九州大学大学院総合理工学府報告第 37 巻 第 2 号 6-11 頁 平成 28 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 37, No. 2, pp. 6- 11, FEB, 2016

Measurement of Electron Density and Electron
Temperature in Decaying Arcs Containing Decomposed
Gas from Polymer
Hassaballa SAFWAT *1 and Kentaro TOMITA *2,†
†E-mail of corresponding author: tomita@ence.kyushu-u.ac.jp(Received January 14, 2016, accepted January 22, 2016)
Electron density (ne) and electron temperature (Te) in arc discharges generated in theatmospheric air were measured using the laser Thomson scattering (LTS) technique. The arcs weregenerated using tungsten electrodes with a gap length of 2 mm. At the tip of the cathode electrode, apolymer material—polyoxymethylene (POM) or polytetrafluoroethylene (PTFE)—was inserted togenerate the arcs containing the decomposed gas from polymer. LTS results showed that ne in thearcs varied with the polymer material used. In particular, with POM, the decay speed of ne was muchhigher than that with PTFE. The LTS results are qualitatively consistent with the results of previous
numerical studies, which indicate that a POM ablation gas can decay the arc conductance more
effectively than a PTFE ablation gas.
Key words: Thomson scattering, Electron density, Arc discharges, Polymer, Gas circuit breaker

1.

Introduction

SF6 gas is generally used in electric circuit
breakers. However, because the global
warming potential of SF6 gas is 22,800 times
higher than that of CO2, studies on alternative
gases are being conducted 1–4). In order to
conduct research to find alternative gases,
understanding the characteristics of SF6 arcs
is a prerequisite. In particular, the electron
density (ne) of the arcs is one of the mostimportant parameters because ne is directly
related with arc conductance. Therefore, wehave measured ne in decaying SF6 gas arcs byusing laser Thomson scattering (LTS) 5, 6).
Understanding of arcs containing metallic
vapor or decomposed gas from polymer is also
important. This is because in gas circuit
breakers (GCBs), they unavoidably mix in thearcs and dramatically change the arc
7).
characteristics
Therefore,
severaltheoreticalandexperimental
studiesregarding the effect of polymers or metallic
vapor on arc discharge have been performed
8–10).
However, there have been few
*1

*2
*3

Department of Applied Science for Electronics andMaterials, Graduate student (Present address:
Physics Department, Faculty of Science, Al-Azhar
University, Cairo, Egypt)
Department of Electrical and Material Science

experimental results showing the effect of thedecomposed gas from polymer on ne of the arcs.
In this study, arcs were generated withsimple tungsten electrodes. In addition, at the
tip of the cathode electrode, a polymer
material—polyoxymethylene(POM)or
polytetrafluoroethylene (PTFE)—was inserted
to investigate the behavior of arcs containing
decomposed gas from polymer. The results of
LTS measurements showed that the spatial
distribution of ne and Te varied with thepolymer material used.

Fig.1 Theoretical curves of the Thomson scattering
electron feature spectrum with ne of 5 × 1021, 2 × 1022, and5 × 1022 m−3. Te was fixed at 1eV.

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2.

九州大学大学院総合理工学報告

MethodIn this study, the LTS technique was used tomeasure ne and Te of the arc discharges.
Because the principle of LTS has been
explained in detail previously 11, 12), only ashort explanation is provided here. In addition,
for convenience, we show several examples of
LTS spectra obtained in this research.
Thomson scattering can be categorized by
the parameter α = (kλD)-1, where k is theabsolute value of the differential wavenumber
vector determined by the incident-laser
wavenumber vector and the scattered-lightwavenumber vector, and λD is the Debye
length of plasma. In this study, almost all
spectra were in the collective regime, because
α was between 0.5 and 2 13). In the collective
regime, Thomson scattering consists of twoparts: an ion feature and an electron feature.
Because the spectral width of the ion feature is
too narrow (<0.1 nm) to be measured with aconventional spectrometer 14, 15), only theelectron feature was detected in this study. In
the range of α from 0.5 to 2, ne and Te were
estimated by the peak wavelength and thespectral shape of the electron feature 13).
Figure 1 shows the spectra predicted underthe experimental condition in this study. The
second harmonic of a Nd:YAG laser (λ = 532nm) was used as a light source and scattered
light at an angle of 90° from the laser path was
detected.

