Atmospheric pressure plasma jets:properties of plasma bullets and the dynamics of the interaction with dielectric surfaces

Cold atmospheric pressure plasma jets, although mostly researched for applications in surface treatment, are rarely investigated in the presence of a surface. This paper presents the properties of plasma bullets formed in the capillary as well as the dynamics of the propagation of the plasma on dielectric surfaces as a function of applied voltage amplitude, gas flow, jet source geometry and the travelling length of the bullet

Atmospheric pressure plasma jets in contact with a dielectric surface:the electric field and the charge at the surface

Atmospheric pressure plasma jets are in the current research focus, as they are cheap and simple to make, yet have shown tremendous potential in material modification or biomedical applications. The common point is the presence of a target in the effluent of the jet. Targets can be metallic, dielectric or made of biological material, however in all cases they modify the plasma to a certain extent and it is therefore necessary to perform research on non-thermal atmospheric pressure plasma sources in the presence of a target.The focus of this paper is the interaction of a kHz helium plasma jet and a dielectric target, more precisely the electric field and the charge at the surface of the target. The Pockels’ method was used to measure the electric field on the surface between 3 mm and 10 mm away from the nozzle of the plasma jet, as a function of the gas flow and applied voltage. The obtained field is of the order of magnitude of 105 V/m.<br/

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

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

A 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-1. 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-1 of travel distance inside the capillary

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

This 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 Bi12SiO20 (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:LiNbO3 (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

Investigation of a plasma–target interaction through electric field characterization examining surface and volume charge contributions: modeling and experiment

Numerical simulations and experiments are performed to better understand the interaction between a pulsed helium plasma jet and a dielectric target. The focus of this work lies on the volume and surface charge influence on the electric field distribution. Experimentally, the electric field due to surface charges is measured inside an electro-optic target under exposure of a plasma jet, using the optical technique called Mueller polarimetry. For the first time, the time-resolved spatial distributions of both the axial and radial components of electric field inside the target are obtained simultaneously. A 2D fluid model is used in a complementary way to the experiments in order to study separately the contribution of volume charges and surface charges to the spatio-temporal evolutions of the electric field during the plasma–surface interaction. The experimental investigation shows that the average axial and radial components of electric field inside the dielectric target, only due to surface charges, are lower than generally reported for electric field values in the plasma plume. Thanks to the phenomenological comparison with experiments, simulations show that during the plasma–surface interaction two effects sequentially determine the electric field inside the target: firstly, a relatively high electric field is observed due to the proximity of the ionization front; afterwards, in longer timescales, lower electric fields are induced due to the contribution of both leftover volume charges close to the target and surface charges deposited on its surface. The experimental technique provides a unique way to examine this second phase of the plasma–surface interaction

Electric field measurements in atmospheric pressure plasmas

Low temperature plasmas at atmospheric pressure offers possibilities that were not accessible to plasma-based technologies for a long time, such as usage on materials sensitive to high temperatures, (bio)materials that are not resistant to vacuuming or even fully drying, (bio)targets that are sensitive to significant current transfer. In addition to the simplicity with which the plasma sources can be built and the ease with which they can be operated, lot temperature plasmas have become very popular in the recent years. A great number of scientific publications has followed this rise in interest for atmospheric pressure plasmas, covering different geometries of mostly dielectric barrier discharges (DBDs), used with or without gas flow and a wide range of excitation frequencies from Hz to MHz. Most commonly reports address the discharge dynamics, densities of heavy species, at times gas temperature measurements, imaging of flow fields and rarely electron densities and electric field but very few on electric field measurements. This paper will give an overview of the recent work in the electric field measurements in atmospheric pressure plasma jets that operate in the ’bullet mode’. A Helium jet with flow rates up to 2 SLM, in a low-power mode (up to 1 W dissipated in the discharge). The jet is run in the bullet mode where one plasma bullet is emitted per voltage period. The results fill focus on the comparison between the jets driven by 30 kHz sine voltage and jets driven by short high voltage pulses. Two measurement methods have been used that allow for comparison between the electric field in the gas phase and on the treated targets, which vary from dielectrics to liquids