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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

Microinstabilities in the Gasdynamic Mirror Propulsion System

The gasdynamic mirror has been proposed as a concept which could form the basis of a highly efficient fusion rocket engine. Gasdynamic mirrors differ from most other mirror type plasma confinement schemes in that they have much larger aspect ratios and operate at somewhat higher plasma densities. There are several types of instabilities which are known to plague mirror type confinement schemes. These instabilities fall into two general classes. One class of instability is the Magnetohydrodynamic or MHD instability which induces gross distortions in the plasma geometry. The other class of instability is the "loss cone" microinstability which leads to general plasma turbulence. The "loss cone" microinstability is caused by velocity space asymmetries resulting from the loss of plasma having constituent particle velocities within the angle of the magnetic mirror "loss cone." These instabilities generally manifest themselves in high temperature, moderately dense plasmas. The present study indicates that a GDM configured as a rocket engine might operate in a plasma regime where microinstabilities could potentially be significant.

Emrich, William↗
NASA NTRS2005

Microinstabilities in the Gasdynamic Mirror Propulsion System

The gasdynamic mirror has been proposed as a concept which could form the basis of a highly efficient fusion rocket engine. Gasdynamic mirrors differ from most other mirror type plasma confinement schemes in that they have much larger aspect ratios and operate at somewhat higher plasma densities. There are several types of instabilities which are known to plague mirror type confinement schemes. These instabilities fall into two general classes. One class of instability is the Magnetohdrodynamic or MHD instability which induces gross distortions in the plasma geometry. The other class of instability is the "loss cone" microinstability which leads to general plasma turbulence. The "loss cone" microinstability is caused by velocity space asymmetries resulting from the loss of plasma having constituent particle velocities within the angle of the magnetic mirror "loss cone." These instabilities generally manifest themselves in high temperature, moderately dense plasmas. The present study indicates that a GDM configured as a rocket engine might operate in a plasma regine where microinstabilities could potentially be significant.

Emrich, William↗
NASA NTRS1971

Ion-acoustic instability of a two-temperature, collisional, fully-ionized plasma.

From a perturbation analysis of the fluid equations for a homogeneous unmagnetized plasma, it is shown that long wavelength ion waves are unstable when the electron temperature exceeds the ion temperature. Thus, the temperature difference can drive a resistive-type ion wave instability in a plasma which on a collisionless basis is stable. The additional destabilizing effect of a current is also investigated.

Rognlien, T. D.↗
NASA NTRS1987

A linear MHD instability analysis of solar mass ejections with gravitation

The linear MHD instability of a cylindrical plasma is used to investigate the origin of solar mass ejections, and the dispersion relation is solved numerically. The initial plasma-flow velocity is found to have a significant effect on the instability criteria and growth rate, and the instability growth-rate is shown to be larger in cases where plasma flow exists, relative to the static case. Results suggest that the plasma column may break into small pieces. Assuming a thin-tube approximation, gravity is found to have little effect on the instability of quasi-horizontal ejection, but to have considerable effect on the vertical ejection. In considering the gravitational force, an exact analytical solution is found for the vertical case, while asymptotic solutions are given for the horizontal and oblique cases.

Song, M. T.↗
NASA NTRS1979

A comparison of intense electrostatic waves near f-UHR with linear instability theory

Intense electrostatic waves near the upper hybrid resonance frequency (f-UHR) have recently been detected near but beyond the plasmapause between + or -50 deg magnetic latitude at all local times. The paper shows that the electrostatic waves observed with the ISEE 1 plasma wave receiver near f-UHR are in qualitative agreement with linear theory of multicyclotron harmonic emissions. The peak amplitudes and bandwidths observed are similar to those calculated for maximum spatial growth rates. Measurements on Hawkeye of similar waves suggest that the electric field polarization is also consistent with theory. For a wide range of plasma parameters, nonconvective instability or large spatial growth rates occur within the cyclotron band encompassing the cold upper hybrid frequency.