Fig.2

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voltage of 7 kV. A 10 Ω resistance was added inseries with the tungsten electrode. The LTS
system has been calibrated using Rayleighscattering signals from a few hundred torr ofnitrogen. For LTS measurements, the second
harmonic of the Nd:YAG laser (λ = 532 nm, 10
ns full width at half maximum) was used as
the probing laser. The scattered light at anangle of 90° from the laser injection wascollected with lenses and focused onto theentrance slit of a triple grating spectrometer(TGS). An intensified CCD camera (Princeton
Instruments, PI-MAX UNIGENII) was used as
the detector. The TGS was used to eliminate
stray light (mainly the scattered laser lightfrom the electrode surface) efficiently. Because
the configuration of the TGS used was almost
the same as that described in a previous report16), here we only describe it briefly. The TGS
consisted mainly of three diffraction gratings
with 2400 lines/mm, six lenses with a focal
length of 250 mm, a laser wavelength stop (0.5mm width) to remove the stray light, anintermediate slit (0.5 mm width), and the
ICCD camera. The entrance slit of the TGS
was set to be 0.2 mm and the total spectral
resolution was 0.3 nm. The timings of the arc
generation, laser injection, and gate opening of
the ICCD camera were well-synchronized witheach other by using a delay pulse generator
and a programmable logic controller. Figure 3
shows the schematic of the electrode setup.
Both electrodes were axially symmetric andthe gap length was 2 mm. The diameters of theanode and cathode were 1 mm and 10 mm,
respectively. At the tip of the cathode, apolymer material, POM or PTFE, with 1 mm
diameter was inserted as shown in this figure.
The material of the electrode was tungsten.

Arc generation and LTS system.

3. Experimental Setup
Figure 2 shows the schematic of the arc
generation system and LTS system. For
generating the arc discharge, a condenser witha capacitance of 2.5 µF was charged at a

7

Fig.3

Schematic of the electrode setup.

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Measurement of Electron Density and Electron Temperature in Decaying Arcs Containing Decomposed Gas from Polymer

4. Results
4.1. Voltage and Current Waveforms
Figure 4 shows the arc voltage waveforms,
which were measured by generating the arcs
with POM and PTFE. In both cases, the decay
constant was 25 µs. The current waveforms
were also measured. As with the voltage
waveforms, the shape of the two current
waveforms was the same, and the decay
constant was 25 µs. The peak voltage andcurrent were 7 kV and 700 A, respectively.

Fig.4

4.2. Self-emission Measurements
Figure5shows
theself-emission
measurements of the arcs generated with thePOM or PTFE polymer by using the ICCD
camera. The gate width was set to 100 ns. The
timings of the measurements were 10, 30, 50,
70, and 100 µs after the start of the discharge.
In these figures, the emission intensity was
artificially colored.

Arc voltage waveforms.

Fig.5 Self-emission of the arcs with POM and PTFE at t
= 10, 30, and 70 µs.

Fig.6 (a) Electron feature spectrum of the arc with POM
measured at t = 10 µs and at z = 0.3 mm. (b) Spectrum
extracted with x ranging from −0.3 to 0.3 mm of Fig. 6 (a).
(c) Spectrum extracted with x ranging from 2.0 to 2.6 mm
of Fig. 6 (a).

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九州大学大学院総合理工学報告第 37 巻第2号

9

ne(×1022 m-3)

Fig.7

Spatial profiles of ne with POM and PTFE measured at t=10, 30, 50, and 70 µs.

POM-W
t=10 μs

Anode

Te

PTFE-W
t=10 μs

Anode

Te
Polymer→

Fig.8

CathodePolymer→

CathodeSpatial profiles of Te with POM and PTFE measured at t = 10, 30, 50, and 70 µs.

10Measurement of Electron Density and Electron Temperature in Decaying Arcs Containing Decomposed Gas from Polymer

4.3. Laser Thomson Scattering
Figure 6 (a) shows an example of the LTS
image obtained in this study. Because an ICCD
camera with 1024×1024 pixels was used as thedetector in this study, the spectral informationand the spatial distribution of the LTS wereobtained simultaneously. Figure 6 (b) and (c)show LTS spectra extracted with x ranging
from −0.3 to 0.3 mm and 2.0 to 2.6 mm, fromthe spectrum shown in Fig. 6 (a), respectively.
The electron feature of collective Thomson
scattering spectra was observed at these
positions. Using theoretical fitting curves
plotted in Fig. 6 (b) and (c), ne and Te were
determined
simultaneously.
The
LTS
measurements were performed at z = 0.3, 0.6,
1.0, 1.4, and 1.7 mm and t = 10, 20, 30, 40, 50,
60, and 70 µs. At all the positions and times, ne
and Te were determined in the same way as
described previously. Figure 7 and 8 show thetwo-dimensional spatial profiles and temporal
evolutions of ne and Te plotted as contour
graphs. Figure 9 shows the temporal evolutionof ne and Te at x = 0.

