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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Hydrodynamic scaling of the deceleration-phase Rayleigh–Taylor instability

The scaling of the deceleration phase of inertial fusion direct-drive implosions is investigated for OMEGA and National Ignition Facility (NIF)-size targets. It is shown that the deceleration-phase Rayleigh–Taylor instability (RTI) does not scale hydro-equivalently with implosion size. This is because ablative stabilization resulting from thermal conduction and radiation transport in a spherically converging geometry is different on the two scales. As a consequence, NIF-scale implosions show lower hot-spot density and mass ablation velocity, allowing for higher RTI growth. On the contrary, stabilization resulting from density-gradient enhancement, caused by reabsorption of radiation emitted from the hot spot, is higher on NIF implosions. Since the RTI mitigation related to thermal conduction and radiation transport scale oppositely with implosion size, the degradation of implosion performance caused by the deceleration RTI is similar for NIF and OMEGA targets. It is found that a minimum threshold for the no-α Lawson ignition parameter of χ{sub Ω} ≈ 0.2 at the OMEGA scale is required to demonstrate hydro-equivalent ignition at the NIF scale for symmetric direct-drive implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Three-dimensional hydrodynamics of the deceleration stage in inertial confinement fusion

The deceleration stage of inertial confinement fusion implosions is modeled in detail using three-dimensional simulations designed to match experiments at the National Ignition Facility. In this final stage of the implosion, shocks rebound from the center of the capsule, forming the high-temperature, low-density hot spot and slowing the incoming fuel. The flow field that results from this process is highly three-dimensional and influences many aspects of the implosion. The interior of the capsule has high-velocity motion, but viscous effects limit the range of scales that develop. The bulk motion of the hot spot shows qualitative agreement with experimental velocity measurements, while the variance of the hot spot velocity would broaden the DT neutron spectrum, increasing the inferred temperature by 400–800 eV. Jets of ablator material are broken apart and redirected as they enter this dynamic hot spot. Deceleration stage simulations using two fundamentally different rad-hydro codes are compared and the flow field is found to be in good agreement.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Passive and active plasma deceleration for the compact disposal of electron beams

Plasma-based decelerating schemes are investigated as compact alternatives for the disposal of high-energy beams (beam dumps). Analytical solutions for the energy loss of electron beams propagating in passive and active (laser-driven) schemes are derived. These solutions, along with numerical modeling, are used to investigate the evolution of the electron distribution, including energy chirp and total beam energy. In the active beam dump scheme, a laser-driver allows a more homogeneous beam energy extraction and drastically reduces the energy chirp observed in the passive scheme. These concepts could benefit applications requiring overall compactness, such as transportable light sources, or facilities operating at high beam power.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Transport and deceleration of fusion products in microturbulence

The velocity-space distribution of alpha particles born in fusion devices is subject to modification at moderate energies due to turbulent transport. Therefore, one must calculate the evolution of an equilibrium distribution whose functional form is not known a priori. Using a novel technique, applicable to any trace impurity, we have made this calculation for fully nonlinear gyrokinetic simulations not only possible but also particularly efficient. We demonstrate a microturbulence-induced departure from the local slowing-down distribution, an inversion of the energy distribution, and associated modifications to the alpha heating and pressure profiles in an ITER-like scenario.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

EARTH’S ROTATIONAL DECELERATION: DETERMINATION OF TIDAL FRICTION INDEPENDENT OF TIMESCALES

This paper determines Earth's rotational deceleration without relying on atomic or ephemeris timescales. Earth's rotation defines the civil time standard called Universal Time (UT). Our previous paper did not examine tidal friction in depth when analyzing the timescale divergence between UT and International Atomic Time (TAI). We examine all available paleontological fossils and deposits for the direct measurements of Earth's past rotation rates, because that record includes all contributing effects. We examine paleontological reports that date Earth's rotation rate using corals, bivalves, brachiopods, rhythmites, and stromatolites. Contributions that vary Earth's moment of inertia, such as continental plate drifts, coastline changes, ice age formations, and viscous glacial rebounds, are superimposed with the secular deceleration. The average deceleration of Earth's rotation rate from all available fossil data is found to be (5.969 ± 1.762) × 10{sup −7} rad yr{sup −2}. Our value is 99.8% of the total rotational deceleration determined by Christodoulidis et al., who used artificial satellite data, and our value is 96.6% of the expected tidal friction value obtained by Stephenson and Morrison. Taking the derivative of conserved angular momentum, the predicted lunar orbital deceleration caused by the average rotational deceleration corresponds closely to lunar models. When evaluating the significant time gaps between UT and TAI, Earth's rotational deceleration is a minor contributing factor. Also, the secular deceleration rate is necessary to correctly date ancient astronomical events. We strongly encourage that more ocean paleontological evidence be found to supplement the record to separate the many periodic variations embedded in these data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

