Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA18 / P30
Time-Resolved Cavity Ring-Down Spectroscopy for Erosion Sensor of Hall Thruster
1

Yusuke Egawa1, Naoji Yamamoto1, Atsushi Yamaguchi1, Yan Huang2
Interdisciplinary Graduate School of Engineering Science, Kyushu University, Fukuoka, Japan
2
School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China
Email: yusuke@aees.kyushu-u.ac.jp

Abstract: The life limiter of Hall thruster is ion bombardment on the acceleration channel. To estimate the lifetime of thethruster, sputtered atoms should be measured. However, the conventional method, the endurance long term test requires
a huge amount of time and money. Therefore, erosion sensor using cavity ring-down spectroscopy has been developed. In
this study, titanium atoms were measured as erosion material by our built sensor. Then, the large noise caused by thedischarge current oscillation was observed so that time-resolved system was introduced and improved the measurement
accuracy.
Keywords: Space Propulsion, Hall thruster, Erosion measurement, Spectroscopy
the ring down signal is attenuated only by the mirrors, on
the other hand, when the atoms exist, its absorb the laserlight. Then, ring down time is given as𝐿
𝜏(𝛾) =(2)𝑐[(1 − 𝑅) + ∫ 𝑘(𝑥, 𝛾) 𝑑𝑥]

1. INTRODUCTION
In past a few decades, electric propulsion achieved morethan 5,000 hours and succeeded in various space
missions. Additional to it, space missions exceeding
10,000 hours has been also increased and several studies
about extending life has been conducted1. To extend thelifetime, it is necessary to optimize thruster’s shape andmagnetic field configuration.
The primary lifetime limiter is wall erosion due to ion
sputtering2. To evaluate the lifetime of the thruster, themeasurement of sputter erosion is necessary. However,the conventional measuring method, the durability test
spends tremendous cost both in the manpower and thetime3. Moreover, measuring parameters is enormous andthe erosion is infinitesimal so that the conventional
method is impractical. Therefore, fast time response andhigh sensitive sputter erosion measurement system isrequired. Considering these requests, in this study,
sputter measurement system using cavity ring-down
spectroscopy (CRDS)4 for electric propulsion has beendeveloped.5-7
2. CRDS
Cavity ring-down Spectroscopy (CRDS) is a highly
sensitive spectroscopic method to which laser absorption
spectroscopy (LAS) is applied. Figure 1 shows theprinciple of CRDS. CRDS has an optical cavity
consisting of two high reflectivity mirrors and it confines
a laser beam. When the absorption species which hassame excitation wavelength with the laser exist in thecavity, the species eats the incident laser and thetransmitted laser is attenuated. Then, the transmitted lightis called ring-down signal and, the ring- down time isdefined when the ring-down signal become 1/e times.
From the ring-down time, the density of the measuring
atoms can be calculated.
In the CRDS, the laser beam reflects in the cavity manytimes so that optical path can be longer and measuring
sensitivity can be enhanced compared to LAS.
The ring-down signal, S(𝑡, 𝛾) is given as𝑡0
𝑆(𝑡, 𝛾) = 𝑆) 𝑒𝑥𝑝 -−(1)𝜏(𝛾)τ(𝛾) is defined as ring down time, which 𝑆) reaches 1⁄𝑒times.
When measuring atoms don’t exist in the optical cavity,

