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Modeling and Optical Diagnostics of Iodine Fed Helicon
Type Thrusters by a Detailed Global Model (DGM)
IEPC-2019-448
Presented at the 36th International Electric Propulsion Conference
University of Vienna • Vienna, Austria
September 15-20, 2019
Konstantinos Katsonis1 and Chloe Berenguer2
DEDALOS Ltd., Thessaloniki, 54645, Greece
Daniele Pavarin3, Fabio Trezzolani4, Nicolas Bellomo5, Marco Manente6, Riccardo Mantellato7, Davide Scalzi8,
Francesco Barato9 and Alessandro Schiavon10
Technology for Propulsion and Innovation - T4i Ltd., Padova, Italy
Abstract: Iodine becomes a common electric thruster propellant due to its well known
advantages. The iodine detailed global model developed by DEDALOS Ltd is used here for
theoretical characterization and optimization of on ground prototypes, on the basis of
plasma components composition diagrams and of functioning diagrams, giving a preview of
the main species present in the thruster and of the electron density and temperature and
allowing for the thruster functioning regimes analysis and optimization. Theoretical spectra,
necessary for non-perturbing optical emission spectroscopy diagnostics are also obtained by
the model. Experimental intensities of the spectral lines from an iodine-fueled RF electric
thruster prototype, under development by T4i S.r.l., are compared with the corresponding
theoretical ones from the model, leading to a detailed optical emission diagnostics.
Nomenclature
xTOT = total ionization percentage
x
'TOT = electrons percentage
C-R = Collisional-Radiative
DGM = Detailed Global Model
DEDALOS = Data Evaluation and Diagnostics ALgorithms Of Systems Ltd.
ET = Electric Thruster
FD = Functioning Diagram
1 Director, katsonis.dedalos@gmail.com.
2 Project Manager, berenguer.dedalos@gmail.com.
3 CEO, d.pavarin@t4innovation.com.
4 Senior R&D engineer, f.trezzolani@t4innovation.com.
5 CTO, n.bellomo@t4innovation.com.
6 Plasma Propulsion Manager, m.manente@t4innovation.com.
7 Project Engineer, r.mantellato@t4innovation.com.
8 Project Engineer, d.scalzi@t4innovation.com.
9 Chemical Propulsion Manager, f.barato@t4innovation.com.
10 Technician, a.schiavon@t4innovation.com.
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
GM = Global Model
HelT = Helicon type ET
IDGM = Iodine Detailed Global Model
ne= electron density
OES = Optical Emission Spectroscopy
p= pressure
PABS = absorbed power
PCC = Plasma Components Composition
QTOT = total flow rate
R= cone base radius
RF = Radio Frequency ET
T4i = Technology for Propulsion and Innovation S.r.l.
Te= electron temperature
ThSpec = theoretical spectrum
H= cone Height
1. Introduction
ODINE has been often chosen nowadays as an Electric Thruster (ET) propellant, in view of its well known
advantages1. The Iodine Detailed Global Model (IDGM)2 developed by DEDALOS Ltd can provide theoretical
characterization and optimization of on ground ET prototypes for this propellant, on the basis of Plasma
Components Composition (PCC) diagrams and of Functioning Diagrams (FD). PCCs give a preview of the main
species present in the ET and of the expected electron density ne and temperature Te values, while FDs allow for the
thruster functioning regimes analysis and optimization. Moreover, theoretical spectra are obtained simultaneously by
IDGM. These are necessary for non-perturbing Optical Emission Spectroscopy (OES) diagnostics of the thruster.
Comparison of the spectral lines experimental intensities with the corresponding theoretical ones calculated by
IDGM constitute the basis of OES.
I
Here we are interested in general in technology of the Helicon Thruster (HelT) type 3. Helicon plasma thruster
technology is developed in Padua by T4i S.r.l., a spin-off of the University of Padua, in the frame of several
international and Italian research projects4. In particular, in the experimental part contained in Section 4 we address
specifically an example of iodine-fueled device belonging to the wider category of RF plasma thrusters, although
largely based on Helicon technology. The considered prototype is a development model by T4i S.r.l. on the basis of
the know how accumulated on Helicon3,4 and more generally RF plasma sources5. By adapting IDGM input
parameters to those prevailing in the RF thruster and after comparison of our theoretical results to experimental ones
obtained recently by T4i S.r.l., characterization and optical diagnostics of the prototype have been addressed.
