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At least 811 records · Page 45

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DOE OSTI2011

The Cosmic Linear Anisotropy Solving System (CLASS) IV: efficient implementation of non-cold relics

We present a new flexible, fast and accurate way to implement massive neutrinos, warm dark matter and any other non-cold dark matter relics in Boltzmann codes. For whatever analytical or numerical form of the phase-space distribution function, the optimal sampling in momentum space compatible with a given level of accuracy is automatically found by comparing quadrature methods. The perturbation integration is made even faster by switching to an approximate viscous fluid description inside the Hubble radius, which differs from previous approximations discussed in the literature. When adding one massive neutrino to the minimal cosmological model, CLASS becomes just 1.5 times slower, instead of about 5 times in other codes (for fixed accuracy requirements). We illustrate the flexibility of our approach by considering a few examples of standard or non-standard neutrinos, as well as warm dark matter models.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Analysis on a general class of holographic type dark energy models

We present a detail analysis on a general class of holographic type dark energy models characterized by the length scale L = 1/a{sup n}(t)∫{sub 0}{sup t}dt' a{sup m}(t'). We show that n ≥ 0 is required by the recent cosmic accelerated expansion of universe. In the early universe dominated by the constituent with constant equation of state w{sub m}, we have w{sub de} ≅ −1−2n/3 for n ≥ 0 and m < 0, and w{sub de} ≅ −(2/3)(n−m)+w{sub m} for n > m ≥ 0. The models with n > m ≥ 0 become single-parameter models like the ΛCDM model due to the analytic feature Ω{sub de} ≅ d{sup 2}/4(2m+3w{sub m}+3){sup 2}a{sup 2(n−m)} at radiation- and matter-dominated epoch. Whereas the cases n = m ≥ 0 should be abandoned as the dark energy cannot dominate the universe forever and there might be too large fraction of dark energy in early universe, and the cases m > n ≥ 0 are forbidden by the self-consistent requirement Ω{sub de} << 1 in the early universe. Thus a detailed study on the single-parameter models corresponding to cases n > m ≥ 0 is carried out by using recent observations. The best-fit analysis indicates that the conformal-age-like models with n = m+1, i.e. L∝1/Ha in early universe, are more favored and also the models with smaller n for the given n−m are found to fit the observations better. The equation of state of the dark energy in models with n = m+1 > 0 transits from w{sub de} < −1 during inflation to w{sub de} > −1 in radiation- and matter-dominated epoch, and then back to w{sub de} < −1 eventually. The best-fit result of the case (n = 0, m = −1) which is so-called ηHDE model proposed in (Huang 2012) is the most favorable model and compatible with the ΛCDM model.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

A class of effective field theory models of cosmic acceleration

We explore a class of effective field theory models of cosmic acceleration involving a metric and a single scalar field. These models can be obtained by starting with a set of ultralight pseudo-Nambu-Goldstone bosons whose couplings to matter satisfy the weak equivalence principle, assuming that one boson is lighter than all the others, and integrating out the heavier fields. The result is a quintessence model with matter coupling, together with a series of correction terms in the action in a covariant derivative expansion, with specific scalings for the coefficients. After eliminating higher derivative terms and exploiting the field redefinition freedom, we show that the resulting theory contains nine independent free functions of the scalar field when truncated at four derivatives. This is in contrast to the four free functions found in similar theories of single-field inflation, where matter is not present. We discuss several different representations of the theory that can be obtained using the field redefinition freedom. For perturbations to the quintessence field today on subhorizon lengthscales larger than the Compton wavelength of the heavy fields, the theory is weakly coupled and natural in the sense of t'Hooft. The theory admits a regime where the perturbations become modestly nonlinear, but very strong nonlinearities lie outside its domain of validity.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Qualitative analysis of Kantowski-Sachs metric in a generic class of f(R) models

