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Effect of magnetic field configuration on the multiply charged ion and plume
characteristics in Hall thruster plasmas
Holak Kim, Youbong Lim, Wonho Choe, Sanghoo Park, and Jongho Seon
Citation: Applied Physics Letters 106, 154103 (2015); doi: 10.1063/1.4918654
View online: http://dx.doi.org/10.1063/1.4918654
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APPLIED PHYSICS LETTERS 106, 154103 (2015)
Effect of magnetic field configuration on the multiply charged ion
and plume characteristics in Hall thruster plasmas
Holak Kim,1 Youbong Lim,1 Wonho Choe,1,a) Sanghoo Park,1 and Jongho Seon2
1
Department of Physics, Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu,
Daejeon 305-701, Republic of Korea
2
Department of Astronomy and Space Science, Kyung Hee University 1732 Deokyoungdaero, Giheung-gu,
Yongin-si, Gyeonggi-do 446-701, Republic of Korea
(Received 5 March 2015; accepted 7 April 2015; published online 15 April 2015)
Multiply charged ions and plume characteristics in Hall thruster plasmas are investigated with
regard to magnetic field configuration. Differences in the plume shape and the fraction of ions with
different charge states are demonstrated by the counter-current and co-current magnetic field configurations, respectively. The significantly larger number of multiply charged and higher charge
state ions including Xe4þ are observed in the co-current configuration than in the counter-current
configuration. The large fraction of multiply charged ions and high ion currents in this experiment
may be related to the strong electron confinement, which is due to the strong magnetic mirror effect
C 2015 AIP Publishing LLC.
in the co-current magnetic field configuration. V
[http://dx.doi.org/10.1063/1.4918654]
Electric propulsion, such as pulsed plasma thrusters, ion,
and Hall thrusters, has been widely studied and developed as
a result of its various advantages, including a high specific
impulse and reduced propellant consumption, in comparison
with chemical propulsion.1 In particular, Hall thrusters,
which have simple and compact device structures, generate
higher thrust and specific impulse at a given power1 on
account of the presence of unlimited space charges. This renders Hall thrusters more valuable for space applications.2
In Hall thrusters, the electrons are confined by a closed
E B drift within crossed electric and magnetic fields, while
the unmagnetized ions are accelerated by the electric field in
quasineutral plasmas.3 The strength and shape of the Hall
thruster magnetic field are strongly linked to the plasma and
beam characteristics.4–6 In addition, the performance parameters, such as the anode efficiency,4,7 channel erosions,8 and
lifetime,7 are strongly dependent on the topology of the magnetic field.7 Therefore, various magnetic field configurations
have been studied and developed in order to improve the
characteristics of Hall thruster plasmas.
The multiply charged ions such as Xe2þ and Xe3þ have
rarely been observed in conventional annular type Hall effect
plasmas, and thus, the effects of multiply charged ions corresponding to the configurations of the magnetic field configurations have not been studied extensively. However, the
presence of multiply charged ions is strongly related to
thruster performance,9–12 power dissipation, wall erosion,13
etc., because of the high momentum and charge state of these
particles. On the other hand, a large multiply charged Xe ion
fraction9,14 has been clearly confirmed in cylindrical-type
Hall thrusters (CHTs) for low-power applications.15–19 As
will be discussed below, we report on the significant influence of the different magnetic field geometries, which are
obtained through a simple modification of the coil polarity,
on the thruster performance and beam characteristics. These
characteristics include the angular distribution of the ion
beam, the ion current, the electron current, and the fraction
of multiply charged ions. In this paper, under identical operational conditions, we investigate the features of the ion
beam, especially in relation to the multiply charged ions, in
accordance with the magnetic field geometry.
The thruster used in this work is a CHT with an outer
channel diameter of 50 mm and a channel depth of 24 mm,
as shown in Fig. 1. It consists of two electromagnetic coils, a
boron nitride ceramic channel and an annular anode that also
acts as a gas distributor. The coils are supplied with currents
by separate power supplies. By adjusting the coil polarity,
the thruster produces two types of magnetic field configurations,3,4 namely, counter-current and co-current configurations. When the current in each coil flows in the opposite
direction, the coils produce the counter-current configuration, as shown in Fig. 1(a), which is often referred to as the
cusp configuration. On the other hand, for the co-current or
direct configuration, the currents in the coils flow in the
same direction, as depicted in Fig. 1(b).
a)
FIG. 1. Schematic of CHT with magnetic field lines for the (a) countercurrent and (b) co-current configurations.
