Tungsten Emitter Research Articles

High-performance three-body near-field thermophotovoltaic energy conversion

Three-body structures have shown great potentials in enhancing the heat transfer rate and tuning radiation spectrum in the near-field region, whereas are rarely considered in improving near-field radiative energy conversion performance. Here, a three-body thermophotovoltaic system configured by a tungsten emitter, a metallic spectrum control layer, and an In0.18Sb0.82Ga photovoltaic cell is considered. By parameter optimization of the spectrum control layer, the efficiency and output power at the gap distance of 10 nm are enhanced from 24.7% and 1.88×105 W/m2 to 35.3% and 3.62×105 W/m2, respectively. The potential mechanism lies in the excitation of coupled surface plasmon polaritons of the metallic spectrum control layer. This work paves the way for applications of three-body structure in thermophotovoltaic systems and designing high-performance energy conversion systems.

ELM Suppression by Boron Powder Injection and Comparison with Lithium Powder Injection on EAST

Type I edge-localized modes (ELMs) in the Experimental Advanced Superconducting Tokamak (EAST) were completely suppressed via boron powder injection into the X-point region of an upper-single null configuration over a wide range of operating conditions (2.8 < Paux < 7.5 MW, 3.8 × 1019 < ne < 6 × 1019 m−3, RF-only and RF + NBI heating scenarios, both grad-B drift directions, and even He ion majority plasmas) (Sun et al. in Nucl. Fusion, 2020). A window of edge B concentration for stable long pulse operation was identified: too low and ELMs return, too high and the discharge suffers radiative collapse. The injection of boron powder above the minimum for ELM suppression coincided with the occurrence of an edge harmonic oscillation detected in magnetics (both on the high-field side and low-field side), in AXUV diodes near the upper X-point, divertor D $${\upalpha }$$ emission, and in a range of other diagnostics (Diallo et al. in: Proceedings of 2020 IAEA fusion energy conference, 2021). No harmonic oscillation was observed when ELMs were present, and stored energy was slightly increased at constant density during ELM suppression. Core tungsten emission during ELM suppression either increased or decreased relative to ELMy H-mode, but the W emission was maintained at acceptable levels. The threshold B injection rate was measured for several conditions, and found to increase with heating power. Li powder injection into comparable discharges also resulted in a short phase of ELM suppression, but density and stored energy both decreased due to the strong pumping effect of lithium; no edge harmonic oscillation was observed with Li injection, indicating that the ELM suppression mechanisms differ. The new set of B-seeded, ELM-suppressed discharges exhibited certain characteristics of quiescent H-mode (Burrell et al. in Phys Plasmas 8:2153, 2001), but did not require high shear, counter beams, etc. The wide operating window and compatibility with RF-only discharges paves the way for future experiments targeting long pulse H-mode discharges with complete ELM suppression.

First analysis of the misaligned leading edges of ITER-like plasma facing units using a very high resolution infrared camera in WEST

Twelve ITER-like plasma-facing units made of tungsten were exposed in the WEST tokamak divertor, with three plasma-facing units (PFUs) significantly overexposed to plasma heat flux: one sharp-edged PFU (vertical misalignment h = 0.8 mm) and two chamfered PFUs (h =0.6 mm and 0.3 mm, respectively). This paper describes the first temperature analysis obtained with a very high spatial resolution infrared camera (pixel size ∼ 0.1 mm) on the misaligned PFU edges and shows the consistency obtained on the parallel heat flux derived from these measurements. The analysis is focused on the hottest areas of the PFU misaligned leading edges, since the temperature detection threshold of the VHR camera is high (Tthreshold,BB ≈ 370 °C). The heat flux parallel to the magnetic field lines is assessed by matching the toroidal temperature profile in the vicinity of the leading edge with 3D finite element modelling. The tungsten emissivity assumed in this study is 0.6, which is consistent with laboratory measurements for damaged PFUs. For the three PFUs studied (with different vertical misalignments, incident angles, geometries), the derived parallel heat flux is similar, and consistent with independent measurements by a Fiber Bragg grating embedded in a graphite PFU at another toroidal location, giving confidence in future experiments using the same settings.

