J. Plasma Fusion Res. SERIES, Vol.3 (2000) 419422

Thomson Scattering System for
Steady State LHD Plasma
NARIHARA Kazumichi*, HAYASHI Hiroshi, YAMADA Ichihiro and YAMAUCHI Kenji
National Institute for Fusion Science, Toki 592-5292, Japan(Received: 19 January 2000 / Accepted: 13 April 2000)

Abstract
We examine the problems that will arise when the present LHD Thomson scattering system operates

for a long pulse plasma and consider countermeasures against them. The viewing window poses mostsevere problems. For an expected heat flux from the plasma of 10kWm2, the present system withstandsthe operation of up to 10 minutes duration, which can be extended to infinity by placing a water cooled
sapphire plate coated with a low-pass-filter in front of the view window. As a whole, the system will
operate without an intemrption at least 16 hours with modest modifications.

Keywords:
steady state plasma, LHD, Thomson scattering

1. lntroduction
Even if a steady state plasma is realized in LHD,

collecting optics, which enables us to take high quality
data even with a small laser energy, which, in turn,

continuous or repetitive plasma diagnostics are stillnecessary to check if the plasma is truly in a steadystate, and sometimes to feedback control the plasmastate. For electron temperature (Te) and density (ne)measurement, repetitive Thomson scattering is mostsuitable because it gives reliable Te and ne in a wide
range of plasma parameters. The Thomson scatteringsystem (TS) currently operating on LHD has alreadydemonstrated its ability to take data for a long pulse
plasma persisting up to 72 s. In below, we study thepermits high repetition operation. The detaildesignconsiderations of the TS is given in [], and theconstruction and the performance of a prototype

problems with which we will confront when theoperation time is prolonged up to hours and consider
countermeasures against them.

2. Description of LHD Thomson Scattering
SystemThe features of the LHD TS are: (1) its obliqueback scattering configuration, which enables us tomeasure whole profiles of Te and ne along a major

Fig. 1 Scattering configuration of the LHD Thomson
scattering system.

radius on the mid plane; (2) its large solid angle of light* C o rre sp o ndin g autho r' s e - mai I@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research: narihara @ nifs. ac j p4r9

Narihara K. et al., Thomson Scattering System for Steady State LHD Plasma

installed on the Compact Helical System (CHS) isdescribed in l2l. Figure I shows the scattering

the view scope extended by 105-120 fibers. The thus
collected light is transported 45 m by the optical fiber tothe diagnostic room and then spectral analyzed by 5-configuration.
The laser beam (pulse energy 0.55 J, repetition rate50 Hz), transported -50 m from the diagnostic room andfilter polychromators. The electric outputs from thepolychromators (up to 1000 channels) are converted todigital data by FASTBUS ADCs (1881M Lecroy). The
digital data are transferred to two I MB fast ECLaligned so as to run along a major radius on the mid
plane, first passes through a focusing lens of / = 7 m,enters the LHD vacuu-m chamber through a laser
window set at the center of 4-O port, runs throughplasma region and finally is dumped in a beam dump
made of carbon block backed by a 3 mm thick SUS 304
plate. The beam diameters at the entrance window andmemories on VME in a flip-flop mode. When the firstmemory becomes full, then the data flow is switched tothe second memory and the data in the first are

viewing window (fused quartz) of 0.65 x 0.4 m2 areatransferred to a 64 MB memory installed on VME. This
process is repeated for two memories alternatively,
enabling continuous data acquisition with acquisition
rate of up to 10 MB/s. Although, the total data size isand 0.05 m width is set on 4-O port, the center of which

limited by the memory size in the VME, this is easily

being separated 0.75 m from the laser beam. The lightbackward scattered is collected by a mosaic mirror of
1.5 x 1.8 m2 area and focused onto the end faces of 200
arrayed optical fibers of 2 mm in diameter. For theincreased substantially.

at the beam dump are l0 mm and 6 mm, respectively. A

This TS has already operated for long pulse
discharge persisting tp to 72 s. An example of thecontour plot of Te of a long-lived plasma is shown inFig.2. The central density is around 2 x l0te/m3. The
slow variation of Te was caused by a unregulated gaspuffing.

standard LHD operation the plasma region is covered by

3. Problems Arising in the Steady State
Operation(i) Laser: Nd:YAG laser (Continuum NY8l-50: 50 Hz
rep. rate, 0.55 J/ l0 ns pulse width, 0.6 mr pointing stability) can operate continuously until the life time offlash lamps (50 million shots = 11.5 days) expires.
(ii) Laser beam transport system This is composed of
6 steering mirrors, one focusing lens. These are placed
in atmosphere without any particular cooling system.
Our concern is if a small fraction of laser power is deposited on the substrate of the miror or lens, thereby introducing thermal distortion to the mirrors or lens. We
have already transported laser beams for more than 10minutes without any noticeable growing degradation in
the transported beam quality.

