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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

ROTATIONAL AND CYCLICAL VARIABILITY IN {gamma} CASSIOPEIAE. II. FIFTEEN SEASONS

The B0.5 IVe star {gamma} Cas is of great interest because it is the prototype of a small group of classical Be stars having hard X-ray emission of unknown origin. We discuss results from ongoing B and V observations of the {gamma} Cas star-disk system acquired with an Automated Photometric Telescope during the observing seasons 1997-2011. In an earlier study, Smith, Henry, and Vishniac showed that light variations in {gamma} Cas are dominated by a series of comparatively prominent cycles with amplitudes of 0.02-0.03 mag and lengths of 2-3 months, superimposed on a 1.21 day periodic signal some five times smaller, which they attributed to rotation. The cycle lengths clustered around 70 days, with a total range of 50-91 days. Changes in both cycle length and amplitude were observed from year to year. These authors also found the V-band cycles to be 30%-40% larger than the B-band cycles. In the present study, we find continued evidence for these variability patterns and for the bimodal distribution of the {Delta}B/{Delta}V amplitude ratios in the long cycles. During the 2010 observing season, {gamma} Cas underwent a mass-loss event ({sup o}utburst{sup )}, as evidenced by the brightening and reddening seen in our new photometry. This episode coincided with a waning of the amplitude in the ongoing cycle. The Be outburst ended the following year, and the light-curve amplitude returned to pre-outburst levels. This behavior reinforces the interpretation that cycles arise from a global disk instability. We have determined a more precise value of the rotation period, 1.215811 {+-} 0.000030 days, using the longer 15-season data set and combining solutions from the V and B light curves. Remarkably, we also find that both the amplitude and the asymmetry of the rotational waveform changed over the years. We review arguments for this modulation arising from transits of a surface magnetic disturbance. Finally, to a limit of 5 mmag, we find no evidence for any photometric variation corresponding to the {gamma} Cas binary period, 203.55 days, or to the first few harmonics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The very high n Rydberg series of Ar 16+ in Alcator C-Mod tokamak plasmas

We report x-ray transitions of the very high-n Rydberg series in Ar 16+ have been observed from Alcator C-Mod tokamak plasmas. Individual emission lines up to 1s16p-1s 2 have been resolved and the central chord line brightnesses with principal quantum number n between 7 and 16 are generally found to decay as 1/n α , with α slightly larger than 3. In the plasma periphery, emission from 1s9p-1s 2 and 1s10p-1s 2 are found to be significantly enhanced relative to this decrease, indicative of selected population of these levels through charge exchange between background neutral deuterium in the ground state and Ar 17+ . An unresolved feature between the wavelengths of 1s27p-1s 2 and 1s30p-1s 2 is also present, which arises through charge exchange with neutral deuterium in the n* = 3 excited state. The brightnesses of transitions populated by charge exchange are spatially up/down asymmetric, with an excess on the side of the magnetic surface X-point. The relative brightness of the unresolved very high-n feature compared to 1s7p-1s 2 is found to increase with electron temperature and decrease with electron density. Simulations of line emission just on the long wavelength side of the Ar 16+ ionization limit indicate that the principal quantum number decay exponent is closer to α = 4 at very high n. The brightness dependence on n below 16 is in excellent agreement with calculations from the flexible atomic code package.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Identification of a network of nonlinear interactions as a mechanism triggering the onset of edge localized modes

Edge localized modes (ELMs) remain a critical issue for the lifetime of in-vessel components of fusion reactors such as ITER. We study ELMs triggered when the pedestal parameters are below the peeling-ballooning limit. These ELMs occur while the pedestal recovers after an ELM crash. We identify a mechanism by which a non-linear perturbation involving many active triads—a network of non-linear interactions – leads to the triggering of these ELMs. Each active triad leads to the non-linear transfer of energy between three waves, such that the network of non-linear interactions lead to a transfer of energy between many waves. The triggering of this non-linear perturbation systematically coincides with a pedestal perturbation induced by the neutral beam injection beams and localized near the magnetic surface of safety factor q = 5. Among the non-linear interactions detected during the ELM by a bi-coherence analysis, the most active triad involves a slow frequency that is qualitatively consistent with a geodesic acoustic mode located at the same location, i.e., near the q = 5 surface.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Computation of linear MHD instabilities with the multi-region relaxed MHD energy principle

