Energy Transmission Factor Research Articles

An investigation on gamma-ray and neutron attenuation properties of multi-layered Al/B4C composite

Nowadays, the concept of multi-layered composite materials is attracting the researchers in terms of enhancing the capabilities of shielding materials as well as extending the scope of utilization towards space sciences and cosmic radiation application. In this study, we report the manufacturing, design, and experimental investigation on newly developed multi-layered Al/B4C shielding composites. Al powder material with a purity of 99% and an average size of 15 µm is used as matrix material. Next, B4C powders with an average size of 9 µm are incorporated into the matrix as reinforcement material. Accordingly, we manufactured several different multilayer Al-B4C shield samples of 3 cm diameter and 1 cm height through powder metallurgy method. Gamma-ray transmission properties are determined using 3 × 3 NaI(Tl) scintillation detector through 60Co and 137Cs point isotropic radioisotopes. Moreover, MCNPX (version 2.7.0) is utilized for deposited energy amount and transmission factor calculations for gamma and neutron radiation. Our results showed that geometric configuration plays a crucial role in shielding efficiency of multi-layered materials. Among the manufactured samples, B4C-Al-B4C sample is reported with promising shielding properties against gamma-ray and neutron radiation. A direct relationship between the overall transmission factor values and deposited energy amount (MeV/g) in each sample is also explored for multi-layered composite shields.It was also seen that combination of layers made significantly improvement of radiation shielding properties of materials.

Genetic Algorithm Integrated Fuzzy AHP-VIKOR Approach for the Investigation of W-Cut Insert Heat Exchanger for Cooling of Dielectric Fluid Used in Ultra-High Voltage Transformer

This study proposes a novel W-cut twisted tape to evaluate and enhance the effectiveness of novel grooved tube heat exchanger. The experimental investigations are carried out with W-cut inserts, which possess two main control factors, namely, twist tape ratio (y = 3.5 to 6.5) and width tape ratio (WR = 0.43 to 0.6). The investigation results prove that in all cases the energy transmission and friction factor of W-cut insert along with the first law and second law efficiency are considerably higher than the conventional tube arrangement. The empirical model equations for friction factor, performance enhancement ratio, Nusselt number, and rational efficiency are developed, and they show good affinity with experimental data. Since the abovementioned factors have conflicting terms, it is essential to select the proper operating configuration of the heat exchanger in order to increase the thermal energy transfer rate with reduced consumption of pumping energy. The multi-objective genetic algorithm (GA) tool is used to identify the optimum operating conditions. Further, the hybrid multi-criteria decision-making model, FAHP-VIKOR, is applied to select the perfect model from the set of non-dominated solution. The ranking of alternatives is as follows: A4 > A11 > A2 > A9 > A21 > A17 > A1 > A24 > A11 > A23 > A7 > A18 > A13 > A10 > A20 > A5 > A19 > A14 > A16 > A22 > A25 > A6 > A15 > A3 > A8. The optimized configuration of W-cut insert is compared with the former inserts, and the optimal configuration exhibits supreme performance than other inserts.

Calculating the energy transmission factor for a couplant layer with nonuniform thickness

The energy transmission factor of a layer of couplant with nonuniform thickness has been calculated for a PET arbitrarily placed with respect to an arbitrarily shaped curved entry surface. The calculation results have been approximated for toroidal, spherical, and cylindrical surfaces. It is shown that in most cases of testing small-diameter articles, partial loss of acoustical contact can be disregarded when calculating the energy transmission factor.

