J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Conceptual Design of Polychromator for Thomson Scattering System
in JT-60SA
Hiroshi Tojo, Takaki Hatae, Takeshi Sakuma, Takashi Hamano, Kiyoshi Itami
Japan Atomic Energy Agency, Naka, Ibaraki, Japan(Received: 18 November 2009 / Accepted: 5 March 2010)

Significant loss in transmissivity of the fiber used for the 18 years operations in JT-60U has been discovered:
thereby making optimization of the filter configuration in the polychromator in Thomson scattering system for thehigh performance plasmas in JT-60SA is an essential task. The loss in the optical components induced by radiationfrom neutrons and γ-rays in particular needs to be examined. This paper concerns the optimization of the filter
configuration in the polychromator with the expected error in measurements due to the radiation damage also being
estimated through comparing between fibers used in JT-60U with radiation resistant fibers.
Keywords: Thomson scattering, polychomator, neutron, JT-60SA, fiberof the fibers requiring replacement also needed identifying.
Another topic concerns radiation resistant fibers being
compared to normal fibers for use in the error analysis. The
Numerical Aperture (NA) of the normal fibers can exceed
0.3, but the materials used in the resistant fibers limited the
NA to 0.2: thereby making the analysis also important inensuring the design of the optical system supported thecollection of the many TS photons. The transmissivity
measurements of the fibers in JT-60U are also provided as
non-resistant fiber examples.

1. Introduction
A Thomson Scattering (TS) system is an essential
diagnostics tool used to measure temporal evolution ofelectron temperature (Te) and density (ne) profiles. The TS
system will not only provides data for used in evaluatingenergy confinement time but also data for use inunderstanding physical phenomena such as the MHD
instability, internal transport barrier, H-mode edge pedestal
and ELM activities. In addition, fusion plasma experiments
have gradually resulted in increased demand for high Te
values to be measured. In JT-60SA [1], the preferred range
is Te = 0.01 – 30 keV. The lower bound is used whenmeasuring runaway electrons with Te ~ 10 eV [2]. The
intensity of scattered with short wave length photons (λ =400 - 700 nm) strongly depends on the accuracy of a highTe value, however there are two notable problems that arise
when analyzing the TS spectrum, one being the lowtransmissivity of the optical fibers in the short wavelength
region (< 700 nm). Neutrons and γ-ray radiation also
reduce the optical transmissivity: thereby, making
estimating the neutron flux and resulting errors in themeasurements essential to the forthcoming highperformance regime. The other problem is optimizing thedesign of the polychromator. The error is sensitive to thefilter configuration in the polychromator used in thedetectors, thus necessitating that the wavelength range at
which each of the detectors needs to measure the light beidentified. Reference [3] describes how to optimize thefilter configurations in a polychromator using theannealing method with the ITER edge TS system. Use ofthis technique enables the error to be examined withrespect to Te and best configuration to be identified.
The objectives of this paper were to optimize the filter
configuration for a targeted Te with JT-60SA plasmas andto estimate the time evolution of the measured error to bechanged by the neutron and γ-ray radiation. The condition

2. Transmissivity measurements of fibers used in
JT-60U
18 years of DD plasma operation in JT-60U(1991-2008) have resulted in significant damage to theoptical fibers in the TS system from the radiation. The
specifications of the TS system were detailed in [2, 4]. The
fibers (GS-180, made by Sumitomo Electric Industries,
Ltd., core diameter: 182 μm, cladding diameter: 200 μm)
are constructed pure quartz with GeO2-doped cladding, NA= 0.3, with a total of 102 fibers being used per bundle(channel). The total neutron rate at the first wall, where it isassumed to be basically the same as the TS optical system,
was roughly estimated to be 8.7×1014 n/cm2. The
fibers-optic bundles transmit the scattered light over ahundred meters to the diagnostic room. The
transmissivities of the target fiber bundles were measured
using a standard light source with an integrating sphere(HR4000CG-UV-NIR, Ocean Optics, Inc.) and a fiber
optic multi-channel spectrometer (HP8153A, Hewlett
Packard). Note that the value of the transmissivity does notinclude the filling rate of the fiber bundle packing. The
spectrum through the target fiber bundles was firstmeasured, thus allowing the relative transmissivity
spectrum to be estimated by using the ratio of the spectrum
through the fiber bundle with that of a 2 meter long test
fiber bundle. Note that apart from the length, theauthor’s e-mail:tojo.hiroshi@jaea.go.jp288©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