IDGM encompasses detailed description of the neutral and of the singly and doubly ionized species present in the
prototype. It also evaluates the absorbed power losses due to the main plasma processes which depend on the
functioning conditions and gives mean values of the thruster plasma parameters.
Global Models (GM) of Iodine plasma have lately acquired a particular interest due to their application in
characterization and in OES diagnostics of ETs of various types when fed by iodine1,6, although using of iodine as
ET propellant was suggested already at the beginning of the century7. The present work is based in a Detailed Global
Model8 (DGM) supported by an extended atomic database of iodine species, rather than using a Collisional-
Radiative (C-R) iodine model, because the latter cannot take account of the sheath formation at the limits of the
plasma and it is not addressing the energy losses detail. DGM allows for detailed characterization of ETs in an
extended domain of absorbed power (PABS) values, provided the data of the expected ionized states are included in
the system. This becomes more important when the ionization energy of the present species is low. For example,
when PABS reaches values about 100 eV the twice ionized species have abundances similar to neutral ones for low
pressures (see Section 2). Our IDGM calculations show that for PABS values of 1 keV, the presence of twice or more
ionized iodine species becomes mandatory for low pressures. Presence of such species play a fundamental role in
the thruster functioning and is essential for the thrust characterization. Study of the concomitant first and second
order iodine spectra is necessary for any propulsion plasma OES diagnostics. Increase of the thruster energy class
leads to considerable presence of more ionized iodine species, in particular when PABS per volume is higher.
Calculation (and acquisition in case OES diagnostics is sought) of higher than the second order spectra should be
addressed in this case.
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
In studying the iodine fed ETs, extensive use of Plasma Components Composition (PCC) diagrams and of
Functioning Diagrams (FD) is made here, as was also the case e.g. in Ref. 1. Although low PABS values are of
interest for the experimental application addressed here, we discuss PCCs belonging to PABS values going up to 100
W. Also, we comment a FD for iodine fed ETs for pressures going from 0.7 up to 5 mTorr, which for the low
pressure region incorporates higher values of the total ionization percentage
x
TOT , an often used parameter. Iodine
propellant being unavailable in situ for interplanetary missions, it is important to insure its efficient use. In so doing,
we also discuss the obtained total ionization during the thruster functioning, on the basis of two forms of FD giving
the
x
TOT dependence either on the pressure or on the absorbed power, two parameters which are very important for
the thruster functioning.
Details of the elaboration of an averaged GM9 and of the iodine homonuclear series structure are not described
here, as both have been often made available in the literature. Iodine homonuclear series data resources and their
evaluation have been discussed elsewhere (see Refs. 1,2 and references therein). In Section 2 we present modeling
results pertaining to iodine fed thrusters obtained by IDGM, depending on the PABS , the pressure p and the electron
temperature Te . Section 3 is devoted to analyze the iodine fed ETs functioning on the basis of FD diagrams which
also result from IDGM exploitation.
Following the review of the main IDGM results addressing the plasma composition and ionization, we compare
obtained theoretical spectra with the corresponding experimental ones, which have been acquired by T4i. Iodine
theoretical spectra given by IDGM are compared in Section 4 with the spectral acquisitions from MET to obtain
OES diagnostics. We focus in the 470 nm to 575 nm spectral region, where important lines of both first and second
order iodine spectra are present according to IDGM analysis. A preview of the expected thruster characteristics
leading to its theoretical characterization are also addressed in this section. IDGM results are useful in controlling
and diagnosing the plasma and contribute to optimize the thruster functioning. At the same time, OES diagnostics
contributes to the model validation for low power class ETs, because theoretical spectra of neutral and singly ionized
species from IDGM are here compared to experimental ones. Section 5 contains the obtained conclusions and the
continuation sought for the present work.
2. Preview of plasma composition of iodine-fueled HelT following PCC diagrams
Plasma composition of iodine-fueled HelTs and RF thrusters in general for fixed flow rate values is investigated in
this section, using PCC diagrams resulting from IDGM. For a given flow rate (QTOT) value, composition depends
mainly on variation of two fundamental parameters, namely the absorbed power (PABS) and the pressure (p).
