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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Final Report for Center for Tokamak Transient Simulations at USU

Providing plasma fluid codes like NIMROD with continuum drift kinetic (CDK) physics that is quantitatively valid and computationally feasible throughout the spatial domain is difficult. Work at Utah State University (USU), in collaboration with the Center for Tokamak Transient Simulations (CTTS), focused on applying CDK closures in disruption-related calculations. Three examples where kinetic physics is paramount are (1) the electron stress tensor closure in Ohms law for accurately describing neoclassical tearing mode (NTM) evolution, (2) runaway electron (RE) density (nRE) and current (jRE) moments in NIMROD’s extended MHD model for self-consistent evolution of RE populations during disruptions and, (3) energetic ion effects on a myriad of MHD instabilities. While NTM simulations and continuum and PIC approaches to energetic ions in NIMROD have been a major goals of USU’s closure work for several years, the development of self-consistent CDK RE capability in NIMROD was started and extended considerably during the CTTS effort. Some goals of CDK RE in NIMROD are to explore the effects of the 2D relativistic phase space in 4D simulations and compare with NIMROD’s fluid RE model. Four publications and two PhD theses came out of the USU CTTS effort.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

The Impact of Alfvénic Shear Flow on Magnetic Reconnection and Turbulence

Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP encounters 4–11 (2020 January–2022 March), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus nonreconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvénic wind streams. We compare with fast, slow non-Alfvénic, and slow Alfvénic streams, finding that the growth rate is suppressed in highly Alfvénic fast and slow wind, and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvénic flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvénic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

slow solar wind↗
DOE OSTI2021

Overview of the TCV tokamak program: Scientific progress and facility upgrades

The TCV tokamak is augmenting its unique historical capabilities (strong shaping, strong electron heating) with ion heating, additional electron heating compatible with high densities, and variable divertor geometry, in a multifaceted upgrade program designed to broaden its operational range without sacrificing its fundamental flexibility. The TCV program is rooted in a three-pronged approach aimed at ITER support, explorations towards DEMO, and fundamental research. A 1 MW, tangential neutral beam injector (NBI) was recently installed and promptly extended the TCV parameter range, with record ion temperatures and toroidal rotation velocities and measurable neutral-beam current drive. ITER-relevant scenario development has received particular attention, with strategies aimed at maximizing performance through optimized discharge trajectories to avoid MHD instabilities, such as peeling-ballooning and neoclassical tearing modes. Experiments on exhaust physics have focused particularly on detachment, a necessary step to a DEMO reactor, in a comprehensive set of conventional and advanced divertor concepts. The specific theoretical prediction of an enhanced radiation region between the two X-points in the low-field-side snowflake-minus configuration was experimentally confirmed. Fundamental investigations of the power decay length in the scrape-off layer (SOL) are progressing rapidly, again in widely varying configurations and in both D and He plasmas; in particular, the double decay length in L-mode limited plasmas was found to be replaced by a single length at high SOL resistivity. Experiments on disruption mitigation by massive gas injection and electron-cyclotron resonance heating (ECRH) have begun in earnest, in parallel with studies of runaway electron generation and control, in both stable and disruptive conditions; a quiescent runaway beam carrying the entire electrical current appears to develop in some cases. Developments in plasma control have benefited from progress in individual controller design and have evolved steadily towards controller integration, mostly within an environment supervised by a tokamak profile control simulator. TCV has demonstrated effective wall conditioning with ECRH in He in support of the preparations for JT-60SA operation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1972

Magnetic models of solar flares.

A solar flare is interpreted as the explosive release of magnetic energy associated with a current sheet in the solar atmosphere. The release is due to field-line reconnection caused by the tearing-mode instability. Current sheets may be classified into closed and open, depending on whether the adjacent field lines are closed (contained in the sun's atmosphere) or open, extending into interplanetary space. A high-energy event requires an open current sheet. It is shown that certain types of photospheric motion may lead to the formation of current sheets. One of these is associated with high-energy events, another with filaments, and a third with surge flares.

