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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 1,549 records · Page 86
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Nonlinear ohmic dissipation in axisymmetric DC and RF driven rotating plasmas

An axisymmetric fully ionized plasma rotates around its axis when a charge separation between magnetic surfaces is produced from DC fields or RF waves. On each magnetic surface, both electrons and ions obey the isorotation law and perform an azimuthal E cross B rotation at the same angular velocity. When Coulomb collisions are taken into account, such a flow displays no Ohmic current short circuiting of the charge separation and thus no linear dissipation. A nonlinear Ohmic response appears when inertial effects are considered, providing a dissipative relaxation of the charge separation between the magnetic surfaces. This nonlinear conductivity results from an interplay between Coriolis, centrifugal, and electron-ion collisional friction forces. This phenomenon is identified, described, and analyzed. Finally, both the quality factor of angular momentum storage and the efficiency of wave driven angular momentum generation are calculated and shown to be independent of the details of the charge separation processes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Reduction of ablated surface expansion in pulsed-power-driven experiments using an aerosol dielectric coating

The quality of warm dense matter samples created by magnetic compression can be largely affected by material ablation. When the ablated material carries currents, local instabilities can grow, which can lead to nonuniformities in the final magnetic pressure. Extending the previous work by Peterson et al., Awe et al., and Hutchison et al., the experiments reported here demonstrate that the expansion of the ablated material can be significantly reduced by using a simple aerosol spray technique. Coating the current-carrying surfaces with a 30–60-μm layer of polyurethane reduced the expansion of the ablated material by a factor of 2 and eliminated material ejections from sharp corners. Here, this technique, tested at the Michigan Accelerator for Inductive Z-Pinch Experiments pulsed power facility at the University of Michigan with currents up to 400 kA, could allow the production of homogeneous warm dense matter samples on pulsed-power drivers. Because of the simplicity of this method, this work brings forth an important contribution to pulsed-power-driven experiments designed to study nuclear fusion, material properties, and radiation science.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

A THz driven split-ring resonator based ultrafast relativistic electron streak camera

The use of sub-wavelength metal structures to locally enhance high frequency electromagnetic fields, generally known as plasmonics, enables breakthrough opportunities across diverse fields of research such as nonlinear optics, biosensing, photovoltaics and others. Here we study the application of sub-wavelength metallic resonators tuned in the THz frequency range for manipulation and diagnostics of relativistic electron beams. In this work, we report on the use of a double-sided split-ring structure driven by a near single cycle THz field generated by optical rectification to impart a time-dependent angular deviation (streak) on a 4.5 MeV electron beam. Electrons passing through the small gap reveal field enhancement factors larger than 10, in good agreement with finite difference time domain simulations. This work paves the way for further application of high frequency metallic structures in compact particle accelerators such as for THz-based relativistic electron streaking at fs and sub-fs temporal resolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Inferring fuel areal density from secondary neutron yields in laser-driven magnetized liner inertial fusion

A technique to infer the areal density ρR of compressed deuterium (D) in cylindrical implosions from the ratio of secondary D–T (deuterium–tritium) neutrons to primary D–D neutrons is described and evaluated. For ρR to be proportional to the ratio of D–T to D–D yield, the increase in the D–T fusion cross section with collisional slowing of the tritium must be small, requiring where TkeV is the electron temperature in keV. The technique is applied to results from laser-driven magnetized liner inertial fusion (MagLIF) targets on OMEGA, where ρR is certainly less than 4 mg/cm 2 . OMEGA MagLIF targets do not achieve a sufficiently high, radially integrated, axial magnetic field BR to confine the tritium, as occurs in Z MagLIF targets, because they are ~10× smaller in radius. The inferred areal densities show that fuel convergence is reduced by preheating, by an applied axial magnetic field, and by increasing the initial fuel density, which are key features of the MagLIF scheme. The results are compared with 1-D and 2-D magnetohydrodynamic simulations for nominal laser and target parameters, which predict areal densities 2× to 3× higher than the measurements.