Addressing a prototype of RF technology, a conical plasma form factor of R = 5 cm base radius and of H =15 cm
height is selected. PCC diagrams illustrate the constitution of the plasma resulting after absorption of PABS , which is
only a part of the available power and for given values of PABS and QTOT . Variation of the species populations is
expressed as a function of p. Alternate presentations of the plasma constitution may also be obtained if p and one of
the two other parameters QTOT or PABS are kept constant, while the other is varying. In general, Q TOT , p, and PABS are
subject to the conditions created in situ. The propellant feed can be conveniently regulated, while p and PABS depend
on the thruster type, HelT, RF, etc. and rely mainly on the vacuum vessel or the space environment and the current
produced by the solar cells or other available power devices.
First, results concerning low PABS values are addressed as we seek primarily application of our theoretical results to
the T4i prototype case. However, IDGM encompasses detailed description of the HT also for high PABS values. We
will tackle a case of 100 W PABS subsequently. In all, the PABS values addressed for the ET plasma having the
aforementioned form factor vary from 25 W to 100 W. These values cover typical examples often considered in case
of iodine-fueled ETs. The influence of the pressure and of the absorbed power on the ET plasma composition are
separately addressed in this section, while the corresponding functioning of the thrusters will be discussed in Section
3. Note that in case of higher ET dimensions than those addressed here and for similar pressure values, the
addressed values of the absorbed power have to be increased in a parametric way if the chosen feed amount remains
the same, in case that sufficiently high ionization percentage is sought.
A. Plasma Composition as a Function of the Pressure p
We address low pressure values, varying from 0.8 mTorr to 5 mTorr while the iodine propellant feed remains at
QTOT = 1 sccm. This feed is of interest to ETs of low power class. Composition of the plasma corresponding to the
chosen feed and pressure region for an absorbed power of PABS = 25 W is shown schematically in Fig. 1. The
presented results are obtained by the corresponding PCC.
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
The index 'TOT' used in Fig. 1 and similar ones, illustrating PCC results, indicate the total plasma density nTOT
(dot broken curve in magenta). For each of the
various iodine neutral / ionized species a sum
value including the densities of the totality of the
corresponding Ground Level (GL) compound plus
these of the corresponding excited ones is given.
Results concerning the electron density ne are
given by a broken curve in red. In the case of Fig.
1 this curve is practically coinciding with the one
giving the total population of the singly ionized
species because of the quasi-neutrality principle in
view of the very low values of the doubly ionized
ones. Plasma component densities resulting from
IDGM calculations are shown in general by curves
in black for neutral species, in red for singly
ionized and in blue for doubly ionized ones.
Populations of the two GL components of the
neutral iodine and of the five GL components of
the singly ionized one are given separately, with
the quantum core description in parenthesis. The
small numbers attributed to the levels correspond
to the order of each one in the corresponding
Grotrian diagram.
It can be seen in Fig. 1 that within the chosen p
variation region the calculated species densities increase in general smoothly with pressure. A notable exception is
the I2+ Ground Level (GL) sum, because the PABS value is very low given the chosen flow rate. Population of the total
neutral iodine ground level I0 GL remains similar to nTOT for all the addressed pressure values. This reflects a rather
low plasma ionization, especially for the higher
pressures, with ne going down to values of two
orders of magnitude lower than nTOT for pressure
values of 5 mTorr.
As expected, populations of the excited states are
clearly lower than the population of the
corresponding GL compound. In order to include in
Fig. 1 at least the more abundant excited species,
their values have been multiplied by ten. Neutral
6s (n° 3) and singly ionized 5d (n° 12) species of
metastable character denoted by “m” become then
also visible. Note that populations of the latter is
diminishing when p increase.
A situation similar to this of Fig. 1, except in
what concerns the absorbed power, which is
doubled to PABS = 50 W is given in Fig. 2. In this
case the plasma is obviously more ionized and
consequently the values of the total density of the
twice ionized iodine I2+TOT shown in the low p
region are higher, with its partial GL populations
also visible.
