Positive Space Charge Research Articles

Ionic wind produced by volume corona discharges and surface dielectric barrier discharges: What role do streamers play?

The present study compares the ionic wind produced by volume DC and AC corona discharges, and by surface dielectric barrier discharges (DBD). On the one hand, in the case of a volume corona discharge ignited between a high-voltage needle and a grounded plate, our measurements highlight that the ionic wind velocity increases in the presence of positive breakdown streamers. On the other hand, in the case of a surface AC DBD, the ionic wind velocity decreases when streamers occur. Why such a difference? The answer is not easy and the debate remains open. However, one answer would be that the streamers occurring in a volume needle-to-plate discharge leave an abundance of positive ions in the inter-electrode space and that these ions drift because of the electric field, just after the streamer propagation. On the other hand, in the case of a surface DBD, the streamers can leave positive ions in their wake but their heads especially deposit positive ions at the location where they stop propagating, i.e. a few millimetres from the electrode (up to about 10–15 mm). Then this positive space charge deposited at a few millimetres from the active electrode edge on the dielectric surface acts as a screen against the electric field due to the applied high voltage, thus preventing the drift of the ions remaining on the surface of the dielectric, close to the electrode edge. Having said that, the reality is that this explanation is certainly very simplistic compared with the very complex phenomena taking place in these two discharges, particularly at the times when the streamers form and propagate.

Non-stationary 3D Perturbation Theory for Describing Nonlinear Interaction of Electric Field with Matter in Inhomogeneous Plasma with Current. Vysikaylo’s Electric Field Shock Waves and Plasma Nozzles

In our works we theoretically prove that the cumulation (self-focusing) of charged particles in a inhomogeneous plasma (with current) is a universal property of cumulative-dissipative structures with characteristic sizes from 10-15 to 1026 m. Electrical phenomena are observed in non-uniform atmospheres and ionospheres of all planets of the Solar System. In this paper, the most complete theory of perturbations to describe phenomena in gas discharge plasma is formulated and, based on this theory, the most complete classification of ambipolar transfer processes in plasma with current and the classification of non-uniform plasma parameter profiles are given. We compare our theoretical results with existing experiments and results obtained in other areas of the natural sciences. In this work, we theoretically (Part 6.1) and experimentally (Part 6.2) we prove that shock waves of electric fields are focusing shells for inhomogeneous plasma cumulative-dissipative positively charged 3D structures. In the Part 6.1 of this work, we provide detailed theoretical justifications for the possibility of the existence of (locally self-focusing by ambipolar drift) Vysikaylo’s electric field shock waves caused by ambipolar diffusion due to a violation of the electrical neutrality of the plasma (in the presence of an electric current). Due to the greater mobility of electrons (ions are more massive), a structure with a positive space charge is formed in the electric field shock waves that self-form in the plasma (with current). Unlike Mach’s shock waves, in closed Vysikaylo’s shock waves transverse electric fields are generated due to the space charge. This makes the problem (in the electric field shock wave region) three-dimensional (in particular, spherically or cylindrical symmetric in this region). In Part 6.1, we will limit ourselves to the study of stationary one-dimensional profiles: 1) parameters in shock waves of the electric field and 2) processes of ambipolar drift, leading to local cumulation of positive charge in the shock wave of the electric field. In Part 6.1, the author will limit himself to obvious remarks arising from the properties of three-dimensional structures with a positive space charge. Based on laboratory 3D experiments (Part 6.2) and theoretical studies of gas-discharge plasma, we prove that ambipolar drift caused by different dependences of the mobility of electrons and positive ions in a simple plasma (with one type of ions) determines the dynamic processes of cumulation of plasma structures – 4D plasmoids in plasma (with current). 4D plasma structures are non-stationary three-dimensional structures. The author draws attention to self-formation in plasma structures (plasmoids) of stationary Vysikaylo’s plasma nozzles - analogues of Laval’s nozzles. A comparison of theoretical 1D and experimental 3D observations of discharge glow (this corresponds to changes in the main parameters) in gas discharge tubes will be presented in Part 6.2. In these experiments, a homogeneous plasma in a gas discharge tube is locally disturbed by a beam of fast electrons. This leads to the self-formation: 1) of electric field shock waves (a layer of positive volume charge) stopped by gas pumping and 2) of transition 3D profiles and Vysikaylo’s plasma 3D nozzles already in a quasi-neutral inhomogeneous plasma. In this work, we were the first to theoretically and experimentally study the processes of nonlinear ambipolar transport caused by the violation of electrical neutrality and 3D interaction of electric fields with matter (charged particles) in an inhomogeneous plasma with current. For the first time it has been proven that the coefficients of ambipolar diffusion due to the violation of electroneutrality are vectors determined by the electric field vector.

