Hot Tungsten Surface Research Articles

ITER relevant multi-emissive sheaths at normal magnetic field inclination

Reliable modeling of macroscopic melt motion induced by fast transients requires the accurate and computationally efficient description of the emitted current density that escapes to the pre-sheath. The ITER sheaths that surround hot tungsten surfaces during edge-localized modes are characterized by important contributions from secondary electron emission and electron backscattering as well as by the coupling between thermionic emission and field electron emission. Under the guidance of systematic particle-in-cell simulations that incorporate a comprehensive analytical electron emission model, a highly accurate semi-empirical treatment of the escaping electron current has been achieved.

Ignition and propagation of nanosecond pulsed plasmas in distilled water—Negative vs positive polarity applied to a pin electrode

Nanosecond plasmas in liquids are being used for water treatment, electrolysis, or biomedical applications. The exact nature of these very dynamic plasmas and, most importantly, their ignition physics are strongly debated. The ignition itself may be explained by two competing hypotheses: ignition in water may occur (i) via field effects at the tip of the electrode followed by tunneling of electrons in between water molecules causing field ionization or (ii) via gaseous processes of electron multiplication in nanovoids that are created from liquid ruptures due to the strong electric field gradients. Both hypotheses are supported by theory, but experimental data are very sparse due to the difficulty in monitoring the very fast processes in space and time. In this paper, we analyze nanosecond plasmas in water that are created by applying a positive and a negative polarity to a sharp tungsten electrode. The main diagnostics are fast camera measurements and fast emission spectroscopy. It is shown that plasma ignition is dominated by field effects at the electrode–liquid interface either as field ionization for positive polarity or as field emission for negative polarity. This leads to a hot tungsten surface at a temperature of 7000 K for positive polarity, whereas the surface temperature is much lower for negative polarity. At ignition, the electron density reaches 4×1025 m−3 for the positive and 2×1025 m−3 for the negative polarity. At the same time, the emission of the Hα light for the positive polarity is four times higher than that for the negative polarity. During plasma propagation, the electron densities are almost identical of the order of 1–2×1025 m−3 followed by a decay after the end of the pulse over 15 ns. It is concluded that plasma propagation is governed by field effects in a low density region that is created either by nanovoids or by density fluctuations in supercritical water surrounding the electrode that is created by the pressure and temperature at the moment of plasma ignition.

Surface ionization of metastable calcium atoms

We report on an experimental study of surface ionization of metastable calcium atoms on a hot polycrystalline tungsten surface in vacuum. We implemented a hollow-cathode discharge to excite a fraction of calcium atoms in an atomic beam to metastable states and collected the resulting calcium ions. We observed that metastable calcium atoms are ionized with a significantly greater efficiency than ground-state atoms, and the results suggest that virtually every metastable atom impacting the hot surface is ionized. These results demonstrate the potential of metastable atom surface ionization as a means of enriching calcium isotopes for applications in medicine, metrology, and fundamental science.

Origin and nature of the emissive sheath surrounding hot tungsten tokamak surfaces

The accurate description of the emitted current that escapes from hot tungsten surfaces is essential for reliable predictions of the macroscopic deformation due to melt motion induced by fast transient events. A comprehensive analytical electron emission model is developed and its implementation in the particle-in-cell 2D3V code SPICE2 is discussed. The properties of emissive sheaths of present tokamaks, where thermionic emission is strongly suppressed by space-charge effects and by prompt re-deposition, are reviewed for arbitrary magnetic field inclination angles. The unique characteristics of emissive sheaths that emerge during ITER ELMs, where weakly impeded thermionic emission is coupled with field emission and competes with electron-induced emission, are revealed. The first ITER simulations are reported for normal inclinations. • The properties of emissive sheaths relevant for present day tokamaks are reviewed. • A comprehensive electron emission model has been implemented in the SPICE2 PIC code. • Electron-induced electron emission is revealed to be important during ITER ELMs. • Thermionic emission is revealed to be coupled to field emission during ITER ELMs.