60@o4r)
c'tr

5.5

3.0

4.0

Mojor rodius (m)2.0c40se69*oo3o0.0
2.5

7t(0.5)2= O.7 Jlcm2, which is much less than the specified damage threshold of l0 J/cm2. High transmittance(>0.975) of the window implies that no heat will be accumulated in the substrate even for long time laser in-tb^@oo&g.dcc cc
Eq

3.0

ness. The laser power density at the window is 0.55/

elot^" c QD^&!*ot
Q-cFo"ooc^

c'Q"
oS-

r.o

0.5

(iii) Laser entrance window is a both side AR coatedfused silica window of 3" in diameter and 0.5" in thick-€j" "^* ^.
q"

1.5

Ba"

3.5

4.O

b
4.5

jection.
(iv) Beam dump: The currently used beam dump is acarbon block backed by a 3 mm thick SUS304 plate.
After -10000 plasma shots including several long pulse
discharges persisting for > 60 s, we observed that there

5.0

Mojor rodius (m)Fig. 2 An example of the evolution of Te profile of thepfasma that persisted lor 72 sec.
420

Narihara K. et al., Thomson Scattering System for Steady State LHD Plasma

formed a burnout hole in the beam dump. We speculatethat the laser sputtering process was accelerated by theraised temperature of the carbon and SUS304. We areplanning to use a water cooled beam dump made ofmolybdenum, which has high heat conductivity and highmelting point.

Cover Glassl--l 12mm

Soft X

WindowIsomm --.]

NeutralParticles(v)View Window: The heat flux from the steady state
LHD plasma is supposed to be around 10 kwm2. If theview window made of fused quartz is exposed to thisheat flux, soft x-ray quanta and neutral particles, thedominant careers of the heat, are deposited in the verythin surface layer (a few microns), thereby introducing

10KW/m2

Radiation

Radiation

5KW/m25KW/mzvery high thermal stress particularly at the startup phaseof plasma. In order to prevent the fracture of the win-

dow due to this thermal stress added to the mechanicalstress caused by vacuum pressure, we set a cover glass(12 mm thick fused quartz) in front of the view window
as shown in Fig. 3. The cover glass is first heated up toa temperature Zgh* while emitting radiation F(Trui =(h)B(h, Tg,.,Jdh from both sides, which, in turn heats
the viewing window. Herc, B(h, D = 2nc2hlh5l(ehc/Mr-l)is the Planck function and 4L) = a(L) = l-(i), whereFig. 3 Side view of the viewing window.

300
250e(/,) is emissivity, a(1.) absorptivity and r(i.)(J^^^:

transmittivity, respectively. Solving the energy transport
equation using z(/") for quartz shown in Fig. 5, we
obtained the time evolutions of the volume averaged
temperatures of the cover glass and the temperature distribution of the view window (76) as shown in Fig. 4.
After 1000 s from the start of plasma discharge, Tsbu ?lmost reaches an equilibrium temperature of 277 "C. The
temperature at the surface facing to the cover glass Z,;,(0) increases with much longer time scale. This temperature inevitably introduces a high temperature gradient at
the window's periphery, where it tightly touches to LHD

E 15oo,$ roo
50o'o,''-o"o3l4oooFig. 4 Evolutions of the temperatures of the cover glassand the viewing port without the cooled sapphire
plate.

flange kept at the room temperature (25 'C). If we makea moderate assumption that T.;n up to 65 oC is permissible, then the current TS system endures 600 sec operation without modifying the view window.
For operation longer than 600 sec, we need toblock the radiation from the cover glass to the view

the periphery temperature fixed and exposed to thesteady state radiation from the cover glass has thetemperature distribution with the peak value at thecenter 40 'C higher than the periphery. The radiation
power emitted by the sapphire plates at this temperatureis negligibly small. Unfortunately, the pass band ofsapphire is wider than that of quartz as shown in Fig. 5,

window. An easily conceivable way to block theradiation is to place a sapphire plate, which has highthermal conductivity (0.36 J/sec-cm:C at 25 "C) andhigh optical transmittance in visible-near infrared
region, between the cover glass and the view window.