Vapor Shield Models for Fusion Reactors Plasma-Facing Components

Radiant heat flux is a dominant mechanism by which energy transfers from the high-temperature core plasma to the interior critical components of the fusion reactor, which result in surface ablation and sever damage to the components. A vapor layer develops at the surface and provides a self-shielding mechanism at the plasma-material interface. Two models for the energy transmission factor through the boundary layer were developed and incorporated in the electrothermal plasma capillary code to predict the effectiveness of these models in surface self-protection. The electrothermal plasma capillary discharge simulates the typical conditions of fusion reactors disruption and quench phase and has been shown to be an adequate technique to evaluate the erosion of plasma-facing component. First model treats the radiant heat transport as it is affected by the variation of the plasma opacity, in which the vapor shield efficiency depends on the plasma optical thickness and the mean plasma opacity. The second model defines the vapor shield by the ratio of the energy reaching the surface to the total radiant energy emitted by the plasma with the inclusion of the plasma kinetic energy. The code can predict the axial and temporal variation of the transmission factor at each time step and mesh point, and predicts the plasma parameters with the effectiveness of the vapor shield at the boundary layer. The code prediction with implementation of both models has been used to compare the results with earlier ones and with some experimental data. Code results are in good correlation with experimentally measured ablation data.

Thermal energy transfer through heat protective clothing during a flame engulfment test

The flame engulfment test according to ISO 13506 assesses the protective performance of ready-made heat and flame protective clothing when exposed to a flash fire condition using an instrumented manikin. It uses a thermal model of the human skin to predict the risk of skin burns. This evaluation method is based on a pass/fail criterion, as either a burn is predicted or not. Therefore, it provides only limited information about the transfer of heat through the protective clothing. In this study, we investigated the use of the total transferred energy as an improved characterization method of the performance of the garments. We defined an energy transmission factor as the quotient between the transferred energy on the clothed manikin divided by the transferred energy registered by the nude manikin during calibration. We analyzed the performance of seven garments and show that the energy transmission factors can be assessed with very high repeatability. When comparing the results of the right and left arms and legs, we found very high correlation coefficients of 0.96 and 0.98, respectively, showing that the thermal insulation of the garments tested was very symmetrical. This new assessment method will be proposed for the revision of ISO 13506.

A close-form solution to predict the total melting time of an ablating slab in contact with a plasma

An exact melt-through time is derived for a one-dimensional heated slab in contact with a plasma when the melted material is immediately removed. The plasma is composed of a collisionless presheath and sheath on a slab, which partially reflects and secondarily emits ions and electrons. The energy transport from plasma to the surface accounting for the presheath and sheath is determined from the kinetic analysis. This work proposes a semi-analytical model to calculate the total melting time of a slab based on a direct integration of the unsteady heat conduction equation, and provides quantitative results applicable to control the total melting time of the slab. The total melting time as a function of plasma parameters and thermophysical properties of the slab are obtained. The predicted energy transmission factor as a function of dimensionless wall potential agrees well with the experimental data. The effects of reflectivities of the ions and electrons on the wall, electron-to-ion source temperature ratio at the presheath edge, charge number, ion-to-electron mass ratio, ionization energy, plasma flow work-to-heat conduction ratios, Stefan number, melting temperature, Biot number and bias voltage on the total melting time of the slab are quantitatively provided in this work.

Effects of Plasma Parameters on the Temperature Field in a Workpiece Experiencing Solid-Liquid Phase Transition

The heat transfer characteristics of a workpiece subject to plasma heating and melting are theoretically and systematically studied. Plasma etching, spray deposition, sputtering, cutting and surface treatment, etc., are usually controlled by energy transfer from plasma to workpieces. In this work, the one-dimensional unsteady conduction equation accounting for solid-liquid phase transition with distinct thermal properties in a workpiece is solved. The plasma is composed of a collisionless presheath and sheath on an electrically floating workpiece that partially reflects or secondarily emits ions and electrons. The energy transport from plasma to the surface is kinetically, analytically and exactly calculated from self-consistent velocity distributions of the ions and electrons. The results show that the predicted surface temperature and energy transmission factor agree well with experimental data. The effects of plasma characteristics and thermal parameters of the workpiece on unsteady temperature profiles and thickness of the molten layer in the workpiece are quantitatively provided in this work. Energy released from recombination of the ions and electrons on the surface is found to play the most important role on heating the workpiece. The deviation of surface temperature contributed by recombination energy can be 1000 K.

Plasma energy transport to an electrically biased surface

The total energy flux, including ion and electron energy fluxes, transport to a biased surface in contact with a plasma is systematically and analytically investigated in this study. Micro-electro-mechanical systems (MEMS), semiconductor technology, plasma etching, spray deposition, sputtering, cutting and surface treatment, etc. are usually controlled by energy transfer from the plasma to workpiece. In this work, the plasma is composed of a collisionless presheath and sheath on an electrically negative biased workpiece partially reflecting and secondly emitting ions and electrons. The presheath is an ionization region that continuously produces ions to supply ion loss to the surface, while the sheath is a space-charge region that accelerates ions and retards electrons toward the surface. Based on the kinetic analysis, the predicted ion density, velocity and energy transmission factor on the surface are found to agree well with experimental data. The effects of dimensionless ion recombination and electron absorption energies, reflectivities and second emissivities of the ions and electrons on the wall, ion-to-electron mass ratio, charge number, electron-to-ion source temperature ratio at the presheath edge, and net current density on energy fluxes across the sheath are obtained. Energies released from recombination of the ions and electrons on the surface play the most important role in energy transfer to the workpiece. The sheath should be accounted for predicting energy transport to a biased surface.

Generation and characteristics of detached recombining plasma and its dynamic behaviour—a bridge between fusion plasmas and low-temperature ionized gases

First, the role of a magnetic divertor in fusion devices is introduced, and it is shown that the energy balance in burning International Thermonuclear Experimental Reactor indicates a strong necessity for reduction of plasma heat load on the divertor target plate. Next, the plasma heat flow to a material surface through sheaths is generally given in terms of the energy transmission factor, in which the energy deposition based on the surface recombination is essential in high-density plasmas. The heat load is independent of any cooling of the plasmas. Then, a variety of interesting characteristics of detached recombining plasmas, which are important in reducing the plasma heat flow, are discussed: generation and structure of plasma detachment, light emission originating from a series of highly excited Rydberg states of atoms, contribution of molecular activated recombination (MAR), transition between conventional electron–ion recombination (EIR) and MAR, relation to dust coagulation processes and the experimental verification of plasma detachment in tokamak fusion devices. Finally, the dynamic behaviour of the detached EIR plasma obtained in the linear divertor plasma simulator NAGDIS-II against the ELM-like heat pulse is shown.

Heat flows through plasma sheaths

Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission (TEE) and secondary electron emission (SEE), in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor δ. An anomaly of δ in detached recombining plasmas is discussed.

Nonlinear collisional monte carlo simulations for high-temperature SOL plasma

A nonlinear Monte Carlo collisional model is applied to to investigate scrape-off layer (SOL) plasmas with high temperatures. In the proposed SOL modeling, A steady state SOL plasma, which satisfies the particle and energy balances and neutrality constraint, is determined in terms of total particle and heat fluxes across the separatrix, the edge plasma temperature, the secondary electron emission coefficient, and the SOL size. A conductive heat flux into the SOL is effectively modeled via random exchange of source particles and the SOL plasma particles. It is found that the potential drop and the electron transmission factor in the collisional SOL plasma are in good agreement with the theoretical prediction. The cooling effect of secondary electrons in the high temperature divertor operation is investigated. In such a collisionless plasma, the present nonlinear collision model is useful because the electron distribution function deviates far from a Maxwellian distribution. In the presence of strong secondary electron emission, the electron sheath energy transmission factor in the collisionless regime is found to be significantly smaller than that in the collisional regime. This fact suggests that a high-temperature divertor operation can be possible.