When measuring atoms don’t exit, 𝜏) , ring down time isgiven as𝐿(3)𝜏) =𝑐(1 − 𝑅)because the term of absorption can be ignored.
When absorption degree, 𝐴𝐵𝑆(𝛾) is defined as integralof absorption coefficient, from equations (2) and (3) it
can be expressed as
𝐿 1
1𝐴𝐵𝑆(𝛾) ≡ A 𝑘(𝑥, 𝛾) 𝑑𝑥 = − 0(4)𝑐 𝜏(𝛾) 𝜏)
Accordingly, absorption degree can be obtained frommeasurement of ring down time. From equation (4),absorption spectrum of measuring atoms can be obtained
when the laser frequency is swept. Line density of lower
level on the optical path of the laser beam can becalculated as
J𝑔C 𝛾IC(5)A 𝑁C dx = 8𝜋
KA 𝐴𝑏𝑠(𝑑𝛾)N𝑔I 𝐴IC 𝑐 J
An estimation of the minimum detectable absorbancecan be obtained from the noise of ring down time in theCRDS. The minimum detectable absorbance, 𝐴𝐵𝑆OCP iscalculated as∆𝜏)(1 − 𝑅)
𝐴𝐵𝑆OCP =(6)𝜏)where ∆𝜏) is standard deviation for 𝜏) .
In this study, titanium atoms were detected as erosion
material by the CRDS system. The detailed parameter for
excitation from ground state is shown in Table 1 and Fig.
2.

- 47 -resolved by the phase of the discharge current oscillation
was developed.
Time-resolved CRDS is to analyze the data with thephase of the discharge current frequency. Then, Volterra
Engine9 which can constantly lock the discharge current
frequency was used. Figure 6 shows the time variation ofthe discharge voltage and discharge current. Then,ON/OFF switching frequency was 20 GHz and the duty
ratio was 20%.
3.3. VACUUM FACILITY
The measurement was conducted in vacuum chamber atKyushu University as shown in Fig. 7. The vacuum
chamber was a cryogenically pumped, cylindrical,
stainless steel vacuum chamber that is 1.0 m in diameterand 1.2 m in length. It has a rotary pump, two turbo
molecular pumps and a cryogenic pump maintaining thepressure below 7.0×10-4 Pa without thruster operation
and 8.8×10-3 Pa (for xenon) during thruster operation.
3.4. HALL THRUSTERIn this study, a 200 W class magnetic layer type Hallthruster developed at Kyushu University was used for ion
source. The overview of the thruster is shown in Fig 8.
The outer diameter is 56 mm and the inner diameter is 40
mm. The anode is located 25 mm upper from the thruster
exit. The acceleration channel is made of boron nitride.
The thruster has 4 solenoidal coils at each corners and asolenoidal coil at the center of the thruster to create andcontrol radial magnetic field in the acceleration channel.
The magnetic flux density and configuration can bechanged by controlling the ratio of the inner coil current
to the outer coil current.
During the measurement, Hall thruster was operated atmass flow rate of 0.68 mg/s.
As an electron source, hollow cathode (Ion Tech,HC252) was used. Operating gas was xenon. When thecathode was ignited, keeper voltage was 14 V and mass
flow rate was 0.27 mg/s. While operation, keeper current
was set at XX A.
3.6. SETUP
The experiment was conducted by using the experiment
setup as shown in Fig. 4. The laser passed 30 mm front
and the titanium target was set 60 mm front from thethruster exit. When the thruster operates, ions areaccelerated and sputtered the target. Then, titanium
atoms were produced and measured by the laser. Thruster
and the CRDS system was mounted in the carbon box forthe protection from the heat and distributed titanium
atoms.

Fig. 1. Principle of CRDS
44p 3D1

Energy, eV32

λ=394.979 nm
3.13900 eV

1
04s2 3F2ground state level

Fig. 2. Grotrian diagram from ground state of Ti8

Table 1. Transition parameter from ground state of Ti8

Transition
4𝑠 𝐹J → 4𝑝 U𝐷X
J U

λ[nm]
394.979

𝐴IC
5.60 × 10^

𝑔C
5𝑔I33. EXPERIMENTAL
3.1 CRDS SYSTEM
Figure 3 shows a schematic diagram of our built CRDS
system. A Littrow external cavity diode laser (DL100 pro,TOPTICA) was used as a laser light source. To detect thetitanium atoms, the wavelength was swept from 394.976
nm to 394.984 nm. The modulated laser was divided into
probe light, reference light and etalon light by beam
splitters. The probe light was modulated to the 1st orderdiffracted light by acousto-optic modulator (AOM), thelight was reduced by ND filter to restrain absorption
saturation, and the light passed through the cavity.
Finally, the signal was amplified by photomultiplier tube(PMT) and detected. The reference light was used foroptogalvanic spectroscopy. The light was chopped by
optical chopper and passed through the HCL. The passed
signal wad amplified by lock-in amplifier to improve S/Nratio. The etalon light was detected after passed throughthe etalon (FSR=1.5 GHz).
3.2 TIME-RESOLVED CRDSHall thruster has a discharge current oscillation. It causes
the fluctuation of ring-down signal and makes themeasurement noise large. When the operating parameterssuch as the magnetic field configuration were slightly
changed to assess the life time of Hall thruster, it is notavailable to recognize the small difference of the effect.
Therefore, the-time resolved CRDS system which can be

