Sheath Resistance Research Articles

Quantifying the Sheath Impedance of the Electric Double Probe Instrument on the Van Allen Probes

AbstractSpherical double probe electric field sensors become electrically coupled to magnetospheric plasma during operation, leading to an instrument response that varies with the local plasma environment. Here, a method is developed for determining this variable coupling impedance for each measurement direction by using periods of favorable boom, wave, and magnetic field geometry. Comparing electric field complex amplitudes between 30 Hz and 10 kHz observed along each boom direction to those predicted from simultaneous magnetic field measurements and cold plasma theory allows for the amplitude and phase response of the instrument to be quantified over the full range of plasma densities encountered on‐orbit. A sheath model is developed to describe how the sheath resistance, sheath capacitance, and relative effective length vary as a function of plasma density. An additional empirical correction is also included to describe the phase response along the spin‐axis. The modeled sheath correction is subsequently tested for case studies of burst‐mode data and statistical analyses of survey‐mode data. It is demonstrated that the levels of agreement between observations and theoretical predictions based on Faraday's Law are substantially greater for the sheath corrected data than for uncorrected observations. Comparisons between observations with oppositely directed Poynting vector directions reveals that the sheath correction reconciles a bifurcated distribution in the uncorrected data to a single peak centered on agreement with Faraday's Law. A full sheath corrected EMFISIS L4 survey mode data set has been produced for final archive. Full details of the sheath correction are also provided for manual data correction.

Improvement of the floating probe method for ion density and electron temperature measurement without compensation due to voltage reduction across the sheath

The floating probe method (FPM) applicable for processing plasma diagnostics was developed for the measurement of ion density and electron temperature (J. Appl. Phys. 101 033305). When an AC voltage is applied to a floating probe, harmonic currents are generated due to the nonlinearity of the sheath. The electron temperature and ion density are obtained using the harmonic currents and the voltage across the sheath. However, in the FPM, when the sensing resistance becomes similar to the sheath resistance, iterative calculations must be performed to compensate for the voltage reduction across the sheath due to the sensing resistor. In this paper, the voltage across a DC blocking capacitor is measured to directly obtain the voltage across the sheath. Therefore, it is not necessary to compensate for the voltage reduction across the sheath through iterative calculations. The electron temperature was increasingly overestimated as the capacity of the DC blocking capacitor became smaller. This overestimation was caused by the capacitive load effect and was compensated for using a correction for the second harmonic current. The measured electron temperature and ion density were compared with those from electron energy distribution functions (EEDFs) in an inductively coupled plasma, and they were in good agreement.

Discharge model and plasma characteristics of high-power pulsed magnetron sputtering titanium target

<sec>High-power pulsed magnetron sputtering has become a popular research tool in surface technology industry because it can prepare the films with excellent surface quality. The plasma density and metal ionization rate are the key factors affecting the quality of the film deposited by high-power pulsed magnetron sputtering. The parameters of high-power pulsed magnetron sputtering (such as applied voltage, pulse width, deposition pressure and peak current) affect the plasma density and metal ionization rate. In this paper, in order to more easily understand the plasma densities and metal ionization rates at the different process parameters, the plasma densities and ionization rates are calculated numerically. An equivalent circuit model established by MATLAB/Simulink software is used to obtain the discharge current curve of high-power pulsed magnetron sputtering titanium (Ti) target. The plasma density near the plasma sheath is calculated by the sheath resistance in the equivalent circuit model. The ionization rate of Ti is calculated by using the semi-cylinder global model theory combined with the discharge current simulated by equivalent circuit model. It is found that under the different high power pulse sputtering voltages, pulse widths and different deposition pressures, the discharge modes are of gas discharge and metal ion discharge, and the gas discharge interacts with metal ion discharge. The equivalent circuit model is produced by the main discharge mode, and the equivalent circuit model composed of capacitor, inductor and resistors in series and in parallel can be used to simulate the discharge current of Ti target. The result shows that the simulated discharge current is accurate in the rising edge and peak value in comparison with experimental data. The value of electron component in the model is related to the saturation ion current.</sec><sec>According to the sheath resistance in the model, the average plasma density in the vacuum chamber increases with increasing sputtering voltage, pulse width and deposition pressure. And the plasma density in the vacuum chamber lies in a range of (2–9) × 10<sup>17</sup> m<sup>–3</sup>. The particle equilibrium equation is established by using the semi-cylinder global model theory. The electron temperature (5 eV) and discharge current are used as boundary conditions to calculate the ionization rate of Ti. The value of the ionization rate of Ti is in a range of 31%–38% at different deposition pressures, and the ionization rate of Ti increases with the increase of deposition pressure.</sec>

