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Chen, J.

Publications and source records attributed to Chen, J..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Fractal boundaries in magnetotail particle dynamics

It has been recently established that particle dynamics in the magnetotail geometry can be described as a nonintegrable Hamiltonian system with well-defined entry and exit regions through which stochastic orbits can enter and exit the system after repeatedly crossing the equatorial plane. It is shown that the phase space regions occupied by orbits of different numbers of equatorial crossings or different exit modes are separated by fractal boundaries. The fractal boundaries in an entry region for stochastic orbits are examined and the capacity dimension is determined.

Chen, J.↗
NASA NTRS1989

Development of efficient computer program for dynamic simulation of telerobotic manipulation

Research in robot control has generated interest in computationally efficient forms of dynamic equations for multi-body systems. For a simply connected open-loop linkage, dynamic equations arranged in recursive form were found to be particularly efficient. A general computer program capable of simulating an open-loop manipulator with arbitrary number of links has been developed based on an efficient recursive form of Kane's dynamic equations. Also included in the program is some of the important dynamics of the joint drive system, i.e., the rotational effect of the motor rotors. Further efficiency is achieved by the use of symbolic manipulation program to generate the FORTRAN simulation program tailored for a specific manipulator based on the parameter values given. The formulations and the validation of the program are described, and some results are shown.

Chen, J.↗
NASA NTRS1989

Collisionless tearing instability of a bi-Maxwellian neutral sheet - An integrodifferential treatment with exact particle orbits

The integrodifferential equation describing the linear tearing instability in the bi-Maxwellian neutral sheet is solved without approximating the particle orbits or the eigenfunction psi. Results of this calculation are presented. Comparison between the exact solution and the three-region approximation motivates the piecewise-straight-line approximation, a simplification that allows faster solution of the integrodifferential equation, yet retains the important features of the exact solution.

Burkhart, G. R.↗
NASA NTRS1989

Microwave time delays for the X/S-band feed system at X-band

The X-band time delays for the X/S feed system of the 34-meter high efficiency antenna are required to refine current X-band downlink (8450 plus or minus 50 MHz) calibrations and prepare for the Magellan uplink (7190 plus or minus 45 MHz). The time delays of elements in the X-band feed system are obtained mostly via computer calculation and partly via measurement. The methods used and results obtained are presented. There is good agreement with theory.

Chen, J.↗
NASA NTRS1989

Microwave time delays for the dual L-C-band feed system

A new dual-frequency feed system at Goldstone is designed to receive the Phobos spacecraft signal at L-band (1668 + or - 40 MHz) and transmit to the spacecraft at C-band (5008.75 + or - 5.00 MHz) simultaneously. Hence, calculations of the time delay from the C-band range calibration coupler to the phase center of the L-C dual feed and back to the L-band range calibration coupler are required to correct the range measurements. Time delays of the elements in the dual-frequency feed system are obtained mostly from computer calculations and partly from experimental measurements. The method used and results obtained are described.

Chen, J.↗
NASA NTRS1989

The effects of gear reduction on robot dynamics

The effect of the joint drive system with gear reduction for a generic two-link system is studied. It is done by comparing the kinetic energy of such a system with that of a direct drive two-link system. The only difference are two terms involving the inertia of the motor rotor and gear ratio. Modifications of the equations of motion from a direct drive system are then developed and generalized to various cases encountered in robot manipulators.

Chen, J.↗
NASA NTRS1988

Cross-guide coupler modeling and design

Modeling of cross-guide couplers based on the theory of equivalent electric and magnetic dipoles of an aperture is described. Additional correction factors due to nonzero wall thickness and large aperture are also included in this analysis. Comparisons of the measured and calculated results are presented for cross-guide couplers with circular or cross-shaped coupling apertures. A cross-guide coupler was designed as a component of the C-band feed to support the Phobos mission.

Chen, J.↗
NASA NTRS1988

Contact regrowth technique for low-resistance nonalloyed contacts to the hot-electron transistor

A novel contact regrowth technique for the formation of extremely low nonalloyed ohmic contacts is reported. The successful demonstration of this technique is reported on an InGaAs/InAlAs hot-electron transistor device. For the investigated InGaAs-based structure, the regrown contacting scheme reported includes an In(0.53)Ga(0.47)As layer, an InAs/GaAs strained-layer superlattice, and an InAs cap, all heavily doped n type with Si. A very low specific contact resistance of 1.8 x 10 to the -7th ohm sq cm to the base layer is obtained. The higher current densities achieved in the transistor characteristics are in close agreement with calculations, and a contact model is presented explaining the poor results of conventional nonalloyed contacts.

Peng, C. K.↗
NASA NTRS1988

A quadratic-form analysis of the collisionless tearing mode

A method for studying collisionless tearing mode properties in cases where complicated orbit integrals must be evaluated is presented. The method is based on a quadratic form of a self-adjoint integrodifferential operator. The collisionless tearing instability in a non-Maxwellian neutral sheet is used to illustrate the advantages of the present method.

Chen, J.↗
NASA NTRS1988

Extremely low nonalloyed and alloyed contact resistance using an InAs cap layer on InGaAs by molecular-beam epitaxy

Extremely low alloyed and nonalloyed ohmic contact resistances have been formed on n-type InAs/In(0.53)Ga(0.47)As/In(0.52)Al(0.48)As structures grown on InP(Fe) by molecular-beam epitaxy. To insure the accuracy of the small contact resistances measured, an extended transmission line model was used to extrapolate contact resistances from test patterns with multiple gap spacings varying from 1 to 20 microns. For a 150-A-thick InAs layer doped to 2 x 10 to the 18th/cu cm and a 0.1-micron-thick InGaAs layer doped to 1 x 10 to the 18th/cu cm, a specific contact resistance of 2.6 x 10 to the -8th ohm-asterisk sq cm was measured for the nonalloyed contact, while a resistance less than 1.7 x 10 to the -8th ohm-asterisk sq cm is reported for the alloyed contact. Conventional Au-Ge/Ni/Au was used for the ohmic metal contact and alloying was performed at 500 C for 50 s in flowing H2. Using a thermionic field emission model, the barrier height at the InAs/InGaAs interface was calculated to be 20 meV.