Complementary information on the
characteristics of the two plasma cases illustrated
by Figs. 1 & 2 is provided by Figs. 3 & 4 which are concomitant to Figs. 1 & 2 correspondingly. Colors attributed to
the iodine species in the figures are similar to those used previously. Figs. 3 & 4 address more specifically
percentages of the main plasma ionized constituents. Corresponding electron temperatures Te are also indicated by
red thin curves bearing full squares ■. Evaluation of Te is mandatory for ET characterization and diagnostics. Total
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
Figure 2. Pressure dependent PCC for PABS = 50 W and
iodine feeding of 1 sccm
Figure 1. Pressure dependent PCC for PABS = 25 W and
iodine feeding of 1 sccm
ionization percentage
x
TOT = nION / nTOT and electrons percentage
x
'TOT = ne / nTOT are also shown by black lines,
continuous with full dots and broken with empty dots correspondingly. As was often the case, the parameter
x
TOT
used here as a measure of the plasma ionization, addresses only the sum of each ion type numbers, irrespectively of
the ionization charges. Then, in calculating the resulting thrust, the number of each ion must be multiplied by the
number of the charges that each ion bears in order to arrive to a value equal to
x
'TOT = ne / nTOT in conformity with the
quasi-neutrality assumption. Consequently,
x
'TOT may exceed 100 % , at least for low pressures where the ionization
is considerable and leads to sustainable presence of at least twice ionized iodine species densities.
Fig. 3 concerning an absorbed power value of PABS = 25 W shows that
x
'TOT has practically similar values with
x
TOT as expected in case of very low double ionization. These values could easily approach 20 % for low pressures,
where the ionization is considerable, while when p increases become negligible. Te increases slowly when the
pressure is diminishing down to 0.8 mTorr. Fig. 4 pertain to an absorbed power of PABS = 50 W. It shows that values
of
x
'TOT become slightly different than those of
x
TOT for low pressures, where the ionization is increasing, with
presence of twice ionized iodine species densities.
Te increases fast for pressure values around 0.8
mTorr.
For increased PABS values even higher ionization
values are reached. Situation pertaining to
absorbed power of 100 W is illustrated in Fig. 5,
with a PCC showing high ionization for low
pressures, although persistent increase of the
neutral species presence in case of high pressures
is manifested.
Percentage of the singly ionized species is
increasing up to pressures of about 2 mTorr,
reaching a plateau for higher pressure values. In
the high pressure region, presence of all the
ionized species with q > 1 is low and I+ follows
practically the total ionization percentage values.
Interestingly, contribution of twice ionized
species to positive charges arrives up to 2x1011 cm-
3 for 0.8 mTorr, although their number is five times
lower than this of the neutral iodine species, as it
can be seen in Fig. 5. Then, an increase of the PABS
values to 100 W allows for a quite improved
utilization of the propellant in case the pressure
stays at the level of a few mTorr, thus allowing for
convenient thrust values.
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
Figure 3. PCC conjugate for PABS = 25 W Figure 4. PCC conjugate for PABS = 50 W
Figure 5. Pressure dependent PCC for PABS = 100 W and
iodine feeding of 1 sccm
We described here ET plasma composition by a
PCC for the typical case of 100 W absorbed power. In
order to obtain a better insight of the situation, we may
address a conjugate diagram, similar to these shown in
Figs. 3 & 4. This diagram is contained in Fig. 6. In this
figure, it can be observed that the electron temperature
varies from 1.5 eV to 5 eV approximately, while the
pressure diminishes from 5 mTorr to 0.8 mTorr.
Inversely, when pressure increases from 0.8 mTorr to 5
mTorr, the percentage of neutral iodine increases
drastically from 30 % up to 90 %. It has also to be
noticed that because of the sufficient absorbed power
value of 100 W, twice ionized iodine species are visible
for low pressure values and have about 7 % presence
for 0.8 mTorr. Consequently, as shown in Fig. 6,
x
'TOT
has a higher value than the
x
TOT one in the low pressure
region, where the ionization is considerable with sustainable presence of twice ionized iodine species densities.
