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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Characteristics of major plasma discharge disruption in the Globus-M spherical tokamak

The characteristics of the major disruption of plasma discharges in the Globus-M spherical tokamak are analyzed. The process of current quench is accompanied by the loss of the vertical stability of the plasma column. The plasma boundary during the disruption is reconstructed using the algorithm of movable filaments. The plasma current decay is preceded by thermal quench, during which the profiles of the temperature and electron density were measured. The data on the time of disruption, the plasma current quench rate, and the toroidal current induced in the tokamak vessel are compared for hydrogen and deuterium plasmas. It is shown that the disruption characteristics depend weakly on the ion mass and the current induced in the vessel increases with the disruption time. The decay rate of the plasma toroidal magnetic flux during the disruption is determined using diamagnetic measurements. Such a decay is a source of the poloidal current induced in the vessel; it may also cause poloidal halo currents.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

HELIOSHEATH MAGNETIC FIELD AND PLASMA OBSERVED BY VOYAGER 2 DURING 2012 IN THE RISING PHASE OF SOLAR CYCLE 24

We discuss magnetic field and plasma observations of the heliosheath made by Voyager 2 (V2) during 2012, when V2 was observing the effects of increasing solar activity following the solar minimum in 2009. The average magnetic field strength B was 0.14 nT and B reached 0.29 nT on day 249. V2 was in a unipolar region in which the magnetic polarity was directed away from the Sun along the Parker spiral 88% of the time, indicating that V2 was poleward of the heliospheric current sheet throughout most of 2012. The magnetic flux at V2 during 2012 was constant. A merged interaction region (MIR) was observed, and the flow speed increased as the MIR moved past V2. The MIR caused a decrease in the >70 MeV nuc{sup −1} cosmic-ray intensity. The increments of B can be described by a q-Gaussian distribution with q = 1.2 ± 0.1 for daily averages and q = 1.82 ± 0.03 for hour averages. Eight isolated current sheets (“PBLs”) and four closely spaced pairs of current sheets were observed. The average change of B across the current sheets was a factor of ≈2, and B increased or decreased with equal probability. Magnetic holes and magnetic humps were also observed. The characteristic size of the PBLs was ≈6 R{sub L}, where R{sub L} is the Larmor radius of protons, and the characteristic sizes of the magnetic holes and humps were ≈38 R{sub L} and ≈11 R{sub L}, respectively.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Detection of coherent structures in photospheric turbulent flows

We study coherent structures in solar photospheric flows in a plage in the vicinity of the active region AR 10930 using the horizontal velocity data derived from Hinode/Solar Optical Telescope magnetograms. Eulerian and Lagrangian coherent structures (LCSs) are detected by computing the Q-criterion and the finite-time Lyapunov exponents of the velocity field, respectively. Our analysis indicates that, on average, the deformation Eulerian coherent structures dominate over the vortical Eulerian coherent structures in the plage region. We demonstrate the correspondence of the network of high magnetic flux concentration to the attracting Lagrangian coherent structures (aLCSs) in the photospheric velocity based on both observations and numerical simulations. In addition, the computation of aLCS provides a measure of the local rate of contraction/expansion of the flow.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Coronal heating driven by a magnetic gradient pumping mechanism in solar plasmas

The heating of the solar corona is a longstanding mystery in astrophysics. Considering that the solar magnetic field is spatially inhomogeneous with a considerable magnetic gradient from the solar surface to the corona, this work proposes a magnetic gradient pumping (MGP) mechanism to try to explain the formation of hot plasma upflows, such as hot type II spicules and hot plasma ejections. In the MGP mechanism, the magnetic gradient may drive the energetic particles to move upward from the underlying solar atmosphere and form hot upflows. These upflow energetic particles are deposited in the corona, causing it to become very hot. Rough estimations indicate that the solar corona can be heated to above 1 million degrees, and the upflow velocity is about 40 km s{sup –1} in the chromosphere and about 130 km s{sup –1} in the corona. The solar magnetic flux tubes act as pumpers to extract energetic particles from the underlying thermal photosphere, convey them, and deposit them in the corona. The deposit of these energetic particles causes the corona to become hot, and the escape of such particles from the photosphere leaves it a bit cold. This mechanism can present a natural explanation to the mystery of solar coronal heating.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Preshot Predictions for LA-43S-1 using the Eulerian AMR Code Roxane

The Ranchero Magnetic Flux Compression Generator (FCG) has been used to create current pulses in the 10-100 MA range for driving both “static” low inductance (0.5 nH) loads1 for generator demonstration purposes and high inductance (10-20 nH) imploding liner loads for ultimate use in physics experiments at very high energy density. This report offers a description of these efforts.

