Speaker
Nicolas Claire
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.4001.pdf
Ion velocity distributions in front of a ceramic surface: an inverse sheath ?
V. Pigeon1 , N. Claire1 , C. Arnas1
1 Aix-Marseille University, CNRS, PIIM, Marseille, France
Plasma-wall interaction is a fundamental field of research in plasma physics for numerous
applications. In Hall thrusters, it is known that the wall properties may influence the discharge,
mainly through electron emission and sputtering. The sheath and pre-sheath in front of an in-
sulator ceramic sample (BNSi02, used in the aforementioned thruster), immersed in the low
temperature Argon plasma of a multipolar device are studied experimentally using an emissive
probe and the laser-induced fluorescence diagnostic. Firstly obtained ion velocity distribution
functions exhibited an unexpected flow of ions directed away from the wall toward the bulk
plasma, and an other one toward the wall. But later experiments involving the laser beam prop-
agation through a 0.8mm drilled hole (slightly smaller than the Debye length λD ' 1cm), see
Fig.1, discarded the wall-directed ion flow. Despite the diffuse laser beam reflection at the ce-
ramic surface, and small detection solid angle of sight, fluorescence transition saturation may
explain the presence of the wall-directed flow in the first measurements.
It seems that the BNSi02 secondary electron emission[1] and/or electrons reflected from an
insulator surface [2] could be the cause of this phenomenon, which would correspond to an
inverse sheath [3]. The ionizing energetic electrons present in the multipolar discharge are sus-
pected to play a major role.
Emissive probe measurements did not show this probable inverse sheath but a monotonic poten-
tial drop. It seems that emissive probes could be too intrusive to measure potential in this thin
high gradient region, especially if electron emission/reflection at the wall is present, leading
biased measurements. Experimental artifacts removal and theoretical modeling (involving the
Campanell’s model [3]) are currently performed in order to confirm the formation of an inverse
sheath in front of the ceramic. 19
25
18
References 17 20
Position(cm)
[1] T. Tondu, M. Belhaj and V. Inguimbert , J. Appl. 16
15
15
Phys. 110, 093301 (2011)
14 10
13
[2] F.X. Bronold and H. Fehske, Phys. Rev. Lett., 115, 5
12
225001 (2015) 11
-5 -4 -3 -2 -1 0 1 2 3 4 5
0
Velocity (km/s)
[3] M.D. Campanell and M.V. Umansky, Phys. Plas-
mas, 24, 057101 (2017) Figure 1: 2D ion velocity distributions in
front of the drilled ceramic. Wall is at 23cm.
Negative velocities are oriented toward the
bulk plasma.