Cooling effect of secondary electrons in high temperature divertor operation

The scrape-off layer (SOL) plasma is studied over a wide range of Coulomb collisionality via particle Monte Carlo simulation with focus on the cooling effect of secondary electrons in the high temperature divertor operation. The electron distribution function in the collisionless regime shows a large departure from a Maxwellian distribution. In the presence of strong secondary electron emission the electron sheath energy transmission factor in the collisionless regime is found to be significantly smaller than that in the collisional regime, which indicates that a higher temperature is obtainable in collisionless SOL plasmas. In both collisional and collisionless SOL plasmas, the change in the conductive energy flux from the core region into the SOL has little influence on the normalized potential drop and electron energy transmission factor, while the electron temperature is raised as the conductive energy flux increases

A Monte Carlo model for velocity space effects in low recycling scrape-off layer plasmas

A Monte Carlo model solving only in velocity space is proposed to investigate a low recycling scrape-off layer (SOL) plasma. In the model, the effects of Coulomb collisions are accurately described by a non-linear Monte Carlo collision operator; a conductive heat flux into the SOL is effectively modelled via randomly exchanging the source particles and the SOL particles; secondary electrons are included. A steady state SOL plasma, which satisfies the particle and energy balances and the neutrality constraint, is determined in terms of the total particle and heat fluxes across the separatrix, the edge plasma temperature, the secondary electron emission coefficient and the SOL size. The model gives gross features of the SOL, such as plasma temperatures and densities, the total sheath potential drop and the sheath energy transmission factor. Simulations neglecting unlike collisions are performed. It is found that the potential drop and the electron energy transmission factor in collisional SOL plasmas are in close agreement with theoretical predictions. The present model primarily provides useful information about collisionless SOL plasmas that is difficult to obtain analytically

Kinetic modeling of scrape-off layer plasmas

Electron transport along open field lines in the diverted scrape-off layer of a tokamak is studied numerically via a kinetic Fokker–Planck approach. The method allows calculation of the distribution function in a situation where large parallel temperature gradients are maintained by collisional relaxation and, at the same time, superthermal electrons stream freely from the midplane of the plasma to the target/sheath boundary. The method also allows calculation of the self-consistent electrostatic field associated with parallel gradients in the distribution function, as well as the potential drop across the target/sheath boundary, where the latter is calculated to enforce appropriate boundary conditions at the target, although the sheath itself is not resolved. The kinetic results are compared to classical fluid results for the case of a simple (nonradiative) divertor. The kinetic solutions exhibit an enhanced superthermal electron population in the vicinity of the target, which results in a larger sheath energy transmission factor, a lower bulk electron temperature, and a smaller sheath potential drop. The sheath potential largely determines the energy with which ions impact the target, thereby affecting the rate of target erosion. Ionization rates and radiation rates from impurities in the vicinity of the target also depend strongly on the local electron temperature and can be sensitive to superthermal tails.

Review of component erosion in electric launcher technology

Component erosion in electric launcher technology (electromagnetic, electrothermal and electrothermal-chemical) plays a major role, which determines the launcher's bore lifetime, launch velocity and operational limits. Erosion studies have been conducted on the electrothermal launcher SIRENS, which simulates expected operational conditions of high heat flux deposition in electric launchers. Various material surfaces have been exposed to SIRENS plasma to test their performance, erosion behavior and lifetime. Tested materials are pure metals, alloys, refractory materials, specially-coated materials, insulators and composites, pure and composite graphites. Results showed that coating reduced erosion. Tungsten-based alloys have no observable erosion. Vapor shield phenomena have been characterized for different materials, and the energy transmission factor through the vapor shielding layer is obtained. >

Pre-feasibility analysis of an energy supply system for Southern Europe: Technical aspects

Technical aspects of the feasibility of energy transmission from hydroelectric power plants in Zaire to southern Europe, and to Italy in particular, have been studied. Three alternatives were considered: electric power transmission, gaseous hydrogen pipeline, and methylcyclohexane/toluene loop cycle. The technical feasibility of each alternative, also for a mid-term prospect, and energy losses were examined and evaluated. A comparison, from an energetic point of view, was also carried out among the three systems by varying the ratio between hydrogen and electric power required by the users. The high values of the energy transmission factor make this energy supply system very attractive. Depending on the sharing of the energy required for final uses, a double transportation system, based on electric power together with hydrogen, could be convenient. Indeed, if a very high amount of hydrogen is required by the users, a system based on hydrogen would be preferable.

