Spot Mode Research Articles

Heating of refractory cathodes by high-pressure arc plasmas: II

Solitary spots on infinite planar cathodes and diffuse and axially symmetric spot modes on finite cathodes of high-pressure arc discharges are studied in a wide range of arc currents. General features are analysed and extensive numerical results on planar and cylindrical tungsten cathodes of atmospheric-pressure argon arcs are given for currents of up to 100 kA. It is shown, in particular, that the temperature of cathode surface inside a solitary spot varies relatively weakly and may be estimated, to the accuracy of about 200–300 K, without actually solving the thermal conduction equation in the cathode body. Asymptotic behaviour of solutions for finite cathodes in the limiting case of high currents is found and confirmed by numerical results. A general pattern of current–voltage characteristics of various modes on finite cathodes suggested previously on the basis of bifurcation analysis is confirmed. A transition from the spot modes on a finite cathode in the limit of large cathode dimensions to the solitary spot mode on an infinite planar cathode is studied. It is found that the solitary spot mode represents a limiting form of the high-voltage spot mode on a finite cathode. A question of distinguishing between diffuse and spot modes on finite cathodes is considered.

Theory and modelling of arc cathodes

Understanding of cathode phenomena in arc discharges has considerably improved in recent years due to the progress both in experiment and in theory. A review of the present understanding of cathode phenomena in high-pressure arc discharges is given. The effect of processes in the arc column on the cathode part of the discharge in most cases is weak, which is a consequence of the fact that the energy flux coming to the cathode is formed in a thin near-cathode plasma region. A theoretical model of the near-cathode region is described. The model can be used both by itself or as a part of a code modelling the whole system of arc electrodes. In particular, the model results in a simple estimate of the upper limit of the cathode temperature and of the lower limit of the temperature inside a cathode spot. A simple asymptotic solution to the thermal conduction equation in the cathode body, describing a steady-state cathode spot, may be obtained making use of the fact that the energy flux from the plasma to the cathode surface is localized in a narrow temperature range. Approaches to a self-consistent modelling of diffuse and spot modes of current transfer to the cathode are discussed.

Observation of different modes of cathodic arc attachment to HID electrodes in a model lamp

Electrodes made of pure and doped tungsten are operated in a special model lamp with a DC current of 1-10 Ain argon or xenon atmosphere within the pressure range of 0.1-1 MPa. Cylindrical electrodes withdifferent designs are investigated with regard to the mode of cathodic arc attachment. Three modes are observed: a diffuse mode, a spot mode and a super-spot mode. The major difference between the diffuse and the spot mode isthe current density, which is low in the case of the diffuse mode and high in the spot mode. The diffuse mode isfavoured by high current, low pressure and weak cooling of the electrode, the spot mode by the oppositeconditions. In a transition region the cathode changes statistically between these modes. Whereas the globalelectrode temperature and the cathode fall of the diffuse and the spot mode differ slightly, the super-spot modeis associated with a significantly decreased global electrode temperature and cathode fall at similarparameters. SEM pictures show that the surface structure of the electrodes has wide influence on the mode ofcathodic arc attachment. Due to the significant difference between the super-spot mode and the two other modesthis paper is concentrated on the comparison between the spot and the diffuse mode.

Observation of an extremely constricted cathodic arc attachment to electrodes of high intensity discharge lamps

Usually two modes of arc attachment to cathodes for high intensitydischarge (HID) lamps are observed, the spot mode for highpressure, low currents and large electrode diameters and thediffuse mode for the opposite conditions. Recently, a veryconstricted attachment to cathodes was observed both in real HIDlamps and in Bochum's model lamp. In the model lamp, the extremelyconstricted arc attachment was observed in argon and xenonatmosphere at a pressure p = 0.26 MPa and for currentsI = 1-6 A. Whereas the global electrode temperature andthe cathode fall of the diffuse and the spot mode differ onlyslightly, the extremely constricted arc attachment (so-called`super spot mode') is associated with a significantly decreasedglobal electrode temperature at similar parameters. The reductionin electrode temperature implies low cathode falls and lowelectrode losses, respectively. Scanning electron micrographs show that the surfacestructure of the electrodes has substantial influence on the modeof cathodic arc attachment.

Arc attachment and fall voltage on the cathode of an ac high-pressure mercury discharge

By observation of arc attachment on the cathode of an ac high-pressure mercury discharge, a transition from a diffuse mode at low current to a spot mode at higher current was found. The transition was accompanied with a sudden decrease in the arc voltage of about 10 V. At decreasing current there is a reverse transition with a voltage increase. A recent cathode-fall model shows quite a similar behaviour. Further, the electrode sheath voltage (essentially the sum of anode and cathode fall) was measured and compared with the calculated cathode fall. The obvious differences especially during the current rise were ascribed to the anode fall which should attain highly negative values in the order of 10 V.

Arc spot formation on cold cathodes in high-intensity discharge lamps

Summary form only given, as follows. We present measurements of arc spot formation during the first 25 microseconds after breakdown by applying high voltage pulses at different currents to arc tubes containing either Ar-only or Ar and Hg vapor. This work has applications to HID lamp starting, since arc spot modes present difficult ballast conditions and can erode electrodes if condensed Hg is not present. Our results show that after breakdown and above a critical current level, the abnormal glow collapses to a low-voltage arc spot in less than a microsecond. Estimates of the critical cathode fields, based on measured abnormal glow I-V characteristics, were in the range of 2 × 10/sup 5/ V/cm. The fields were weakly dependent on buffer gas composition. Such fields require at least a factor of one hundred enhancement to reach the field-emission regime. High-speed imaging of the evolving spot confirmed the presence of a remnant abnormal glow emission during the first hundred nanoseconds after breakdown for the Ar-Hg case. Transient spectroscopic measurements of the cathode region and monochromatic high-speed imaging revealed the presence of strong Hg and Hg/sup +/ emissions in much less than one-microsecond in the Ar-Hg case. This suggests that ionized Hg vapor aids in the formation of the arc spot and sustains it. No W emissions were observed for this case. In the Ar-only discharge, W emissions evolved over many microseconds and had very low excitation temperatures. Thus while W was evaporated from the spot region, its role in forming the ionizable vapor for the arc spot in the Ar-only case was less clear. Finally, we discuss implications of this work on theories of arc spot formation and the electrode erosion.

Spatial variation of luminescence of InGaN alloys measured by highly-spatially-resolved scanning cathodoluminescence

Cathodoluminescence (CL) measurements were performed on thick InGaN samples over a wide range of indium concentrations ([ln]=0.03–0.20). The layers were grown on c-plane sapphire using an AlGaN buffer layer, followed by 4 μm GaN, and 100 nm InGaN films. These thick InGaN layers were exceptionally specular, especially at low indium concentrations. Some degree of microscopic roughness was observed for x>0.15. With the objective of correlating the microstructure to the spatial dependence of the luminescence, average CL spectra over large areas were compared with high-spatial-resolution CL measurements. The variation of the indium-related peaks over large areas was observed under low magnification scanned CL. The full width at half-maximum in the spot mode is typically smaller than wide-area scans by a factor of 2 to 3. The size of the areas with constant emission wavelength changes significantly with indium content. It has also been observed that the distribution of the local CL peak wavelength changes from single-mode Gaussian to a multimodal distribution for x>0.1. A shoulder of the main peak at the low energy side appears at higher indium content. Spot-mode spectra show that this low energy shoulder consists of single lines at specific wavelengths. In highly resolved monochromatic images we correlate the low energy side shoulder to two structural defects: pinholes and grooves. Particularly, bright luminescence occurs at the center of the grooves.

Cathodoluminescence Investigations of Interfaces in InGaN/GaN/Sapphire Structures

Scanning electron microscopy and cathodoluminescence (CL) in spot and depth-profiling modes were used to evaluate the in-plane and in-depth uniformity of light emission from InGaN/GaN quantum well (QW) structures. The structures were grown by MOCVD on sapphire with a low-temperature (LT) GaN buffer. Depth-profiling CL investigations were used to identify the observed CL emissions, which show a complicated in-depth evolution. The influence of a LT GaN buffer on the structural and optical properties of the GaN/sapphire interface is discussed.

Cathodoluminescence Investigations of Interfaces in InGaN/GaN/Sapphire Structures

Scanning electron microscopy and cathodoluminescence (CL) in spot and depth-profiling modes were used to evaluate the in-plane and in-depth uniformity of light emission from InGaN/GaN quantum well (QW) structures. The structures were grown by MOCVD on sapphire with a low-temperature (LT) GaN buffer. Depth-profiling CL investigations were used to identify the observed CL emissions, which show a complicated in-depth evolution. The influence of a LT GaN buffer on the structural and optical properties of the GaN/sapphire interface is discussed.

Numerical modelling of a dynamic mode change of arc attachmentto cathodes of high-intensity discharge lamps

A commercial finite-element analysis code is used to calculateaxisymmetric transient temperature fields, current density distributionsand energy fluxes in the cathode of a high-intensity discharge lamppowered by a unipolar current I(t). The calculation is based on a sheathmodel of the near-cathode layer and the assumption that the resultingcathode fall voltage U(t) is spatially constant. So a calculation of thefields in the arc region is not necessary. Depending on the electrodedimensions and the frequency of I(t) hysteresis-like periodictransitions between the diffuse and spot modes of arc attachment areobserved in the numerical solutions for 0.62 MPa mercury discharges withpure, as well as thoriated, tungsten cathodes. These findings are comparedwith measurements from the literature. The simulation shows that thelatent heat has negligible influence on spot formation and that theaverage cathode power loss can be smaller for a time-varying current thanfor the direct current with equal mean value. The presented method can beused as a guide to better electrode geometries and current forms.

Near-cathode phenomena in HID lamps

A brief review is given of the present understanding of cathode phenomena in arc discharges. A model of a near-cathode plasma region of high-pressure arcs with hot cathodes is described. Possible ways of integration of the model into numerical codes describing a lamp on the whole are discussed. An approach is suggested allowing one to estimate the upper limit of the cathode temperature and the lower limit of the temperature inside a cathode spot without detailed calculations of a temperature distribution inside the cathode. Solutions describing a diffuse and spot modes of current transfer to a thin thermionic cathode are presented and found to compare favorably with available experimental data.

The hot refractory anode vacuum arc: a new plasma source for metallic film deposition

A new mode of the vacuum arc, the Hot Refractory Anode Vacuum Arc (HRAVA), was investigated as a plasma source for depositing coatings. Arc currents of 155–340 A were sustained for periods of up to 120 s between a water-cooled Cu source cathode, and a non-consumable refractory anode, which was heated by the arc. Cu coatings were deposited on ground stainless steel and glass substrates. A shutter controlled when and how long the substrate was exposed to the plasma. The coating rates were measured by weighing the substrates, and the macro particles (MPs) presence on the coating surface was examined by optical microscopy. Films that formed in a 30-s exposure at the beginning of a 175-A arc, when it operated in the cathode spot mode, were heavily contaminated with MPs. The density of MPs with diameters of between 3 μm and 50 μm was ∼10 3 mm −2. However, with a 30-s exposure, which began 30 s after arc initiation, by which time the arc was in the HRAVA mode, the MP density was reduced to ∼1 mm −2. The HRAVA deposition rate was ∼1–2μm/min onto substrates placed at distances of ∼110–120 mm from the arc axis. The HRAVA deposition rate is comparable to filtered cathode spot vacuum arc deposition, but over a much larger deposition area.

Quantitative Analysis of Metal Sequestering in Different Cell Components of Gloeocapsa alpicola in the Overplus Phase

Abstract The elemental composition of polyphosphate bodies (PPB's) and other components of the cell have previously been determined by using energy dispersive X-ray spectrometers(EDX). In this present study we perform a quantitative analysis of normally grown cells and cells that were grown in the overplus phenomenon. Cells of Gloeocapsa alpicola were grown in modified Fitzgerald's media and harvested after a logarithmic growth phase of 14 days. The overplus cells were grown, as has been previously described. The cells were exposed to 20ppm of Al, Cd, Cu, Mn, Ni, Pb, and Zn, as well as a control, for one hour. The cells were then air dried on formvar coated grids or fixed and embedded in EPON according to Luft's procedure. For Xn-ay analysis cells were first located using the TEM mode, and then the microscope was switched to the STEM mode. Analyses of cell components were carried out using the spot mode (75Kv).

Measurements of Ions from High-Current Hollow Cathodes Using Electrostatic Energy Analyzer

Energy distributions of ions emanating from high-current, xenon hollow cathodes were measured using an electrostatic energy analyzer. Ions with energies much greater thanmaximumpotential differences between electrodes were observed. This result is consistent with previous retarding-potential-analyzer data, but the electrostaticenergy-analyzer data show much more complicated structures. To characterize these distributions, they were Ž t with simple Gaussian functions, and the effects of discharge conditions on high-energy-ion characteristics were investigated. The energy of ions emanating from cathodes appears to increase as discharge current is increased or cathode  ow rate is reduced. Higher discharge currents cause the ion populations in the high-energy tails of the distributions to increase in the plume mode while the high-energy tails decrease in the spot mode.

Thermal load of laser aperture masks in nonmechanical trephination for penetrating keratoplasty with the Er:YAG laser: comparison between stainless steel and ceramic masks.

