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NASA NTRS2010

Fabrication and Modification of Nanoporous Silicon Particles

Silicon-based nanoporous particles as biodegradable drug carriers are advantageous in permeation, controlled release, and targeting. The use of biodegradable nanoporous silicon and silicon dioxide, with proper surface treatments, allows sustained drug release within the target site over a period of days, or even weeks, due to selective surface coating. A variety of surface treatment protocols are available for silicon-based particles to be stabilized, functionalized, or modified as required. Coated polyethylene glycol (PEG) chains showed the effective depression of both plasma protein adsorption and cell attachment to the modified surfaces, as well as the advantage of long circulating. Porous silicon particles are micromachined by lithography. Compared to the synthesis route of the nanomaterials, the advantages include: (1) the capability to make different shapes, not only spherical particles but also square, rectangular, or ellipse cross sections, etc.; (2) the capability for very precise dimension control; (3) the capacity for porosity and pore profile control; and (4) allowance of complex surface modification. The particle patterns as small as 60 nm can be fabricated using the state-of-the-art photolithography. The pores in silicon can be fabricated by exposing the silicon in an HF/ethanol solution and then subjecting the pores to an electrical current. The size and shape of the pores inside silicon can be adjusted by the doping of the silicon, electrical current application, the composition of the electrolyte solution, and etching time. The surface of the silicon particles can be modified by many means to provide targeted delivery and on-site permanence for extended release. Multiple active agents can be co-loaded into the particles. Because the surface modification of particles can be done on wafers before the mechanical release, asymmetrical surface modification is feasible. Starting from silicon wafers, a treatment, such as KOH dipping or reactive ion etching (RIE), may be applied to make the surface rough. This helps remove the nucleation layer. A protective layer is then deposited on the wafer. The protective layer, such as silicon nitride film or photoresist film, protects the wafer from electrochemical etching in an HF-based solution. A lithography technique is applied to pattern the particles onto the protective film. The undesired area of the protective film is removed, and the protective film on the back side of the wafer is also removed. Then the pattern is exposed to HF/surfactant solution, and a larger DC electrical current is applied to the wafers for a selected time. This step removes the nucleation layer. Then a DC current is applied to generate the nanopores. Next, a large electrical current is applied to generate a release layer. The particles are mechanically suspended in the solvent and collected by filtration or centrifuge.

Ferrari, Mauro↗

NASA NTRS2004

Compact Plasma Accelerator

A plasma accelerator has been conceived for both material-processing and spacecraft-propulsion applications. This accelerator generates and accelerates ions within a very small volume. Because of its compactness, this accelerator could be nearly ideal for primary or station-keeping propulsion for spacecraft having masses between 1 and 20 kg. Because this accelerator is designed to generate beams of ions having energies between 50 and 200 eV, it could also be used for surface modification or activation of thin films.

Foster, John E.↗

NASA NTRS1995

Alfven waves, alpha particles, and pickup ions in the solar wind

Past studies of the properties of Alfven waves in the solar wind have indicated that (1) the amplitude of the velocity fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles do not participate in the wave motions because they 'surf' on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the 'observed' wave speed, B(sub o)(delta v squared / delta B squared)exp 1/2, as determined from the ratio of the amplitudes of the velocity and magnetic fluctuations. It is proposed that the modification of Alfven wave propagation speed is due to pressure anisotropies resulting from asymmetric distributions of interstellar pickup ions. If the proposed explanation is correct, it indicates that scattering of pickup ions onto a (bi)spherical shell may not be as complete as generally supposed.

Goldstein, B. E.↗

NASA NTRS1984

Thermal evaporation of spherical clouds - Effects of viscous stresses

The thermal evaporation of a cold spherical cloud embedded in hot gas is analyzed, including the effects of viscous stresses as well as 'saturation' of the conductive heat flux, for the case of equal electron and ion temperatures. The relevant transport properties for an H-He plasma are discussed, including the modification of these transport coefficients when temperature or velocity gradients become large. The fluid-dynamical equations for steady spherical evaporation are given and cast into dimensinless form. Numerical integration of these equations and the results are discussed, emphasizing the novel features of the viscous solutions, astrophysical implications, and uncertainties in the physics.

Giuliani, J. L., Jr.↗

NASA NTRS2005

Implementation of the dawn ion propulsion system

The Dawn ion propulsion system (IPS) was intended to be simply a larger version of the ion propulsion system that flew on Deep Space 1 (DS1). Implementation of this system to meet the needs of the Dawn mission, however, required modification of some IPS components and completely new developments of others.

ion propulsion↗

NASA NTRS1978

Observed and theoretical profiles of the Si II lines at lambda 1814

Emission line spectra observed at the limb of the solar disc are presented for transitions of singly ionized silicon at lambdas 1816.93, 1817.45 and 1808.01. The profiles show self-reversals in the line cores. A non-LTE analysis for a simplified model of the silicon ion is presented, and calculated line profiles are compared with the observations. The study indicates that some modification of the Vernazza et al. (1973) model of the solar chromosphere in the transition region is needed to reconcile the calculated and observed profiles.

