Sheath Interface Research Articles

Wall current closure effects on plasma and sheath fluctuations in Hall thrusters

The excitation of negative energy, ion sound type modes driven by the E × B drift and the reactive/dissipative response of the wall sheath interface is analyzed for conditions typical in a Hall thruster. Such sheath impedance modes are sensitive to the dielectric properties of the thruster wall material, which therefore may have direct influence (other than via the secondary electron emission) on fluctuations and transport. Our results predict mode frequencies consistent with the frequencies of fluctuations observed experimentally.

Evaluating a New Core‐Sheath Procedure for Processing Hard Metals by Equal‐Channel Angular Pressing

A new design of billet, based on a core–sheath configuration, was used for the processing of Ni, Fe, and a NiTi alloy by equal‐channel angular pressing (ECAP) at room temperature. This configuration involves inserting metal cores within Fe sheaths prior to processing and it is designed especially for use with hard‐to‐deform materials. Billets were processed through one or two ECAP passes at room temperature and the microhardness values were recorded across the transverse directions within the cores to evaluate the flow process. As in conventional ECAP, the hardness increased significantly after the first pass and there were regions of lower hardness along the bottom surfaces of each core. The gradient of hardness decreased with increasing core diameter but the average microhardness values remained unchanged. Three‐dimensional finite element simulations were used to evaluate the flow behavior after one pass of ECAP using different core metals. These simulations show the lower areas of the cores undergo less deformation than the upper areas and the homogeneity increases with increasing levels of friction at the core–sheath interface.

Ultrahigh-Speed Coating of DLC at Over 100 $\mu{\rm m}/{\rm h}$ by Using Microwave-Excited High-Density Near Plasma

In this paper, ultrahigh-speed deposition of diamond-like carbon (DLC) at deposition rates over 100 m/h is investigated. DLC coatings to steel substrates are conducted by using a plasma CVD apparatus employing new-type high-density plasma, which is sustained by microwaves propagating as surface waves along plasma–sheath interface. For plasma generation during coating in a mixture gas of Ar, methane, and tetra-methyl-silane (TMS), injection of 2.45-GHz microwave and application of substrate bias are conducted in a pulsed manner at a frequency of 500 Hz. DLC coatings with a microwave peak power of 1000 W, and a bias voltage of ${-}{200}~{\rm V}$ show that the deposition rate and hardness of DLC are increased and decreased, respectively, with increasing duty ratio from 6% to 50% for pulsed plasma generation. The hardness of DLC is increased by increasing the gas flow rate of TMS from 10 to 20 sccm. In addition, the hardness of DLC is increased almost linearly with increasing substrate current averaged over coating time. As an example of DLC coating in this paper, the deposition rate and hardness of DLC are 188 $\mu{\rm m/h}$ and 12.9 GPa, respectively, where plasma is generated at a duty ratio of 50% with the gas flow rates of 40, 200, and 20 sccm in Ar, methane, and TMS, respectively.

Measurements of the parallel ion velocity distribution at the plasma–sheath interface

The ion velocity distribution parallel to magnetic field lines, f is extracted from retarding field analyzer (RFA) measurements in the ASDEX Upgrade scrape-off layer. The RFA ion current–voltage characteristic is transformed into a system of linear equations, from which f is unfolded using a standard regularization method. The algorithm is validated on numerically generated data. The experimentally measured f compares favorably with the ion velocity distributions calculated from a quasi-neutral kinetic simulation of the plasma pre-sheath. The technique can be used to measure f by RFAs in plasma processing, high-energy particle accelerators or by satellite-born RFAs.

Observation of source gas depletion in narrow metal tube during internal diamond-like carbon coating with microwaves

For stable plasma generation inside mm-sized metal tubes and holes, we developed a new method of plasma generation using microwave propagation along plasma–sheath interface; “Microwave–sheath Voltage combination Plasma (MVP)” method. In this work, we tried to homogenize the axial distribution of film thickness and property in internal diamond-like carbon (DLC) coating using MVP by further employing the method proposed by Segner; the characteristic of the method is to repeat the depletion and refill of source gas in a coated tube during plasma-on time and plasma-off time of pulsed plasma generation inside the tube, respectively. The variability of film thickness was improved from 35% in our previous result to 14% in this work. It was further demonstrated that the source gas depletion can be visualized by observing optical emission from Ar atom and C2 dimer.

