九州大学大学院総合理工学府報告第 36 巻 第 2 号 1-5 頁 平成 27 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 36, No. 2, pp. 1-5, FEB, 2015

Development of Thomson Scattering Diagnostics for
Cathode-Directed Streamer Discharges in Helium
Nima BOLOUKI*1,† Safwat HASSABALLA*2 Kentaro TOMITA*1
and Kiichiro UCHINO*1
E-mail of corresponding author: bolouki.nima.721@m.kyushu-u.ac.jp†(Received December 27, 2014, accepted December 29, 2014)
Laser Thomson scattering diagnostics has been developed for the study of the streamer moving
toward the cathode in helium near atmospheric pressure. In order to avoid laser perturbation andobtain sufficient intensity of the Thomson scattering signal, two cylindrical lenses were used to
optimize the spot size of a probing laser at a focusing point. At a position of 2 mm from the anode(needle electrode), the electron density and electron temperature were successfully measured to be ~1018 m-3 and ~ 2 eV, respectively, in the initial stage of the primary streamer.
Key words: Non-thermal atmospheric-pressure plasmas, streamers, laser Thomson scattering

diagnostics

1.

Introduction

Atmospheric pressure plasmas produced bypulsed discharges are currently being
investigated as an effective source of
non-equilibrium plasmas. It is well known thatthe electrical breakdown near atmosphericpressure is described by streamer theory.1)
Based on streamer dynamics, the initial stageof electrical breakdown is the development of
nonlinear ionization waves, known as
streamer discharges or primary streamers, ina non-ionized medium,
followed by thecreation of a weakly-ionized non-equilibrium
plasma.2)-4) Then, the streamers cross the gap.
This final stage of evolution is also known as asecondary streamer.
In the study of streamers, some reports havedealt with the structures and propagation of
streamers. For instance, Ref. 5 measured thedevelopment of neon streamers as a function ofelectric field strength and streamer length.
The propagation of air streamers and theirdependence on applied voltage, distance
between electrodes,6) and pressure7) have also
been investigated. In addition, time-resolved
measurements8) have been successfully
performed. Reference 9 measured andsimulated pulsed streamers in three spatial
*1
*2
*3
*4

Department of Electrical and Material Science
Department of Physics, Faculty of Science, Al-Azhar
University, Cairo, Egypt

dimensions.
Furthermore,
spectroscopic
studies of streamer discharges have beenreported. One of these studies utilizedmeasurements and analyses of the absoluteintensities of the second and first negative
systems of molecular nitrogen to determine thereduced electric field and electron number
density.10),11) Also, Ono et al. applied optical
diagnostic techniques, such as laser-induced
florescence, laser absorption, and opticalemission spectroscopy, to measure thedensities and temperatures of different active
species produced inside the pulsed streamer
discharge at atmospheric pressure.12),13)
However, until the present study there has
been no direct measurement of the primary
streamer to examine the temporal evolution ofelectron density (ne) and electron temperature(Te).
Measurements of ne and Te are important tounderstand the basic structure of the streamer.
Also, for the industrial applications ofstreamers, the active species generated inside
the streamers play key roles. Generation ofsuch species is governed by free electrons
inside the plasmas. Therefore, measurementsof ne and Te are essential to understand the
behavior of active species.
Laser Thomson scattering (LTS) has been
known as one of the most reliable methods tomeasure the ne and Te of plasmas. Moreover, it
provides a high spatial resolution.14)-19) The

2

Development of Thomson Scattering Diagnostics for Cathode-Directed Streamer Discharges in Helium

successful application of LTS diagnostics tomicro discharges (secondary streamers) has
been reported previously.20) However, there are
two challenges when the LTS method isapplied to primary streamers. The first one isthat, because of the small Thomson scatteringcross-section and the low electron density ofthe primary streamer (on the order of 1018 m-3),
the scattering signal level becomes extremely
low. The second one is the laser perturbation
against the plasma electric state. Because theprimary steamer accompanies a very highelectric field, the superposition of the strong
electromagnetic field in the laser spot may
change the electric state in the streamer.
The current investigation aims to solve theabove-mentioned problems of the LTS method
to diagnose the primary streamer and present
the temporal evolution of ne and Te in theinitial stages of the cathode-directed streamer.

