Search

Multiplication of primary electrons at a dielectrics/gas interface exposed to an external electric field leads to the formation of ultra-fast contracted ionisation waves called surface streamers with high local electric field in their microscopic heads. We study this phenomenon in coplanar barrier discharge in atmospheric pressure air where streamers of both polarities are generated simultaneously. The electric field strength development in the discharge is determined experimentally using time-correlated single photon counting enhanced optical emission spectroscopy with high spatiotemporal resolution. We show that very high values of the electric field of even above 1200 Td (approx. 300 kV cm(-1)) can be a physical reality for positive streamer directly at the dielectric surface. The peak value for negative streamer lies under 200 kV cm(-1). In parallel, a two-dimensional fluid model describes the rapidly changing electric field and the spatiotemporal discharge dynamics in good agreement with the experimental results. We also present the theoretical results of spatiotemporal development of electron density and of charge transfer to the dielectric surface via a collapsing sheath as the discharge structure decays within a very few nanoseconds.

Liquid organic scintillators are important devices for measurements of neutron radiation. This work aims to develop and optimize the composition of liquid organic scintillators so it can be used for fast neutron spectrometry. As the neutron radiation is usually accompanied with γ ray radiation, it is important to have quality γ/n discrimination. The new cocktail for house made liquid organic scintillator is prepared and studied with intention of being able to separate gamma and neutron for neutron energies above 0.5 MeV while keeping lower constraints on practical use (e.g., sealing because of oxygen) than commercial liquid scintillators. In preceding work the composition of liquid scintillators was optimized. Two twocomponent scintillators were selected for further studies. Solvent DIPN (Di-iso-propyl-naphthalene Mixed Isomers) is selected for both. First is mixed with luminophore PYR (1-Phenyl-3-(2,4,6-trimethyl-phenyl)-2-pyrazoline) of concentration 5 g/l. Second is mixed with luminophore THIO (2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene) of concentration 5 g/l. In this work the response of scintillator to monoenergetic beam of neutrons was measured for multiple neutron energies at PTB in Braunschweig. The two parameter spectrometric system NGA-01 is used to analyze the energy and discrimination characteristics. 137 Cs and 60 Co are used as radiation sources for calibration with pure γ rays. Then the response of scintillator for same neutron energies was simulated using GEANT4. The dissipated energy in the scintillator in response to monoenergetic neutrons is obtained. Both, measured and simulated responses are compared. Functional dependence for yield of recoiled products is estimated. It is seen that main recoil product hydrogen proton is well observed in both. From the edge of proton response one can assume the yield for given neutron energy. The recoiled carbon ion (from elastic collision) is on the other side difficult to observe in measured results but clearly seen in dissipated energy plots. It suggests that yield of carbon ion is very small relatively to proton yield. These results will serve as basis for response function evaluation of scintillator which is necessary for evaluation of unknown neutron spectra from measurements with scintillator.

Electron and ion currents to a cylindrical Langmuir (electrostatic) probe were calculated using the particle-in-cell/Monte Carlo (PIC/MC) self-consistent simulation for a neutral gas in the pressure range 2–3,000 Pa. The simulation enables us to calculate the probe currents even at high neutral gas pressures when the collisions of collected charged particles with neutral gas particles near the probe are important. The main aim of this paper is the calculation of probe currents at such high gas pressures and the comparison of the results with experimentally measured probe currents. Simulations were performed for two cases: (a) probes with varying radii in a non-thermal plasma with high electron temperature at low neutral gas pressure of 2 Pa (in order to verify the correctness of our simulations), and (b) probe with the radius of 10 um in the afterglow plasma with low electron temperature and a higher neutral gas pressure (up to 3,000 Pa). The electron probe currents obtained in case (a) show good agreement with those predicted by the orbital motion limited current (OMLC) theory for probes with radii up to 100 um for the given plasma conditions. At larger probe radii and/or at higher probe voltages, the OMLC theory incorrectly predicts too high an electron probe current for the plasma parameters studied. Additionally, a formula describing the spatial dependence of the electron density in the presheath in the collisionless case is derived. The simulation at higher neutral gas pressures, i.e. case (b), shows a decrease of the electron probe current with increasing gas pressure and the creation of a large presheath around the probe. The simulated electron probe currents are compared with those of measurements by other authors, and the differences are discussed.

