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PPS/ICOPS 2001 CONFERENCE
Las Vegas, Nevada
June 17-22, 2001
RUBIDIUM FILTERED THOMSON SCATTERING MEASUREMENT IN AN
ATMOSPHRIC PRESSURE ARGON ARC
S.H. Zaidi, Z. Tang, R.B. Miles
Department of Mechanical and Aerospace Engineering
Princeton University, Princeton NJ 08540
Abstract
High temperature, atmospheric pressure plasmas
represent a significant challenge for diagnostics. The
temperature is too high for physical probes, the plasmas
are filamentary with dimensions too small to be resolved
by microwave techniques, and the plasma luminosity and
Rayleigh scattering background limit optical diagnostics.
We report here the measurement of electron temperature
and number density in an atmospheric arc discharge by
Thomson scattering collected through an optically thick
Rb filter. A narrow linewidth, frequency-tunable pulsed
Ti:Sapphire laser was employed. The Thomson scattering
is frequency broadened by the thermal motion and the ion
acoustic coherent motion of the electrons. The linewidth
of the Thomson scattering is much greater than the
absorption linewidth of the Rb, so it passes through the
Rb filter and into a spectrometer. The detector is timegated synchronized with the laser to suppress the plasma
luminosity. The frequency spectrum of the Thomson
scattered light is fitted to a theoretical model in order to
determine the electron temperature and number density.
I.INTRODUCTION
Experimental diagnostic techniques play a
significant role for the understanding of plasma
properties and the validation of theoretical predictions.
These diagnostics are in high demand in plasma
aerodynamics where both intrusive and non-intrusive
techniques are used to conduct plasma studies. The use of
Langmuir probes is the most popular method for the
measurement of electron temperature and electron
density in low temperature and weakly ionized plasmas
[1]. In spite of their usefulness, the intrusive nature and
their restricted application to the low temperature
plasmas make these probes unsuitable for high
temperature plasma studies.
Among the non-intrusive techniques, Thomson
scattering has emerged as an important diagnostic tool
which can be used to make unambiguous measurements
of electron temperature and electron number density in
plasmas [2]. Experiments in plasmas with electron
densities in the range of 1017 to 1021 cm-3 and electron
temperatures in the range of 1eV to 5eV are now being
performed [3]. Recent attempts have been made to lower
limit for Thomson scattering measurements to below
1012 cm-3 . This density regime is commonly found in
glow discharges which are used in industrial application
including etching and depositions.
In spite of several advantages, Thomson
scattering technique does have few limitations which
mainly come from the plasma luminosity and the
Rayleigh light which is scattered by the system as a
back ground light. The Thomson signal has to compete
with plasma emission and the elastic background
radiation, both of which can mask the Thomson signal.
To overcome the problem of interference from the
undesired radiation, atomic dispersive resonance filters
can be employed. In the present work a Rubidium filter
along with a narrow width, frequency-tunable, pulsed
Ti:Sapphire laser was used. The laser was tuned to the
780 nm absorption line of Rb vapor. The Rb filter was
made optically thick at the laser wavelength, so that the
light that is Rayleigh scattered from the neutral species
in the plasma and background scattering are filtered
out. As the Thomson signal is frequency broadened by
the thermal motion and the ion acoustic coherent
motion of the electrons, its linewidth becomes much
greater than the absorption linewidth of the rubidium.
This way the Thomson signal passes through the Rb
filter whereas the undesired Rayleigh and background
radiation is blocked. Further details of the Thomson
technique and the related apparatus are presented in the
following sections.
II.THOMSON SCATTERING
When a laser beam is passed through a plasma,
the Thomson component of the scattered light arises
form free electrons. The bound electrons scatter the
Rayleigh component whereas the stray light comes
from elastic scattering from various surfaces in the
system. Thomson and Rayleigh components are
Doppler broadened but have very different spectral
widths because of the much lower mass of electrons
compared with that of atoms.
The line shape of the Thomson scattering
profile is a complicated function of the parameter α
which, in turn, is a function of the scattering angle and
the plasma characteristics (electron number density and
electron temperature). The parameter α is defined as:
α = 1/kλD ≈ λ0 /(4πλD sin (θ/2))
(1)
where λD is the Debye’s length in the plasma and k is the
magnitude of the difference between the scattered wave
vector KS and the incident wave vector KO . Since the
Thomson scattering is elastic, KS = KO . If α <<1,
the scattering comes from the uncorrelated electron
motion and leads to incoherent Thomson scattering
whereas for α>1 collective electron motion plays a
dominant role and leads to coherent Thomson scattering.