Thermal effects on the laser aperture mask may play a major role in the thermal loading of the cornea during nonmechanical trephination in penetrating keratoplasty. The purpose of this study was to assess the temperature increase on the laser mask using the 2.94-microm Er:YAG laser in order to find suitable parameters for avoidance of thermal damage to the cornea. Thermal load measurements were performed on donor (7.5 mm trephination diameter, 0.7 mm thickness, central hole 3.0 mm) and recipient (7.5 mm trephination diameter, 0.7 mm thickness, outer diameter 13.0 mm) aperture masks. The masks were either mounted on a thermal isolator or fixed directly on porcine corneal samples. Temperature increase was measured either under static conditions in the ablation area (setup 1) and at the opposite side of the mask (setup 2) or in the ablation area under dynamic conditions, rotating the whole globe to simulate a constant trephination speed with the mask positioned directly on a porcine cornea (setup 3). We used the NWL Er:YAG solid-state laser in a 1.3-mm free-running spot mode focused on the trephination margin (half of the beam on the mask and half of it on the cornea) with a pulse energy of 200 or 400 mJ and 18CrNi10 stainless steel versus three different types of ceramic masks (silicium carbide, silicium nitrite, aluminum oxide). Temperature was assessed using an infrared pyrometer with automatic data acquisition software for a personal computer. Overall, the temperature rise ranged between 43.6 K (metal donor mask at the trephination area with 400 mJ pulse energy) and 3.3 K (silicium carbide recipient mask at the opposite side of the mask with 200 mJ pulse energy). With all setups and both energy levels, the heating of the metal mask was significantly higher (P<0.02) than the heating of the three types of ceramic masks. The silicium carbide masks revealed the lowest temperature rise. Comparing the three setups, the temperature rise was maximal under static conditions in the ablation area and minimal at the opposite side, with the dynamic setup ranging in between. Temperature rise was significantly greater (P<0.04) in donor masks than in recipient masks for each mask material and both energy levels. The physical characteristics of silicium carbide masks seem superior to those of metal masks with regard to minimizing the thermal load of the epithelium or superficial stroma during Er:YAG laser trephination of the cornea for penetrating keratoplasty.

Numerical modelling of arc attachment to cathodes of high-intensity discharge lamps

The axisymmetric energy fluxes in an arc cathode are calculated by a commercial finite-element analysis code. The model decouples the sheath from the arc. Exclusively a spot mode exists at small integral loads by heat flux in the steady state for a cathode in 1 MPa xenon and direct currents below 10 A. The diffuse mode occurs at high loads by heat flux or current. In-between multiple solutions are possible. A spot mode is favoured by lower thermal conductivity, higher emissivity, lower support temperature, greater diameter and a smaller length of the cathode. This conforms with experiments. The method provides a design tool for cathodes.

Laser-induced fluorescence characterization of ions emitted from hollow cathodes

Laser-induced fluorescence (LIF) was used to measure the mean and variance of the velocity distribution of xenon ions emitted from two hollow cathode assemblies operating at low power. High-energy ions detected in plume-mode and spot-mode operation are consistent with the potential-hill model of high-energy ion production. The distributions of velocities were modeled, yielding temperatures on the order of a few electronvolts in plume mode and 1 eV in spot mode. The LIF of neutral xenon atoms in the plumes indicated thermal temperatures significantly less than the temperatures associated with the ion velocity distributions.

Improvement in Quantitative X-ray Microanalysis of Biological Cryosections.

: The accuracy of Na measurements in biological cryosections has been improved through replacement of the 8 µm Be window of the Si(Li) detector with a Super Atmospheric Thin Window (SuperATW) window mounted on a germanium electron microscope (GEM) detector. Greater accuracy of Ca measurements was also attained when a GEM detector instead of an Si(Li) detector was used for analysis of total Ca concentrations in cardiac muscle. Although the advantage of improved sensitivity for Na and Ca is lost in spot mode analysis because of the overriding contribution of biological variability, this advantage becomes important in X-ray mapping. We compared the two systems by analyzing cryosections of identical biological material under comparable conditions. We found that Si contamination in thin biological cryosections is unpredictable; it may be widespread and differ in degree in various cell compartments of the same cell. Hence, a correction for Si contamination should be included in the calculations of elemental concentrations. We suggest here a procedure for measuring and subtracting Si contamination from elemental spectra.

Characterization of the arc evaporation of a hot boron cathode

Cathodic arc discharge with a hot boron cathode has been characterized by high-speed photography, mass spectroscopy and ion energy analysis. During operation, the cathode surface exhibits an extended molten area in which moving cathode spots could be detected by the high-speed cine-camera. This leads to the conclusion that the hot boron cathode exhibits a mixed arc mode including features both of thermionic and spot modes. The charge state distribution, as well as the kinetic energy of the boron ions emerging from the cathode, were measured and conclusions have been drawn with respect to the underlying mechanisms.

Observation of tungsten particles in vacuum arcs by laser induced fluorescence

We have clarified the relation between the decay of tungsten ion density in the vicinity of current zero and vacuum arc mode in high current period by using a laser induced fluorescence method in tungsten vacuum arcs of 60 Hz sinusoidal current with the peak value of 3.3, 6.7, and 9.8 kA. In the case of 6.7 kA, the arc mode was the anode spot mode. Because of the generation of the anode spot, the tungsten ion density near the anode was higher than near the cathode and the density near the anode was about ten times as high as the case of 3.3 kA which was the diffuse mode. In the case of 9.8 kA, which was the intense arc mode, the density near the anode was not significantly different from the case of 6.7 kA. The density near the cathode was higher than near the anode and tungsten ions were observed till about 30 /spl mu/s after current zero while they disappeared at current zero in the other cases.