SO1-3
Thomson Scattering Diagnostics of Plasmas for Various Applications
様々な応用プラズマのトムソン散乱計測
Kiichiro Uchino, Weiting Chen, Nima Bolouki and Kentaro Tomita
内野喜一郎, 陳韋廷，ボロウキ ニマ, 富田健太郎
Graduate School of Engineering Sciences, Kyushu University
6-1, Kasuga Koen, Kasuga, Fukuoka 816-8580, Japan
九州大学大学院総合理工学府 〒816-8580 福岡県春日市春日公園6-1
The laser Thomson scattering technique has been developed as a diagnostics method for plasmas forvarious applications. Those plasmas includes low-pressure discharge plasmas such as an ECR plasma, an
inductively-coupled plasma, and a capacitively-coupled plasma, and high pressure micro-plasmas such as
the capacity-coupled-discharge plasma, a dielectric-barrier-discharge plasma and the plasma for theextreme ultra-violet light source. In this presentation, it is reviewed how successfully those Thomson
scattering diagnostics have been performed.

1. Introduction
Thomson scattering is the light scattering by freeelectrons in plasmas. Quantitative measurements of
the intensity and the spectrum of the scattered lights
give the absolute values of electron density andelectron temperature. [1]The authors have been developing the laserThomson scattering (LTS) as a diagnostic method
for industrially applied plasmas. First, we haveapplied LTS to low-pressure processing plasmas,and recently, are extending to micro-discharge
plasmas. In this article, we will describe about these
our efforts. For the application of LTS toindustrially applied plasmas, we are often facedwith following problems. First, signals are
sometimes difficult to be detected due to low signal
intensities. In order to solve this problem, we adopt
the signal accumulation over many laser shots
because plasmas are maintained continuously or
produced repeatedly at a high frequency. Second,
the strong stray lights which are the laser lights
scattered by windows and chamber walls can easily
swamp the scattered signal lights. Because thespectral spread of the stray lights is determined by
the instrumental function of the spectrometer used
for the detection optics system, the problem can beresolved by using a triple-grating spectrometer
which has high stray-light rejection characteristics.
Third, the high laser energy may change values ofelectron density and temperature. For this problem,we must examine the laser conditions (energy,wavelength, pulse width, and so on) which do not
induce laser perturbations.
The application of LTS to low pressure plasmas
is reviewed in Refs. [2], [3] and described briefly insection 2. For micro-plasmas which are recently
being developed, LTS is the unique diagnostic

method for some cases. As an example, theapplication of LTS to a pulsed micro-discharge
produced at around atmospheric pressure ispresented in section 3.
2. Application to Low Pressure Plasmas
We first applied LTS to an ECR processing
plasma produced in an argon gas about twenty years
ago. [4] Because the electron density was of theorder of 1018 m-3, the analog detection was adopted
and the signals were observed using an oscilloscope.
Since the plasma was continuously maintained,
signal accumulations over many laser shots were
demonstrated to be effective to obtain an enoughsignal-to-noise ratio. The photon-counting method
was used to measure the detailed structure of thescattered spectrum. When the electron energy
distributionfunction(EEDF)
ofthe
inductively-coupled plasma was examined, it was
found that argon plasmas have the Maxwellian
EEDF and, on the other hand, EEDFs of plasmas
produced in molecular gases such as methane andoxygen deviated remarkably from the Maxwellian.
[5] For capacitively-coupled plasmas, the lowest
detection limit was pursued and the electron density
as low as 5×1015 m-3 was proved to be detectable by
LTS after a longtime signal accumulation. [6]In the reactive plasmas, the interaction between
laser lights and plasma particles sometimes induces
problems for LTS measurements. We must examine
the particle composition of the plasma and evaluate
the possible laser perturbations to the plasma. The
strategy for such situation is reviewed in Ref. [3].
When the Raman scattering, whose spectrum
overlaps with the LTS spectrum, was the problem,
the LTS spectrum was separated from the Ramanspectrum by observing both spectra from twodifferent scattering angles. [7]3. Application to Micro-discharge Plasmas
We are now applying LTS to diagnose somemicro-plasmas such as a capacity-coupleddischarge (CCD) plasma, a dielectric-barrierdischarge (DBD) plasma, and a laser produced
plasma for the extreme-ultraviolet light source.[8]In the followings, the experiment for the CCD
plasma is described.
The CCD plasma was created between a set oftungsten electrodes composed of a needle and ahemisphere (=2.4 mm) with a gap of 500 m. The
discharge was triggered by a high-voltage
semiconductor
switchwithfastrisingcharacteristics and we could produce the CCD
plasma with excellent reproducibility. The
discharge gas was a neon gas at a pressure of 400
Torr. For the LTS measurement, the Nd:YAG laser
beam with a output energy of about 5 mJ wasinjected to the discharge perpendicularly to thedischarge axis. Then, we could obtain LTS spectra
along with the laser beam path in the radial
direction of the discharge. Figure 1 (a) shows a
two-dimensional image of the LTS signal recordedby an ICCD camera when the laser beam wasinjected to the center of the electrode gap. In orderto obtain enough SN ratios, LTS signals wereaveraged over 2000 laser shots for this image. The
measurement time was at 25 ns after the beginningof the discharge. From this image, we can obtain(a)radiusdirection

1.4 mmwavelength
Wavelength stop （±0.8 nm）

 (nm)Fig. 1 (a) Two-dimensional image of the LTS
signal measured at 25 ns after the start of thedischarge. (b) Thomson scattering spectrum at
r=0 extracted from Fig. 1 (a).

radial profiles of electron density (ne) and electron
temperature (Te). Figure 1 (b) shows an example ofa LTS spectrum obtained at the radial centre
position (r=0). Since the LTS spectrum was in thecollective regime, ne and Te could be evaluated fromthe spectral shape and the peak wavelength. It canbe seen from Fig. 1 (b) that the theoretical curve
which corresponds to ne = 4.6×1022 m-3 and Te = 1.0
eV is most close to the measured spectrum and so
those were fixed as measured values. The laser
beam was injected at 5 different axial positions andtwo-dimensional distributions of ne and Te wereobtained with the spatial resolution of 50×50×50
m3.
Based on the results obtained for the CCD
plasma, the lower detection limit of the LTS signalof the present detection system was estimated. Then,the result suggested that the lower limit of theelectron density could be as low as 1018 m-3 if weuse the ICCD camera in the photon-counting mode.
Therefore, we have a possibility to examine thestreamer phase of the pulsed discharges at
atmospheric pressure.
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