The factor governing the spectral profile of the
Thomson scattering is referred to as the form factor and
is denoted by S(K,ω). The simplified expression for the
form factor can be obtained by using the Salpeter’s
approximations [4] which assume that for most of the
practical cases, the electron mass is much less than that
of the ion and the electron temperature and the ion
temperature are comparable.
The experimental detection of the line shapes
provides information on plasma temperature and electron
number and ion number densities. It would be worth
mentioning that the frequency spectrum of the scattered
light is strongly dependent on the scattering angle and the
motion of the charged particles in the plasma. As
compared to large scattering angles, spectral features at
small scattering angles are compressed. In this regime
stray light is a major interference that must be rejected
and attenuated for accurate measurement of the scattering
profile. This rejection can be achieved by employing
atomic vapor filters as is explained n the next section.
III. ATOMIC VAPOR FILTERS
Atomic vapor filters have been widely used to
suppress Rayleigh signals and the back ground noise
which arises from the elastic scattering from medium
particles and surfaces. The initial applications involve the
successful measurement of Raman intensities of gases
only a few GHz away from the Rayleigh line [5].
Recently atomic filters have also been used in Thomson
scattering measurements for the same reason. Bakker et.
al. [6] used a sodium notch filter to suppress the stray
light intensity in performing 90° Thomson scattering
experiments in a low density plasma. In the current work
an optically thick Rb filter has been employed in the
system.
Figure 1: A schematic of the experimental arrangement.
IV. EXPERIMENTAL RESULTS
Figure 1 describes a schematic diagram of the
experimental setup. The arc source plasma was an
atmospheric argon MAXI-ARC lamp form NIST. In
this water-cooled lamp, the arc constricting section is
6.3 mm long with a 4.0 mm diameter dis k. The design
of the lamp ensures stable burning of the argon flow.
Pure atmospheric argon was supplied to the arc
chamber. From the emission spectrum of the lamp it
was found that the emission in the infra-red was
significantly lower than in the visible region. Hence, it
was advantageous to use the infra-red radiation source
for Thomson scattering to reduce noise from the plasma
emission background.
In contrast to Snyder et. al. [7], Bentley [8]
and Bakker et. al [6] who performed 90° Thomson
experiments, a backward layout has been adopted in
this work. With the restricted optical access of the
structure of the argon arc lamp, only the forward or
backward scattering experiments were possible. The
advantages of backward layout lie in a relatively less
stray light radiation and the appropriate α parameter for
plasma diagnostics. As mentioned earlier, a narrow
linewidth pulsed injection seeded Ti: Sapphire laser
was used as the light source. The output of the laser was
about 50-60 mj/pulse with a line-width of and about
100 MHz which made it an ideal choice for Thomson
scattering. However the optical purity of this laser
system was only about 99%. Besides the injection seed
component, laser emission also consists of a broad ASE
component. The elastic scattering from this ASE was
found much stronger than the weak Thomson
scattering. An ASE reduction filtered was constructed
to overcome this problem. This filter consists of two
spatial filters and 20-fold prisms which are used as the
dispersive elements. Figure 2 shows both the ASE and
the filtered ASE emission spectrum when the laser was
tuned to the Rb D2 line. The full width at half maximum
of the ASE component is about at 2.0 nm whereas the
corresponding value for the filtered ASE is about 0.7
nm which is spectrally narrow enough and can be
suppressed by an optically thick Rb filter.
signal was reduced dramatically when the laser arc was
extinguished. Once the Thomson signal was observed, a
scan of its spectrum was obtained. The measurement
results have been shown in figure 3 which also included
the plasma emission spectrum without the laser and the
laser emission spectrum without the plasma. The
electron number density and the electron temperature
was extracted from the Thomson scattering by
matching the experimental line profile with the
Figure 2: The ASE and the filtered ASE spectrum of the
Ti:Sapphire laser.
It is worth mentioning that the Thomson scattering is
linearly polarized whereas the plasma radiation is unpolarized. Therefore a Glan-Taylor polarizer was
employed to pass the light which was polarized parallel
to the electric field of the incident laser. The backward
scattered light from the argon arc was spatially filtered
and was passed through the Rb filter which absorbed the
elastic and Rayleigh scattered light. The Thomson
component of the scattered light was focused into a
monochromator for spectral analysis where a
photomultiplier tube (PMT) detected the required signal.