An ITER Challenge Absolute Surface Temperature Measurements of Low and Varying Emissivity Tungsten Plasma-Facing Components

One of the challenges that International Thermonuclear Experimental Reactor (ITER) will face during plasma operation is to determine the absolute divertor surface temperature of the actively water-cooled (70 °C, 3 MPa) tungsten plasma-facing units (PFUs) to ensure their integrity. The expected steady-state heat flux up to 10 MW/m2 is close to the operational limit and so one of the goals of the thermographic system is to make reliable measurements (wavelength band: 3.5– $4.5~\mu \text{m}$ ) with relatively low error bars to avoid tungsten melting and material damages. The tungsten emissivity is low and dependent on wavelength, temperature, and surface state (roughness, cracks, oxidation, and erosion/deposition processes) which can evolve during the time along with plasma operation. An error on the absolute emissivity can lead to large absolute temperature errors, and consequently to either a reduction of the operational window to fulfill safety limits, or an increased risk regarding the integrity of the components. For the past years, a number of emissivity measurements have been performed with ITER-like PFU mock-ups made of different grades of tungsten, with different techniques, from different material makers, and from different manufacturers. The overall emissivity discrepancy is large, indicating that tungsten emissivity is a major issue for temperature measurement and wall protection. The accuracy of the temperature measurement is investigated for both monocolor and bicolor IR thermography techniques. We present laboratory tests performed with an IR camera equipped with a rotating filter wheel for bicolor temperature measurement. It shows that the bicolor technique can reach high precision $\Delta \text {T}/\text {T} at high temperature assuming the emissivity ratio is constant regarding the two selected wavelengths.

Effects of the Chodura sheath on tungsten ionization and emission in tokamak divertors

AbstractThe decay of the electric potential in the sheath region in tokamak divertors occurs on a scale length on the order of the main ion gyroradius (Chodura sheath) due to magnetic fields lines intersecting the divertor plates at grazing incidence. As a consequence, high‐Z impurities like tungsten ionize within the sheath region in attached plasma conditions. The modification of the electron distribution in the sheath region must thus be taken into account to accurately model ionization and emission of impurities within the sheath region. To that end, an analytical expression of the distribution of the vertical ionization path for impurities sputtered from divertor plasma‐facing components is derived. This expression is then used to estimate the fraction of neutral impurities ionizing within the sheath and the average vertical ionization path, and to derive an effective SXB (the number of ionizations per emitted photon) coefficient which includes the effects of the variation of the electron distribution in the sheath region. These results are applied to tungsten impurities sputtered from divertor plates. It is shown that the SXB coefficient for neutral tungsten is significantly reduced in high‐density attached divertor plasma conditions (ne ≳ 5 × 1013 cm−3) because of the ionization of neutral tungsten well within the sheath region.

Strongly Luminescent Tungsten Emitters with Emission Quantum Yields of up to 84 %: TADF and High‐Efficiency Molecular Tungsten OLEDs

AbstractMetal‐TADF (thermally activated delayed fluorescence) emitters hold promise in the development of next generation light‐emitting materials for display and lighting applications, examples of which are, however, largely confined to CuI and recently AuI, AgI, and AuIII emitters. Herein is described the design strategy for an unprecedented type of metal‐TADF emitter based on inexpensive tungsten metal chelated with Schiff base ligand that exhibit high emission quantum yields of up to 56 % in solutions and 84 % in thin‐film (5 wt % in 1,3‐bis(N‐carbazolyl)benzene, mCP) at room temperature. Femtosecond time‐resolved emission (fs‐TRE) spectroscopy and DFT calculations were undertaken to decipher the TADF properties. Solution‐processed OLEDs fabricated with the W‐TADF emitter demonstrated external quantum efficiency (EQE) and luminance of up to 15.6 % and 16890 cd m−2, respectively.

Strongly Luminescent Tungsten Emitters with Emission Quantum Yields of up to 84 %: TADF and High-Efficiency Molecular Tungsten OLEDs.

Metal-TADF (thermally activated delayed fluorescence) emitters hold promise in the development of next generation light-emitting materials for display and lighting applications, examples of which are, however, largely confined to CuI and recently AuI , AgI , and AuIII emitters. Herein is described the design strategy for an unprecedented type of metal-TADF emitter based on inexpensive tungsten metal chelated with Schiff base ligand that exhibit high emission quantum yields of up to 56 % in solutions and 84 % in thin-film (5 wt % in 1,3-bis(N-carbazolyl)benzene, mCP) at room temperature. Femtosecond time-resolved emission (fs-TRE) spectroscopy and DFT calculations were undertaken to decipher the TADF properties. Solution-processed OLEDs fabricated with the W-TADF emitter demonstrated external quantum efficiency (EQE) and luminance of up to 15.6 % and 16890 cd m-2 , respectively.

