Nascap-2k Spacecraft Charging Code Overview › ... › docs › 6-3_MMandell.pdf · Nascap-2k...

Nascap-2k Spacecraft Charging Code Overview

Myron J. Mandell and Victoria A. Davis, Science Applications International Corporation

David L. Cooke and Adrian WheelockAir Force Research Laboratory, Space Vehicles Directorate

S ft Ch i T h l C fSpacecraft Charging Technology ConferenceAlbuquerque, New Mexico, USA

September 22, 2010

Outline

• Nascap-2kNascap 2k– What is it?– Core capabilities– Recent progressRecent progress

• Object Toolkit• Nascap-2k user interface

E l• Examples:– Geosynchronous charging– LEO current collection

El t t ti th t l– Electrostatic thruster plumes– Antenna-induced currents

• Summary

Energy | Environment | National Security | Health | Critical Infrastructure

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2SAIC and the SAIC logo are registered trademarks of Science Applications International Corporation in the U.S. and/or other countries.

2SCTC 2010 N2k Acronyms defined on last slide

What Is Nascap-2k?

• Nascap-2k is a fully three-dimensional computer code to calculate the interaction of a spacecraft with its plasma environmentinteraction of a spacecraft with its plasma environment

• Interactions include:– Surface potentials

• Charged particle spectraSecondary electron emission• Secondary electron emission

• Bulk and surface conductivity– Current collection

• Applied potentials– Perturbation of near-field environment

• Plasma sheaths– Thruster plumes

• Charge exchange ion flow

• Calculations are analytic where possible, PIC (particle-in-cell) where required• Code features

– Object and grid definition– Simplified user interface for problem definition and execution– Graphical display of results


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p p y• Air Force Research Laboratory and NASA sponsorship

Nascap-2k Core Capabilities

• Define spacecraft surface geometryDefine spacecraft surface geometry• Grid surrounding space• Calculate environmentally induced

time-dependent surface potentials• Calculate external potentials:• Calculate external potentials:

– Analytic space charge (5 models)– Macroparticle space charge

(4 models)• Generate and track macroparticles• Generate and track macroparticles

– Uniform with boundary injection– Sheath generation– Charge exchange

• Post processing:• Post-processing:– Time-dependent surface potentials

and currents– Time-dependent volume potentials,

currents, and densities


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currents, and densities

Progress Since 2005

• Included in current release (Version 3.2)Thruster plume interactions– Thruster plume interactions

• M. J. Mandell et al., “Modeling the NEXT Multithruster Array Test with Nascap-2k,” IEEE Trans. Plasma Science 36, 2309, October 2008

– Macroparticle splitting• Accounts for thermal effects• Accounts for thermal effects

– Orbit averaged PIC– Multiple cutplane display– ScriptRunner for calculations outside user interface (for long-running PIC)

• In preparation for next release (Version 4.1)– New flexible database– Environment: Kappa distribution function implemented; flowing Maxwellian

and tabular under developmentand tabular under development– Surface and volume currents

• See Poster Paper “Pseudopotential Algorithms for Simulation of VLF Plasma Antenna Current Flow”


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– Release expected early 2011

Acronyms defined on last slide

New Nascap-2k Database

• More capabilities– No limit on grid size; no limit on number of particlesg ; p– No limit on number of stored quantities. This allows for better postprocessing,

more complex plasma models, and more timesteps– More robust

• Flexible– Accommodate future Nascap-2k growth– Available for other Air Force programs

• N2kDB Tool provides for direct access to the database– Console version

J ® i t f d C++ d i ll li k d lib i– Java® interface and C++ dynamically linked library version• Can be used to write to database

• N2kDB– C++ dynamically linked library– Interfaces to Fortran C++ and Java– Interfaces to Fortran, C++, and Java

• Files– Random access with ASCII and/or integer keys– Separate files for general information, special elements, pages of particles,

and time-dependent information


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p– Specified data item names with up to 16 characters and/or an integer

Java is a registered trademark of Oracle and/or its affiliates

Object Toolkit

B ild ft f• Build spacecraft surface models

• Intrinsic building blocksI t f fi it l t• Import from finite-element preprocessors

• Surface attributesM t i l– Material name

• Conductivity• Secondary electron emission

– Conductor numberCo ducto u be• Customizable to other

applications via an external file


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Object Toolkit Examples


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Nascap-2k GUI Input tabsp

Problem

Environment

Ch D it Charge Density Formulation

Particle Initialization and Tracking


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Nascap-2k Results Tabs

Examine & Edit Script & Run Script

View Surface & Space Potentials & Trajectories in 3D

View Time History Plots & Tables


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3-D Results DisplaySurface Potentials, Space Potentials, and Ion Trajectories, p , j


