S. N. Tskhai

Experimental verification of the method for detection of water microleakages in plasma vacuum chambers by using the hydroxyl spectrum

Experimental determination of the sensitivity of the method for detection of water microleakages in the cooling systems of the plasma vacuum chambers of complex electrophysical devices (such as tokamaks, fuel elements of nuclear reactors, and plasmachemical reactors) is considered. It was shown that the spectroscopic method for detection of water microleakages by using the hydroxyl radiation spectrum makes it possible to detect leakages at a level of 10−5 Pa m3 s−1. The spatial resolution of the method allows one to localize defects with an accuracy of several centimeters.

Methods of Reference Signal and Phase Shifts in the Multipass Laser Schemes for the Detection of Trace Gas Impurities

The application of the methods using the phase shift of modulated radiation and ICOS for the measurement of relatively low concentrations of gas impurities is considered. The measurement sensitivities of 0.01 ppbv and 6 × 10−11 cm−1 with respect to concentration and absorption coefficient, respectively, are obtained for the NO2 molecules.

Recording weak absorption spectra by the phase-shift method with deep amplitude and frequency modulation using a diode laser and a high Q cavity

A method is suggested and developed for recording absorption spectra of particles in a high Q cavity from the phase delay of amplitude-modulated radiation generated by a tunable diode laser. The method features high spectral resolution and sensitivity. It is based on the experimentally established possibility of recording spectra in conditions of high-percentage modulation, under which the concomitant frequency deviation exceeds the bandwidth. This is achieved by processing in a special way the signals in the absorption recording channel of the spectrometer and concurrently measuring the radiation frequency variation parameters in real time. The method is experimentally implemented for the case of square-pulse modulation of the laser injection current. The absorption spectrum of water molecules is recorded in the region of 644 nm with a spectral resolution of 4:5 10 3 cm 1 and a sensitivity of 7 10 11 cm 1 in terms of the absorption coefficient. Comparative measurements are taken by another well-known high-sensitivity laser technique ICOS. The broadening coefficients for some weak absorption water lines by helium are determined. Possible ways are discussed of further development of the method suggested. (Some figures may appear in colour only in the online journal)

Measurements of the atmospheric concentrations of nitrogen dioxide using the radiation absorption in a spectral range of 415 nm

The trace atmospheric concentrations of NO2 are measured with the methods of the diode laser absorption spectroscopy and the phase shift of the modulated radiation. An electronic transition in a range of 415 nm is employed. For a measurement time of 20 ms, the sensitivity of the concentration measurements is about 10−11 relative to the concentration of air particles (0.02 ppbv). At a time interval of 5–10 min, the reproducibility of the absolute concentration measurements (0.1 ppbv) is limited by the low-frequency fluctuations of the optical parameters of the spectrometer and the calibration system and corresponds to the level determined by the Allan variance.

The influence of dust particles on the intensities of plasma emission lines

Emission spectra of glow-discharge plasma in Ar–He gas mixture have been measured. It is established that the introduction of Al2O3 dust particles into plasma significantly changes the relative intensities of spectral lines.

Study of water molecule decomposition in plasma by diode laser spectroscopy and optical actinometry methods

The methods of diode laser radiation absorption at vibrational–rotational molecule transitions and optical actinometry with measurements of its electron emission spectra are used independently to study water molecule dissociation in glow discharge plasma in a mixture of water vapor and inert gases at reduced pressure. The methods yield close results. The dissociation reaches 98%.

Fast recording of weak absorption spectra in optical cavity using tunable laser

The possibilities of new spectral technique R-ICOS are demonstrated by an example of weak absorption spectra of methane. The technique uses the combination of the intensities of laser beams transmitted through and reflected by the gas-filled resonant optical cavity. Such an approach leads to damping signal fluctuations due to mismatching of laser and cavity modes. With cavity mirrors of moderate reflectivity of 0.8–0.99, the spectra of the quality enough to measure the trace methane concentrations in atmosphere can be recorded within 320 μs. For the acquisition time of 20 s, the sensitivity to absorption is α = 2 × 10−8 cm−1 (1σ), which is 40 times better than needed for the background methane monitoring in standard atmosphere.

Generation of Intense UV Radiation during High-Current Breakdown over a Ferrite Surface

The dynamics and emission characteristics of pulsed breakdown over a ferrite surface at a current amplitude of 270 kA and current rise time of 80 ns were studied experimentally. It is shown that the characteristic transverse size of the discharge region in visible radiation is ~3 mm, while that in vacuum UV (VUV) radiation is ~200 μm. The duration of the VUV pulse with an average power of ≈0.275 GW is about 80 ns.

Optimisation of external cavity parameters of a weak absorption laser spectrometer

We consider some peculiar features of the optimisation procedure of external optical cavity parameters of a laser spectrometer, caused by the use of a three-beam measurement scheme and the presence of losses in the mirrors. It is found that the maximum sensitivity to the absorption of an intracavity medium can be achieved only at a certain choice of the value and the ratio of the reflection coefficients of the mirrors. For example, registration of the spectra of the methane impurity in the atmosphere shows that in accordance with the calculation model, for the same resonator Q-factor the use of an input mirror with a smaller reflection coefficient allows the measurement sensitivity to be increased by approximately two times.

Measurements of electron density and electric field in plasma produced in nanosecond discharge in pressurized gases

Summary form only given. This talk outlines the main experimental results regarding the research of nanosecond time-scale discharge in gases as air, H2 and He, conducted at pressures P≥105 Pa. Discharges were ignited in gas filled chambers, with interelectrode gap 0.5-3 cm, by application of high-voltage (HV) pulses with amplitudes ~100-200 kV and duration 1-7 ns at FWHM to a blade shaped cathode with sharp edge. The discharge in such conditions is ignited by high-energy runaway electrons (RAE), which are considered responsible for pre-ionization of the gas in the interelectrode gap.We have investigated this discharge using a variety of non-disturbing, experimental techniques with temporal resolution close to a single nanosecond, such as: fast-framing imaging, x-ray foil spectroscopy, electron beam imaging and electron beam foil spectroscopy. Profound conclusions were drawn according the experimental results, regarding the dynamics of the discharge evolution and dependence on the gas and pressure, energy spectrum of RAE and the x-ray radiation, RAE emission mechanism. Temporal evolution of plasma parameters such as electron density and temperature were determined using Optical Emission Spectroscopy. These results predicted a presence of significant electric fields (~10 kV/cm) in the plasma. Lately we have measured the intensity of electric fields using Coherent anti-Stokes Raman Scattering, which also allows calculating the conductivity of plasma.