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NASA NTRS1983

The effect of solar-wind convection on charged particle transport in interplanetary space

In the formulation of focused transport without convection, approximate solutions of the Boltzmann equation are described in terms of small perturbations of its two fundamental steady-state solutions. In the presence of convection, the same approach is applicable, provided that the analysis is carried out in a system moving with the solar wind. The approximate transport equations that apply in this system are very similar to those that apply in the absence of convection. The new equations assume that the wind blows in the direction of the guiding field and is constant in space and time. Nevertheless, these equations are more general than existing formulations, for they retain their validity when the solar wind velocity is large, and they describe the coherent modes of transport that appear when the mean free path is greater than 1 AU. An interesting implication of the new formulation is that coherent disturbances are swept along with the wind. In this respect, they differ from truly scatter-free modes, which are not affected by the wind.

Earl, J. A.↗

NASA NTRS1985

Shuttle vertical fin flowfield by the direct simulation Monte Carlo method

The flow properties in a model flowfield, simulating the shuttle vertical fin, determined using the Direct Simulation Monte Carlo method. The case analyzed corresponds to an orbit height of 225 km with the freestream velocity vector orthogonal to the fin surface. Contour plots of the flowfield distributions of density, temperature, velocity and flow angle are presented. The results also include mean molecular collision frequency (which reaches 1/60 sec near the surface), collision frequency density (approaches 7 x 10 to the 18/cu m sec at the surface) and the mean free path (19 m at the surface).

Hueser, J. E.↗

NASA NTRS1985

A mechanism for the local concentration enhancement of the shuttle atmosphere

Preliminary calculations suggest that collisions between instreaming atmospheric constituents and secondary backscattered molecules can generate unexpectedly large enhancements in neutral gas concentrations in the vicinity of the shuttle. This effect is a result of a rapid decrease in the mean free path length of the scattered components following the initial expected concentration enhancement. A study will be outlined for the theoretical investigation of this mechanism. The shape dependence of associated glow halos on vehicle configuration will be discussed.

Rantanen, R.↗

NASA NTRS1985

Plasma-gas interactions studies in a hybrid plume plasma rocket

Plasma-gas interaction was investigated and the basic mechanisms for energy and particle transport. The solution approach assumes cylindrical geometry and includes a multiplicity of atomic reactions, and the presence of a strong magnetic field is described. The principal reactions are electron and ion impact ionization, and charge exchange between hot ions and cold neutrals. Radial particle and energy transport is mainly by diffusion. A modified Bohm diffusion model for plasma in the core of the plume, and classical neutral particle diffusion in the cooler regions of the flow are presented. Neutrals are allowed to free stream in the low density regions, where the collision mean-free-path becomes comparable, or even larger than the characteristic dimensions of the system.

Chang, F. R.↗

NASA NTRS1986

Coronal and interplanetary propagation, interplanetary acceleration, cosmic-ray observations by deep space network and anomalous component

The purpose is to provide an overview of the contributions presented in sessions SH3, SH1.5, SH4.6 and SH4.7 of the 19th International Cosmic Ray Conference. These contributed papers indicate that steady progress continues to be made in both the observational and the theoretical aspects of the transport and acceleration of energetic charged particles in the heliosphere. Studies of solar and interplanetary particles have placed emphasis on particle directional distributions in relation to pitch-angle scattering and magnetic focusing, on the rigidity and spatial dependence of the mean free path, and on new propagation regimes in the inner and outer heliosphere. Coronal propagation appears in need of correlative multi-spacecraft studies in association with detailed observation of the flare process and coronal magnetic structures. Interplanetary acceleration has now gone into a consolidation phase, with theories being worked out in detail and checked against observation.

Ng, C. K.↗

NASA NTRS1985

Numerical modeling of the hybrid plume plasma rocket

This paper reports on ongoing research in the area of numerical simulations of hybrid plume dynamics, and describes the basic mechanisms for energy and particle transport. The solution approach assumes cylindrical geometry and includes a multiplicity of atomic reactions, as well as the presence of a strong magnetic field. The principal reactions are electron impact ionization, and resonant charge exchange between hot ions and cold neutrals. Radial particle and energy transport is mainly by diffusion. The present description includes a modified Bohm diffusion model for plasma in the core of the plume, and classical neutral particle diffusion in the cooler regions of the flow. Neutrals are allowed to 'free stream' in the low density regions, where the collision mean-free-path becomes comparable, or even larger than the characteristic dimensions of the system. A parameteric scan of the various regimes of interest has been completed and the most significant results are presented.