Over the last decade, a variational principle based on a generalisation of Taylor's relaxation, referred to as multi-region relaxed magnetohydrodynamics (MRxMHDs) has been developed. The numerical solutions of the MRxMHD equilibria have been constructed using the Stepped Pressure Equilibrium Code (SPEC) (Hudson et al 2012 Phys. Plasmas 19 112502). In principle, SPEC could also be established to describe the MRxMHD stability of an equilibrium. In this work, a theoretical framework is developed to relate the second variation of the energy functional to the so-called Hessian matrix, enabling the prediction of MHD linear instabilities of cylindrical plasmas, and is implemented in SPEC. The negative and positive eigenvalues of the Hessian matrix predict the stability of an equilibrium. Verification studies of SPEC stability results with the M3D-C 1 code and the tearing mode $\Delta^{^{\prime}}$ criterion have been conducted for ideal and resistive MHD instabilities, respectively, in a pressureless cylindrical tokamak, and have shown good agreement. Overall, our stability analysis is a critical step towards understanding the MHD stability of three-dimensional MHDs where nested flux surfaces, magnetic islands and stochastic regions co-exist.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Superconducting low-beta Nb 3 Sn cavity for ATLAS and future ion accelerators

We report on a Nb 3 Sn-coated low-beta superconducting radio frequency (RF) cavity intended for accelerating ions. We aim to apply the cavity in ATLAS, our Argonne National Laboratory user facility for nuclear physics studies with ion beams in the energy range of 5–20 MeV u −1 . The Nb 3 Sn-coated cavity, a 145 MHz quarter-wave optimized for ions moving with velocity β = $v/c$ = 0.08 exhibits an order-of-magnitude reduction in RF losses into helium at 4.4 K compared to a superconducting niobium (Nb) cavity at the same frequency and temperature. Experimentally measured fields are among the highest to date for any Nb 3 Sn-coated cavity, reaching a peak surface magnetic field of 105 mT. We also present a practical solution to the problem of cavity frequency tuning. Tuning by mechanical deformation has been a challenge with Nb 3 Sn due to its brittle nature, however, using a set of techniques tailored to the properties of thin-film Nb 3 Sn on Nb, we can repeatably tune the cavity to the ATLAS master clock frequency after it is cooled, while maintaining the excellent performance characteristics. The same Nb 3 Sn cavity technology offers broad benefits for future ion accelerators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Self-driven current generation in turbulent fusion plasmas

Global gyrokinetic simulations with self-consistent coupling of neoclassical and turbulent dynamics demonstrate that turbulence can greatly affect plasma self-driven mean current generation in tokamaks. The current amplitude, profile and associated phase space structures can all be modified. Turbulence can critically reduce the current generation in the collisionless regime, generate current profile corrugation near the rational magnetic surface and nonlocally drive current in the linearly stable region—all these are expected to have a radical impact on broad tokamak physics. Both electron parallel acceleration and residual stress from turbulence play tremendous roles in turbulence-induced current generation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

A model investigation of the impact of lower hybrid wave scattering angle on current drive profile in EAST and Alcator C-Mod

Lower hybrid current drive (LHCD) is beneficial for developing a steady-state operation scenario in a tokamak. This paper conducts a modeling investigation to identify an optimum rotation angle of the initial lower hybrid perpendicular (to the background magnetic field $\overrightarrow{B}$) wavevector for best matching the experimental RF current profile. It is hypothesized that central RF power deposition widely observed in the present-day LHCD experiments arises from wave scattering by turbulence. In a standard model without considering such interactions, the predicted power deposition profile is generally broad with off-axis peaking, not in agreement with experimental observations. A heuristic approach is adopted by introducing a spectral broadening mechanism by modifying the initial orientation of the perpendicular wavevector. The ray-tracing/Fokker–Planck solver GENRAY/CQL3D is utilized within the python-based π-scope framework. A focus is given to identify the perpendicular wavenumber orientation angle with respect to the magnetic surface normal vector at the initial ray location. Our modeling study shows that rotating the perpendicular wavevector in such a way as to increase the initial poloidal component is effective in reproducing the centrally peaked current profile observed in normal shear plasmas on both EAST and C-Mod. These waves can readily be absorbed to the central plasma, which reduces the sensitivity of the power deposition profile to a slight change of the plasma condition. The same approach is also found to help broaden the off-axis power deposition profile in a reverse-shear EAST plasma, leading to a better agreement with the experiment. The results presented here suggest that spectral modification arising from edge density fluctuations in a tokamak may need to be considered in understanding wave propagation and absorption. A further experimental and theoretical/modeling study is vital as a reverse approach is adopted in this study. Finally, our work suggests that mitigation or control measures are critical for parasitic effects occurring on the first pass in a reactor regime.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Transition in particle transport under resonant magnetic perturbations in a tokamak