B. Influence of the absorbed power PABS on the plasma composition illustrated by an 1 mTorr modified PCC
The mandatory role which the absorbed power plays on the plasma equilibrium conditions of ET functioning can
be appreciated through Fig. 7, presenting a power depending PCC diagram which gives the plasma composition for
the same form factor as before in case of an
indicative pressure of 1 mTorr, with the feed set
always at 1 sccm. PABS varies from 25 W to 100
W. Symbols and colors in Fig. 7 are the same
with those of Figs. 1 to 6. As the pressure must
stay constant at 1 mTorr, nTOT diminishes slowly
with absorbed power in Fig. 7, divided by about
three when PABS is increasing four times. As was
illustrated in Figs. 1 & 2, the bulk of the plasma
ionization for about 1 mTorr pressure is following
the curve giving the density of the singly ionized
species, with values of ne and of I+ GL compound
being about the same and only for high absorbed
power appears a small difference. Both ne and I+
GL increase slowly to approach the nTOT values
with increasing PABS . It is to be noted that,
beginning with about PABS = 80 W, the I1+ GL
compound species become more abundant than
the neutral ones. The fact that for higher PABS
values the difference between ne and I+TOT
becomes distinguishable was already observed in
Fig. 6. For 1 mTorr, trebly ionized I3+ species are
so scarce that are not included in Fig. 7.
Populations belonging to the various cores of
the neutral and of the ionized species are shown
separately by thin curves of various types,
marked in parenthesis by 1 to 5 following the atomic descriptions except for twice ionized species. Expressing the
plasma density as a function of the absorbed power gives an interesting insight of the plasma behavior.
A diagram concomitant to the PCC shown in Fig. 7, is contained in Fig. 8 and gives additional aspects of the
plasma properties under the same conditions prevailing for Fig. 7. Fig. 8 is somewhat similar to Figs. 3, 4 & 6 and
uses the same symbols. However, species percentages and Te values are given here as a function of the absorbed
power, instead of using the pressure as ordinate and results are concomitant to those presented in Fig. 7.
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
Figure 7. Power dependent PCC for p = 1 mTorr and
iodine feeding of 1 sccm
Figure 6. PCC conjugate for PABS = 100 W
It is to be noticed that for about PABS = 75 W
neutral and singly ionized species have similar
values, e.g. 50 % of the provided propellant is
practically not used for thrust. Doubly ionized
species contribute very little to the total plasma
ionization even for high PABS values and appear
at the bottom of the figure, with trebly ionized
species practically absent.
As reported previously,
x
TOT and
x
'TOT
percentages, shown by continuous and dashed
black curves correspondingly, are very similar
for high PABS values but have quite
distinguishable values when absorbed power
values exceed, say, PABS = 50 W.
3. Characterization and optimization of iodine-fueled HelTs using FD diagrams
A detailed description of the expected functioning of a prototype thruster and its theoretical characterization can
be obtained by using of a dedicated FD. Typically, FDs provide the propellant ionization percentage as a function of
the pressure. They contain parametric curves for various values belonging to concomitant electron temperature and
PABS values. Thus, a FD provides in condensed form a set of PCCs. Alternatively, FDs illustrating the propellant
ionization according to the variation of PABS or of QTOT can be obtained. The IDGM model has been used to obtain
both the FD diagram types presented in this section, belonging to an iodine-fueled thruster with the form factor
addressed previously in Section 2.
In Part A we show how a FD can be used in order to obtain a detailed insight on the thruster functioning. Optimal
functioning conditions of iodine-fueled HelTs are also investigated by a modified FD diagram giving ionization vs
absorbed power. Such a FD will be shown in Part B.
A. Preview of iodine-fueled HelT devices characteristics by ionization vs pressure FD diagrams
A typical FD is shown in Fig. 9, in
which the ionization percentage TOT is
given for pressure values varying from
0.8 mTorr to 5 mTorr. This figure
contains iso-power curves for PABS
values varying from 25 W to 100 W and
iso-thermic curves for Te values varying
from 1.6 eV to 4 eV. As expected,
curves of this figure illustrating TOT
values vary considerably as a function
of the pressure in the region of low
pressure values, while when the
pressure is quite high TOT remains low
even for high PABS values. Hence,
functioning of the thruster under
conditions corresponding to the right
region of Fig. 9 is not to be considered
because even with high absorbed power
values big part of the propellant quits
the thruster without contributing to the
thrust. The temperatures corresponding
to the given iso-thermic curves increase
faster for lower pressures because TOT
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
Figure 9. FD for QTOT(I) = 1 sccm and pressure range from 0.8
mTorr up to 5 mTorr
Figure 8. Power dependent PCC conjugate for p = 1 mTorr
is quite high there. With diminishing p, iso-power curves approach to draw lines for lower power values,
corresponding to faster TOT increase. This indicates that the pressure diminishing is more efficient in the low
pressures region.