Comparative Transcriptome Analysis of Rhizoctonia solani-resistant and -Susceptible Rice Cultivars Reveals the Importance of Pathogen Recognition and Active Immune Responses in Host Resistance

Rice sheath blight (SB), caused by Rhizoctonia solani (R. solani), is a major threat to rice production worldwide. The molecular mechanisms of the SB resistance in rice are poorly understood. The transcriptomes of the SBresistant rice cultivar YSBR1 and the susceptible cultivar Lemont were analyzed after R. solani infection. A total of 7624 differentially expressed genes (DEGs) were identified at one or more timepoints in a cultivar. 5526 and 5618 DEGs were differentially expressed in Lemont and YSBR1, respectively. YSBR1 exhibited stronger and earlier transcriptional response to R. solani than Lemont. Gene ontology enrichment analysis revealed that genes that encode cell wall-modifying and glycosyl-degrading enzymes or anti-microbial proteins were specifically induced in YSBR1 at 6 hpi. MapMan analysis revealed that more DEGs related with cell wall, β-glucanses, respiratory burst, phenylpropanoids and lignin were highly induced by R. solani in YSBR1 than in Lemont. The results also showed that receptor-like kinases and jasmonic acid signaling may play important roles in host resistance to R. solani. This study highlights potential candidate genes and signaling pathways involved in rice sheath resistance and can help to further clarify the mechanistic events underlying resistance and susceptibility to R. solani.

High-resolution disruption halo current measurements using Langmuir probes in Alcator C-Mod

Halo currents generated during disruptions on Alcator C-Mod have been measured with Langmuir ‘rail’ probes. These rail probes are embedded in a lower outboard divertor module in a closely-spaced vertical (poloidal) array. The dense array provides detailed resolution of the spatial dependence (~1 cm spacing) of the halo current distribution in the plasma scrape-off region with high time resolution (400 kHz digitization rate). As the plasma limits on the outboard divertor plate, the contact point is clearly discernible in the halo current data (as an inversion of current) and moves vertically down the divertor plate on many disruptions. These data are consistent with filament reconstructions of the plasma boundary, from which the edge safety factor of the disrupting plasma can be calculated. Additionally, the halo current ‘footprint’ on the divertor plate is obtained and related to the halo flux width. The voltage driving halo current and the effective resistance of the plasma region through which the halo current flows to reach the probes are also investigated. Estimations of the sheath resistance and halo region resistivity and temperature are given. This information could prove useful for modeling halo current dynamics.

A finite element procedure for radio-frequency sheath–plasma interactions based on a sheath impedance model

A finite element code that solves self-consistent radio-frequency (RF) sheath–plasma interaction problems is improved by incorporating a generalized sheath boundary condition in the macroscopic solution scheme. This sheath boundary condition makes use of a complex sheath impedance including both the sheath capacitance and resistance, which enables evaluation of not only the RF voltage across the sheath but also the power dissipation in the sheath. The newly developed finite element procedure is applied to cases where the background magnetic field is perpendicular to the sheath surface in one- and two-dimensional domains filled by uniform low- and high-density plasmas. The numerical results are compared with those obtained by employing the previous capacitive sheath model at a typical frequency for ion cyclotron heating used in fusion experiments. It is shown that for sheaths on the order of 100 V in a high-density plasma, localized RF power deposition can reach a level which causes material damage. It is also shown that the sheath–plasma wave resonances predicted by the capacitive sheath model do not occur when parameters are such that the generalized sheath impedance model substantially modifies the capacitive character of the sheath. Possible explanations for the difference in the maximum RF sheath voltage depending on the plasma density are also discussed.