Chang, F. R.↗

NASA NTRS1986

The acceleration and propagation of solar flare energetic particles

A review of the most pertinent data on solar energetic particles is presented, and the implications of the data are discussed, taking into account radio emissions, hard X-rays, gamma rays, energy spectra and electron-proton correlations, chemical compositions, and isotopic and ionic compositions. The mechanisms of solar flare particle acceleration are considered along with solar flare particle spectra in interplanetary space. Attention is given to stochastic acceleration, shock acceleration, acceleration in direct electric fields, the mean free paths of solar electrons and protons in interplanetary space, and an illustration of the probable effect of adiabatic deceleration on the spectra of solar flare ions at the time of maximum.

Forman, M. A.↗

NASA NTRS1986

Time-dependent coronal shock acceleration of energetic solar flare particles

A global time-dependent model is presented for the coronal and interplanetary shock acceleration and propagation of energetic solar flare particles. The calculations are carried out to help prove that coronal shock acceleration of solar flare particles is responsible for energetic solar flare event data gathered in interplanetary space. The model is based on the theory of diffusive shock acceleration, and requires particle speeds to be much greater than bulk velocities. Also, sufficient scattering must occur upstream and downstream of the shock for the particle scattering mean free path to be smaller than the characteristic scale lengths, which causes the same particles to encounter the shock repeatedly. A spherically symmetric shock wave is assumed, which leads to the same emission configuration for impulsively and monoenergetically emitted particles. Consideration is given to acceleration by compression at the shock front, adiabatic deceleration in the divergent downstream flow, the temporal evolution of the shock and the three-dimensional geometry of the corona. The model is used to generate normalized proton omnidirectional distributions at 1 AU and at the shock front. The spectral exhibit trends similar to those in observational data, especially proton acceleration times and the proton distribution profiles at 1 AU.

Lee, M. A.↗

NASA NTRS1986

On the thermal properties of Martian fines

Theoretical models are developed for the thermal conductivity of grains smaller than 30 microns diameter and over 1 mm diameter in order to aid in the understanding of the thermal inertia of the surface regolith of Mars. The thermal inertia is a function of the thermal conductivity of the surface materials, and is discussed in terms of pore sizes below and above the mean free path of gas molecules (GMFP). The models are limited to surface deposits which consist of non-bonded materials at Martian surface pressures. The calculations indicate that a particulate surface with grains having pores smaller than the GMFP will be proportional to the pore or grain size. The thermal conductivity of a surface made of grains larger than the GMFP will be independent of the particle size. The results for the larger grain sizes is supported by available laboratory data. Further experiments are recommended in order to test the validity of the models for the smaller grain surfaces.

Jakosky, B. M.↗

NASA NTRS1986

InGaAs/InAlAs hot-electron transistor

Using InGaAs for the base and InAlAs for the emitter and collector barriers, the first hot-electron transistor in this material system is fabricated. It is shown that 1.6 percent of the injected hot electrons can be transported ballistically through a 0.3 micron thick In(0.53)Ga(0.47)As plus 800-A-thick InAlAs barrier layer at 77 K giving rise to an average mean free path of 920 A. An energy spread of 130 MeV was observed for the ballistic electrons injected at about 700 MeV above the thermal equilibrium conditions. The value of collector barrier heights measured are in reasonable agreement with those deduced independently from thermionic emission studies in InGaAs gate, InAlAs/InGaAs capacitor structures.

Reddy, U. K.↗

NASA NTRS1987

Effect of hydrogen on deformation structure and properties of CMSX-2 nickel-base single-crystal superalloy

Material used in this study was a heat of the alloy CMSX-2. This nickel-based superalloy was provided in the form of oriented single crystals, solutionized for 3 hrs at 1315 C. It was then usually heat treated as follows: 1050 C/16h/air cool + 850 C/48h/air cool. The resulting microstructure is dominated by cuboidal, ordered gamma precipitates with a volume fraction of about 75% and an average size of 0.5 microns. In brief, the most compelling hydrogen induced-changes in deformation structure are: (1) enhanced dislocation accumulation in the gamma matrix; and (2) more extensive cross-slip of superdislocations in the gamma precipitates. The enhanced dislocation density in gamma acts to decrease the mean free path of a superdislocation, while easier cross slip hinders superdislocation movement by providing pinning points in the form of sessile jobs. Both processes contribute to the increase of flow stress and the notable work hardening that occurs prior to fracture.

Dollar, M.↗

NASA NTRS1986

Focusing anisotropy of solar cosmic rays

Analysis of 50-MeV particle data from ICE and 1-GeV data from eight high-latitude neutron monitors of the February 16, 1984 solar cosmic ray event indicates that an exponential with scattering mean free path of greater than 2 AU and with index in the 1.3-1.7 range provides a better description of particle anisotropies than the first-order form, confirming the prediction from cosmic ray transport theories which incorporate the effect of adiabatic focusing into the usual pitch angle scattering formalism. The time profile of the event is largely a reflection of coronal transport processes, with good fits provided by a coronal diffusion coefficient of about 10 to the 18th sq cm/s and with velocity-dependent escape. The simple method of predicting the interplanetary scattering parameters has application where particle injection is nearly continuous, but where previously accelerated particles have filled the heliosphere with a large omnidirectional background.

