Crystal Tungsten Research Articles

Investigations on a hybrid positron source with a granular converter

Promising results obtained with crystal targets for positron production led to the elaboration of a hybrid source made of an axially oriented tungsten crystal, as a radiator, and an amorphous tungsten converter. If the converter is granular, made of small spheres, the heat dissipation is greatly enhanced and the thermal shocks reduced, allowing the consideration of such device for the future linear colliders. A positron source of this kind is investigated. Previous simulations have shown very promising results for the yield as for the energy deposition and the PEDD (Peak Energy Deposition Density). Here, we present detailed simulations made in this granular converter with emphasis on the energy deposition density, which is a critical parameter as learned from the breakdown of the SLC target. A test on the KEKB linac is foreseen; it will allow a determination of the energy deposited and the PEDD in the converter through temperature measurements. Four granular converters, made of W spheres of mm radius have been built at LAL-Orsay; they will be installed at KEK and compared to compact converters. A description of the experimental layout at KEK is provided. Applications to future linear colliders as CLIC and ILC are considered.

Direct observation of electron propagation and dielectric screening on the atomic length scale.

The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations--periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology (1 as = 10(-18) seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10(-15) seconds) extreme-ultraviolet light pulses to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few ångströms. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer--constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.

A Flexible Quasi-Solid-State Electrochromic Device with Polymeric Electrolyte and WO3/NiO Complementary System

A flexible quasi-solid-state electrochromic device (ECD), assembled with a polymeric crystal composite electrolyte and tungsten oxide/nickel oxide (WO3/NiO) complementary system, is demonstrated in this study. The polymer composite, which is composed of a UV-cured ethoxylated trimethylolpropane triacrylate (ETPTA), propylene carbonate (PC) and ferrocene (Fc), is used as the electrolyte in the flexible quasi-solid-state ECD. The optimal composition ration of this composite is at the weight ratio of 15/85 for ETPTA/PC with 1 M LiClO4. For the cathodic electrochromic electrode, a flexible polyethylene terephthalate (PET) is used as the substrate, and indium-tin- oxide (ITO) and WO3 are deposited sequentially on the PET by sputtering. A NiO/ITO/PET electrode prepared by sputtering is used as the anodic coloring electrode. The thickness of ITO, WO3 and NiO film is 110, 70 and 60 nm, respectively. This flexible all-solid-state ECD fabricated with the polymeric crystal composite electrolyte shows an optical contrast of ca. 37.5% at 550 nm. The optical transmittance of the ECD at 550 nm can be reversibly modulated from 47.5% (bleached) to 10.0% (darkened), by applying potentials of 2.5 and 2.5 V, respectively.

Stability and mobility of rhenium and osmium in tungsten: first principles study

We report a series of ab initio studies based upon density functional theory for the behavior of rhenium and osmium atoms in body-centered-cubic tungsten crystal. Contrary to the fast one-dimensional migration of self-interstitial atoms, interstitials of these solute elements in tungsten have three-dimensional motion because they form a mixed dumbbell having a low rotation energy barrier. The migration of these solute elements strongly influences the effects of radiation upon the materials, and our results suggest that the low rotation energy barrier leading to three-dimensional migration is a property that is key to the explanation of the radiation effects experimentally observed in tungsten-rhenium and tungsten-osmium alloys.

Thermal Magnetoconductivity of Tungsten Below 6 K: Combining the Zero-, Low- and High-Field Cases

We present a phenomenological expression to describe the thermal magnetoconductivity of tungsten which unifies the zero-field thermal conductivity with the low and high-field cases. Its basic form is derived from semi-classical magnetoresistance theory with fitting parameters allowing adjustment to particular samples. Thermal conductivity measurements at 5.5 K and in fields from 0 to 2 T are presented on two single crystals of tungsten with significantly different purities. We find that—with suitable adjustment of the fitting parameters—the equation is an excellent fit to the data in both cases. The significance of the fitting parameters and differences in their values are discussed, and we describe how they can be related to the individual properties of the samples. It is concluded that the equation should be applicable to any field below that at which orbit quantisation occurs and temperatures from 6 K down to tungsten’s superconducting transition.

Detection of high-energy electrons by a directional spectrometer

A method for the detection of 26- and 28-GeV electrons by a directional spectrometer based on oriented tungsten, tungstate, and garnet crystals has been reported. In spite of an anomalous character of the development of an electromagnetic shower at the initial stage in a crystal oriented along the 〈111〉 axis, the relative energy release of the shower in the spectrometer behind the crystal at the final stage is described by the standard dependence of the development of the electromagnetic shower.

Critical energy of anomalous electron-induced showers

The results of experimental studies of anomalous electromagnetic showers caused by electrons with energies of tens of GeV, emerging from oriented tungsten crystals and detected by an electromagnetic spectrometer, are presented. An approximately twofold increase in the critical energy ɛc of the anomalous shower development in comparison with the similar quantity in the case of the standard shower development is detected. Its behavior is studied as a function of the crystal orientation, thickness, temperature, and the electron energy.

