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Papers by Ilija Stefanovic

HAL (Le Centre pour la Communication Scientifique Directe), 2008

ABSTRACT While steady state Townsend discharges may provide data for ionization coefficients thos... more ABSTRACT While steady state Townsend discharges may provide data for ionization coefficients those are often not as accurate as those produced in dedicated pulsed current growth experiments. In this paper we show that one may be able to measure ionization coefficients in DC microdischarges that are of excellent quality. Measurements were made for argon and argon/nitrogen mixtures with different gas flow rates. The technique based measuring the spatial profile of emission a Townsend discharge. In spite of having the drift length of only 1 mm, excellent agreement has been found between our new measurements and the data for low-pressure, larger dimension (2-4cm) discharges in argon (Jelenak et al) for the E/N in the range from 300 Td to 4000 Td, where E/N is normalized electrical field strength. Below 300 Td our measured values are larger then those by Jelenak et al. This discrepancy with previous measurements will be discussed. The influence of the gas flow-rate and nitrogen concentration on the radial discharge profile in the Townsend mode will also be presented and discussed. Jelenak et al 1993 Phys. Rev. E 47 3566

Bulletin of the American Physical Society, Oct 8, 2010

sized hydrocarbon particles through RF plasma polymerization of Ar diluted C 2 H 2 could be suppr... more sized hydrocarbon particles through RF plasma polymerization of Ar diluted C 2 H 2 could be suppressed or enhanced by proper plasma-pulsing, depending upon the pulsing frequency. Thus, the processes in afterglow play important role for dust generation. We applied Laser Absorption Spectroscopy to trace the time evolution of metastable density in different gas mixtures: pure Ar, Ar/ C 2 H 2 , Ar/C 2 H 2 /dust and Ar/dust. By introducing C 2 H 2 in argon plasma, a very fast decay of metastables in the afterglow was detected, caused by a strong quenching rate in Ar*-C 2 H 2 collisions. From the decay time we deduced that about 98% of C 2 H 2 was dissociated in the plasma. After formation of the dust particles, the metastable density enhanced and reached the ten time larger density in argon/dust mixture, for the same pressure and input power. The reason is the formation of dusts, which give the additional source of particle losses. These losses are compensated by increasing the electron temperature, thus the ionization rate.

Bulletin of the American Physical Society, Oct 5, 2010

The chemical phenomena in hydrocarbon containing Ar/He dusty plasmas have been studied combining ... more The chemical phenomena in hydrocarbon containing Ar/He dusty plasmas have been studied combining MIR tuneable diode laser absorption spectroscopy (TDLAS), Fourier transform infra-red (FTIR) and mass spectroscopy (MS) techniques. The experiments were done in a rf cc-coupled parallel plate reactor, f=13.56 MHz. Using TDLAS, the temporal evolution of C 2 H 2 , C 2 H 4 , CH 3 , CH 4 , CO and CO 2 were measured. Simultaneously, FTIR spectroscopy was used to monitor the growth process of the particles whereas MS was used to analyse the exhaust gas. The concentrations of the species were in the range of 10 10 to 10 14 molecules cm −3. The change in plasma conditions, in particular the sudden decrease of the electron energy with the disappearance of the dust after it reach a critical size, leads to a reduced degree of dissociation of CH 4 , while the density of C 2 H 2 is enhanced.

New Journal of Physics, Apr 29, 2003

We present the results of time resolved spectroscopy measurements of the Hα line of atomic hydrog... more We present the results of time resolved spectroscopy measurements of the Hα line of atomic hydrogen in an Ar-C2H2 radiofrequency plasma. The presence of the fast (high-energy) and slow (low-energy) components of the hydrogen atoms can be deduced from the Doppler broadening of the Hα line. With the appearance of the dust particles, the Hα profile became narrower, indicating reduction of the fast component. We discuss several mechanisms for the formation of the fast hydrogen atoms in our plasma. The main cause for the reduction of the hydrogen atom fast component is the plasma change from electropositive to electronegative, decreasing the sheath's width and voltage. The change of plasma conditions coincides with the end of the dust coagulation/agglomeration when the dust particles become multiply negatively charged. We propose here a new experimental technique for detection of the dust coagulation/agglomeration phase.

HAL (Le Centre pour la Communication Scientifique Directe), Sep 30, 2013

Experiments concerning different aspects of plasma influence on low pressure diamond growth in lo... more Experiments concerning different aspects of plasma influence on low pressure diamond growth in low pressure microwave plasmas are discussed. A planar microwave plasma source is used to perform deposition experiments under well defined afterglow conditions dominated by atomic hydrogen. Experiments with variable atomic hydrogen content indicate that the presence of sufficient amounts of atomic hydrogen is a necessary condition for diamond growth. But further increasing atomic hydrogen content doesn't influence crystal quality remarkablely. It is therefore concluded that further improvements in crystal quality can be achieved only achieved with the help of additonal control parameters which exhibit considerable influence on surface energy of the growing nulei. This conclusion is in good correspondence to the investigated plasma effects. Plasmas are best suitable to generate such additional parameters, e.g. plasma induced particle bombardment or external substrate bias. Besides special usefulness for atomic hydrogen generation and low thermal stress of substrates this is a further advantages of low pressure plasma activation.