An improved sheath impedance model for the Van Allen Probes EFW instrument: Effects of the spin axis antenna

AbstractA technique to quantitatively determine the sheath impedance of the Van Allen Probes Electric Field and Waves (EFW) instrument is presented. This is achieved, for whistler mode waves, through a comparison between the total electric field wave power spectra calculated from magnetic field observations and cold plasma theory and the total electric field wave power measured by the EFW spherical double probes instrument. In a previous study, a simple density‐dependent sheath impedance model was developed in order to account for the differences between the observed and calculated wave electric field. The current study builds on this previous work by investigating the remaining discrepancies, identifying their cause, and developing an improved sheath impedance correction. Analysis reveals that anomalous gains are caused by the spin axis antennas measuring too much electric field at specific densities and frequencies. This is accounted for in an improved sheath impedance model by introducing a density‐dependent function describing the relative effective length of the probe separation, Leff, in addition to the sheath capacitance and resistance values previously calculated. Leff values vary between between 0.5 and 1.2, with values >1 accounting for the anomalous gains and values <1 accounting for the shorting effect at low densities. Applying this improved sheath impedance model results in a significant increase in the agreement level between observed and calculated electric field power spectra and wave powers over the previous model.

Identification of Blast Resistance in a Core Collection of Foxtail Millet Germplasm.

Blast, also known as leaf spot, caused by Pyricularia grisea (teleomorph: Magnaporthe grisea), is a serious disease affecting both forage and grain production in foxtail millet in India. For the identification of new and diverse sources of blast resistance, a foxtail millet core collection comprising 155 accessions was evaluated against the Patancheru isolate (Fx 57) of M. grisea. In a field screen during 2009 and 2010, 21 accessions were identified with neck and head blast resistance against Fx 57. In a greenhouse screen, 11 of the 155 accessions exhibited seedling leaf blast resistance to the same isolate. Further evaluation of the selected 28 accessions (found resistant to neck and head blast under field conditions during 2009 and 2010 or leaf blast in the greenhouse screen) against four M. grisea isolates (Fx 57, Fx 58, Fx 60, and Fx 62 from Patancheru, Nandyal, Vizianagaram, and Mandya, respectively) led to the identification of 16 accessions with leaf, sheath, neck, and head blast resistance to at least one isolate. Two accessions (ISe 1181 and ISe 1547) were free from head blast infection and showed resistance to leaf (score ≤3.0 on a 1-to-9 scale), neck, and sheath blast (score ≤2.0 on a 1-to-5 scale) against all four isolates. In addition, ISe 1067 and ISe 1575 also exhibited high levels of blast resistance. Blast-resistant accessions with superior agronomic and nutritional quality traits can be evaluated in multilocation yield trials before releasing them for cultivation to farmers.

Resistência física de bainhas de plantas de arroz supridas com silício e infectadas por Rhizoctonia solani

Physical resistance of leaf sheaths of rice plants supplied with silicon and infected by Rhizoctonia solani This study aimed to associate the silicon (Si) deposition on sheaths of rice plants with an increase in physical resistance against infection by Rhizoctonia solani. Plants from the susceptible cultivars BR-Irga 409 and Labelle were grown in nutrient solution containing (+Si) or not (-Si) Si. Si concentration on sheaths of plants supplied with Si and inoculated with R. solani significantly increased compared to plants non-supplied with Si, reducing the relative lesion length at 96 hours after inoculation (hai). There were significant differences between -Si and +Si treatments for the force required of a metal needle to pass through the sheaths at 48 and 72 hai. The greater physical resistance at 24 hai was due to the fact that R. solani was growing on sheath tissue without destroying it, which could be proved by the absence of symptoms. After this period, the resistance decreased due to the rapid R. solani growth. However, on sheaths of plants supplied with Si, the force required for the needle to penetrate was higher at 48 and 72 hai. The sheath resistance of plants supplied with Si to infection by R. solani can be explained partly by the greater physical resistance due to Si deposition.