Studies of the cathode sheath of a low pressure hollow cathode discharge

In this work, a theoretical model for the cathode sheath using experimental data of a low pressure hollow cathode discharge (HCD) is derived considering it to be non-collisional. Secondary electrons emitted from the cathode surface are taken into account on the discharge model and their influence on the theoretical sheath potential profile is investigated. The plasma parameters and the floating potential profile along the discharge axis were inferred from the current-voltage characteristics of a single Langmuir probe positioned at the inter-cathode space of the HCD. For a low pressure HCD, typical values of the electron density and electron temperature are ne ≈ 1016 m−3 and Te = 4 eV, respectively. By using the probe data the floating potential profile was determined to verify the position of the plasma-cathode sheath interface and to promote a qualitative discussion between theoretical and experimental results.

Capillary electrophoresis‐mass spectrometry methods for tryptic peptide mapping of therapeutic antibodies

Tryptic peptide mapping is routinely used in the biotech industry to confirm primary sequence, cell line stability, and to analyze posttranslational modifications. Peptide analysis is generally done by reverse phase liquid chromatography with UV or mass spectrometric detection. This method provides excellent resolution and sequence coverage. However, traditional methods are slow, and generally cannot detect small, hydrophilic peptides due to coelution with the column void volume. In this work, complementary CE-MS peptide analysis methods have been developed. The analyses are performed on a traditional CE-MS instrument with a sheath interface, and also on a novel sheathless interface that promises improved resolution and limit of detection. The methods were performed on a tryptic digest of a therapeutic monoclonal antibody for which LC-MS detects 97% sequence coverage. The 3% not covered consists of 11 peptides containing three amino acids or fewer, including two in the critical complementarity binding domain. Without further processing, the same tryptic digest was analyzed by CE-MS. Separation and detection of the 11 small peptides was achieved on CE-MS systems with both interfaces. The sheathless system produced better peak capacity and gave mass spectra with significantly less noise, while the sheath system proved to have better repeatability.

Influence of Ni and Cu contamination on the superconducting properties ofMgB2 filaments

Technical MgB2 wires usually have a sheath composite consisting of different metals. For the inner sheathwith direct contact to the superconducting filament, chemically inert Nb maybe used as a reaction barrier and thermal stabilization is provided by a highlyconductive metal like Cu. A mechanical reinforcement can be achieved by theaddition of stainless steel. In order to illuminate the influence of defects in thereaction barrier, monofilament in situ wires with direct contact between theMgB2 filament and frequently applied reactive sheath metals like Cu, Ni or Monel are studied. Reactionsof Mg and B with a Cu-containing sheath lead to Cu-based by-products penetrating the wholefilament. Reactions with Ni-containing sheaths lead to Ni-based by-products which tend toremain at the filament–sheath interface. Cu and/or Ni contamination of the filament lowers theMgB2-forming temperature due to the eutectic reaction between Mg, Niand Cu. Thus, for the samples heat-treated at low temperaturesJC and(partly) TC are increased compared to stainless-steel-sheathed wires. At high heattreatment temperatures uncontaminated filaments lead to the highestJC values. From the point of view of broken reaction barriers in real wires, the contaminationof the filament with Cu and/or Ni does not necessarily constrain the superconductivity; itmay even improve the properties of the wire, depending on the desired application.

Plasma and sheath

In the analysis of a finite plasma system in contact with an absorbing wall separate model zones of a quasi-neutral plasma body and a thin space charge sheath may be distinguished. This subdivision goes back to Langmuir and is based on a small ratio of the Debye length λD and the plasma dimension L. It is strictly valid only in the asymptotic limit λD/L → 0. In this limit the plasma–sheath interface, the ‘sheath edge’, is characterized by a formal singularity of the plasma solution and closely related to the Bohm criterion. For finite λD, in contrast, there are difficulties in the interpretation of the plasma–sheath transition, in the application of the Bohm criterion, and in a reasonable definition of the plasma–sheath boundary. These difficulties are discussed both in the framework of a fluid and of a kinetic analysis on the basis of an ‘intermediate scale’ analysis. Special emphasis is given to the matching problem and its relation to the eigenvalue problem caused by the plasma balance.

Filament Microstructure and Superconducting Properties of ${\rm MgB}_{2}$ Wires With Thin Filaments

Technical conductors for magnet applications require a highly conductive sheath component and multifilament geometry to ensure sufficient thermal stability. As the filament size of such wires is in the order of 100 mum and below, impurity phases can easily lead to a decrease of the current carrying capability. Not only the quality of precursor powders plays an important role in achieving homogeneous filaments, but also reaction products emerging during heat treatments. With decreasing filament diameter the influence of impurity phases on the current carrying capability becomes more and more pronounced. In this paper we present filament microstructures and superconducting properties of mono- and multifilamentary MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires with thin filaments. In SiC doped multifilament wires with Fe sheath and without Nb barrier a thick reaction layer emerged during heat treatments and changed the composition of the filament close to the sheath. These conductors showed a strong degradation of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (B)with decreasing filament diameter. We show that a Nb barrier can easily prevent reaction layers at the filament sheath interface. For undoped and SiC doped monofilament wires with Nb barrier, Cu stabilizer and stainless steel (SS) reinforcement, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values even increased with decreasing filament diameter. We obtained J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> as high as 387 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 3 Tesla for undoped and 100 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 6 Tesla for SiC doped wires with only 164 mum outside diameter and 65 mum filament diameter.

Capillary electrophoresis‐mass spectrometry of citrus endophytic bacteria siderophores

CE-ESI-MS with a liquid sheath interface and IT mass analyzer was used for analysis of siderophores from different strains of Methylobacterium spp. citrus endophyte extracts. Three Methylobacterium strains were investigated according to positive bioassay tests. Bacteria cultures were grown under Fe(III) absence (siderophore producing cultures) and under Fe(III) presence (control cultures). Siderophores were extracted from culture supernatant with polystyrene resins. BGE and sheath-liquid composition were optimized, respectively, in order to assure both, best peak resolution and ESI-MS sensitivity. The best analysis conditions were obtained with 100 mmol/L ammonium bicarbonate at pH 8 as BGE and methanol:H(2)O 25:75 + 0.05% formic acid as sheath liquid. CZE-ESI-MS analysis revealed two possible siderophores, according to bacterium species, presenting M(r) of 1004.3 and 798.3 Da.

Subsurface cisterna-lined axonal invaginations and double-walled vesicles at the axonal–myelin sheath interface

The axonal–myelin sheath interface of vertebrate myelinated axons possesses special structural complexities, and there may be an intercellular macromolecular traffic transversing the periaxonal cleft that spans the internodal axon. By conventional electron microscopy and serial sectioning, we observed a category of double-walled vesicles at the axonal–myelin sheath interface, which often contained ribosome-like particles or endoplasmic reticulum. Some of them were demonstrated to continue with the subjacent axon with a thin stalk. In addition, we described a special category of axonal invaginations, probably mediated by subsurface cisternae. The functional implications of these specialized structures were discussed.

Investigating mechanical behaviour at a core–sheath interface in peripheral nerve

As peripheral nerves bend and stretch, internal elements need to move in relation to each other. However, the way in which intraneural components interact is poorly understood. Previous work identified a distinct core and sheath in the rat sciatic nerve and provides a useful model with which to investigate this interaction. Here we have focused on identifying the mechanical and anatomical characteristics of the interface between core and sheath. Nerve samples, 15 and 20 mm long, of rat sciatic nerves were harvested and placed in a purpose-built jig, and a tensile testing machine was used to pull core from sheath. Mechanical tests of specimens in which core had been previously pulled from sheath by 25% of its initial length achieved a mean pull-out force approximately six times smaller than that achieved using intact controls. These results are consistent with the proposal that core-sheath interactions involve physical connections rather than a viscous fluid interface. Anatomical features of this interface were characterised using transmission electron microscopy. It appeared that sheath was derived from epineurium and most of the perineurium, whilst core consisted of endoneurium and a small proportion of the perineurium: the plane of cleavage appeared to involve the innermost perineurial cell layer.

Changes in plasmodesmata frequency in vascular bundles of maize seedling leaf induced by growth at sub-optimal temperatures in relation to photosynthesis and assimilate export

The frequency of plasmodesmata at the cell interfaces important for C4 photosynthesis was studied in two inbred lines of maize, KW 1074 (flint, chilling-tolerant) and CM 109 (dent, chilling-sensitive), grown until the third leaf stage at sub-optimal temperature (14/12 °C) in relation to the photosynthesis and assimilate export. The number of plasmodesmata linking mesophyll and bundle sheath as well as bundle sheath and parenchyma cells in the third leaf increased in both lines compared to control leaves at the same physiological stage grown at the optimal temperature (24/22 °C). The number of plasmodesmata on the mesophyll–bundle sheath interface increased relatively more in the KW 1074 line than in CM 109. At the sub-optimal temperature, there was also an increase in the plasmodesmata frequency between vascular parenchyma cells, but only in KW 1074 the differences were statistically significant. Plants of both lines grown at low temperature exhibited a decrease of Φ PSII. However, the light response curve of Φ PSII as well as the gas exchange rate in leaves of plants grown at sub-optimal temperature showed values higher in the KW 1074 than in the CM 109 line. KW 1074 seedlings also showed shorter the time interval between 14C incorporation into a leaf and 14C-assimilate appearance in the transport path. The genotypic difference in the photosynthetic competence of plants grown at sub-optimal temperatures might thus be related to the ability to increase the number of plasmodesmata at the interfaces between mesophyll, bundle sheath, and parenchyma cells. Such an increase in plasmodesmata density enable more efficient exchange of photosynthetic metabolites as well as export of sucrose at the temperatures which slow down the diffusion from cell to cell.

Particle-in-cell/Monte Carlo simulation of a capacitively coupled radio frequency Ar/CF4 discharge: Effect of gas composition

A one-dimensional particle-in-cell/Monte Carlo model is developed to study a capacitively coupled radio frequency discharge in a gas mixture of argon and CF4. The simulation takes into account the following charged particles: electrons, two kinds of positive ions (Ar+, CF3+), and two kinds of negative ions (F−, CF3−). The model considers electron–Ar collisions, electron−CF4 collisions, various kinds of collisions of CF3+, F−, CF3−, or Ar+ with Ar or CF4, and positive–negative ion recombination. The probability for the positive–negative ion recombination is determined from a recombination rate constant. The ion–neutral elastic and reactive collisions are simulated by an ion–molecule collision model for endothermic reactions. The typical results of this model are electron and ion densities, fluxes and energy distributions, collision rates, and electric field and potential distributions. The simulation is performed for 0.1/0.9, 0.5/0.5, and 0.9/0.1 ratios of a Ar/CF4 mixture, as well as for pure Ar and pure CF4 discharges at a pressure of 200 mTorr. It is observed that at high CF4 concentration the discharge behaves as a typical electronegative discharge and that CF3+ is the major positive ion. At low CF4 concentration, keeping the other operating parameters the same, the double layer structure and the electron density maxima at the bulk–sheath interface, which are representative for an electronegative discharge, disappear and the Ar+ density exceeds the CF3+ density by more than 1 order of magnitude. The results show that the F− ions are the dominant negatively charged species for all Ar/CF4 ratios investigated.