Fig. 3. Schematic diagram of CRDS system- 48 -

Absorbance, ppm

120100806040
200-12 -10 -8 -6 -4 -2Fig. 4. Experimental setup024

6

Absolute frequency, GHz
Fig. 6. Absorption spectrum versus absolute frequency (alldata)

4. RESULTS AND DISCUSSIONFigure 5 shows time variation of mirror reflectivity. It isclear that the mirror reflectivity was gradually decreasing
during the measurement. The data was alternatelyobtained under vacuum and thruster operation 10 times
each. Then, about 100 ring-down signals under eachconditions were obtained at a time.
The mirror reflectivity under vacuum was higher thanthat under thruster operation because the mirrors werecontaminated by the titanium atoms which was produced
from the target. While the mirror reflectivity wasdecreasing under thruster operation, the mirror
reflectivity was slightly increasing under vacuum. This
would be because the titanium atoms on the mirrors wereevacuated.
Figure 6 shows absorbance when the time-resolved
CRDS was not used. Figure 7 shows absorbance when
the time-resolved CRDS was used. When the timeresolved CRDS was used, it is clear that the dispersion
was reduced.
Figure 8 shows absorbance distribution calculated by
300 ring-down signals. Figure 9 shows absorbance
distribution calculated by 10,000 ring-down signals withtime-resolved CRDS. The two has peak at absolute
frequency of around -4 GHz, however, compared to fig.
8, the dispersion seems to be small. Therefore, the timeresolved effect could be observed.

Absorbance, ppm

120100806040
200-12 -10 -8 -6 -4 -2

02

46

Absolute frequency, GHz
Fig. 7. Absorption spectrum versus absolute frequency(time-resolved)
Absorbance, ppm

2.0

120.0

105.0

1.6Phase, π

90.00

75.00

1.2

60.00

0.8

45.00

30.00

0.4
15.00

0.0-12 -10 -8 -6 -4 -2 0

Mirror reflectivity, %99.975
VacuumThruster operating

99.9700.000

246Absolute frequency, GHz
Fig. 8. Absorbance distribution

99.96599.9602.099.955

1.6Absorbance, ppm
120.0

105.0

90.00

050

100150Phase, π99.950200

Data number
Fig. 5. Time variation of mirror reflectivity

75.00

1.2

60.00

0.8

45.00

30.00

0.4
15.00

0.000

0.0
-12 -10 -8 -6 -4 -2 0246

Absolute frequency, GHz
Fig. 9. Absorbance distribution (time-resolved)- 49 -5. CONCLUSION
In this study, CRDS system was developed for theerosion assessment in Hall thruster. It was observed thatthe mirror reflectivity was gradually decreased during theHall thruster operation because of the contamination of
the sputtered particles on the mirror surface. On the otherhand, the mirror reflectivity was improved when Hallthruster was not operating.
In addition, to improve the measurement accuracy, thetime-resolved system was introduced. As a result, thedispersion could be reduced and our built time-resolved
CRDS system was validated.
As a future work, this time-resolved CRDS system will
be arranged to be able to detect aluminum atoms andmeasure the wall erosion of 200 KW class Hall thrusterwhich was developed at Kyushu University.
Acknowledgments
This work was supported by JSPS KAKENHI Grant
Number, JP18H03815 and JAXA. The contributions ofProf. Ikoh Funaki and Prof. Ryudo Tsukizaki are greatly
appreciated.
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