B. Optimal functioning conditions of iodine-fueled HelTs by ionization vs absorbed power FD diagrams
Using of an alternate form of FD
diagrams can address the optimizing of
an iodine-fueled thruster. This is
illustrated in Fig. 10, addressing
variations of Te and of PABS values in the
same intervals with those of Fig. 7, but
with ionization variations given as a
function of the PABS instead of the
pressure. A feed of 1 mTorr is
considered throughout.
Pressure values (of 0.7 mTorr, 0.8
mTorr, 1.0 mTorr, 1.5 mTorr, 2.0 mTorr,
3.0 mTorr and 5.0 mTorr), given in Fig.
10 by starred curves follow loosely the
temperature ones, but higher pressures
belong to lower temperatures. A novel
view of a thruster functioning is given
by Fig. 10, illustrating the obligation
for the pressure values to stay small if
we want to obtain sufficient ionization
of the provided propellant without
increasing excessively the absorbed
power amount.
4. Analysis of iodine experimental and theoretical spectra aiming OES diagnostics
It is well known that the DGMs allow also for theoretical evaluation of the spectral line intensities, which by
comparison with the experimental ones give the ne and Te values of the thruster plasma. Argon theoretical plasma
spectral lines, obtained by Collisional-
Radiative or Global models have been used
previously in OES diagnostics10,11. In the
same direction, analysis of theoretical xenon
spectra has been made, leading to interesting
results12. Because of the similarities between
Xe and I atoms, we expect results which
eventually should be compared
advantageously, even if their chemical
properties are clearly different. Aiming OES
diagnostics of the aforementioned iodine-
fueled prototype developed in Padua, we
focused on plasma conditions expected to
prevail during its functioning. The employed
thruster has an envelope contained in a ϕ 55 x
l 120 mm cylinder, weights approximately 0.4
kg and is characterized by a very simple
structure, including a ceramic discharge
chamber, an RF antenna for plasma ionization
and a magnetic system based on SmCo
permanent magnets, all housed within a
metallic structure.
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
Figure 11. Part of the experimental iodine I & II spectrum
Figure 10 : Modified FD for QTOT(I) = 1 sccm and PABS from 25 W to
100 W.
Acquisition of the plasma experimental spectra, together with the PCC and the FD diagrams and theoretical
spectra which are obtained from IDGM, contribute efficiently to theoretical characterization and to OES diagnostics.
This has been demonstrated previously for other electric thruster types fed by iodine1. In Fig. 11 we address a typical
spectrum acquired recently by T4i from the thruster plasma, in the spectral region which is contained between 470
nm and 620 nm shown.
Measured intensities are given in Fig. 11 with the prevailing conditions also reported. In particular, the spectra
were observed in the thruster plasma, placing an optical fiber approximately 5 mm downstream its exhaust section,
oriented orthogonally to the axis of the thruster. The plasma has roughly a conical form and expands rapidly
downstream the thruster, thus making it difficult to precisely estimate the actual pressure and size of the discharge in
the region of interest, although an
approximated estimation can be performed.
Iodine I & II transitions are contained,
with some prominent neutral lines in blue and
ionic ones in red, following analysis by
IDGM. Overall comparison of I I with I II
intensities indicates a substantial ionization.
Rates of experimental vs theoretical lines
intensities allow for OES. Experimental
spectra as this of Fig. 11, together with the
PCC and the FD diagrams obtained from
IDGM contribute efficiently to theoretical
characterization and to diagnostics of iodine
fed HelTs. This has been demonstrated
previously for other electric thruster types3
fed by iodine.
Part of the provided experimentally
acquired spectrum contained in Fig. 11 is
reported in Fig. 12, in which we noted also
prominent spectral lines of the first and the
second iodine spectrum by blue and red lines
respectively. Lines belonging to the 6s – 7p
multiplet of the neutral iodine and to the 6 s –
6p multiplet of singly ionized iodine are
shown, together with two inner-shell lines of
the latter. In Fig. 13 we focus in the 470 nm
to 575 nm spectral region containing
important lines of both the neutral and the
ionized iodine spectra in order to optimize the
OES diagnostics. Information about the
plasma dimensions, the pressure and the
iodine flow rate is also provided in Fig. 13.