DRIFT-INDUCED DECELERATION OF SOLAR ENERGETIC PARTICLES

We investigate the deceleration of solar energetic particles (SEPs) during their propagation from the Sun through interplanetary space, in the presence of weak to strong scattering in a Parker spiral configuration, using relativistic full orbit test particle simulations. The calculations retain all three spatial variables describing particles’ trajectories, allowing us to model any transport across the magnetic field. Large energy change is shown to occur for protons, due to the combined effect of standard adiabatic deceleration and a significant contribution from particle drift in the direction opposite to that of the solar wind electric field. The latter drift-induced deceleration is found to have a stronger effect for SEP energies than for galactic cosmic rays. The kinetic energy of protons injected at 1 MeV is found to be reduced by between 35% and 90% after four days, and for protons injected at 100 MeV by between 20% and 55%. The overall degree of deceleration is a weak function of the scattering mean free path, showing that, although adiabatic deceleration plays a role, a large contribution is due to particle drift. Current SEP transport models are found to account for drift-induced deceleration in an approximate way and their accuracy will need to be assessed in future work.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Getting a grip on the transverse motion in a Zeeman decelerator

Zeeman deceleration is an experimental technique in which inhomogeneous, time-dependent magnetic fields generated inside an array of solenoid coils are used to manipulate the velocity of a supersonic beam. A 12-stage Zeeman decelerator has been built and characterized using hydrogen atoms as a test system. The instrument has several original features including the possibility to replace each deceleration coil individually. In this article, we give a detailed description of the experimental setup, and illustrate its performance. We demonstrate that the overall acceptance in a Zeeman decelerator can be significantly increased with only minor changes to the setup itself. This is achieved by applying a rather low, anti-parallel magnetic field in one of the solenoid coils that forms a temporally varying quadrupole field, and improves particle confinement in the transverse direction. The results are reproduced by three-dimensional numerical particle trajectory simulations thus allowing for a rigorous analysis of the experimental data. The findings suggest the use of a modified coil configuration to improve transverse focusing during the deceleration process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Simulation of decelerating streamers in inhomogeneous atmosphere with implications for runaway electron generation

The dynamics of positive and negative streamers is numerically simulated in atmospheric pressure air in the range of parameters corresponding to the streamer deceleration and termination in the middle of the discharge gap. A detailed comparison with experiments in air at constant and variable density demonstrates good agreement between the 2D simulation results and the observations. It is shown that positive and negative streamers behave in radically different ways when decelerating and stopping. When the head potential drops, the negative streamer transits to the mode in which the propagation is due to the forward electron drift. In this case, the radius of the ionization wave front increases, whereas the electric field at the streamer head decreases further and the streamer stops. Its head diameter continues to increase due to the slow drift of free electrons in the residual under-breakdown field. On the contrary, the only advancement mechanism for a positive streamer with a decreasing head potential is a decrease in the effective radius of the ionization wave, leading to a local increase in the electric field. This mechanism makes it possible to compensate for the reduction in the efficiency of gas photoionization at small head diameters. A qualitative 1D model is suggested to describe streamer deceleration and stopping for different discharge polarities. Estimates show that, during positive streamer stopping, the local electric field at the streamer head can exceed the threshold corresponding to the transition of electrons to the runaway mode when the head potential (relative to the surrounding space) decreases to~1.2 kV in atmospheric pressure air. In this case, pulsed generation of a beam of runaway electrons directed into the channel of a stopping positive streamer can occur. The energy of the formed pulsed electron beam depends on the intensity of photoionization in front of the streamer head. This energy can vary from 700 V (when increasing the photoionization rate by a factor of 10 with respect to the value in atmospheric pressure air) to 2.6 kV (when decreasing the photoionization rate by a factor of 1000). It is possible that this behavior of decelerating positive streamers can explain the observed bursts of x-ray radiation during the streamer propagation in long air gaps.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Deceleration of alpha particles in the solar wind by instabilities and the rotational force: Implications for heating, azimuthal flow, and the Parker spiral magnetic field