Bulletin of the American …, 2010

2019 IEEE Pulsed Power & Plasma Science (PPPS)

Plasma jets belong to the category remote plasma. This means that the discharge conditions and th... more Plasma jets belong to the category remote plasma. This means that the discharge conditions and the chemical effect on samples can be tuned separately, this being a big advantage compared to standard low-pressure reactors. The inductive coupling brings the advantage of a pure and dense plasma. The microwave excitation allows furthermore miniaturization and generation of low temperature plasmas. The present paper shows the state of the art of the research on such sources, demonstrating their work up to atmospheric pressure.

Le Centre pour la Communication Scientifique Directe - HAL - CEA - Commissariat à l’énergie atomique, Jun 21, 2021

We analyze how properties of an argon/dust pulsed plasma depend on the shape of the electron ener... more We analyze how properties of an argon/dust pulsed plasma depend on the shape of the electron energy probability function (EEPF) and the pulsing frequency. The study is carried out using a spatially averaged model, which assumes that the plasma has R = 15 cm radius and L = 7 cm height and consists of electrons with density ne, singly charged positive ions (Ar +) with density ni, dust particles with density nd = 3.0×10 7 cm-3 , radius ad = 50 nm and negative charge eZd (e is the elementary charge), ground-state argon atoms (Ar0) with density na, metastable argon atoms (Arm) with density nm, argon atoms in the resonance 4s states (3 P1 and 1 P1) as well as argon atoms in 4p states. The neutral gas pressure P is 10 Pa, and the EEPF is described as 12

Plasma Sources Science and Technology, 2021

The MMWICP (miniature microwave ICP) is a new plasma source using the induction principle. Recent... more The MMWICP (miniature microwave ICP) is a new plasma source using the induction principle. Recently Klute et al presented a mathematical model for the electromagnetic fields and power balance of the new device. In this work the electromagnetic model is coupled with a global chemistry model for nitrogen, based on the chemical reaction set of Thorsteinsson and Gudmundsson and customized for the geometry of the MMWICP. The combined model delivers a quantitative description for a non-thermal plasma at a pressure of p = 1000 Pa and a gas temperature of T g = 650–1600 K. Comparison with published experimental data shows a good agreement for the volume averaged plasma parameters at high power, for the spatial distribution of the discharge and for the microwave measurements. Furthermore, the balance of capacitive and inductive coupling in the absorbed power is analyzed. This leads to the interpretation of the discharge regime at an electron density of n e ≈ 6.4 × 1018 m−3 as E/H-hybridmode ...

Bulletin of the American Physical Society, 2016

Bulletin of the American Physical Society, 2019

A recent survey of the available data on voltage current characteristics revealed a serious lack ... more A recent survey of the available data on voltage current characteristics revealed a serious lack of data and discrepancies which are well beyond the acceptable. Recently a new series of measurements has been completed for the low current obstructed discharges both in diffuse (Townsend) and constricted regime. It has been shown that previous measurements were not affected by the local heating of the cathode as a possible cause of the large scatter of the data. Smooth V-I characteristics were obtained both in steady state measurements and in the pulsed measurements made in an attempt to minimize the perturbation of the cathode surface by ions. Negative differential resistance was obtained from V-I characteristics which explained the occurrence of self sustained oscillations below the onset of constriction. A model was developed which managed to explain all the observables in the experiment including the negative differential resistance (NDR), onsets of oscillation and the shape of osc...

Journal of Physics D: Applied Physics, 2020

The properties (densities of electrons and metastable argon atoms, effective electron temperature... more The properties (densities of electrons and metastable argon atoms, effective electron temperature and dust charge) of argon/dust and pure argon pulsed plasmas are studied using a spatially-averaged model. The calculated time-dependencies for the densities of electrons and metastable atoms are compared with the experimental measurements and are found to be in a good qualitative agreement. It is analyzed how the plasma properties depend on the shape of the electron energy probability function (EEPF), the pulsing frequency and the duty cycle for both dust-free and dusty plasma. The analysis reveals that the agreement between theory and experiment is better with Druyvesteyn EEPF than the Maxwellian EEPF. Further, the variation in the pulsing frequency ν p differently affects the metastable density n m in a dust-free and in a dusty plasma. For large ν p, the metastable density in the dust-free pulsed plasma is larger than in the continuous-wave (CW) discharge, while the opposite is obtai...