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At least 199 records · Page 11

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NASA NTRS1980

Numerical studies of new stellarator concepts

A three-dimensional computer code has been developed to study the magnetohydrodynamic equilibrium and stability of a diffuse or sharp boundary plasma in toroidal geometry. It is shown how equilibria with net toroidal current identically zero can be determined and how growth rates of instabilities can be calculated. Applications are made to an l = 2, 3 stellarator configuration that offers the possibility of achieving a critical value as high as 10% for the plasma parameter

Bauer, F.↗

NASA NTRS1981

Adiabatic and nonadiabatic processes in thermal models of solar hard X-ray bursts

The temporal evolution of thermal-bremsstrahlung-emitting material at the top of a solar coronal loop under various processes is examined as a model of solar hard X-ray bursts. The behavior of emission measure and temperature with time is analyzed for the hot plasma confined in a segment at the top of a loop, taking into account the effects or the time variation of the confining toroidal magnetic field, the longitudinal expansion of the source through the motion of a pair of collisionless conduction fronts and unspecified heating or cooling processes. Analytic expressions for the behavior of the toroidal magnetic field and the heating rate necessary to reproduce a given behavior of emission measure and temperature with time are obtained in terms of the initial magnetic field strength and density of the heated region and a characteristic longitudinal expansion time. Results are applied to observations of two impulsive solar hard X-ray bursts, and it is shown that the contribution of heating is negligible after the first few seconds of the events, although significant nonadiabatic heating occurs early in the events.

Emslie, A. G.↗

NASA NTRS1981

Solutions of the Helmholtz equation with boundary conditions for force-free magnetic fields

It is shown that the solution, with one ignorable coordinate, for the Taylor minimum energy state (resulting in a force-free magnetic field) in either a straight cylindrical or a toroidal geometry with arbitrary cross section can be reduced to the solution of either an inhomogeneous Helmholtz equation or a Grad-Shafranov equation with simple boundary conditions. Standard Green's function theory is, therefore, applicable. Detailed solutions are presented for the Taylor state in toroidal and cylindrical domains having a rectangular cross section. The focus is on solutions corresponding to the continuous eigenvalue spectra. Singular behavior at 90 deg corners is explored in detail.

Rasband, S. N.↗

NASA NTRS1981

Fluctuations and turbulence in an electric field Bumpy Torus plasma

Fluctuation characteristics of plasma number density and electrostatic potential below the ion plasma and ion cyclotron frequencies in an electric field Bumpy Torus plasma were investigated experimentally, using digitally implemented spectral analysis techniques. The toroidal plasma was biased to high potentials by applying positive or negative voltage to electrodes located in the midplanes of two sectors of the toroidal array. The plasma was observed to be biased to 80 or 90% of the potential on the midplane electrodes, regardless of polarity. The radial electric field exceeded 1 kV/cm at the plasma boundary and penetrated inward to at least one-half of the plasma radius. When the imposed radial electric fields reached values characteristic of the experiment, the E/B drift velocities were comparable to the particle thermal velocities. The amplitude statistics of both the density and the potential fluctuations were found to be Gaussian for the most part, with near-zero skewness and a kurtosis of about 3.0. The spectral index of the density and potential fluctuations ranged from 2 to 6. The higher frequency components were found to propagate faster than the E/B drift velocity, which is the characteristic speed of the lower-frequency components.

Roth, J. R.↗

NASA NTRS1982

Non-axisymmetric shapes of a rotating drop in an immiscible system

The nonaxisymmetric shapes of a rotating drop in an immiscible system were studied. Five basic families of shapes (axisymmetric, two-lobed, three-lobed, four-lobed, and toroidal) were observed. The sequence (axisymmetric to two-lobed to three-lobed to four-lobed to toroidal) seems to be linked to increasing spin-up velocity. For the axisymmetric case, direct comparisons of experiments with the theory of a free rotating drop were surprisingly good the equatorial area differs from theory by only 30%. Furthermore, the non-axisymmetric shapes are in good qualitative agreement with the theory, although the theory does not address the presence of an outer fluid.

Wang, T. G.↗

NASA NTRS1982

Spherical harmonic representation of the main geomagnetic field for world charting and investigations of some fundamental problems of physics and geophysics

Progress in the harmonic analysis of MAGSAT data is reported. Single-day data sets were subdivided into information on the sunrise side of the Earth and information on the sunset side of the Earth. Data for the main and external fields each demonstrate a clear and consistent systematic difference between the sets of data which was determined to be, due to ionospheric currents which differ from the sunset to the sunrise terminator. A toroidal field was analyzed for and determined to be an apparent toroidal field resulting from electric currents concentrated in the two terminators. Progressive elimination of auroral zone data demonstrates that the information presented does not arise from complications due to Birkeland currents.

