Effect of multiply charged ions on the performance and beam characteristics in annular and cylindrical type Hall thruster plasmas Holak Kim, Youbong Lim, Wonho Choe, and Jongho Seon Citation: Applied Physics Letters 105, 144104 (2014); doi: 10.1063/1.4897948 View online: http://dx.doi.org/10.1063/1.4897948 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An axially propagating two-stream instability in the Hall thruster plasma Phys. Plasmas 21, 072116 (2014); 10.1063/1.4890025 Effect of the annular region on the performance of a cylindrical Hall plasma thruster Phys. Plasmas 20, 023507 (2013); 10.1063/1.4793741 Electron cross-field transport in a low power cylindrical Hall thruster Phys. Plasmas 11, 4922 (2004); 10.1063/1.1791639 Plasma flow and plasma–wall transition in Hall thruster channel Phys. Plasmas 8, 5315 (2001); 10.1063/1.1421370 One-dimensional model of the plasma flow in a Hall thruster Phys. Plasmas 8, 3058 (2001); 10.1063/1.1371519 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.248.6.145 On: Fri, 10 Oct 2014 14:54:27
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Effect of multiply charged ions on the performance and beam characteristics in annularand cylindrical type Hall thruster plasmasHolak Kim, Youbong Lim, Wonho Choe, and Jongho Seon Citation: Applied Physics Letters 105, 144104 (2014); doi: 10.1063/1.4897948 View online: http://dx.doi.org/10.1063/1.4897948 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An axially propagating two-stream instability in the Hall thruster plasma Phys. Plasmas 21, 072116 (2014); 10.1063/1.4890025 Effect of the annular region on the performance of a cylindrical Hall plasma thruster Phys. Plasmas 20, 023507 (2013); 10.1063/1.4793741 Electron cross-field transport in a low power cylindrical Hall thruster Phys. Plasmas 11, 4922 (2004); 10.1063/1.1791639 Plasma flow and plasma–wall transition in Hall thruster channel Phys. Plasmas 8, 5315 (2001); 10.1063/1.1421370 One-dimensional model of the plasma flow in a Hall thruster Phys. Plasmas 8, 3058 (2001); 10.1063/1.1371519
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
Effect of multiply charged ions on the performance and beam characteristicsin annular and cylindrical type Hall thruster plasmas
Holak Kim,1 Youbong Lim,1 Wonho Choe,1,a) and Jongho Seon2
1Department of Physics, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu,Daejeon 305-701, Republic of Korea2Department of Space Science and Astronomy, Kyung Hee University, 1732 Deokyoungdaero, Giheung-gu,Yongin-si, Gyeonggi-do 446-701, Republic of Korea
(Received 3 September 2014; accepted 29 September 2014; published online 10 October 2014)
Plasma plume and thruster performance characteristics associated with multiply charged ions in a
cylindrical type Hall thruster (CHT) and an annular type Hall thruster are compared under identical
conditions such as channel diameter, channel depth, propellant mass flow rate. A high propellant
utilization in a CHT is caused by a high ionization rate, which brings about large multiply charged
ions. Ion currents and utilizations are much different due to the presence of multiply charged ions.
A high multiply charged ion fraction and a high ionization rate in the CHT result in a higher
specific impulse, thrust, and discharge current. VC 2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4897948]
Low-power Hall thrusters have, recently, received much
attention as one of the most promising electric propulsion
systems for space applications, particularly in conjunction
with advanced space missions such as formation flying and
micro-spacecraft constellation.1 The Hall thrusters have been
developed to a relatively high level of efficiency from 45%
to 55% in the power range from 0.5 to 5 kW.2 However, the
scaling down to low-power Hall thrusters with high effi-
ciency presents many difficulties in optimizing the magnetic
field, particle losses in the discharge channel, and channel
wall erosion, etc.2–6 In contrast to the conventional annular
type Hall thruster (AHT), cylindrical type Hall thrusters
(CHT) have mainly been studied for scaling down of the
Hall thruster for low power.3 The size reduction of the
thruster for low power inevitably involves an augmented
effect of the plasma-facing surface, and a high volume-to-
surface ratio of the discharge channel is obtained by remov-
ing the inner magnetic core in the CHT. Otherwise, the
operation principle of the CHT is basically similar to that of
the AHT.7,8 Both the high magnetic mirror ratio and the vol-
ume-to-surface ratio in the CHT are beneficial for better
electron confinement by a magnetic mirror effect and a
reduced particle loss effect at the dielectric channel wall.