Liquid organic scintillators are important materials for measurements of neutron radiation. This work aims to develop and optimize the composition of liquid organic scintillators so they can be used for fast neutron spectrometry . As neutron radiation is usually accompanied with γ ray radiation, this work is focused on γ /n discrimination. It is shown that Di-iso-propylnaphthalene Mixed Isomers and 1-Methylnaphthalene are capable of good γ /n discrimination. Afterwards, the concentration of luminophores is varied and it is shown that the PYR and THIO luminophores with concentrations above 3 g/l have Figure of Merit FOM > 1 . 27 above the energy threshold of 0 . 14 MeVee and 0 . 17 MeVee , respectively.

Formation of filamentary gas discharge forms, commonly referred to as streamers, is one of the conditions required for initiation and subsequent propagation of lightning leaders. It is quantitatively demonstrated that streamers can be initiated under thunderstorm conditions when two precipitation particles cause an enhancement of the electric field by passing in close vicinity of each other. Conditions for avalanche to streamer transition are documented using a model of two spherical hydrometeor particles placed in uniform ambient field. The results are presented in scaled form using similarity relations for gas discharges and can be applied for a wide range of thunderstorm conditions, including different air pressures, electric fields and particle dimensions.

The Particle In Cell/Monte Carlo Collisions (PIC/MCC) simulation was used for the calculation of electron and ion currents to a spherical Langmuir (electrostatic) probe. This simulation took into account the collisions of collected charged particles with neutral gas particles around the probe and it can calculate the probe currents at higher neutral gas pressures.

This work analyzes different current source and conductivity parameterizations and their influence on the diurnal variation of the global electric circuit (GEC). The diurnal variations of the current source parameterizations obtained using electric field and conductivity measurements from plane overflights combined with global Tropical Rainfall Measuring Mission satellite data give generally good agreement with measured diurnal variation of the electric field at Vostok, Antarctica, where reference experimental measurements are performed. An approach employing 85 GHz passive microwave observations to infer currents within the GEC is compared and shows the best agreement in amplitude and phase with experimental measurements. To study the conductivity influence, GEC models solving the continuity equation in 3‐D are used to calculate atmospheric resistance using yearly averaged conductivity obtained from the global circulation model Community Earth System Model (CESM). Then, using current source parameterization combining mean currents and global counts of electrified clouds, if the exponential conductivity is substituted by the conductivity from CESM, the peak to peak diurnal variation of the ionospheric potential of the GEC decreases from 24% to 20%. The main reason for the change is the presence of clouds while effects of 222Rn ionization, aerosols, and topography are less pronounced. The simulated peak to peak diurnal variation of the electric field at Vostok is increased from 15% to 18% from the diurnal variation of the global current in the GEC if conductivity from CESM is used. This research was supported by NSF under grant AGS-1135446. CESM can be obtained through http://www.cesm.ucar.edu. RBF model is available at https:// bitbucket.org/vbayona/gec_rbffd. FVS model is fully described in cited references. Ali data representing figures, current sources, and conductivity profiles used for simulations are available at https://doi.org/10.18113/033958.

This paper presents 2D simulations showing the dynamics of interaction between two counter-propagating helium discharges at atmospheric pressure. The discharges were generated in two glass tubes separated by a few centimeters in ambient air. First, we applied the same voltage to each electrode wrapped around each discharge tube. Simulation results show that discharges ignite in tubes and then propagate, approaching each other without merging, as observed in experiments. Using the optical emission of discharges, we have determined the minimum distance of approach of counter-propagating discharges. This minimum distance is of the order of the tube diameter, and varies with the tube radius and inversely to the applied voltage. For conditions with different applied voltages on the electrodes, simulation results show a connection of both counter-propagating discharges with an increase in the discharge emission in the interaction region. For conditions with a time delay between the voltage applied on both electrodes, based on the optical emission of discharges, we have observed that counter-propagating discharges approach each other without merging. The minimum distance is of the order of the tube diameter, as in the reference case, without time delay between both applied voltages. However, simulation results show that the electron density in the interaction region between both discharge channels is about one order of magnitude higher than that for the reference case.

