The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden to Department SPONSORINGMONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)AFRLNSBXT SPONSORMONITOR'S REPORT NUMBER(S) DISTRIBUTIONIAVAILABILITY STATEMENTApproved for Public Relase; distribution unlimited. SUPPLEMENTARY NOTES - ABSTRACTNascap-2k is a modem spacecraft charging code, replacing the older codes NASCAP/GEO, NASCAP/LEO, POLAR, and DynaPAC. The code builds on the physical principles, mathematical algorithms, and user experience developed over three decades of spacecraft charging research. Capabilities include surface charging in geosynchronous and interplanetary orbits, sheath and wake structure and current collection in low-Earth orbits, and auroral charging. External potential structure and particle trajectories are computed using a finite element method on a nested grid structure and may be visualized within the Nascap-2k interface. Space charge can be treated either analytically, self-consistently with particle trajectories, or by importing plume densities from an external code such as EPIC (Electric Propulsion Interactions Code). Particle-in-cell capabilities are available to study dynamic plasma effects. Auxiliary programs to Nascap-2k include Object Toolkit (for developing spacecraft surface models) and GridTool (for constructing nested grid structures around spacecraft models). The capabilities of the code are illustrated by way of three examples: charging ofa geostationary satellite, self-consistent potentials for a negative probe in a LEO spacecraft wake, and potentials associated with thruster plumes. SUBJECT TERMS Spacecraft chargingPlasma simulation, Nascap-2k AbstractNascap-2k is a modem spacecraft charging code, replacing the older codes NASCAP/GEO, NASCAP/LEO, POLAR, and DynaPAC. The code builds on the physical principles, mathematical algorithms, and user experience developed over three decades of spacecraft charging research.Capabilities include surface charging in geosynchronous and interplanetary orbits, sheath and wake structure and current collection in low-Earth orbits, and auroral charging. External potential structure and particle trajectories are computed using a finite element method on a nested grid structure and may be visualized within the Nascap-2k interface. Space charge can be treated either analytically, self-consistently with particle trajectories, or by importing plume densities from an external code such as EPIC (Electric Propulsion Interactions Code). Particle-in-cell (PIC) capabilities are available to study dynamic plasma effects.Auxiliary programs to Nascap-2k include Object Toolkit (for developing spacecr...

In situ spectroelectrochemistry demonstrates stability of electrografted diazonium interfaces on conductive oxides & their suitability as anchoring groups for molecular species.

A new model of the plasma plume from Hall Effect Thrusters (HET's) is presented. The model includes the self-expansion of the main beam by density gradient electric fields, lowenergy ions produced by resonant charge exchange between beam ions and neutral atoms (ambient and thruster-induced), and angle-dependent elastic scattering of beam ions off neutral atoms. The variation of radial velocities across the annular thruster beam is also included. The model is an advance over previous plume models in the way it numerically models the self-expansion of the main beam, and in particular, the treatment of elastic scattering using recently calculated differential cross sections. The results are compared with recent measurements of the energy and angledependent plume from the BPT4000 Hall-Effect Thruster. Both the intensity and energy dependence of the scattering peaks are compared. The principal result is that elastic scattering is the source of the majority of ions with energy greater than E/q=50V that are observed at angles greater than 45° with respect to the thrust axis. The model underscores the need for elastic scattering cross sections for multiply charged ions, as well as a better understanding of HET propellant utilization.

1] Historically, the characterization of the magnetospheric environment has limited our ability to determine spacecraft surface charging levels. One difficulty lies in the common practice of fitting the plasma data to a Maxwellian or Double Maxwellian distribution function, which may not represent the data well for use in spacecraft charging simulations. We use electron and ion flux spectra measured by the Los Alamos National Laboratory (LANL) Magnetospheric Plasma Analyzer (MPA) during eclipse in September 2001 to examine how the use of different spectral representations of the charged particle environment in computations of spacecraft potentials during magnetospheric substorms affects the accuracy of the results. We examine charging and noncharging flux spectra and the relationships between the density and temperature moments. We then calculate the spacecraft potential (zero net current) using both the measured fluxes and several different fits to these fluxes. The potential computed using the measured fluxes and secondary and backscattered fluxes computed for graphite carbon, with a constant fraction of 81% of secondary electrons escaping, is within a factor of three of the measured potential for 87% of the data. Potentials calculated using a Kappa function fit to the electron flux and a Maxwellian function fit to the ion flux agree with measured potentials nearly as well. Alternative spectral representations give less accurate estimates. The use of all the components of the net flux, along with spacecraft specific average material properties, gives a better estimate of the spacecraft potential than the measured flux from a single high-energy channel.Citation: Davis, V. A., M.

The Space Power Experiment Aboard Rockets I (SPEAR I) biased two 10‐cm radius spheres as high as 46,000 V positive with respect to an aluminum rocket body. The experiment measured the steady state current to the spheres and the floating potential of the rocket body. Three‐dimensional calculations performed using NASCAP/LEO and POLAR 2.0 show that both ion‐collecting and electron‐collecting sheaths were formed. The rocket body potential with respect to the ionospheric plasma adjusted to achieve a balance between the electron current collected by the spheres and the secondary electron‐enhanced ion current to the rocket body. This current balance was obtained with a large ion‐collecting sheath that enveloped most of the electron‐collecting sheath and reduced the area for collection of ionospheric electrons. The calculated current is in agreement with the flight measurement of a steady state current of less than 1/10 A. The calculations show that the rocket body was driven thousands of volts negative with respect to the ionospheric plasma. The calculated rocket potential is within the uncertainty of that inferred from ion spectrometer data. The current flowed through the space plasma. There was almost no direct charge transport between the spheres and the rocket body.

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Recently, the debate on what is the best daytime Geosynchronous Earth Orbit spacecraft charging index has been reopened. In this paper, the conclusions of one of the recent papers on the subject are verified by comparing Nascap-2k results with charging and fluxes measured on the Spacecraft Charging at the High Altitudes, Intelsat, Defense Satellite Communications System, and Los Alamos National Laboratory Geosynchronous Earth Orbit satellites. In addition, a refined measure of charging is presented as the total thermal electron flux above a certain minimum energy that is well above the second crossover point in secondary electron emission. The use of this type of index is justified by correlations between Nascap-2k simulation results and total fluxes above a range of energies. The best minimum energy to use is determined for spacecraft of different design and surface materials. Finally, the optimum Geosynchronous Earth Orbit daytime spacecraft charging index is obtained, and its use for predicting and resolving spacecraft anomalies in real time is justified.