Ana Sobota - Academia.edu (original) (raw)

Papers by Ana Sobota

Plasma Sources Science and Technology, 2016

Journal of Physics D: Applied Physics, 2015

ABSTRACT This work investigates the flow dynamics of a radio-frequency (RF) non-equilibrium argon... more ABSTRACT This work investigates the flow dynamics of a radio-frequency (RF) non-equilibrium argon atmospheric pressure plasma jet. The RF power is at a frequency of 50 Hz or 20 kHz. Combined flow pattern visualizations (obtained by shadowgraphy) and gas temperature distributions (obtained by Rayleigh scattering) are used to study the formation of transient vortex structures in initial flow field shortly after the plasma is switched on and off in the case of 50 Hz modulation. The transient vortex structures correlate well with observed temperature differences. Experimental results of the fast modulated (20 kHz) plasma jet that does not induce changes of the gas temperature are also presented. The latter result suggests that momentum transfer by ions does not have dominant effect on the flow pattern close to the tube. It is argued that the increased gas temperature and corresponding gas velocity increase at the tube exit due to the plasma heating increases the admixing of surrounding air and reduces the effective potential core length. With increasing plasma power a reduction of the effective potential core length is observed with a minimum length for 5.6 W after which the length extends again. Possible mechanisms related to viscosity effects and ionic momentum transfer are discussed.

Review of Scientific Instruments, 2013

Plasma Sources Science and Technology, 2014

An extensive electrical study was performed on a coaxial geometry atmospheric pressure plasma jet... more An extensive electrical study was performed on a coaxial geometry atmospheric pressure plasma jet source in helium, driven by 30 kHz sine voltage. Two modes of operation were observed, a highly reproducible low-power mode that features the emission of one plasma bullet per voltage period and an erratic high-power mode in which micro-discharges appear around the grounded electrode. The minimum of power transfer efficiency corresponds to the transition between the two modes. Effective capacitance was identified as a varying property influenced by the discharge and the dissipated power. The charge carried by plasma bullets was found to be a small fraction of charge produced in the source irrespective of input power and configuration of the grounded electrode. The biggest part of the produced charge stays localized in the plasma source and below the grounded electrode, in the range 1.2-3.3 nC for ground length of 3-8 mm.

Journal of Physics D: Applied Physics, 2012

The role of the driving field frequency f d of a cool atmospheric plasma (CAP) is investigated fo... more The role of the driving field frequency f d of a cool atmospheric plasma (CAP) is investigated for values around f * d = 13.56 MHz using a two-dimensional fluid numerical model applied to a parallel plate configuration. It is found that keeping the voltage constant the current amplitude roughly scales with f 2 d ; a tendency that can be understood using a RC circuit model of the plasma-sheath configuration. Moreover, it is seen that the electron density increases faster than the density of the excited species. This implies that plasma heating will increase relatively more than plasma reactivity so that the basic feature of the CAP, to be cool and reactive, will weaken for increasing f d -values.

Journal of Physics D: Applied Physics, 2011

Electrical breakdown in near-atmospheric pressure noble gases requires voltages that are quite hi... more Electrical breakdown in near-atmospheric pressure noble gases requires voltages that are quite high, which is undesirable for a large number of possible applications. Metallic structures (antennas) were used on the outer side of the lamp burner to enhance the electric field locally while keeping the same potential difference across the electrodes. Optical and electrical measurements were performed in an argon or xenon atmosphere at 0.3 or 0.7 bar, with 4 or 7 mm between the electrode tips. We used rod-shaped tungsten electrodes of 0.6 mm in diameter. We found that both active and passive antennas facilitate breakdown, and we demonstrated the differences between the two types and their effects on the breakdown process.

Journal of Physics D: Applied Physics, 2011

Journal of Physics D: Applied Physics, 2009

A pin-pin electrode geometry was used to study the velocities of streamers propagating over a fla... more A pin-pin electrode geometry was used to study the velocities of streamers propagating over a flat dielectric surface and in gas close to the dielectric. The experiments were done in an argon atmosphere, at pressures from 0.1 to 1 bar, with repetitive voltage pulses. The dielectric surface played a noticeable role in discharge ignition and propagation. The average speed of the discharge decreased with higher pressure and lower voltage pulse rise rate. It was higher when the conductive channel between the electrodes was formed over the dielectric, rather than through the gas. Space resolved measurements revealed an increase in velocity of the discharge as it travelled towards the grounded electrode.