Previous studies also demonstrated unusually high propellant
utilization and enhanced electron transport compared to
AHTs.7–10 As will be discussed in more detail below, we
found the existence of a high fraction of multiply charged Xe
ions, such as Xe2þ and Xe3þ in CHT plasmas, which is
closely related to performance characteristics, such as a high
specific impulse. In this paper, we describe the overall
thruster performance and Xe beam characteristics associated
with multiply charged ions in the AHT and CHT under iden-
tical conditions, including the diameter and depth of the dis-
charge channel and Xe flow rate.
Illustrated in Fig. 1 are schematics of the CHT and the
AHT used for the experiment, showing an anode with a gas
distributor, two electromagnetic coils, and a boron nitride ce-
ramic channel for each thruster. The outer diameter (50 mm)
and channel depth (24 mm) are identical for both thrusters,
but the inner core is retracted in the CHT as noted above.
The radial (Br) and axial magnetic fields (Bz) are also shown
in the figure. For the CHT, the currents in the two coils flow
in the same direction. The radial magnetic field lines along
the outer channel surface are mostly concentrated near the
channel exit for the AHT, but distributed over a relatively
wide region in the channel for the CHT.
The thruster was mounted on a thruster stand that is a
simple pendulum type with two pivots, and the displacement
of the stand calibrated by small weights was measured as
thrust using a laser and a position-sensitive detector combi-
nation. Experiments were carried out in a 3 m long and 1.5 m
diameter vacuum vessel, and the operation pressure was
maintained at 33 lTorr (for Xe gas) at a total Xe flow rate of
8 sccm. The anode mass flow rate was fixed at 7 sccm for
both thrusters. A commercial hollow cathode (Heatwave
HWPES-250) was used as an external neutralizer, and the
keeper current and Xe mass flow rate were kept at 1.58 A
and 1 sccm, respectively. Plume characteristics were studied
by a Faraday probe and an E�B probe. The Faraday probe
measures the angular distribution of the ion current density
and the total ion current collected by a commercial picoam-
meter (KEITHLEY 6485). The Faraday probe was installed
on the rotation stage with a radius of 48 cm centered at the
thruster exit, which can be rotated from �100� to þ100�
with respect to the thruster axis. An E�B probe is a velocity
filter that selects ions satisfying the Lorentz force equation
by balancing the perpendicular electromagnetic forces.11,12
It consists of an entrance collimator, a velocity filter, an exit
collimator, and a collector. Both collimators, made of stain-
less steel, are 70 mm in length and 4 mm in diameter. In the
velocity filter section, whose length is 140 mm, a magnetic
field is provided by two permanent magnets and its strength
a)Author to whom correspondence should be addressed. Electronic mail:
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is 0.23 T at the center of the filter body. An electric field per-
pendicular to the magnetic field is established by a pair of
aluminum plates separated by 10 mm. The casing of the
E�B probe is electrically grounded and its inside is kept
under vacuum via several small through holes. The E�B
probe was placed at 64 cm from the thruster exit on the
thruster axis.13
Shown in Fig. 2 are plume pictures and angular distribu-
tions of the ion current density for the AHT and CHT,
respectively. Both the plume pictures and the ion current
density profiles indicate that the angular distribution of the
ion beam is narrower for the AHT than for the CHT. In addi-
tion, the ion current density for the AHT and CHT increases
mainly near the thruster axis as the anode voltage is raised
FIG. 1. A schematic diagram and mag-
netic field profiles of (a) the AHT and
(b) the CHT. (c) Radial magnetic field
strength and (d) axial magnetic field
strength of the CHT and AHT along
the outer channel surface.
FIG. 2. Pictures of the plasma plume
of (a) the AHT and (b) the CHT.
Angular distribution of the ion current
density of (c) the AHT and (b) the
CHT at anode voltages from 220 V to
340 V with intervals of 20 V.
144104-2 Kim et al. Appl. Phys. Lett. 105, 144104 (2014)
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from 220 V to 340 V. This large plume angle for the CHT
could be attributed to its distinct magnetic field topology and
the corresponding electric field normal to the magnetic
field.4,5,14 Because the broad plume angle is related to the
wall erosion and the thrust loss, various ways to reduce the
plasma plume angle have been investigated.8
The discharge current Id, ion current Ii, electron current
Ie, and propellant utilization Up under various voltages are
plotted in Fig. 3. Ion current Ii was obtained by angular inte-
gration of the measured ion current density, and Ie¼ Id � Ii.