Nanosecond pulsed discharges in distilled water: I. Continuum radiation and plasma ignition

Nanosecond plasmas in liquids are an important method to trigger the water chemistry for electrolysis or for biomedical applications in plasma medicine. The understanding of these chemical processes relies on knowing the variation of the temperatures in these dynamic plasmas. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages at 20 kV and pulse lengths of 15 ns applied to a tungsten tip with 50 μm diameter immersed in water. Plasma emission is analyzed by optical emission spectroscopy ranging from UV wavelengths of 250 nm to visible wavelengths of 850 nm at a high temporal resolution of 2 ns. The spectra are dominated by the black body continuum from the hot tungsten surface and line emissions from the hydrogen Balmer series. Typical temperatures from 6000 K up to 8000 K are reached for the tungsten surface corresponding to the boiling temperature of tungsten at varying tungsten vapor pressures. The analysis of the ignition process and the concurrent spectral features indicate that the plasma is initiated by field ionization of water molecules at the electrode surface. At the end of the pulse, field emission of electrons can occur. During the plasma pulse, it is postulated that the plasma contracts locally at the electrode surface forming a hot spot. This causes a characteristic contribution to the continuum emission at small wavelengths.

Catalytic dissociation of tris(dimethylamino)silane on hot tungsten and tantalum filament surfaces.

The dissociation of tris(dimethylamino)silane (TrDMAS) on hot tungsten and tantalum surfaces was studied under collision-free conditions. The products from the hot-wire decomposition of TrDMAS were monitored using a 10.5 eV vacuum ultraviolet laser single-photon ionization in tandem with time-of-flight mass spectrometry. Formation of a methyl radical and N-methyl methyleneimine (NMMI) was detected. A transition from a surface reaction rate-limiting regime at filament temperatures lower than 1800-2000 °C to mass transport regime at higher temperatures (>1800-2000 °C) was observed for the formation of both products. In the surface reaction regime, the Arrhenius behavior was followed in two separate temperature regions with different activation energies. It was found that low temperatures (900-1300 °C) favor the production of the methyl radical and high temperatures (1400-2000 °C) favor the production of NMMI with lower activation energies. A theoretical investigation using ab initio calculations of the concerted and stepwise formation of NMMI along with the homolytic cleavages of N-CH3 and Si-H in the gas phase has shown that the concerted pathway to form NMMI is the most energetically favorable one of all four routes with an activation barrier of 328 kJ mol-1. The lower activation energy values determined experimentally for the formation of NMMI and ˙CH3 as compared to those obtained from theoretical calculations indicate that the dissociation of TrDMAS, an N-containing organosilicon molecule, on the W and Ta surfaces is a catalytic cracking process.

Graphene oxide film reduction using atomic hydrogen annealing

Atomic hydrogen annealing (AHA) was investigated as a novel method to reduce graphene oxide (GO). In this method, high-density atomic hydrogen is generated on a hot tungsten (W) surface through a catalytic cracking reaction. The X-ray photoelectron spectra show that the GO films were reduced by AHA at low temperature. The GO film resistance, measured using the four-point probe method, decreased by 6 orders of magnitude when treatment was carried out with a W mesh temperature of 1780°C, a sample temperature of 241°C, and for a treatment time of 3600s. A reduced graphene oxide (r-GO) film having a low resistance of 272Ω was obtained by AHA. AHA allows fine control over the obtained physical properties. We expect that these r-GO films obtained by using AHA at low temperature will be used for producing electrical devices.

Secondary electron emission from plasma-generated nanostructured tungsten fuzz

Recently, several researchers [e.g., Yang et al., Sci. Rep. 5, 10959 (2015)] have shown that tungsten fuzz can grow on a hot tungsten surface under bombardment by energetic helium ions in different plasma discharges and applications, including magnetic fusion devices with plasma facing tungsten components. This work reports the direct measurements of the total effective secondary electron emission (SEE) from tungsten fuzz. Using dedicated material surface diagnostics and in-situ characterization, we find two important results: (1) SEE values for tungsten fuzz are 40%–63% lower than for smooth tungsten and (2) the SEE values for tungsten fuzz are independent of the angle of the incident electron. The reduction in SEE from tungsten fuzz is most pronounced at high incident angles, which has important implications for many plasma devices since in a negative-going sheath the potential structure leads to relatively high incident angles for the electrons at the plasma confining walls. Overall, low SEE will create a relatively higher sheath potential difference that reduces plasma electron energy loss to the confining wall. Thus, the presence or self-generation in a plasma of a low SEE surface such as tungsten fuzz can be desirable for improved performance of many plasma devices.

