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At least 127 records · Page 7

Dense hydrogen layers for high performance MagLIF

We report that Magnetized Liner Inertial Fusion (MagLIF) experiments driven by the Z machine produce >10 13 deuterium-deuterium fusion reactions. Simulations indicate high yields and gains (1000) with increased current and deuterium-tritium layers for burn propagation. Such a coating also isolates the metal liner from the gaseous fuel, which should reduce mixing of liner material into the fuel. However, the vapor density at the triple point is only 0.3 kg/m3, which is not high enough for MagLIF operation. We present two solutions to this problem. First, a fuel wetted low-density plastic foam can be used to form a layer on the inside of the liner. The desired vapor density can be obtained by controlling the temperature. This does however introduce carbon into the layer which will enhance radiation losses. Simulations indicate that this wetted foam layer can significantly contribute to the fusion yield when the foam density is less than 35 kg/m 3 . Second, we show that a pure frozen fuel layer can first be formed on the inside of the liner and then low temperature gaseous fuel can be introduced just before the implosion without melting a significant amount of the ice layer. This approach is the most promising for MagLIF to produce high yield and gain.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Flame balls - Past, present and future

This paper discusses analytical and numerical work that has been carried out in order to understand flame-balls and related phenomena that have been observed in microgravity experiments. The importance of heat losses is identified, whether from conduction, convection, or radiation. Accurate numerical simulations for hydrogen-air mixtures with radiation losses reveal a flammability limit of 3.5 percent of hydrogen by volume, a value close to the experimental one. The important role of stability analyses is emphasized, with particular attention to the role of three-dimensional instabilities in explaining unsteady spheroidal flames and flame-strings, objects that are observed in the experiments. We speculate that the dynamics of flame-strings is affected by in-depth radiation absorption for mixtures containing SF6, and report on some preliminary calculations in which this phenomenon is accounted for.

Buckmaster, J.↗

Demonstration of low-mode shape control in indirect-drive double shell implosions at the NIF

Double shell inertial confinement fusion is a concept for achieving robust thermonuclear burn that uses dense metal shells to compress deuterium-tritium (DT) fuel to fusion conditions. Double shell implosions are typically indirectly driven and involve a target that consists of a low-Z ablator, a foam layer, and a high-Z pusher surrounding the DT fuel. The goal of the campaign is to achieve a volumetric burn as radiation losses from the DT fuel are trapped by the opaque high-Z shell. The overall performance of double shell implosions relies on the efficient collisional transfer of kinetic energy between layers. The efficiency of this transfer (and therefore the overall performance of a given implosion) is degraded by the presence of low-mode asymmetries. P2 asymmetries are often observed in spatially resolved 2D radiographs of nominal double shell implosions. This work discusses three such experiments: one with an oblate P2 asymmetry, one with a prolate P2 asymmetry, and one with an approximate spherical symmetry. After performing a shape analysis of the oblate and prolate implosions to quantify asymmetries, these experimental results were compared with the results of hydrodynamic simulations for the two experiments. Differences between the experiment and simulation were then used to design an approximately spherical implosion by altering the incident laser cone fraction. Radiographs from the experiment that implemented the modified cone fraction show evidence of an implosion that is approximately spherical until bang time. This design is intended to serve as a point design for future studies that will seek to optimize various aspects of the double shell target.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Coronal plasma parameters in a long-duration X-ray event observed by Skylab

Physical conditions in the solar coronal X-ray-emitting plasma are analyzed for an event observed by the S-056 X-ray telescope on Skylab in August 1973. The event was located at the west limb and consisted of a series of high looplike structures that underwent dramatic changes in structure and intensity for more than 50 hours. A total X-ray energy (8 to 20 A) of about 5 by 10 to the 28th power erg is obtained by integrating over the lifetime and is shown to be equivalent to that reported previously for a 'typical' flare. It is found that a first major peak in total X-ray flux and thermal energy density was due to an increase in temperature, a major secondary peak in these parameters corresponded to the time of maximum in density, and conductivity energy losses near maximum intensity were much greater than radiative losses. Models dealing with sources of heating and energy balance are evaluated.

Vorpahl, J. A.↗

The decay of coronal loops brightened by flares and transients

Observations of X-ray emitting loops derived from Skylab S-054 photographs, and combined with temperature and brightness estimates from Solrad data, are used to determine brightness decay times resulting from various coronal energy loss mechanisms. Conductive losses are found to be more rapid than radiative losses. Attention is given to the role of geometrical inhibition of conduction as a possible mechanism of brightness decay. Soft X-ray observations are consistent with the continuation of the 'evaporation' driven by thermal conduction late into the decay phase of the event.