Usage of Displaced Current in Langmuir Probe Measurement

We investigate the possibility of the usage of displaced current in Langmuir probe measurement theoretically and experimentally The displaced current flows through the self-generated or artificial capacitance of the reference electrode under an alternative sweep voltage. It can increase the total current afforded by the reference electrode, and eliminate the interference deriving from the finite plasma sheath resistance of the reference electrode. The results in both theory and experiment lead to the conclusion that the usage of displaced current is valid and more efficient with a floating probe method. This method is applied in some harsh plasma environments when the grounded chamber wall can easily be contaminated by the dielectric.

Factors Affecting the Sheath Losses in Single-Core Underground Power Cables with Two-Points Bonding Method

Single-core underground power cables with two-points bonding induce currents in their metallic sheaths. The sheath induced currents are undesirable and generate power losses and reduce the cable ampacity. This paper has shown that the values of the sheath losses in some cases could be greater than conductor losses, depending on various factors. Such these factors are type of cable layouts, cable parameters, cable spacing, sheath resistance, phase rotation, conductor current and cable armoring. In this paper the above factors have been investigated. The calculations are carried out depending mainly on IEC 60287 by a proposed computer program using MATLAB. Keywords: Single-core cables, two-point bonding method, sheaths losses, flat, trefoil, armoring DOI: http://dx.doi.org/10.11591/ijece.v2i1.115

Using rf impedance probe measurements to determine plasma potential and the electron energy distribution

Earlier work has demonstrated the usefulness of a network analyzer in plasma diagnostics using spherical probes in the thin sheath limit. The rf signal applied to the probe by the network analyzer is small in magnitude compared to probe bias voltages, and the instrument returns both real and imaginary parts of the complex plasma impedance as a function of frequency for given bias voltages. This information can be used to determine sheath resistance, sheath density profiles, and a technique for measuring electron temperature. The present work outlines a method for finding plasma potential and the electron energy distribution within a limited energy range. The results are compared to those using conventional Langmuir probe techniques. The rf method has general application to diverse areas of plasma investigations when the plasma is uniform and probe dimensions are much less than the size of the plasma. These applications include laboratory and space environments.

Radio frequency-compensated Langmuir probe with auxiliary double probes

A radio frequency (rf) compensation design using auxiliary double probes connected in parallel with a main measurement probe was developed for Langmuir probe diagnostics. This probe structure can reduce the sheath impedance of the main probe. In our probe design, the sheath capacitance of the probe can be increased and its sheath resistance can be decreased with increasing dc bias differential voltage between the auxiliary double probes. The I-V characteristic curve and electron energy distribution functions measured by our probe system had sufficient rf compensation performance in inductively coupled plasmas.

On the use of the metallic nozzle of a cutting arc torch as a Langmuir probe

The region inside the nozzle (bore diameter ≈ 1 mm) of a cutting arc torch is inaccessible to most plasma diagnostics, and numerical simulations are the only means to find out the relative importance of several physical processes. In this work, a study of electrostatic (Langmuir) probes applied to the inside of a high energy density 30 A cutting arc torch nozzle is presented. The metallic nozzle was used as a Langmuir probe, so the plasma flow is not perturbed by the probe as a solid body. Biasing the nozzle through an electric circuit that employs appropriate resistors together with the arc power source, the i–V nozzle characteristic was built. It was found that under a large positively biased nozzle, the electron current drained from the arc was relatively small, ≈ 1 A, notwithstanding the fact that the size of the nozzle was relatively large. On the other hand, an almost linear ion current was found for the ion branch for nozzle voltages well below the floating value. Based on the magnitude of inverse slope of the ion current, an estimation of the average electron temperature of the plasma in the vicinity of the nozzle wall was estimated from an ion sheath resistance model using a non-equilibrium two-temperature Saha-equation. An average electron temperature of about 4200 K and a corresponding plasma density of 4 × 1017 m- 3 were found.

Sheath Voltage Estimation for Inductively Coupled Plasma Etcher by Impedance Analysis

A thick quartz glass is etched using an inductively coupled plasma device under constant antenna power. Sheath voltage, which is the RF voltage from the plasma edge to the sample surface, is estimated by impedance analysis. Sheath resistance and sheath capacitance are determined from some experimental bias voltage, which is the RF voltage applied to the sample bed electrode. Once they are determined, the bias and sheath voltages for arbitrary thick samples and bias power can be calculated. The calculated bias voltages agree with the experimental ones within 5%. Etching rate is proportional to sheath voltage for various etching conditions. A quartz sample 5-mm-thick can be etched at the same etch rate as a thin quartz sample using a thick quartz sample bed with a hole where the quartz sample is placed.

High‐voltage antenna‐plasma interaction in whistler wave transmission: Plasma sheath effects

We study the plasma sheath surrounding an antenna that transmits whistler mode waves in the inner magnetosphere in order to investigate the feasibility of conducting controlled experiments on the role of wave‐particle interactions in the pitch angle diffusion of relativistic radiation belt electrons. We propose a model for an electrically short antenna‐sheath‐plasma system with transmission frequencies below the electron characteristic frequencies and much higher than the ion characteristic frequencies. The ion current is neglected. We analytically solve a time‐dependent one‐dimensional situation by neglecting the effects of the wave's magnetic field. In our model, the antenna is charged to a large negative potential during a steady transmission. Positive charge occurs in the sheath and the sheath is free of electrons and conduction current. The net charge on the antenna and in the sheath is zero. The volume, or the radius in a cylindrical case, of the sheath varies in response to the charge/voltage variation on the antenna. The oscillating radius of the sheath translates to a current in the plasma, which radiates waves into the plasma. A whistler wave transmission experiment conducted by the RPI‐IMAGE has shown that the model may describe the most important physical processes occurring in the system. The system response is predominately reactive, showing no evidence for significant sheath current or sheath resistance. The negligibly small sheath conduction electron current can be understood if the antenna is charged to a substantial negative potential, as described by the model. Quantitatively, the model may underestimate the sheath capacitance by about 20%.

Genotypic Correlation and Multivariate QTL Analyses for Cell Wall Components and Resistance to Stalk Tunneling by the European Corn Borer in Maize

ABSTRACTCorrelations between concentrations of cell wall components (CWCs) in the leaf sheath and stalk and resistance to stalk tunneling by the European corn borer (ECB) [Ostrinia nubilalis (Hübner)] have been reported in some maize (Zea mays L.) populations. Evaluations of resistance to ECB stalk tunneling (ECB‐ST) and concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) have been performed on recombinant inbred lines (RILs) developed from the cross of maize inbred lines B73 (susceptible to ECB‐ST, low to moderate CWC concentrations) and DE811 (resistant to ECB‐ST, high CWC concentrations). The objective of this study was to estimate genotypic correlations between ECB‐ST and CWC concentrations and compare locations and effects of quantitative trait loci (QTL) for those traits. Genotypic correlations between ECB‐ST and CWCs were not significant, but clustering of QTL for ECB‐ST and CWCs was observed. Negative genotypic correlations between ECB‐ST and CWC concentrations were observed at some loci, and resistance to ECB‐ST may be associated with a subset of the QTL observed for CWCs and ADF in particular. Resistance to ECB‐ST may also be associated with starch concentration in the stalk, which could explain the detection of resistance alleles contributed by B73. Examination of temporal differences in CWC and starch concentrations, and marker‐assisted transfer of select alleles, could provide more information on mechanisms of resistance to ECB‐ST.