The PMT was time gated synchronously with the laser to
suppress the plasma luminosity. The signal was
BOXCAR averaged, A/D converted, and was recorded
by a computer.
Before making any Thomson measurements,
the detection system was aligned by capturing the Raman
features of the carbon oxide gas. Once the satisfactory
Raman signals were obtained, the system was ready to
measure the Thomson signal. The monochromator was
set to 783.7 nm and the laser was tuned to 780.02 nm.
Firstly, when the lamp was off, the laser radiation was
blocked to check the zero level. Then the arc was ignited
and the emission signal strength was monitored. The
laser light was again blocked to check the zero drift, and
the time gate of the BOXCAR was checked to make sure
that the gate captured all the scattered light from the laser
radiation. When the laser was focused into the arc, a big
signal due to Thomson scattering was observed. The
Figure 3: The plot of the measured Thomson scattering
spectrum along with the fitted model.
theoretical models. The α parameter was 1.17 and the
measurement results gave an electron temperature of
0.82 ± 0.06 eV, and an electron number density of 1.61
× 1016 ± 0.05 × 1016 electrons /cm3 . The errors were
estimated by the asymptotic standard errors from the
fitting. It must be noted that all the Thomson scattering
electron features observed so far were asymmetrical.
The blue-wing hump was more than 10% to 30% larger
than the red-wing hump. This asymmetry feature has
also been observed by Snyder and Bentley [7,8]. An
example of such a Thomson scattering line profile is
shown in figure 4. Although the integrated scattered
light intensity is stronger in the blue wing than I the red
wing, they have comparable spectral line-shapes and
the same value of electron number density and electron
temperature can be extracted from both sides. I n the
graph, the two sides are separately fitted. The arc lamp
was operated at an operating condition different from
that used in figure 3. In this case the electron number
density was found 5.48 × 1015 cm-3 whereas the
electron temperature was about 2.42 eV.
[6]: Bakker L.P., Kroesen G.M.W., Thoms on scattering
using an atomic notch filter, Review of scientific
instruments, Vol. 71, No 5, May 2000, pp 2007-2014.
[7] S.C. Snyder, L.D. Reynolds, J.R. Fincke, G.D.
Lassahn, J.D. Grandy, T.E. Repetti, Electron
temperature and electron number density profiles in an
atmospheric press argon plasma jet. Phys. Rev. E.,
50:519, 1994.
[8] R.E. Bentley, A departure from local
thermodynamic equilibrium within a freely burning arc
and asymmetrical Thomson electron features, J. Phys.
D: Appl. Phys., 30:2880, 1997.
Figure 4: The plot of the measured Thomson signal along
with the fitted model.
V. SUMMARY
An optically thick Rb filter has been
successfully used to suppress the elastic and Rayleigh
scattering background while measuring a Thomson signal
from an argon arc plasma which was operating at an
atmospheric pressure. The spectral purity of a narrow
linewidth Ti:Sapphire laser was achieved by using an
ASE filter. The information on the electron temperature
and the electron number density was extracted from the
Thomson signal by fitting a model curve to the data
VI. REFERENCES
[1] D. Batani, S. Alba, P. Lombardi, A. Glassi, Use of
Langmuir probes in a weakly ionized, steady state plasma
with strong magnetic field, Rev. Sci. Instrum. 68 (11),
Nov. 1997.
[2] J. Sheffield, Plasma scattering of electromagnetic
radiation, Academic Press, New York, 1975.
[3] C.J. Barth, M.N.A. Beurskens, C.C Chu, A.J.H.
Donne, N.J.L. Cardozo, J. Herranz, H.J.V.D. Meiden,
F.J. Pijper, A high resolution multiposition Thomson
scattering system for the Rijnhuizen Tokamak Project,
Rev. Sci. Instrum. 68 (9), Sep. 1997.
[4] E.E. Salpetter, Electron Density Fluctuations in a
plasma, Physical Review, Vol. 120, No. 5, Dec. 1960, pp
1528-1535.
[5] Tang Z., Zaidi S.H., Miles R.B., Density gradient
rubidium dispersive absorption filter for low wave
number Raman and Thomson scattering, AIAA 20000644, 38th Aero. Scien. Meeting and Exhib., 10-13
January, 2000, Reno, NV.