Journal of Physics D: Applied Physics, 2011

The effect of frequency on the characteristics of ac-driven breakdown processes in 0.7 bar argon ... more The effect of frequency on the characteristics of ac-driven breakdown processes in 0.7 bar argon is investigated by means of a two-dimensional fluid model. The geometry represents the high intensity discharge lamp burner with a pin-pin electrode system forming a 7 mm electrode gap. The breakdown process is considered in the frequency range between 60 kHz and 1 MHz. The appearance of the discharge and the influence of the voltage frequency on its characteristics obtained in the simulations is in good agreement with the experimental data (accompanying paper-Sobota et al 2011 J. Phys. D: Appl. Phys. 44 224002, special issue on LS12/WLED3 symposium). The role of the secondary electron emission from the electrode surfaces is demonstrated and linked to the lowering of the threshold voltage with the increase in frequency observed both in experiment and model.

Journal of Physics D: Applied Physics, 2013

We report on experimentally obtained values of the electric field magnitude on a dielectric surfa... more We report on experimentally obtained values of the electric field magnitude on a dielectric surface induced by an impinging atmospheric pressure plasma jet. The plasma plume was striking the dielectric surface at an angle of 45 • , at 5 mm from the surface measured at the axis of the jet. The results were obtained using Pockels technique on a BSO (Bi 12 SiO 20 ) crystal. A coaxial configuration of the plasma jet was used, operating in a stable mode with one bullet per voltage period, at 30 kHz and amplitude of 2 kV. The electric field was shown to be a function of the gas flow (He, at 300, 500 and 700 SCCM) and the manner in which the discharge spreads over the dielectric surface. The maximum value of 11.6 × 10 5 V m −1 was obtained at the negative half-period of the discharge current measured at the grounded electrode, at the flow of 300 SCCM. The largest electric field averaged over the area of the spreading of the discharge (3.6 × 10 5 V m −1 ) was found in the same conditions.

Journal of Applied Physics, 2013

Simulation of an Ar/NH3 low pressure magnetized direct current discharge Phys. Plasmas 20, 013508... more Simulation of an Ar/NH3 low pressure magnetized direct current discharge Phys. Plasmas 20, 013508 Simulation benchmarks for low-pressure plasmas: Capacitive discharges Phys. Plasmas 20, 013507 Spatiotemporal structure of a millimetric annular dielectric barrier discharge plasma actuator Phys. Fluids 25, 017103 Electron and ion thermal forces in complex (dusty) plasmas Phys. Plasmas 20, 013703 Temporal evolution of the re-breakdown voltage in small gaps from nanoseconds to milliseconds Appl. Phys. Lett. 102, 012904 (2013) Additional information on J. Appl. Phys.

IEEE Transactions on Plasma Science, 2000

Antenna structures are not often used as a way to influence the breakdown process in gases. Howev... more Antenna structures are not often used as a way to influence the breakdown process in gases. However, they do show great potential for lowering the ignition voltage of high-intensity discharge (HID) lamps. We used HID lamp burners filled with pure argon to test the effects that a simple active antenna can have on the breakdown process. We show the photographs of the process and explain why it is different from the case without antenna structures.

IEEE Transactions on Plasma Science, 2000

ABSTRACT Plasma bullets are ionization fronts created in atmospheric-pressure plasma jets. The pr... more ABSTRACT Plasma bullets are ionization fronts created in atmospheric-pressure plasma jets. The propagation behavior of those bullets is, in the literature, explained by the formation of an interface between the inert gas and the ambient air created by the gas flow of the plasma jet, which guides these discharges in the formed gas channel. In this paper, we examine this ionization phenomenon in uniform gases at atmospheric pressure where this interface between two gases is not present. By changing electrical parameters and adding admixtures such as oxygen, nitrogen, and air to the gas flow, the conditions for which plasma bullets are present are investigated. Nanosecond time-resolved images have been taken with an ICCD camera to observe the propagation behavior of these discharges. It is argued that the inhomogeneous spatial concentration of metastable atoms and ions, due to the laminar gas flow and the operation frequency of the discharge in the range of a few kilohertz, is responsible for the guidance of the ionization fronts. Furthermore, conditions have been observed at where the branching of the discharge is stable and reproducible over time in the case of a helium plasma by adding admixtures of oxygen. Possible mechanisms for this phenomenon are discussed.

IEEE Transactions on Plasma Science, 2000

Electrical breakdown in noble gas near a dielectric is an important issue in lighting industry. I... more Electrical breakdown in noble gas near a dielectric is an important issue in lighting industry. In order to investigate the influence of the dielectric on the ignition process, we perform measurements in argon, with pressure varying from 0.1 to 1 bar, using a pin-pin electrode geometry. Here, we present timeresolved images of ignition process for two different distances from electrodes to the dielectric.

