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On the interpretation of Langmuir probe data inside a spacecraft sheath

dc.contributor.authorOlson, J.
dc.contributor.authorBrenning, N.
dc.contributor.authorWahlund, J.-E.
dc.contributor.authorGunell, H.
dc.date2010
dc.date.accessioned2016-03-30T12:01:17Z
dc.date.available2016-03-30T12:01:17Z
dc.identifier.urihttps://orfeo.belnet.be/handle/internal/3191
dc.descriptionIf a Langmuir probe is located inside the sheath of a negatively charged spacecraft, there is a risk that the probe characteristic is modified compared to that of a free probe in the ambient plasma. We have studied this probe-in-spacecraft-sheath problem in the parameter range of a small Langmuir probe (with radius rLP⪡λD) using a modified version of the orbit motion limited (OML) probe theory. We find that the ambient electron contribution Ie(ULP) to the probe characteristic is suitably analyzed in terms of three regions of applied probe potential ULP. In region I, where the probe is negatively charged (i.e., ULP<U1, where U1 is the potential in the sheath at the probe position), the probe characteristicIe(ULP) is close to that of OML theory for a free probe in the ambient plasma. In the probe potential range ULP>U1, there is first a transition region II in applied potential, U1<ULP<U2, in which the key factor to determine the shape of Ie(ULP) is a potential minimum UM between the probe and the ambient plasma. This minimum gives the depth Upl−UM of a potential barrier that prevents the lowest energy ambient electrons from reaching the probe. For a high enough positive probe potential, in region III, the barrier becomes small. Here, Ie(ULP) again approaches OML theory for a free probe. The boundary U2 between regions II and III is somewhat arbitrary; we propose a condition on the barrier, Upl−UM⪡kBTe/e, as the definition of region III. The main findings in this work are qualitative rather than quantitative. The existence of the transition region points to that special care must be taken to extract plasma parameters from measuredI(ULP) as the probe characteristic is likely to depart from usual OML in crucial respects: (1) the ambient plasma potential Upl falls into the transition region, but there is no obvious knee or other feature to identify it, (2) there is in this region no exponential part of Ie(ULP) that can be used to obtain Te, instead, (3) the probe size is important in determining the curve shape. We have tentatively applied our simplified probe-in-sheath model to real probe data from the Cassini spacecraft, taken in the dense plasma of Saturn’s magnetosphere. We propose that our model gives a better description than OML of measuredLangmuir probe sweeps in space plasmas where the Langmuir probe is situated within the spacecraft sheath, i.e., for long Debye lengths. The understanding of these probe sweep effects in such regions may improve by self-consistent particle simulations of the spacecraft environment.
dc.languageeng
dc.titleOn the interpretation of Langmuir probe data inside a spacecraft sheath
dc.typeArticle
dc.subject.frascatiPhysical sciences
dc.audienceScientific
dc.subject.freeAmbient plasmas
dc.subject.freeApplied potentials
dc.subject.freeCassini spacecraft
dc.subject.freeCurve shape
dc.subject.freeDebye length
dc.subject.freeDense plasma
dc.subject.freeKey factors
dc.subject.freeOML theory
dc.subject.freeParameter range
dc.subject.freeParticle simulations
dc.subject.freePlasma parameter
dc.subject.freePotential barriers
dc.subject.freePotential minima
dc.subject.freePotential range
dc.subject.freeProbe characteristics
dc.subject.freeProbe position
dc.subject.freeProbe size
dc.subject.freeSheath problem
dc.subject.freeSpace plasmas
dc.subject.freeTransition regions
dc.subject.freeLangmuir probes
dc.subject.freeMagnetosphere
dc.subject.freePlasmas
dc.subject.freeSpacecraft
dc.subject.freeProbes
dc.source.titleReview of Scientific Instruments
dc.source.volume81
dc.source.issue10
dc.source.page105106
Orfeo.peerreviewedYes
dc.identifier.doi10.1063/1.3482155
dc.identifier.scopus2-s2.0-78149444107


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