Channeling radiation: Past achievements and future prospects

Abstract

When a relativistic charged particle passes through a single crystal very nearly along a major crystalline plane or axis so that it is channeled in that direction, it undergoes periodic motion in the plane transverse to this direction and hence it can radiate. Quantum mechanically, this channeling radiation corresponds to a radiative transition between two eigenstates of the transverse crystalline potential; when the transition occurs between two bound states, a sharp spectral line is emitted. When there are only two bound states (for incident electrons), or when the interplaner potential is nearly harmonic (as for incident positrons), the emitted radiation is nearly monochromatic. Since the discovery of channeling radiation at the LLNL Electron-Positron Linear Accelerator, many of its properties have been delineated, both there and elsewhere. For example, channeling radiation is very intense, forward-directed, easily tunable, and for the planar case, linearly polarized. Channeling radiation has been used as a probe both of the interplanar potentials and other properties of perfect crystals and of the effects of impurities and defects in imperfect crystals. Finally, channeling radiation has great potential use as a photon source for numerous other applications in several fields of science and technology.

This paper is intended to keynote the first International Conference on Coherent Radiation Processes in Strong Fields by recalling some history of the discovery and exploitation to date of channeling radiation. Studies of channeling radiation, in addition to elucidating the physics of the process itself, its application to the determination of properties of perfect and imperfect crystals, and its potential application to a large variety of fields by its use as an intense, monochromatic, forward-directed, tunable, and polarized photon source, have spawned an entire industry of studies of other coherent radiation processes, all consisting of photon production from beams of relativistic charged particles traversing periodic structures, which constitute the principal subject matter of this Conference. This paper will be limited to the discussion of channeling radiation and some of its applications. It will be in the nature of an illustrative exposition, showing many of the features of channeling radiation and its applications in a qualitative way. Several detailed studies of channeling radiation will be presented later in the Conference.     

Sintering mechanism of CaO during carbonation reaction in the presence of water vapor

In this paper, CaO sintering in the presence of water vapor for CO₂ capture were carried out by ReaxFF(Reactive Force Field) molecular dynamics. The CaO sintering model was simulated at different temperatures (873 K-1273 K) and atmospheres (CO₂, H₂O), respectively. The results showed that water vapor could significantly promote the sintering process of CO₂ capture by CaO. The Mean-square displacement (MSD) and Boltzmann–Arrhenius dependency were used to study the diffusion properties of CaO particles. The decreased diffusion activation Ea and increased pre-exponential factor D₀ indicate that CaO particles have a stronger initial diffusivity and a lower diffusion barrier in the presence of CO₂ and H₂O. The inner and outer regions of CaO atoms were analyzed and it was found that the activation energy is the main factor to enhance the diffusion in the presence of CO₂ for CaO sintering process, whereas the pre-exponential factor dominates with both CO₂ and H₂O. Water vapor enhanced the sintering pf CaO carbonation reaction is mainly achieved by promoting atoms in the inner layers of CaO particles. The types and numbers of sintering atoms during the sintering process were counted, and the distances between Ca and O atoms were calculated, which found that water vapor first dissociates into hydroxyl and H protons on the CaO surface, and the hydroxyl group will stay on the surface of CaO and combine with CO₂, while the H proton will combine with O inside CaO to promote the sintering of CaO further.     

染料敏化太阳电池中TiO2颗粒界面接触对电子输运影响的研究

采用溶胶-凝胶法制备TiO₂浆料,通过丝网印刷技术印刷和不同温度曲线烧结TiO₂薄膜,并应用于染料敏化太阳电池(DSC).高分辨透射电子显微镜发现,低温下多孔薄膜中TiO₂颗粒之间呈现点接触,510 ℃烧结后TiO₂颗粒间由点接触变为面接触,近邻颗粒数增多,接触面积增大.同时采用强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)技术,研究了不同颗粒接触方式和接触面积对电子传输与复合的影响.结果表明:在420- 510 ℃之间,随着烧结温度提高,颗粒接触面积增大,电子传输时间(τ _d)缩短,电子有效扩散长度(L _n)增大,暗电流减小;当烧结温度达到550 ℃时,薄膜比表面积减小,多孔结构坍塌,表面态密度增大,电子传输时间(τ _d)增大.电池光伏特性研究表明:在480-510 ℃范围内烧结得到的TiO₂薄膜,电池短路电流密度(J_sc)最佳,电池效率(η)最好. 关键词： 界面接触 电子输运 暗电流 染料敏化太阳电池     

Radiation from 39 and 45 MeV electrons channeled in Lithium Niobate

Abstract

Channeling radiation from 39 and 45 MeV electrons channeled along the <0001> axis, the (0110) plane and the (1210) plane of a 30 μm thick LiNbO ₃ crystal has been measured. Calculations of the planar crystal potentials were performed by means of the many-beam formalism. Good agreement between theory and experiment is obtained for the planar channeling radiation. Associated with channeling additional radiation lines have been observed, which may be explained by a periodic perturbation of the continuum potential.     