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0003-6951/2015/106(15)/154103/5/$30.00
106, 154103-1
C 2015 AIP Publishing LLC
V
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154103-2
Kim et al.
In Fig. 2, we show the radial magnetic field strength
along the outer channel wall and the axial magnetic field
strength along the thruster axis. Along the outer channel wall,
the radial magnetic field Br has a large peak in the middle of
the channel in the counter-current configuration, and has the
maximum value near the channel exit in the co-current configuration. It is noted that the maximum value of Br is larger
by a factor of 1.6 with respect to the value in the countercurrent configuration at the outer channel wall. Along the
thruster axis, the axial magnetic field Bz is 1.3–2.7 times
larger for the co-current configuration than for the countercurrent configuration, as illustrated in Fig. 2(b). As regards
conventional annular-type Hall thrusters, a strong Br near the
channel exit contributes to the confinement of electrons by an
E B drift within crossed electric and magnetic fields. In
comparison, the large axial magnetic field gradient in CHTs
provides additional electron confinement near the axis
through the magnetic mirror effect.4,15,20 The mirror ratios
along the magnetic field lines from the inner core to the outer
wall of the channel shown in Fig. 1 are 1.5–3.0 for the
counter-current and 2.0–4.5 for the co-current configuration.
The mirror ratios are greater by 1.2–1.5 for the co-current
configuration than for the counter-current configuration.
In this experiment, the operating pressure was maintained below 35 lTorr in a 3 m long and 1.5 m diameter
FIG. 2. (a) Radial magnetic field strength along the outer channel wall and
(b) axial magnetic field strength along the thruster axis for the countercurrent and co-current configurations.
Appl. Phys. Lett. 106, 154103 (2015)
vacuum chamber, and the thruster was operated at a 7 sccm
Xe flow rate through the anode. A commercial hollow cathode (Heatwave HWPES-250) was used to provide electrons
for the discharge, and the Xe mass flow rate was kept at
1 sccm. For co-current and counter-current magnetic field
configurations, the currents of coil 1 and coil 2 were þ1.5 A/
þ1.5 A and þ1.5 A/1.5 A, respectively (Fig. 1). The characteristics of the plasma plume were investigated using
E B and Faraday probes. The former is a velocity filter,
which distinguishes between ions with different charge states
by selecting those satisfying the Lorentz force equation by
the perpendicular electromagnetic forces.21,22 This probe
consists of an entrance collimator, a velocity filter, an exit
collimator, and a collector. The collimators were 70 mm in
length and 4 mm in diameter and were composed of stainless
steel. The magnetic field of the velocity filter was provided
by two permanent magnets of 0.23 T in strength, and an electric field was applied through a pair of metal plates separated
by 10 mm. The collector was shaped as a cone with a cylindrical tube, and the filter body was covered by carbon steel.
The probe casing was electrically grounded, and its interior
was evacuated through several holes.9 The E B probe was
located at 540 mm from the thruster exit plane and on the
thruster axis, aligned by a laser alignment system at the
thruster axis. The Faraday probe consisted of a collector and
a guard ring, and the current density was collected by a commercial picoammeter (KEITHLEY 6485). This probe was
mounted on a rotation stage of radius 480 mm at the thruster
exit, and the total ion current was obtained by integrating the
angular current density from 90 to 90 with respect to the
thruster axis.
The type of magnetic field configuration could be recognized from photographs of the plasma plume. These images
were captured for both field configurations using a commercial digital single-lens reflex (DSLR) camera installed in a
side window of the vacuum chamber, as shown in Fig. 3(a).
