Thomas Nilsson 18jan2011
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Transcript of Thomas Nilsson 18jan2011
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Simulations of spacecraftpotential measurements onCassini
Thomas Nilsson
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Diploma work done at the Swedish Institute of SpacePhysics (IRF Uppsala)
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Overview
IntroductionSimulation software (SPIS)Cassini spacecraft resultsLangmuir probe results OML and Sheath model.Conclusions and outlookQuestions
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What has been investigated?
How does the Cassini s/c affect thespacecraft potential measurements
made by the Langmuir probe? Simulations of full s/c without probe
How does the connecting parts (thestub) between s/c and the Langmuirprobe affect the probe bias sweeps?Simulations of probe and stub only (no s/c)
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Intro
SPIS Cassiniresults LP results Conclusions
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The Cassini spacecraft
Showing the Huygens probe being releaseddown through the atmosphere of Saturn'smoon Titan.
Position of the Langmuirprobe on the Cassini
spacecraft.
One of the largestinterplanetary spacecraftsever. (6 m long)
High gain antenna (4m indiameter)
The Langmuir probe is on a
1.5 m boom.
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SPIS Cassiniresults LP results Conclusions
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The Langmuir probe
Made of titanium with atitanium nitride coating
Boom The stub, radius 3.175 mmlength 109 mm, at same
potential as the sphere
The sphere,
radius 25 mm
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The simulation software
Spacecraft Plasma Interaction System,
(SPIS v.3.7rc09)Assumptions/settings: Only hydrogen ions
PIC electrons Ti = Te B = 0
stationary plasma wrt S/C no photoelectrons
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SPIS Cassiniresults LP results Conclusions
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Simplified model of the S/C
The Cassini s/c is modelled asa cylinder with varying radius,
with the booms for the Langmuir
probe and the magnetometerpresent.
The LP and stub are omitted.
Simulated in various plasmas.
n: 100; 10; 1; 0.1; 0.01[cm^-3]
Te: 10; 1; 0.1 [eV]
The nested boxes are there tohelp optimize the 3D meshing of
the simulation volume, finer andmore accurate close to thespacecraft. Wider and lessaccurate at the boundary.
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SPIS Cassiniresults LP results Conclusions
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Example result from the Spacecraft
simulations
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Intro
SPIS Cassiniresults
LP results Conclusions
The plasma potential in the vicinity of the spacecraft for aplasma with ne = 1 cm-3 and Te = 1 eV.
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Result of the Spacecraft simulations
Long Debye length =>78.5% of the S/Cpotential will remain atthe position of theprobe.
This means that thetrue Vsc relates to the
measured probefloating potential asVsc = Vps /0.215
4.65 Vps
Agrees well withfactor ~5 derived fromcomparisons of LP toELS data
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Intro
SPIS Cassiniresults
LP results Conclusions
Note: boom but not probe included inthe simulation. The potential plotted isthe potential at the position where theprobe should have been.
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Langmuir probe simulations
The Langmuir probe with sphereand stub. Different size cylindersto optimize the 3D meshing of thesimulation volume.
Two different cases;
- one in OML, where rp
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The Langmuir probe theory
Orbit motion limited, OML, where rp > D
For the case in between a model of the
sheath is required. The model used isthe one published by Walker, 1964.
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SPIS Cassiniresults LP results Conclusions
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Results of OML case
The collected currentto the probe in the
simulations with thestub is slightly lower(4%) than when thestub is absent.
Ip Ap, => we couldexpect 1% decrease with
the stub due to the
interface.
Hence, the stub stealssome electrons that
should otherwise have
hit the probe.
The simulated sweep for the Langmuir probe with(green) and without the stub (blue). The OMLpredicted curve dashed in red.
Each * represents one simulation. 12Thomas Nilsson
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SPIS Cassiniresults LP results Conclusions
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Results of case in between OML and SL
The one without the stub collects far more than the Walkermodel states and agrees well with the work by Laframboise.About 10% current reduction when stub is attached.
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SPIS Cassiniresults LP results Conclusions
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Results of case in between OML and SL
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SPIS Cassiniresults LP results Conclusions
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Sheath around the Langmuir probe
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Intro
SPIS Cassiniresults LP results Conclusions
Showing the plasma potential around the Langmuir probe withits stub present for a plasma with ne = 1000 cm-3 and Te =0.05 eV.
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ConclusionsWalker analytical model needs someimprovements.
The probe current when the stub is
present is lower than without the stub.(worse with shorter Debye length)
75-80% of Vs/c remains at the probeposition for long Debye lengths. Factor ~5 should be used for calculating s/c
potential from probe-to-s/c potential.
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SPIS Cassiniresults LP resultsConclusions
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Outlook / Problems
Future work may include:
Moving plasma with respect to the
spacecraft (and/or probe). Photoelectrons, SPIS v3.7rc09 have some
difficulties with shadows.
Model of the probe and stub ready for this, only asmall modification in SPIS is needed. (materialproperties)
Cassini model would need bigger modifications.
(shadows on a curved surface)To fully simulate s/c influence on LP, SPIS must
include e.g backtracking possibility to probe to
improve statistics 17Thomas Nilsson
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SPIS Cassiniresults LP resultsConclusions
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Questions?
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