Effects of NaPO3 concentration in Na2WO4–WO3 system on pulse electro-deposition tungsten coating from oxide molten slat

Electro-deposition technique is an attractive and promising technique to fabricate tungsten armor-coatings. Metallic tungsten coatings were obtained by pulsed electro-deposition on CuCrZr alloy used as a heat sink material. In this study, the effects of NaPO3 concentration and temperature on the crystal structure and morphologies of the coatings were investigated by XRD and SEM. Metallic tungsten coating was successfully obtained as the NaPO3 concentration is 30mol% at the temperature of 1073K in air atmosphere. XRD results show that the deposited coatings have the metallic tungsten crystal with a body centered cubic (BCC) structure. (211) orientation was favored most likely for each sample obtained at different temperatures. The average grain size of tungsten coatings was varies in the region of 3–15μm. And the addition of NaPO3 have an obviously influence on the decrease of tungsten’s grain size.

Estimates of the transverse development of anomalous electromagnetic showers

Estimates of the transverse development of anomalous electromagnetic showers induced by 26-GeV electrons and oriented along the 〈111〉 axis in tungsten, tungstate, and garnet crystals show the differences of shower development profiles from similar profiles of standard shower development. The Moliere length estimates are ∼8X0, ∼7.5X0, and ∼6X0 or ∼28 mm, ∼67 mm, and 87 mm, respectively.

Modeling Load-displacement Curve and Pop-in Effect in Nanoindentation Tests

The nanoindentation test of a single crystal of tungsten is simulated by a multiscale model based on a nonlinear elastic finite element analysis coupled with both ab initio calculations of the ideal shear strength and crystallographic considerations. The onset of microplasticity, associated with the pop-in effect identified in experimental nanoindentation tests (creation of first dislocations), is assumed to be related to the moment of achieving the value of the ideal shear strength for the copper crystal under a superimposed hydrostatic stress. The calculated value of the critical indentation depth is in a very good correspondence with that of the experimentally observed pop-in on the load-displacement curve. The value of the Young modulus of tungsten received from the reduced modulus of elasticity of the original Hertz's model is also in an excellent agreement with experimental data. This offers us a possibility to assess the concentration of alloying and impurity elements in the surface and sub-surface layers.

Estimation of the parameters of the cascade curve of electromagnetic shower development in a spectrometer with oriented crystal converter

Based on the experimental results on the development of electromagnetic showers from 26-GeV electrons in misoriented and oriented along the 〈111〉 axis tungsten crystals 2.7, 5.8, 8.4mm thick and then in a composite Cherenkov shower spectrometer, the parameters of the cascade curve of shower development in a spectrometer with 1-mm tungsten crystal converter were estimated.

Phase transformation research of fusion reactor first wall material tungsten

Tungsten is remarkable for its robustness; especially it has the highest melting point of all the non-alloyed metals. Tungsten and tungsten alloys have been widely used in aerospace, weapon, nuclear industries and fusion reactor. Tungsten is expected to become fusion reactor first wall material for this reason. In this paper, phase transformation processes of fusion reactor first wall material tungsten have been investigated via molecular dynamics simulation based on the modified embedded atom model. Surface melting velocities at different temperatures are calculated as V(T) = −5.082 + 0.00136T and thermodynamic melting point is determined by fitting front advance velocities. Structure changes, thermal expansion coefficient, radial distribution function, static structure factor and average atomic energy for uniform melting processes are studied to simulate plasma thermal shock heating to superheat state of tungsten in fusion reactor. The superheat limit of tungsten crystals can be gotten according to simulation results. The superheat limit for tungsten crystal melting is about 27.2%. Tungsten is the best plasma-facing material because of its highest melt point and highest limiting superheating of all the non-alloyed metals.

Thorium in tungsten: construction of interatomic EAM potentials from ab initio data

The interatomic interaction potential of tungsten and thorium crystals and those of hypothetical tungsten and thorium alloys within the embedded atom approach are considered. The corresponding Ansatz functions are fitted against full potential linear augmented plane wave data of real tungsten- and thorium- and hypothetical tungsten-thorium-crystals. The result is interatomic potentials, ready for use within classical molecular dynamics schemes. A cross check of the resulting force scheme derived by comparison of ab initio and classical molecular dynamics data is provided. Furthermore, we used the potentials to calculate the phonon dispersion relations, which then serve as an additional check.