Temporal evolution and mechanism of secondary streamers in N2/O2 mixtures at atmospheric pressure under DC voltage

In this work, the temporal evolution of secondary streamers in N2/O2 mixtures at atmospheric pressure under DC voltage were investigated using a two-dimensional axisymmetric fluid model. The effects of voltage amplitude, polarity and oxygen concentration were analyzed. The propagation of secondary streamers is divided into two stages: fast propagation followed by slow propagation. The interior of secondary streamers is filled with positive space charges, while the exterior contains negative space charges. The reduced electric field in the secondary streamer is initially linearly proportional to the applied voltage and decreases linearly with the length of secondary streamers. The length of secondary streamers is primarily determined by the applied voltage, regardless of the oxygen concentration. A reversed propagation direction is simulated under negative applied voltage. We propose a different mechanism for secondary streamers. The high-density positive charge layer near the electrode initiates the propagation of secondary streamers, and the extension of positive charge region dominates the propagation. The distribution of electric field along the channel is mainly determined by the distribution of net space charge. The propagation direction is opposite to the direction of electron drift. The secondary streamer eventually stagnates due to the decrease in electron density in high oxygen concentration, while it is caused by the uniform current density at the front of the secondary streamer in low oxygen concentration.

Large-volume CdZnTe bar detectors characterized by laser-induced transient currents

In this work, we studied the free-carrier transport properties and space-charge formation/polarization in a CdZnTe bar-shaped radiation detector using the Laser-Induced Transient Current technique. We found out that a steady-state space charge ranging from 8 × 108 to 2.1 × 109 cm−3 is formed throughout the detector at about 0.5 s following biasing. The measured current waveforms were modeled by Monte Carlo simulations after taking into account the electric and weighting fields inside the detector and shielding box. Modeling of the unpolarized waveforms revealed an exceptionally high electron mobility-lifetime product μeτe≥0.095cm2V−1. The observed formation of positive space charge in the biased detector and linear scaling of the current waveform shapes on applied bias revealed that the detector polarization is attributed to carrier injection at the anode. Measurements in pulsed and DC bias in modified electrode geometry proved the surface-charge formation.

Cathode sheath parameters and their influences on arc root behavior after liquid metal bridge rupture in atmospheric air

The cathode sheath (CS) formation of the direct current air circuit breaker is simulated by a fluid model, and the influence of metal vapor concentration between the contacts after liquid metal bridge rupture is considered. The CS conductivity increases with the increasing concentration of copper vapor. The copper vapor concentration increases from 5% to 95%, and the thickness of the positive space charge layer and ionization layer increases from 22.3 and 49.1 μm to 51.8 and 81.7 μm, respectively. Increasing the CS conductivity is beneficial for the motion of arc roots in a certain range.

Dimensional analysis on microscale gas breakdown with electric field nonuniformity and positive space charge effects

A dimensional method was employed to evaluate the microscale gas breakdown characteristics at atmospheric pressure, resulting in a universal breakdown curve applicable to different types of gases (e.g., Ar, Xe, Ne, and N2). As the gap distance decreases, the breakdown mode transitions from ion-induced secondary electron emission to the field emission regime. In the field emission regime, the positive space charge effect becomes more significant. We discovered that incorporating the positive space charge effect in the field emission regime can be achieved by modifying the local electric field enhancement factor β. Consequently, we propose an effective electric field enhancement factor, βeff, which scales linearly with β, to accurately reproduce the breakdown curve while considering the positive space charge effect. This proposed approach significantly simplifies the numerical model. Additionally, we examined the effects of gas pressure, gap distance, cathode properties (e.g., work function and secondary electron emission coefficient), and electric field nonuniformity.

Simulation analysis on microscopic discharge characteristics of the bipolar corona of a floating conductor

A floating conductor exhibits a bipolar corona phenomenon with microscopic discharge characteristics that are still unclear. In this study, a plasma simulation model of the bipolar corona with 108 chemical reaction equations is established by combining hydrodynamics and plasma chemical reactions. The evolution characteristics of electrons, positive ions, negative ions and neutral particles, as well as the distribution characteristics of space charges are analyzed, and the evolutionary flow of microscopic particles is summarized. The results indicate that the positive end of the bipolar corona initiates discharge before the negative end, but the plasma chemistry at the negative end is more vigorous. The electron generation rate can reach 1240 mol (m3 s)−1, and the dissipation rate can reach 34 mol (m3 s)−1. The positive ion swarm is dominated by O+ 4, and the maximum generation rate can reach 440 mol (m3 s)−1. The negative ion swarm is mainly O− 2 and O− 4. The O− 2 content is approximately 1.5–3 times that of O− 4, and the maximum reaction rate can reach 51 mol (m3 s)−1. The final destination of neutral particles is an accumulation in the form of O3 and NO, and the amount of O3 produced is approximately 4–6 times that of NO. The positive end of the bipolar corona is dominated by positive space charges, which continue to develop and spread outwards in the form of a pulse wave. The negative end exhibits a space charge distribution structure of concentrated positive charges and diffused negative charges. The validity of the microscopic simulation analysis is verified by the macroscopic discharge phenomenon.

Thermal Aging Properties of 500 kV AC and DC XLPE Cable Insulation Materials.

Despite similar material composition and insulation application, the alternating current (AC) cross-linked polyethylene (XLPE) and direct current (DC) XLPE materials cannot replace each other due to different voltage forms. Herein, this work presents a systematical investigation into the effects of thermal aging on the material composition and properties of 500 kV-level commercial AC XLPE and DC XLPE materials. A higher content of antioxidants in the AC XLPE than in the DC XLPE was experimentally demonstrated via thermal analysis technologies, such as oxidation-induced time and oxidation-induced temperature. Retarded thermal oxidation and suppression of space charge effects were observed in thermally aged AC XLPE samples. On the other hand, the carbonyl index of DC XLPE dramatically rose when thermal aging was up to 168 h. The newly generated oxygen-containing groups provided deep trapping sites (~0.95 eV) for space charges and caused severe electric field distortion (120%) under -50 kV/mm at room temperature in the aged DC XLPE samples. For the unaged XLPE materials, the positive space charge packets were attributed to the residue crosslinking byproducts, even after being treated in vacuum at 70 °C for 24 h. Thus, it was reasoned that the DC XLPE material had a lower crosslinking degree to guarantee fewer crosslinking byproducts. This work offers a simple but accurate method for evaluating thermal oxidation resistance and space charge properties crucial for developing high-performance HVDC cable insulation materials.

Revealing pre-breakdown processes of a single-gap pseudospark switch triggered from the anode side

Pseudospark switch is a special kind of low-pressure (typically 1–100 Pa) high-voltage pulsed discharge switch with the hollow electrode configuration. This paper experimentally investigates the pre-breakdown processes of a single-gap pseudospark switch triggered from the anode side. The species and contents of impurities on copper electrode surface are firstly analyzed, which mainly include C, N and O. Then, at the same conditions, the trigger delay and jitter of triggering from the anode side are significantly higher than the cathode side, as well as the minimum voltage and pressure that the switch can be triggered. The evolution of the discharge region of the pseudospark switch triggered from the anode side is observed by the ICCD camera for the first time in the literature. The time-resolved images show that the luminescence firstly appears in the hollow anode where the trigger discharge happens and then in the hollow cathode, the cathode hole, the main gap and the anode hole successively, which is completely different from triggering from the cathode side. Finally, a model considering the migration and collisions of ions and fast atoms is established, and it points out that the ion flux from the hollow anode plays an important role in triggering from the anode side. The key processes also include the formations of the positive space charge column inside the hollow cathode and the intense secondary electron emission as an electron source for the main gap.

Analysis of Impact Ionization Effects on Current Collapse of AlGaN/GaN HEMTs

Computer-aided analysis of impact ionization effects on turn-on characteristics or current collapse of AlGaN/GaN HEMTs is performed. Here, an intrinsic semi-insulating buffer is adopted in which deep donors are assumed to compensate deep acceptors, then the ionized deep donor usually plays a role as an electron trap. Calculated turn-on characteristics show that when impact ionization is not included, the drain current begins to increase relatively slowly because electrons are emitted from the deep donors, showing a large current collapse. On the other hand, when impact ionization is included and an ON-state drain voltage <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {D} {{ \text {ON}}}}$ </tex-math></inline-formula> is high, generated holes between the gate and drain flow toward the buffer and are captured by neutral deep donors, particularly at the source side. The hole capturing time becomes relatively short when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {D} {{ \text {ON}}}}$ </tex-math></inline-formula> is high and the hole density is high. Because of these increases in positive space charges in the buffer, the drain current increases relatively fast before the electron emission under the gate starts. Therefore, the current collapse becomes weaker when the impact ionization is considered and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {D} {{ \text {ON}}}}$ </tex-math></inline-formula> is higher. The situation may be similar with a different type of buffer such as an Fe-doped semi-insulating buffer in which the Fe-originated level (deep acceptor) is above the midgap and it usually plays a role as an electron trap.

Fast space charge packet characteristics within low-density polyethylene affected by epitaxial crystallization

The formation of space charge packet increases the maximum electric field within HVDC polymeric cable insulation. However, the influence of epitaxial crystallization on transient characteristics of space charge packet behavior in polyethylene (LDPE) under high temperature is still unrevealed. Therefore, polytetrafluoroethylene (PTFE), isotactic polypropylene (iPP), and glass (G) were adopted as the substrates to prepare LDPE samples with different epitaxial crystallization, denoted as PE-PTFE, PE-iPP, and PE-G respectively, for the measurement of space charge packet at 60 °C under −125 kV/mm. The experimental results show that space charge packets with both positive and negative polarity occurred immediately in LDPE samples after polarization, whose mobility and rising rate of electric field are at least 10 times higher than that at normal temperature. The injection amount of positive space charge packet arranged as PE-PTFE > PE-G > PE-iPP. According to the epitaxial crystallization characteristics of parallel (PE-PTFE), crossed (PE-iPP), and random (PE-G) arrangement of molecular chain, as well as the feature that hole migration is tend to be affected by molecular chain arrangement, the epitaxial crystallization of PE-iPP is contributed to the transport within inter-molecules, resulting in the difficulty of the appearance of positive space charge packet. The obtained results in this paper are important to have deep understanding for space charge behavior within cable insulation.

Investigation of transport properties of perovskite single crystals by pulsed and DC bias transient current technique

Time-of-flight (ToF) transient current method is an important technique to study the transport characteristics of semiconductors. Here, both the direct current (DC) and pulsed bias ToF transient current method are employed to investigate the transport properties and electric field distribution inside the MAPbI3 single crystal detector. Owing to the almost homogeneous electric field built inside the detector during pulsed bias ToF measurement, the free hole mobility can be directly calculated to be about 22 cm2⋅V−1⋅s−1, and the hole lifetime is around 6.5 μs–17.5 μs. Hence, the mobility-lifetime product can be derived to be 1.4 × 10−4 cm2⋅V−1–3.9 × 10−4 cm2⋅V−1. The transit time measured under the DC bias deviates with increasing voltage compared with that under the pulsed bias, which arises mainly from the inhomogeneous electric field distribution inside the perovskite. The positive space charge density can then be deduced to increase from 3.1 × 1010 cm−3 to 6.89 × 1010 cm−3 in a bias range of 50 V–150 V. The ToF measurement can provide us with a facile way to accurately measure the transport properties of the perovskite single crystals, and is also helpful in obtaining a rough picture of the internal electric field distribution.

Investigation of the interaction of dense noble gas plasmas with cold cathodes: I—Experimental setup and application to Al, Cu, Ti, and graphite cathodes

AbstractThe formation of arc spots on cold cathodes, called arcing, is an interference factor in all kinds of plasma technical devices. It is investigated by igniting an arc in a clean atmospheric pressure Ar or Kr filling of a stainless steel vessel. It is brought into interaction by a magnetic blast field with a negatively biased commutation electrode (CE), formed by a rod with a diameter of 2 mm consisting of pure Al, Cu, Ti, or graphite; its end face is polished with diamond grinding powder. The arc commutation is observed by short‐time photography, streak camera records, and temporally highly resolved optical spectroscopy for different applied voltages. The emission of ion lines of the filling gas and of vaporized electrode material by the plasma in front of the cathode is indicating the formation of a positive space charge layer in front of its surface. It provides the ions, which induce secondary electron emission of the cathode surface and with it, the arc commutation. The commutation time tc elapsing between a signal generated by the arc plasma in front of the CE and the beginning of a current flow through the electrode increases for low voltages with increasing permittivity of a surface layer formed by electrically insulating metal oxide and decreases with increasing electrical conductivity of the layer. It is lowest for graphite without an oxide layer. On scanning electron microscope pictures, taken of the end face of the electrode after arc commutation, craters with diameters of less than 1 μm are visible.

Space charge formation in the high purity semi-insulating bulk 4H–silicon carbide

We investigated the carrier transport and space charge formation in the high purity semi-insulating 4H-SiC bulk single crystal. Using the Laser–induced Transient Current Technique we observed anomalous short current waveform oscillations that are caused by very short carrier lifetime ≤ 1 ns. We conclude from a detailed inspection of current waveform shape completed by the analysis of collected charge that the electron mobility decreases in an increasing electric field. The saturation value of electron drift velocity 8.7 × 106 cm2s-1 was evaluated. The Hall effect measurement revealed the n-type electrical conductivity and dominant donor energy of ED = 0.85 eV. We also observed the blocking character of prepared Au contacts, which induce the electron depletion and positive space charge formation in biased sample. The continuous increase of the space charge in a wide time interval of 10-1 s – 104 s, attested to testified on a presence of a dispersed impurity band localized below the Fermi energy. Based on these findings, we proposed a theoretical model describing the observed phenomena very well.

Low-Temperature Annealing of CdZnTeSe under Bias.

We performed a gradual low-temperature annealing up to 360 K on a CdZnTeSe radiation detector equipped with gold and indium electrodes under bias at both polarities. We observed significant changes in the detector’s resistance and space-charge accumulation. This could potentially lead to the control and improvement of the electronic properties of the detector because the changes are accompanied with the reduction in the bulk dark current and surface leakage current. In this article, we present the results of a detailed study of the internal electric field and conductivity changes in CdZnTeSe detector for various annealing steps under bias taking into account different polarities during annealing and subsequent characterization. We observed that low-temperature annealing results in an increase in the barrier height at the contacts that, in general, reduces the dark current and decreases the positive space charge present in the sample compared to the pre-annealed condition.

EMERGING TRENDS IN WALL-FREE HALL THRUSTERS DEVELOPMENT

2D-hybrid model was created for the proposed new type accelerator with a virtual cathode which allows to avoid sputtering of the cathode surface and to preserve the dynamics of accelerated ions. In the framework of the model, it was shown that ions first form a positive space charge in the system center, and eventually, under an ac-tion of created own electric field, emerge from both ends of the system.

Revisiting the theory of positive glow corona with a comprehensive kinetic scheme

The theory of positive glow corona is revisited by performing a detailed simulation based on the fluid model with a comprehensive kinetic scheme (CKS). The kinetic scheme includes 28 species, 127 chemical reactions for N2/O2 mixture. The simulation results are compared with that from an averaged kinetic scheme (AKS) which considers only five species and seven reactions. The two models give similar predictions in terms of discharge current. However, several differences in details of the results bring new physical insights to the theory of positive glow corona. It is shown that, the model with the AKS underestimates the detachment effect that plays an important role in positive glow corona discharge. Moreover, the wave-front of the current pulse calculated by the model with the CKS has a smaller steepness, a lower peak-to-peak value, and a longer rise time. These results are in better agreement with the experimental measurement reported in the literature. It is also found that the positive space charge of glow corona is dominated by O2 + in the ionization layer and by O4 + in the drift region far away from the anode. Negative ions are produced in the ionization layer close to the anode and the main species are O− and O3 −.

The characteristics of ion wind thruster based on dielectric barrier discharge in near space

The experimental results show that when the pressure is constant, with the increase of power, the thrust-power ratio of the thruster decreases, and the wind speed and thrust increase, but the increase rate gradually decreases and finally tends to be stable；When the pressure is 5.5 kPa, The rate of increase in thrust decreases the slowest, that is, the thruster has the maximum thrust potential at 5.5 kPa, and when the input power is greater than 75 W, the relative maximum thrust can be generated, reaching 14.8 μN; At the same time, with the increase of input power, the thrust - power ratio increases first and then decreases; When the input power is constant in the range of 15–65 W, as the air pressure increases, the wind speed decreases, and the thrust and thrust-power ratios all have local extremes, that is, when the air pressure is 8 kPa, it reaches the regional maximum, the maximum is 12.7 μN and 0.45 μN/W, respectively, at 16 kPa, it reaches the minimum value of the area, and the minimum is 4 μN and 0.14 μN/W, respectively. By comparing the discharge images under different atmospheric pressures, it can be concluded that the thrust generated by the accelerated collision of particles with a single short-time large current pulse is less than that generated by the accelerated collision of ions with a long-time small high-frequency pulse discharge, and the particle collision probability is the main reason affecting the wind speed; Meanwhile, with the increase of input power, the “virtual electrode” formed by the accumulation of positive ion space charge reduces the increase rate of wind speed and thrust.

Expeditionary research of electrical processes on a high mountain plateau taking into account aerosols and humidity

The work continues the analysis of the results of complex expeditionary measurements in the surface atmosphere, some of which have recently been published. It uses data from expeditions of the Laboratory of Geophysical Research of the Faculty of Physics of the Southern Federal University at research stations of the Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences: 2017 at the Tsimlyansk Research Station in the steppe zone, 2018 and 2019 at the Kislovodsk alpine station located on the Shadzhatmaz plateau, 2.100 m above sea level. With the help of regression analysis, empirical relationships of the electric potential gradient near the earth’s surface and the concentration of aerosols with a diameter of more than 0.1 μm were obtained for four measurement periods. The investigated dependence has a linear character. Judging by the results obtained, the space charge of aerosols for the arid steppe zone is negative. For the Shadzhatmaz plateau, the results indicate the presence of a positive space charge in the atmosphere under high humidity conditions. The discovered facts are considered as a consequence of the differences in the physicochemical properties of aerosol particles under different conditions. Proceeding from the results of measurements at the alpine station, regression analysis revealed a close relationship between the concentration of aerosols and relative humidity for different size ranges of particles, which is well approximated by an exponential function.

Simulation of positive streamer propagation in an air gap with a GFRP composite barrier

The puncture of glass fibre reinforced polymer (GFRP) laminate is a primary damage pattern of wind turbine blades due to lightning strikes. A numerical simulation model of positive streamer propagation in a needle-to-plate air gap with a GFRP laminate is established to investigate the breakdown mechanism of GFRP laminate. The model not only considers the dynamics of charged particles in the air and the composite laminate, but also the current continuity at gas–solid interfaces. The simulated streamer discharge pattern and the surface streamer length are in good agreement with the observation results. The distributions and evolutions of the electron number density, electric field, and surface charge densities during streamer propagation are obtained. It is found that the enhancement of the electric field on the GFRP laminate is caused by the rapid deposition of positive and negative space charges on the GFRP laminate after a secondary streamer incepts on the lower surface of the GFRP laminate. The effects of the applied voltage, relative permittivity, and thickness of the GFRP laminate on the electric field on the GFRP laminate are investigated. The obtained results could assist in further understanding of the mechanism of GFRP wind blade breakdown due to lightning strikes.