42 ENGINEERING↗
DOE OSTI2015

SOLAR CYCLE VARIATION OF THE INTER-NETWORK MAGNETIC FIELD

The solar inter-network magnetic field is the weakest component of solar magnetism, but it contributes most of the solar surface magnetic flux. The study of its origin has been constrained by the inadequate tempospatial resolution and sensitivity of polarization observations. With dramatic advances in spatial resolution and detecting sensitivity, the solar spectropolarimetry provided by the Solar Optical Telescope on board Hinode in an interval from the solar minimum to maximum of cycle 24 opens an unprecedented opportunity to study the cyclic behavior of the solar inter-network magnetic field. More than 1000 Hinode magnetograms observed from 2007 January to 2014 August are selected in the study. It has been found that there is a very slight correlation between sunspot number and magnetic field at the inter-network flux spectrum. From solar minimum to maximum of cycle 24, the flux density of the solar inter-network field is invariant, at 10 ± 1 G. The observations suggest that the inter-network magnetic field does not arise from flux diffusion or flux recycling of solar active regions, thereby indicating the existence of a local small-scale dynamo. Combining the full-disk magnetograms observed by the Solar and Heliospheric Observatory/Michelson Doppler Imager and the Solar Dynamics Observatory/Helioseismic and Magnetic Imager in the same period, we find that the area ratio of the inter-network region to the full disk of the Sun apparently decreases from solar minimum to maximum but always exceeds 60%, even in the phase of solar maximum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Instrument for in-situ orientation of superconducting thin-film resonators used for electron-spin resonance experiments

When used in electron-spin resonance measurements, superconducting thin-film resonators must be precisely oriented relative to the external magnetic field in order to prevent the trapping of magnetic flux and the associated degradation of resonator performance. We present a compact design solution for this problem that allows in-situ control of the orientation of the resonator at cryogenic temperatures. Tests of the apparatus show that when proper alignment is achieved, there is almost no hysteresis in the field dependence of the resonant frequency.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

MEASUREMENTS OF NON-THERMAL LINE WIDTHS IN SOLAR ACTIVE REGIONS

Spectral line widths are often observed to be larger than can be accounted for by thermal and instrumental broadening alone. This excess broadening is a key observational constraint for both nanoflare and wave dissipation models of coronal heating. Here we present a survey of non-thermal velocities measured in the high temperature loops (1–4 MK) often found in the cores of solar active regions. This survey of Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) observations covers 15 non-flaring active regions that span a wide range of solar conditions. We find relatively small non-thermal velocities, with a mean value of 17.6 ± 5.3 km s{sup −1}, and no significant trend with temperature or active region magnetic flux. These measurements appear to be inconsistent with those expected from reconnection jets in the corona, chromospheric evaporation induced by coronal nanoflares, and Alfvén wave turbulence models. Furthermore, because the observed non-thermal widths are generally small, such measurements are difficult and susceptible to systematic effects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

CHALLENGING SOME CONTEMPORARY VIEWS OF CORONAL MASS EJECTIONS. I. THE CASE FOR BLAST WAVES

Since the closure of the “solar flare myth” debate in the mid-1990s, a specific narrative of the nature of coronal mass ejections (CMEs) has been widely accepted by the solar physics community. This narrative describes structured magnetic flux ropes at the CME core that drive the surrounding field plasma away from the Sun. This narrative replaced the “traditional” view that CMEs were blast waves driven by solar flares. While the flux rope CME narrative is supported by a vast quantity of measurements made over five decades, it does not adequately describe every observation of what have been termed CME-related phenomena. In this paper we present evidence that some large-scale coronal eruptions, particularly those associated with EIT waves, exhibit characteristics that are more consistent with a blast wave originating from a localized region (such as a flare site) rather than a large-scale structure driven by an intrinsic flux rope. We present detailed examples of CMEs that are suspected blast waves and flux ropes, and show that of our small sample of 22 EIT-wave-related CMEs, 91% involve a blast wave as at least part of the eruption, and 50% are probably blast waves exclusively. We conclude with a description of possible signatures to look for in determining the difference between the two types of CMEs and with a discussion on modeling efforts to explore this possibility.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Generalized investigation of the rotation-activity relation: favoring rotation period instead of Rossby number

