Improvements of data quality of the LHD Thomson scattering diagnostics
in high-temperature plasma experimentsa…
I. Yamada,1,b?K. Narihara,1H. Funaba,1H. Hayashi,1T. Kohmoto,1H. Takahashi,1
T. Shimozuma,1S. Kubo,1Y. Yoshimura,1H. Igami,1and N. Tamura2
1National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
2Department of Energy Science and Technology, Nagoya University, Nagoya 464-8463, Japan
?Presented 17 May 2010; received 15 May 2010; accepted 10 July 2010;
published online 18 October 2010?
In Large Helical Device ?LHD? experiments, an electron temperature ?Te? more than 15 keV has
been observed by the yttrium-aluminum-garnet ?YAG? laser Thomson scattering diagnostic. Since
the LHD Thomson scattering system has been optimized for the temperature region, 50 eV?Te
?10 keV, the data quality becomes worse in the higher Teregion exceeding 10 keV. In order to
accurately determine Tein the LHD high-Teexperiments, we tried to increase the laser pulse energy
by simultaneously firing three lasers. The technique enables us to decrease the uncertainties in the
measured Te. Another signal accumulation method was also tested. In addition, we estimated the
influence of high-energy electrons on Teobtained by the LHD Thomson scattering system. © 2010
American Institute of Physics. ?doi:10.1063/1.3483189?
The Thomson scattering system is one of the most reli-
able diagnostics for measuring the electron temperature ?Te?
and density ?ne? profiles of fusion plasmas. We constructed a
Thomson scattering system and installed on the Large Heli-
cal Device ?LHD? in 1989.1,2The measured Terange has
been optimized for Te=50 eV–10 keV. Currently, high-
temperature plasmas whose Teexceeds 10 keV have been
generated by the strong electron cyclotron resonance heating
?ECRH?.3,4In such a high-temperature region, the data qual-
ity of measured Tebecomes worse. In addition, the electron
density is low, ne?1018m−3, in usual high-TeECRH experi-
ments. This makes data quality further degraded. To accu-
rately determine Tein the LHD high-Te, low-neexperiments
is one of the key issues. We tried to improve the Tedata
quality by two methods. Both of them are based on an at-
tempt to increase the signal intensity and decrease the statis-
tical uncertainties. In this paper, we describe the methods for
improving Tedata quality. In addition, we estimated the in-
fluence of high-energy electrons produced by strong ECRH
on Teobtained by the LHD Thomson scattering system.
II. LHD THOMSON SCATTERING SYSTEM
The LHD Thomson scattering system measures the Te
and neprofiles of LHD plasmas along the LHD major radius
at a horizontally elongated section, as shown in Fig. 1. Typi-
cal specifications are listed in Table I. Since the LHD Thom-
son scattering system has several yttrium-aluminum-garnet
?YAG? lasers, flexible multilaser operations are possible. Th-
omson scattered light is collected with a large ?1.5 m
?1.8 m? spherical mirror and analyzed by polychromators
that have five wavelength channels. The five filter transmis-
sions are optimized for the Terange, Te=50 eV–10 keV.
III. IMPROVEMENT OF TeDATA QUALITY
A. Near simultaneous laser firing
The LHD Thomson scattering system has three 2.3 J/10
Hz high-energy YAG lasers ?Thales SAGA 230-10?. By us-
ing more lasers, flexible multilaser operations are possible.
For example, three 2.3 J/10 Hz lasers can be used as a
6.9 J/10 Hz laser by firing the lasers simultaneously. Increas-
ing the laser pulse energy is expected to be useful for the
measurements in low density plasma experiments in which
both signal intensity and signal-to-noise ratio are low. Figure
2 shows an example of the raw signal waveform detected by
a wavelength channel in a polychromator, and a gate pulse
applied to the analog-to-digital converter. In order to cause
a?Contributed paper, published as part of the Proceedings of the 18th Topical
Conference on High Temperature Plasma Diagnostics, Wildwood, New
Jersey, May 2010.
b?Electronic mail: yamadai@LHD.nifs.ac.jp.FIG. 1. ?Color online? Schematic of the LHD Thomson scattering system.
REVIEW OF SCIENTIFIC INSTRUMENTS 81, 10D522 ?2010?
0034-6748/2010/81?10?/10D522/3/$30.00© 2010 American Institute of Physics
no damages to optics such as beam guiding mirrors and laser
windows, we shifted the peak positions to control the maxi-
mum intensity. Figure 3 shows a comparison of Teprofiles
obtained by 1 laser and 3 lasers. The plasma performances of
the two discharges were almost the same, and the line den-
sities were ?0.3?1019m−2. As shown in the left figure, the
Teerror is large in the temperature above ?10 keV, whereas
Teerrors are small below Te?8 keV. By using three lasers,
the Teerror bars have been successfully decreased by 50%
around the plasma center. When two lasers are used, Teerror
bars were decreased by 45%. The degree of improvement of
Tedata quality is more significant around the plasma center,
Te?10 keV. In the previous paper, we discussed on data
quality improvement using four YAG lasers.5The central
temperature and density were 2.5 keV and 0.6?1019m−3,
respectively. The experimental uncertainty was decreased by
65% in the experiment. The degree of improvement in the
previous experiment is somewhat better than that in this ex-
periment, but the difference is not large.