In this paper we investigate, from the dynamical systems perspective, the evolution of a Kantowski-Sachs metric in a generic class of f(R) models. We present conditions (i.e., differentiability conditions, existence of minima, monotony intervals, etc.) for a free input function related to the f(R), that guarantee the asymptotic stability of well-motivated physical solutions, specially, self-accelerated solutions, allowing to describe both inflationary- and late-time acceleration stages of the cosmic evolution. We discuss which f(R) theories allows for a cosmic evolution with an acceptable matter era, in correspondence to the modern cosmological paradigm. We find a very rich behavior, and amongst others the universe can result in isotropized solutions with observables in agreement with observations, such as de Sitter, quintessence-like, or phantom solutions. Additionally, we find that a cosmological bounce and turnaround are realized in a part of the parameter-space as a consequence of the metric choice.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Tensor perturbations in a general class of Palatini theories

We study a general class of gravitational theories formulated in the Palatini approach and derive the equations governing the evolution of tensor perturbations. In the absence of torsion, the connection can be solved as the Christoffel symbols of an auxiliary metric which is non-trivially related to the space-time metric. We then consider background solutions corresponding to a perfect fluid and show that the tensor perturbations equations (including anisotropic stresses) for the auxiliary metric around such a background take an Einstein-like form. This facilitates the study in a homogeneous and isotropic cosmological scenario where we explicitly establish the relation between the auxiliary metric and the space-time metric tensor perturbations. As a general result, we show that both tensor perturbations coincide in the absence of anisotropic stresses.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Primordial gravitational waves for universality classes of pseudoscalar inflation

Current bounds from the polarization of the CMB predict the scale-invariant gravitational wave (GW) background of inflation to be out of reach for upcoming GW interferometers. This prospect dramatically changes if the inflaton is a pseudoscalar, in which case its generic coupling to any abelian gauge field provides a new source of GWs, directly related to the dynamics of inflation. This opens up new ways of probing the scalar potential responsible for cosmic inflation. Dividing inflation models into universality classes, we analyze the possible observational signatures. One of the most promising scenarios is Starobinsky inflation, which may lead to observational signatures both in direct GW detection as well as in upcoming CMB detectors. In this case, the complementarity between the CMB and direct GW detection, as well as the possibility of a multi-frequency analysis with upcoming ground and space based GW interferometers, may provide a first clue to the microphysics of inflation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

Radiative pulsed L-mode operation in ARC-class reactors

Abstract A new ARC-class, highly-radiative, pulsed, L-mode, burning plasma scenario is developed and evaluated as a candidate for future tokamak reactors. Pulsed inductive operation alleviates the stringent current drive requirements of steady-state reactors, and operation in L-mode affords ELM-free access to ∼ 90 % core radiation fractions, significantly reducing the divertor power handling requirements. In this configuration the fusion power density can be maximized despite L-mode confinement by utilizing high-field to increase plasma densities and current. This allows us to obtain high gain in robust scenarios in compact devices with P fus > 1000 MW despite low confinement. We demonstrate the feasibility of such scenarios here; first by showing that they avoid violating 0D tokamak limits, and then by performing self-consistent integrated simulations of flattop operation including neoclassical and turbulent transport, magnetic equilibrium, and radiofrequency current drive models. Finally we examine the potential effect of introducing negative triangularity with a 0D model. Our results show high-field radiative pulsed L-mode scenarios are a promising alternative to the typical steady state advanced tokamak scenarios which have dominated tokamak reactor development.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Wide class of logarithmic potentials with power-tower kink tails

We present a wide class of potentials which admit kinks and corresponding mirror kinks with either a power law or an exponential tail at the two extreme ends and a power-tower form of tails at the two neighbouring ends. We analyse kink stability equation in all these cases and show that there is no gap between the zero mode and the beginning of the continuum. Finally, we provide a recipe for obtaining logarithmic potentials with power-tower kink tails and estimate kink–kink interaction strength.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2024

Cryogenic aspects of a 20 MW class low-temperature superconducting generator for the renewables industry