Flux threshold measurements of He-ion beam induced nanofuzz formation on hot tungsten surfaces

We report measurements of the energy dependence of flux thresholds and incubation fluences for He-ion induced nano-fuzz formation on hot tungsten surfaces at UHV conditions over a wide energy range using real-time sample imaging of tungsten target emissivity change to monitor the spatial extent of nano-fuzz growth, corroborated by ex situ SEM and FIB/SEM analysis, in conjunction with accurate ion-flux profile measurements. The measurements were carried out at the multicharged ion research facility (MIRF) at energies from 218 eV to 8.5 keV, using a high-flux deceleration module and beam flux monitor for optimizing the decel optics on the low energy MIRF beamline. The measurements suggest that nano-fuzz formation proceeds only if a critical rate of change of trapped He density in the W target is exceeded. To understand the energy dependence of the observed flux thresholds, the energy dependence of three contributing factors: ion reflection, ion range and target damage creation, were determined using the SRIM simulation code. The observed energy dependence can be well reproduced by the combined energy dependences of these three factors. The incubation fluences deduced from first visual appearance of surface emissivity change were (2–4) × 1023 m−2 at 218 eV, and roughly a factor of 10 less at the higher energies, which were all at or above the displacement energy threshold. The role of trapping at C impurity sites is discussed.

Surface-morphology changes and damage in hot tungsten by impact of 80 eV – 12 keV He-ions and keV-energy self-atoms

We report results of measurements on the evolution of the surface morphology of a hot tungsten surface due to impacting low-energy (80 – 12,000 eV) He ions, performed at the ORNL Multicharged Ion Research Facility (MIRF). Surface-morphology changes were investigated over a broad range of fluences, energies and temperatures for both virgin and pre-damaged W-targets. At low fluences, ordered coral-like and ridge-like surface structures are observed, with great grain-to-grain variability. At the largest fluences, individual grain characteristics disappear in FIB/SEM scans, and the entire surface is covered by a multitude of near-surface bubbles with a broad range of sizes, and disordered whisker growth, while in top-down SEM imaging the surface is virtually indistinguishable from the nanofuzz produced on linear plasma devices. These features are evident at progressively lower fluences as the He-ion energy is increased. In addition, simulations were carried out of damage caused by cumulative bombardment of 1 keV W self-atoms, using LAMMPS at the Kraken supercomputing facility of the University of Tennessee. The simulations show strong defect-recombination effects that lead to a saturation of the total defect number after a few hundred impacts, while sputtering and implantation lead to an imbalance of the vacancy and interstitial numbers.

He-ion and self-atom induced damage and surface-morphology changes of a hot W target

We report results of measurements on the evolution of the surface morphology of a hot tungsten surface due to impacting low-energy (80–12 000 eV) He ions and of simulations of damage caused by cumulative bombardment of 1 and 10 keV W self-atoms. The measurements were performed at the ORNL Multicharged Ion Research Facility, while the simulations were done at the Kraken supercomputing facility of the University of Tennessee. At 1 keV, the simulations show strong defect-recombination effects that lead to a saturation of the total defect number after a few hundred impacts, while sputtering leads to an imbalance of the vacancy and interstitial number. On the experimental side, surface morphology changes were investigated over a broad range of fluences, energies and temperatures for both virgin and pre-damaged W-targets. At the lowest accumulated fluences, small surface-grain features and near-surface He bubbles are observed. At the largest fluences, individual grain characteristics disappear in focused ion beam/scanning electron microscopy (FIB/SEM) scans, and the entire surface is covered by a multitude of near-surface bubbles with a broad range of sizes, and disordered whisker growth, while in top-down SEM imaging the surface is virtually indistinguishable from the nano-fuzz produced on linear plasma devices. These features are evident at progressively lower fluences as the He-ion energy is increased.

