenerate four-wave mixing laser spectroscopy and laser Thomson scattering to studies of thermal argon plasma

We investigated laser-based spectroscopic techniques: phase-conjugate degenerate four-wave mixing (PC-DFWM) and laser Thomson scattering (LTS) in terms of their application to Stark profile studies in thermal plasma. PC-DFWM was used for the Stark profile determination while LTS served for plasma diagnostic purpose. Experiments were performed for thermal plasma generated in the transferred arc and burning in pure argon at atmospheric pressure. The spectral profile of the 696.543 nm ArI line was studied. Our attention was paid to the problem of the laser power influence on the measured quantities and plasma state. In the case of PC-DFWM the influence of the laser power manifests itself as broadening of the line profile that can be corrected by a simple extrapolation of the results to the zero laser intensity. On the other hand the results of the Thomson scattering show strong dependence of the electron temperature on the laser intensity and its variations during the laser pulse.