Pre-heating Phase Research Articles

Laser Assisted Machining Process of HIPed Silicon Nitride

In this study, fiber coupled HPDL was employed for Laser-Assisted Machining (LAM) of Hot Isostatically Pressed (HIPed) silicon nitride workpieces. In LAM, the intense energy of laser was used to enhance the machinability by locally heating the workpiece and thus reducing its yield strength. In this experiment, the laser power ranged from 200 W to 800 W while the diameter of the workpiece was 16mm. While machining, the surface temperature was kept nearly constant by laser heating after a preheat phase of 58 seconds. Microscopy observations of machined surfaces and chips were performed to understand the mechnical behavior of silicon nitride with temperature. The effects of chip formation temperature on machining characteristics were assessed by measuring the cutting forces and flank wear. At a temperature of about 1153℃, a good quality machined surface on the HIPed silicon nitride workpiece was obtained with no grain pullout regions and little oxidation layer.

The coupled thermal-hydraulic-mechanical behaviour of a large-scale in situ heating experiment

The thermo-hydraulic-mechanical behaviour of a large in situ heating test, the Buffer/Container Experiment (BCE), carried out at Atomic Energy of Canada's underground research laboratory, is considered. The work can be seen as an extension of the authors' previous research, investigating the behaviour of a large-scale test, the so-called ‘isothermal test’, to now include heating effects. Suggestions related to the micro–macro behaviour of bentonitic buffer materials subjected to resaturation in confined non-isothermal conditions are explored. Simulation of pre-heating phases of the experiment demonstrates the ability of the model to describe the hydraulic regime in the host rock and isothermal infiltration into the buffer. Consideration of the heating phase confirms that the temperature field is well understood, with a good correlation between the numerical and experimental results. In the moisture field it is found that the interaction between the emplaced buffer and host rock is of considerable importance. It is found that inclusion of micro–macro interaction effects, by way of consideration of the impact of swelling and micro–macro interactions on the hydraulic conductivity of the material, yielded significantly improved correlations between observed and calculated results in the moisture field. The results of the numerical simulations suggest that the interaction between the emplaced buffer and host rock is strongly influenced by the micro–macro behaviour of the bentonitic buffer material. It is concluded that the consideration of the impact of micro–macro interactions on moisture flow in bentonitic buffers is of considerable importance, and may be of some significance in considering the total resaturation time as part of the performance assessment of a nuclear waste disposal repository.

Predictions of dynamically emerging brane inflation models

We confront the recent proposal of emerging brane inflation with $\mathrm{WMAP}3+\mathrm{SDSS}$, finding a scalar spectral index of ${n}_{s}={0.9659}_{\ensuremath{-}0.0052}^{+0.0049}$ in excellent agreement with observations. The proposal incorporates a preceding phase of isotropic, nonaccelerated expansion in all dimensions, providing suitable initial conditions for inflation. Additional observational constraints on the parameters of the model provide an estimate of the string scale. A graceful exit to inflation and stabilization of extra dimensions is achieved via a string gas. The resulting preheating phase shows some novel features due to a redshifting potential, comparable to effects due to the expansion of the universe itself. However, the model at hand suffers from either a potential overproduction of relics after inflation or insufficient stabilization at late times.

Evolution of the Loop‐Top Source of Solar Flares: Heating and Cooling Processes

We present a study of the spatial and spectral evolution of the loop-top (LT) sources in a sample of 6 flares near the solar limb observed by {\it RHESSI}. A distinct coronal source, which we identify as the LT source, was seen in each of these flares from the early ``pre-heating'' phase through the late decay phase. Spectral analyses reveal an evident steep power-law component in the pre-heating and impulsive phases, suggesting that the particle acceleration starts upon the onset of the flares. In the late decay phase the LT source has a thermal spectrum and appears to be confined within a small region near the top of the flare loop, and does not spread throughout the loop, as is observed at lower energies. The total energy of this source decreases usually faster than expected from the radiative cooling but much slower than that due to the classical Spitzer conductive cooling along the flare loop. These results indicate the presence of a distinct LT region, where the thermal conductivity is suppressed significantly and/or there is a continuous energy input. We suggest that plasma wave turbulence could play important roles in both heating the plasma and suppressing the conduction during the decay phase of solar flares. With a simple quasi-steady loop model we show that the energy input in the gradual phase can be comparable to that in the impulsive phase and demonstrate how the observed cooling and confinement of the LT source can be used to constrain the wave-particle interaction.

