|
|
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).
|
|
| |
|
| |
|
|
Study of Post-Discharge Processes
Soural, Ivo ; Hrachová, Věra (referee) ; Brablec, Antonín (referee) ; Krčma, František (advisor)
The decaying plasma was studied by the optical emission spectroscopy. DC discharge created at 45 – 200 mA in Pyrex and Quartz tubes in flowing regime was used. The emission of three nitrogen spectral systems (1st and 2nd positive and 1st negative) were studied in time evolution for pressures of 500 – 5 000 Pa at two wall temperatures – ambient and liquid nitrogen (150 K inside the decaying plasma). Results showed that all three nitrogen systems (respectively N2(B, v), N2(C, v) and N2+(B, v) states as their origins) had their population maxima called pink-afterglow in the afterglow part. These maxima decreased with the increase of pressure for all systems, and moved to the later decay time. Maxima increased with discharge current (respectively power) and moved to shorter time. Populations at temperature of 150 K were measured due to the experimental arrangement from 17 ms, only, and thus pink aftergow maximum wasn’t observed (only at 5 000 Pa some maximum was recognized). Populations were smaller at 150 K that populations measured at laboratory temperature at the middle decay time (50-100 ms). At the late time, the populations were higher at lower temperature at lower pressure. Higher shifts (in intensity and decaytime) of pink afterglow maxima were observed in Quartz tube in comparison with their values in Pyrex tube. Besides the populations, rotational temperatures of selected bands of three observed spetral systems (for 1st negative 0-0 band, 1st positive 2-0 band and for 2nd positive 0-2 band) were measured. Rotational temperatures were monitored from presumption that this kind of temperature is equal to temperature of neutral gas (at local thermodynamic equilibrium). Results from 1st negative and 1st positive system showed strong decreasing of rotational temperatures up to about 10 ms at post-discharge begin, then temperatures were constant up to 20 ms of decay time and after that they grew up. Temperatures increased with the increase of current. The part with decreased temperature correlated with pink-afterglow part of post-discharge. Unfortunately, rotational temperatures of 2nd positive system had bad reproducibility and the time profile shape was opposite. Experimental results were compared with numerical kinetic model created by group of prof. Vasco Guerra at Instituto Supetior Técnico in Portugal. Several sets of conditions for simulation at 500 and 1 000 K in active discharge were applicable for the calculation corresponding to the experiment. Comparison of numerical simulation and experimental data done for N2(B) state demonstrated that maxima populations in pink afterglow are depended on the temperature difference between active discharge and post discharge. Maxima populations were supposed in pink afterglow disappeared if the same temperatures in active and post discharges were supposed. Temperature in active discharge is higher at higher apllied power, as it was showed from rotational temperatures observation. The results clearly showed that real temperature profile must be included into the kinetic model.
|
guest ::