Advances in neutral tungsten ultraviolet spectroscopy for the potential benefit to gross erosion diagnosis

A spectral survey of tungsten emission in the ultraviolet region has been completed in the DIII-D tokamak and the CTH torsatron to assess the potential benefit of UV emission for the diagnosis of gross W erosion. A total of 29 W I spectral lines are observed from the two experiments using survey spectrometers between 200 and 400 nm with level identifications provided based on a structure calculation for many of the excited states that produce strong emission lines. Of the 29 observed lines, 20 have not previously been reported in fusion relevant plasmas, including an intense line at 265.65 nm which could be important for benchmarking the frequently exploited line at 400.88 nm. Nearly all of the observed spectral lines decay down to one of the six lowest energy levels for neutral W, which are likely to be long-lived metastable states. The impact of metastable level populations on the W I emission spectrum and any erosion measurement utilizing a spectroscopic technique is potentially significant. Nevertheless, the high density of W I emission in the UV region allows for the possibly of determining the relative metastable fractions and plasma parameters local to the erosion region. Additionally, the lines observed in this work could be used to perform multiple independent gross erosion measurements, leading to more accurate diagnosis of gross tungsten erosion.

An overview of the in-vessel ICRF-diagnostics in the ASDEX Upgrade tokamak

Radio Frequency (RF) waves in the Ion Cyclotron Range of Frequency (ICRF) are successfully used to heat fusion plasmas. For a better understanding of the ICRF wave propagation, absorption and other processes in the plasma, the ICRF-related in-vessel diagnostics has been extended recently in the ASDEX Upgrade tokamak.A number of high frequency B-field probes have been installed which, arranged as an array, pick up the magnetic field components of the radiated wave, and can thus be used to characterize the radiation spectrum of the ICRF-antennas during plasma discharges. Dual channel logarithmic RF-detectors with phase detection capability have been designed and installed to track the fast variations of the probe signals during events like edge localized modes (ELMs). The same probes are also used to measure the ion-cyclotron emission (ICE) of plasmas. A fast, open source data acquisition system, which digitizes the RF-signal with 125 MSamples/s and 14 bit amplitude resolution, records the ICE signals. To allow ICE measurements during ICRF operation notch filters are used to suppress the ICRF signal by 50 dB. In order to characterize drive mechanisms and consequences of RF sheaths on the ICRF-antenna structures, 12 antenna limiter tiles are equipped with shunts to allow the measurements of RF- and DC-currents during operation, with some of the locations also covered by the spectroscopic lines of sight measuring the emission of neutral tungsten at 400.9 nm (WI). The antenna-embedded X-mode reflectometry with 3 channels operating simultaneously in the extended U-band helps to study ICRF wave coupling, whereas the RF current probes in the 3-strap ICRF antennas provide information on the phasing between central and outer straps.

Thermionic-enhanced near-field thermophotovoltaics

Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ∼ 30% and ∼ 70 W/cm2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (∼10% and ∼10 W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.

Approximation methods used for processing measurements of the intensity spectra of radiation sources of SF-56 spectrophotometers

The experimental results of testing the operation features of a SF-56 spectrophotometer are presented. Methods for processing the results of measuring the intensity of radiation sources using Maxima and Gnuplot software packages are developed to increase the accuracy of measurements. The use of the Gaussian function in approximation of the emission curve of low-pressure gas lamps for a nominal slit width of 0.3 nm increases the accuracy of determination of the wavelength of the maximum of emission lines by 2.5 times compared to the passport data. However, the effective width of the slit defined as the half-width of the reference line 486 nm (or 656.1 nm) always exceeds the passport values by 15 - 20%, and for a nominal slit of 1 nm by a factor of 1.5. The wavelength of the maximum of the SF-56 reference line (during the measurement time up to 6000 sec) linearly depends on time. The dependence of the reference line intensity on the heating time turned out to be nonlinear and is well approximated by the formula for an oscillator with a damping factor equal to the natural frequency. An analytical expression for the spectrum of the tungsten emission coefficient in the wavelength range of 300 - 1100 nm was derived. The Planck curve for the tungsten emissivity describes the spectrum of the signal of a standard incandescent lamp only in the wavelength range of 300 - 420 nm. The spectral function of the instrument is also determined. It is noted that SF-56 spectrophotometers can be used to measure the spectra of radiation placed in the lighting unit. The presented data can be used both to improve this type of devices, and to improve the accuracy and reliability of measurement results.