Erosion of a tungsten limiter under high heat flux in TEXTOR

Erosion characteristics of a tungsten plate heated up in TEXTOR by the plasma load have been investigated at temperatures extending to the melting point. No enhancement of atomic release exceeding physical sputtering and normal thermal sublimation for temperatures below 3700K was observed. The liquid tungsten moved fast along the plate in the direction perpendicular to the magnetic field lines. The motion is caused by the Lorentz force due to the thermoelectron current emitted from the hot tungsten surface. The motion of liquid tungsten caused a material loss of 2.85g during two discharges. The material redistribution due to the melt layer motion is compared with a MEMOS-1.5D simulation.

Quantification of a radical beam source for methyl radicals

A radical beam source for methyl radicals (CH3) was characterized applying ionization-threshold mass spectrometry. The beam source is based on thermal dissociation of methane (CH4) or azomethane (N2(CH3)2) in a heated tungsten capillary. A flux of (3±1)×1013 cm−2 s−1 CH3 radicals is produced using methane as precursor gas and a capillary temperature of 1650 K. Alternatively, a flux of (3±1)×1014 cm−2 s−1 CH3 is produced using azomethane as precursor gas and a capillary temperature of 1150 K. The dominant production of methyl from the precursor methane occurs due to reaction 2 CH4+M→2 CH3+H2+M at the hot tungsten surface. The dominant production of methyl from azomethane occurs due to the reaction N2(CH3)2→2 CH3+N2. Besides methyl radicals, only stable molecules contribute to the emitted flux; within the detection limit, no atomic hydrogen is observed. From the comparison of ionization-threshold mass spectrometry and standard mass spectrometry, it is concluded that the cracking pattern of methane varies with the methane gas temperature. This is explained by the vibrational assisted dissociation of methane.

Thermionic emission of hydrogen isotopes (Mu, H, D and T) from W and interpretation of a role of the hot W as an atomic hydrogen source

Resonant ionization was applied to learn about the production mechanism of thermal neutral hydrogen isotopes including muonium (Mu; μ +e − ) from a hot tungsten surface as reaction products with pulsed 500 MeV protons. The activation energy E a obtained in the temperature dependence of production of neutral muonium (1.72 (5) eV) as well as that of T atom (1.89 (1) eV), is consistent with the emission of the atoms directly from bulk sites in the solid rather than from the surface for which one should expect E a≈3 eV. Since hydrogen molecules are easily dissociated into hydrogen atoms interacting with a surface of the hot tungsten, even residual H 2 in the ultra high vacuum chamber of 3×10 −10 mbar at 1200 K was found to be easily detected and extracted by this method. From the results obtained through the temperature and pressure dependence measurements for D 2 and H 2, we propose a view of the role of hot W as a catalyzer to dissociate H 2 molecules and producing hydrogen atoms. After dissociation on the surface, the hydrogen atoms are dissolved into the bulk solid, and are thermionically emitted from the bulk to the vacuum at high temperatures.

Rate of H 2 atomization over the surface of a hot tungsten filament

A new adsorption-desorption method for determining the efficiency of H 2 thermal dissociation on a hot metal surface is proposed. The results obtained by means of this method confirm the validity of Langmuir's equation describing H 2 atomization on a hot tungsten surface within the temperature range 1020–1315 K at a H 2 pressure of 2 × 10 −3 torr. A comparison of our results with Brennan-Fletcher's equation gives lower values for the efficiency of H 2 atomization.

Application of the two-temperature Maxwellian model to describe re-emission of gas molecules from surfaces

Experimental data of Carpenter and Watts (1975) on the re-emission of helium and argon from hot polycrystalline tungsten surfaces are reprocessed using a more general gas-surface interaction model than that used by the authors. The effects of different assumptions concerning the degree of accommodation at the surface of the momentum and energy due to the tangential component of velocity of the gas molecule are discussed.