At the same Xe mass flow rate, the CHT has higher Ie and Ii,
which could imply that the ionization rate of Xe is higher for
the CHT. The propellant utilization Up (¼MIi=e _m, where M,
_m, and e are the mass of a Xe atom, Xe mass flow rate, and
electron charge, respectively), which stands for ionization ef-
ficiency, is also higher for the CHT by a factor of 1.6–1.8
than for the AHT and well exceeds unity at anode voltages
from 220 V to 340 V as shown in Fig. 3(d). This large Up
exceeding unity for the CHT was reported previously9,10 and
was explained by the predicted presence of multiply charged
ions that were generated due to the increased ion residence
time inside the discharge channel. The high ionization rate
of the CHT also enables the thruster not only to operate at
low _m but also to keep a high Id even at the same _m in com-
parison with the AHT.
In order to experimentally investigate the multiply
charged ions in detail, the ion species fractions were meas-
ured at several different anode voltages. As illustrated in Fig.
4(a), the presence of multiply charged ions in the CHT is
clearly demonstrated in the E�B spectra that have three
prominent peaks corresponding to Xeþ, Xe2þ, and Xe3þ,
respectively. The figure shows the much higher normalized
peak current for Xe2þ in the CHT than in the AHT and the
distinct manifestation of the Xe3þ peak in the CHT. This
result can explain the reason Up> 1 for the CHT.
The plume attenuation as a result of charge exchange
(CEX) collisions can occur due to background neutrals
existing between the thruster exit and the E�B probe. This
effect was taken into account through the simplified CEX
correction model,15 which is a way to correct the attenuation
fraction between ions and background neutrals. The attenua-
tion fraction of each charge state can be written as
j
j0
� �Xeþ¼ e�n0r1z; r1 ¼ 87:3� 13:6 log Vdð Þ; (1)
j
j0
� �Xe2þ¼ e�n0r2z; r2 ¼ 45:7� 8:9 log 2Vdð Þ; (2)
j
j0
� �Xe3þ¼ e�n0r3z; r3 ¼ 16:9� 3:0 log 3Vdð Þ; (3)
where j is the E�B collector current, j0 is the current at the
thruster exit, ðj=j0ÞXekþ is the attenuation fraction, rk is the
CEX cross section for Xekþ (k¼ 1, 2, 3), n0 is the neutral
density, and z and Vd are the distance from the thruster exit
to the E�B probe and the discharge voltage of 300 V,
respectively. Here, the collision is regarded as a symmetric
reaction between an ion and a background neutral, i.e.,
XeþþXe ! XeþXeþ and Xe2þþXe ! XeþXe2þ. In
addition, Xe4þ and Xe5þ ions are neglected because their
fractions are considered to be small compared with Xe2þ and
Xe3þ and the calculation of the CEX correction for Xe4þ
and Xe5þ is complicated. The calculated attenuation frac-
tions for Xeþ, Xe2þ, and Xe3þ are 0.88, 0.95, and 0.98,
respectively. We define the ion current fraction Xk as
Xk ¼ Ik=Ii ¼ Ik=P3
k¼1 Ik ¼ nkZ3=2k =
P3k¼1 nkZ
3=2k , where nk,
Zk, and Ik are the number density, the charge state, and the
current of Xekþ (k¼ 1, 2, 3) ions, and Ii ¼P3
k¼1 Ik. The frac-
tions Xk were calculated by the triangle fitting method15 that
takes each triangle area as the amount of collected current at
each charge state, and are plotted in Figs. 4(b) and 4(c). The
fraction of the multiply charged ions X2 þ X3, which is the
sum of the current fractions of Xe2þ and Xe3þ, is 40%–52%
for the CHT and 10%–17% for the AHT. This large fraction
FIG. 3. (a) Discharge current, (b) ion
current, (c) electron current, and (d)
propellant utilization in relation to an-
ode voltages.
144104-3 Kim et al. Appl. Phys. Lett. 105, 144104 (2014)
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for the CHT could be caused by the existence of slow ions
that have a long residence time inside the discharge chan-
nel.4 Since the large fraction of the multiply charged ions is
strongly linked to both the high thrust density and channel
erosion12 due to their high energy, the study on multiply
charged ions according to their fraction is important for the
thruster performance.
