|
|
Study of Positive Column in Glow Discharge under Medium Pressures Using Computational Experiment
Laca, Marek ; Hrachová, Věra (advisor) ; Novák, Stanislav (referee) ; Roučka, Štěpán (referee)
The positive column of the oxygen and argon-oxygen direct current glow discharge was investigated using a fluid plasma model at pressures around hundreds of pascals and discharge currents from ten to forty miliampers. The model describes the infinitely long positive column in cylindrical discharge tube. It is based on the continuity equation for particle concentration with the collisional right hand side. The model utilises the drift-diffusion approximation of particle flux and the mean-electron-energy approximation for the description of the electron interaction. The radial profile of particle concentration and interaction with the glass wall of the discharge tube is taken into account. The model predicts the electric field and the particle concentration in the positive column. The simulation results were compared with the measured intensity of electric field strength in the oxygen and argon-oxygen mixture. The impact of discharge conditions, like the pressure and gas composition, on the properties of the positive column was investigated. The model confirmed that the strength of longitudinal electric field at medium pressures is about 40 V/cm in oxygen, molecular gas, and it is about 3 V/cm in argon, noble gas.
|
|
|
Study of Glow Discharge in Oxygen and its Mixtures at Medium Pressures
Schmiedt, Lukáš ; Hrachová, Věra (advisor) ; Mazánková, Věra (referee) ; Novák, Stanislav (referee)
Title: Study of Glow Discharge in Oxygen and its Mixtures at Medium Pressures Author: Lukáš Schmiedt Department: Department of Surface and Plasma Science Supervisor: Doc. RNDr. Věra Hrachová, CSc. Department of Surface and Plasma Science Abstract: The positive column of DC glow discharge sustained in oxygen and oxygen-argon mixtures has been studied in two discharge tubes of the same shape made from different materials (Silica and Pyrex glass) for total pressures 200 - 1750 Pa and discharge currents up to 40 mA. Parameters of the discharge - electric field strength and emission spectra - were studied with respect to existence of the low- and high-gradient forms of the positive column of the discharge. We have focused on the qualitative analysis of the differences in emission spectra for both particular forms, as well as on possibility of determination of the rotational temperature of oxygen molecule under various discharge conditions, particularly with respect to the relative amount of the admixture. Keywords: glow discharge; oxygen; medium pressures; emission spectra
|
|
|
Measurement of the plasma potential by means of the ball-pen and Langmuir probe
Zanáška, Michal ; Tichý, Milan (advisor) ; Hrachová, Věra (referee)
The ball-pen probe represents a new unique probe recently developed at the Institute of Plasma Physics in Prague for direct measurement of plasma potential at the CASTOR tokamak. Presently is the probe utilized also on several other high-temperature devices in Europe. In recent years one has aimed to use ball- pen probe also in conditions for which it was not originally intended. The aim of this bachelor thesis is to experimentally demonstrate that the ball-pen probe is suitable for the direct measurement of the plasma potential in low-temperature weakly magnetized plasma generated by a DC cylindrical magnetron by perfor- ming comparative measurements using a Langmuir probe. 1
|
|
|
Study of laser mixture in the large pressure region
Morávek, Matěj Jan ; Hrachová, Věra (advisor) ; Kudrna, Pavel (referee)
This work studies discharge plasma in a mixture of gases, similar to that used in the so-called CO2-lasers. This mixture consists of CO2, N2 and He. The effect of the mixture composition and discharge parameters (especially pressure, in the range of 266 Pa - 100 kPa) on the distribution of energy in the vibrational levels of nitrogen was examined. This is important parameter for modelling of the discharge plasma. The effect of the mixture composition on the degree of dissociation of the CO2 molecules was also studied. The relative concentration of CO was applied to find the conditions leading to a minimal dissociation of the carbon dioxide. Measurements of radial profiles were also made. Results from two discharge tubes made from different materials were compared. Two types of discharge were utilized to acquire a wide pressure range - low pressure DC glow discharge in the range of 266 Pa to 1330 Pa and dielectric barrier discharge in the range of 5 kPa to 100 kPa. Both discharges are used in commercial CO2-lasers. We observed a descending dependence of the vibrational temperature on the pressure and a big step caused by increased occurrence of standing ionizing waves in the mixtures with low nitrogen ratio. Vibrational temperature in the DBD was markedly lower than in the DC GD, because of the...
|
|
|
Modelling of glow discharge positive column of oxygen in the middle pressures region
Laca, Marek ; Hrachová, Věra (advisor) ; Plašil, Radek (referee)
The aim of the presented work was to create model of the glos discharge positive column of the oxygen in the middle pressures (in range 100-1000 Pa) with the methods of the computer physics, specifically with the method of the chemical kinetics. I have focused on the existence of two forms of the discharge positive column in the oxygen, low and high gradient form with axial electrical field about 100 V/m and 1 kV/m. I developed continuous model of the low temperature plasma as the steady state solution of the continuity equation with the neglected divergent member. The model involves twelve different kinds of the particles and reactions among them: electrons, ground and excited states of the atomic and molecular oxygen, of course negative and positive ions. I had numerically solved time evolution of the particle concentration at constant pressure, until the system reached equilibrium state. My numerical results were compared with experimental measurements of the electron density, which had been done in the Department of Surface and Plasma Science. To get a consistency between my results and experimental ones for electron density, I had to variate reaction coefficients of the reactions in which charged particles are produced.
|
|
| |
|
| |
|
| |
|
|
Influence of the laser mixture composition on the existence of the dinitrogen bands
Morávek, Matěj Jan ; Hrachová, Věra (advisor) ; Kudrna, Pavel (referee)
This project studies mixtures of helium, nitrogen and carbon dioxid, widely used in so called CO2 lasers. These lasers are classified as discharge lasers. In the case of low output lasers is often used a DC glow discharge. We can find bands so called first and second positive systems of a dinitrogen molecule in an emission spectrum of the glow discharge. It is possible to deduce energetic balance and a vibrational temperature from these bands. The vibrational temperature was studied by the vibrational spectroscopy of dinitrogen molecule for various discharge currents and pressures and for various distribution of nitrogen in the mixture (we will start with an industrial mixture LASAL 63).
|
|
| |
guest ::