Davies, J. R.↗
DOE OSTI2019

The modeling of delayed-onset Rayleigh-Taylor and transition to mixing in laser-driven HED experiments

In this paper, we discuss simulations, along with a benchmarking experiment, performed using the xRAGE code which demonstrate the ability of a laser model to predict laser-driven, high-energy-density shock hydrodynamics with good fidelity. This directly contributes to our ability to model hydrodynamic-instability dynamics produced by a laser drive typical of those available at OMEGA, OMEGA-EP, NIF, and similar facilities. In particular, we show how the laser model is essential for predicting deceleration-phase Rayleigh-Taylor arising from laser turn-off. We do this using the experimental case of a seeded single-mode perturbation. Then, we turn to a seeded multimode perturbation to show how the above result permits us to access the modeling of hydrodynamic mixing, a topic of interest for future work.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Absolute calibration of GafChromic film for very high flux laser driven ion beams

We report on the calibration of GafChromic HD-v2 radiochromic film in the extremely high dose regime up to 100 kGy together with very high dose rates up to 7 × 10 11 Gy/s. The absolute calibration was done with nanosecond ion bunches at the Neutralized Drift Compression Experiment II particle accelerator at Lawrence Berkeley National Laboratory (LBNL) and covers a broad dose dynamic range over three orders of magnitude. We then applied the resulting calibration curve to calibrate a laser driven ion experiment performed on the BELLA petawatt laser facility at LBNL. Here, we reconstructed the spatial and energy resolved distributions of the laser-accelerated proton beams. The resulting proton distribution is in fair agreement with the spectrum that was measured with a Thomson spectrometer in combination with a microchannel plate detector.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Interplay between fluctuation driven toroidal axisymmetric flows and resistive ballooning mode turbulence

An interplay between fluctuation driven toroidal axisymmetric flows (convective cell modes) and resistive ballooning mode turbulence after the pedestal collapse is numerically studied by a four-field reduced MHD model in the BOUT++ framework. The strong flow shear suppresses the radial transport of pressure filaments, and the pressure profile in the pedestal region is partially recovered. As a result, a secondary instability is quasilinearly excited, which yields a secondary collapse. Here, the subsequent damped oscillation is also analyzed by phase diagram analysis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Synchrotron based X-ray radiography of convergent shock waves driven by underwater electrical explosion of a cylindrical wire array

We present x-ray radiography images showing the propagation of shock waves generated by electrical explosion of a cylindrical arrangement of wires in water driven by pulsed power. In previous experiments [S. N. Bland et al., Phys. Plasmas 24, 082702 (2017)] the merger of shock waves from adjacent wires has produced a highly symmetrical, cylindrical shock wave converging on the axis, where it is expected to produce a high density, strongly coupled plasma ideal for warm dense matter research. However, diagnostic limitations have meant that much of the dynamics of the system have been inferred from the position of the front of the cylindrical shock and timing/spectra of light emitted from the axis. Here we present synchrotron-based radiography of such experiments – providing direct quantitative measurements on the formation of the convergent shock wave, the increased density of water on axis caused by its arrival, and its ‘bounce’ after arrival on axis. The obtained images are compared with twodimensional hydrodynamic simulations which reproduce the observed dynamics with a satisfactory agreement in density values.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Field reconstruction from proton radiography of intense laser driven magnetic reconnection

Magnetic reconnection is a process that contributes significantly to plasma dynamics and energy transfer in a wide range of plasma and magnetic field regimes, including inertial confinement fusion experiments, stellar coronae and compact, highly magnetized objects like neutron stars. Laboratory experiments in different regimes can help refine, expand and test the applicability of theoretical models to describe reconnection. Laser-plasma experiments exploring magnetic reconnection at moderate intensities (I L ~ 10 14 Wcm -2 ) have been performed previously, where the Biermann battery effect self-generates magnetic fields and the field dynamics studied using proton radiog- raphy. At high laser intensities (I L λ$2\atop{L}$ > 10 18 Wcm -2 μm 2 ), relativistic surface currents and the time-varying electric sheath fields generate the azimuthal magnetic fields. Numerical modeling of these intensities has shown the conditions within the magnetic field region can reach the threshold where the magnetic energy can exceed the rest mass energy such that σ cold = B 2 /(μ 0 n e m e c 2 ) > 1 [A. E. Raymond, et al., Phys. Rev. E, 98, 043207 (2018)]. Presented here is the analysis of the proton radiography of a high-intensity (~ 10 18 Wcm -2 ) laser driven magnetic reconnection geometry. Lastly, the path integrated magnetic fields are recovered using a “field-reconstruction algorithm” to quantify the field strengths, geometry and evolution.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Laser-driven semiconductor switch for generating nanosecond pulses from a megawatt gyrotron