Finn, G. D.↗

NASA NTRS1984

Performance capabilities of the 12-centimeter Xenon ion thruster

The 8- and 12-cm mercury ion thruster systems were developed primarily to provide N-S station keeping of satellites with masses up to about 1800 to 3600 kg respectively. The on-orbit propulsion requirements of recently proposed Large Space Systems (LSS) are beyond the thrust capabilities of the baseline 8- and 12-cm thruster systems. This paper presents a characterization of the performance capabilities of the 12-cm Xenon ion thruster to enable an evaluation of its application to LSS auxiliary propulsion requirements. With minor thruster modifications and simplifications the thrust was increased to 64 mN, a factor of six over the baseline 12-cm mercury thruster performance. The thruster was operated over a range of specific impulse of about 2000 to 4000 seconds and at total efficiencies up to 68.0 percent. The operating levels reached in this study were found to be close to the operating limits of the thruster design in terms of perveance, grid breakdown voltage and thruster component temperatures such as those of the magnets and cathode baffle.

Mantenieks, M.↗

NASA NTRS1984

Performance capabilities of the 12-centimeter Xenon ion thruster

Mantenieks, M.↗

NASA NTRS1989

Effects of unequal particle number densities on Alfven waves

Analytic plasma theory and numerical solutions of the dispersion equation are used to show that the assumption that the linear properties of the waves are determined by a charge-neutral plasma in the absence of the nonthermal particles, while the nonthermal particles cause growth or additional damping superposed onto the background, is seriously flawed even for stable plasmas. Even when the nonthermal particles do not contribute significantly to the dispersion equation, unequal thermal electron and ion number densities (due to the presence of the nonthermal particles) may cause fundamental low wave number modifications to the Alfven modes, including the creation of a new resonance and severely modified dispersion. These results are found for both cold and warm plasmas. Previous work on Alfven waves should be reevaluated in view of these results.

Cairns, I. H.↗

NASA NTRS1988

Simulation of ion-cyclotron-like modes in a magnetoplasma with transverse inhomogeneous electric field

The new kinetic ion-cyclotronlike (IC-like) instability driven by a localized electric field perpendicular to the ambient magnetic field is investigated using particle simulation. The existence of the new mode is demonstrated and the mode is distinguished from the well-known Kelvin-Helmholtz branch and the kinetic modification of this branch. The issue of the initial loading of particles in phase space in the simulation is clarified. The simulation results show that the growing ion waves are associated with the small vortices in the linear state, which evolve into a nonlinear stage dominated by large vortices with lower frequencies.

Nishikawa, K.-I.↗

NASA NTRS1978

Modification of a very large thermal-vacuum test chamber for ionosphere and plasmasphere simulation

No large-volume chamber existed which could simulate the ion and electron environment of near-earth space. A very large thermal-vacuum chamber was modified to provide for the manipulation of the test volume magnetic field and for the generation and monitoring of plasma. Plasma densities of 1 million particles per cu cm were generated in the chamber where a variable magnetic flux density of up to 0.00015 T (1.5 gauss) was produced. Plasma temperature, density, composition, and visual effects were monitored, and plasma containment and control were investigated. Initial operation of the modified chamber demonstrated a capability satisfactory for a wide variety of experiments and hardware tests which require an interaction with the plasma environment. Potential for improving the quality of the simulation exists.

Pearson, O. L.↗

NASA NTRS2007

Increasing the Life of a Xenon-Ion Spacecraft Thruster

A short document summarizes the redesign of a xenon-ion spacecraft thruster to increase its operational lifetime beyond a limit heretofore imposed by nonuniform ion-impact erosion of an accelerator electrode grid. A peak in the ion current density on the centerline of the thruster causes increased erosion in the center of the grid. The ion-current density in the NSTAR thruster that was the subject of this investigation was characterized by peak-to-average ratio of 2:1 and a peak-to-edge ratio of greater than 10:1. The redesign was directed toward distributing the same beam current more evenly over the entire grid andinvolved several modifications of the magnetic- field topography in the thruster to obtain more nearly uniform ionization. The net result of the redesign was to reduce the peak ion current density by nearly a factor of two, thereby halving the peak erosion rate and doubling the life of the thruster.

Goebel, Dan↗

NASA NTRS2011

Biogeochemical Impact of Long-Range Transported Dust over Northern South China Sea

