thomas nilsson 18jan2011

8/12/2019 Thomas Nilsson 18jan2011

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Simulations of spacecraftpotential measurements onCassini

Thomas Nilsson

1

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

Thomas Nilsson 2


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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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Intro



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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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Intro



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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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Intro



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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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Intro



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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

Intro



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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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Intro



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Results of case in between OML and SL

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Intro



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Sheath around the Langmuir probe

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Intro


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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Intro

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

Intro

SPIS Cassiniresults LP resultsConclusions


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Questions?

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thomas nilsson 18jan2011

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