implying that some portion (30Vo) of the radiation fromthe cover glass can not be blocked. Hence, the safe
operation time is prolonged only about 3 times. Much
longer operation time will be achieved by coating thesapphire plates with a thin film filter whose bandwidthis narrower than that of the fused quartz but is wide
enough for Thomson scattered light. For example, alow-pass filter with the cutoff wavelength at 2.5 1tm,

On a water-cooled frame at 25 "C,24 sapphire plates of
0.1 x 0.1 m2 in area and 5 mm in thickness are patched
to form a 0.4 x 0.6 m2 mosaic plate. The opening
percentage of 8O7o seems to be easily realized. A
numerical calculation shows that the sapphire plate with421

Narihara K. et al., Thomson Scattering System for Steady State LHD Plasma

rently used, is 1.6 MB/s, which is too high to be stored
in the disk continuously. If we reduce the number ofbackgrounds to 2, the birthrate is reduced to 0.53 MB/s,

1.00.8which permits a continuous data acquisition by transfer-ring data in the 64 MB memory as soon as the data size
reaches 1 MB. Accordingly, a I MB data file is being
created sequentially every 1.9 s, until a hard disk of,0.)3 o.oqg 0.4

say, 100 GB, which is available at a reasonable cost, be-6F o.z+F-ra-come full in 53 hours. For continuous Te deduction
from these data files, four task sharing PCs (Pentium IVIntel) are necessary.

Elack Body Rad. (550K)
ouartzSapph ire

Transmitted Rad.

4. Discussionsllave length (pm)Up to this, we considered the problems that disable
the operation of the TS. Here, we discuss a problem thatis related to degradation of the data quality. Impurity
particles from plasma pile up on the surface of the coverglass forming a thin layer of film, which distorts thespectrum of the Thomson scattered light, introducing
systematic errors in Te. For exposure to -10000 shots ofaverage 2 sec duration plasmas, no appreciable change
in the transmittance of the cover glass was observed.
Then, the data quality is preserved at least for 20000 sec(5.5 hours). The operation time thus limited is prolonged
by replacing the cover glass with a backup in vacuum.
We have another two backup cover glasses housed

Fig. 5 Transmittances of fused quartz and sapphire asfunctions of wavelength. Spectra of black bodyradiation from the cover glass and the radiationthat pass through the sappier plate are alsoshown.

which is easily fabricated, almost block the radiationfrom the cover glass.
The temperature coefficient of refractive index of
13 x l0-6/'C makes the sapphire plates focusing lenses,

but they are too weak to degrade the image quality ofthe laser beam.

The cover glass, being faced with the plasma, isdarkened gradually and then backups are indispensablebeneath (Fig. l). Another method is to correct thefor periodic replacement in vacuum, Therefore, aspectrum of Thomson scattered light with thescheme to cool the cover glass made of sapphire directlytransmittance obtained by measuring the spectrum of theis very expensive.
For the same reasoning, replacing the view window
with the cooled sapphire plate will do well. However, it

blackbody radiation from a heater set inside of thevacuum through the cover glass plus the sapphire plate
plus the view window. This method, however, inevitably

will be much difficult technically.
(vi) Data acquisition/processing: A sequence of processes involving data acquisition, data storage in amemory, data transfer to a computer and calculation

introduces intemrptions of the measurement. Altemative

method is to occasionally clean the cover glass by using
laser blow off method [3], which also requires someintemrption of the measurement.

without an intemrption during a long pulse plasma discharge is our concern. If the deduced data is used for
feedback control of the plasma state, a real time dataprocessing with a minimum delay is required. Thepresent TS system can be modified so as to afford a realtime data processing by implementing multi processors
into the VME frame. Here, we only examine the feasibility of continuous data acquisition for a long pulse
plasma discharge. The most time consuming process inthe present data acquisition scheme is the data transferfrom the 64 Mb memory to the memory on a PC andsubsequent creation of a data file which is stored in a5. Conclusions
From the above considerations, it seems quite
feasible to operate the present 50 Hz repetition rateThomson scattering system for at least l6 hours withmodest modifications.

tllReferences
K. Narihara er al., Fusion Eng. Des. 34-35, 67(re97).
t2l K. Narihara et al., Rev. Sci. Instrum. 66, 4607
(1995).

disk. It takes I sec for I MB data. The data birthrate forthe single laser plus 8 backgrounds mode, that is cur-t3l K. Narihara et al., Rev. Sci. Instrum. 63, 3527
(1992\.
422