Homoepitaxial silicon growth in a non-ultra-high vacuum environment by ion-assisted deposition on Si wafer and seeded glass substrates

Eliminating the requirement of ultra-high vacuum (UHV) conditions and achieving high-rate crystalline silicon (c-Si) growth are important targets for cheap mass production of semiconductor devices such as thin-film solar cells. In this paper we report on the achievement of high-quality, high-rate, epitaxial Si growth in a non-UHV environment by ion-assisted deposition (IAD) on (100) Si wafer substrates as well as on glass substrates featuring a polycrystalline Si seed layer. This was achieved by high-rate growth to suppress the contamination in the growing Si film and by a sacrificial protective layer which protects the initial growth surface prior to start of epitaxy. On (100) Si wafer substrates good structural quality of the epitaxially grown Si films was achieved, resulting in 1-Sun open-circuit voltages of up to 550 mV. On planar seeded glass substrates the heating procedure had to be modified to take the large thermal mass and the thermal stress caused by the underlying glass substrate into account. It is shown that the pre-heating phase must be extended by more than a factor of 4 and that high growth temperatures have to be avoided to minimise thermal stress. Taking these considerations into account and using two post-deposition treatments (rapid thermal annealing and hydrogenation), open-circuit voltages of 420 mV and short circuit-current densities of about 10 mA/cm 2 under 1-Sun illumination are achieved on glass substrates.

Comparison of TRANSP-evolved q-profiles with MSE constrained equilibrium fits on JET

The pitch angle predicted by the transport analysis code TRANSP was compared with motional Stark effect (MSE) data. TRANSP was initialized in the early ramp-up phase using a measured pitch angle profile but evolved its current profile thereafter using the magnetic field diffusion equation. The comparison was carried out over a wide range of pulses, including cases where a current hole was established in the pre-heating phase. Besides confirming that a neoclassical model of the resistivity is superior to the classical one in modelling the evolution of the current profile in JET, the results also indicate that the TRANSP prediction is nearly always within the 95% confidence band of the MSE data. In addition, it is shown that the TRANSP q-profile is consistent with MHD data and Faraday rotation polarimetry data. The results presented here underpin the accuracy of the current profile predicted by a well-diagnosed TRANSP run.

Dynamic modeling of the combined heat and mass transfer during the adsorption/desorption of water vapor into/from a zeolite layer of an adsorption heat pump

This paper presents a one-dimensional model describing the combined heat and mass transfer during the adsorption/desorption of water vapor into/from a zeolite layer in a small-scale adsorption heat pump. The model is utilized to study the dynamics of both adsorption and desorption processes as well as to examine the influence of both the layer thickness and the volume of the vapor phase of the adsorber/desorber heat exchanger on their time requirements. The model equations have been numerically solved using the software packet gPROMS ( general PROcess Modeling System) [ http://www.psenterprise.com/gPROMS]. The duration of the adsorption process is found to be generally longer than that of the desorption process, depending on the zeolite layer thickness. Reducing the layer thickness by 50% results in a reduction to 25% in the duration of the adsorption process and to about 33% of the desorption process. In order to get reasonable power densities from an adsorption heat pump using zeolite layers, the layer thickness must not exceed 2.5 mm. Moreover, the volume of the vapor phase of the adsorber/desorber heat exchanger has to be minimized, in order to minimize the unavoidable losses of the coefficient of performance of the periodical adsorption heat pump due to the pre-cooling as well as the pre-heating phases.

Low power BL Lacertae objects and the blazar sequence

The spectral properties of blazars seem to follow a phenomenological sequence according to the source luminosity. By inferring the source physical parameters through (necessarily) modeling of the blazar spectra, we have previously proposed that the sequence arises because the particles responsible for most of the emission suffer increasing radiative losses as the luminosity increases. Here we extend those results by considering the widest possible range of blazar spectral properties. We find a new important ingredient for shaping the spectra of the lowest power objects, namely the role of a finite timescale for the injection of relativistic particles. Only high energy particles radiatively cool over such a timescale leading to a break in the particle distribution: particles with this break energy are those emitting most of the power, and this gives rise to a link between blazar spectra and total energy density inside the source, which controls the cooling timescale. The emerging picture requires two phases for the particle acceleration: a first pre–heating phase in which particles reach a characteristic energy as the result of balancing heating and radiative cooling, and a more rapid acceleration phase which further accelerate these particles to form a power law distribution. While in agreement with standard shock theory, this scenario also agrees with the idea that the luminosity of blazars is produced through internal shocks, which naturally lead to shocks lasting for a finite time.

