Milka Nikolic

Measurements of population densities of metastable and resonant levels of argon using laser induced fluorescence

We present a new approach to measure population densities of Ar I metastable and resonant excited states in low temperature Ar plasmas at pressures higher than 1 Torr. This approach combines the time resolved laser induced fluorescence technique with the kinetic model of Ar. The kinetic model of Ar is based on calculating the population rates of metastable and resonant levels by including contributions from the processes that affect population densities of Ar I excited states. In particular, we included collisional quenching processes between atoms in the ground state and excited states, since we are investigating plasma at higher pressures. We also determined time resolved population densities of Ar I 2 p excited states by employing optical emission spectroscopy technique. Time resolved Ar I excited state populations are presented for the case of the post-discharge of the supersonic flowing microwave discharge at pressures of 1.7 and 2.3 Torr. The experimental set-up consists of a pulsed tunable dye lase...

Characterization of the supersonic flowing microwave discharge using two dimensional plasma tomography

A tomographic numerical method based on the two-dimensional Radon formula for a cylindrical cavity has been employed for obtaining spatial distributions of the argon excited levels. The spectroscopy measurements were taken at different positions and directions to observe populations of excited species in the plasmoid region and the corresponding excitation temperatures. Excited argon states are concentrated near the tube walls, thus, confirming the assumption that the post discharge plasma is dominantly sustained by travelling surface wave. An automated optical measurement system has been developed for reconstruction of local plasma parameters of the plasmoid structure formed in an argon supersonic flowing microwave discharge. The system carries out angle and distance measurements using a rotating, flat mirror, as well as two high precision stepper motors operated by a microcontroller-based system and several sensors for precise feedback control.

On a dielectric-barrier and a microwave-cavity discharge in synchronized operation?the case of a helium/oxygen mixture

We have investigated the performance of a microwave-cavity discharge (MWD) operating in tandem with a fast rise-time pulsed dielectric-barrier discharge (DBD). The tandem discharge operated in a helium/oxygen mixture, where metastable molecular oxygen could be produced efficiently using MWD in proportionally large quantities (order of 20% of total oxygen number density). In this new arrangement, a DBD operating at high E/N provided a metastables-rich mixture, thereby modifying the discharge kinetics of the MWD, which operated in the E/N range centered around 10?Td. Both discharges operated in synchronized pulse-repetitive mode, which was tailored to maximize the oxygen metastable production efficiency. The system operated at pressures up to 350?Torr with an average power between 3 and 20?W.

Electron density measurements in a pulse-repetitive microwave discharge in air

We have developed a technique for absolute measurements of electron density in pulse-repetitive microwave discharges in air. The technique is based on the time-resolved absolute intensity of a nitrogen spectral band belonging to the Second Positive System, the kinetic model and the detailed particle balance of the N2C3Πu (ν = 0) state. This new approach bridges the gap between two existing electron density measurement methods (Langmuir probe and Stark broadening). The electron density is obtained from the time-dependent rate equation for the population of N2C3Πu (ν = 0) using recorded waveforms of the absolute C3Πu → B3Πg (0-0) band intensity, the forward and reflected microwave power density. Measured electron density waveforms using numerical and approximated analytical methods are presented for the case of pulse repetitive planar surface microwave discharge at the aperture of a horn antenna covered with alumina ceramic plate. The discharge was generated in air at 11.8 Torr with a X-band microwave gener...

Resonant-frequency discharge in a multi-cell radio frequency cavity

We are reporting experimental results on a microwave discharge operating at resonant frequency in a multi-cell radio frequency (RF) accelerator cavity. Although the discharge operated at room temperature, the setup was constructed so that it could be used for plasma generation and processing in fully assembled active superconducting radio-frequency cryo-module. This discharge offers a mechanism for removal of a variety of contaminants, organic or oxide layers, and residual particulates from the interior surface of RF cavities through the interaction of plasma-generated radicals with the cavity walls. We describe resonant RF breakdown conditions and address the issues related to resonant detuning due to sustained multi-cell cavity plasma. We have determined breakdown conditions in the cavity, which was acting as a plasma vessel with distorted cylindrical geometry. We discuss the spectroscopic data taken during plasma removal of contaminants and use them to evaluate plasma parameters, characterize the process, and estimate the volatile contaminant product removal.

Tomographic analysis of plasma sources with distorted cylindrical symmetry

The tomographic reconstruction of local plasma parameters for nonequilibrium plasma sources is an indispensable tool for understanding the fundamental processes and phenomena that follow the dynamics of plasmas with complex geometry. We present practical approaches that are developed on the case-to-case basis and applied to several plasma sources in our laboratories.

Electron density measurements in supersonic flowing discharges

Local measurements of the electron density in high pressure discharges have commonly been performed through use of an electrical probe, such as a Langmuir probe, or by the Stark broadening of the hydrogen lines. However in supersonic flowing discharges electrical probes can cause shocks to form, which is unwanted. In addition, these types of discharges do not often contain the hydrogen needed to determine the electron density through Stark broadening where the measurements are hampered by the lack of intensity and breadth of the hydrogen Balmer lines. An alternative approach is to use the intensity of the rotational bands of the N2 second positive system. We performed detailed measurements of the population densities of the N2 C3nu-B3ng system and the hydrogen Balmer lines in a supersonic flow of weakly ionized Ar/H2/Air. Gases were premixed in the stagnation chamber at room temperature by adding up to 10% hydrogen and up to 45% air to pure argon. A cylindrical cavity was used to sustain a discharge in the pressure range of 100-700 Pa. Absolute emission spectroscopy was used to determine the gas temperature in the flow from the N2 system. Comparison was made between the results obtained from the N2 band intensity technique and Stark broadening of the hydrogen Balmer lines. The difference between line broadening technique and the nitrogen band absolute intensity technique is found to be by a factor of two to three.

Experimental and kinetic study of the martian atmospheric entry plasma

In this paper we discuss the properties of the hot, ionized gas behind the bow shock during the Martian descent and/or aerobraking mostly from the perspective the system being a plasma reactor that processes large volume of the dilute atmospheric weakly ionized gas in the freestream. It becomes compressed, heated, and ionized in the shock layer. There are a few concepts of small subsystems that could be used to harvest a part of the kinetic energy converted during the entry or aerobraking phase, or to redistribute the heat flux, thereby acting as a thermal protection system. We describe the results of a kinetic model created based on free stream gas temperature, density, and velocity data from the Viking, Pathfinder, and MER Opportunity Landers. Altitudinal distribution of atmospheric composition was taken from Viking data. The density and temperature across the shock front were calculated for each probe. A temperature spread was observed due to the imprecision in the atmospheric models and data for the free stream temperature. Conditions in Mars ionosphere provide seeding electrons, which contribute to the formation of the ionizing wave in Martian atmospheric entry plasma (MAEP). Electron energy distribution function was calculated for the Martian atmosphere by solving the Boltzmann transport equation. For atmospheric gas mixtures, electron temperature and dissociation rate coefficients were then calculated. The electron density was determined from the Saha equation for a simple model with the main species for the ionized gas. A gas composition model was then used to estimate the dissociation of C02 in the Martian atmosphere for steady state and non-steady state conditions and a comparison was made. All this triggers many ideas for subsystems utilizing MAEP as plasma reactor.