Bieber, J. W.↗

NASA NTRS1986

Knudsen diffusion - The effect of small pore size and low gas pressure on gaseous transport in soil

The analytical principles and applications of the theory of Knudsen diffusion are reviewed, with emphasis on gas transport in the soils of planetary bodies. Knudsen diffusion occurs when the mean free path of diffusing gas molecules surpasses the size of the pores through which diffusion proceeds. The process is then dominated by collisions with the pore walls. Computational techniques for deriving the Knudsen coefficient for soils with a nonreentrant cross-section shape are reviewed, along with methods of deriving a coefficient for soils which permit both Knudsen and bulk diffusion. Sample calculations for three pore-size distributions are provided to illustrate the decrease in transport efficiency with increasingly smaller soil pore sizes.

Clifford, S. M.↗

NASA NTRS1986

Observations of the soft X-ray diffuse background at 0.1 keV

The results of a sounding rocket flight that observed the very soft X-ray diffuse background in a section of the northern Galactic hemisphere are presented. The ratio of the measured Be band count rates to the Wisconsin sky survey B band count rates is nearly constant over a 120 deg arc on the sky, strongly suggesting a common emission mechanism for both Be and B band X-rays. The mean free path is about seven times smaller for the Be band than for the B band. The present results imply that the neutral hydrogen column density between the emitting material and the earth varies by less than 2 x 10 to the 18th H I/sq cm over the observed region. It is concluded that such variations are unlikely unless the total intervening column density is less than a few times this value.

Bloch, J. J.↗

NASA NTRS1987

Properties of a large-scale interplanetary loop structure as deduced from low-energy proton anisotropy and magnetic field measurements

Correlated particle and magnetic field measurements by the ISEE 3 spacecraft are presented for the loop structure behind the interplanetary traveling shock event of Nov. 12, 1978. Following the passage of the turbulent shock region, strong bidirectional streaming of low-energy protons is observed for approximately 6 hours, corresponding to a loop thickness of about 0.07 AU. This region is also characterized by a low relative variance of the magnetic field, a depressed proton intensity, and a reduction in the magnetic power spectral density. Using quasi-linear theory applied to a slab model, a value of 3 AU is derived for the mean free path during the passage of the closed loop. It is inferred from this observation that the proton regime associated with the loop structure is experiencing scatter-free transport and that either the length of the loop is approximately 3 AU between the sun and the earth or else the protons are being reflected at both ends of a smaller loop.

Tranquille, C.↗

NASA NTRS1987

Temperature dependence (4K to 300K) of the electrical resistivity of methane grown carbon fibers

Experimental measurements of the electrical resistivity vs temperature of methane vapor grown carbon fibers are presented. The fibers are heat treated from 1100 C (as-grown) to 3000 C. Data are fit to a standard two band model, which yields values for boundary scattering limited electron mobility, in-plane mean free path, energy band overlap, and total carrier density. The data are also fit to an ellipsoidal band model, where data fits yield effective masses, band overlap, Fermi velocity, phonon contributions to scattering, and ionized impurity scattering rates.

Woollam, John A.↗

NASA NTRS1987

Particle acceleration at shocks - A Monte Carlo method

A Monte Carlo method is presented for the problem of acceleration of test particles at relativistic shocks. The particles are assumed to diffuse in pitch angle as a result of scattering off magnetic irregularities frozen into the fluid. Several tests are performed using the analytic results available for both relativistic and nonrelativistic shock speeds. The acceleration at relativistic shocks under the influence of radiation losses is investigated, including the effects of a momentum dependence in the diffusion coefficient. The results demonstrate the usefulness of the technique in those situations in which the diffusion approximation cannot be employed, such as when relativistic bulk motion is considered, when particles are permitted to escape at the boundaries, and when the effects of the finite length of the particle mean free path are important.

Kirk, J. G.↗

NASA NTRS1988

Ballistic transport in II-VI semiconductor compounds and alloys

Realistic band structures are used in calculating the group velocity and scattering rates for electrons with injection energies up to 1 eV in ZnTe, CdTe, and the low-effective-mass alloy Hg(0.7)Cd(0.3)Te. Scattering from longitudinal optical phonons, ionized impurities, and alloy disorder have been included in the full band-structure calculation, which automatically includes both intra- and intervalley scattering. Of the II-VI materials considered, at 77 K HgCdTe is superior for low injection energies (up to 0.25 eV) while CdTe is superior at higher injection energies (1 eV) at room temperature. The attainable mean free paths (equal to or greater than 1000 A) and group velocities (equal to or greater than 10 to the 8th cm/s) for both systems are comparable to values found in III-V systems.

Berding, M. A.↗