Transpacific transport and impact of Asian dust aerosols have been well documented (e.g., results from ACE-Asia and regional follow-on campaigns), but little is known about dust invasion to the South China Sea (SCS). On 19-21 March 2010, a fierce Asian dust storm affected large areas from the Gobi deserts to the West Pacific, including Taiwan and Hong Kong. As a pilot study of the 7-SEAS (Seven South East Asian Studies) in the northern SCS, detailed characteristics of long-range transported dust aerosols were first observed by a comprehensive set of ground-based instruments deployed at the Dongsha islands (20deg42'52" N, 116deg43'51" E). Aerosol measurements such as particle mass concentrations, size distribution, optical properties, hygroscopicity, and vertical profiles help illustrate the evolution of this dust outbreak. Our results indicate that these dust particles were mixed with anthropogenic and marine aerosols, and transported near the surface. Satellite assessment of biogeochemical impact of dust deposition into open oceans is hindered by our current inability in retrieving areal dust properties and ocean colors over an extensive period of time, particularly under the influence of cloudy conditions. In this paper, we analyze the changes of retrieved Chlorophyll-a (Chl-a) concentration over the northern SCS, considered as oligotophic waters in the spring, from long-term SeaWiFS measurements since 1997. Over the past decade, six long-range transported dust events are identified based on spatiotemporal evolutions of PM10 measurements from regional monitoring stations, with the aid of trajectory analysis. Multi-year composites of Chl-a imagery for dust event and non-dust background during March-April are applied to overcome insufficient retrievals of Chl-a due to cloudy environment. Due to anthropogenic modification within a shallow boundary layer off the densely populated and industrial southeast coast of China, the iron ion activation of deliquescent dust particles enhances the efficiency of fertilization for biological productivity. Compared to the West Pacific, the marine ecosystem in the northern SCS is much more susceptible to the biogeochemical impact of long-range transported Asian dust.

Tsay, Si-Chee↗

NASA NTRS2013

Farfield Plume Measurement and Analysis on the NASA-300M

NASA is developing a 15-kW Hall thruster to support future NASA missions. This activity is funded under the Space Technology Mission Directorate Solar Electric Propulsion Technology Demonstration project. As a part of the development process, the far-field plume characteristics of the NASA-300M, a 20-kW Hall thruster, were studied. The results will be used to study how various aspects of the operation of this thruster affect the overall performance. This data will be used to guide future design work and serve as a baseline for comparison to a magnetically shielded version of the NASA-300M that will be tested in the future. For this study, a far-field Faraday probe was swept in a polar fashion to map the ion current density. An ExB probe (Wien filter), two retarding potential analyzers, and a Langmuir probe were mounted at a fixed location on the thruster axis in the far-field plume. The data reduction method followed recommendations in recent studies by Brown, Reid, and Shastry with modifications that are tailored to the plasma plume environment of high-power Hall thrusters. Results from this and prior testing show that the plume is richer in doubly-charged ions, larger in spatial extent, and capable of greater probe heating than lower power thrusters. These characteristics require special treatment in experimental setup and data analysis, which will be the main focus of this paper. In particular, covered topics will include a new, more accurate, method of integration for analysis of the ExB probe data and effect of secondary electron emission on the Faraday probe data.

Huang, Wensheng↗

NASA NTRS1976

An endurance test of a 900 series 30-cm engineering model ion thruster

The 10,000 hour endurance test of the first engineering model 30-cm thruster (S/N 701) identified discharge chamber erosion as a life limiting problem. Modifications to minimize this problem have been implemented in the 900 series thrusters. This paper describes a test conducted to evaluate the success of these modifications. The post-test analysis results of thruster 701 are summarized and the erosion reducing modifications described. Test results for thruster 901 up to 4165 hours, when the test was interrupted by a shorted wire, are described. The conclusions of a post-test examination of thruster 901 are included. Also being evaluated was a thermal vacuum breadboard SCR series inverter power processor which is described.

Collett, C. R.↗

NASA NTRS1981

Performance capabilities of the 8-cm Mercury ion thruster

The paper presents an initial characterization of the performance capabilities and constraints of the 8-cm Hg ion thruster system with a view to evaluating its application to large space system propulsion requirements. With minor thruster modifications, the thrust was increased by about a factor of four, while the discharge voltage was reduced from 39 to 22 volts. The thruster was operated over a range of specific impulse of 1950 to 3040 seconds, and a maximum total efficiency of about 54% was attained at a discharge voltage of 24 volts and thruster input power of 0.49 kW.

Mantenieks, M. A.↗

NASA NTRS1992

Modifications of magnetohydrodynamics as applied to the solar wind

The effect of including the Braginskii viscous stress tensor in magnetohydrodynamics is remarked upon. It is shown that semiquantitative agreement with a recently observed anisotropy in the turbulent solar wind spectrum can be achieved in this way. The modifications of the dynamical equations are simple enough to permit their inclusion in numerical codes. The effects of large 'ion parallel viscosities' also may be significant for plasmas in quite different regimes than the solar wind.

Montgomery, David↗

NASA NTRS1975

Experimental investigations of argon and xenon ion sources

The multipole thruster was used to investigate the use of argon and xenon propellants as possible alternatives to the electric thruster propellants of mercury and cesium. The multipole approach was used because of its general high performance level. The design employed, using flat and cylindrical rolled sections of sheet metal, was selected for ease of fabrication, design, assembly, and modification. All testing was conducted in a vacuum facility and the pumping was accomplished by a 0.8 m diffusion pump together with liquid nitrogen cooled liner. Minimum discharge losses were in the 200-250 ev. ion range for both argon and xenon. Flatness parameters were typically in the 0.70-0.75 range.

Kaufman, H. R.↗