A laser-driven semiconductor switch (LDSS) employing silicon (Si) and gallium arsenide (GaAs) wafers has been used to produce nanosecond-scale pulses from a 3 μs, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532nm laser, which produced 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with a 9 ns width and >70% reflectance. Under the same conditions, a single GaAs wafer yielded 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. Using two active wafers, pulses of variable length down to 3 ns duration were created. The switch was tested at incident 110 GHz RF power levels up to 600 kW. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS has many potential uses in high power millimeter-wave research, including testing of high-gradient accelerator structures.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Thermal-gradient-driven elemental segregation in Ge 2 Sb 2 Te 5 phase change memory cells

Thermal gradients have been predicted to play a large role in compositional segregation leading to failure in phase change memories. We have developed a methodology for isolating thermal-gradient driven segregation effects without interference from the electric field. In Ge 2 Sb 2 Te 5 functional layers, Sb and Te move along the temperature gradient, while Ge segregates in the opposite direction. The direction of segregation is consistent for devices that were repeatedly melted, as well as for devices that were never melted and remained in the polycrystalline state. Here, the results have implications for the reliability of phase change memories.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Probing ion species separation and ion thermal decoupling in shock-driven implosions using multiple nuclear reaction histories

Simultaneously measured DD, DT, and D 3 He reaction histories are used to probe the impacts of multi-ion physics during the shock phase of inertial confinement fusion implosions. In these relatively hydrodynamiclike (burn-averaged Knudsen number ⟨N K ⟩ ~0.3) shock-driven implosions, average-ion hydrodynamic DUED simulations are able to reasonably match burnwidths, nuclear yields, and ion temperatures. However, kinetic-ion FPION simulations are able to better simulate the timing differences and time-resolved reaction rate ratios between DD, DT, and D 3 He reactions. FPION simulations suggest that the D 3 He/DT reaction rate ratio is most directly impacted by ion species separation between the 3 He and T ions, whereas the D 3 He/DD reaction rate ratio is affected by both ion species separation and ion temperature decoupling effects.

Hydrodynamic codes↗
DOE OSTI2019

Thresholds of absolute instabilities driven by a broadband laser

Absolute instability thresholds for stimulated Raman scattering and two-plasmon decay driven by a broadband laser pulse are evaluated numerically. The scalings of the calculated thresholds with density scale length, temperature, and central wavelength are qualitatively similar to existing analytic results. The threshold values, yet, reflect significant quantitative differences. Comparisons between thresholds calculated for various broadband power spectra suggest a universal scaling of the threshold intensity with laser coherence time ($\tau_{c}$). For SRS I thr ∝ $\tau_{c}^{-1/3}$, and for TPD I thr ∝ $\tau_{c}^{-1/2}$ .

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Analytic gradients for the single-reference driven similarity renormalization group second-order perturbation theory

Herein we derive and implement analytic energy gradients for the single-reference driven similarity renormalization group second-order perturbation theory (DSRG-PT2). The resulting equations possess an asymptotic scaling that is identical to that of the second-order Møller–Plesset perturbation theory (MP2), indicating that the exponential regularizer in the DSRG equations does not introduce formal difficulties in the gradient theory. We apply the DSRG-PT2 method to optimizing the geometries of 15 small molecules. The equilibrium bond lengths computed with DSRG-PT2 are found similar to those of MP2, yielding a mean absolute error of 0.0033 Å and a standard deviation of 0.0045 Å when compared with coupled cluster with singles, doubles, and perturbative triples.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Verification of the global gyrokinetic stellarator code XGC-S for linear ion temperature gradient driven modes