Kurth, W. S.↗
NASA NTRS1988

A mirror instability associated with newly created ions in a moving plasma

Using a plasma model in which the ambient magnetic field is parallel to the z axis and the wave vector, an instability that resembles the usual hydromagnetic mirror instability is demonstrated. The instability is caused by freshly created ions in a moving plasma; the source of its free energy is associated with the ring distribution of the newly created ions, resulting in a high ion kinetic temperature in the direction transverse to the ambient magnetic field. The results of a stability analysis indicate that this instability can lead to the amplification of magnetosonic waves.

Wu, C. S.↗
NASA NTRS1983

Computations of the current profile influence on tearing instability

Results of computations of tearing instability of a plasma column in a strong longitudinal magnetic field are presented for different current profiles. The power current profiles j=j sub 0(1-(r/a)(P))(nu) are considered and analyzed for various values of the parameters p and v. The tearing instability of the smoothed two-step profiles is studied. Optimal smoothed two-step profiles are constructed which are stable with respect to all helical largescale perturbations.

Blekher, P. M.↗
NASA NTRS1984

Voyager 1 evidence for ion-cyclotron instability in the vicinity of the Io plasma Torus

The Voyager 1 traversal of the Io plasma torus was marked by pronounced enhancement in low frequency wave activity with especially strong wave intensification below the proton gyrofrequency, suggesting the excitation of ion-cyclotron waves over the same radial range in which the energetic ring current ions exhibit phase space density depletion due to strong pitch angle scattering loss to the atmosphere. If the more intense emissions observed intermittently by Voyager 1 are representative of waves in the high latitude source region, the resulting pitch angle scattering would be sufficiently rapid to explain the observed energetic ion losses and the excitation of intense Jovian auroral emissions of field lines passing through the torus.

Thorne, R. M.↗
NASA NTRS2008

The Formation of Coronal Loops by Thermal Instability in Three Dimensions

Plasma loops in solar active regions have been observed in EUV and soft X-rays for decades. Their formation mechanism and properties, however, are still not fully understood. Predictions by early models, based on 1D hydrostatic equilibria with uniform plasma heating, are not consistent with high-resolution measurements. In this Letter, we demonstrate, via 3D simulations, that a class of heating models can lead to the dynamic formation of plasma loops provided the plasma is heated sufficiently to match SXT soft X-ray measurements. We show that individual flux tubes in a 3D magnetic structure tend to stand out against their neighbors. The loops have large aspect ratios and nearly uniform cross sections in the corona, similar to those observed by EIT and TRACE. The coronal EUV emission from these thermally unstable solutions is roughly consistent with EIT measurements. The solution oscillates in time through a large-amplitude, nonlinear cycle, leading to repeated brightening and fading of the loops.

Mok, Yung↗
NASA NTRS1983

Numerical simulations of loops heated to solar flare temperatures. I - Gasdynamics. II - X-ray and UV spectroscopy

The NRL's Dynamic Flux Tube Model is used to numerically simulate the dynamic response of a coronal magnetic loop to an energy input of the order encountered in solar flares. The coronal plasma is heated by the deposition of flare energy at the top of the loop to more than 10 million K, yielding a conduction front that moves toward the chromosphere, where the plasma is heated by the large downward conductive flux and ablates upward to the coronal part of the loop at velocities of a few hundred km/sec. The conduction front simultaneously produces chromospheric ablation and compresses the material ahead of it. With the aid of compressional instabilities, the compressed plasma grows throughout the flare heating phase, presenting a possible source of the flare optical continuum emission which is correlated with soft X-ray radiation. The observational consequences of rapidly heated loop gas dynamic processes are discussed. In the second part of this presentation, the dynamical calculation results previously obtained are used to predict the spectral line intensities, profiles and wavelengths of several X-ray lines and the UV line of Fe XXI at 1354.1 A. Three different viewing orientations of the loop are considered.

Cheng, C.-C.↗
NASA NTRS1972

Instability of ring current protons beyond the plasmapause during injection events.

