Engineering topics

Singh, Nagendra

Publications and source records attributed to Singh, Nagendra.

61 records · Page 4

NASA NTRS1987

Electric fields and double layers in plasmas

Various mechanisms for driving double layers in plasmas are briefly described, including applied potential drops, currents, contact potentials, and plasma expansions. Some dynamical features of the double layers are discussed. These features, as seen in simulations, laboratory experiments, and theory, indicate that double layers and the currents through them undergo slow oscillations which are determined by the ion transit time across an effective length of the system in which double layers form. It is shown that a localized potential dip forms at the low potential end of a double layer, which interrupts the electron current through it according to the Langmuir criterion, whenever the ion flux into the double is disrupted. The generation of electric fields perpendicular to the ambient magnetic field by contact potentials is also discussed. Two different situations were considered; in one, a low-density hot plasma is sandwiched between high-density cold plasmas, while in the other a high-density current sheet permeates a low-density background plasma. Perpendicular electric fields develop near the contact surfaces. In the case of the current sheet, the creation of parallel electric fields and the formation of double layers are also discussed when the current sheet thickness is varied. Finally, the generation of electric fields and double layers in an expanding plasma is discussed.

Singh, Nagendra↗

NASA NTRS1987

Electric fields and double layers in plasmas

Various mechanisms for driving double layers (DLs) in plasmas are described, including applied potential drops, currents, contact potentials, and plasma expansions. Somne dynamic features of the DLs are discussed; and it is demonstrated that DLs and the currents through them undergo slow oscillations, determined by the ion transit time across an effective length of the system in which the DLs form. It is shown that a localized potential dip forms at the low potential end of a DL, which interrupts the electron current through it according to the Langmuir criterion whenever the ion flux into the DL is disrupted. Also considered is the generation of electric fields perpendicular to the ambient magnetic field by contact potentials.

Singh, Nagendra↗

NASA NTRS1987

Simulations of auroral plasma processes - Electric fields, waves and particles

Plasma processes driven by current sheets of finite thicknesses in an ambient magnetized plasma are studied using a 2 1/2 dimensional particle-in-cell code, and similarities are found between simulated plasma processes and those observed in the auroral plasma. Current sheets are shown to be bounded by large perpendicular electric fields occurring near their edges above the conducting boundary. Shaped potential structures form when the current sheets are narrow, and when the current sheets are wide, potential structures develop a significant parallel potential drop such that the electrons are accelerated upwards. Downward parallel electric fields of variable strength are noted in the downward current region, and double layer formation is seen in both narrow and wide current sheets. High frequency oscillations near the electron plasma frequency and its harmonic are seen, and low frequency waves are observed.

Singh, Nagendra↗

NASA NTRS1986

Temporal features of the refilling of a plasmaspheric flux tube

The refilling of plasmaspheric flux tubes was studied by assuming that the protonosphere provides an ionospheric boundary where the H(+) density can be assumed; the supersonic flow in the flux tube is driven by the depletion of the plasma from the flux tube, while the base density and pressure in the protonosphere remain constant. The time-dependent continuity and momentum equations for the H(+) ions were solved. The electron gas was assumed to obey the Boltzmann law, and the proton gas was assumed to be isothermal. In agreement with the postulate of Banks et al. (1971), it was found that an important feature of the refilling is the formation of a shock pair at the equator; as the shocks propagate toward the ionosphere, the refilling occurs. Depending on the density at the ionospheric boundaries, a fair agreement was found between the refilling rates obtained for L = 6.6 and those from the GEOS 2 observations.

Singh, Nagendra↗

NASA NTRS1986

Plasma processes driven by current sheets and their relevance to the auroral plasma

Plasma processes dealing with ac and dc electric fields, the formation of ion beams and conics, and electron acceleration are considered, and similarities between simulation results and satellite-based observations are discussed. Electrostatic shock-type electric fields are found to occur near the current sheet edges, and double layers having upward electric fields form inside the sheet and are distinguishable from the large perpendicular electric fields only in wide sheets with thicknesses much greater than the ion Larmor radius. It is found that the most energetic ions have pitch angles near 90 deg, indicating a large perpendicular acceleration of the ions, and that the downward accelerating electrons inside the sheet are neither monoenergetic nor perfectly field aligned.

Singh, Nagendra↗

NASA NTRS1986

Numerical simulations of auroral plasma processes. I - Ion beams and conics. II - Electric fields

The characteristics of ion beams and conics as seen in two-dimensional numerical simulations in which the plasmas are driven by current sheets of a finite thickness are described. It is shown that the most energetic ions in the simulations have pitch angles near 90 deg, implying a large perpendicular acceleration of the ions. Large perpendicular electric fields, similar to those measured in electrostatic shocks, are mainly confined near the edges of the current sheets, where E-perpendicular is much larger than E-parallel. Double layers with E-perpendicular about equal to, and spatially separated from, E-parallel form in the interior of wide current sheets. Such double layers have upward electric fields in the region of the upward current. Downward electric fields develop outside the current sheet.

Singh, Nagendra↗

NASA NTRS1986

Ion acceleration in expanding ionospheric plasmas

Plasma expansion along the ambient magnetic field in regions of density gradients provides a mechanism for accelerating ions. A brief review of the basic phenomenon of plasma expansion is given. Estimates of the energies of the accelerated ions in an expanding ionospheric plasma along geomagnetic flux tubes are obtained by solving the time-dependent hydrodynamic equations. It is found that, over certain altitude ranges, each ion species can be the most energetic; the maximum energies of the different ions are found to be limited to less than about 10 eV for H(+), 5 eV for He(+), and less than about 1.5 eV for O(+).

Singh, Nagendra↗