Voitenko L. Relaxation processes in the plasma of high-voltage pulsed discharges in water.

The thesis submitted to be awarded the scientific degree of a Candidate of Physical and Mathematical Sciences, Speciality 01.04.08 – Plasma Physics. – Taras Shevchenko National University of Kyiv within the Ministry for Education and Science of Ukraine, Kyiv, 2018. This work is an experimental study of a plasma decay of pulsed discharges in water. There have been made experimental measurements of the temperature and plasma concentration and their changes after the termination of the energy contribution to a discharge. The spectrum dynamics versus time has been observed in the optical range. Decay coefficients have been determined, when the electron concentration of the discharge plasma varies from Ne·= 5·1021 cm– 3 to Ne = 2·1017 cm– 3. The work has demonstrated for the first time the following: 1. In a dense plasma values of the decay factors are significantly reduced. There is a correlation between the moments of increasing the recombination rate with the appearance of lines of radiation in the spectrum. 2. The distribution and magnitude of the intensity of the radiation observed in a dense plasma (at the electron density ≥ 1019 cm-3) do not always correspond to the temperature and concentration of the dense plasma. 3. In the dense plasma the «-9/2» -Principle of the dependence of the coefficients of recombination on the temperature for a three-part recombination has not been fulfilled. The decay factor depends only on the concentration, and it does not depend on the plasma temperature. The empirical formula for the dependence of the decay factor on the concentration of electrons in the concentration range of 1022 cm-3 ≥ Ne ≥ 1017 cm-3 and on the temperature from 6 000 K to 50 000 K has been is obtained. 4. At significant impurities of metal atoms, at a discharge in water the density of electrons in the plasma increases to 1022 cm-3. This allows to receive a plasma with a large degree of non-ideality Г ~ 3. 5. It has been shown, that in the visible range of the spectrum there is a failure in the intensity at the wavelengths, which correspond to the plasma frequency. There is a smooth displacement of the failure in the intensity at the plasma frequency in the red region of the spectrum as plasma decays. 6. No evidence of the dependence of the decay factor on the degree of nonideality has been found experimentally. Besides experimental measurements, the work included making calculations of decay coefficients due to different models of the recombination for the same values of the temperature concentration, that in the experiment were used. The available theoretical models do not explain the differences between the experimental measurements and the calculated values of the decay coefficients. The dynamics of the spectra changes is accounted for the effect of the plasma electrical microfields on the structure of separate atoms. A decrease in the recombination speed is accounted for by a change in the level structures of separate atoms at high densities of electrons in the plasms.