Ageing of ceramics in a Hall Effect
Plasma thruster
Nadia PELLERIN1,2, Lahib BALIKA1,3, Stéphane PELLERIN3, Cristian. FOCSA4, Emmanuel VÉRON1,2, Daniel
PAGNON5 and M. DUDECK6
1
2
CNRS, UPR 3079 CEMHTI, 1D avenue de la recherche scientifique, 45071 Orléans cedex 2, France
Université d’Orléans, Faculté des Sciences, Avenue du Parc Floral, BP 6749, 45067 Orléans cedex 2, France
3
Laboratoire GREMI, UMR 6606 (Université d’Orléans – CNRS), BP 6744, 45067 Orléans cedex 2, France
4
5
Laboratoire PhLAM, UMR CNRS 8523 Université Lille 1, 59655 Villeneuve d’Ascq, France
Laboratoire LPGP, bât 210, Université Paris-Sud11, 15 rue G. Clémenceau, 91405 Orsay cedex, France
6
Institut d’Alembert, Université Pierre et Marie Curie, 75252 Paris cedex, France
Abstract: Hall Effect Thrusters (HET) constitute very promising devices for
space applications. The insulators erosion phenomena of the accelerating
chamber by the xenon ions exhibit a great complexity, and not only strongly
affect the performances of HET, but also are the main limiting cause of their
lifetime. An actinometric method has been developed to measure the HET
ceramic wall erosion from means of Optical Emission Spectroscopy using the
relationship between XeI, XeII and BI lines. We observed an asymmetric
behavior of the re-deposits on the ceramic and on the anode of the HET. To
complete this in-situ approach, the behavior and the ageing of insulators in a
SPT100-ML have been studied ex-situ. Ceramic (BN-SiO2) rings constituting the
channel of the HET were manufactured with inserts periodically set all around
the channel wall. Simultaneously, silicon substrates were set in the channel to try
to de-correlate the effects of re-deposition of products from the ceramics erosion
in the channel (visible on the silicon substrates) and anode, from the
microstructural and chemical changes of the ceramics (visible on the inserts)
during HET working. Substrates and inserts are then analyzed with an
environmental SEM.
Keywords: thruster – ceramic – erosion – ageing – OES
1. Introduction
The Hall Effect Thruster is a type of plasmabased propulsion devices for space vehicles. It is
now recognized to have attractive performances for
global efficiency, specific impulsion, and reliability.
This propulsion system allowing multiple re-starts
are being used increasingly giving due to their
propellant efficiency and suitability for low
acceleration applications for geostationary satellites
(satellite communications systems for example) and
orbital station-keeping with high sensitive orbit
maneuvers, orbit transfer, or for interplanetary
missions. The Hall thruster was invented in the late
1950’s and was essentially developed by Russian
teams for efficient propulsion device, the vast
majority of satellites worldwide relying chemical
thrusters. One common type of Hall Effect Thruster
developed in the Soviet Union is the Stationary
Plasma Thruster (SPT) for the first time in
December 1971 on the Soviet Meteor spacecraft.
Some generations of SPT engines with increasing
thrust have been built and used for satellite
stabilization essentially. Hall Thruster has also been
the subject of a large number of researches in USA
or France. ESA launched two space probes equipped
with plasma thrusters, the first one (Stentor in
December 2002) used a PPS-1350 from SNECMA
(France) and a SPT-100 from Fakel (Russia), and the
second used a PPS-1350G (SNECMA) on Smart-1
spacecraft for the primary propulsion towards a
lunar orbit reached in 2004 1. During this mission,
the electric thruster has accumulated about 5000
hours in-flight and was started 800 times.
HET uses a partially magnetized plasma discharge
(rLarmor electronic << channel dimension << rLarmor
ionic), standing in an annular discharge chamber
with a radial magnetic field (around 200 Gauss)
generated by a set of external coils. Electrons are
emitted by an external hollow cathode and then
driven towards the channel bottom where an anode
stands. Xenon gas is emitted from the anode which
plays also the part of gas-distributor. The electrons
are focused by the magnetic field to produce xenon
ions by inelastic collisions (mainly single charge
ions). The electrons move towards the anode through
the magnetic lines. The produced ions are then
accelerated by the self-consistent axial electric field
generated by the decrease of the electron mobility
due to the magnetic field at the channel exit. Here,
ion acceleration is obtained without the use of a set
of polarized grids to extract and accelerate the ions
as for gridded ion thruster.
The insulated ceramics constituting the channel
walls of the accelerating chamber play an important
role in the HET, regarding the plasma discharge
properties. Composite ceramic BN-SiO2 is often
used. Some authors have shown that ceramic
chemical nature has consequence on discharge
current and global energetic efficiency. The HET life
time is very depending of the ageing of insulator
ceramics. Xenon particles are indeed responsible of
sputtering and chemical transformations of these
ceramics. S. Khartov et al 4 measured by RBS a
surface composition change on a thickness of 2.5 µm
for Borosil (or BGP) ceramics with main
components BN and SiO2, thruster operating during
44 hours. During the first 100 hours, erosion is
estimated at around 0.1 nm/h. After this time, the
rate is strongly decreased, correlated to geometric
correlation between exit area and ion velocity
direction 2,3.
The goal of this study is to analyze the ceramics
change during the running first hours. An in-situ
actinometric method is developed to quantify the
erosion rate thanks to Optical Emission
Spectroscopy (OES) measurements. Results of this
indirect approach are compared to ex-situ analysis of
the microstructure and chemical composition of the
ceramics by conventional methods of material
science (ESEM, EDX, X-ray diffraction).
2. Experimental set-up
A PPS-100ML (laboratory model) Hall Effect
Thruster has been used for ageing ceramics study.