Preheating—cosmic magnetic dynamo?

We study the amplification of large-scale magnetic fields during preheating and inflation in several different models. Preheating can resonantly amplify seed fields on cosmological scales. In the presence of conductivity, however, the effect of resonance is typically weakened and the amplitude of produced magnetic fields depends sensitively on the evolution of conductivity during the preheating and thermalization phases. In addition we discuss geometric magnetization, where amplification of magnetic fields occurs through coupling to curvature invariants. This can be efficient during inflation due to a negative coupling instability. Finally we discuss the breaking of the conformal flatness of the background metric whereby magnetic fields can be stimulated through the growth of scalar metric perturbations during metric preheating.

The Lamé equation in parametric resonance after inflation

We show that the most general inflaton potential in Minkowski spacetime for which the differential equation for the Fourier modes of the matter fields reduces to Lame's equation is of the form V(phi)=lambda phi^4/4+Kphi^2/2+mu/(2 phi^2)+V_0. As an application, we study the preheating phase after inflation for the above potential with K=0 and arbitrary lambda,mu >0. For certain values of the coupling constant between the inflaton and the matter fields, we calculate the instability intervals and the characteristic exponents in closed form.

Determination of refractive indexes of In 0.52Al 0.48As on InP in the wavelength range from 250 to 1900 nm by spectroscopic ellipsometry

In a previous paper, we found that between InP and In 0.52Al 0.48As grown by molecular beam epitaxy, an interface layer exists, owing to the interaction of arsenic with InP in the preheat phase of the MBE growth. To determine the refractive index and thicknesses of the In 0.52Al 0.48As and the interface layer in the wavelength range from 300 to 1900 nm, layers 10, 20, 200 and 2000 nm thick of MBE-grown In 0.52Al 0.48As on InP were investigated using spectroscopic ellipsometry.

Determination of the refractive index of In 0.53Al 0.11Ga 0.36As on InP in the wavelength range from 280 to 1900 nm by spectroscopic ellipsometry

The refractive index of the quarternary material In 0.53Al 0.11Ga 0.36As/InP is measured for the first time by spectroscopic ellipsometry in the wavelength range from 280 to 1900 nm. In previous papers we found out that between InP and MBE-grown In 0.52Al 0.48As or In 0.53Ga 0.47As an interface layer exists, due to the interaction of arsenic with InP in the preheat phase of the MBE growth. Therefore we grew three layers of In 0.53Al 0.11Ga 0.36As on InP with different thicknesses to determine the refractive indices and exact values of the thicknesses.

Determination of refractive indices of In 0.53Al 0.40Ga 0.07As and In 0.53Al 0.11Ga 0.36As on InP in the wavelength range from 280 to 1900 nm by spectroscopic ellipsometry

The refractive indices of In 0.53Al 0.40Ga 0.36As layers on InP are measured for the first time by spectroscopic ellipsometry in the wavelength range from 280 to 1900 nm. In previous papers we found out that between InP and MBE-grown In 0.52Al 0.48As and In 0.53Ga 0.47As layers an interface layer exists, due to the interaction of arsenic with InP in the preheat phase of the MBE growth. We investigated three layers of each composition with different thicknesses for the determination of the refractive indices and the exact values of the thicknesses.

Application Of Numerical Modelling To The Simulation Of The Electric-Preheat Steam-Drive (Epsd) Process In Athabasca Oil Sands