Upgrade of the Vapor Shield Modeling Code MAGFIRE

An upgrade of the MAGFIRE code used to model the vapor shield (VS) effect was needed to include: higher disruption heat fluxes of interest to ITER, a comparison of carbon to beryllium erosion depths and the effects of ion/electron beam charge separation in the VS. Results show that higher heat fluxes give a lower total energy transmission factor (f) as expected. Beryllium divertor plates have much higher erosion depths than do carbon as well as a higher f. Charge separation has a small effect on the VS stopping power for electrons and on the distribution of deposited energy in the VS. However, the effect will be more important as the disruption particle energy increases.

The use of an electrothermal plasma gun to simulate the extremely high heat flux conditions of a tokamak disruption

Disruption damage conditions for future large tokamaks like ITER are nearly impossible to simulate on current tokamaks. The electrothermal plasma source SIRENS has been designed, constructed, and operated to produce high density (> 1025/m3), low temperature (1–3 eV) plasma formed by the ablation of the insulator with currents of up to 100 kA (100 μs pulse length) and energies up to 15 kJ. The source heat fluence (variable from 0.2 to 7 MJ/m2) is adequate for simulation of the thermal quench phase of plasma disruption in future fusion tokamaks. Different materials have been exposed to the high heat flux in SIRENS, where comparative erosion behavior was obtained. Vapor shield phenomena has been characterized for different materials, and the energy transmission factor through the shielding layer is obtained. The device is also equipped with a magnet capable of producing a parallel magnetic field (up to 16 T) over a 8 msec pulse length. The magnetic field is produced to decrease the turbulent energy transport through the vapor shield, which provides further reduction of surface erosion (magnetic vapor shield effect).

Comparative study of component erosion for electromagnetic and electrothermal launchers

The electrothermal launcher SIRENS has been used to study the erosion of critical components (rails and insulators) of plasma-driven launchers. SIRENS can produce high-density (>10/sup 25//m/sup 3/) low-temperature (1-3 eV) plasma, formed by the ablation of the insulator (Lexan), with currents up to 100 kA. The incident heat flux varies between 2 and 90 GW/m/sup 2/ over 100 mu s duration, for input energies of 1-10 kJ. Erosion studies have been performed on several insulators, pure and coated metals, alloys, and several graphite grades. The fraction of the total incident energy transmitted to the eroded surface varies from 12 to 30% for the materials tested and decreases to 5-7% as the incident energy fluence increases. Such reduction in erosion for a given incident fluence is due to the vapor shield effect. The scaling law for the energy transmission factor through the vapor shield layer was obtained for the exposed materials. >

Vapor shield protection of plasma facing components under incident high heat flux

Disruption damage to plasma facing components has been found to be a limiting design constraint in ITER and other large fusion devices. A growing data base is confirming the role of the vapor shield in protecting ablated surfaces under disruption-like conditions, which would imply longer lifetimes for plasma facing components. We present new results for exposure of various material surfaces to high heat fluxes up to 70 GW/m 2 over 100 μs (7 MJ/m 2 ) in the SIRENS high heat flux test facility. Tested materials are graphite grades, pyrolytic graphite, refractory metals and alloys, refractory coatings on copper substrates, boron nitride and preliminary results of diamond coating on silicon substrates. An empirical scaling law of the energy transmission factor through the vapor shield has been obtained. The application of a strong external magnetic field, to reduce turbulent energy transport in the vapor shield boundary, is shown to decrease f by as much as 35% for fields of 8 T.