Mimura, Mikio

Namba, Shin-ichi

Sato, Kuninori

Tsuboi, Hiroshi

Tsuji, Hiroshi

Osaka City University Repository

Plasma Electron Temperature Estimation using Line Intensity Ratio
of Low Ionized Oxygen Ions
Mikio MIMURA*, Hiroshi TSDn*,
Hiroshi TSUBOI**, Shin-ichi NAMBA** and Kuninori SATO**
(Received September 30, 2000)
Synopsis
For the optical measurement of electron temperature of stationary plasma, a line intensity ratio method
that utilizes the lines from low ionized oxygen ions is tested. This method is applicable to the low electron
temperature between 4 eV and 11 eV, which can not be measured by the usual intensity ratio method. The
line intensity of lower ionized oxygen ions, 011, 0111 and OIV from TPD-ll plasma are measured by a VUV
monochromator. ,Then a calculation of the corresponding line intensity is performed using the rate equations.
By comparing their intensity ratios, the electron temperature of the plasma is estimated. The obtained
electron temperature agrees well to the result of a Thomson scattering measurement.
KEYWORDS: electron temperature, intensity ratio, rate equation, oxygen ion, TPD-ll
Introduction
One of the well-known optical measurements of plasma electron temperature is the line intensity ratio
method (1), which utilizes the intensity ratio of two spectral lines emitted by the same ionized state ion. Such
a line pair whose intensity ratio is sensitive to electron temperature, however, comes from only higher
ionized states of ions. For example, if the oxygen is used, the temperature sensitive lines comes from OV or
OVI, which are created in plasma with the electron temperature higher than 10 eV or so. Therefore such an
ordinary intensity ratio method can not be used for the plasma with low electron temperature below 10 eV.
Here we try to develop another intensity ratio method for low temperature plasma. In this method, the
line intensity ratio from different ionized state ions is used, which is sensitive to electron temperature below
10 eVe The measured intensity ratio is compared with the calculated result derived from rate equations. By
making the best fit between them, the electron temperature is estimated. This method is applicable even
when the plasma electron temperature is less than 10 eVe
Experimental Setup and Obtained Data
A stationary plasma machine, TPD-II is shown in Fig.I. The He plasma is created by the discharge
between a cathode and an anode with floating electrodes to stabilize the plasma. The plasma flows through
the hole of the anode and the floating electrodes into the plasma region, where the magnetic field of 5 kG is
applied to confine the plasma in axial direction. The trace gas, oxygen is fed through a leak valve into the
plasma region.
A two-meter grazing incidence VUV monochromator is used to detect the oxygen' lines. It uses a 600
grooves/nun grating, and the. angle of incidence is 860 • The entrance slit width is 10 Jl m and exit slit width
is 20 Jl m, which results in a resolution of 0.2 A. The detector is Hamamatsu Photonics R595, a 20-stage
*Department ofApplied Physics, Faculty of Engineering
**National Institute for Fusion Science
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electron multiplier with CsI coated first dynode. The signal is recorded on a XT recorder chart. The spectral
sensitivity calibration was performed by using the branching ratio method in the VUV region (2).
target
-190
He gas
~j
~~ ~ ~I ~ cathode
-:glass
orifice O2 j anode _ :metal
plasma region' discharge regio~
I I
o +190[CIIl]
Fig. 1. Schematic drawing ofTPD-II machine.
Figure 2 shows examples of the spectral lines near the wavelength of 830 A and near 790 A, where the
discharge current is 110 A. The resonance lines 011 834.462 A, 0111 835.292 A, and OIV 790.103/790.203 A
lines are used to get the intensity ratio. OV lines are not detected in the present discharge current. The
parameters ofthese lines are summarized in Table. I.
(~) . 011 832.754A
~a OIIT 835 292A 0111 832.927A
::J 011 834.462A ,
~ \OIII 833.742A.~ \~ 011833.326A
~ \ I
Wavelength [A]
(B)
DIY 790.103A
OIY 790 20M
OIV 787.710A
01791.516A
Wavelength [Al
Fig. 2. Examples of spectral lines of 011, 0111 (A) and OIV (B).
Wavelength(A) ilE(eV) fy Transition J-J
on 834.462 14.858 0.21 2s22p3 4S _ 2s12p44p 3/2 - 5/2
om 835.292 14.843 0.12 2s22p23p _ 2s12p33D 2-3
OIY 790.103 15.690 0.014 2s22pl 2p _ 2s12p2 2D 3/2 - 3/2
790.203 15.688 0.13 2s22pl 2p _ 2s12p22D 3/2 - 5/2
Table I . Parameters of lines for intensity ratio method
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...... 0.
Q..:.: : : ....... X ... ·1. 110A
. .
........... 'i:-90A"O
.. e··
.... I=70A····
I=50A:. ·
./::). X
The intensity of these lines normalized to the 011 line intensity is plotted as a function of the discharge
current in Fig.3. As the discharge current increases, the intensity of 0111 and OIV lines increases, which
shows qualitatively the increase in the electron temperature.
~
~
o 101 ..-----""P""----.......-----.-----~
~
~Q).~ 10°
ta
E
~ 10-1
.0
• ...,04
(/)
5 10-2
.:5
Q)
~ 10-31.....---.......&.....---1..------------'()II ()!I! ()l'I
Fig.3. Intensity of 011, 0111, OIV lines as a function of discharge current.
Line Intensity Calculation using Rate Equations
To get the intensity ratio using rate equations, the following simplification is used. First, the electron
velocity distribution is Maxwellian, therefore no electron beam exists in the plasma. Second, the plasma is
uniform in the radial direction. Then the density of each ionized state ion is calculated by the following zero
dimensional rate equations.
where ne is the electron density, Sj is the ionization rate coefficient, ri is the recombination rate coefficient