Peng, C. K.↗
NASA NTRS1987

Characteristics of annealed p/n junctions between GaAs and Si(100)

Electrical characteristics of a GaAs(p)/Si(n) interface were determined from capacitance-voltage, current-voltage, and secondary ion mass spectroscopy (SIMS) measurements and compared to those on GaAs(p) epitaxial layers on GaAs(n) substrates. The comparison was made between the junctions as grown, and after an anneal at 850 C for 20 min in 10 percent forming gas under an As overpressure. For the GaAs/Si junction the ideality factor changed from 2 or larger to 1.5 and the apparent intercept voltage changed from 2.5 to 1.3 V after annealing. For the GaAs homojunction, the intercept voltage increased from 1.1 to 1.3 V. In addition, the excess current in the forward and reverse bias conditions dropped drastically in the heterojunction. No movement of the metallurgical junction was discernible to within the resolution capability of SIMS. The junction properties obtained by annealing suggest an atomic restructuring of the Si(100) interface during growth or annealing. These new results raise the possibility that the GaAs/Si interface can be made into an electrically viable junction and incorporated into active devices.

Unlu, M. S.↗
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 NTRS1986

A variational assimilation method for satellite and conventional data: Development of basic model for diagnosis of cyclone systems

A three-dimensional diagnostic model for the assimilation of satellite and conventional meteorological data is developed with the variational method of undetermined multipliers. Gridded fields of data from different type, quality, location, and measurement source are weighted according to measurement accuracy and merged using least squares criteria so that the two nonlinear horizontal momentum equations, the hydrostatic equation, and an integrated continuity equation are satisfied. The model is used to compare multivariate variational objective analyses with and without satellite data with initial analyses and the observations through criteria that were determined by the dynamical constraints, the observations, and pattern recognition. It is also shown that the diagnoses of local tendencies of the horizontal velocity components are in good comparison with the observed patterns and tendencies calculated with unadjusted data. In addition, it is found that the day-night difference in TOVS biases are statistically different (95% confidence) at most levels. Also developed is a hybrid nonlinear sigma vertical coordinate that eliminates hydrostatic truncation error in the middle and upper troposphere and reduces truncation error in the lower troposphere. Finally, it is found that the technique used to grid the initial data causes boundary effects to intrude into the interior of the analysis a distance equal to the average separation between observations.

Achtemeier, G. L.↗
NASA NTRS1986

Chaos and nonlinear dynamics of single-particle orbits in a magnetotaillike magnetic field

The properties of charged-particle motion in Hamiltonian dynamics are studied in a magnetotaillike magnetic field configuration. It is shown by numerical integration of the equation of motion that the system is generally nonintegrable and that the particle motion can be classified into three distinct types of orbits: bounded integrable orbits, unbounded stochastic orbits, and unbounded transient orbits. It is also shown that different regions of the phase space exhibit qualitatively different responses to external influences. The concept of 'differential memory' in single-particle distributions is proposed. Physical implications for the dynamical properties of the magnetotail plasmas and the possible generation of non-Maxwellian features in the distribution functions are discussed.

Chen, J.↗
NASA NTRS1986

Modal test/analysis correlation for Centaur G Prime launch vehicle

A modal test was performed on the Centaur G Prime launch vehicle for the purpose of verifying the loads analysis model. This paper describes the procedure by which modal parameters obtained in this test were correlated with the corresponding analytical predictions. Based on this correlation the stiffness model of the shuttle trunnion system has been modified. The evolution of the model updating and the final results are described.

Chen, J.↗
NASA NTRS1985

Collsionless tearing instability in a non-Maxwellian neutral sheet - An integro-differential formulation

In the present investigation, a general integro-differential formalism is derived for the study of the collisionless tearing mode in a highly non-Maxwellian neutral sheet in which both electrons and ions are treated kinetically. The obtained formalism is applied to a specific non-Maxwellian distribution. The dispersion relation for the considered system is determined, taking into account the fundamental harmonic of the orbital frequency. It is found that the dispersion relation is dominated by the electrons. The results are presented in a number of graphs. The growth rates of non-Maxwellian distributions are generally much greater than the growth rate of the conventional isotropic tearing instability.

Chen, J.↗
NASA NTRS1985

Sixty GHz IMPATT diode development

The objective of this program is to develop 60 GHz GaAs IMPATT Diodes suitable for communications applications. The performance goal of the 60 GHz IMPATT is 1W CW output power with a conversion efficiency of 15 percent and 10 year life time. During the course of the program, double drift (DD) GaAs IMPATT Diodes have been developed resulting in the state of the art performance at V band frequencies. A CW output power of 1.12 W was demonstrated at 51.9 GHz with 9.7 percent efficiency. The best conversion efficiency achieved was 15.3 percent. V band DD GaAs IMPATTs were developed using both small signal and large signal analyses. GaAs wafers of DD flat, DD hybrid, and DD Read profiles using molecular beam epitaxy (MBE) were developed with excellent doping profile control. Wafer evaluation was routinely made by the capacitance versus voltage (C-V) measurement. Ion mass spectrometry (SIMS) analysis was also used for more detailed profile evaluation.

Ma, Y. E.↗