Back discharge refers to any discharges initiated at or near a dielectric layer covering a passive electrode (Czech et al 2011 Eur. Phys. J. D 65 459–74). Back discharge activity is commonly observed in electrostatic precipitators. This study aims to contribute to increasing the fundamental understanding of back discharge phenomena by using a plasma fluid model. The modelling strategy only considers the region of back discharge development as a first approach, and the numerical simulation is complemented by an experimental study. Back discharge ignition is studied with a pinhole of radius 100 µm set in a dielectric layer. First, we have considered the criterion for back discharge ignition from an electrostatic point of view, and the numerical results confirm the major role of the surface charge density deposited on the dielectric layer. Then the dynamics of back discharge in the 'onset-streamer' regime (Masuda and Mizuno 1977/1978 J. Electrostat. 2 375–96) is described: the discharge ignites inside the pinhole, develops outside as a cathode-directed ionizing wave, before stopping. This regime is characterized by a current pulse and the corresponding optical emission. Results obtained in experiments and simulations are in good agreement. Furthermore, this discharge regime is independent of the pinhole radius (ranging from 75 to 150 µm) despite a change in the discharge shape. Finally, an increase in the initial negative ion density or Laplacian electric field is found to be responsible for the transition from 'onset-streamer' to 'space streamer' regime, which corresponds well with experimental observations.

International audience; This paper presents an experimental and numerical study of a pulsed air plasma discharge at atmospheric pressure propagating in a capillary glass tube. In this work, we have compared the discharge structures and the axial propagation velocities of discharges. First, we have studied a needle-to-plane configuration without tube. For applied voltages in the range 7-18 kV, we have observed in experiments and in simulations that a plasma ball starts to develop around the needle tip. Then, for applied voltages less than 14 kV, in experiments, the discharge rapidly splits into several streamer channels with a main axial streamer. In simulations, we have computed only the main axial discharge. For applied voltages higher than 14 kV, in experiments and in simulations, we have observed that the discharge propagates with a cone shape in the gap. For all studied voltages, a good experiment/modelling agreement is obtained on the axial propagation velocity of the discharge, which increases with the applied voltage. Then, we have studied the propagation of discharges inside capillary tubes with radii in the range 37.5-300 μm. In experiments and simulations, we have observed that for small tube radius, the discharge front is quite homogeneous inside the tube and becomes tubular when the tube radius increases. Experimentally, we have observed that the velocity of the discharge reaches a maximum for a tube radius slightly less than 100 μm. We have noted that for a tube radius of 100 μm, the discharge velocity is three to four times higher than the velocity obtained without tube. This clearly shows the influence of the confinement by a capillary tube on the discharge dynamics. In this work, we have only simulated discharges for tube radii in the range 100-300 μm. We have noted that both in experiments and in simulations, the velocity of the discharge in tubes increases linearly with the applied voltage. As the radius of the tube decreases, the discharge velocity derived from the simulations slightly increases but is less than the experimental one. We have noted that the discrepancy on the discharge velocity between experiments and simulations increases as the voltage increases.

International audience; This paper presents simulations of the dynamics of surface charging by an air plasma discharge at atmospheric pressure initiated by a needle anode inside a capillary glass tube. During the discharge propagation in the tube, the highest positive surface charge density is observed close to the point electrode. We have shown that during the discharge propagation, the positive surface charge is increasing behind the discharge front, while the electric field at the surface is decreasing. Then, we have studied the influence of the tube radius, its permittivity and the applied pulsed voltage on surface charges. We have shown that the surface charge density during the discharge propagation is inversely proportional to the tube radius and surface charge densities of 30-50 nC/cm for a tube with = 100 m and an applied voltage of 12 kV have been obtained. We have also noted that a higher permittivity results in a higher surface charge density and a faster surface charge deposition. Then we have shown that the surface charge deposited is proportional to the applied voltage. Finally, at the end of the voltage pulse, our simulations indicate that the positive surface charge deposited during the discharge propagation in the tube decreases to very low values in few nanoseconds.

This paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity εr on the discharge structure and dynamics. As a reference, we have used a relative permittivity εr = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100 µm and εr = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300–600 µm, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700 µm, the tube has no influence on the discharge which propagates axially. For a tube radius of 100 µm, when εr increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease in the tube radius and εr. Then, we have compared the relative influence of the value of the tube radius and εr on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of εr on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge.