Tungsten Emitter Research Articles

Influence of roughness and composition on the total emissivity of tungsten, rhenium and tungsten–25% rhenium alloy at high temperature

Abstract Measurements of the total directional and hemispherical emissivity of tungsten, rhenium and tungsten with 25% rhenium samples of various surface states was carried out in two wavelength ranges, 0.6–2.8 and 0.6–40 μm, from 1300 up to 2500 K in high vacuum. A clear trend was identified for the ratio of the 0.6–2.8 μm to the 0.6–40 μm hemispherical emissivity (assumed to be close to the true α/e ratio, which is the ratio of the solar absorptivity α to the total emissivity e) as a function of the RMS roughness. This ratio is decreasing with increasing RMS roughness rapidly up to around 1 μm and then tends to stabilize for higher roughness. It suggests that increasing the roughness, starting from a flat surface, will progressively give more weight to the long wavelength range emissivities in comparison to the short wavelength ones. Beside the RMS roughness, a sharp surface structure on a finer scale found on some samples also led to an additional decrease of the α/e ratio, but these structures are not stable at high temperature and disappear due to a natural high temperature smoothing. Finally our results also show that the α/e ratio of rhenium is lower than the one of tungsten.

Method for the calibration of the spectral irradiance of tungsten filament transfer standard sources traceable to synchrotron radiation

The spectral irradiance calibration of tungsten strip and spiral filament lamps applying synchrotron radiation revealed that the spectral irradiance in the wavelength range from 280 to 400nm can be well approximated by blackbody radiation according to Planck's law. Consequently, the spectral irradiance of the filament lamp can then be described by an effective irradiance temperature, which would be beneficial for practical measurements. Including the emissivity of tungsten into the approximation, the model can be expanded to visible and near-infrared wavelength regions. The effective irradiance temperature dependence of the lamp current was investigated and appeared to be close to linear.

Simulation of spectroscopic patterns obtained in W/C test-limiter sputtering experiment at TEXTOR

On the TEXTOR tokamak various experiments aimed at investigation of tungsten erosion and transport are performed. In one experiment a spherical W/C twin limiter positioned close to the last-closed flux surface in the near scrape-off layer was exposed to a number of comparable plasma discharges with stepwise variations of edge plasma parameters. Spatial distribution of tungsten and carbon light emission was recorded with two dimensional CCD cameras and spectrometer systems with high spectral and spatial resolution. Penetration depths, tungsten sputtering fluxes and erosion yields were measured. Comparison between experimental data and the results of modelling with the 3D Monte-Carlo code ERO is performed. The main objective of this study was to test the adequacy of the existing atomic data for neutral tungsten. The modelled penetration depths of the light emission of tungsten are a factor of 2–3 smaller than in experiment, which may indicate the overestimation of ionization rates.

Double-coiled tungsten filament lamps as absolute spectral irradiance reference sources

We have developed a physical model for the spectral behaviour of the radiation of tungsten filament lamps. The model is based on Planck's radiation law, published values for the emissivity of tungsten, measurement geometry and measured values for a residual correction function. In this paper, we study the physical interpretation of the model. The studied properties influencing the lamp spectra include the emissivity data used, light recycling effect due to the cylindrical shape of the filament, transmittance of the glass envelope and the halogen gas and temperature dependence of the transmittance. It appears that in addition to tungsten emissivity, one needs to account for the bulb transmittance and the light recycling within the cylindrical filament. We did not observe any wavelength dependence of the absorption of the filling gas.

Melt-layer ejection and material changes of three different tungsten materials under high heat-flux conditions in the tokamak edge plasma of TEXTOR

The behaviour of tungsten (W) plasma-facing components (PFCs) has been investigated in the plasma edge of the TEXTOR tokamak to study melt-layer ejection, macroscopic tungsten erosion from the melt layer as well as the changes of material properties such as grain-size and abundance of voids or bubbles. The parallel heat flux at the radial position of the exposed tungsten tile in the plasma ranges around q‖ ∼ 45 MW m−2 causing samples to be exposed at an impact angle of 35° to 20–30 MW m−2. Locally the temperature reached up to 6000 K, high levels of evaporation and boiling are causing significant erosion in the form of continuous fine spray or droplet ejection. The amount of fine-spray tungsten emission depends strongly on the material properties: in the case of the tungsten–tantalum alloy the effect of spraying and droplet emission is significantly higher at even low temperatures when compared with regular tungsten or even ultra-high purity tungsten which shows almost no spraying at all. Differences in the material composition, grain structure and size may be related to the different evolution of macroscopic erosion. In addition the re-solidified material is studied and strong differences in terms of re-crystallized grain size and evolution of the grain structure and grain orientation are observed. The build up of large voids has been observed.

Regenerable Field Emission Cathodes: Surface Morphology and Performance

This paper reports on a field-emission cathode for use in electric propulsion that has the potential for very long lifetime due to its ability to be regenerated when the emitter tip(s) become damaged. The field-emitting tips were formedby applying an ion-extracting electric potential to a heated indium-coated tungsten needle, knownas a liquidmetal ion source. The liquid-metal ion source was then cooled, freezing in a solid nanotip at the apex. When the modified emitter was then subjected to electron-extracting potentials, stable and long-lived electron emission was observed. The first goal of this investigation was to operate and quench a liquid-metal ion source at ion emission currents from 1 to 30 A to acquiremicrographs of the surfacemorphology as a function of the ion emission current at quench.Micrographs of the quenched emitter tips revealed Taylor-cone-shaped structures. Some of the quenched emitters exhibited multiple nanoprotrusions on the tapered surface of the microscale Taylor cone, which were capable of electronfield emission.The secondgoal of this investigationwas to compare regenerablefield emitterswith single-needle tungsten field emitters. Each type of emitter was used to obtain electron emission at a vacuum chamber pressure of 10 8 Torr, and then the emitters were exposed to increased pressure, up to 10 5 Torr, to observe how long they could sustain emission. In all cases, emission from the regenerable emitters lasted 10s of hours longer at increased pressure, and it was demonstrated that the tungsten emitters would eventually permanently fail, whereas the regenerable emitters could be repaired when they became damaged.

Tungsten measurement on Alcator C-Mod and EBIT for future fusion reactors1This article is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

Tungsten will be an important element in nearly all future fusion reactors because of its presence in plasma facing components. This makes tungsten a good candidate for a diagnostic element for ion temperature and toroidal velocity measurement, and it makes understanding tungsten emissions important for tokamak power balance. The effect of tungsten on tokamak plasmas is investigated at the Alcator C-Mod tokamak using VUV, bolometry, and soft X-ray spectroscopy. Tungsten was present in Alcator C-Mod as a plasma facing component and through laser blow-off impurity injection. Quasi-continuum emission previously seen at other tokamaks has been identified. Theoretical predictions are presented of tungsten emission that could be expected in future Alcator C-Mod measurements. Furthermore, spectra of highly charged tungsten ions have been studied at the SuperEBIT electron beam ion trap. This emission could prove useful for spectroscopic diagnostics of future high-temperature fusion reactor plasmas.

Permanent magnet finger-size scanning electron microscope columns

This paper presents permanent magnet scanning electron microscope (SEM) designs for both tungsten and field emission guns. Each column makes use of permanent magnet technology and operates at a fixed primary beam voltage. A prototype column operating at a beam voltage of 15kV was made and tested inside the specimen chamber of a conventional SEM. A small electrostatic stigmator unit and dedicated scanning coils were integrated into the column. The scan coils were wound directly around the objective lens iron core in order to reduce its size. Preliminary experimental images of a test grid specimen were obtained through the prototype finger-size column, demonstrating that it is in principle feasible.

Electron field emission from a gold tip under laser irradiation at the plasmon-resonant wavelength

Electron field emission properties of a sharp gold tip under continuous wave laser irradiation at the plasmon-resonant wavelength are investigated. A gold-coated tungsten emitter with a tip radius of 100 nm is used. The plasmon-resonant wavelength of the tip obtained by a finite-difference time-domain calculation is around 530 nm. Thus, I-V characteristics of the tip under laser irradiations at 442, 532, and 633 nm were measured. The intensity of 10 mW, 532 nm irradiation at the focal point was 13.9 kW/cm2. TE-polarized 532 nm irradiation shows the maximum current enhancement despite its smaller photon energy and intensity compared with the irradiation at 442 nm. The current enhancement factor ranged from 13.7 to 217. From the energy dispersive x-ray analysis, these results can be attributed to plasmon resonance. The possible mechanisms of plasmon-resonant current enhancement are considered to be the effective photoassisted field emission and thermal field emission.

Study of the emissivity of rough surfaces periodic using the method of coupled waves analysis (CWA) compared with method of geometrical optics approximation (GOA)

We present in this paper a numerical study of the validity limit of the geometrical optics approximation compared with a differential method which is established according to rigorous formalisms based on the electromagnetic theory. The precedent studies show that this method is adapted to the study of diffraction by periodic rough surfaces. We determine by two methods the emissivity of gold and tungsten for surfaces with a rectangular or sinusoidal profile, for a wavelength equal to 0.55 microns. The monochromatic directional emissivity of these surfaces clearly depends on the angle of incidence, the surface profile, height, period and the nature of the material. We perform our calculations by a method of coupled wave analysis (CWA) and a geometric optics method (GOA). The latter method is theoretically valid only when the dimensions of the cavities are very large compared to the wavelength, while the CWA is theoretically correct whatever these dimensions. The main purpose of this work is to investigate the validity limit of GOA compared with CWA. The obtained results for a fixed height of the grating, allowed us to delimit the validity domain of the optic geometrical approximation for the treated cases. Finally, the agreement between the emissivity calculated by the differential method and that given on the basis of the homogenization theory is satisfactory when the period is much smaller than the wavelength.

Carbon Nanotube Electron Source for Field Emission Scanning Electron Microscopy

A carbon nanotube (CNT) is one of the promising candidates for next generation field emission materials. We have been studied emission properties of CNTs and reported that even a thick (φ∼20 nm) multi-walled nanotube (MWNT) had a small emission area of about 5×10-14 cm2, which resulted in brightness enhancement by 1-2 orders as compared with conventional tungsten field emission electron source. In this study, a single MWNT emitter is mounted on a commercially available field emission scanning electron microscope (FE-SEM). Focused ion beam nano-machinning and nano-manipulation methods are used to prepare MWNT emitters, and practicalities of these emitters for FE-SEM use (beam alignment, beam stability, life time, image quality and resolution) are investigated. As a result, it is proved that the MWNT emitter can be used as a replacement of a single crystalline tungsten emitter without any modification of an existing FE electron gun and its control system. [DOI: 10.1380/ejssnt.2011.400]

Nanostructure formation onto a tip of field gas ion emitter by field‐assisted oxygen etching

AbstractFor focused ion beam technology in the next generation, an adequate emitter shape for field gas ion source was investigated to obtain a higher angular current density, dI/dΩ. A < 111 > ‐oriented tungsten emitter with a trimer‐terminated nanostructure was fabricated by field‐assisted oxygen etching method. Since this method does not utilize thermodynamic phenomena, e.g. faceting of crystal plane with a thermal treatment or pseudomorphic growth of surface layer, shape of the emitter tip has a rotational symmetry with no ridge lines which brings about a decrease of axial beam current. In the preliminary experiment for helium gas, an extraction of ion current was attempted from the trimer at the apex tip, and the increases in helium ion current and dI/dΩ in proportion to the gas pressure were observed. Copyright © 2010 John Wiley & Sons, Ltd.

Tungsten blow-off in response to the ignition of arcing: Revival of arcing issue in future fusion devices

Behaviors of an arc spot ignited on a tungsten electrode in response to a transient heat load are investigated in the linear divertor simulator NAGDIS-II using a pulsed laser as a transient heat source. Tungsten neutral emission from the arc spot is observed with an image intensified fast framing camera. When arcing is initiated, the tungsten emission continues much longer than the laser pulse width of ∼0.6ms. The ignition of arcing becomes much easier compared to clean surface when the tungsten surface is exposed to the helium plasma and morphology is slightly changed.

Plasma surface interaction on the surface of tungsten divertor tiles in LHD

Four graphite divertor tiles coated with a 100μm thick layer of tungsten by using a vacuum plasma spray (VPS) method were installed in the LHD (Large Helical Device) in the 2008FY campaign (a total of 1700 pieces of graphite tiles are used in LHD). Tungsten emission of the plasma and the reliability of the coated tungsten layer and graphite substrate have been investigated by using real-time spectroscopic analysis and post-mortem surface analysis, respectively. The tungsten coated tiles could adequately withstand for a single experimental campaign, the heat and particle loads from the divertor plasma which were highly localized at the strike points. Tungsten accumulation in the core plasma could not be seen for the operation schemes in this campaign, and sputtered tungsten impurities did not have any influence on plasma discharges.