The f-Ratio Quantification Method for X-ray Microanalysis Applied to Mg-Al-Zn Alloys.

The f-ratio quantitative X-ray microanalysis method has been recently developed for binary systems based on a scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) system. This method incorporates traditional EDS experiments and Monte Carlo simulations, and calibration factors are calculated with standard samples to evaluate the differences between them. In this work, the f-ratio method was extended to Mg-Al-Zn multi-element systems using a cold field emission SEM and a tungsten emission SEM. Results show that the stability of the beam current does not influence the f-ratio quantification accuracy. Thus, the f-ratio method is suitable for quantitative X-ray mapping with a long-time acquisition or even an unstable beam current. Comparing with other quantitative techniques including the routine standardless analysis and the standard-based k-ratio method, the f-ratio method is a simple and accurate quantification method.

Determination of tungsten sources in the JET-ILW divertor by spectroscopic imaging in the presence of a strong plasma continuum

The identification of the sources of atomic tungsten and the measurement of their radiation distribution in front of all plasma-facing components has been performed in JET with the help of two digital cameras with the same two-dimensional view, equipped with interference filters of different bandwidths centred on the W I (400.88 nm) emission line. A new algorithm for the subtraction of the continuum radiation was successfully developed and is now used to evaluate the W erosion even in the inner divertor region where the strong recombination emission is dominating over the tungsten emission. Analysis of W sputtering and W redistribution in the divertor by video imaging spectroscopy with high spatial resolution for three different magnetic configurations was performed. A strong variation of the emission of the neutral tungsten in toroidal direction and corresponding W erosion has been observed. It correlates strongly with the wetted area with a maximal W erosion at the edge of the divertor tile.

Research on the normal spectral band emissivity of tungsten between 150 and 500 °C.

Tungsten is the baseline divertor material for a tokamak. Due to the effect of the interaction of plasma and the first wall, the first wall accumulates a large amount of heat flux and suffers irreversible damages. Therefore, the diagnosis of the first wall temperature of the tokamak by using a radiation thermometer is the premise to ensure the safe and stable operation of the whole facility. However, in order to achieve high precision of temperature measurement, the emissivity of tungsten must be measured accurately. In this paper, we built a set of emissivity measurement system and a method for accurate calculation of emissivity is proposed. This method effectively eliminated the interference of background radiation and improves the accuracy of emissivity measurement. By using this method, the author measured the emissivity of tungsten under the conditions of different surface roughness in the range of 150 °C-500 °C and discussed the uncertainty of the measurement at the end.

Near-field thermophotovoltaic energy conversion using an intermediate transparent substrate.

We propose a scheme for near-field thermophotovoltaic (TPV) energy conversion, where thermal emission from an emitter is extracted by an intermediate transparent substrate attached to the top of a photovoltaic (PV) cell. The addition of an intermediate transparent substrate suppresses the unwanted heat transfer from the emitter to the PV cell due to the surface modes on the PV cell while maintaining the enhancement in the interband absorption. We confirm that our scheme is applicable for near-field TPV systems using a silicon (Si) or tungsten (W) emitter. As a specific example, we designed a near-field TPV system composed of a one-dimensional Si photonic crystal thermal emitter, an InGaAs PV cell, and an intermediate Si substrate, and displayed that our scheme could realize both high power density (>5 × 104 W/m2) and high power conversion efficiency (>40%) at a 50-nm gap between the emitter and the intermediate substrate.

Using spectral distribution of radiance temperature and relative emissivity for determination of the true temperature of opaque materials

A method for determining the thermodynamic (true) temperature of opaque materials by the registered spectrum of thermal radiation under the conditions when we do not know emissivity of a free-radiating body is presented. A special function, which is a product of relative emissivity of tungsten by the radiation wavelength, was used as the input data. The accuracy of results is analyzed. It is shown that when using relative emissivity, the proposed algorithm can be used both within the range of applicability of the Wien approximation and the Planck formula.