Anti-diffusive radiation flow in the cooling layer of a radiating shock

This paper shows that for systems with optically thin, hot layers, such as those that occur in radiating shocks, radiation will flow uphill: radiation will flow from low to high radiation energy density. These are systems in which the angular distribution of the radiation intensity changes rapidly in space, and in which the radiation in some region has a pancaked structure, whose effect on the mean intensity will be much larger than the effect on the scalar radiation pressure. The salient feature of the solution to the radiative transfer equation in these circumstances is that the gradient of the radiation energy density is in the same direction as the radiation flux, i.e. radiation energy is flowing uphill. Such an anti-diffusive flow of energy cannot be captured by a model where the spatial variation of the Eddington factor is not accounted for, as in flux-limited diffusion models or the P₁ equations. The qualitative difference between the two models leads to a monotonic mean intensity for the diffusion model whereas the transport mean intensity has a global maximum in the hot layer. Mathematical analysis shows that the discrepancy between the diffusion model and the transport solution is due to an approximation of exponential integrals using a simple exponential.     

Oscillations and island evolution in radiating diffusion flames

We show how an island (isola) evolves out of the usual S-curve of steady states of diffusion flames when radiation losses are accounted for and how it eventually disappears when radiation increases further. At small activation temperatures there are never any islands. We show that stable oscillations evolve first out of perturbations of steady states on the S-curve at large Damköhler numbers. Only if the activation temperature is large enough do they also appear on the islands. The region of the stable oscillations grows larger as activation temperature decreases.     

Electromagnetic radiation of MeV electrons in thick aligned single crystals

Abstract

Earlier published theoretical models for MeV electrons are generalized in this paper. Different theoretical predictions for planar channeling and accompanying electromagnetic radiation in thick crystals are presented. A comparative analysis of theoretical and experimental spectra of photon radiation is given. Thickness dependences of channeling quantum state populations, radiation line broadening and photon flux intensities are obtained. The existence of planar electron channeling at 54 MeV in a silicon single crystal with a thickness of several millimetres is shown.     

Modification of the theory of diffracted channeling radiation for axial electron and positron channeling

A new type of combinational channeling radiation induced by subbarrier (interband) transitions for the transverse motion of relativistic electrons (positrons) is studied. It is known as diffracted channeling radiation (DCR). The formula describing the DCR angular distribution in the case of axial channeling is obtained by taking into account the band structure of energy levels for the transverse motion of electrons (positrons). It is shown that, in the two-wave approximation of the wave function A(r) of virtual photons, the DCR matrix elements in the dipole approximation for axial and plane channeling coincide formally (with the dimension of the problem taken into account). However, the formulas for DCR angular distributions in the cases of axial and plane channeling differ considerably.     

Investigation of mass and energy coupling between soot particles and gas species in modelling ethylene counterflow diffusion flames

A numerical model is developed aiming at investigating soot formation in ethylene counterflow diffusion flames. The mass and energy coupling between soot solid particles and gas-phase species is investigated in detail. A semi-empirical two-equation model is chosen for predicting soot mass fraction and number density. The model describes particle nucleation, surface growth, and oxidation. A detailed kinetic mechanism is considered for the gas phase and the effect of considering radiation heat losses is also evaluated. Simulations were done for a range of conditions that produce low-to-significant amounts of soot using three strategies: first by changing the strain rate imposed on the flow field, second, by changing the oxygen content in the oxidant stream, and third, by changing the pressure. Additionally, the effect of the transport model chosen was analysed. The results showed that, for the flames studied and within the limits of the present work, the soot and gas radiation terms are of primary importance for numerical simulations. Additionally, it was shown that the soot mass and thermodynamic properties coupling terms are, in general, a second-order effect, with an importance that increases as soot amount increases. As a general recommendation, the radiation terms have always to be considered, whereas full coupling has to be employed only when the soot mass fraction, Y_S, is equal to or larger than 0.008. If a higher precision is required, with errors less than 1%, full coupling should be taken into account for Y_S ≥ 0.002. For lower soot amounts, the coupling through soot mass and thermodynamic properties may be neglected as a first approximation, but an error on the total mass conservation will be present. Additionally, discrepancies from considering different transport models (detailed or simplified) are larger than those found from not fully coupling the phases.     

Resonant influence of hypersound on the radiation of an axially channeled electron

Abstract

The spectral intensity of the radiation emitted by an axially channeled electron in a single crystal excited by a longitudinal hypersonic wave propagating along the channeling direction has been calculated for the energy range 10MeV ≤ E ≤ 100 MeV. It has been shown that under the influence of acoustic vibrations excited in the single crystal a resonant intensification of the electron channeling radiation, a variation of its spectral distribution as well as inverse radiative transitions are possible.     

A nondiffusion mechanism for writing shifted holograms in LiNbO3

This paper attempts to explain the experimental facts that the resolution of LiNbO₃ crystals in writing holograms is poor, and that large discrepancies exist between the measured characteristics of diffraction efficiency and predictions based on current and widely used models. A new model for charge transport is proposed that takes into account macro-and mesoscopic nonuniformities. It is shown that the electric fields arising from modulation of the spontaneous polarization by defects lead to asymmetric diffusion of photoexcited electrons, which makes it highly unlikely that the writing of shifted components of holograms is via a diffusion mechanism. Possible characteristics of shifted and non-shifted holograms written via photogalvanic currents are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1636–1642 (September 1997)     

Surface diffusion of Ni on Si(111) with coadsorption of Co

The effect of the coadsorption of Co and Ni on an Si(111) surface structure and on the diffusion of adsorbed atoms is investigated by low-energy electron diffraction and Auger electron spectroscopy. It is established that surface structures similar to those formed with the adsorption of Co alone are formed with the Ni and Co coadsorption on an Si(111) surface. It is found that the contribution of surface diffusion to the transport of Ni atoms is sharply higher on an Si(111) surface with submonolayer Co concentrations in the temperature range 500–750 °C than for a pure surface, where the main mechanism of Ni transport along the surface is diffusion of Ni atoms through the bulk of Si. Fiz. Tverd. Tela (St. Petersburg) 41, 1489–1494 (August 1999)     

Channeling radiation: Theory,semi-classical simulations

Abstract

An introduction to channeling and channeling (Kumakhov) radiation is given. Relativistic and quantum-mechanical effects are discussed in function of the electron or positron energy. Phenomena of Quantum Electrodynamics in strong macroscopic field, which can be tested in channeling conditions, are described. Recipees for semi-classical Monte-Carlo simulations are presented and one of them compared with experiments at 10 and 150 GeV.     

The importance of thermal radiation transfer in laminar diffusion flames at normal and microgravity

The importance of radiation heat loss in laminar and turbulent diffusion flames at normal gravity has been relatively well recognized in recent years. There is currently lack of quantitative understanding on the importance of radiation heat loss in relatively small scale laminar diffusion flames at microgravity. The effects of radiation heat transfer and radiation absorption on the structure and soot formation characteristics of a coflow laminar ethylene/air diffusion flame at normal- and microgravity were numerically investigated. Numerical calculations were conducted using GRI-Mech 3.0 combustion chemistry without the NO_x mechanism and complex thermal and transport properties, an acetylene based soot formation model, and a statistical narrow-band correlated-k non-grey gas radiation model. Radiation heat transfer and radiation absorption in the microgravity flame were found to be much more important than their counterparts at normal gravity. It is important to calculate thermal radiation transfer accurately in diffusion flame modelling under microgravity conditions.     

Flash Sintering of Oxide Ceramics under Microwave Heating

We report on the results of the analysis of the effect of flash sintering, which is observed upon heating compacted powder materials by high-intensity microwave radiation. Ceramic samples of Y₂O₃, MgAl₂O₄, and Yb: (LaO)₂O₃ were sintered to a density exceeding 98–99% of the theoretical value during 0.5–5 min without isothermal hold. The specific microwave power absorbed volumetrically in the samples was 20–400 W/cm³. Based on the analysis of the experimental data (microwave radiation power and heating and cooling rates) and of the microstructure of the obtained materials, we propose a mechanism of flash sintering based on the evolution of the thermal instability and softening (melting) of the grain boundaries. The proposed mechanism also explains the flash sintering effect observed when a dc or a low-frequency ac voltage is applied to the samples. The microwave heating makes it possible to implement flash sintering without using electrodes for supplying energy to the articles being sintered.     