Tungsten spectroscopy relevant to the diagnostics of ITER divertor plasmas

The possibility of using extreme ultraviolet emission from low charge states of tungsten ions to diagnose the divertor plasmas of the ITER tokamak has been investigated. Spectral modelling of Lu-like W3+ to Gd-like W10+ has been performed by using the Flexible Atomic Code, and spectroscopic measurements have been conducted at the Sustained Spheromak Physics Experiment (SSPX) in Livermore. To simulate ITER divertor plasmas, tungsten was introduced into the SSPX spheromak by prefilling it with tungsten hexacarbonyl prior to the usual hydrogen gas injection and initiation of the plasma discharge. The tungsten emission was studied using a grazing-incidence spectrometer.

Calculation of the performance parameters of tungsten filament lamps using the exponent rules

The experimentally known values of the 14 exponents associated with the exponent-rules for vacuum and gas-filled tungsten filament lamps are utilized to obtain the temperature dependence of performance parameters such as power demand, lumen output, life and efficacy. It is also shown that the data on emissivity of tungsten reported by Jones and Langmuir are consistent with theory in contrast to those obtained by Roser and Wensel. Further, the analysis predicts a higher power loss factor and a longer life for gas-filled tungsten filament lamps than for vacuum lamps.

Experimental evaluation of the influence of shank shape of field ion emitter on the angular current density

An adequate emitter shape for a gas field ion source was investigated to obtain a higher angular current density, dI/dΩ. Single-crystalline ⟨111⟩-oriented tungsten emitters having a similar tip radius of curvature and a different taper angle of emitter shank were prepared by dc- and ac-electrochemical polishing. The tip apex of the emitters was terminated with a tungsten trimer by means of remolding and a field evaporation processes. The helium ion currents emitted from these trimers were measured as a function of helium pressure. The ion current and the dI/dΩ obtained from the emitter with a taper angle of 1.5° were enhanced by factors of 2.7 and of 3.9, respectively, compared to those from an emitter with a taper angle of 14.5°. These experimental results suggest that to obtain a higher dI/dΩ, the emitter having a narrower taper angle is better.

Spectral irradiance model for tungsten halogen lamps in 340–850 nm wavelength range

We have developed a physical model for the spectral irradiance of 1 kW tungsten halogen incandescent lamps for the wavelength range 340-850 nm. The model consists of the Planck's radiation law, published values for the emissivity of tungsten, and a residual spectral correction function taking into account unknown factors of the lamp. The correction function was determined by measuring the spectra of a 1000 W, quartz-halogen, tungsten coiled filament (FEL) lamp at different temperatures. The new model was tested with lamps of types FEL and 1000 W, 120 V quartz halogen (DXW). Comparisons with measurements of two national standards laboratories indicate that the model can account for the spectral irradiance values of lamps with an agreement better than 1% throughout the spectral region studied. We further demonstrate that the spectral irradiance of a lamp can be predicted with an expanded uncertainty of 2.6% if the color temperature and illuminance values for the lamp are known with expanded uncertainties of 20 K and 2%, respectively. In addition, it is suggested that the spectral irradiance may be derived from resistance measurements of the filament with lamp on and off.

Development and qualification of a bulk tungsten divertor row for JET

A bulk tungsten divertor row has been developed in the frame of the ITER-like Wall project at JET. It consists of 96 tiles grouped in 48 modules around the torus. The outer strike point is located on those tiles for most of the ITER-relevant, high triangularity plasmas. High power loads (locally up to 10–20 MW/m 2) and erosion rates are expected, even a risk of melting, especially with the transients or ELM loads. These are demanding conditions for an inertially cooled design as prescribed. A lamella design has been selected for the tungsten, arranged to control the eddy and halo current flows. The lamellae must also withstand high temperature gradients (2200 to 220 °C over 40 mm height), without overheating the supporting carrier (600–700 °C maximum). As a consequence of the tungsten emissivity, the radiative cooling drops appreciably in comparison with the current CFC tiles, calling for interleaved plasma scenarios in terms of performance. The compromise between shadowing and power handling is discussed, as well as the consequences for operation. Prototypes have been exposed in TEXTOR and in an electron beam facility (JUDITH-2) to the nominal power density of 7 MW/m 2 for 10 s and, in addition, to higher loads leading to surface temperatures above 2000 °C.