Magnetic activity in Sun-like and low-mass stars causes X-ray coronal emission which is stronger for more rapidly rotating stars. This relation is often interpreted in terms of the Rossby number, i.e., the ratio of rotation period to convective overturn time. We reconsider this interpretation on the basis of the observed X-ray emission and rotation periods of 821 stars with masses below 1.4 M {sub ☉}. A generalized analysis of the relation between X-ray luminosity normalized by bolometric luminosity, L {sub X}/L {sub bol}, and combinations of rotational period, P, and stellar radius, R, shows that the Rossby formulation does not provide the solution with minimal scatter. Instead, we find that the relation L {sub X}/L {sub bol}∝P {sup –2} R {sup –4} optimally describes the non-saturated fraction of the stars. This relation is equivalent to L {sub X}∝P {sup –2}, indicating that the rotation period alone determines the total X-ray emission. Since L {sub X} is directly related to the magnetic flux at the stellar surface, this means that the surface flux is determined solely by the star's rotation and is independent of other stellar parameters. While a formulation in terms of a Rossby number would be consistent with these results if the convective overturn time scales exactly as L{sub bol}{sup −1/2}, our generalized approach emphasizes the need to test a broader range of mechanisms for dynamo action in cool stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

EXPLAINING THE COEXISTENCE OF LARGE-SCALE AND SMALL-SCALE MAGNETIC FIELDS IN FULLY CONVECTIVE STARS

Despite the lack of a shear-rich tachocline region, low-mass fully convective (FC) stars are capable of generating strong magnetic fields, indicating that a dynamo mechanism fundamentally different from the solar dynamo is at work in these objects. We present a self-consistent three-dimensional model of magnetic field generation in low-mass FC stars. The model utilizes the anelastic magnetohydrodynamic equations to simulate compressible convection in a rotating sphere. A distributed dynamo working in the model spontaneously produces a dipole-dominated surface magnetic field of the observed strength. The interaction of this field with the turbulent convection in outer layers shreds it, producing small-scale fields that carry most of the magnetic flux. The Zeeman–Doppler-Imaging technique applied to synthetic spectropolarimetric data based on our model recovers most of the large-scale field. Our model simultaneously reproduces the morphology and magnitude of the large-scale field as well as the magnitude of the small-scale field observed on low-mass FC stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

DROPOUT OF DIRECTIONAL ELECTRON INTENSITIES IN LARGE SOLAR ENERGETIC PARTICLE EVENTS

In the “gradual” solar energetic particle (SEP) event during solar cycle 23 we have observed the dispersionless modulation (“dropout”) in directional intensities of nonrelativistic electrons. The average duration of dropout periods is ∼0.8 hr, which is consistent with the correlation scale of solar wind turbulence. During the dropout period electrons could display scatter-free transport in an intermittent way. Also, we have observed a decrease in the anisotropic index of incident electrons with increasing electron energy (E{sub e}), while the index of scattered/reflected electrons is nearly independent of E{sub e}. We hence perform an observational examination of the correlation between the anisotropic index of low-energy scattered/reflected electrons and the signature of the locally measured solar wind turbulence in the dissipation range, which is responsible for resonant scattering of nonrelativistic electrons. Since during the dropout period the slab turbulence fraction is dominant (0.8 ± 0.1), we pay close attention to the effect of slab fraction on the correlation examined. Our observation is consistent with the simulation result that in the dominance of the slab turbulence component there should exist a dispatched structure of magnetic flux tubes, along which electrons could be transported in a scatter-free manner. Since a similar phenomenon is exhibited in the “impulsive” SEP event, electron dropout should be a transport effect. Therefore, being different from most ion dropout events, which are due to a compact flare source, the dropout of directional electron intensities should be caused by the change of turbulence status in the solar wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE NONPOTENTIALITY OF CORONAE OF SOLAR ACTIVE REGIONS, THE DYNAMICS OF THE SURFACE MAGNETIC FIELD, AND THE POTENTIAL FOR LARGE FLARES