2. Experimental setup and procedure
The electrode set consists of a needle
electrode (diameter 1.6 mm) and ahemispherical electrode (diameter 2.4 mm)with a gap distance of 16 mm. These electrodes
are made of tungsten and were placed in a
vacuum chamber. Power was applied byrepeated nanosecond high-voltage pulses witha rise time of 10 ns and a peak voltage of 3 kV.
The mono-polar high-voltage pulses weregenerated by a DC power supply using a fasthigh-voltage semiconductor switch (BEHLKE
HTS50-60) with repetition rate of 10 Hz. The
mono-polar pulse generated pulsed discharge
in a pure helium gas at pressure of 250 Torr.
Our previous research report of thecapacity-coupled discharge20) describes asimilar experimental setup and electricalcircuit. The polarity of the voltage applied to
the needle electrode was positive, so that it led
to the formation of a positive streamer moving
toward the cathode.
The principles of Thomson scattering
diagnostics have been reviewed already, 19),20)
and therefore, we only mention that theThomson scattering in this study is in theincoherent regime and that the scattering is
induced by electrons moving independently.
The scattered light intensity yields theelectron density ne. The thermal motion ofelectrons causes the scattered spectrum to be
Gaussian due Doppler broadening. Thus, thespread of the scattered spectrum gives theelectron temperature Te.
The light source of the laser Thomson

scattering was the second harmonic beam (λ =532 nm) of a Nd:YAG laser (Continuum,
Surelite III). The laser beam (pulse duration of
10 ns, repetition rate of 10 Hz) was focused bya lens and injected in the gap between
electrodes inside the chamber. First, aplano-convex lens with focal length of 200 mm
was employed to focus the 8mJ laser beam.
This setting provided a spot size of 50 µm anda power density of 5×1014 W/m2 and induced
the perturbation of the electric state in thestreamer. Then, this lens was replaced with a
combination of two cylindrical lenses with focal
length of 300 mm and 400 mm to avoid laser
perturbation. Further explanation of this issue
will follow in the next section. The Thomson
scattering spectrum was analyzed by atriple-grating spectrometer (TGS) whose
configuration, which was designed to reduce
the stray light, was almost the same as thatused in other experiments.14)-20) Finally, thespectrum was detected by a gated ICCD
camera (Princeton, PI-MAX UNIGEN II).
The propagation of the pulsed streamer was
directly observed by the gated ICCD camera,
without the TGS. Figure 1 shows the opticalemission image of the streamer at different
times. In this case, the gated width of the
ICCD was set to be 2 ns and the accumulation
number was 50.
In order to perform temporally-resolved
measurements, the timing of the laser system
and ICCD camera, which were synchronizedwith the laser repetition rate of 10 Hz, werevaried with respect to the pulsed dischargeusing two delay generators (SRS, DG-535). In
order to increase the signal-to-noise ratio, the
LTS spectrum was accumulated by the ICCD
camera over several thousand pulsed
discharges and recorded by a computer-based
data acquisition system.

3. Results and discussion
Based on the ICCD pictures shown in Fig. 1
and the waveforms of the applied voltage andcurrent shown in Fig. 2, two phases can beobserved in the streamer: the primary
streamer and the secondary streamer.12) At theearly stage, the primary steamer formed as a
cloud between 0 and 70 ns. The streamer
discharge began with an ionization cloud at
the tip of the needle electrode (Figs. 1 (a), (b)).
The time in Fig.1 (a) is defined as a time origin(t = 0) in this paper. Then, the streamer
expanded as a spherical shell toward the平成 26 年度

九州大学大学院総合理工学報告

hemispherical electrode at a distance of 16 mm,
as shown in Figs. 1 (c) and (d). After that, it
reached the hemispherical electrode (Fig. 1 (e))
and finally, by increasing the current, thesecondary streamer was created (Fig. 1 (f)).
The streamer velocity was calculated to be 105
m/s, which is in good agreement withpreviously reported values.21)
The radius of curvature of the needle
electrode tip was around 40 µm, and therefore,
the electric field intensity when the voltage of
3 kV was applied was estimated to be 8 × 107
V/m. The streamer discharge starts from thispoint, so in order to observe the effect of thehigh electric field associated with the steamer,
it is desirable to set the observation point as
close to the needle tip as possible. However, ata point 1 mm from the needle tip, for example,
stray lights produced by needle electrodeobstructed signal detection. Finally, we
selected a position 2 mm from the needle tip
for the Thomson scattering measurement inthis paper.

第 36 巻第2号

3

measurement sufficiently high, the photoncounting detection technique was employed.
The Thomson scattering signals wereaccumulated by the ICCD camera over 5000
laser shots. It can be seen from Fig. 3 that themeasured spectrum is well fitted by aGaussian curve. Therefore, we can determine
the values of ne and Te.

Fig. 2 Voltage and current waveforms.

Fig.3 An example of the measured LTS spectrum. The fullline is a fitted Gaussian curve.

Fig.1 Time evolution of the cathode-directed streamer inhelium gas. The images were captured by an ICCD
camera. The colors of the images are artificial andshow signal intensities. The ICCD camera covereda wavelength range from ultra-violet to nearinfra-red.