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Geosynchronous Orbit Charging (I)Geometric Model and Problem Specification

• Spacecraft covered with insulating material• High secondary emission on OSRsHigh secondary emission on OSRs• No grid needed because BEM

(Boundary Element Method) provides electric fieldsprovides electric fields

CGlaz


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Geosynchronous Orbit Charging (II)Environment and Timestepping Specificationspp g p

• Environment tab sets• Environment tab sets geosynchronous environment distribution functionhere a MaxwellianCharging tab sets timestep


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• Charging tab sets timestepparameters

Geosynchronous Orbit Charging (III)

• Script (automatically generated) specifies calculation steps– Script can be edited internally or externally

• Monitors progress of calculation


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Monitors progress of calculation• Results in 30 minutes

Geosynchronous Orbit Charging (IV)Results3D Tab Displayp y

• Positive differential charging:charging: – Outboard region of

solar panel– Center of sunward

panel– OSR radiator panels

• Detailed information available for selectedavailable for selected surface element

• With grid, could superpose potential p p pcontours and particle trajectories

• Results tab provides ti hi t f f


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time history of surface currents and potentials


Nascap-2k Adaptable to Real-time Calculations

• Nascap-2k charging algorithms fast• Algorithms used in prototype of Nascap 2k RT• Algorithms used in prototype of Nascap-2k RT• Nascap-2k RT will compute surface potentials in MSM-calculated

charged particle environments in operational charging forecast systemsys e

• Figures compare measured potentials with those computed by Nascap-2k RT using MSM environments (R. Hilmer, Fall Meeting of the American Geophysical Union, 2005)

-1600

-1400

-1200

DSCSMSM AMSM B

-1600

-1400

-1200

DSCSMSM AMSM B

-350

-300

-250

DSCSMSM AMSM B

LOW CHARGING (~0.2 keV peak) HIGH CHARGING (~0.6 keV peak)MEDIUM CHARGING (~0.3 keV peak)

-1000

-800

-600

-400

-200

MSM C-1000

-800

-600

-400

-200

MSM C-200

-150

-100

-50

MSM C


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0215.0 215.1 215.3 215.4 215.6 215.7 215.8 216.0

045.0 45.1 45.3 45.4 45.6 45.7 45.8 46.0

0217.0 217.1 217.3 217.4 217.6 217.7 217.8 218.0


LEO Current Collection (I)Charging Hazards and Wake Studies Experiment (CHAWS)g g p ( )

• Flown twice on Wake Shield Facility (WSF)STS 60 (F b 1994)– STS-60 (Feb. 1994)

– STS-69 (Sept. 1995)• Negative probe bias up to 2 kV• Instrumented for total current and

current distribution on probe• V.A. Davis, et al., J Geophys Res,• p. 12445, 1999

CHAWS Probe


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LEO Current Collection (II)Spatial Gridding with GridToolp g

Fine resolution around probe and near edge of disk


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Multiply nested cubic grids with “special” elements containing object for computational speed


LEO Current Collection (III)Steady-state Potential Solutiony

• CHAWS probe at 2 kV• CHAWS probe at -2 kV• PIC ions and fluid electrons:

ion

b0 0

1 exp

• Charge stabilization permits

0 0

ionb ln

en

mesh >> Debye• High voltage in wake requires tracking.• Strictly continuous electric fields and

third order particle tracking for efficiencythird order particle tracking for efficiency• Potential penetrates substantially into ram flow• Ions near WSF edge accelerated in ram direction before attracted to probe• O+ ions, H+ ions, rear surface of WSF, and WSF experiment all serve to


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, , , pshield the probe potential, limiting the range of high potentials in the downstream wake


LEO Current Collection (IV)Selected Particle Trajectoriesj

• Ions generated at problem boundary with thermal spread about ramIons generated at problem boundary with thermal spread about ram velocity

• Ions strike probe at tip or on far side


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LEO Current Collection (V) Orbit Averaged Particle-In-Cellg

• Macroparticle charge distributed over trajectory sub-steps, allows longer timesteps in dynamic calculationsa o s o ge es eps dy a c ca cu a o s

Full Trajectory (steady-state)M ti l t

Particle-in-Cell (dynamic)M ti l h

Orbit averagedMacroparticles carry chargeMacroparticles carry current

Share current sub-step timeto grid each sub-step

Macroparticles carry charge

Share charge to grid at end of timestep

Macroparticles carry charge

Share

to grid each sub-step

sub-step timechargetimestep


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10 iters (with sharing) in 2 hours 900 2-s timesteps in 140 hours 90 20-s timesteps in 12 hours