Flares and eruptions from solar active regions (ARs) are associated with atmospheric electrical currents accompanying distortions of the coronal field away from a lowest-energy potential state. In order to better understand the origin of these currents and their role in M- and X-class flares, I review all AR observations made with Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager and SDO/Atmospheric Imaging Assembly from 2010 May through 2014 October within ≈40° from the disk center. I select the roughly 4% of all regions that display a distinctly nonpotential coronal configuration in loops with a length comparable to the scale of the AR, and all that emit GOES X-class flares. The data for 41 regions confirm, with a single exception, that strong-field, high-gradient polarity inversion lines (SHILs) created during emergence of magnetic flux into, and related displacement within, pre-existing ARs are associated with X-class flares. Obvious nonpotentiality in the AR-scale loops occurs in six of ten selected regions with X-class flares, all with relatively long SHILs along their primary polarity inversion line, or with a long internal filament there. Nonpotentiality can exist in ARs well past the flux-emergence phase, often with reduced or absent flaring. I conclude that the dynamics of the flux involved in the compact SHILs is of pre-eminent importance for the large-flare potential of ARs within the next day, but that their associated currents may not reveal themselves in AR-scale nonpotentiality. In contrast, AR-scale nonpotentiality, which can persist for many days, may inform us about the eruption potential other than those from SHILs which is almost never associated with X-class flaring.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The transverse and rotational motions of magnetohydrodynamic kink waves in the solar atmosphere

Magnetohydrodynamic (MHD) kink waves have now been observed to be ubiquitous throughout the solar atmosphere. With modern instruments, they have now been detected in the chromosphere, interface region, and corona. The key purpose of this paper is to show that kink waves do not only involve purely transverse motions of solar magnetic flux tubes, but the velocity field is a spatially and temporally varying sum of both transverse and rotational motion. Taking this fact into account is particularly important for the accurate interpretation of varying Doppler velocity profiles across oscillating structures such as spicules. It has now been shown that, as well as bulk transverse motions, spicules have omnipresent rotational motions. Here we emphasize that caution should be used before interpreting the particular MHD wave mode/s responsible for these rotational motions. The rotational motions are not necessarily signatures of the classic axisymmetric torsional Alfvén wave alone, because kink motion itself can also contribute substantially to varying Doppler velocity profiles observed across these structures. In this paper, the displacement field of the kink wave is demonstrated to be a sum of its transverse and rotational components, both for a flux tube with a discontinuous density profile at its boundary, and one with a more realistic density continuum between the internal and external plasma. Furthermore, the Doppler velocity profile of the kink wave is forward modeled to demonstrate that, depending on the line of sight, it can either be quite distinct or very similar to that expected from a torsional Alfvén wave.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

3-D effects on viscosity and generation of toroidal and poloidal flows in LHD

Three-dimensional effects on plasma flows have been experimentally studied in the large helical device with 3D configurations. Spontaneous toroidal flow without net driving force using the combination of perpendicular neutral beam injection (NBI) heating and balanced tangential NBI heating has been investigated with two magnetic configurations. Co- and counter-directed spontaneous flows have been observed depending on the collisionality. Toroidal flow shear changes the sign at 0.4<r{sub eff}<0.6 between co- and counter-flowing plasmas, where r{sub eff} is a averaged minor radius. The detailed flow structures have been also examined at the edge region with stochastic magnetic field. A poloidal flow driven by the positive radial electric field has been observed at the open field region just outside of the plasma boundary. At the stochastic region, a shear of poloidal flow has been observed, while no shear of toroidal flow has been observed. The toroidal flow shear changes only in the nested magnetic flux surface region. A difference of density fluctuation property has been observed between co- and counter-flowing plasmas, where toroidal flow shears are formed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