H. Tojo et al., Conceptual Design of Polychromator for Thomson Scattering System in JT-60SA

2-meter-test fibers had the same specification as the
100-meter fiber. Use of the spectrometer required theincident light to be input into another fiber(P400-2-UV-VIS). However, the effective size of the fiber
input was small when compared to the image size of theend section of the 100-meter-fibers, with the NA value of ~0.2 of the fiber also being inadequate. The error caused byany misalignment of the fibers bundles therefore reliedsignificantly on the power measured by the spectrometerand the prevention of any of the incident light being
collected. The transmissivities were therefore correctedusing another measurement with a power meter that has thewider sensitive area of ~ 6.3 mm2 (referred as “power
calibration”). Four narrow band-passed filters (FWHM <20 nm) at 502.4, 566.1, 599.6 and 694.6 nm were utilized.
The ratio of the power of the test fiber bundle to that of the
100-meter-fiber bundles for each filter was used inprecisely evaluating the transmissivities. Wide range
transmissivities (400 - 1000 nm) were then finallydetermined using the least square method to identify thebest fit between the two measurements. Figure 1 provides
an example of the transmissivity spectrum of a fiber
installed in 1991 and located at z = 0.458 m (verticalcoordinates of the target), together with the transmissivity
measured in 1991 (green). The 3 points of data recorded in1991 confirms decreases of -1.3 dB (700 nm), -2.8 dB (600nm) and -7.3 dB (500 nm) to have taken place. It isbelieved that that degradation resulted from neutron and
γ-ray radiation because it is particularly severe in the shortwavelength (λ) region. The noises at λ < 480 nm and λ >900 nm was caused by lack of output intensity ofintegrating sphere and/or detector in the spectrometerlacking sufficient sensitivity.

Fig. 1 Measured spectral transmissivity of a usedfiber in JT-60U (PMT#8). The four blue points are
the results from power calibrations. The light green
line is data obtained in 1991.
As mentioned above low transmissivity can be clearlyseen in the fiber data in the short wavelength region, witheven further decay being expected from the severe
damage caused by radiations in JT-60U experiments.
Radiation resistant fiber (Mitsubishi Cable Industries,Ltd) were therefore utilized in the calculations. Decreases
in transmissivity in the short wavelength region are seenfrom 0 C/kg to 258 C/kg (e.g. at 600 nm changed from
0.8 to 0.5). At more than 258 C/kg, however, some
improvements were observed in the middle wavelength
range of 600 – 900 nm (e.g. increased from 0.6 to 0.8 at
700 nm). One possibility for that recovery may be theradiation hardening that has been observed in irradiation
experiments such as [6]. It is believed that impurities in
the fiber tend to bond the Si and O radicals generated by
the radiation thus curing the broken structures. The core
diameter of the fiber is 200 μm with an NA value of 0.2
and a length of 100 m. Note that a lack of data resulted inassumption being made for the transmissivity at 0 C/kg in
ensuring that the loss in dB/km had a linear relation from
420 - 500 nm. Notch filters are being planned to be used
to prevent dominant line emissions being treated as
background signals in the measurements. In this paper theassumption was made that filters with 20 nm of FWHM
(Full Width at Half Maximum) against Dα at 656 nm,
which will be the dominant line radiation, were being
used. The filling rate in the fibers was also assumed to be
60 %. The transmissivity and reflection rate of eachlenses and band-pass filters were considered to be 95 %with the channels being placed in the order from theshortest wavelength region to ensure that the first channel
could be used to detect the strongest light.

3. Thomson scattering system in JT-60SA
The TS system is one of the backbone diagnostic
systems used in JT-60SA. In the present conceptual
design a diagnostic laser beam is injected tangentially tothe toroidal direction, with three viewing fields covering
entire plasma radial space area. Figure 2 presents aschematic of the viewing field of the system. Port P1(outboard edge system), P2 (core system), and P5(inboard edge system) cover the entire radial position.
The optical systems are located in the port-plugs where
no vacuum is necessary. The planned specifications are
summarized in Table 1. An existing YAG laser system (λi= 1064 nm) with 7.4 J (Max.) of output energy and a 50
Hz repetition rate [5] will be used as the diagnostic laser.
The corrected light will be transmitted to by 100 meterlong fiber to the polychromators in the diagnostic room.
The polychromators consists of Avalanche PhotodiodeDetectors (APDs) with band-pass filters being used toacquire the appropriate wavelength regions as presented
in Fig.3.The quantum efficiency of the APDs, thetransmissivity of the 100 meter long fiber, and the opticalcomponents are shown in Fig. 4.

4. Optimization of Polychromators
4.1 Photon numbers from Thomson scattering andevaluation of relative error
The error to be calculated in determining Te and ne can be
estimated using the photon numbers from TS,
bremsstrahlung, and impurity line radiation. The
calculations required the following differential photon
cross section provided in [7]:289

H. Tojo et al., Conceptual Design of Polychromator for Thomson Scattering System in JT-60SA

d 2σ pdεd Ω s= q (ε ,θ ,2α )re2 ×

exp(−2α 1 + ε 2 (2(1 + ε )(1 − cos θ )) )(1)

2 K 2 (2α )(1 + ε ) 2 ε 2 + 2(1 + ε )(1 − cos θ )α=me c 22Te(2)where, q (ε, θ, 2α) is the depolarization factor,
ε=(λs−λi)/λi the normalized wavelength expressed by theincident laser wavelength (λi = 1.064 μm), and scattered(λs) and Ωs, θ , re, K2(x) and c are the solid angle are thescattering angle, electron radius, modified Bessel functionof the second kind and the light velocity, respectively. Notethat the assumption was made that the wave electric fieldof incident laser (Ei) being perpendicular to the scattering
plane and electric field parallel to the incident would beobserved. Figure 5 (top) provides the waveforms of thedifferential cross section, revealing the wavelength
dependence on the scattered photons. It indicates atconstant scattered angle (θ = 135°), generations of intenselight with short wavelength are expected in high Te.

