25F12
Design, Construction, Operation, Improvement and Future Prospects of LHD
Thomson Scattering Diagnostic
LHDトムソン散乱の設計・建設・改良と将来展望
Kazumichi Narihara, Hiroshi Hayashi, Ichihiro Yamada, Hisayoshi Hunaba,
Ryo Yasuhara, Toshikazu Kohmoto
成原一途、林浩、山田一博、舟場久芳、安原亮、河本俊和
National Institute for Fusion Science, Toki 509-5292, Japan
核融合研究所 〒509-5292 岐阜県土岐市下石町322-6
Looking back on our past activities in the LHD Thomson scattering diagnostic, we examine what weresucesses and what were failures. Based on these, we propose a somewhat different TS system.

1. Introduction
It is almost 20 years since we started the designof the Thomson scattering (TS) diagnostic for
LHD experiment. We tried to construct a TSsystem which would give useful information as
much as possible for understanding LHD plasma.
On the occasion that the first author retires thisfiscal year, we would like to look back on what
we strove and examine what were successes, andwhat were failures [1]. We also propose a revised
TS system that would be more attractive for theLHD experiment.
2. Basic Design
2-1 Choice of TS Type: Twenty years ago, and still
now, there were three types of TS: TV-TS, YAG-TA
and LIDAR. In view of the LHD port configuration,
LIDAR seemed to be very attractive, but weabandoned it because we could not anticipate a big
progress in its spatial resolution of 10 cm. In
considering the overall performance, i.e., brightness,spatial resolution and repetition, we chose YAG-TS.
2-2 Scattering configuration: Looking at the LHD
port configuration, we considered several TS
configurations, i.e., the combination of theobservation region and the view window. Amongthem, we chose the heavily oblique back scattering
configuration that can measure whole plasma fromedge-to-edge along a major radius passing themagnetic axis. This choice was proved to be a
success by the fact that the observed electron
temperature (Te) and density (ne) profiles directly
give the positions of magnetic axis and themagnetic islands without a time-consuming
magnetic-surface reconstruction.
2.3 Light collection optics: We compared a lenssystem and a mirror system. Lens system has muchbetter imaging performance than mirror. However,the normally used lens materials are likely to be
darkened by intense neutron flux expected in theDD-experiment, hence we abandoned lens system.
The poor imaging problem of the mirror was partly
solved by setting the center of the mirror curvature
at the center of the viewing window. A rectangularmirror of 1.5m×1.8 m size was fabricated bypatching 11×12=132 hexagonal mirrors of 88 mmside length. Each hexagonal mirror was glued by
Epoxy on a mirror mount, which introduced a
distortion on the mirror surface and degraded theimaging performance, which was a big failure.
2.4 Optical fibers: Large spherical aberration of thelight-collection mirror necessitated the large
diameter (2 mm) of the optical fibers onto the endof which the image of laser beam is focused. Our
serious concern was a large transmission loss
induced by an intense neutron irradiation expectedin the DD-experiment. We made threefiber-companies to develop optical fiber that has astrong resistance against radiation. Finally wepurchased 200 optical fibers with the specifications:length 45m; N.A~0.22; the transmission loss
induced by 1MR CO-60 γ ray irradiation <20dB/km
for λ>650nm.
2.5 Observation region: The end tips of 200
optical fibers cut at 60º were placed on a fiber
holder and precisely aligned along the image of thelaser beam running in the region 2.2m<R<5.3m.
This expected observation length was much longer
than the real plasma region (2.5m<R<4.7 m), thus
making ~1/3 of fibers and polychromators useless,which is a bitter failure caused by relying on an
unreliable information. If we want to see the plasma
region with 200 fibers, we should set the viewing
window a little farther away from the laser input
window.
Wefabricated200
2.6
Polychromators:
conventional 5-color-filter polychromators. The
gain of photo detectors (APD) is very sensitive bothto the bias voltage and to the ambient room

temperature, thereby introducing systematic errors
in Te and ne. In addition to the conventional room
temperature and voltage regulation, we took thefollowing measure: we first classified 1000 APDs
by their recommended bias voltages VC into 200
groups, and then allotted five APDs in each groupto each polychromator. Like a ‘common mode
rejection’ method, this helped appreciably to reduce
systematic errors in Te, which is deduced from
relative magnitudes of APD’s outputs. This may bea reason why Te profile is better in quality than neprofile.