Sturrock, P. A.↗
NASA NTRS1974

Instabilities connected with neutral sheets in the solar wind

A preliminary study is presented of two sets of data obtained by HEOS 1 and OGO 5 in the solar wind, which reveal the internal structure of two neutral sheets and their two-dimensional structure. HEOS 1 observations of the effects of the tearing mode instability in one of the sheets are described, including complicated structures connected with the sector boundary, sharp increases and decreases in the magnetic field intensity, and the presence of closed loops. HEOS 1 and OGO 5 observations of large oscillations due to plasma pressure imbalances are discussed, and it is concluded that an interchange instability may have been observed.

Formisano, V.↗
NASA NTRS1976

High conductivity magnetic tearing instability

Linearized equations of magnetohydrodynamics are used to investigate the tearing mode, for arbitrary values of the conductivity, through a consideration of the additional effect of the electron-inertia contribution to Ohm's law. A description is provided of the equilibrium and subsequent instability in the magnetohydrodynamic approximation. A method for solving the perturbation equations in the linear approximation is discussed and attention is given to the results in the high conductivity limit.

Cross, M. A.↗
NASA NTRS1977

Microstructure of a magnetotail fireball

We use high time resolution data from the magnetometer, LEPEDEA and plasma wave analyzer on IMP-7 to examine the microstructure of a long-duration magnetotail fireball event observed during an extended quiet period on November 9, 1972. We demonstrate that the magnetic field is turbulent with fluctuation time scales down to several seconds. These findings suggest that the magnetic merging process may be a highly turbulent one, possibly associated with a form of the tearing mode instability.

Coroniti, F. V.↗
NASA NTRS1978

Magnetic dips in the solar wind

Using magnetic data from the HELIOS 1 fluxgate magnetometer, with a 0.2 sec resolution, the structures of several interplanetary discontinuities involving magnetic dips and rotations of the magnetic field vector were investigated. A minimum variance analysis illustrates the behavior of the magnetic field through the transition in the plane of its maximum variation. Using this analysis, quite different structures have been individuated and, in particular, narrow transitions resembling almost one dimensional reconnected neutral sheets. For the thinner cases (scale lengths of the magnetic rotation of the order or smaller than 1,000 km), results show the observed structures could be the nonlinear effect of a resistive tearing mode instability having developed on an originally one dimensional neutral sheet at the solar corona.

Dobrowolny, M.↗
NASA NTRS1979

Magnetopause structure from satellite observations

Observations on magnetopause structure are reported. Major topics covered include: classical reconnection, transport mechanisms, magnetospheric boundary layers, tearing modes, and Jupiter's magnetopause.

Sonnerup, B. U. O.↗
NASA NTRS1978

On the fine structure of cometary plasma tails

It is argued that the small-scale structures observed close to the plasma tail axis of comets may be a consequence of well-known finite-resistivity instabilities in the cometary cross-tail current sheet. While the 'tearing' mode instability, which causes a breakup of the layer along the current flow lines, may be responsible for the observed small condensation or 'knots' near the axis, the 'ripple' mode instability may give rise to the small-scale wavy structures.

Morrison, P. J.↗
NASA NTRS1979

Magnetic dips in the solar wind

Using magnetic data from the Helios-1 fluxgate magnetometer, with a 0.2 s resolution, we have investigated the structure of several interplanetary discontinuities involving magnetic dips and rotations of the magnetic field vector. A minimum variance analysis illustrates the behaviour of the magnetic field through the transition. Using this analysis, quite different structures have been isolated and, in particular, narrow transitions resembling almost one dimensional reconnected neutral sheets. For the thinner cases (scale lengths of the magnetic rotation of the order or smaller than 1000 km), we find that the observed structures can be the nonlinear effect of a resistive tearing mode instability having developed on an originally one dimensional neutral sheet at the solar corona.

Dobrowolny, M.↗
NASA NTRS1979

Electromagnetic structure of the magnetopause and boundary layer

After a review of the properties and predictions of the closed and open models of the magnetopause, OGO-5 magnetometer data are used to illustrate various observed signatures of the magnetopause current layer and the adjacent plasma boundary layer. Among the topics touched upon are: fluctuations, diamagnetic effects, and field aligned currents in the boundary layer; one dimensionality of the magnetopause; presence and absence of a magnetic field component perpendicular to the magnetopause; finite ion gyroradius effects. A brief summary is given of existing Vlasov theory for the description of tangential, rotational, and contact discontinuities. Special attention is paid to the tangential momentum balance and the jump conditions at a rotational discontinuity. Low frequency fluctuations are discussed with emphasis on the signatures of the tearing mode.