Protons and alpha particles in the fast solar wind are only weakly collisional and exhibit a number of non-equilibrium features, including relative drifts between particle species. Two non-collisional mechanisms have been proposed for limiting differential flow between alpha particles and protons: plasma instabilities and the rotational force. Both mechanisms decelerate the alpha particles. In this paper, we derive an analytic expression for the rate Q{sub flow} at which energy is released by alpha-particle deceleration, accounting for azimuthal flow and conservation of total momentum. We show that instabilities control the deceleration of alpha particles at r r{sub crit}, where r{sub crit}≃2.5 AU in the fast solar wind in the ecliptic plane. We find that Q{sub flow} is positive at r<r{sub crit} and Q{sub flow}=0 at r⩾r{sub crit}, consistent with the previous finding that the rotational force does not lead to a release of energy. We compare the value of Q{sub flow} at r<r{sub crit} with empirical heating rates for protons and alpha particles, denoted Q{sub p} and Q{sub α}, deduced from in situ measurements of fast-wind streams from the Helios and Ulysses spacecraft. We find that Q{sub flow} exceeds Q{sub α} at r<1 AU, and that Q{sub flow}/Q{sub p} decreases with increasing distance from the Sun from a value of about one at r = 0.29–0.42 AU to about 1/4 at 1 AU. We conclude that the continuous energy input from alpha-particle deceleration at r<r{sub crit} makes an important contribution to the heating of the fast solar wind. We also discuss the implications of the alpha-particle drift for the azimuthal flow velocities of the ions and for the Parker spiral magnetic field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A scheme for reducing deceleration-phase Rayleigh–Taylor growth in inertial confinement fusion implosions

It is demonstrated that the growth of acceleration-phase instabilities in inertial confinement fusion implosions can be controlled, especially in the high-foot implosions [O. A. Hurricane et al., Phys. Plasmas 21, 056314 (2014)] on the National Ignition Facility. However, the excessive growth of the deceleration-phase instabilities can still destroy the hot spot ignition. A scheme is proposed to retard the deceleration-phase Rayleigh–Taylor instability growth by shock collision near the waist of the inner shell surface. Two-dimensional radiation hydrodynamic simulations confirm the improved deceleration-phase hot spot stability properties without sacrificing the fuel compression.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Constraining the cosmic deceleration-acceleration transition with type Ia supernova, BAO/CMB and H(z) data

We revisit the kink-like parametrization of the deceleration parameter q(z) [1], which considers a transition, at redshift z{sub t}, from cosmic deceleration to acceleration. In this parametrization the initial, at z >> z{sub t}, value of the q-parameter is q{sub i}, its final, z=−1, value is q{sub f} and the duration of the transition is parametrized by τ. By assuming a flat space geometry we obtain constraints on the free parameters of the model using recent data from type Ia supernovae (SN Ia), baryon acoustic oscillations (BAO), cosmic microwave background (CMB) and the Hubble parameter H(z). The use of H(z) data introduces an explicit dependence of the combined likelihood on the present value of the Hubble parameter H{sub 0}, allowing us to explore the influence of different priors when marginalizing over this parameter. We also study the importance of the CMB information in the results by considering data from WMAP7, WMAP9 (Wilkinson Microwave Anisotropy Probe—7 and 9 years) and Planck 2015. We show that the contours and best fit do not depend much on the different CMB data used and that the considered new BAO data is responsible for most of the improvement in the results. Assuming a flat space geometry, q{sub i}=1/2 and expressing the present value of the deceleration parameter q{sub 0} as a function of the other three free parameters, we obtain z{sub t}=0.67{sup +0.10}{sub −0.08}, τ=0.26{sup +0.14}{sub −0.10} and q{sub 0}=−0.48{sup +0.11}{sub −0.13}, at 68% of confidence level, with an uniform prior over H{sub 0}. If in addition we fix q{sub f}=−1, as in flat ΛCDM, DGP and Chaplygin quartessence that are special models described by our parametrization, we get z{sub t}=0.66{sup +0.03}{sub −0.04}, τ=0.33{sup +0.04}{sub −0.04} and q{sub 0}=−0.54{sup +0.05}{sub −0.07}, in excellent agreement with flat ΛCDM for which τ=1/3. We also obtain for flat wCDM, another dark energy model described by our parametrization, the constraint on the equation of state parameter −1.22 < w < −0.78 at more than 99% confidence level.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Compact disposal of high-energy electron beams using passive or laser-driven plasma decelerating stage

A plasma decelerating stage is investigated as a compact alternative for the disposal of high-energy beams (beam dumps). This could benefit the design of laser-driven plasma accelerator (LPA) applications that require transportability and or high-repetition-rate operation regimes. Passive and laser-driven (active) plasma-based beam dumps are studied analytically and with particle-in-cell (PIC) simulations in a 1D geometry. Analytical estimates for the beam energy loss are compared to and extended by the PIC simulations, showing that with the proposed schemes a beam can be efficiently decelerated in a centimeter-scale distance.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Compact disposal of high-energy electron beams using passive or laser-driven plasma decelerating stage