The overall performances of the thrusters, which are the
specific impulse Isp (¼T= _mg ¼ vi _mi= _mg, where vi, _mi, and gare the ion velocity, the ion mass flow rate, and the gravita-
tional acceleration, respectively), thrust T, and anode effi-
ciency g (¼T2=2 _mIdVa, where Va is the anode voltage), are
plotted in Fig. 5. Although the anode efficiency g is only
slightly (about 7%) higher for the CHT, Isp and T of the CHT
are much (about 40%) higher than those of the AHT at
Va¼ 300 V. At the same Va and _m for both thrusters, the
much higher Isp and T for the CHT are considered to be
attributed to the multiply charged ions and the high ioniza-
tion rate. In order to further understand the effect of Xe2þ
and Xe3þ in Isp and T, we define the effective ion speed
vi; eff ¼P3
k¼1 Nkvk=P3
k¼1 Nk and the effective ion mass flow
rate _mi; eff ¼P3
k¼1ðM _NkÞ ¼ MP3
k¼1ðAnkvkÞ, where vk and
A are the speed of the Xekþ ion and the hemispherical area,
respectively, and Nk is the number of Xekþ ions. Then,
the thrust and specific impulse can be expressed as
T ¼ vi; eff _mi; eff and Isp ¼ vi; eff _mi; eff= _mg. By assuming that
the ion current fractions measured at the thruster axis repre-
sents the most probable value in the entire plume, we use vk
obtained from the E�B spectra and nk from Xk, i.e.,
nk ¼ CXk=Z3=2k , where C is a constant. The calculated vi; eff
and _mi; eff for the CHT are 1.10 and 1.34 times higher,
respectively, than those for the AHT, which suggests that the
FIG. 4. (a) Normalized collector current versus bias potential of the E�B
probe. Ion current fraction of (b) the AHT and (b) the CHT corrected by
CEX collisions as a function of the anode voltage.
FIG. 5. (a) Specific impulse, (b) thrust, and (c) anode efficiency versus the
anode voltage for the AHT and CHT.
144104-4 Kim et al. Appl. Phys. Lett. 105, 144104 (2014)
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143.248.6.145 On: Fri, 10 Oct 2014 14:54:27
high Isp of the CHT is more influenced by _mi; eff than vi; eff .
By following the calculation, both Isp and T of the CHT are
higher by 1.47 times than that of the AHT, which is
consistent with the measurement shown in Fig. 5. In addi-
tion, at the same anode voltage, the anode efficiency g is
about 3%–4% higher for the CHT due to the high Ii as shown
in Fig. 5(c).
On the other hand, another aspect is that the high ioniza-
tion rate and high multiply charged ion fraction in the CHT
give rise to a high discharge current Id that raises the power
consumption, and the broad plume angle causes the less effi-
cient use of thrust. It is also noted that a higher magnetic field
is required in the CHT to produce a thrust value similar to that
in the AHT in our experiment. By taking the coil power into
account, the thrust efficiencies (gt ¼ T2=2 _mðPa þ PcoilÞ,where Pa ¼ IdVa and Pcoil is the coil power) for AHT and
CHT are 34% and 30%, respectively, at the same anode
power of 300 W.
The high ionization rate and multiply charged ion frac-
tion have merits and demerits depending on limitations in ei-
ther the power or the propellant consumption. When there is
a limitation of the power consumption, AHTs show higher
efficiency, but if the limitation is in the propellant consump-
tion, CHTs have higher propellant efficiency.
In summary, performance and plume characteristics for
AHT and CHT were investigated under identical operating
conditions, such as channel diameter, channel depth, and Xe
flow rate. The ion current distribution was broader for the
CHT due to its distinct magnetic field lines forming equipo-
tential surfaces. The propellant utilization of the CHT
exceeded unity because of the high multiply charged ion frac-
tion and high ionization rate. The thrust and specific impulse
were much higher for the CHT at the same anode voltage
because of the higher _mi; eff and vi; eff . However, the large frac-
tion of multiply charged ions also caused the high Id that
raised the power consumption. Therefore, both thrusters can
be selectively used for low power or low propellant missions
depending on the criteria for each situation.
The authors thank Dr. Mihui Seo for her substantial
support. This work was partly supported by the Space Core
Technology Program (Grant No. 2014M1A3A3A02034510)
through the National Research Foundation of Korea funded
by the Ministry of Science, ICT and Future Planning. This
work was also partly supported by the Korea Institute of
Materials Science (KIMS) (Grant No. 10043470) funded by
the Ministry of Trade, Industry and Energy.
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