Potentials in Thruster Plumes (I)

• Thruster plumesThruster plumes– Produce potentials that modify contaminant trajectories– Produce charge-exchange ions that lead to enhanced plasma

density around the spacecraftdensity around the spacecraft– Interact with spacecraft surfaces– Interact with other thruster plumes

• Import plume ion densities from external file– Densities created by PlumeTool, part of EPIC (Electric Propulsion

Interactions Code, a NASA SEE (Space Environments Effects) te act o s Code, a S S (Space o e ts ects)product)

• Calculate potentials self-consistently with charge exchange ion generation and transport


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exchange ion generation and transport

Potentials in Thruster Plumes (II)Charge-Exchange Ion Generationg g

Plume potentials of three active thrusters Charge Exchange return to three active and one dormant thrusterand one dormant thruster

• Used to calculate self-consistent interaction of ion thruster plumes with other plumes or spacecraft

• Used to simulate plume of NEXT Multi-Thruster Array


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p y• Results published in IEEE Transactions on Plasma Science, 2008• Implementation funded by NASA

Potentials in Thruster Plumes (III)Space Versus Labp


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Space Conditions Lab Conditions

Antenna-induced Currents (I)

• DSX (Demonstration and Science Experiments)S ( e o st at o a d Sc e ce pe e ts)– Air Force Research Laboratory experiment– High-voltage VLF (0.1-50 kHz) transmitter– Large oscillating sheathsLarge oscillating sheaths– Poster papers

• Nascap-2k Self-consistent Simulations of a VLF Plasma Antenna• Pseudopotential Algorithms for Simulation of VLF Plasma Antenna Current

Flow

• Surface currents and volume electron currents obtained using pseudopotential methods– Consistent with Nascap-2k simulations and physically reasonable

boundary conditions• Volume ion currents obtained directly from PIC calculations


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– Enabled by capabilities of new database


Antenna-induced Currents (II)Surface Currents

• Cones indicate direction of currentof current

• Colors indicate magnitude of current

• Numeric results show capacitive loading near boom root and tip

0.12 0.006Surface current

0.08

0.1

ce c

urre

nt (A

/m)

0.004

0.005

(A/m

^2)

Surface currentDerivative

0.02

0.04

0.06

Tran

sver

se s

urfa

c

0.001

0.002

0.003

Der

ivat

ive


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00 10 20 30 40 50

Distance from boom root (m)

0

Antenna-induced Currents (III)Volume Electron Currents

• Upper antenna sheath growingpp g g– Electrons moving away from antenna,

indicated by currents toward antenna• Lower antenna sheath shrinking

ee-g– Electrons moving toward antenna,

indicated by currents away from antenna• Counterstreaming electron currents in

e-e

cold plasma associated with whistler waves e- e-


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Antenna-induced Currents (IV)Volume Ion Currents

• Obtained directly from PIC resultsU t• Upper antenna– Just switched to near zero potential– Energetic ions that had been orbiting

t l d d b i iantenna are released and begin moving outward

– Ions released during previous cycle are approaching grid boundaryapproaching grid boundary

• Lower antenna– Just switched to negative potential

Energetic ions released during near zero– Energetic ions released during near zero phase have moved into low field region of outer sheath

– Stragglers are being reattracted to


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gg gantenna by high fields


Summary

• Nascap-2kascap– User-friendly integrated code– Study and analysis of a wide variety of spacecraft-plasma interactions– Variety of important space environments.Variety of important space environments.– Uses efficient algorithms– Builds on heritage going back to late 1970s

• Examples presentedExamples presented– Charging in geostationary orbit– Current collection in low-Earth orbit

Charge exchange generation and potentials in thruster plumes– Charge exchange generation and potentials in thruster plumes– Surface and volume currents generated by antenna

• Nascap-2k is supported by Air Force Research Laboratory and the NASA Space Environments and Effects program


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NASA Space Environments and Effects program– Distributed through http://see.msfc.nasa.gov

Acronyms

• CHAWS: Charge Hazards And Wake Studiesg• GEO: Geosynchronous• LEO: Low-Earth orbit• MLI: Multi-layer insulationy• MSM: Magnetospheric Specification Model• OSR: Optical Solar Reflector• PIC: Particle-in-cell• VLF: Very low frequency• WSF: Wake Shield Facility


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