Krieger, A. S.↗

Investigation in the 7-by-10 Foot Wind Tunnel of Ducts for Cooling Radiators within an Airplane Wing

Report presents the results of an investigation made in the NACA 7 by 10-foot wind tunnel of a large-chord wing model with a duct to house a simulated radiator suitable for a liquid-cooled engine. The duct was expanded to reduce the radiator losses, and the installation of the duct and radiator was made entirely within the wing to reduce form and interference drag. The tests were made using a two-dimensional-flow setup with a full-span duct and radiator. Section aerodynamic characteristics of the basic airfoil are given and also curves showing the characteristics of the various duct-radiator combinations. An expression for efficiency, the primary criterion of merit of any duct, and the effect of the several design parameters of the duct-radiator arrangement are discussed. The problem of throttling is considered and a discussion of the power required for cooling is included.

Harris, Thomas A↗

Transport and Chemical Effects on Concurrent and Opposed-flow Flame Spread at Microgravity

Flame spread over flat solid fuel beds is a useful means of understanding more complex two-phase non-premixed spreading flames, such as those that may occur due to accidents in inhabited buildings and orbiting spacecraft. The role of buoyant convection on flame spread is substantial, especially for thermally-thick fuels. The conventional view, as supported by computations and space experiments, is that for quiescent mu-g conditions, the spread rate must be unsteady and decreasing until extinction occurs due to radiative losses. However, this view does not consider that radiative transfer to the fuel surface can enhance flame spread. In this work we suggest that radiative transfer from the flame itself, not just from an external source, can lead to steady flame spread at mu-g over thick fuel beds.

Son, Y.↗

Transport And Chemical Effects On Concurrent And Opposed-Flow Flame Spread At Microgravity

Flame spread over flat solid fuel beds is a useful means of understanding more complex two-phase non-premixed spreading flames, such as those that may occur due to accidents in inhabited buildings and orbiting spacecraft. The role of buoyant convection on flame spread is substantial, especially for thermally-thick fuels. With suitable assumptions, deRis showed that the spread rate (S(sub f)) is proportional to the buoyant or forced convection velocity (U) and thus suggests that S(sub f) is indeterminate at mu g (since S(sub f) = U) unless a forced flow is applied. (In contrast, for thermally thin fuels, the ideal S(sub f) is independent of U.) The conventional view, as supported by computations and space experiments, is that for quiescent g conditions, S(sub f) must be unsteady and decreasing until extinction occurs due to radiative losses. However, this view does not consider that radiative transfer to the fuel surface can enhance flame spread. In recent work we have found evidence that radiative transfer from the flame itself can lead to steady flame spread at mu g over thick fuel beds. Our current work focuses on refining these experiments and a companion modeling effort toward the goal of a space flight experiment called Radiative Enhancement Effects on Flame Spread (REEFS) planned for the International Space Station (ISS) c. 2007.

Son, Y.↗

Investigation in the 7-By-10 Foot Wind Tunnel of Ducts for Cooling Radiators Within an Airplane Wing, Special Report

An investigation was made in the NACA 7- by 10-foot wind tunnel of a large-chord wing model with a duct to house a simulated radiator suitable for a liquid-cooled engine. The duct was expanded to reduce the radiator losses, and the installation of the duct and radiator was made entirely within the wing to reduce form and interference drag. The tests were made using a two-dimensional flow set-up with a full-span duct and radiator. Section aerodynamic characteristics of the basic airfoil are given and also curves showing the characteristics of the various duct-radiator combinations. An expression for efficiency, the primary criterion of merit of any duct, and the effect of the several design parameters of the duct-radiator arrangement are discussed. The problem of throttling is considered and a discussion of the power required for cooling is included. It was found that radiators could be mounted in the wing and efficiently pass enough air for cooling with duct outlets located at any point from 0.25c to 0.70c from the wing leading edge on the upper surface. The duct-inlet position was found to be critical and, for maximum efficiency, had to be at the stagnation point of the airfoil and to change with flight attitude. The flow could be efficiently throttled only by a simultaneous variation of duct inlet and outlet sizes and of inlet position. It was desirable to round both inlet and outlet lips. With certain arrangements of duct, the power required for cooling at high speed was a very low percentage of the engine power.