In this paper we give a progress update on the cryogenic design for cooling a 20 MW class, partially superconducting generator with stationary field coils for offshore wind renewables industry [1-2]. This is a continuation of an earlier program on a 10 MW system dating back ten years. Whereas this new power rating increase leads to a radial diameter expansion of the superconducting field coils from 4 to > 9.5 m, it maintains the axial length. We show the design based on this scaled up with enlarged diameter. The field coil size increase asks for higher cooling power that leads to a bigger cold box size to accommodate the cryocoolers. In the design, as many as 8 cryocoolers can be accessed and serviced from the nacelle that houses the main cryogenic components. A typical thermal load balance sheet with all components is given and compared against the available cryocooler cooling power at different operating conditions. Due to the cryocoolers’ local point of contact cooling within the nacelle, the temperature gradient of the thermal shield that fully encloses the cold mass with its embedded field coils needs to be balanced out to minimize the heat burden on the field coils. This requires additional analytical efforts and implementation of further design features. The extended nacelle houses the cold box with its cryogenic infrastructure. The interface for the envisaged cryogenic pushbutton closed-loop circulating system remains invisible, requires no handling of cryogenic liquids and is hermetically closed. The field coil diameter increase also leads to a greater initial helium gas storage volume. In addition, it requires higher initial room temperature fill pressure within the toroidal helium vapor storage tanks and in cooling tubes connecting to the cryocoolers. The toroidal helium gas tanks are thermally coupled with the thermal shield so that helium convection inside the storage tank can improve heat transfer and reduce the temperature gradient of the thermal shield. Besides those heat transfer challenges, additional mechanical strain within this large structure is exerted on the torque tubes during initial cooldown and when energizing field coils. Some of those design challenges are quite unexpected, leading to novel workarounds in order to maintain the chosen cooling strategy. Finally, we assess those design limitations in view of further cryogenic scalability with emphasis on manufacturability and assembly.

cryogenics↗

DOE OSTI2011

EXPANDED VERY LARGE ARRAY DETECTION OF 44.1 GHz CLASS I METHANOL MASERS IN SAGITTARIUS A

We report on the detection of 44 GHz Class I methanol (CH{sub 3}OH) maser emission in the Sagittarius A (Sgr A) complex with the Expanded Very Large Array (EVLA). These EVLA observations show that the Sgr A complex harbors at least four different tracers of shocked regions in the radio regime. The 44 GHz masers correlate with the positions and velocities of previously detected 36 GHz CH{sub 3}OH masers, but less with 1720 MHz OH masers. Our detections agree with theoretical predictions that the densities and temperatures conducive for 1720 MHz OH masers may also produce 36 and 44 GHz CH{sub 3}OH maser emission. However, many 44 GHz masers do not overlap with 36 GHz methanol masers, suggesting that 44 GHz masers also arise in regions too hot and too dense for 36 GHz masers to form. This agrees with the non-detection of 1720 MHz OH masers in the same area, which are thought to be excited under even cooler and less dense conditions. We speculate that the geometry of the 36 GHz masers outlines the current location of a shock front.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2012

OBSERVATIONS OF ENHANCED EXTREME ULTRAVIOLET CONTINUA DURING AN X-CLASS SOLAR FLARE USING SDO/EVE

Observations of extreme ultraviolet (EUV) emission from an X-class solar flare that occurred on 2011 February 15 at 01:44 UT are presented, obtained using the EUV Variability Experiment (EVE) on board the Solar Dynamics Observatory. The complete EVE spectral range covers the free-bound continua of H I (Lyman continuum), He I, and He II, with recombination edges at 91.2, 50.4, and 22.8 nm, respectively. By fitting the wavelength ranges blueward of each recombination edge with an exponential function, light curves of each of the integrated continua were generated over the course of the flare, as was emission from the free-free continuum (6.5-37 nm). The He II 30.4 nm and Ly{alpha} 121.6 nm lines, and soft X-ray (SXR; 0.1-0.8 nm) emission from GOES are also included for comparison. Each free-bound continuum was found to have a rapid rise phase at the flare onset similar to that seen in the 25-50 keV light curves from RHESSI, suggesting that they were formed by recombination with free electrons in the chromosphere. However, the free-free emission exhibited a slower rise phase seen also in the SXR emission from GOES, implying a predominantly coronal origin. By integrating over the entire flare the total energy emitted via each process was determined. We find that the flare energy in the EVE spectral range amounts to at most a few percent of the total flare energy, but EVE gives us a first comprehensive look at these diagnostically important continuum components.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