Design of ITER divertor VUV spectrometer and prototype test at KSTAR tokamak

Design and development of the ITER divertor VUV spectrometer have been performed from the year 1998, and it is planned to be installed in the year 2027. Currently, the design of the ITER divertor VUV spectrometer is in the phase of detail design. It is optimized for monitoring of chord-integrated VUV signals from divertor plasmas, chosen to contain representative lines emission from the tungsten as the divertor material, and other impurities. Impurity emission from overall divertor plasmas is collimated through the relay optics onto the entrance slit of a VUV spectrometer with working wavelength range of 14.6–32 nm. To validate the design of the ITER divertor VUV spectrometer, two sets of VUV spectrometers have been developed and tested at KSTAR tokamak. One set of spectrometer without the field mirror employs a survey spectrometer with the wavelength ranging from 14.6 nm to 32 nm, and it provides the same optical specification as the spectrometer part of the ITER divertor VUV spectrometer system. The other spectrometer with the wavelength range of 5–25 nm consists of a commercial spectrometer with a concave grating, and the relay mirrors with the same geometry as the relay mirrors of the ITER divertor VUV spectrometer. From test of these prototypes, alignment method using backward laser illumination could be verified. To validate the feasibility of tungsten emission measurement, furthermore, the tungsten powder was injected in KSTAR plasmas, and the preliminary result could be obtained successfully with regard to the evaluation of photon throughput.

Development of GEM detector for plasma diagnostics application: simulations addressing optimization of its performance

The advanced Soft X-ray (SXR) diagnostics setup devoted to studies of the SXR plasma emissivity is at the moment a highly relevant and important for ITER/DEMO application. Especially focusing on the energy range of tungsten emission lines, as plasma contamination by W and its transport in the plasma must be understood and monitored for W plasma-facing material. The Gas Electron Multiplier, with a spatial and energy-resolved photon detecting chamber, based SXR radiation detection system under development by our group may become such a diagnostic setup considering and solving many physical, technical and technological aspects. This work presents the results of simulations aimed to optimize a design of the detector's internal chamber and its performance. The study of the effect of electrodes alignment allowed choosing the gap distances which maximizes electron transmission and choosing the optimal magnitudes of the applied electric fields. Finally, the optimal readout structure design was identified suitable to collect a total formed charge effectively, basing on the range of the simulated electron cloud at the readout plane which was in the order of ∼ 2 mm.

The temperature dependence of optical properties of tungsten in the visible and near-infrared domains: an experimental and theoretical study

The knowledge of optical properties of tungsten at high temperatures is of crucial importance in fields such as nuclear fusion and aerospace applications. The optical properties of tungsten are well known at room temperature, but little has been done at temperatures between 300 K and 1000 K in the visible and near-infrared domains. Here, we investigate the temperature dependence of tungsten reflectivity from the ambient to high temperatures (<1000 K) in the 500–1050 nm spectral range, a region where interband transitions make a strong contribution. Experimental measurements, performed via a spectroscopic system coupled with laser remote heating, show that tungsten’s reflectivity increases with temperature and wavelength. We have described these dependences through a Fresnel and two Lorentz–Drude models. The Fresnel model accurately reproduces the experimental curve at a given temperature, but it is able to simulate the temperature dependency of reflectivity only thanks to an ad hoc choice of temperature formulae for the refractive indexes. Thus, a less empirical approach, based on Lorentz–Drude models, is preferred to describe the interaction of light and charge carriers in the solid. The first Lorentz–Drude model, which includes a temperature dependency on intraband transitions, fits experimental results only qualitatively. The second Lorentz–Drude model includes in addition a temperature dependency on interband transitions. It is able to reproduce the experimental results quantitatively, highlighting a non-trivial dependence of interband transitions as a function of temperature. Eventually, we use these temperature dependent Lorentz–Drude models to evaluate the total emissivity of tungsten from 300 K to 3500 K, and we compare our experimental and theoretical findings with previous results.

Combined selective emitter and filter for high performance incandescent lighting

The efficiency of incandescent light bulbs (ILBs) is inherently low due to the dominant emission at infrared wavelengths, diminishing its popularity today. ILBs with cold-side filters that transmit visible light but reflect infrared radiation back to the filament can surpass the efficiency of state-of-the-art light-emitting diodes (LEDs). However, practical challenges such as imperfect geometrical alignment (view factor) between the filament and cold-side filters can limit the maximum achievable efficiency and make the use of cold-side filters ineffective. In this work, we show that by combining a cold-side optical filter with a selective emitter, the effect of the imperfect view factor between the filament and filter on the system efficiency can be minimized. We experimentally and theoretically demonstrate energy savings of up to 67% compared to a bare tungsten emitter at 2000 K, representing a 34% improvement over a bare tungsten filament with a filter. Our work suggests that this approach can be competitive with LEDs in both luminous efficiency and color rendering index (CRI) when using selective emitters and filters already demonstrated in the literature, thus paving the way for next-generation high-efficiency ILBs.