Harris, Thomas A.↗

Quasi-Periodic Fluctuations and Chromospheric Evaporation in a Solar Flare Ribbon Observed by Hinode/EIS, IRIS, and RHESSI

The Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS) obtained rapid cadence (11.2 s) EUV stare spectra of an M7.3 flare ribbon in AR 12036 on 2014 April 18. Quasi-periodic (P approx. = 75.6 +/- 9.2 s) intensity fluctuations occurred in emission lines of O IV, Mg VI, Mg VII, Si VII, Fe XIV, and Fe XVI during the flare's impulsive rise, and ended when the maximum intensity in Fe XXIII was reached. The profiles of the O IV- Fe XVI lines reveal that they were all redshifted during most of the interval of quasi-periodic intensity fluctuations, while the Fe XXIII profile revealed multiple components including one or two highly blueshifted ones. This indicates that the flare underwent explosive chromospheric evaporation during its impulsive rise. Fluctuations in the relative Doppler velocities were seen, but their amplitudes were too subtle to extract significant quasi-periodicities. RHESSI detected 25-100 keV hard-X-ray sources in the ribbon near the EIS slit's pointing position during the peaks in the EIS intensity fluctuations. The observations are consistent with a series of energy injections into the chromosphere by nonthermal particle beams. Electron densities derived with Fe XIV (4.6 x 10(exp 10) per cu cm) and Mg VII (7.8 x 10(exp 9) per cu cm) average line intensity ratios during the interval of quasi-periodic intensity fluctuations, combined with the radiative loss function of an optically thin plasma, yield radiative cooling times of 32 s at 2.0 x 10(exp 6) K, and 46 s at 6.3 x 10(exp 5) K (about half the quasi-period); assuming Fe XIV's density for Fe XXIII yields a radiative cooling time of 10(exp 3) s (13 times the quasi-period) at 1.4 x 10(exp 7) K.

Brosius, Jeffrey W.↗

Thermal characterization of an AMTEC recirculating test cell

An alkali metal thermoelectric converter (AMTEC) recirculating test cell has been operated in order to determine the magnitudes of the primary heat losses of the cell and the value of the emissivity of the condenser surface. The energy balance included radiation losses, conductive losses, and losses due to the flow of sodium into the cell. The radiative heat flux dominated the heat loss mechanism of the cell at open circuit, and the condenser emissivity was calculated to be about 0.1. It is shown that, if this emissivity can be reduced to 0.02, then parasitic losses in an AMTEC recirculating test cell operating near peak power would be less than 40 percent of the heat required by the cell. The condenser emissivity decreases with elapsed time, resulting in improved thermal performance of the cell.

Underwood, M. L.↗

Radiative-dominated cooling of the flare corona and transition region

Recent observations of some compact flares indicate that the differential emission measure, q, of flare coronal and transition region plasma has a much steeper dependence on temperature than in nonflare regions. It is noted that this result is not compatible with models for a flare loop in which conduction to the chromosphere dominates the cooling, even in the case where the loop has a large divergence in its cross-sectional area. Only by a combination of many loops is it possible to reproduce the observations. Hence, models in which radiation dominates the evolution are investigated. It is found that the radiative models predict that q varies as T to the power (l + 1) where l measures the dependence of the radiative loss coefficient on temperature. It is concluded that the radiative models are also incapable of explaining the observations (unless, again, a combination of many loops is postulated) and it is suggested that large mass motions with velocities of the order of the sound speed may be required.

Antiochos, S. K.↗

Fe II emission lines. I - Chromospheric spectra of red giants

A 'difference filtering' algorithm developed by Ayers (1979) is used to construct high-quality high-dispersion long-wavelength IUE spectra of three giant stars. Measurements of all the emission lines seen between 2230 and 3100 A are tabulated. The emission spectrum of Fe II is discussed in comparison with other lines whose formation mechanisms are well understood. Systematic changes in the Fe II spectrum are related to the different physical conditions in the three stars, and examples are given of line profiles and ratios which can be used to determine conditions in the outer atomspheres of giants. It is concluded that most of the Fe II emission results from collisional excitation and/or absorption of photospheric photons at optical wavelengths, but some lines are formed by fluorescence, being photoexcited by other strong chromospheric lines. Between 10 and 20 percent of the radiative losses of Fe II arise from 10 eV levels radiatively excited by the strong chromospheric H Ly-alpha line.

Judge, P. G.↗

Shaping High Efficiency, High Temperature Cavity Tubular Solar Central Receivers

High temperature solar receivers are developed in the context of the Gen3 solar thermal power plants, in order to power high efficiency heat-to-electricity cycles. Since particle technology collects and stores high temperature solar heat, CNRS (French National Center for Scientific Research) develops an original technology using fluidized particles as HTF (heat transfer fluid). The targeted particle temperature is around 750 °C, and the walls of the receiver tubes, reach high working temperatures, which impose the design of a cavity receiver to limit the radiative losses. Therefore, the objective of this work is to explore the cavity shape effect on the absorber performances. Geometrical parameters are defined to parametrize the design. The size and shape of the cavity, the aperture-to-absorber distance and its tilt angle. A thermal model of a 50 MW hemi-cylindrical tubular receiver, closed by refractory panels, is developed, which accounts for radiation and convection losses. Parameter ranges that reach a thermal efficiency of at least 85% are explored. This sensitivity analysis allows the definition of cavity shape and dimensions to reach the targeted efficiency. For an aperture-to-absorber distance of 9 m, the 85% efficiency is obtained for aperture areas equal or less than 20 m2 and 25 m2 for high, and low convection losses, respectively.