THE MAGNETIC FIELD MORPHOLOGY OF THE CLASS 0 PROTOSTAR L1157-mm

We present the first detection of polarization around the Class 0 low-mass protostar L1157-mm at two different wavelengths. We show polarimetric maps at large scales (10'' resolution at 350 {mu}m) from the SHARC-II Polarimeter and at smaller scales (1.''2-4.''5 at 1.3 mm) from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). The observations are consistent with each other and show inferred magnetic field lines aligned with the outflow. The CARMA observations suggest a full hourglass magnetic field morphology centered about the core; this is only the second well-defined hourglass detected around a low-mass protostar to date. We apply two different methods to CARMA polarimetric observations to estimate the plane-of-sky magnetic field magnitude, finding values of 1.4 and 3.4 mG.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

PRODUCTION OF THE EXTREME-ULTRAVIOLET LATE PHASE OF AN X CLASS FLARE IN A THREE-STAGE MAGNETIC RECONNECTION PROCESS

We report on observations of an X class flare on 2011 September 6 by the instruments on board the Solar Dynamics Observatory. The flare occurs in a complex active region with multiple polarities. The Extreme-Ultraviolet (EUV) Variability Experiment observations in the warm coronal emission reveal three enhancements, the third of which corresponds to an EUV late phase. The three enhancements have a one-to-one correspondence to the three stages in flare evolution identified by the spatially resolved Atmospheric Imaging Assembly observations, which are characterized by a flux rope eruption, a moderate filament ejection, and the appearance of EUV late phase loops, respectively. The EUV late phase loops are spatially and morphologically distinct from the main flare loops. Multi-channel analysis suggests the presence of a continuous but fragmented energy injection during the EUV late phase resulting in the warm corona nature of the late phase loops. Based on these observational facts, we propose a three-stage magnetic reconnection scenario to explain the flare evolution. Reconnections in different stages involve different magnetic fields but show a casual relationship between them. The EUV late phase loops are mainly produced by the least energetic magnetic reconnection in the last stage.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

DETECTION OF 36 GHz CLASS I METHANOL MASER EMISSION TOWARD NGC 253

We have used the Australia Telescope Compact Array to search for emission from the 4{sub –1} → 3{sub 0} E transition of methanol (36.2 GHz) toward the center of the nearby starburst galaxy NGC 253. Two regions of emission were detected, offset from the nucleus along the same position angle as the inner spiral arms. The emission is largely unresolved on a scale of 5'', has a FWHM line width of <30 km s{sup –1}, and an isotropic luminosity orders of a magnitude larger than that observed in any Galactic star formation region. These characteristics suggest that the 36.2 GHz methanol emission is most likely a maser, although observations with higher angular and spectral resolution are required to confirm this. If it is a maser, this represents the first detection of a class I methanol maser outside the Milky Way. The 36.2 GHz methanol emission in NGC 253 has more than an order of magnitude higher isotropic luminosity than the widespread emission recently detected toward the center of the Milky Way. If emission from this transition scales with the nuclear star formation rate, then it may be detectable in the central regions of many starburst galaxies. Detection of methanol emission in ultra-luminous infrared galaxies would open up a new tool for testing for variations in fundamental constants (particularly the proton-to-electron mass ratio) on cosmological scales.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

THE CONFINED X-CLASS FLARES OF SOLAR ACTIVE REGION 2192

The unusually large active region (AR) NOAA 2192, observed in 2014 October, was outstanding in its productivity of major two-ribbon flares without coronal mass ejections. On a large scale, a predominantly north–south oriented magnetic system of arcade fields served as a strong top and lateral confinement for a series of large two-ribbon flares originating from the core of the AR. The large initial separation of the flare ribbons, together with an almost absent growth in ribbon separation, suggests a confined reconnection site high up in the corona. Based on a detailed analysis of the confined X1.6 flare on October 22, we show how exceptional the flaring of this AR was. We provide evidence for repeated energy release, indicating that the same magnetic field structures were repeatedly involved in magnetic reconnection. We find that a large number of electrons was accelerated to non-thermal energies, revealing a steep power-law spectrum, but that only a small fraction was accelerated to high energies. The total non-thermal energy in electrons derived (on the order of 10{sup 25} J) is considerably higher than that in eruptive flares of class X1, and corresponds to about 10% of the excess magnetic energy present in the active-region corona.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

QUASI-PERIODIC SLIPPING MAGNETIC RECONNECTION DURING AN X-CLASS SOLAR FLARE OBSERVED BY THE SOLAR DYNAMICS OBSERVATORY AND INTERFACE REGION IMAGING SPECTROGRAPH