Transport properties of a binary mixture of CO2ben N2 from the pair potential energy functions based on a semi-empirical inversion method

The potential energy surface of a CO₂-N₂ mixture is determined by using an inversion method, together with a new collision integral correlation [J. Phys. Chem. Ref. Data 19 1179 (1990)]. With the new invert potential, the transport properties of CO₂-N₂ mixture are presented in a temperature range from 273.15 K to 3273.15 K at low density by employing the Chapman-Enskog scheme and the Wang Chang-Uhlenbeck-de Boer theory, consisting of a viscosity coefficient, a thermal conductivity coefficient, a binary diffusion coefficient, and a thermal diffusion factor. The accuracy of the predicted results is estimated to be 2% for viscosity, 5% for thermal conductivity, and 10% for binary diffusion coefficient.     

Strahlungstransportmechanismus und Transportkoeffizienten im Ar-Hochdruckbogen

TheE(J)-characteristics, the total radiated powerU(J), and the radial temperature distributionsT(r, J) measured in a 5 mm diam. Ar cascade arc between 1 and 10 atms are used to evaluate the transport properties of Ar with special attention to the radiation transport mechanism. The electrical conductivity σ(T,p) is determined from the characteristics and temperature distributions by means of Ohm's law. Comparing the results with Langevin's formula, the cross-sections for electron-ion and electron-atom collisions are calculated. The transparent emission coefficiente _tr (T, p) in the transmission range of quartz is extracted in a similar way from the radiation characteristicsU(J); its continuum contribution, which amounts up to 90%, is obtained by integrating the measured spectral continuum distribution. Inserting σ(T, p)E ² ande _tr (T, p) into the energy balance yields the thermal conductivityk _therm (T,p) as well as the uv radiation contribution. The latter can be described by ordinary radiation diffusion above 3 atms and by long range radiation at atmospheric pressure. So the electric power supplied to the arc leaves it in radial direction partially by heat conduction, partially by transparent radiation in the visible and adjacent spectral regions, and partially by ordinary radiation diffusion in the far uv atp≧ 3 atms or by long range diffusion at about atmospheric pressure.     

Infrared radiation from hot holes during spatial transport in selectively doped InGaAs/GaAs heterostructures with quantum wells

The infrared radiation from hot holes in In_xGa_1−x As/GaAs heterostructures with strained quantum wells during lateral transport is investigated experimentally. It is found that the infrared radiation intensities are nonmonotonic functions of the electric field. This behavior is due to the escape of hot holes from quantum wells in the GaAs barrier layers. A new mechanism for producing a population inversion in these structures is proposed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 478–482 (10 October 1996)     

Energy losses of electrons in oriented crystals

Abstract

Within the framework of the multistring model of axial channeling of electrons the method for calculating the losses to radiation in channeling is proposed. The physical model includes the multiple scattering, the beam and target characteristics. The simulated results are compared with the experimental data.     

Silica on Silicon Carbide

Silicon carbide (SiC) as both the most important non-oxide ceramic and promising semiconductor material grows stoichiometric SiO ₂ as its native oxide. During passive oxidation, a surface transformation of SiC into silica takes place causing bulk volume and bulk mass increase. This review summarizes state-of-the-art information about the structural aspects of silicon carbide, silica, and SiC–SiO ₂ interfaces and discusses physicochemical properties and kinetics of the processes involved. A special section describes the electronic properties of carbide–oxide interfaces, which are inferior compared to Si–SiO ₂ interfaces, limiting the use of SiC-based electronics. In the oxidation of SiC there is a variety of parameters (e.g., porosity, presence of sintering aids, impurities, crystallographic orientation, surface treatment, and atmospheric composition) influencing the process. Therefore, the kinetics can be complex and will be discussed in detail. Nonetheless, a general linear-parabolic time-law can be found for most SiC materials for passive oxidation, thus indicating a mainly diffusion-controlled mechanism. The pronounced anisotropy of SiC expresses itself by quite different oxidation rates for the various crystallographic faces. Manifold impact factors are reflected by oxidation rate-constants for silicon carbide that vary over orders of magnitude. The understanding of SiC oxidation and silica formation is still limited; therefore, different oxidation models are presented and evaluated in the light of current knowledge.