Figure 3 shows an example of the obtained
LTS spectrum. The detailed conditions for thisspectrum will be described later. In order tokeep the signal-to-noise ratio of theSince there is a high electric field in the
primary streamer particularly near theneedle electrode, frequent electron-impact
excitations of the gas atoms may occur in thisregion. Hence, perturbation is possible due tothe strong electromagnetic field by the
probing laser with a high power density.
Yamamoto et al.22) reported that the Thomson
scattering signal is affected by thephoto-ionization of metastable xenon atoms
when the laser power density is greater than
1015 W/m2. Chen et al.23) also showed that thephoto-ionization of metastable argon atoms
affects the Thomson scattering spectrum
when the power density of the probing laser isgreater than 4 × 1013 W/m2. In the case of thestreamer, due to the very high electric field,

4Development of Thomson Scattering Diagnostics for Cathode-Directed Streamer Discharges in Helium

the effects of the photo-ionization of all
excited atoms must be taken into account. In
order to examine the effects of the laser
perturbation, the dependence of the signal
intensity on the laser power density wasinvestigated. In this investigation, the laser
beam focusing system has been changed froma typical plano-convex lens to a combinationof two cylindrical lenses whose setup will beshown later. The essential point here is thatthe beam spot of the probing laser by two
cylindrical lenses had a diameter 0.2 mm
which is much larger than the 50 µm spot
diameter created by the original plano-convex
lens. Using the new laser focusing system, theprobing laser energy was changed in therange from 10 mJ to 100 mJ. The result isshown in Fig. 4. This measurement was
carried out at t = 16 ns when the laser beam
hit to the streamer head. The figure showsthat the signal intensity is linear up to 1.3 ×
1014 W/m2 and that the signals clearly deviate
from the linearity at greater power densities.
Therefore, Thomson scattering measurements
must be performed below the laser power
density of 1 × 1014 W/m2.

order to obtain a clear LTS system, the laser
energy was increased up to 100 mJ, whichcorresponded to a power density of 5 × 1013
W/m2. This value is well below than thethreshold for the laser perturbation. The LTS
spectrum shown in Fig. 3 was obtained using
this setup.

Fig.4 Signal intensity of the Thomson scattering as afunction of the laser power density.

Fig.5 Setup of cylindrical lens system used to shape thelaser spot, which measured 1 mm vertically and
0.2 mm horizontally. The axis of the second lens
(focal length 400 mm) is perpendicular to that of
the first lens (focal length 300 mm).

Based on above result, we decided to set thelaser spot size to be 0.2 mm × 1 mm (so thatthe actual shape was elliptical) for the further
measurements of this study. The setup of thetwo cylindrical lens system was as illustrated
in Fig. 5. As shown in Fig. 1 (b), the laser beam
was injected from the lower side of the paper tothe upper side. The laser spot size of 0.2 mm
was in the direction of the streamer
development (left to right). The scattered light
was collected using the line of sightperpendicular to the paper surface, and thespot size of 1 mm was in this direction. In

Figure 6 shows the temporal evolution of ne
and Te at the point of 2 mm from the needle
electrode. At t = 16 ns, the electron
temperature was around 2 eV. At this time,
the laser beam hit the head of the streamer asshown in Fig. 1 (b). As time passed, Te
decreased to reach a value of 0.83 eV at t = 36
ns. On the other hand, ne was about 3 × 1018
m-3 at this time and subsequently decreased to1 × 1018 m-3 at t = 56 ns. This behavior showsthat the electron temperature and electron
density have their maximum values at thehead of the streamer.

It has been reported2) that, in front of thestreamer head, the external and internal
electric fields combine to make a stronger total
field, which boosts ionization. Thus, intense
ionization and excitation of gas atoms occur inthis region. This could be a strong reason why
the light emitted in the streamer head appears
in the ICCD images of this experiment (Figs. 1.
(b), (c), (d)). On the other hand, inside thestreamer and in the plasma channel behindthe streamer head, the electric field becomes平成 26 年度

九州大学大学院総合理工学報告

weaker, which may slow down theionization.21) These features of the streamer
are consistent with our observation. Also, themeasured values of ne are close to thesimulated helium streamer results at
atmospheric pressure in Refs. 21 and 24. At t =72 ns, the streamer channel reached thehemispherical electrode (Fig. 1 (e)), so theelectrical current significantly increase to forma secondary streamer. At this time, Te and neincreased dramatically until 140 ns (Figs. 6 (a)and (b)).

(a)(b)
Fig.6 Temporal evolution of a) electron density and b)
electron temperature of the streamer. The
measured position is at 2 mm from the needle
electrode.

4. Conclusion
In this study, the LTS method was developed
to diagnose streamers. In order to avoid laser
perturbation, the laser spot size was optimized
by using two cylindrical lenses. Because we
could utilize a relatively large laser energy
without introducing laser perturbation, we
could clearly detect LTS signals and measure
the ne and Te values of the streamer for the
first time. Also, measurements of the temporal
evolution of ne and Te at a point 2 mm from theneedle electrode are consistent with reported

第 36 巻第2号

5

results from streamer simulations.
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