Gueguen, Ronny (ORCID:0000000252368638)↗

Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the closed DIII-D SAS configuration

A comparative study of nitrogen versus neon has been carried out to analyze the impact of the two radiative species on power dissipation, SOL impurity distribution, divertor and pedestal characteristics. The experimental results show that N remains compressed in the divertor, thereby providing high radiative losses without affecting the pedestal profiles and displacing carbon as dominant radiator. Neon, instead, radiates more upstream than N thus reducing the power flux through the separatrix leading to a reduced ELM frequency and compression in the divertor. A significant amount of neon is measured in the plasma core leading to a steeper density gradient. The different behavior between the two impurities is confirmed by SOLPS-ITER modeling which for the first time at DIII-D includes multiple impurity species and a treatment of full drifts, currents and neutral–neutral collisions. The impurity transport in the SOL is studied in terms of the parallel momentum balance showing that N is mostly retained in the divertor whereas Ne leaks out consistent with its higher ionization potential and longer mean free path. This is also in agreement with the enrichment factor calculations which indicate lower divertor enrichment for neon. The strong ionization source characterizing the SAS divertor causes a reversal of the main ions and impurity flows. The flow reversal together with plasma drifts and the effect of the thermal force contribute significantly in the shift of the impurity stagnation point affecting impurity leakage. This work provides a demonstration of the impurity leakage mechanism in a closed divertor structure and the consequent impact on pedestal. Since carbon is an intrinsic radiator at DIII-D, in this paper we have also demonstrated the different role of carbon in the N vs Ne seeded cases both in the experiments and in the numerical modeling. Here, carbon contributes more when neon seeding is injected compared to when nitrogen is used. Finally, the results highlight the importance of accompanying experimental studies with numerical modeling of plasma flows, drifts and ionization profile to determine the details of the SOL impurity transport as the latter may vary with changes in divertor regime and geometry. In the cases presented here, plasma drifts and flow reversal caused by high level of closure in the slot upper divertor at DIII-D play an important role in the underlined mechanism.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Energy deposition in low-power coaxial plasma thrusters

An experimental examination of energy deposition in self-field, coaxial plasma thrusters revealed that the thrust efficiency ranged from 2-9 percent and that the dominant losses resulted from electrode heating and propellant ionization. Sensible enthalpy and radiative losses were negligible. Thruster specific impulse increased with current, ranging from 550-1750 seconds. Spectroscopic studies of the plume plasma showed that the electron temperature ranged from 0.5-2.5 eV and that the dominant species were singly and doubly ionized argon. Attempts to raise thruster efficiency by increasing the chamber pressure resulted in reduced electrode losses and lowered I(sp), but the thrust efficiency decreased because of a current redistribution that lowered the thrust beyond expectations.

Myers, R. M.↗

Impact of Temperature and Optical Error on the Combined Optical and Thermal Efficiency of Solar Tower Systems for Industrial Process Heat: Preprint

Concentrating solar thermal (CST) power towers can provide high flux concentrations at commercial scale. As a result, CST towers exhibit potential for high-temperature solar industrial process heat (SIPH) applications. However, at higher operating temperatures, thermal radiation losses can be significant. This study explores the trade-off between thermal and optical losses for SIPH applications using a collection of three case studies at operating temperatures that range from 900-1,550 degrees C. We assume blackbody radiation to represent the thermal losses at the receiver and we use ray tracing to estimate the optical losses. The results show the impact of process temperature on the maximum attainable system efficiency, as well as the higher flux concentration requirements as the temperature increases.

concentrating solar power↗

Time-dependent shock acceleration of energetic electrons including synchrotron losses

The present investigation of the time-dependent particle acceleration problem in strong shocks, including synchrotron radiation losses, solves the transport equation analytically by means of Laplace transforms. The particle distribution thus obtained is then transformed numerically into real space for the cases of continuous and impulsive injections of particles at the shock. While in the continuous case the steady-state spectrum undergoes evolution, impulsive injection is noted to yield such unpredicted features as a pile-up of high-energy particles or a steep power-law with time-dependent spectral index. The time-dependent calculations reveal varying spectral shapes and more complex features for the higher energies which may be useful in the interpretation of outburst spectra.

Fritz, Klaus-Dieter↗