Barraclough, D. R.↗

NASA NTRS1981

High-sensitivity circular polarimetry of the Sun, 0.5 - 1.7 microns

Using an isometric (symmetrical) photoelastic-modulator polarimeter and a Cassegrain telescope, broadband polarization measures of the Sun were obtained with unprecedented sensitivity. With an instrumental background of x 10 to the minus 6th power differential effects delta q = V/I of 1 x 10 to the minus 7th power were detectable. Some results: (1) the lambda dependence of broadband circular polarization in spots was observed out to 1.7 mu, extending the visible-light measurements of Illing, Landman, and Mickey. The q drops to of the order of 1 x 10 to the minus 5 power at 1.2 mu (in a strong spot), then rises at 1.6 mu. While molecules or other special mechanisms must play a role in the visible-light polarization, at lambda 1.5 mu the q values seem consistent with gray magneto-opacity. (2) By surveying inactive regions upper limits are set to broadband polarization due to global or deep-seated magnetic fields. For a double-toroidal pattern a differential measure is found, q sub b = (q sub ne + q sub sw - q sub nw - q sub - se) 14, at 1.7 mu, of (+1.2 + or = 0.7) x 10 to the minus 7th power a mean from three series in 1981. Based on a simple estimate by L. Biermann this would correspond to deep-down toroidal fields of or approx 2000 gauss. Extension of this work are discussed.

Kemp, J. C.↗

NASA NTRS1983

Thin, rotating plasma disks

The MHD equilibrium of thin, rotating plasma disks is considered. An axisymmetric steady state is assumed. A corotation lag is included, as is a nonzero toroidal field with Bphi varies as rBr. Rigorous expressions are obtained for a characteristic Alfven Mach number and temperature, both of which involve vertical averages. If there is strict corotation, the temperature always diverges at large radii. If there is a corotation lag, the temperature need not diverge, and the magnetodisk solutions can extend to arbitrarily large radii. With weak additional assumptions, it is shown that M sub A is less than 2 exp 1/2 for a mainly poloidal field and M sub A is less than unity for a mainly toroidal field. For the Jovian magnetodisk, M sub A = 0.8 from Voyager 1 inbound data, and the observation of a temperature nearly constant or slowly decreasing with r from 20 R sub J to 80 R sub J on the outbound pass implies a corotation lag there.

Cheng, A. F.↗

NASA NTRS1985

Rapid Adhesive Bonding for Metals and Composites

Toroidal induction heater is key element in rapid bonding technique. Rapid adhesive bonding equipment features toroid induction heater for quick bonding of composites and metals. Process avoids damaging composites that cannot withstand high temperatures for long time intervals. Process scaled up for production. Technique suited to bonding of end tabs onto material test specimens.

Stein, B. A.↗

NASA NTRS1984

The evaluation of a deformable diffraction grating for a stigmatic EUV spectroheliometer

A second toroidal replica grating was found to have an aspect ratio within 10% of the desire value. The aspect ratio of replica toroidal gratings is coming out somewhat low. Whether this is caused by some relaxation in the epoxy or some residual error in the setup when the replication is made from the deformable submaster grating will be determined. The first imaging tests at EUV wavelengths near 500 A were carried out. The initial results are most encouraging with blur sizes of the order of, or less than, 40 microns. Further focussing and additional photographic tests will be carried out, and preparations made for the first photoelectric measurements of the image quality using the (256 x 1024)-pixel MAMA detection system. The visible-light (256 x 1024)-pixel MAMA detector tube has completed initial evaluation.

Timothy, J. G.↗

NASA NTRS1984

The evaluation of a deformable diffraction grating for a stigmatic EUV spectroheliometer

Two crucial areas in the development of the high resolution EUV spectroheliometer are the elastically deformable toroidal diffraction grating and the open-structure imaging pulse counting detector system. The optical performance of a high frequency toroidal grating at EUV wavelengths was verified with a 3600 groove/mm master grating on a spherical concave blank. An open-structure (256 x 1024) pixel, multi-anode microchannel array was developed and tested as a pulse counting detector system. These two systems verify the proof of concept of the spectroheliometer design.

Timothy, J. G.↗

NASA NTRS1986

Field line twist and field-aligned currents in an axially symmetric equilibrium magnetosphere

Field-aligned Birkeland currents and the angle of the magnetic line twist were calculated for an axially symmetric pole-on magnetosphere (assumed to be in MHD equilibrium). The angle of the field line twist was shown to have a strong radial dependence on the axisymmetric magnetotail as well as on the ionospheric conductivity and the amount of thermal plasma contained in closed magnetotail flux tubes. The field line twist results from the planetary rotation, which leads to the development of a toroidal magnetic B-sub-phi component and to differentially rotating magnetic field lines. It was shown that the time development of the toroidal magnetic B-sub-phi component and the rotation frequency are related through an induction equation.

Voigt, Gerd-Hannes↗

NASA NTRS1987

Mercury's magnetic field - A thermoelectric dynamo?