MONTE CARLO SIMULATION MODEL OF ENERGETIC PROTON TRANSPORT THROUGH SELF-GENERATED ALFVEN WAVES

A new Monte Carlo simulation model for the transport of energetic protons through self-generated Alfven waves is presented. The key point of the model is that, unlike the previous ones, it employs the full form (i.e., includes the dependence on the pitch-angle cosine) of the resonance condition governing the scattering of particles off Alfven waves-the process that approximates the wave-particle interactions in the framework of quasilinear theory. This allows us to model the wave-particle interactions in weak turbulence more adequately, in particular, to implement anisotropic particle scattering instead of isotropic scattering, which the previous Monte Carlo models were based on. The developed model is applied to study the transport of flare-accelerated protons in an open magnetic flux tube. Simulation results for the transport of monoenergetic protons through the spectrum of Alfven waves reveal that the anisotropic scattering leads to spatially more distributed wave growth than isotropic scattering. This result can have important implications for diffusive shock acceleration, e.g., affect the scattering mean free path of the accelerated particles in and the size of the foreshock region.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

MULTI-WAVELENGTH OBSERVATIONS OF THE SPATIO-TEMPORAL EVOLUTION OF SOLAR FLARES WITH AIA/SDO. II. HYDRODYNAMIC SCALING LAWS AND THERMAL ENERGIES

In this study we measure physical parameters of the same set of 155 M- and X-class solar flares observed with AIA/SDO as analyzed in Paper I, by performing a differential emission measure analysis to determine the flare peak emission measure EM{sub p} , peak temperature T{sub p} , electron density n{sub p} , and thermal energy E{sub th}, in addition to the spatial scales L, areas A, and volumes V measured in Paper I. The parameter ranges for M- and X-class flares are log (EM{sub p}) = 47.0-50.5, T{sub p} = 5.0-17.8 MK, n{sub p} = 4 × 10{sup 9}-9 × 10{sup 11} cm{sup –3}, and thermal energies of E{sub th} = 1.6 × 10{sup 28}-1.1 × 10{sup 32} erg. We find that these parameters obey the Rosner-Tucker-Vaiana (RTV) scaling law T{sub p}{sup 2}∝n{sub p} L and H∝T {sup 7/2} L {sup –2} during the peak time t{sub p} of the flare density n{sub p} , when energy balance between the heating rate H and the conductive and radiative loss rates is achieved for a short instant and thus enables the applicability of the RTV scaling law. The application of the RTV scaling law predicts power-law distributions for all physical parameters, which we demonstrate with numerical Monte Carlo simulations as well as with analytical calculations. A consequence of the RTV law is also that we can retrieve the size distribution of heating rates, for which we find N(H)∝H {sup –1.8}, which is consistent with the magnetic flux distribution N(Φ)∝Φ{sup –1.85} observed by Parnell et al. and the heating flux scaling law F{sub H} ∝HL∝B/L of Schrijver et al.. The fractal-diffusive self-organized criticality model in conjunction with the RTV scaling law reproduces the observed power-law distributions and their slopes for all geometrical and physical parameters and can be used to predict the size distributions for other flare data sets, instruments, and detection algorithms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

A Non-contact Apparatus for Measuring Longitudinal Critical Current of REBCO Tapes Using the Magnetic Circuit Method

This paper details the design and construction of a non-contact apparatus that can measure the longitudinal critical current of REBCO tapes based on magnetic circuit method. A 3D finite element model was proposed to study the relation between I{sub c} (critical current of the REBCO tape) and B{sub H} (magnetic field at the Hall sensor). The proportionality coefficient of I{sub c} and B{sub H} was obtained by using the said model. Several experiments on the REBCO tapes were conducted to determine the repeatability, reliability, and accuracy of the apparatus. Four parameters (average value, minimum value, the range, and the coefficient of critical current variation (COV)) were used to determine the repeatability and accuracy of the apparatus. It was observed that transmission speed affects test results because of relaxation of the magnetic flux. Another tape was tested at different transmission speeds to study the relation between B{sub H} and decay time. From the results, a principle formula was established to explain the relation between B{sub H} and decay time, and it was shown that the test results also take the same form as the principle formula.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