(10 Ω) and capacitance (2.5 µF) of the electrodecircuit.
From the self-emission measurements, it isconfirmed that the emission intensity
gradually decreased after the arcs were
produced. With time, the spatial distributionof the self-emission became asymmetric.
However, clear differences were not observed
when the different polymers were used.
Here, we discuss the results of the LTS
measurements. As shown in Fig. 6 (a), theshape of the LTS spectra measured in thisstudy has a dip around x = 0. Such a dip incollective Thomson scattering means that ne
and Te were low at around x = 0. In ourprevious research, which was performed
without a polymer material, the LTS spectra
did not show such a dip 17). Therefore, it seems
that this spatial profile of LTS spectra is due tothe influence of polymer ablation.
We estimated the region where the polymer
material expanded from the cathode surface bymeasuring the change in the length of thepolymer material. After producing arcs 200
times, the lengths of both the POM and PTFE
decreased by approximately 0.2 mm. Thismeans that 0.001 mm of POM and PTFE wasablated by the arc production on an average.
The mass flux Γabl of the ablated polymer was
calculated by the formula shown below:

Fig.9 Temporal evolutions of ne and Te at z = 0.3, 1.0, and
1.7 mm (x is fixed to 0).

5. Discussion
The measurements of voltage and current
waveforms show that the characteristics of thearc current and the arc voltage were almost
the same even when the arcs were generated
with different polymers. The decay constantsof the voltage and current were both equal to
25 µs. The results indicate that their decay
constants were determined from the resistance

abl



 V
pol

abl

m pol st(1),where ρpol is the mass density in the solidstate of the polymer material, ΔVabl is thechange in the volume of the polymer material,
Δs is the surface area of the polymer material,
and Δt is the time scale for polymer ablation.
ρpol of POM and PTFE are 1.41×103 and2.20×103 kg/m3, respectively 18). From theexperimental results, ((∆Vabl)/(∆s)) = 0.001 mm.
We assumed that Δt = 50 µs and that Γabl wasconstant in this time scale. The flow velocity
vabl was calculated as follows:

v 
abl

 m
abl



pol

(2),

where ρ is the mass density of the gas phaseof the polymer materials. The value of ρ
depends on the gas temperature. From theresultsofLTS
measurements,
gas
temperatures on the surface of the polymer
material were estimated as 0.3 eV for POM

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九州大学大学院総合理工学報告

and 0.7 eV for PTFE, respectively. Under these
conditions, ρ = 5×10−2 kg/m3 for POM and3×10−2 kg/m3 for PTFE. Accordingly, vabl was
calculated to be 3×102 m/s for POM and1.5×103 m/s for PTFE. Although the method of
estimation of vabl is quite crude, the calculated
result implies that the ablated polymer
expands to the region where the LTS was
performed.
As can be seen from Figure 7 and Figure 8,
near the cathode region, ne of the arc with
POM was smaller than that with PTFE. In
particular, at z = 0.3 mm, ne of the arc with
POM was approximately 10 times smaller
than that with PTFE.
In Ref. [18], the conductance of arcs
containing POM and PTFE has been
investigated by numerical calculations. The
results show that POM can decay the arc
conductance more effectively than PTFE. It isnoted that this difference is caused by thethermalconductivity
andelectricalconductivity of POM and PTFE. The LTS
results are qualitatively consistent with theresults of the numerical analysis and thethermal and electrical properties of POM andPTFE.

6. Conclusion
This paper presented the experimental
approach to understand the characteristics ofarcs containing decomposed gas from polymer.
POM and PTFE were used as the polymer
materials. The measurements of voltage,current, and self-emission did not show any
clear differences when the polymer used as theablation gas was changed. On the contrary, the
LTS results clearly showed that ne and Techanged in the arcs with different polymers
because of ablation. These results indicatethat Thomson scattering can be a powerful
method to study arcs containing decomposedgas from polymer.

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11

Acknowledgments
The authors wish to thank Professor
Kiichiro Uchino and Mr. Daisuke Gojima fromthe Interdisciplinary Graduate School ofEngineering Sciences, Kyushu University for
their substantial support of the study.

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