The ground test national facility PIVOINE-2G of the
ICARE laboratory (Orléans, France) has allowed to
drive experiments. It is constituted with a large
vacuum chamber (diameter 2 m, length 5 m)
equipped with a cryogenic pumping system
(70 000 L/s).
substrate
insert
Figure 1. External ceramic of the HET equipped with four
inserts and silicon substrates.
The plasma cylindrical channel (width 2 cm) is
limited by two ceramic rings: the inner and the
external of diameter respectively 69 mm and
109 mm. For ex-situ analysis, the rings have been
machined with 4 (0°, 90°, 180° and 270°) specific
openings so that interchangeable inserts could be
disposed all around the channel, for inner and
external wall. Four pairs of inserts (face to face in
the channel) can then be set on the device. Rings and
inserts have been elaborated in boron nitride – silica
ceramic 60 BN – 40 SiO2 mol. %. (M26 grade – by
Saint-Gobain Ceramics).
Furthermore, silicon substrates are also glued on the
walls along cylinder symmetry axis for the specific
analysis of deposited materials during thruster
running.
For ageing study, successive runs have been
performed, between 4 and 24 hours in same working
conditions, for successive cycles of 4 hours duration.
The obtained static pressure in the chamber is
around 2.10-5 mbar for a xenon mass flow rate of 5
mg/s. The electric parameters are (nominal
conditions): a discharge voltage of 300 V and a
current of 5 A, for a thrust of 88 mN and a maximal
radial magnetic field of around 200 G. The global
efficiency is 55 %.
The erosion in-situ measurement by actinometric
method has been described elsewhere 5. The light
emitted by the plasma plume allows spectroscopic
analysis. The intensities of Xe I (828 nm), Xe II
(484 nm), and B I (250 nm) lines are recorded by
OES, thanks to an Acton spectrometer (Spectra Pro
2750, focal length 0.750 m, Triple Grating
Monochromator).
After runs of given duration, inserts are extracted
and analyzed for microstructure change study by
Environmental Scanning Electron Microscope
(ESEM), and for chemical change by Energy
Dispersive X-ray Spectrometer (EDS), with a
Philips- XL-40 microscope. Thanks to geometric set
of inserts on the rings, data are then available versus
time and space coordinates (z dimension along the
channel axis and θ angle all around the chamber), for
inner and external ceramics.
The insert weight loss is also measured versus run
duration by mechanical balance (accuracy ± 1 mg).
In another time, silicon substrate deposits are
measured from a profilometer DEKTAK 6M Stulys
profiler (LPGP laboratory – Orsay, France).
3. Results and discussion
The in-situ actinometric method for ceramic
erosion investigation is based on the correlation
between erosion rate and a ratio deduced of OES
measurements:
I ( BI 250 nm)  I ( XI 828nm)
OES Erosion 
I ( XII 484 nm)
This method has been yet validated from QCM
(Quartz Cristal Microbalance) erosion measurements
in some conditions 6. Thanks to irradiation by pulsed
laser of the inner ceramic, a new direct calibration of
the optical signal is developed, measuring the
ablated volume (hole) by profilometry method 7.
Ex-situ analyses are performed on inserts
after operating of the Hall Effect thruster during n
cycles of 4 hours (n = 1 to 6). Each insert is
examined by ESEM and EDS according to the
following way: microstructure and global
composition are studied for areas called A (side
towards the anode), B and C (side towards the exit
channel). Furthermore, evolution of the composition
is analyzed according to z dimension thanks to 12
analyze points (1 µm3) separated from 1 mm each
other (figure 2).
a/
b/
Figure 2. (a) Image of an insert after 24 hours of thruster
running. (b) Scheme of an insert repairing A, B and C areas,
and points of EDS analysis versus z axis.
A composition gradient versus z is observed from
the first cycle of 4 hours. In particular, the C region
is impoverished in silicon, whereas the channel
bottom is impoverished with boron and nitrogen and
enriched with oxygen, and carbon to a lesser extent
(figure 3). Silicon impoverishment is fast (figure 4),
whereas boron and nitrogen impoverishment are
continuous with time. Results for inner and external
ceramics are generally similar. However, it is
observed that impoverishment in boron is more
important for external ceramic, and correlated to the
detection of xenon in more important amount on this
ring.
a
b
c
d
Figure 5. BSE mode - ESEM micrograph of the BN-SiO2 inserts
(a) pristine ceramic, (b) A region for external insert after 4 h of
thruster running, (c) B region for external insert after 12 h of
thruster running,(d) C region for external insert after 16 h of
thruster running.
4. Conclusion
Figure 3. Evolution of Si/B ratio in inner and external ceramics,
versus A (1), B (2) and C (3) regions of the insert, and running
duration of the Hall Effect Thruster.
Hall Effect Thruster operating involves significant
change of the ceramics channel walls during the first
hours. Erosion has been investigated by
complementary ways to analyze the chemical and
physical processes involved.
Acknowledgments
This research is supported by the French Research Group,
GDR CNRS/CNES/SNECMA/Universités n°3161 "
Propulsion par plasma dans l’espace".
References
Figure 4. Silicon rate detected in inner and external ceramics,
versus z dimension and running duration of the Hall Effect
thruster.
The analysis of the microstructure change with
running duration shows specific evolution of the
ceramic depending of the z dimension in the
channel. The bottom undergoes a deposit (figure 5b,
c), and the part C corresponding to the exit channel
is undergone to sputtering which involves SiO2
grains pulling out, in agreement with EDS results.
These results allow submitting hypothesis
concerning chemical reactions on ceramics surface
during HET operating. Profilometry and weight loss
measurements confirm erosion phenomena and
allow quantifying erosion rate in C region.
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