Plasma Sources Science and Technology, 2016

Journal of Physics D: Applied Physics, 2015

ABSTRACT This work investigates the flow dynamics of a radio-frequency (RF) non-equilibrium argon... more ABSTRACT This work investigates the flow dynamics of a radio-frequency (RF) non-equilibrium argon atmospheric pressure plasma jet. The RF power is at a frequency of 50 Hz or 20 kHz. Combined flow pattern visualizations (obtained by shadowgraphy) and gas temperature distributions (obtained by Rayleigh scattering) are used to study the formation of transient vortex structures in initial flow field shortly after the plasma is switched on and off in the case of 50 Hz modulation. The transient vortex structures correlate well with observed temperature differences. Experimental results of the fast modulated (20 kHz) plasma jet that does not induce changes of the gas temperature are also presented. The latter result suggests that momentum transfer by ions does not have dominant effect on the flow pattern close to the tube. It is argued that the increased gas temperature and corresponding gas velocity increase at the tube exit due to the plasma heating increases the admixing of surrounding air and reduces the effective potential core length. With increasing plasma power a reduction of the effective potential core length is observed with a minimum length for 5.6 W after which the length extends again. Possible mechanisms related to viscosity effects and ionic momentum transfer are discussed.

Review of Scientific Instruments, 2013

Plasma Sources Science and Technology, 2014

An extensive electrical study was performed on a coaxial geometry atmospheric pressure plasma jet... more An extensive electrical study was performed on a coaxial geometry atmospheric pressure plasma jet source in helium, driven by 30 kHz sine voltage. Two modes of operation were observed, a highly reproducible low-power mode that features the emission of one plasma bullet per voltage period and an erratic high-power mode in which micro-discharges appear around the grounded electrode. The minimum of power transfer efficiency corresponds to the transition between the two modes. Effective capacitance was identified as a varying property influenced by the discharge and the dissipated power. The charge carried by plasma bullets was found to be a small fraction of charge produced in the source irrespective of input power and configuration of the grounded electrode. The biggest part of the produced charge stays localized in the plasma source and below the grounded electrode, in the range 1.2-3.3 nC for ground length of 3-8 mm.

Journal of Physics D: Applied Physics, 2012

The role of the driving field frequency f d of a cool atmospheric plasma (CAP) is investigated fo... more The role of the driving field frequency f d of a cool atmospheric plasma (CAP) is investigated for values around f * d = 13.56 MHz using a two-dimensional fluid numerical model applied to a parallel plate configuration. It is found that keeping the voltage constant the current amplitude roughly scales with f 2 d ; a tendency that can be understood using a RC circuit model of the plasma-sheath configuration. Moreover, it is seen that the electron density increases faster than the density of the excited species. This implies that plasma heating will increase relatively more than plasma reactivity so that the basic feature of the CAP, to be cool and reactive, will weaken for increasing f d -values.

Journal of Physics D: Applied Physics, 2011

Electrical breakdown in near-atmospheric pressure noble gases requires voltages that are quite hi... more Electrical breakdown in near-atmospheric pressure noble gases requires voltages that are quite high, which is undesirable for a large number of possible applications. Metallic structures (antennas) were used on the outer side of the lamp burner to enhance the electric field locally while keeping the same potential difference across the electrodes. Optical and electrical measurements were performed in an argon or xenon atmosphere at 0.3 or 0.7 bar, with 4 or 7 mm between the electrode tips. We used rod-shaped tungsten electrodes of 0.6 mm in diameter. We found that both active and passive antennas facilitate breakdown, and we demonstrated the differences between the two types and their effects on the breakdown process.

Journal of Physics D: Applied Physics, 2011

Journal of Physics D: Applied Physics, 2009

A pin-pin electrode geometry was used to study the velocities of streamers propagating over a fla... more A pin-pin electrode geometry was used to study the velocities of streamers propagating over a flat dielectric surface and in gas close to the dielectric. The experiments were done in an argon atmosphere, at pressures from 0.1 to 1 bar, with repetitive voltage pulses. The dielectric surface played a noticeable role in discharge ignition and propagation. The average speed of the discharge decreased with higher pressure and lower voltage pulse rise rate. It was higher when the conductive channel between the electrodes was formed over the dielectric, rather than through the gas. Space resolved measurements revealed an increase in velocity of the discharge as it travelled towards the grounded electrode.