Sonnerup, B. U. O.↗
NASA NTRS1979

Data reduction and analysis for the TRW IMP-7 plasma wave experiment

Highlights from the IMP 7 plasma wave experiment are briefly outlined. The measurements in the tail revealed great complexities involving substorms and fireballs and provided insight into the overall dynamics of the magnetosphere. The low levels of magnetic turbulence may be of significance with respect to the development of tearing mode dissipation for reconnection events. The IMP wave observations in the distant magnetosphere boundary region showed that the boundaries were quite diffuse and frequently purely defined. Finally, nearly simultaneous IMP 7 and 8 comparisons were used to achieve an understanding of shock structure and propagation.

Scarf, F. L.↗
NASA NTRS1980

Primary energy release

The physical processes by which the magnetic energy of a solar active region is converted to other forms of energy in the appearance of a solar flare are discussed. Observations of the secondary manifestations of flare energy release, such as thermal plasmas and energetic particle emissions, are presented, with particular attention given to the temporal variations of flare radiation, the various forms of energy release, flare energy density, flare locations and sizes, energy distributions and H alpha, hard X-ray and microwave burst events. Current models of the primary energy release process are surveyed, and the models of Spicer (1976, 1977), which explains rapid flare energy release in terms of multiple tearing modes causing reconnection in sheared magnetic fields, and Uchida and Sakurai (1976, 1978), which attributes primary energy release to dynamic collapse caused by the interchange instability of the neutral sheet, are examined in detail.

Kahler, S.↗
NASA NTRS1980

Dynamic spectral characteristics of thermal models for solar hard X-ray bursts

It is proposed that the source for solar hard X-ray bursts may consist of a distribution of many small impulsively-heated kernels, each cooled by anomalous conduction, with lifetimes shorter than current burst data temporal resolution. The dynamic spectra of bursts are governed by the dynamic evolution of the kernel production process, such as magnetic-field dissipation in the tearing mode. An integral equation is formulated, the solution of which yields information on this kernel production process from dynamic burst spectra, for any kernel model.

Brown, J. C.↗
NASA NTRS1979

Magnetospheric reconnection, substorms, and energetic particle acceleration

The steady state reconnection model of the terrestrial magnetosphere predicts a maximum potential drop of about 100 kV across the tail. During substorms particles are accelerated to energies above 1 MeV. At substorm onset, large inductive emfs may be generated by explosive tearing mode reconnection which is driven nonlinearly unstable by the solar wind and convection stresses on the tail plasma sheet. In the inner magnetosphere, energetic particles are also produced by stochastic wave turbulent acceleration and by convection driven inward radial diffusion.

Coroniti, F. V.↗
NASA NTRS1980

The stability properties of cylindrical force-free fields - Effect of an external potential field

A large-scale potential field with an embedded smaller-scale force-free structure gradient x B equals alpha B is studied in cylindrical geometry. Cases in which alpha goes continuously from a constant value alpha 0 on the axis to zero at large r are considered. Such a choice of alpha (r) produces fields which are realistic (few field reversals) but not completely stable. The MHD-unstable wavenumber regime is found. Since the considered equilibrium field exhibits a certain amount of magnetic shear, resistive instabilities can arise. The growth rates of the tearing mode in the limited MHD-stable region of k space are calculated, showing time-scales much shorter than the resistive decay time.

Chiuderi, C.↗
NASA NTRS1981

Magnetospheric substorm models - Comparison with neutral sheet magnetic field observations

Four models for geomagnetic substorms, a quiet tail model, and models incorporating structural effects of the tail are examined for consistency with magnetic-field data during satellite crossings of the tail neutral sheet/plasma sheet. For this data the tearing mode instability model is always consistent, and inward moving distant neutral line is sometimes consistent, quasi-steady reconnection with slow shock and intermediate wave structure and locally quiet tail rarely consistent, and an outward propagating rarefaction wave is never consistent with the magnetic observations. In several cases structural effects of the tail are consistent with key features of the magnetic signatures.

Speiser, T. W.↗