A plasma decelerating stage is investigated as a compact alternative for the disposal of high-energy beams (beam dumps). This could benefit the design of laser-driven plasma accelerator (LPA) applications that require transportability and or high-repetition-rate operation regimes. Passive and laser-driven (active) plasma-based beam dumps are studied analytically and with particle-in-cell (PIC) simulations in a 1D geometry. Analytical estimates for the beam energy loss are compared to and extended by the PIC simulations, showing that with the proposed schemes a beam can be efficiently decelerated in a centimeter-scale distance.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Proton irradiation-decelerated intergranular corrosion of Ni-Cr alloys in molten salt

The effects of ionizing radiation on materials often reduce to "bad news". Radiation damage usually leads to detrimental effects such as embrittlement, accelerated creep, phase instability, and radiation-altered corrosion. Here we report that proton irradiation decelerates intergranular corrosion of Ni-Cr alloys in molten fluoride salt at 650 °C. We demonstrate this by showing that the depth of intergranular voids resulting from Cr leaching into the salt is reduced by proton irradiation alone. Interstitial defects generated from irradiation enhance diffusion, more rapidly replenishing corrosion-injected vacancies with alloy constituents, thus playing the crucial role in decelerating corrosion. Our results show that irradiation can have a positive impact on materials performance, challenging our view that radiation damage usually results in negative effects.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Deceleration, precooling, and multi-pass stopping of highly charged ions in Be{sup +} Coulomb crystals

Preparing highly charged ions (HCIs) in a cold and strongly localized state is of particular interest for frequency metrology and tests of possible spatial and temporal variations of the fine structure constant. Our versatile preparation technique is based on the generic modular combination of a pulsed ion source with a cryogenic linear Paul trap. Both instruments are connected by a compact beamline with deceleration and precooling properties. We present its design and commissioning experiments regarding these two functionalities. A pulsed buncher tube allows for the deceleration and longitudinal phase-space compression of the ion pulses. External injection of slow HCIs, specifically Ar{sup 13+}, into the linear Paul trap and their subsequent retrapping in the absence of sympathetic cooling is demonstrated. The latter proved to be a necessary prerequisite for the multi-pass stopping of HCIs in continuously laser-cooled Be{sup +} Coulomb crystals.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Development of new platforms for hydrodynamic instability and asymmetry measurements in deceleration phase of indirectly driven implosions on NIF

Hydrodynamic instabilities and asymmetries are a major obstacle in the quest to achieve ignition at the National Ignition Facility (NIF) as they cause pre-existing capsule perturbations to grow and ultimately quench the fusion burn in experiments. This paper reviews the development of two new experimental techniques to measure high-mode instabilities and low-mode asymmetries in the deceleration phase of indirect drive inertial confinement fusion implosions. In the first innovative technique, self-emission from the hot spot was enhanced with an argon dopant to “self-backlight” the shell in-flight, imaging the perturbations in the shell near peak velocity. Experiments with pre-imposed two-dimensional perturbations showed hydrodynamic instability growth of up to 7000× in areal density. These experiments discovered unexpected three-dimensional structures originating from the capsule support structures. These new 3-D structures became one of the primary concerns for the indirect drive ICF program that requires their origin to be understood and their impact mitigated. In a second complementary technique, the inner surface of the decelerating shell was visualized in implosions using x-ray emission of a high-Z dopant added to the inner surface of the capsule. With this technique, low mode asymmetry and high mode perturbations, including perturbations seeded by the gas fill tube and capsule support structure, were quantified near peak compression. Using this doping method, the role of perturbations and radiative losses from high atomic number materials on neutron yield was quantified.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Deceleration-stage Rayleigh–Taylor growth in a background magnetic field studied in cylindrical and Cartesian geometries

Experiments have identified the Rayleigh–Taylor (RT) instability as one of the greatest obstacles to achieving inertial confinement fusion. Consequently, mitigation strategies to reduce RT growth and fuel–ablator mixing in the hotspot during the deceleration phase of the implosion are of great interest. In this work, the effect of seed magnetic fields on deceleration-phase RT growth are studied in planar and cylindrical geometries under conditions relevant to the National Ignition Facility (NIF) and Omega experiments. The magnetohydrodynamic (MHD) and resistive-MHD capabilities of the FLASH code are used to model imploding cylinders and planar blast-wave-driven targets. Realistic target and laser parameters are presented that suggest the occurrence of morphological differences in late-time RT evolution in the cylindrical NIF case and a measurable difference in spike height of single-mode growth in the planar NIF case. The results of this study indicate the need for target designs to utilize an RT-unstable foam–foam interface in order to achieve sufficient magnetic field amplification to alter RT evolution. Benchmarked FLASH simulations are used to study these magnetic field effects in both resistive and ideal MHD.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