2.7 Data Acquisition: To acquire a large number ofsignals of 200*5=1000 in a short time, we adopted
Lecroy 1881M charge ADC modules running on
FASTBUS, which is now not available. We shouldhave purchased much more ADC modules for thefuture extension of the TS system.
3. FIRST DATA and Subsequent Improvements
Soon after the start of the 2nd LHD run, theLHD-TS began to yield Te and ne profile data.
Initial failures were solved step by step, but thefollowing two problems remained to be solved for along time.
3-1 Effect of background radiation: In the earlyphase of the series of LHD-runs, fairly good Te
profiles were obtained. However, as plasma heating
power increased, Te and ne profiles sometimes
showed up irregularities on the channels which‘see’ the region where there is carbon divertor
plates in the background view scope. Guessingthis was caused by the background radiation fromthe divertor plates, we made grooves on the surfaceof the divertor plates to decrease the reflectivity,
which had some favorable effect for a few years.
However, as the heating power increased further,
the irregularities on profiles began to appear again.
Next, we replaced the divertor plates with thosewith a higher heat removal ability. The surfaces
were grooved again. This replacement had
favorable effect for a few years, but as the heating
power increased further, the same phenomenon
reappeared. To see what was happening, wemodified the APD circuits so that the DC level aswell as AC (pulse) can be read. Afterwards wefound that when Te and ne profiles become irregular,
the APD’s DC outputs get large and often exceed
the dynamic range of the circuits. The blackbody
radiation temperature up to 2700K was estimatedfrom the APD’s DC outputs. Considering that thegrooved structure is likely to be heated upon intenseheat flux, we removed the grooves on the surface,which resulting in less frequent appearance of theprofile irregularities. Tungsten diverter plates withhigh heat removal ability will be necessary if theplasma heating power is further increased.
3.2 Density calibration: In the adopted heavily
oblique backscattering configuration, the lightcollection optics is likely to pickup intense straylight, invalidating the conventional Rayleighscattering calibration of the absolute sensitivity ofeach polychromator. Hence, we intended tocalibrate by applying hydrogen Raman scatteringand checked its validity in CHS, a pilot machine for
LHD. However, for a safety reason it was forbidden
without any scientific discussion to fill H2 gas in theLHD chamber after the LHD-TS construction wasalmost completed. This was the start of ourtime-consuming painful efforts. Firstly, we tried
N2-Raman scattering, but in vein: the #1 filter catchonly small portion of the N2-Raman spectra and theresultant calibrated ne profile fluctuated channel tochannel. Next we replaced the #1 filter with new
one that can cache wider N2-Raman spectra, butalso in vein. This effort, however, gave a rewardthat the minimum detectable Te lowered down to~3eV. Thereafter an idea of “high-pressure low
laser energy Rayleigh scattering” came to ourminds. Firstly we tried with a tunable laser (OPO)
tuned to the #3 filter, but in vein. There was adifficulty in shaping the OPO beam envelope as theYAG laser. Finally, we newly added #6 filter-APD
combinations dedicated for YAG laser Rayleighscattering in 2007. After two years intense efforts tostabilize the beam transverse positioning, we finallyare able to obtain ne profile, the line integral ofwhich agrees fairly well with the HCN laserinterferometer data in 2010.
4. Possible Revised TS
If we were able to go back to the past, we would
like to construct a TS somewhat differently.
Disregarding the neutron radiation effects on theoptics, we use a lens system and set polychromatorsin the LHD room, thereby saving much expenditureon the optical fibers. A lens system enables us to set
arrays of fiber-tip along the focal lines of manylaser beams, one of which is moved synchronouslywith the laser beam transverse position, thereby
giving 2-D profiles. The plasma region along R
passing the magnetic axis is dissected by 300
optical fibers of ~1.5 mm in core diameter. An arrayof fibers with different length can distinguish thescattering positions by the time delay in fibers like a
LIDAR. This LIDAR-like scheme, which has ahigh spatial resolution of ~1 cm, will reduce thenumber of the APDs necessary for a 2D-profilemeasurement. The radiation effects on optics, if any,
would rather give us valuable information.
References[1] K.Narihara et al.: Rev. Sci. Instrum. 72 (2001) 1122.