The upper and lower parts of the photograph show the
counter-current and the co-current configurations, respectively. During the experiment, the generated plasma plumes
were distinguishable by the naked eye for both field configurations. The emission intensity profile along the thruster axis
was calculated from RGB pixel values of plume images at
the same scale, as shown in Fig. 3(b). Here, the beam image
captured by the camera in Fig. 3(a) spreads farther along the
thruster axis for the co-current configuration. The emission
intensity calculated from the RGB image in Fig. 3(b) is also
higher for the co-current configuration.
In Fig. 4, we plot the discharge current Id , the ion current Ii , the electron current Ie , and the current and propellant
efficiencies for both field configurations as measured by
the Faraday probe at various anode voltages. Here, Id and Ii
are typically higher in the co-current configuration than in
the counter-current configuration, for the entire set of anode
voltages. The propellant efficiency gp , which is defined as
_ where M, m,
_ and e are the mass of a Xe atom,
gp ¼ MIi =em,
the Xe mass flow rate, and the electron charge, respectively,
is higher than unity. This value increases with increasing anode voltage in both field configurations. An gp value exceeding unity implies that a large number of multiply charged
ions are present in the discharge channel,9,18 since Ii is
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Kim et al.
FIG. 3. (a) Photograph of plume for counter-current (top) and co-current (bottom) configurations. (b) Emission intensity analyzed from the color map along
the thruster axis. The thruster was operated at an anode voltage of 300 V.
proportional to Zi , where Zi is the ion charge state. The electron current Ie (¼Id Ii ) of the co-current configuration is
lower than that of the counter-current configuration, although
Appl. Phys. Lett. 106, 154103 (2015)
both Ii and Id are higher in the co-current configuration. The
reason for the lower Ie for the co-current configuration may
be attributed to the strong magnetic mirror effects in the
channel, which aid electron trapping4,16–19 and which may
also prevent electrons from traveling toward the anode.15 As
a result, both gp and the current efficiency gc (¼ Ii =Id Þ are
higher in the co-current configuration than in the countercurrent configuration, by 8%–12% and 6%–8%, respectively.
The enhanced performances, which are related to different
characteristics of the ion beam, were investigated in our previous study.9
The fraction of multiply charged ions, primarily Xe2þ
and Xe3þ, was obtained using an E B probe, whose measured spectrum is shown in Fig. 5. It can be seen that the
E B probe voltages at the peaks, which are proportional to
the ion energies, are similar for both the co-current and the
counter-current configurations. However, we note that the
ion current fractions are significantly different in the highenergy range. In the co-current configuration, the fractions of
Xe2þ and Xe3þ are much higher than those in the countercurrent configuration. In addition, highly charged ions such
as Xe4þ were also observed. The detailed fractions of the
multiply charged ions were calculated by integrating the
area of the measured current distributions.23 Under the
assumption that the ion current fractions at the thruster
axis represent those in the plume, the ion current P
fraction
4
is
defined
as
X
¼
I
=I
¼
I
=
Xk (k ¼ 1,2,3,4)
k
k i
k
k¼1 Ik
3=2 P3
3=2
¼ nk Zk = k¼1 nk Zk , where Ik , Zk , and nk are the current,
charge state, and number density of the Xekþ ions in the
plume regions, respectively.9 The calculated ion current fraction Xk plotted in Fig. 5(b) clearly demonstrates the higher
population of Xe2þ and Xe3þ and the existence of Xe4þ
(3%) for the co-current configuration.
The sums of the
P4
multiply charged ion fractions
k¼2 Xk are approximately
46% and 33% for the co-current and the counter-current configurations, respectively. The larger fraction of multiply
charged ions in the co-current configuration may be closely
FIG. 4. (a) Discharge current, (b) ion
current, (c) electron current, and (d)
propellant and current efficiencies with
regard to anode voltages.
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154103-4
Kim et al.
FIG. 5. (a) Normalized E B spectra, (b) histogram of ion current fractions,
and (c) modified propellant efficiency of the counter-current and co-current
configurations at an anode voltage of 300 V.
related to the enhanced electron confinement, which is attributed from the strong magnetic mirror effect that retains electrons inside the channel15 and increases the ionization rate.
Furthermore, the high ion density in the channel due to the
increased ionization could also give rise to increased residence
time through an enhanced ambipolar potential.4 Therefore,
those aforementioned effects may contribute to the generation
of multiply charged ions in the co-current configuration.