After searching among the set of our
theoretical spectra in the same region, we
identified the theoretical spectrum shown in
Fig. 13 as corresponding to the experimental.
In this figure theoretical I I & II lines are
shown for iodine feed of QTOT = 1 sccm.
Pressure is of 2.0 mTorr and PABS of 25 W.
Comparing Fig. 13 with Fig. 12, it is seen
that for both the I0 and I+1 species the
intensities of the theoretical lines are
analogous to the experimental ones given in
Fig. 12. Also the ratio of the neutral lines
intensities versus the ionic ones is similar in the experiment and in the theory. As the 1 sccm feed is about 0.1 mg/s
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
Figure 12. Experimental iodine I & II spectrum in the 470 nm
to 575 nm region
Figure 13. Iodine I & II spectrum in the 470 nm to 575 nm
region, obtained by IDGM
in case of Xe or I feed, we infer that the absorbed power is 25 W for a pressure of 2 mTorr. Following Fig. 3 of
Section 2, the approximate electron temperature is 1.8 eV with an ionization percentage of 5 % for an approximate
pressure value of 2 mTorr. Because in the addressed region the theoretical spectrum shown in Fig. 13 is very similar
to the experimental one of Fig. 12, we conclude that the experimental conditions are comparable with the ones
corresponding to the spectral calculations. The global comparison of the theoretical spectra multiplets with the
experimental ones illustrated here, minimize errors coming from the atomic data and gives also an evaluation of the
ionization percentage.
In order to check the model output, another estimate of the ionization percentage was performed by T4i by
estimating the total ion current ejected by the thruster using a Faraday probe, which gave substantially higher values,
in the range of 50-85%. Such wide uncertainty margin accounts for several factors hampering the accuracy of
Faraday probe measurements, such as ion focusing and secondary electron emission, which typically lead to an
over-estimation of the measured current. Other minor effects usually come from geometrical uncertainties in the set-
up.
Even considering all this, however, Faraday probe estimations exceed spectroscopic ones by a considerable
margin; this may be due to a number of reasons, among which the most probable are
i) uncertainty in the estimation of the plasma discharge characteristics (pressure, size) in the section of interest,
ii) inhomogeneities in plasma distribution within the plume, leading to zones of locally low density and
iii) the relatively small wavelength span employed in the first spectroscopic acquisitions. Plume inhomogeneity,
in particular, is likely the main source of disagreement, since the spectroscopic measurements considered in this
work are related to a relatively small, fixed spot in the plume, while Faraday probe measurements involved radial
scans of the plume for ion current integration and are thus of a global nature.
Further testing is required in order to rule out these and possibly other elements, in order to achieve a reasonable
agreement between Faraday probe and spectroscopic measurements which, in our opinion, is required for a sound
testing procedure. More detailed comparisons aiming to conclude with the RF plasma thruster characterization and
diagnostics, based to additional experimental spectra, is in progress.
5. Conclusions and perspectives
We conclude that the PCC and the FD diagrams together with the theoretical spectra obtained from IDGM
contribute efficiently in characterizing theoretically iodine-fueled HelT and more generally RF plasma thrusters and
allow for a support for their OES diagnostics. Moreover, as was also the case previously with other thruster types
fed by iodine, experimental support by acquisition of experimental spectra corresponding to theoretical ones gives
the possibility to obtain OES diagnostics of the thruster plasma. It also corroborates the theoretical results already
obtained by the IDGM model on the basis of the experimental results.
In particular, T4i is willing to continue the spectroscopic analysis on its RF thrusters and will likely extended it
in the future to the REGULUS propulsion system, currently under development and based on an iodine-fed RF
thruster5,13,14. The new tests will include spectral acquisitions with a wider wavelength span, in order to provide more
data to tune the model, as well as spectroscopic measurements at multiple radial locations in the plume.
References
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
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The 36th International Electric Propulsion Conference, University of Vienna, Austria
September 15-20, 2019