Permanent magnetism and conventional dynamo theory are possible but problematic explanations for the magnitude of the Mercurian magnetic field. A new model is proposed in which thermoelectric currents driven by temperature differences at a bumpy core-mantle boundary are responsible for the (unobserved) toroidal field, and the helicity of convective motions in a thin outer core (thickness of about 100 km) induces the observed poloidal field from the toroidal field. The observed field of about 3 x 10 to the -7th T can be reproduced provided the electrical conductivity of Mercury's semiconducting mantle approaches 1000/ohm per m. This model may be testable by future missions to Mercury because it predicts a more complicated field geometry than conventional dynamo theories. However, it is argued that polar wander may cause the core-mantle topography to migrate so that some aspects of the rotational symmetry may be reflected in the observed field.

Stevenson, D. J.↗

NASA NTRS1988

Ray tracing optical analysis of offset solar collector for space station solar dynamic system

OFFSET, a detailed ray tracing computer code, was developed at NASA Lewis Research Center to model the offset solar collector for the Space Station solar dynamic electric power system. This model traces rays from 50 points on the face of the Sun to 10 points on each of the 456 collector facets. The triangular facets are modeled with spherical, parabolic, or toroidal reflective surface contour and surface slope errors. The rays are then traced through the receiver aperture to the walls of the receiver. Images of the collector and of the Sun within the receiver produced by this code provide insight into the collector receiver interface. Flux distribution on the receiver walls, plotted by this code, is improved by a combination of changes to aperture location and receiver tilt angle. Power loss by spillage at the receiver aperture is computed and is considerably reduced by using toroidal facets.

Jefferies, Kent S.↗

NASA NTRS1988

The dynamical regime of fluid flow at the core surface

An alternative method for determining the fluid motion immediately beneath the core-mantle boundary is presented which is based on solving the full nonlinear core motions problem. This method is used to examine three dynamical hypotheses about the flow: (1) the steady motions hypothesis; (2) the geostrophic hypothesis; and (3) the toroidal flow hypothesis. Better fits to the field are obtained with the toroidal flows than with geostrophic flows, casting considerable doubt on the validity of the geostrophic hypothesis. Additionally, some indication is found that failure of the frozen-flux approximation, a concomitant assumption, may be a serious obstacle to obtaining reliable maps of the core fluid flow.

Bloxham, Jeremy↗

NASA NTRS1988

Driven, steady-state RFP computations

The pseudospectral three-dimensional MHD code of Dahlburg et al. (1986 and 1987) is used to compute the dynamical behavior of a channel of magnetofluid carrying an axial current and magnetic flux. This situation contains the essential MHD behavior of the reversed-field pinch (RFP). An externally imposed electric field is applied to an initially current-free magnetofluid and drives currents that rise and eventually fluctuate about values corresponding to pinch ratios Theta of about 1.3, 2.2, and 4.5. A period of violent turbulence leads to an approximately force-free core, surrounded by an active MHD boundary layer that is not force-free. A steady state is reached that can apparently be sustained indefinitely (for several hundred Alfven transit times or longer). The turbulence level and time variability in the steady state increase with increasing Theta. The average toroidal magnetic field at the wall reverses for Theta = 2.2 and 4.5, but not for Theta = 1.3. Negative toroidal current filaments are observed. The Lundquist numbers are of the order of a few hundred.

Dahlburg, J. P.↗

NASA NTRS1988

Ray tracing optical analysis of offset solar collector for Space Station solar dynamic system

OFFSET, a detailed ray tracing computer code, was developed at NASA Lewis Research Center to model the offset solar collector for the Space Station solar dynamic electric power system. This model traces rays from 50 points on the face of the sun to 10 points on each of the 456 collector facets. The triangular facets are modeled with spherical, parabolic, or toroidal reflective surface contour and surface slope errors. The rays are then traced through the receiver aperture to the walls of the receiver. Images of the collector and of the sun within the receiver produced by this code provide insight into the collector receiver interface. Flux distribution on the receiver walls, plotted by this code, is improved by a combination of changes to aperture location and receiver tilt angle. Power loss by spillage at the receiver aperture is computed and is considerably reduced by using toroidal facets.

Jefferies, Kent S.↗

NASA NTRS1990

A statistical study of Pc 3-5 pulsations observed by the AMPTE/CCE magnetic fields experiment. I - Occurrence distributions

A classification system based on dynamic power spectra of AMPTE/CCE magnetic field data has been developed which allows semiquantitative specification of pulsation activity at all times. Fifteen months of 6.24-s AMPTE/CCE magnetic field data have been analyzed using this scheme to generate a comprehensive data base of all pulsation activity observed from L = 5 to L = 9 at all local times in the frequency range 0-80 mHz. The pulsation divide naturally into five basic categories: toroidal fundamental mode resonances, toroidal harmonic resonances, compressional low-frequency waves, radially polarized waves, and disturbed intervals. Additional distinctions among members of these classes are made to arrive at 14 categories which describe 95 percent of the pulsations observed. Normal spatial distributions in local time and L shell are determined for each category. Magnetic latitude distributions are determined for four coherent categories of activity.

Anderson, B. J.↗