Abstract Nonlinear 3D MHD simulations and validations reveal that the hybrid particle-MHD transport is a key process for driving the pump-out in the presence of Resonant Magnetic Perturbations (RMPs) in the KSTAR tokamak. Particle transport and the resulting density pump-out by RMPs are shown to be composed of not only the classical flow convection near magnetic islands due to polarization but also the neoclassical ion diffusion across perturbed magnetic surfaces. The latter is known as the Neoclassical Toroidal Viscosity (NTV) and is integrated into nonlinear MHD simulations here for the first time, revealing that the two-stage pump-outs observed in KSTAR experiments are reproduced only with such integrated nonlinear MHD and transport evolution. Near-resonant responses, which have received less attention than the resonant response, play distinct roles in the pump-out along with the island formation. In addition, this modeling is used to investigate the pump-outs in double-null-like plasmas and numerically capture the effect of the double-null shape on the pump-outs, which may explain the difficulty of Edge Localized Mode (ELM) suppression access in double-like plasmas. This reveals new aspects of the impact toroidal geometry and mode coupling have on 3D physics and reveals the importance of near-resonant components in suppressing ELMs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Recent advance progress of HL-3 experiments

Since the first plasma realized in 2020, a series of key systems on HL-3 (known as HL-2M before) tokamak have been equipped/upgraded, including in-vessel components (the first wall, lower divertor, and toroidal cryogenic/water-cooling/baking/glow discharge systems, etc.), auxiliary heating system of 11 MW, and 28 diagnostic systems (to measure the plasma density, electron temperature, radiation, magnetic field, etc.). Magnet field systems were commissioned firstly for divertor plasma discharges. During the 2nd experimental campaign of HL-3 tokamak, several great progresses have been achieved. Firstly, the successful operation with plasma current larger than 1 MA was achieved under a divertor configuration. Secondly, the advanced divertor concept with two distinct snowflake configurations was realized. It is found that the distribution of ion saturation current and heat flux on bottom plate becomes wide due to magnetic surface expansion, demonstrating the advantage of such configuration in the heat flux mitigation. In addition, using the combination of NBI, ECRH and LHCD, the standard sawtoothing high confinement mode of megampere plasma was firstly accessed on the HL-3. The successful commissioning of HL-3 is beneficial for the initial operation of ITER.

1 MA plasma↗
DOE OSTI2014

DISCOVERY OF THE TRANSIENT MAGNETAR 3XMM J185246.6+003317 NEAR SUPERNOVA REMNANT KESTEVEN 79 WITH XMM-NEWTON

We report the serendipitous discovery with XMM-Newton that 3XMM J185246.6+003317 is an 11.56 s X-ray pulsar located 1' away from the southern boundary of supernova remnant Kes 79. The spin-down rate of 3XMM J185246.6+003317 is <1.1 × 10{sup –13} s s{sup –1}, which, together with the long period P = 11.5587126(4) s, indicates a dipolar surface magnetic field of <3.6 × 10{sup 13} G, a characteristic age of >1.7 Myr, and a spin-down luminosity of <2.8 × 10{sup 30} erg s{sup –1}. Its X-ray spectrum is best-fitted with a resonant Compton scattering model and also can be adequately described by a blackbody model. The observations covering a seven-month span from 2008 to 2009 show variations in the spectral properties of the source, with the luminosity decreasing from 2.7 × 10{sup 34} erg s{sup –1} to 4.6 × 10{sup 33} erg s{sup –1}, along with a decrease of the blackbody temperature from kT ≈ 0.8 keV to ≈0.6 keV. The X-ray luminosity of the source is higher than its spin-down luminosity, ruling out rotation as a power source. The combined timing and spectral properties, the non-detection of any optical or infrared counterpart, together with the lack of detection of the source in archival X-ray data prior to the 2008 XMM-Newton observation, point to 3XMM J185246.6+003317 being a newly discovered transient low-B magnetar undergoing an outburst decay during the XMM-Newton observations. The non-detection by Chandra in 2001 sets an upper limit of 4 × 10{sup 32} erg s{sup –1} to the quiescent luminosity of 3XMM J185246.6+003317. Its period is the longest among currently known transient magnetars. The foreground absorption toward 3XMM J185246.6+003317 is similar to that of Kes 79, suggesting a similar distance of ∼7.1 kpc.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