Orientation dependences of the response of an electromagnetic spectrometer with oriented crystal converter

The experimental results of the study of orientation dependences of the response of a composite Cherenkov shower spectrometer with a converter made of a 1-mm tungsten crystal oriented along the 〈111〉 axis at an electron energy of 28 GeV and two converter temperatures, 293 and 77 K, are presented. The parameters of the cascade curve of the shower development are varied depending on the orientation angle and crystal temperature. It is found that there is a point S in the cascade curve of the shower development in the spectrometer, at which all cascade curves intersect at any converter orientation. The position of this point in the spectrometer depth depends on the converter temperature.

Multiplicity of charged particles in electron-induced showers developing in oriented silicon and tungsten crystals

A more than twofold increase in the average multiplicity of charged particles in electromagnetic showers initiated by electrons with an energy of 26 GeV in tungsten crystals 2.7, 5.8, and 8.4 mm thick, oriented along the 〈111〉 axis, in comparison with misoriented crystals is shown. For a silicon crystal 20 mm thick, oriented along the 〈110〉 axis, at an electron energy of 28 GeV, the average multiplicity of charged particles increases by a factor of ∼1.6. The widths of the orientation dependences of the average multiplicity of charged particles in electron-induced showers in silicon and tungsten crystals are proportional to the crystal thickness and depend on the electron energy as E −1/2.

Coherent bremsstrahlung from neutrons

Theory of coherent bremsstrahlung from neutrons in crystals is developed in the framework of method of virtual photons. For this purpose we reduced the cross-section of scattering of a photon by a neutron. On the basis of derived equations we calculated the differential over an emitted photon energy cross-section for coherent bremsstrahlung from neutrons in the tungsten crystal.

Total yield of channeling radiation from relativistic electrons in thin Si and W crystals

Orientation dependences of channeling radiation total yield from relativistic 155–855MeV electrons at both 〈100〉 axial and (100) planar channeling in thin silicon and tungsten crystals are studied by means of computer simulations. The model as well as computer code developed allows getting the quantitative results for orientation dependence of channeling radiation that can be used for crystal alignment in channeling experiments and/or for diagnostics of initial angular divergence of electron beam.

A positron source using an axially oriented crystal associated to a granular amorphous converter

A non-conventional positron source using the intense γ radiation from an axially oriented monocrystal which materializes into e+e− pairs in a granular amorphous converter is described. The enhancement of photon radiation by multi-GeV electrons crossing a tungsten crystal along its <111> axis is reported. The resulting enhancement of pair production in an amorphous converter placed 2 meters downstream, is also reported. Sweeping off the charged particles from the crystal by a bending magnet upstream of the converter allows a significant reduction of the deposited energy density. Substituting a granular target made of small spheres for the usual compact one, makes the energy dissipation easier. The deposited energy and corresponding heating are analyzed and solutions for cooling are proposed. The configurations studied here for this kind of positron source allow its consideration for unpolarized positrons for the ILC.

An event generator for crystal source Application of the CLIC positron baseline

A high energy electron beam propagating at glancing angles along a mono crystal axis experience a collective scattering from many crystal atoms and may hence, be channeled; it performs an oscillating trajectory and emits a large number of photons. This property is used to obtain an intense beam of photons which generate a large number of e+e− pairs in an amorphous target. The radiation in crystals is handled by dedicated programs as FOT while the interaction between charged particles and amorphous material is well described in Monte Carlo codes such as Geant4 or EGS. Thus a Monte Carlo code associating the crystal program FOT with Geant4 has been worked out and a presentation is given for this new program named G4Fot. An application to the future positron baseline of the Compact Linear Collider (CLIC) using a tungsten crystal oriented on its ⟨111⟩ axis and an amorphous material, the so called hybrid source is also presented.

Properties of RE–W cathode and its application in electrostatic precipitation for high temperature gas clean-up

In this paper, three rare earth tungsten (RE–W) electrostatic precipitation cathodes (cerium–tungsten (Ce–W), lanthanum–tungsten (La–W) and yttrium–tungsten (Y–W)) were prepared by powder metallurgy. The microstructure and the electron emission properties of the three cathodes were examined. The application of the Y–W cathode in high temperature electrostatic precipitation was investigated. Results show that the addition of the rare earth reduces the tungsten crystal size, with yttrium (Y) affecting the crystal size more significantly than lanthanum (La) and cerium (Ce). The rare earth (cerium, lanthanum or yttrium) exists, in the form of crystalline tungstates, at the tungsten (W) crystal boundary. The effective work function of the La–W cathode has the lowest value of 2.88eV at vacuum condition, but the Y–W cathode shows the best emission properties at atmosphere pressure. The collection performance of an electrostatic precipitator (ESP) using the Y–W cathode was investigated by ash particles with various resistivity and size distribution at various temperatures. The particles with higher resistivity tend to be collected effectively at high ambient temperature, and the collection efficiency rise with the increase in temperature. If the resistivity of dusts is not high enough, corresponding to the highest collection efficiency, there exists an optimum temperature value. As in a conventional corona ESP, the larger particles are collected more effectively in an ESP with Y–W cathode.