XGC (X-point Gyrokinetic Code) is a whole-volume, total-f gyrokinetic particle-in-cell code developed for modeling tokamaks. Recently, XGC has been extended to model more general 3D toroidal magnetic configurations, such as stellarators. These improvements have concluded in the XGC-S version. In this paper, XGC-S is benchmarked in the reduced delta-f limit for linear electrostatic ion temperature gradient-driven microinstabilities, which can underlie turbulent transport in stellarators. An initial benchmark of XGC-S in tokamak geometry shows good agreement with the XGC1, ORB5, and global GENE codes. A benchmark between XGC-S and the EUTERPE global gyrokinetic code for stellarators has also been performed, here in the geometry of the optimized stellarator Wendelstein 7-X. Good agreement has been discovered for the mode number spectrum, mode structure, and growth rate.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

A next-generation inverse-geometry spallation-driven ultracold neutron source

The physics model of a next-generation spallation-driven high-current ultracold neutron (UCN) source capable of delivering an extracted UCN rate of around an order of magnitude higher than the strongest proposed sources, and around three orders of magnitude higher than existing sources, is presented. This UCN-current-optimized source would dramatically improve cutting-edge UCN measurements that are currently statistically limited. A novel “Inverse Geometry” design is used with 40 l of superfluid 4He (He-II), which acts as the converter of cold neutrons to UCNs, cooled with state-of-the-art subcooled cryogenic technology to ∼1.6K. Our source design is optimized for a 100 W maximum heat load constraint on the He-II and its vessel. In this paper, we first explore modifying the Lujan Center Mark-3 target for UCN production as a benchmark. In our Inverse Geometry, the spallation target is wrapped symmetrically around the cryogenic UCN converter to permit raster scanning the proton beam over a relatively large volume of tungsten spallation target to reduce the demand on the cooling requirements, which makes it reasonable to assume that water edge-cooling only is sufficient. Our design is refined in several steps to reach a UCN production rate PUCN=2.1×109s−1 under our other restriction of 1MW maximum available proton beam power. We then study the effects of the He-II scattering kernel used as well as reductions in PUCN due to pressurization to reach PUCN=1.8×109s−1. Finally, we provide a design for the UCN extraction system that takes into account the required He-II heat transport properties and implementation of a He-II containment foil that allows UCN transmission. We estimate a total useful UCN current from our source of Ruse≈5×108s−1 from an 18cm diameter guide ∼5m from the source. Under a conservative “no return” (or “single passage”) approximation, this rate can produce an extracted density of >1×104UCNcm−3 in <1000l external experimental volumes with a 58Ni (335neV) cutoff potential.

Leung, K. K. H. (ORCID:0000000203285326)↗
DOE OSTI2019

Geodesic modes driven by untrapped resonances of NB energetic ions in tokamaks

Geodesic modes are typically excited by a minor concentration of energetic ions, but unstable mode frequencies are substantially different from Geodesic Acoustic Modes (GAMs) and are named EGAM (Energetic particle GAM). The EGAM instability driven by Neutral Beam Injection (NBI) has been observed in DIII-D tokamak experiments. The problem of the geodesic mode instability is analytically studied using a full drift kinetic equation. To analyze the instability condition, an ionization NBI location is assumed to be on the high field side of tokamaks. Here, a minority NBI ion distribution is modeled by an energetic ion tail in the untrapped-passing region that remains between a magnetic axis and the trapped NBI boundary. The EGAM instability condition is defined by the parallel NBI ion velocity v | | ≈ ( 1.2 – 1.5 ) ω R 0 q 0 that has to be above the effective EGAM phase velocity. In this case, the EGAM frequency is ≈ 50 % below the standard stable GAM frequency, which is reduced by a small concentration of energetic NBI ions. Qualitative comparison of the developed geodesic mode theory with NBI heating experiments in the midregion of the tokamak plasma is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Self-organized standing waves generated by AC-driven electron cyclotron drift instabilities

The formation of self-organized standing wave structures is observed due to the electron cyclotron drift instability (ECDI) driven by a time-varying external electric field and a crossed magnetic field. Using a particle-in-cell simulation, two standing wave mechanisms are identified: the linear mode and the beating mode. In the former, a standing wave emerges as a superposition of two counterpropagating ion acoustic waves predicted by the linear theory of ECDI. On the other hand, in the beating mode, the plasma wave is in resonance with the applied frequency. Nonlinear resonance of such standing waves results in a change in the dominant wavenumber and frequency. Such counterpropagating plasma waves are consistent with the experimental observations using coherent Thomson scattering in a crossed-field plasma discharge.

42 ENGINEERING↗