Atmospheric pressure plasma jets in Helium – the electric field and the charge delivered to a dielectric surface

The family of non-thermal atmospheric pressure discharges has been the focus of intense research of a large number of research groups in the last fifteen years, as they are easy and cheap to assemble and run, and exhibit properties that can be used in surface treatment or biological applications. In this discharge family the non-thermal atmospheric pressure plasma jet commands a good deal of attention.\u3cbr/\u3eThe mentioned applications all involve the presence of a surface (target for treatment) in the vicinity of the discharge, and it has been shown that in many cases the presence of the surface alters the properties of the discharge. There are many types of surfaces to consider, from metals, dielectrics, to liquid surfaces, and they all leave a different mark on the discharge properties. Still, most of the research has been done on plasma jets expanding freely into the open air. \u3cbr/\u3eThis paper reports on the electric field and charge measurements delivered to a dielectric surface by an atmospheric-pressure plasma jet working in the bullet mode in helium. Imaging will be presented alongside the measurements of charge, as it will be evident that the charge distribution on the dielectric surface will mirror the observations obtained by imaging. The central results contain the charge packed in one ionization wave leaving the capillary towards the target, around 20 pC. The associated electric fields vary between 3×105 and 6×105 V/m

Cold atmospheric pressure plasma jets - charge carried by plasma bullets

Cold atmospheric pressure plasma jets are re-searched for applications in surface modification, synthesis, sterilization, medicine. Partially due to the relative ease of constructing a plasma jet, a great amount of work has been published on jets in a va-riety of gases, using excitation in a wide frequency range and in several typical geometries. Most com-monly reported on are descriptions of discharge dy-namics, densities of various reactive species, fol-lowed by gas temperature measurements, imaging of flow fields, and rarely electron densities and associ-ated electric fields. The first reported measurement of the electric field associated with plasma bullets has been performed by using a spectroscopic technique and published in 2011 by Sretenovic ́ et al [1], fol

Charge transfer to a dielectric target by guided ionization waves using electric field measurements

\u3cp\u3eA kHz-operated atmospheric pressure plasma jet is investigated by measuring charge transferred to a dielectric electro-optic surface (BSO crystal) allowing for the measurement of electric field by exploiting the Pockels effect. The electric field values, distribution of the surface discharge and amount of deposited charge are obtained for various parameters, including gas flow, applied voltage, target distance and the length of the capillary from ground to the end. A newly formed surface discharge emerges at the target when enough charge is deposited at the impact point and electric fields are high enough, i.e. 200 pC and 9 ±2 kV cm\u3csup\u3e-1\u3c/sup\u3e. The maximum amount of charge transferred by a single ionization wave ('plasma bullet') is 350 ±40 pC. Due to the emerging new surface discharge behind the impact point, the total charge deposited on the surface of the dielectric target can increase up to 950 pC. The shape of the secondary discharge on the target is found to be mainly driven by gas flow, while the applied voltage allows us to utilize longer distances within the boundaries set by this gas mixing. Finally the ionization wave is found to lose charge along its propagation on the inner walls of the capillary. The loss is estimated to be approximately 7.5 pC mm\u3csup\u3e-1\u3c/sup\u3e of travel distance inside the capillary.\u3c/p\u3

Electric field and temperature in a target induced by a plasma jet imaged using Mueller polarimetry

Mueller polarimetry is used to investigate the behavior of an electro optic target (BSO crystal) under exposure of guided ionization waves produced by an atmospheric pressure plasma jet. For the first time, this optical technique is time resolved to obtain the complete Mueller matrix of the sample right before and after the impact of the discharges. By analyzing the induced birefringence, the spatial profiles and local values are obtained of both the electric field and temperature in the sample. Electric fields are generated due to deposited surface charges and a temperature profile is present, due to the heat transferred by the plasma jet. The study of electric field dynamics and local temperature increase at the target, due to the plasma jet is important for biomedical applications, as well as surface functionalization. This work shows how Mueller polarimetry can be used as a novel diagnostic to simultaneously acquire the spatial distribution and local values of both the electric field and temperature, by coupling the external source of anisotropy to the measured induced birefringence via the symmetry point group of the examined material

Imaging axial and radial electric field components in dielectric targets under plasma exposure

\u3cp\u3eThis work presents new ways to investigate the individual electric field components in a dielectric target induced by a non thermal atmospheric pressure plasma jet. Mueller polarimetry is applied to investigate electro-optic crystals under exposure of guided ionization waves produced by a plasma jet. Three different cases are examined to visualize the different electric field components induced in the crystals by charges deposited on the surface by impact of the ionization waves. Investigating a Bi\u3csub\u3e12\u3c/sub\u3eSiO\u3csub\u3e20\u3c/sub\u3e (BSO) crystal at normal incidence allows measurement and visualization of the axial field, while if the crystal is examined at 45° both radial and axial electric field components are combined. For the first time, a Fe:LiNbO\u3csub\u3e3\u3c/sub\u3e (Felinbo) crystal is examined using Mueller polarimetry under influence of a plasma jet. In this case, exclusively the patterns and local values of the radial field are obtained and not the axial field. These unique imaging options in the target for the individual electric field components allow a new and more complete investigation of the dynamics of surface discharges on dielectric materials.\u3c/p\u3

Revealing plasma-surface interaction at atmospheric pressure:imaging of electric field and temperature inside the targeted material

\u3cp\u3eThe plasma-surface interaction is studied for a low temperature helium plasma jet generated at atmospheric pressure using Mueller polarimetry on an electro-optic target. The influence of the AC kHz operating frequency is examined by simultaneously obtaining images of the induced electric field and temperature of the target. The technique offers high sensitivity in the determination of the temperature variation on the level of single degrees. Simultaneously, the evolution of the electric field in the target caused by plasma-driven charge accumulation can be measured with the threshold of the order of 10\u3csup\u3e5\u3c/sup\u3e V/m. Even though a specific electro-optic crystal is used to obtain the results, they are generally applicable to dielectric targets under exposure of a plasma jet when they are of 0.5 mm thickness, have a dielectric constant greater than 4 and are at floating potential. Other techniques to examine the induced electric field in a target do not exist to the best of our knowledge, making this technique unique and necessary. The influence of the AC kHz operating frequency is important because many plasma jet designs used throughout the world operate at different frequency which changes the time between the ionization waves and hence the leftover species densities and stability of the plasma. Results for our jet show a linear operating regime between 20 and 50 kHz where the ionization waves are stable and the temperature increases linearly by 25 K. The charge deposition and induced electric fields do not increase significantly but the surface area does increase due to an extended surface propagation. Additionally, temperature mapping using a 100 μm GaAs probe of the plasma plume area has revealed a mild heat exchange causing a heating of several degrees of the helium core while the surrounding air slightly cools. This peculiarity is also observed without plasma in the gas plume.\u3c/p\u3

Excitation and relaxation of the asymmetric stretch mode of CO2 in a pulsed glow discharge

The excitation and relaxation of the vibrations of CO2; as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed, being (1) time-resolved in situ Fourier transform nfrared (FTIR) spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3-6.7 mbar and a current range of 10-50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T3) to the rotational temperature (Trot) is found to be only dependent on Trot, in the conditions under study. The cay rate ρT3-Trot follows the relation ρT3-Trot = 388 s-1 exp(Trot - 273 K)/(154 K). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05 to 0.18 for 10 mA to 50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge