Space Charge Build-up Research Articles

The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys

Zr-Nb alloys are known to perform better in corrosion and hydrogen pick-up than other Zr alloys but the mechanism by which this happens is not well understood. Atomistic simulations using density functional theory of both tetragonal and monoclinic ZrO2 were performed, with intrinsic defects and Nb dopants. The overall defect populations with respect to oxygen partial pressure were calculated and presented in the form of Brouwer diagrams. Nb is found to favour 5 + in monoclinic ZrO2 at all partial pressures, but can exist in oxidation states ranging from 5 + to 3 + in the tetragonal phase. Nb5+ is charge balanced by Zr vacancies in both phases, suggesting that contrary to previous assumptions, Nb does not act as an n-type dopant in the oxide layer. Clusters containing oxygen vacancies were considered, Nb2+ was shown to exist in the tetragonal phase with a binding energy of 2.4 eV. This supports the proposed mechanism whereby low oxidation state Nb ions (2 + or 3+) charge balance the build-up of positive space-charge in the oxide layer, increasing oxygen vacancy and electron mobility, leading to near-parabolic corrosion kinetics and a reduced hydrogen pick-up. Previous experimental work has shown that tetragonal ZrO2 transforms to the monoclinic phase during transition, and that during transition a sharp drop in the instantaneous hydrogen pick-up fraction occurs. The oxidation of lower charge state Nb defects to Nb5+ during this phase change, and the consequent temporary n-doping of the oxide layer, is proposed as an explanation for the drop in hydrogen pick-up during transition.

Experimental and numerical simulation of a TPC like set up for the measurement of ion backflow

Ion backflow is one of the effects limiting the operation of a gaseous detector at high flux, by giving rise to space charge which perturbs the electric field. The natural ability of bulk Micromegas to suppress ion feedback is very effective and can help the TPC drift volume to remain relatively free of space charge build-up. An efficient and precise measurement of the backflow fraction is necessary to cope up with the track distortion due to the space charge effect. In a subtle but significant modification of the usual approach, we have made use of two drift meshes in order to measure the ion backflow fraction for bulk Micromegas detector. This helps to truly represent the backflow fraction for a TPC. Moreover, attempt is taken to optimize the field configuration between the drift meshes. In conjunction with the experimental measurement, Garfield simulation framework has been used to simulate the related physics processes numerically.

Space charge formation in polyimide films and polyimide/SiO2 double-layer measured by LIMM

For Polyimides (PI) used as insulating inter-layers or passivation layers in microelectronics and power electronics, reliability and failure mechanisms are intimately linked to phenomena such as non-linear conduction, distortion of field distribution and electrical ageing that have all some relation to space charge processes. Knowledge about these space charge distributions is therefore of importance to optimize structures and designs involving PIs. However, in the thickness scale of interest for such applications, of the order of 10 μm, no results were reported for PI, and only scarce studies for other materials. Space charge measurements carried out on PI using the laser intensity modulation method (LIMM) are reported in this work. PI-containing metal-insulator-semiconductor structures were pre-stressed under DC fields up to 2 MV/cm. The impact of the doping type of the substrate (either n-type or, p-type silicon) has been evaluated to address the mechanisms of space charge formation. The nature of electrode substrate substantially impacts the measured charge amount and charge nature, pointing towards charge injection as the origin of space charge build-up. The interpretation is substantiated by results obtained using a SiO2 layer as a barrier against electronic injection from the substrate into the PI layer.

Methods for spectroscopic measurement of electric field in atmospheric pressure helium discharges

A short overview of the emission spectroscopy methods for measuring the macroscopic electric field in high pressure discharges with helium is given. The occurrence of macroscopic electric field is a consequence of the space charge buildup. It is a common feature of discharge sheaths, streamer heads and double layers. The spectroscopic methods are based on polarization-dependent Stark splitting and shifting of atomic lines in the presence of a relatively strong electric field. For high pressure discharges Stark shifting of helium lines and their forbidden counterparts is used. The advantage of Stark methods is their ab initio basis which makes them independent on other plasma parameters. A different method for field measurement, based on the helium line ratio, can be applied in cases where the Stark method cannot be used.

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter.

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.

Coulomb explosion of “hot spot”

The study presented in this paper has shown that the generation of hard x rays and high-energy ions, which are detected in pinch implosion experiments, may be associated with the Coulomb explosion of the hot spot that is formed due to the outflow of the material from the pinch cross point. During the process of material outflow, the temperature of the hot spot plasma increases, and conditions arise for the plasma electrons to become continuously accelerated. The runaway of electrons from the hot spot region results in the buildup of positive space charge in this region followed by a Coulomb explosion. The conditions for the hot spot plasma electrons to become continuously accelerated have been revealed, and the estimates have been obtained for the kinetic energy of the ions generated by the Coulomb explosion.

Combination of two Gas Electron Multipliers and a Micromegas as gain elements for a time projection chamber

We measured the properties of a novel combination of two Gas Electron Multipliers with a Micromegas for use as amplification devices in high-rate gaseous time projection chambers. The goal of this design is to minimize the buildup of space charge in the drift volume of such detectors in order to eliminate the standard gating grid and its resultant dead time, while preserving good tracking and particle identification performance. To characterize this micro-pattern gas detector configuration, we measured the positive ion back-flow and energy resolution at various element gains and electric fields, using a variety of gases, and additionally studied crosstalk effects and discharge rates. At a gain of 2000, this configuration achieves an ion back-flow below 0.4% and an energy resolution better than σ/E=12% for 55Fe X-rays.

Absolute determination of radiation bursts and of proportional counters space charge effect through the influence method

When proportional counters are employed in charge integration mode to determine the magnitude of a radiation pulse, so intense that individual detection events take place in a time too short to produce individual output pulses, mostly in pulsed neutron sources, the strong build-up of positive space charge reduces the electric multiplication factor of the proportional detector. Under such conditions the ensuing measurement underestimates the amount of radiation that interacted with the detector. If the geometric characteristics, the filling gas pressure and the voltage applied to that detector are known, it becomes possible to apply an analytical correction method to the measurement.In this article we present a method that allows to determine the absolute value of the detected radiation burst without the need to know the characteristics of the employed detectors. It is necessary to employ more than one detector, taking advantage of the Influence Method.The “Influence Method” is conceived for the absolute determination of a nuclear particle flux in the absence of known detector efficiency and without the need to register coincidences of any kind. This method exploits the influence of the presence of one detector in the count rate of another detector, when they are placed one behind the other and define statistical estimators for the absolute number of incident particles and for the efficiency (Rios and Mayer, 2015 [1,2]). Its practical implementation in the measurement of a moderated neutron flux arising from an isotopic neutron source was exemplified in (Rios and Mayer, 2016 [3]) and the extension for multiple detectors in (Rios and Mayer 2016 [4]).

Metal nanoparticle mediated space charge and its optical control in an organic hole-only device

We reveal the role of localized space charges in hole-only devices based on an organic semiconductor with embedded metal nanoparticles (MNPs). MNPs act as deep traps for holes and reduce the current density compared to a device without MNPs by a factor of 104 due to the build-up of localized space charge. Dynamic MNPs charged neutrality can be realized during operation by electron transfer from excitons created in the organic matrix, enabling light sensing independent of device bias. In contrast to the previous speculations, electrical bistability in such devices was not observed.

Electric field inhomogeneity in ohmic‐type CdTe detectors measured by time‐of‐flight technique

AbstractThe bias‐induced instability of the internal electric field in Ohmic‐type CdTe gamma‐ray detectors is investigated by using the time‐of‐flight technique. Based on the solution of one dimensional Poisson equation, transient current waveforms at different delays of laser excitation from the bias application and at different temperatures are analysed It is shown that under DC bias application, the internal electric field decreases with depth because of the positive space charge buildup in the detectors. Further, the origin of the space charge is attributed to the injection and successive trapping of holes from the anode side at the early stage of evolution and then later to the thermal release of electrons from a deep donor at 0.64 eV. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A parametric study on the PD pulses activity within micro-cavities

A two-dimensional kinetic model has been used to parametric investigation of the spark-type partial discharge pulses inside the micro-cavities. The model is based on particle-in-cell methods with Monte Carlo Collision techniques for modeling of collisions. Secondary processes like photo-emission and cathode-emission are considered. The micro-cavity may be sandwiched between two metallic conductors or two dielectrics. The discharge within the micro-cavity is studied in conjunction with the external circuit. The model is used to successfully simulate the evolution of the discharge and yield useful information about the build-up of space charge within the micro-cavity and the consequent modification of the applied electric field. The phase-space scatter plots for electrons, positive, and negative ions are obtained in order to understand the manner in which discharge progresses over time. The rise-time and the magnitude of the discharge current pulse are obtained and are seen to be affected by micro-cavity dimensions, gas pressure within the micro-cavity, and the permittivity of surrounding dielectrics. The results have been compared with existing experimental, theoretical, and computational results, wherever possible. An attempt has been made to understand the nature of the variations in terms of the physical processes involved.

A novel technique for compensation of space charge effects in the LUPIN-II detector

A new method for improving REM counter performance in Pulsed Neutron Fields (PNFs) has been developed. This method uses an analysis of the build-up of space charge in the counter to compensate for an underestimation of Ambient Dose Equivalent (H*(10)) in intense pulsed fields. It was applied to three sets of experimental data acquired using the LUPIN-II REM counter device, which is designed for use in PNFs. The data was acquired using the cyclotron at Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB), at the HiRadMat facility at CERN and at the ‘Elettra Sincrotrone Trieste’ (ELETTRA), Italy. A comparison of the data with and without this compensation method is used to highlight its effectiveness. The LUPIN-II performance, which has already been shown to be able to cope with fields of up to hundreds of nSv/burst, is improved by at least one order of magnitude, with further potential for improvement.

Graphics processing unit accelerated three-dimensional model for the simulation of pulsed low-temperature plasmas

A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 × 106 particles with dynamic weighting and a total mesh size larger than 108 cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPU-implementation versus a CPU-implementation is considered, and a speed-up factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speed-up is realized for parallelization of the electron processes.

Temporal and temperature evolution of electric field in CdTe:In radiation detectors

We employed measurement of the Pockels electro-optic effect to study the electric field and space charge dynamics in semi-insulating CdTe doped with indium. We performed measurements of time and temperature dependence of the electric field. The polarization due to space charge build-up decreases with increasing temperature. Increase of temperature, therefore, leads to de-polarization in CdTe:In detectors which are opposite to the CdTe:Cl samples studied to date. We have shown that the thermally activated depolarization cannot be explained by the conventional model used for the description of space charge formation so far and an alternative model involving a recombination level was suggested and successfully used.

$$\hbox {H}^{-}$$ H - -Like Centers and Space-Charge Effects in Cryogenic Germanium Detectors for Dark Matter Search

Space-charge build-up phenomena are investigated in germanium detectors for dark matter search, associated with the thermal emission of carriers by the H\(^{-}\)-like, dopant-related A\(^{+ }\)and D\(^{-}\) shallow trap centers. Evidence for such processes follows from a combined study of the time evolution of the pattern of charge sharing between the different collection electrodes of the device, and of the kinetics of carrier emission by the A\(^{+ }\)and D\(^{- }\)centers. Implications for detector physics are discussed.

Comparative Study on Space Charge Distribution Measurements using PEA and PWP Methods on High Voltage Insulation

Major drawbacks to high voltage insulator especially on electrical aging process have been an issue for decades. The internal electric field distribution due to the presence of local space charge will distort, which in turn may cause the insulating material to degrade and finally can lead to accelerated aging and electrical breakdown. Comprehension of space charge buildup in solid insulator is crucial to determine how aging occurs, thus enabling the prediction of insulator lifetime. Space charge measurement has been extensively researched in recent years and has become a common method of investigating solid insulator properties. Pressure Wave Propagation (PWP) and Pulsed Electro-Acoustic (PEA) are non-destructive dielectric testing methods used to determine the space charge distribution within the insulation. This paper presents the basic principles, typical measurement setup and comparison of results from several published papers on application of PWP and PEA methods to investigate space charge accumulation. The space charge distribution in the sample is measured as current and voltage signals obtained from PWP and PEA methods respectively. Judging from extracted results of other research, both methods show the capability to detect premature aging activities in the insulator, which is necessary for condition monitoring of high voltage insulators.

Study of electric field distribution and low frequency noise of CdZnTe radiation detectors

Polarization phenomena in a metal-semiconductor-metal (M-S-M) structure of metallic Schottky contacts deposited on CdZnTe radiation detectors were studied. We evaluate the distribution of the electric field along the biased M-S-M structure by Pockels measurements. The results show that almost all the electric field is developed across the depletion layer of the reverse-biased contact. The noise measurements of the CdZnTe detectors studied show that the dominant noise is 1/fm noise. The 1/fm noise, with the parameter m close to one, is present at frequencies below 100 Hz and its bandwidth decreases in the course of the polarization process. At higher frequencies, we observed an increase of the m parameter to 2, which indicates a strengthened effect of the generation-recombination processes. In the frequency band of dominating 1/fm = 1 noise, the increase of magnitude of the noise spectral density was proportional to the power of 6, in relation to the current through the detector. This high value is explained as a result of a screening effect of the space charge buildup during the polarization.

Simulation of space-charge effects in an ungated GEM-based TPC

A fundamental limit to the application of Time Projection Chambers (TPCs) in high-rate experiments is the accumulation of slowly drifting ions in the active gas volume, which compromises the homogeneity of the drift field and hence the detector resolution. Conventionally, this problem is overcome by the use of ion-gating structures. This method, however, introduces large dead times and restricts trigger rates to a few hundred per second. The ion gate can be eliminated from the setup by the use of Gas Electron Multiplier (GEM) foils for gas amplification, which intrinsically suppress the backflow of ions. This makes the continuous operation of a TPC at high rates feasible.In this work, Monte Carlo simulations of the buildup of ion space charge in a GEM-based TPC and the correction of the resulting drift distortions are discussed, based on realistic numbers for the ion backflow in a triple-GEM amplification stack. A TPC in the future p¯anda experiment at FAIR serves as an example for the experimental environment. The simulations show that space charge densities up to 65fCcm−3 are reached, leading to electron drift distortions of up to 10mm. The application of a laser calibration system to correct these distortions is investigated. Based on full simulations of the detector physics and response, we show that it is possible to correct for the drift distortions and to maintain the good momentum resolution of the GEM-TPC.

Space Charge Measurement on XLPE Cable for HVDC Transmission using PEA Method

Abstract The use of cross-linked polyethylene (XLPE) cables for High Voltage Alternating Current (HVAC) transmission has been successful in the world. However polymer insulated cables for High Voltage Direct Current (HVDC) transmission are still in development due to space charge build-up that occurs in cables under DC stress. Knowledge must be gained on the processes of charge build-up, including nature, kinetics and amount of accumulated charges, and on the consequences of charge accumulation regarding field distribution in the insulation. One way to determine the space charge distribution in XLPE cables for HVDC transmission non-destructively is the Pulse Electro Acoustic (PEA) method, a direct method for measuring space charge distribution in dielectric materials. The PEA method was chosen because it is suitable for a wide range of sample geometry (thin and thick samples), and is available for cable geometry. The present contribution concerns space charge distribution measurements in sections of HVDC model cables. To account for the situation of cables in service, measurements were performed with a temperature gradient across the insulation thickness, and this could be achieved by heating through the cable conductor. Measurements performed on XLPE insulated model cables with 4.5 mm insulation thickness were realized with -40 kV DC applied for 150 minutes. Injecting a current of 200A in the conductor produced theoretically a temperature gradient of 8 °C between the core and the outer face of the cable, with a surface temperature of the cable of 50 °C. A deconvolution procedure, wherein attenuation, dispersion and cylindrical geometry are accounted for, was used to recover space charge profiles at different times under stress. It is shown that considerable field redistribution occurs within the cable, compared to the Laplacian field distribution predicted in cylindrical geometry, due in part to the conductivity gradient associated with the thermal gradient, and for the other part to charge build-up occurring within the cable.

High temperature breakdown strength and voltage endurance characterization of nanofilled polyamideimide

For high temperature wire enamel application, the dielectric properties of nanoscale silica and alumina filled polyamideimide were characterized as a function of filler loading, and temperature. It was found that although the dc breakdown strength improved for both composite systems, only the alumina also exhibited an increase in ac breakdown strength. In addition, the voltage endurance at elevated temperature in both a needle/plate and twisted wire geometry was improved over a narrow range of filler loading (5-7.5 wt%). At room temperature, pulsed electroacoustic analysis (PEA) data does not indicate any difference in space charge buildup in the composite for fields up to 25 kV/mm over a period of 2 h. This is in contrast to other nanocomposite systems, which tend to show mitigation of space charge buildup with the addition of nanoparticles. Dielectric spectroscopy shows that the addition of nanoparticles increases the real permittivity above that expected for volumetric mixing rules. This enhancement is associated with high effective permittivity interfacial areas. No systematic changes in losses in the frequency range between 10-4 and 105 Hz were seen. The results suggest that carrier scattering is the primary mechanism leading to the improved breakdown strength.