DISCOVERY OF X-RAY PULSATIONS FROM THE INTEGRAL SOURCE IGR J11014–6103

We report the discovery of PSR J1101–6101, a 62.8 ms pulsar in IGR J11014–6103, a hard X-ray source with a jet and a cometary tail that strongly suggests it is moving away from the center of the supernova remnant (SNR) MSH 11–61A at v > 1000 km s{sup –1}. Two XMM-Newton observations were obtained with the EPIC pn in small window mode, resulting in the measurement of its spin-down luminosity E-dot =1.36×10{sup 36} erg s{sup –1}, characteristic age τ {sub c} = 116 kyr, and surface magnetic field strength B{sub s} = 7.4 × 10{sup 11} G. In comparison to τ {sub c}, the 10-30 kyr age estimated for MSH 11–61A suggests that the pulsar was born in the SNR with initial period in the range 54 ≤ P {sub 0} ≤ 60 ms. PSR J1101–6101 is the least energetic of the 15 rotation-powered pulsars detected by INTEGRAL, and has a high efficiency of hard X-ray radiation and jet power. We examine the shape of the cometary nebula in a Chandra image, which is roughly consistent with a bow shock at the velocity inferred from the SNR age and the pulsar's E-dot . However, its structure differs in detail from the classic bow shock, and we explore possible reasons for this.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The Soft X-ray research instrument at the Linac Coherent Light Source

The Soft X-ray Research instrument provides intense ultrashort X-ray pulses in the energy range 280–2000 eV. A diverse set of experimental stations may be installed to investigate a broad range of scientific topics such as ultrafast chemistry, highly correlated materials, magnetism, surface science, and matter under extreme conditions. A brief description of the main instrument components will be given, followed by some selected scientific highlights.

43 PARTICLE ACCELERATORS↗
DOE OSTI2024

RF AND MECHANICAL DESIGN OF A 915 MHz SRF CAVITY FOR CONDUCTION-COOLED CRYOMODULES

Conduction-cooled SRF niobium cavities are being developed for use in compact, continuous-wave electron linear accelerators for a variety of industrial applications. A 915MHz two-cell cavity has been designed to achieve an energy gain of 3.5 MeV. The design of the cell shape aims at minimizing the peak surface magnetic field. Field flatness is achieved by adjusting the length of the outer end half-cells. The higher-order mode analysis shows that absorbers are not required for a moderate beam current of 5 mA. One of the beam tubes has two side-ports for insertion of coaxial fundamental power couplers. The mechanical design and analysis were done to maintain a stress near or less than 15.5 MPa for all anticipated loading conditions. This is half the measured yield strength and is to provide relief from creep when cavity is evacuated and stored with outside atmospheric pressure.

Ciovati, Gianluigi↗
DOE OSTI2018

High Speed Tribology of the Head-Disk Interface by Computer Simulation Final Report CRADA No. TC-297-92-C

The purpose of this project is to develop and validate a molecular dynamics model of nanoscale tribological processes occurring inside the head-disk interface of magnetic recording devices. Within the three years of the project the partners will: write a new molecular dynamics computer simulation code for growth, sliding and impact inside the head-disk interface; identify accurate models for chemical bonding within coatings used to protect magnetic surfaces, especially amorphous carbon thin films; simulate the growth of amorphous carbon thin films and analyse the resulting chemical bonding; simulate friction and wear processes relevant to the head-disk interface; and validate the model through comparison with AFM and nanoindenter studies of friction and wear of amorphous carbon thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2024

RF and Mechanical Design of a 915 MHz SRF Cavity for Conduction-Cooled Cryomodules

Conduction-cooled SRF niobium cavities are being developed for use in compact, continuous-wave electron linear accelerators for a variety of industrial applications. A 915 MHz two-cell cavity has been designed to achieve an energy gain of 3.5 MeV. The design of the cell shape aims at minimizing the peak surface magnetic field. Field flatness is achieved by adjusting the length of the outer end half-cells. The higher-order mode analysis shows that absorbers are not required for a moderate beam current of 5 mA. One of the beam tubes has two side-ports for insertion of coaxial fundamental power couplers. The mechanical design and analysis were done to maintain a stress near or less than 15.5 MPa for all anticipated loading conditions. This is half the measured yield strength and is to provide relief from creep when cavity is evacuated and stored with outside atmospheric pressure.