Effects of boundary conditions and flow on the kink instability in a cylindrical plasma column

An experimental investigation of the kink instability is presented in a linear plasma column where one end is line-tied to the plasma source, and the other end is not line-tied and therefore free to slide over the surface of the end-plate. This latter boundary condition is a result of plasma sheath resistance that insulates, at least partially, the plasma from the end-plate. The helical m=1 kink mode is observed to grow when the plasma current exceeds a threshold and, close to the criticality, is characterized by an axial mode structure with maximum displacement at the free axial boundary. Azimuthal rotation of the mode is observed such that the helically kinked column always screws into the free axial boundary. The kink mode structure, rotation frequency and instability threshold are accurately reproduced by a recent kink theory [D. D. Ryutov et al., Phys. Plasmas 13, 032105 (2006)], which includes axial plasma flow and one end of the plasma column that is free to move due to a perfect non line-tying boundary condition which is experimentally verified. A brief review of the kink theory and its predictions for the boundary conditions relevant in the present experiments are presented.

Ion extraction from magnetically driven shunting arc and estimation of ion density at sheath boundary

Ion density of a magnetically driven shunting arc discharge is estimated from the target voltage characteristics of the pulse modulator circuit in plasma-based ion implantation (PBII). The voltage recovery time constant directly reflects the ion sheath characteristics, and the sheath resistance is related to the ion density inside the transient sheath. The measured characteristics are analyzed using an equivalent circuit of the pulse modulator in PBII. The estimated ion density decreases from 3 × 10 8 to 4 × 10 7 cm − 3 with time after the arc ignition from 100 to 400 μs. This characteristic almost agrees with that of an ion current extracted from the arc plasma by applying a negative pulse voltage to the target.

Comprehensive analysis of chlorine-containing capacitively coupled plasmas

Capacitively coupled discharges of strongly reactive atmospheres containing mixtures of boron trichloride (BCl3) and chlorine (Cl2) are investigated employing spatially resolved Langmuir probe measurements, and three probes that are spatially integrating methods: optical emission spectroscopy (OES), self-excited electron resonance spectroscopy (SEERS), and impedance characteristics of the discharge. The analysis covers the pure gases including some mixtures, discharge pressure, and rf power over nearly two orders of magnitude, and their impact on important plasma parameters of “first order,” such as plasma density, plasma potential, electron temperature, temperature of the plasma bulk, electron collision rate with neutrals, and actual rf power coupled into the discharge. From these, other properties (electrical conductivity, capacitance, plasma bulk resistance, sheath resistance, and its electrically defined thickness) can be derived. Since the methods are partially complementary, a mutual control of the obtained data is made possible, and we finally obtain a self-consistent model for capacitive coupling connecting data obtained with electrical and optical probes. Compared to electropositive discharges of inert atomic gases (Ar) and molecular gases (H2), which are used as calibration standard for BCl3 and Cl2, the electron plasma density ne is definitely lower, whereas the electron temperature Te is significantly higher, which would be expected by electron attachment to the electronegative molecules—at least at higher discharge pressures. Furthermore, we compared values for Te and ne obtained with OES and SEERS, respectively, and with the Langmuir-probe system. The agreement in electron plasma density and electron temperature for Ar is surprisingly good, despite the fact that the electron energy distribution would be described with two temperatures. For argon plasma, the variation of the calculated dc conductivity for nearly pure capacitive coupling either from impedance measurements or SEERS is within 30%. This is a result of uncertainties in current path rather than principal faults of the various methods. For the reactive, molecular gases, however, the results vary significantly. These data serve to determine several derived properties. Among these, are the sheath thickness, which is compared with optical and electrical data, and the conductivity of the plasma bulk. As they are derived from simultaneous, but independent measurements, they confirm the relative simple model of an electropositive discharge (argon and argon/krypton), and stress the difficulty to describe plasmas consisting of electronegative constituents (Cl2, BCl3, and their mixtures) which is due mainly to a pressure-dependent transition from stochastic to ohmic heating and from electropositive to electronegative behavior.