We have also examined the effect of the multiply
charged ions on the propellant efficiency. To this end, the
modified propellant efficiency gp;k for each ion charge k,
_ k =Zk ¼ ðM=emÞ
_
their sum gp;m are defined as gp;k ¼ ðM=emÞI
P4
1,22
Ii Xk =Zk and gp;m ¼ k¼1 gp;k .
The calculated gp;k and gp;m
with the determined Xk are depicted in Fig. 5(c) for both
field configurations at an anode voltage of 300V. Note that
Appl. Phys. Lett. 106, 154103 (2015)
gp;m is less than gp as a consequence of the definition of gp;k .
The level of contribution of the multiply charged ions
P4
k¼2 gp;k to gp;m is approximately 15% and 25% in the
counter-current and the co-current configurations, respectively. High gp;m can enhance the thruster performance, in
terms of thrust, specific impulse, and efficiency, because gp;m
is closely related to the ion flux in the plume. The overall ion
momentum is also increased by a high fraction of multiply
charged ions with high energies.9
The effect of the multiply charged ion and high ionization
rate on the overall thruster performance is studied by defining
ion mass
the effective ion speed vi; eff and effective
P4
P4 flow rate
are
defined
as
v
¼
N
v
=
m_ i; eff ; these
i;
eff
k
k
k¼1
k¼1 Nk and
P
P
m_ i; eff ¼ 4k¼1 ðMN_ k Þ ¼ MA 4k¼1 ðnk vk Þ, where vk and Nk are
the speed of the Xekþ ion and the number of Xekþ ions, respectively, and A is the hemispherical area of the plume
with respect to the thruster exit and radial center.9 Using the
above values, the thrust T and specific impulse Isp can be
_ Here, vk
expressed as T ¼ vi; eff m_ i; eff and Isp ¼ vi; eff m_ i; eff =mg.
is obtained from the E B probe spectra shown in Fig. 5(a)
and nk can be calculated from Xk .9 At an anode voltage of
300 V, the calculated vi; eff and m_ i; eff in the co-current configuration are higher by a factor of 1.06 and 1.05, respectively,
than those in the counter-current configuration. Thus, T
and Isp are 1.11 times higher in the former than in the
latter. Furthermore, the measured T and the anode efficiency
_ d Va Þ in the co-current configuration are approxig ð¼ T 2 =2mI
mately 12 mN and 35%, respectively, which are 9% and 12%
higher than those in the counter-current configuration. Our
result regarding the higher anode efficiency g in the co-current
configuration is consistent with reported previously.4 All the
characteristics reported here exhibited better performance in
the co-current configuration. We believe that it is attributed to
the additional electron confinement and the enhanced multiply
charged ion fraction generated by the magnetic field arrangement, which has a stronger axial magnetic field and a mirror
effect. This may also suggest that the magnitude of the magnetic field, compared to the shape of the magnetic field, has
more significant effects on the thruster performance than the
magnetic field shape, in our experiment. A detailed analysis of
the measured parameters and the related characteristics will be
given in a subsequent paper.
In summary, the ion beam characteristics in conjunction
with multiply charged ions were investigated according to
the magnetic field configuration under identical operational
conditions of the cylindrical Hall thrusters. Compared to the
counter-current field configuration, significant differences
were observed for the co-current configuration: (i) the shape
of the plasma plume based on the camera images was further
extended in the axial direction, (ii) the emission intensity
was higher, (iii) the ion current Ii , discharge current Id , and
the propellant gp and current gc efficiencies were higher,
whereas the electron current Ie was lower, (iv) we also
observed that, among the entire ions, the total fraction of
multiply charged ions (Xe2þ, Xe3þ, and Xe4þ) was higher
by 11%, and Xe4þ ions also appeared, and finally, (v) the
measured values for Isp and T were higher, which is consistent with the calculated values. In contrast with the countercurrent field configuration, the strong axial magnetic field
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154103-5
Kim et al.
and the magnetic mirror effect are believed to be responsible
for the better thruster performance.
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 of Korea.
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