Journal of Physics D: Applied Physics, 2011

The effect of frequency on the characteristics of ac-driven breakdown processes in 0.7 bar argon ... more The effect of frequency on the characteristics of ac-driven breakdown processes in 0.7 bar argon is investigated by means of a two-dimensional fluid model. The geometry represents the high intensity discharge lamp burner with a pin-pin electrode system forming a 7 mm electrode gap. The breakdown process is considered in the frequency range between 60 kHz and 1 MHz. The appearance of the discharge and the influence of the voltage frequency on its characteristics obtained in the simulations is in good agreement with the experimental data (accompanying paper-Sobota et al 2011 J. Phys. D: Appl. Phys. 44 224002, special issue on LS12/WLED3 symposium). The role of the secondary electron emission from the electrode surfaces is demonstrated and linked to the lowering of the threshold voltage with the increase in frequency observed both in experiment and model.

Journal of Physics D: Applied Physics, 2013

We report on experimentally obtained values of the electric field magnitude on a dielectric surfa... more We report on experimentally obtained values of the electric field magnitude on a dielectric surface induced by an impinging atmospheric pressure plasma jet. The plasma plume was striking the dielectric surface at an angle of 45 • , at 5 mm from the surface measured at the axis of the jet. The results were obtained using Pockels technique on a BSO (Bi 12 SiO 20 ) crystal. A coaxial configuration of the plasma jet was used, operating in a stable mode with one bullet per voltage period, at 30 kHz and amplitude of 2 kV. The electric field was shown to be a function of the gas flow (He, at 300, 500 and 700 SCCM) and the manner in which the discharge spreads over the dielectric surface. The maximum value of 11.6 × 10 5 V m −1 was obtained at the negative half-period of the discharge current measured at the grounded electrode, at the flow of 300 SCCM. The largest electric field averaged over the area of the spreading of the discharge (3.6 × 10 5 V m −1 ) was found in the same conditions.

Journal of Applied Physics, 2013

Simulation of an Ar/NH3 low pressure magnetized direct current discharge Phys. Plasmas 20, 013508... more Simulation of an Ar/NH3 low pressure magnetized direct current discharge Phys. Plasmas 20, 013508 Simulation benchmarks for low-pressure plasmas: Capacitive discharges Phys. Plasmas 20, 013507 Spatiotemporal structure of a millimetric annular dielectric barrier discharge plasma actuator Phys. Fluids 25, 017103 Electron and ion thermal forces in complex (dusty) plasmas Phys. Plasmas 20, 013703 Temporal evolution of the re-breakdown voltage in small gaps from nanoseconds to milliseconds Appl. Phys. Lett. 102, 012904 (2013) Additional information on J. Appl. Phys.

IEEE Transactions on Plasma Science, 2000

Antenna structures are not often used as a way to influence the breakdown process in gases. Howev... more Antenna structures are not often used as a way to influence the breakdown process in gases. However, they do show great potential for lowering the ignition voltage of high-intensity discharge (HID) lamps. We used HID lamp burners filled with pure argon to test the effects that a simple active antenna can have on the breakdown process. We show the photographs of the process and explain why it is different from the case without antenna structures.

IEEE Transactions on Plasma Science, 2000

ABSTRACT Plasma bullets are ionization fronts created in atmospheric-pressure plasma jets. The pr... more ABSTRACT Plasma bullets are ionization fronts created in atmospheric-pressure plasma jets. The propagation behavior of those bullets is, in the literature, explained by the formation of an interface between the inert gas and the ambient air created by the gas flow of the plasma jet, which guides these discharges in the formed gas channel. In this paper, we examine this ionization phenomenon in uniform gases at atmospheric pressure where this interface between two gases is not present. By changing electrical parameters and adding admixtures such as oxygen, nitrogen, and air to the gas flow, the conditions for which plasma bullets are present are investigated. Nanosecond time-resolved images have been taken with an ICCD camera to observe the propagation behavior of these discharges. It is argued that the inhomogeneous spatial concentration of metastable atoms and ions, due to the laminar gas flow and the operation frequency of the discharge in the range of a few kilohertz, is responsible for the guidance of the ionization fronts. Furthermore, conditions have been observed at where the branching of the discharge is stable and reproducible over time in the case of a helium plasma by adding admixtures of oxygen. Possible mechanisms for this phenomenon are discussed.

IEEE Transactions on Plasma Science, 2000

Electrical breakdown in noble gas near a dielectric is an important issue in lighting industry. I... more Electrical breakdown in noble gas near a dielectric is an important issue in lighting industry. In order to investigate the influence of the dielectric on the ignition process, we perform measurements in argon, with pressure varying from 0.1 to 1 bar, using a pin-pin electrode geometry. Here, we present timeresolved images of ignition process for two different distances from electrodes to the dielectric.