Abstract An implicit three-dimensional electrothermal simulator has been developed to numerically model multi-phase electric-preheat steam-drive (EPSD) processes in the Athabasca deposit. Two versions of the model are discussed. In the first version (Model 1), the electric preheat phase of process is modelled assuming that fluid expansion and flow can be neglected. The second model (Model 2) accounts for the flow of oil, water, and steam, as well as heat transfer due to conduction and convection during both the electric preheat and steam-drive phases of the recovery process. Data from two simple two-dimensional reservoir simulations using both models are presented and compared. The results suggest that in most circumstances the expansion and flow of fluids during the electric preheat may have negligible effect on the temperature distributions established in the reservoir, and hence, that the computationally less complex Model 1 is generally adequate for simulating EPSD processes in the Athabasca deposit. Finally, an EPSD process in a representative reservoir is simulated using a 1 ha repeated 5-spot pattern with an electric preheat of 365 days. The preheat is followed by a combination of steam and hot waterflooding extending for a period of approximately eight years. Cumulative recovery is shown to be about 60% original-oil-in-place (OOIP) at an OSR of 0.42. Introduction Large portions of the Athabasca oil sand deposits of Alberta have porosities, oil saturations and pay thicknesses that should make them excellent candidates for economical steamflooding. However, to inject steam and produce bitumen continuously at practical rates it is necessary to have some initial fluid communication between injector and producer wells. Under in situ conditions the reservoir fluids in these formations are virtually immobile. In an electric-preheat steam-drive (EPSD) process the reservoir is first selectively heated electrically so as to lower the viscosity of the oil along predetermined paths between wells. Fluid mobilities along these heated interwell channels are generally several hundred times greater than in the original reservoir, and steam can be injected at satisfactory rates, at pressures well below fracture pressure. Electrode wells (which may also be required to serve as injectors or producers), excited at power frequency), are located with due consideration given to reservoir lithology and electrical properties so as to cause current to pass through the regions that are desired to be heated. Electric current flows primarily through the interconnected water paths in the oil sand and the electrical power dissipated along these filamentary paths is rapidly transferred to the surrounding bitumen and to the sand matrix. Detailed considerations for the selection and placement of electrode wells for various reservoir lithologies have been previously reported(1–3). As well, a considerable literature relating to the numerical and physical modelling of electric-preheat processes has developed(1–16). Recently the economics of such processes have been examined by the Alberta Oil Sands Technology and Research Authority(17) and by Petrotec Systems Inc. of Denver(18). Few of the referenced articles consider the problem of fluid flow during electrical preheating(4, 6, 7, 12, 15, 16, 17). Only three of these(12, 15, 17) specifically address modelling of the electric-preheat steam-drive process.

Experimental and theoretical investigation of the gas embedded Z-pinch

The paper reports observations of a sub-millimetre diameter Z-pinch discharge initiated in hydrogen gas at high pressure (0.33-2 bar) and powered by a fast rising current (dI/dt ≈ 1 kA·ns−1). The conditions for heating the pinch ohmically under pressure balance are established by using a small preheat current of short duration (5 kA, 70 ns). Measurements of the radial density profiles of the electrons in the pinch and the surrounding neutral gas are presented. The peak electron temperature in the pinch is estimated from the soft X-ray emission. The observations show that during the preheat phase, when there is no equilibrium, the plasma column expands radially and drives a shock into the neutral gas. This results in an increase of the neutral gas density in a thin layer surrounding the pinch. When the main current is applied, a pinch in radial equilibrium, with a radius of about 400 μm and peak electron density on axis (2.5 × 1024 m−3 at 0.33 bar), is observed. The estimated peak electron temperature on axis (69 eV at 0.33 bar) is about twice the Bennett temperature. An increase of the line density is observed during the equilibrium. This is ascribed to a flow of neutral particles from the high density layer surrounding the pinch column. The equilibrium is destroyed by an m = 1 instability which grows from a helical perturbation on axis. The experimental results are compared with calculations based on ideal MHD stability theory, and it is shown that the existence of a centrally peaked current density profile can be inferred from the nature of the observed instabilities. An explanation is offered for the absence of the m = 0 mode, on the basis of cooling of the outer region of the plasma.

Fuel preconditioning studies for e-beam fusion targets

Fuel temperature and density conditions, achieved during the preheat phase of electron-beam fusion compression experiments, must be accurately known to understand experimental results via numerical simulations. We present studies of discharge preheating in a simplified cylindrical geometry which compare measured quantities with results from the one-dimensional Lagrangian CHARTB magnetohydrodynamic code. Experimental measurements included schlieren photography and ultraviolet through visible time- and space-resolved spectroscopy in various configurations. It is seen that an 8-kA 500-ns heating pulse in 100 Torr of D2+10% O2 produces 10–12 eV temperatures, 1018 cm−3 electron densities, and 7×105 cm/s expansion velocities in the heated discharge channel. These results are consistent with previous claims for neutron-producing targets, although the target geometry is different.