We first report the quasi-periodic slipping motion of flare loops during an eruptive X-class flare on 2014 September 10. The slipping motion was investigated at a specific location along one of the two ribbons and can be observed throughout the impulsive phase of the flare. The apparent slipping velocity was 20–110 km s{sup −1}, and the associated period was 3–6 minutes. The footpoints of flare loops appeared as small-scale bright knots observed in 1400 Å, corresponding to fine structures of the flare ribbon. These bright knots were observed to move along the southern part of the longer ribbon and also exhibited a quasi-periodic pattern. The Si iv 1402.77 Å line was redshifted by 30–50 km s{sup −1} at the locations of moving knots with a ∼40–60 km s{sup −1} line width, larger than other sites of the flare ribbon. We suggest that the quasi-periodic slipping reconnection is involved in this process and the redshift at the bright knots is probably indicative of reconnection downflow. The emission line of Si iv at the northern part of the longer ribbon also exhibited obvious redshifts of about 10–70 km s{sup −1} in the impulsive phase of the flare, with the redshifts at the outer edges of the ribbon larger than those in the middle. The redshift velocities at post-flare loops reached about 80–100 km s{sup −1} in the transition region.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2019

Regulation of LC3 lipidation by the autophagy-specific class III phosphatidylinositol-3 kinase complex

Autophagy is a conserved eukaryotic pathway critical for cellular adaptation to changes in nutrition levels and stress. The class III phosphatidylinositol (PI)3-kinase complexes I and II (PI3KC3-C1 and -C2) are essential for autophagosome initiation and maturation, respectively, from highly curved vesicles. We used a cell-free reaction that reproduces a key autophagy initiation step, LC3 lipidation, as a biochemical readout to probe the role of autophagy-related gene (ATG)14, a PI3KC3-C1-specific subunit implicated in targeting the complex to autophagy initiation sites. We reconstituted LC3 lipidation with recombinant PI3KC3-C1, -C2, or various mutant derivatives added to extracts derived from a CRISPR/Cas9-generated ATG14-knockout cell line. Both complexes C1 and C2 require the C-terminal helix of VPS34 for activity on highly curved membranes. However, only complex C1 supports LC3 lipidation through the curvature-targeting amphipathic lipid packing sensor (ALPS) motif of ATG14. Furthermore, the ALPS motif and VPS34 catalytic activity are required for downstream recruitment of WD-repeat domain phosphoinositide-interacting protein (WIPI)2, a protein that binds phosphatidylinositol 3-phosphate and its product phosphatidylinositol 3, 5-bisphosphate, and a WIPI-binding protein, ATG2A, but do not affect membrane association of ATG3 and ATG16L1, enzymes contributing directly to LC3 lipidation. These data reveal the nuanced role of the ATG14 ALPS in membrane curvature sensing, suggesting that the ALPS has additional roles in supporting LC3 lipidation.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2025

Improved laser-plasma accelerator stability via high-bandwidth longitudinal focal position stabilization of a 100 TW-class laser system

Laser-plasma accelerators (LPAs) offer an attractive alternative to conventional accelerators for the development of compact electron sources and next-generation light sources. Due to orders-of-magnitude larger accelerating gradients, LPAs enable the acceleration of high-brightness electron beams to ultrarelativistic energies in millimeter- to centimeter-scale distances. However, LPA stability is limited by shot-to-shot fluctuations of the driving laser system. Specifically, fluctuations in the final-focus longitudinal position result in correlated instability in LPA electron beam qualities, including total beam charge, average beam energy, and energy spread. We demonstrate active stabilization of the longitudinal focal position for a 100 TW-class laser system. This repetition-rate scalable stabilization system leverages noninvasive wave front monitoring of a copropagating, unamplified kHz pulse train to guide corrective adjustments to the focal position of a 1 Hz amplified drive laser via an upstream telescope on millisecond timescales. In this demonstration, the approach limits standard deviation fluctuations of the amplified drive beam’s longitudinal focal position to ±0.22⁢ mm (representing at least a 53% reduction), achieved with a correction bandwidth that reaches the Nyquist frequency limit. Consequently, we observe marked improvements in both long-term and shot-to-shot LPA stability in terms of charge and spectrum.

Beam control↗