The stability of ring current protons with an injection spectrum modeled by a loss cone distribution function is examined for typical ring current parameters. It is found that a quasi-electrostatic ion loss cone mode can be excited with frequencies just below and growth rates of the order of 0.01 times the ion plasma frequency. The instability is strongest in the moderate beta about equal to 1, low-density region just outside the plasmapause; for the beta much greater than 1 auroral regions and the high-density plasmasphere the mode is nearly stable. For the same ring current parameters the electromagnetic ion cyclotron wave is almost nonconvectively unstable, with growth rates of the order of 0.1 times the ion cyclotron frequency. The combination of the two unstable modes results in a large quasi-linear diffusion coefficient throughout most of the proton velocity space. Unless it is maintained by rapid inward convection, the ring current injection anisotropy will be reduced by diffusion toward the loss cone on time scales short in comparison to the minimum precipitation lifetime.

Coroniti, F. V.↗
NASA NTRS1987

The emissions of broadband electrostatic noise in the near vicinity of the Shuttle Orbiter

Measurements of the Space Shuttle environment from the STS-3 and Spacelab 2 missions indicate the presence of oblique ion streams and broadband electrostatic noise. A two-dimensional theoretical model is applied to study a possible causal relationship between the ion streams and the broadband noise, especially in terms of the ion acoustic wave and ion-ion wave modes. This model predicts the generation of waves with frequencies ranging from the ion cyclotron frequency up to values greater than the ion plasma frequency, with the maximum growth rate occurring in the 10-kHz range. These results are consistent with the observational data from the STS-3 mission. The model also shows that these two wave modes can coexist only when the wave vectors of the two wave modes are nearly perpendicular. The parametric dependence of the wave instabilities on the plasma parameters and the inclination of the wave propagation vector is also studied.

Hwang, K. S.↗
NASA NTRS1986

General Bohm and Langmuir conditions for a strong double layer in space plasmas

The appropriate Bohm and Langmuir conditions when there are three beam-type particles passing through the double layer in addition to the usual two species of trapped particles are derived. It is shown that the Langmuir conditions, which normally imply a net current through the double layer, are consistent with zero net current when the class of accreting ions is included. It is also found that the Bohm criterion is quite different from its usual form, and that it has nothing to do with instabilities of the plasma at some critical level, as is believed to be true.

Williams, A. C.↗
NASA NTRS1985

Plasma heating at collisionless shocks due to the kinetic cross-field streaming instability

Heating at collisionless shocks due to the kinetic cross-field streaming instability, which is the finite beta (ratio of plasma to magnetic pressure) extension of the modified two stream instability, is studied. Heating rates are derived from quasi-linear theory and compared with results from particle simulations to show that electron heating relative to ion heating and heating parallel to the magnetic field relative to perpendicular heating for both the electrons and ions increase with beta. The simulations suggest that electron dynamics determine the saturation level of the instability, which is manifested by the formation of a flattop electron distribution parallel to the magnetic field. As a result, both the saturation levels of the fluctuations and the heating rates decrease sharply with beta. Applications of these results to plasma heating in simulations of shocks and the earth's bow shock are described.

Winske, D.↗
NASA NTRS1993

On the instability and energy flux of lower hybrid waves in the Venus plasma mantle

Waves generated near the lower hybrid resonance frequency by the modified two stream instability have been invoked as a possible source of energy flux into the topside ionosphere of Venus. These waves are observed above the ionopause in a region known as the plasma mantle. The plasma within the mantle appears to be a mixture of magnetosheath and ionospheric plasmas. Since the magnetosheath electrons and ions have temperatures of several tens of eV, any instability analysis of the modified two stream instability requires the inclusion of finite electron and ion temperatures. Finite temperature effects are likely to reduce the growth rate of the instability. Furthermore, the lower hybrid waves are only quasi-electrostatic, and the energy flux of the waves is mainly carried by parallel Poynting flux. The magnetic field in the mantle is draped over the ionopause. Lower hybrid waves therefore cannot transport any significant wave energy to lower altitudes, and so do not act as a source of additional heat to the topside ionosphere.

Strangeway, R. J.↗