Investigation of the precursors of a-Si:H films produced by decomposition of silane on hot tungsten surfaces

A-Si:H films were deposited by the hot filament method. In order to study the film precursors, hydrogen content and bonding of the films were examined by IR spectroscopy and compared to films prepared by glow discharge. The degree of silane decomposition was measured as a function of filament temperature. The results suggest that silicon atoms are the major species desorbing from the hot filament surface and that the film properties depend strongly on the reaction of the silicon atoms with undecomposed silane in the gas phase.

Selectivity loss during tungsten chemical vapor deposition: The role of tungsten pentafluoride

Our previous studies found evidence for a selectivity loss mechanism that is initiated by desorption of tungsten subfluorides formed by the reaction of WF6 with metallic tungsten surfaces. When a hot (≂600 °C) tungsten surface is exposed to WF6, a nonvolatile subfluoride, apparently WF4 , will accumulate on a nearby SiO2 surface held at ≂30 °C. Disproportionation of this tungsten subfluoride upon heating above 350 °C produces a reactive state of tungsten that induces rapid selectivity loss when exposed to chemical vapor deposition conditions. The net effect of this tungsten subfluoride desorption–disproportionation mechanism is the transport of tungsten from a tungsten surface to the silicon dioxide surface. Isolation of a higher tungsten subfluoride, apparently WF5, has been accomplished by cooling the SiO2 surface to ≂−45 °C under otherwise similar experimental conditions. Preliminary Auger spectroscopic results yield a stochiometry of F/W=4.9±0.5 for this subfluoride, and temperature programmed desorption results are consistent with the behavior of tungsten pentafluoride. Upon heating WF5 multilayers, most of the compound desorbs, but some reaction and disproportionation occurs leaving a reduced state of tungsten on the SiO2 surface. It seems likely that tungsten pentafluoride is the key volatile species in a tungsten transport mechanism that can initiate selectivity loss.

Proximity Effect in the Low Pressure Chemical Vapor Deposition of Tungsten.

ABSTRACTThe self-limiting effect during Low Pressure Chemical Vapor Deposition of tungsten is manifested by a sudden interruption of the reduction of tungsten hexafluoride WF6 by silicon, so that only very thin (self-limited) films of silicon-reduced tungsten can be grown. It has been shown that the self-limiting effect is caused by formation of the non-volatile subfluoride WF4. The temperature dependence of the self-limiting thickness of the tungsten films grown in a hot-wall reactor exhibits a characteristic maximum at temperatures near 350°C, which indicates that at this temperature the rate of formation of WF4 is lower than at temperatures above and below. In the present paper we attempt to explain this peculiar dependence. We suggest a mechanism responsible for formation of the blocking agent WF4. We demonstrate that the presence of a hot tungsten surface is essential for the strong self-limiting effect. This leads us to the discussion of the proper selection of the reactor type (hot-wall vs. cold-wall) for different process requirements.

Non-adiabatic processes at metal surfaces

When an atom or other particle is moving sufficiently rapidly near a metal surface the state of the electron gas may deviate from the adiabatic one which would obtain if the particle moved infinitely slowly along its trajectory. In fast ion scattering from surfaces the ion fraction in the scattered beam is non-adiabatic, and the simple semi-classical theory of this is explained. An example of a surface reaction occurring at thermal energies is surface chemiluminescence where the non-adiabatic emission of a photon occurs. The semi-classical theory of this is discussed. The more rigorous coherent or quantum mechanical approach is developed by considering the Heisenberg equations of motion for the field operators, and is necessary for example in sputtering (e.g. in SIMS) and in post ionization in field evaporation. The theory is applied to ionization of hyperthermal sodium atoms from a hot tungsten surface. Finally, the possibility of energy loss to electron hole pairs providing a trapping mechanism for adsorbing particles is considered. This problem is approached by solving the Heisenberg equations of motion for a slowly moving particle. It is found that while this mechanism is a significant one, there are several competing mechanisms capable of masking it in individual cases.