and 1: is the confinement time.
The neutral oxygen is fed constantly through the leak valve, which is the source term. In the present case,
since the plasma is stationary, the time derivative in the left side are set to zero. The differential equations
reduce to simple simultaneous equations.
The intensity of a line is obtained by the following corona model equation.
I =nelli < av > ex
The intensity ratio of two lines calculated in this way depends on two parameters, electron temperature
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Te , and the product of the electron density and the confinement time ne r . According to a Thomson
scattering measurement, the typical electron density of TPD-ll plasma is 2 X I012/cm3. As for the
confinement time, the usual definition can not be used because TPD-II 'plasma has no confinement in the
axial direction. So the confinement time is interpreted here to be the time while an ion with the room
temperature passes the discharge tube length, which is 3.8 m. It is 2.8 ms for helium ion and 5.6 ms for
oxygen ion. Here the average time 4.2 ms is used as the typical confinement time.
101 _--..,------r----,------,
100
o.~ 10-1
" 10-2C.~ 10-3
=] 10·(
10-6
10-6 L-..__.J----..J.---~---
011 0111 OIV
Fig.4. Calculated intensity ratio of 011, 0111, OIV lines. (A) Dependence on Te , (B) Dependence on ne r
Figure 4(A) shows the dependence of the intensity ratio on the electron temperature, where the above
values of ne and r are substituted in the equations. When the electron temperature increases by factor
four from 2.5 eV to 10 eV, the intensity ratio changes drastically.
Figure 4(B) shows the dependence of the intensity ratio on ne r, when the electron temperature is 5 eV
The center values show when ne r is 2X 1012/cm3 X4.2 ms. When ne r is increased or decreased by factor
two, the intensity ratio does not change so much compared to the change with Te as shown in (A). So even if
there is some ambiguity in ne r, rough estimation of electron temperature becomes possible.
Temperature Estimation by Intensity Ratio
Figure 5 shows the calculated electron temperature as a function of the intensity ratio IonlIonand IoIVlIon.
The error bar means the extent of variation due to the change of ne 7: by factor two.
Because of the background noise and the saturation level of the detecting system, the measurable signal
ratio is from 1/100 to 2.5. When Te is below 4 eV, the 0111 line cannot be detected. Therefore the ratio
IomlIon is applicable when Te is between 4 eV and 9 eV. When Te is below 7 e~ the OIV line cannot be
detected. Therefore the ratio IOIvlIon is applicable when Te is between 7 eVand 11 eVe Using both line ratios,
the present method is applicable when Te is between 4 eV and 11 eV. It might be thought that the measurable
Te range is very narrow. But since the TPD-II plasma has the temperature in this region, this method is useful
for TPD type plasma. When electron temperature. exceeds 10 eV, the OV line can be detected, so the usual
intensity ratio method becomes applicable.
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loIV
1011~ I .. -!--~ ..f _........ - ..~ ......-....-.-~~tII,
I
Intensity Ratio
Fig.5. Calculated electron temperature as a function of intensity ratio.
Using the curves in Fig. 5, the electron temperature of TPD-II plasma is estimated from the measured
intensity ratio data in Fig. 3. The estimated electron temperature is shown as the white circles in Fig. 6. The
error bar shows again the variance due to the change of n. r by factor 2. When the discharge current
changes from 40 A to 110 A, the electron temperature changes from 4 eV to 10 eV.
To check the accuracy of the estimated electron temperature, the result of a Thomson scattering
measurement in a similar discharge condition is shown as the black circles in the same figure. When the
discharge current is between 40 A and 70 A, they agree in 16 % error. When the discharge current is about 80
A, the error becomes 33 %. Since the agreement is quite well, the present intensity ratio method is useful for
electron temperature monitoring.
120
••
m
40 60 80 100
Discharge Current [A]
5
Fig.6. Estimated electron temperature as a function of discharge current (white circle),
and result ofThomson scattering measurement (black circle).
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Conclusions
In a typical discharge of TPD-II plasma, the calculation of rate equations shows that 0111 line is detected
when Te is higher than 4 eV, and OIV line is detected when Te is higher than 7 eVe Using the intensity ratio of
these lines to 011 line, the electron temperature ofTPD-II plasma is estimated. When the discharge current is
40 A to 110 A, the electron temperature is estimated to be 4 eV to 10 eV, which agrees well to the result of a
Thomson scattering measurement. Since the usual intensity ratio method is applicable o'nly when Te is higher
than 10 eV, the present method, which is applicable when Te is between 4 eV and 11 eV, will become a
useful method as a supplement to the usual intensity ratio method.
Acknowledgements
This work was carried out under a collaborating research program at the National Institute for Fusion
Science.
References
1) H. R. Griem:, Plasma Spectroscopy (McGraw-Hill, New York, ,1964).
2) K. Sato, M. Otsuka and M. Mimura, Appl. Opt. 23 (1984) 3336.
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Plasma Electron Temperature Estimation using Line Intensity Ratio of Low Ionized Oxygen Ions

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Plasma Electron Temperature Estimation using Line Intensity Ratio of Low Ionized Oxygen Ions

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