On the consistency of the collisionless sheath model

It is shown that the collisionless sheath model found in textbooks is internally inconsistent. It generates results that contradict the main assumption of the sheath model, namely the negligible ionization. The source of the inconsistency is in the improper boundary condition at the plasma–sheath interface and in the inconsistent derivation of the sheath equations.

How to patch active plasma and collisionless sheath: A practical guide

Most plasmas have a very thin sheath compared with the plasma dimension. This necessitates separate calculations of the plasma and sheath. The Bohm criterion provides the boundary condition for calculation of plasma profiles. To calculate sheath properties a value of the electric field at the plasma–sheath interface has to be specified in addition to the Bohm criterion. The value of the boundary electric field and a robust procedure to approximately patch plasma and collisionless sheath with a very good accuracy are reported.

Plasma flow and plasma–wall transition in Hall thruster channel

In this paper a model of the quasineutral plasma and the transition between the plasma and the dielectric wall in a Hall thruster channel is developed. The plasma is considered using a two-dimensional hydrodynamic approximation while the sheath in front of the dielectric surface is considered to be one dimensional and collisionless. The dielectric wall effect is taken into account by introducing an effective coefficient of the secondary electron emission (SEE), s. In order to develop a self-consistent model, the boundary parameters at the sheath edge (ion velocity and electric field) are obtained from the two-dimensional plasma bulk model. In the considered condition, i.e., ion temperature much smaller than that of electrons and significant ion acceleration in the axial direction, the presheath scale length becomes comparable to the channel width so that the plasma channel becomes an effective presheath. It is found that the radial ion velocity component at the plasma–sheath interface varies along the thruster channel from about 0.5Cs (Cs is the Bohm velocity) near the anode up to the Bohm velocity near the exit plane dependent on the SEE coefficient. In addition, the secondary electron emission significantly affects the electron temperature distribution along the channel. For instance in the case of s=0.95, the electron temperature peaks at about 16 eV, while in the case of s=0.8 it peaks at about 30 eV. The predicted electron temperature is close to that measured experimentally. The model predictions of the dependence of the current–voltage characteristic of the E×B discharge on the SEE coefficient are found to be consistent with experiment.

Boundary Conditions and Particle Loading for the Modeling of a Semi-infinite Plasma

We present a method for the treatment of the boundary conditions and the particle loading in a self-consistent semi-infinite Particle-In-Cell simulation. We assume a nonionizing, collisional plasma in contact with an electrode. The simulation is planar and one-dimensional (1d-2v), and is driven by an rf and/or DC current source. The particle loading is done by appropriate drifting velocity distributions, both for ions and electrons. Limiting the simulation to the region immediately adjacent to the electrode dramatically improves run-times and diagnostic resolution and is ideal for studying the sheath and plasma-sheath interface independently from the rest of the plasma. The method can be generalized easily to model cylindrical and spherical geometries, and surface processes can also be included. The control parameters of the simulation are the values for the current drive (Irf, ωrf, IDC), the species temperatures in the bulk plasma Ts, and the values of the ion fluxes Γs.

Fast ultrahigh-pressure liquid chromatography: On-column UV and time-of-flight mass spectrometric detection

An ultrahigh-pressure liquid chromatography (UHPLC) system capable of generating 52,000 psi (3,600 bar) was constructed. Fused-silica capillaries with inner diameters of 29 μm and lengths up to 70 cm were packed with nonporous 1.5-μm isohexylsilane-modified (C6) silica particles using a carbon dioxide enhanced slurry packing method. The system performance was evaluated by monitoring the UV absorbance of small organic compounds on-column. At optimum linear velocity, efficiencies as high as 570,000 plates m−1 (k=0.5) were obtained. Analysis times could be reduced significantly, both by operating at higher than optimum linear velocities and by shortening the column. Selected benzodiazepines and triazine herbicides were separated in less than 60 s. The UHPLC system was also coupled to a time-of-flight mass spectrometer for the separation and detection of selected benzodiazepines. A loss in efficiency due to the liquid sheath interface was observed. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 631–643, 1999