B. Raw signal accumulation methods
Next, we tried the raw data accumulation method on
fixed plasma discharges to decrease statistical uncertainties.
A total of 29 fixed plasma discharges were carried out. The
line electron density was ?0.3?1019m−2, and the repro-
ducibility of the 29 plasma discharges was within ?10%. In
this case, each Teprofile was measured with 1 laser pulse.
Figure 4 shows the Teprofiles obtained by a raw data accu-
mulation of fixed 1, 2, 5, and 29 plasma discharges. Relative
errors around the plasma center, dTe?0?/Te?0?, are 26%,
18%, 11%, and 5.5%, respectively. The data quality, i.e.,
smallness of error bars, becomes better as the number of
accumulated shots increases, as expected. Further, we tried
the other data accumulation method. In this method, raw
signals in a few time frames during the period when the
plasma is almost stationary are summed up. By using this
method, the data quality has been improved by 54% and 39%
when 3 and 5 temporal frames were added, respectively. As
the case of the simultaneous laser firing, the degree of im-
provement of Tedata quality is more significant around the
plasma center, Te?10 keV.
Figure 5 shows the summary of the degree of improve-
ment of Tedata quality. Horizontal axis stands for the num-
ber of laser pulses or accumulated raw data, and vertical axis
shows the uncertainty of Te?0? normalized to unity at n=1.
The results in the near simultaneous laser firing mode are
plotted as diamonds. A series of circles and triangles are
obtained by the shot accumulation method and the frame
accumulation method, respectively. The solid curve shows
1/?n. Roughly speaking, normalized Teerrors in the three
different methods show a similar behavior, as 1/?n. This
suggests that the Tedata quality is mainly determined by the
statistical uncertainty in low-ne, high-Teplasma experiments.
Since above the methods are not exclusive, then they can be
applied jointly. The normalized Teerror has been success-
fully decreased down to 10% by combing the shot accumu-
lation and frame accumulation methods.
FIG. 2. Thomson scattered signal from three laser pulses ?lower waveform?.
Upper waveform is the ADC gate pulse.
FIG. 3. Teprofiles measured by single laser pulse and three laser pulses.
FIG. 4. Comparison of Teprofiles obtained by raw data accumulation of 1,
2, 5, and 29 fixed plasma discharges.
TABLE I. Typical specification of the LHD TS system.
Number of observable spatial points
Temporal sampling frequency
Up to 200
5 eV–20 keV
50 eV–10 keV
10D522-2Yamada et al. Rev. Sci. Instrum. 81, 10D522 ?2010?
IV. ON THE INFLUENCE FROM HIGH-ENERGY
During strong ECRH plasma discharges, the electron
distribution function may consist of two components, the
lower energy bulk component and high-energy component.6
The LHD Thomson scattering system mainly observes the
Thomson scattered light by bulk electrons, and that by high-
energy electrons ?HEEs?, is hardly detected. However, the Te
obtained by the Thomson scattering diagnostics is affected
by them when a considerable amount of HEEs is generated.
Since the wavelength resolution of the polychromators is
poor for accurately observing the two components, it is dif-
ficult to obtain the information on HEEs by the Thomson
scattering diagnostics. However, considering the influence of
HEEs on Temeasured by Thomson scattering diagnostics is
useful for the check of accuracy and reliability. Therefore,
we estimate Teunder an assumption that the temperature of
high-energy component is 69 keV and the population range
is 0%–50%. An example is shown in Fig. 6. The upper figure
shows Teprofiles at the HEE population of 0%, 20%, and
50%. As expected, Teobtained by the Thomson scattering
system decreases as the population of HEEs increases. In
other words, bulk temperatures obtained by the Thomson
scattering system under the assumption that no HEEs exist
are overestimates. However the error caused from ignoring
HEE effect is not large in this work. Even when the popula-
tion of HEEs is assumed to be 50%, the error has been esti-
mated to be 14%. If accurate and reliable information on
HEEs is provided from another diagnostics and/or theoretical
calculation, more practical data analysis taking the effect of
HEE into account will be possible.
In order to accurately measure Teand neprofiles of
low-ne, high-TeLHD plasmas by the LHD Thomson scatter-
ing diagnostics, we tried two methods: near simultaneous
laser firing method and data accumulation method. Experi-
mental uncertainty has been successfully reduced by both
methods. The results suggest that experimental error is
mainly determined by the statistical uncertainty. We also
considered the influence of HEEs generated in ECRH experi-
ments on Thomson scattering diagnostics. It has been esti-
mated to be small. More accurate estimation of that needs the
help of another diagnostics and/or theoretical works.
This work was supported by the NIFS LHD project bud-
NIFS09ULHH502? and the Grant-in-Aid for Scientific Re-
search ?C?, Grant No. 21560860. We are grateful to the LHD
experiment group colleagues.
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FIG. 5. ?Color online? Normalized experimental error of Teas a function of
the number of laser pulses or accumulated plasma shots.
FIG. 6. ?Color online? Comparison between electron temperature profiles
that takes HEE effects into account Te
?and the difference between Te?0??
10D522-3Yamada et al.Rev. Sci. Instrum. 81, 10D522 ?2010?