Tungsten and Cobalt: Sheppard et al. Respond

First and foremost, our data from Nevada (Sheppard et al. 2007c) should not be quantitatively compared with that from Oregon. We did not make such a comparison in our article; to reinforce separation of these two studies, we described the results in separate subsections and presented data in separate figures. Our Oregon study was an independent test of dendrochemistry for establishing temporal patterns of environmental tungsten in a town with known emission of airborne tungsten. Tungsten emission in Sweet Home, Oregon, began in November 2000, and tree-ring tungsten in cottonwoods near the emission source increased at that time relative to comparison towns in central Oregon. This test demonstrated that dendrochemistry accurately depicts tungsten availability, at least when using cottonwoods. Therefore, dendrochemistry, especially when using cottonwood trees, can be trusted to accurately depict tungsten availability in Fallon, Nevada, where timing of emission of tungsten particles is not known with certainty. The comparison of real interest was between Fallon and other towns of west-central Nevada. Tree-ring tungsten in Fallon was not significantly different from that of other towns of west central Nevada during the tree-ring period centered on 1991 [Figure 4 in our article (Sheppard et al. 2007c)], before the onset of the leukemia cluster. During the tree-ring period centered on 1995, corresponding to just before the onset of the leukemia cluster, Fallon tree-ring tungsten began trending upward and was significantly higher than Nevada comparison towns. During the following two time periods, overlapping temporally with the childhood leukemia cluster, Fallon tree-ring tungsten continued trending upward and remained higher than Nevada comparison towns, with significance levels at or near p = 0.05 (we provided p-values in place of error bars), thus indicating the temporal correspondence between elevated tungsten in Fallon and the childhood leukemia cluster. The Centers for Disease Control and Prevention (CDC) conclusion that tungsten is not mathematically associated with the leukemia cases of Fallon (CDC 2003) is based on case-comparison testing within Fallon. This does not rule out that an underlying association actually exists but is not detectable by the case-comparison technique. Granted, no relation was reported between leukemia and tungsten exposure, but exposure to tungsten was found to be community-wide, with levels being high both in case children and families and in comparison children and families (CDC 2003). In other words, there was little to no variability in exposure at the community scale (i.e., most everyone in Fallon has been exposed) but high variability in onset of disease (i.e., some people got leukemia but others have not). When variability of an exposure is low relative to individual susceptibility to a disease, genetic studies are needed to identify gene polymorphisms that might make sick children more susceptible to effects of the exposure (Steinberg et al. 2007). Our environmental research in Fallon has followed an ecologic approach with the philosophy that greater variability in exposure between different towns is more important than the minor variability in exposure within communities (Sheppard et al. 2007a). The entire town of Fallon has been compared environmentally with other towns of west-central Nevada. Multiple environmental indicators have been used, such as outdoor airborne particulates (Sheppard et al. 2006), lichens (Sheppard et al. 2007d), surface dust (Sheppard et al. 2007b), and tree rings (Sheppard et al. 2007c). These indicators incorporate environmental conditions differently from one another, yet they have corroborated one another in showing that airborne tungsten is elevated in Fallon relative to other towns of west-central Nevada or the surrounding desert. Additionally, airborne tungsten particles in Fallon have been identified as anthropogenic in origin, and not natural (Sheppard et al. 2007e). Even with this preponderance of evidence showing spatial and temporal patterns of airborne tungsten in Fallon, we still have not concluded in any of our reports on Fallon that exposure to tungsten causes leukemia. Quite the opposite: We have acknowledged that environmental data alone cannot lead to such a conclusion and that direct biomedical testing is needed to establish a causal linkage between tungsten and leukemia. Years ago in an article on disease cluster research, Shimkin (1965) stated that cooperation of industrial management is needed to identify and reduce environmental carcinogens. This comment still rings true today. Kennametal Inc. (Fallon, NV) claims that it supports research in Fallon aimed at understanding the childhood leukemia cluster there (Goodale 2005), but its support is apparently selective. We hereby encourage Kennametal to engage in reasonable dialogue about research in Fallon related to the childhood leukemia cluster.