Ciovati, Gianluigi↗
DOE OSTI2020

Epitaxial Stabilization of Single-Crystal Multiferroic YCrO 3 Thin Films

We report on the growth of stoichiometric, single-crystal YCrO 3 epitaxial thin films on (001) SrTiO 3 substrates using pulsed laser deposition. X-ray diffraction and atomic force microscopy reveal that the films grew in a layer-by-layer fashion with excellent crystallinity and atomically smooth surfaces. Magnetization measurements demonstrate that the material is ferromagnetic below 144 K. The temperature dependence of dielectric permittivity shows a characteristic relaxor-ferroelectric behavior at T C = 375–408 K. A dielectric anomaly at the magnetic transition temperature indicates a close correlation between magnetic and electric order parameters in these multiferroic YCrO 3 films. These findings provide guidance to synthesize rare-earth, chromite-based multifunctional heterostructures and build a foundation for future studies on the understanding of magnetoelectric effects in similar material systems.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with NuStar

We present NuSTAR observations of neutron star (NS) low-mass X-ray binaries: 4U 1636-53, GX 17+2, and 4U 1705-44. We observed 4U 1636-53 in the hard state, with an Eddington fraction, F{sub Edd}, of 0.01; GX 17+2 and 4U 1705-44 were in the soft state with fractions of 0.57 and 0.10, respectively. Each spectrum shows evidence for a relativistically broadened Fe K{sub α} line. Through accretion disk reflection modeling, we constrain the radius of the inner disk in 4U 1636-53 to be R{sub in}=1.03±0.03 ISCO (innermost stable circular orbit), assuming a dimensionless spin parameter a{sub ∗}=cJ/GM{sup 2}=0.0, and R{sub in}=1.08±0.06 ISCO for a{sub ∗}=0.3 (errors quoted at 1σ). This value proves to be model independent. For a{sub ∗}=0.3 and M=1.4 M{sub ⊙}, for example, 1.08 ± 0.06 ISCO translates to a physical radius of R=10.8±0.6 km, and the NS would have to be smaller than this radius (other outcomes are possible for allowed spin parameters and masses). For GX 17+2, R{sub in}=1.00--1.04 ISCO for a{sub ∗}=0.0 and R{sub in}=1.03--1.30 ISCO for a{sub ∗}=0.3. For a{sub ∗}=0.3 and M=1.4 M{sub ⊙}, R{sub in}=1.03--1.30 ISCO translates to R=10.3--13.0 km. The inner accretion disk in 4U 1705-44 may be truncated just above the stellar surface, perhaps by a boundary layer or magnetosphere; reflection models give a radius of 1.46–1.64 ISCO for a{sub ∗}=0.0 and 1.69–1.93 ISCO for a{sub ∗}=0.3. We discuss the implications our results may have on the equation of state of ultradense, cold matter and our understanding of the innermost accretion flow onto NSs with low surface magnetic fields, and systematic errors related to the reflection models and spacetime metric around less idealized NSs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Very High n Rydberg Series of Ar^16+ in Alcator C-Mod Tokamak Plasmas

X-ray transitions of the very high-n Rydberg series in Ar^16+ have been observed from Alcator C-Mod tokamak plasmas. Individual emission lines up to 1s16p - 1s^2 have been resolved and the central chord line brightnesses with principal quantum number n between 7 and 16 are generally found to decay as 1/n^alpha, with alpha slightly larger than 3. In the plasma periphery, emission from 1s9p - 1s^2 and 1s10p - 1s^2 are found to be significantly enhanced relative to this decrease, indicative of selected population of these levels through charge exchange between background neutral deuterium in the ground state and Ar^17+. An unresolved feature between the wavelengths of 1s27p - 1s^2 and 1s30p - 1s^2 is also present, which arises through charge exchange with neutral deuterium in the n^* = 3 excited state. The brightnesses of transitions populated by charge exchange are spatially up/down asymmetric, with an excess on the side of the magnetic surface X-point. The relative brightness of the unresolved very high-n feature compared to 1s7p - 1s^2 is found to increase with electron temperature and decrease with electron density. Simulations of line emission just on the long wavelength side of the Ar^16$ ionization limit indicate that the principal quantum number decay exponent is closer to alpha = 4 at very high n. The brightness dependence on n below 16 is in excellent agreement with calculations from the Flexible Atomic Code package.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