Fig. 4 Quantum efficiency of APD (dot line) andassumed transmissivity of notch filter (dashed line).
Fiber transmissivity with and without γ-ray irradiation.
The irradiation doses are 258 C/kg, 1290 C/kg and6450 C/kg.
The photon numbers observed in one channel measuring
the photons with εj-1 < ε < ε j can be estimated using
following expression:N s , j = ne ΔL

λi Ei
hc

εj

Ωs ³

ε j −1d 2σ pd ε dΩ sη (ε )T (ε )dε(3)εj≡ ne ³ S (ε , θ ,2α )dε ≡ ne g j (Te )ε j −1

where j denotes the channel number, εj the boardernormalized wavelength between the wavelength region of
the jth and (j+1)th channels. And, ΔL, h, η(ε) and T(ε) are
the scattering length, Planck’s constant, quantum efficiencyof the APD, and the total transmissivity of the optics,
including the fiber bundle, respectively. When the signal
was measured, however, other photons generated by thebremsstrahlung and impurity line radiations could also be
observed as background light. The back ground light in the
εj-1 < ε < ε j region in particular can be written by [8], or:Fig. 2 Schematic of three optics from P1 (blue), P2(green) and P5 (pink) in JT-60SA.
Table 1 Main parameters of optics system

Radialposition
Scattering
LengthFig. 3

P1(outboard
edge)
0.349
–
0.416 m
8 mm

P2(core)
0.211
0.353 m
25 mm–

N B, j = n
P5(inboard
edge)
0.174
0.214 m25 mm

2e

me c 2 8 2π me c 2 3
re ΔΩ s dΔL
Te 3 3 hεj× sin θDΔtg ff Z eff ³–

ε j −1η (ε )T (ε ) K (ε )
dε
1+ ε

(4)

εj

= ne2 ³ B (ε , θ ,2α )dε ≡ ne2 h j (Te )ε j −1

where d, D, Δt, gff and Zeff are the diameter of the incident
beam, the effective pass length of the plasma, the dataacquisition time (longer than the laser pulse duration),
Gaunt factor, and effective ionic charge, respectively. K(ε)
is the enhancement factor used to denote the difference
resulting from theoretical estimation of the bremsstrahlung.
That difference derives from the impurity lines,
recombination emissions and refraction light from the wall.
Note that the line integration effect is ignored in theformula, or in other words a uniform distribution (same ne
and Te along the sight line) is assumed. Ωs = 5 msr, θ =Image of polychromator for JT-60SA

290

H. Tojo et al., Conceptual Design of Polychromator for Thomson Scattering System in JT-60SA

135°, Ei = 3 J, ΔL = 0.02 m, d = 8 mm, D = 2.5 m, Δt = 300
ns, gff = 3, Zeff = 3.5, K (ε) = 3 (constant) and ne = 6.0×1019
m-3 were assumed to be the parameters used in
optimizations.
The photons generated by the TS are estimated bysubtracting the total signal from background signal.
Derivation (σj2) of the measured number photons cantherefore be expressed using [3], or:σ 2j = N 0 + Fnoise [ne g j (Te ) + 2ne2 h j (Te )](5)

where N0 and Fnoise are the dark noise and excess noise
factor of the APD, respectively. In this analysis N0 was
assumed to be 225 and Fnoise 3.0 which were expectedwith the detector gain of 100 provided in reference [9].
After determining Te and ne in the data analysis thefollowing χ2 is minimized in finding the best fit of theparameters.
J(ne g (Te ) − N j ) 2

j =1

σ 2j

χ =¦
2

Fig. 5 (Top): Differential cross section of Thomson
scattering for various Te (1 keV – 50 keV) at θ = 135°.
(Bottom): optimized filter configuration in thepolychromator with Nch values.
where E1 and E2 are errors defined as <σTe /Te> for eachdifferent pattern of the optical filters, p the probability that
E2 < E1 and TSA the constant temperature value. Use of thenatural log of both sides results in the following
expression:(10)
E1 = E 2 + TSA ln p

(6)

The associate errors in Te and ne can be estimated through astandard analysis [8]¦ (g j σ j )2(7)
σ T2e =222 2′′
¦ (ne g j σ j ) ¦ (g j σ j ) − ¦ ne g j g j σ j[ (This formula indicates that some thermal noise(proportional to the temperature) is included as error in thecalculations. The TSA then needed to be reduced infocusing on global minimum value for E. The downhill
simplex method was utilized in examining different
patterned filter setup [10]. In the calculations, the TSA wasconstantly reduced using 0.5(7+n), where n denotes thechange in the value of TSA.
4.3 Effect from radiation and optimizations
The planned annual limitations of the neutron yield witheach of the research phases and the JT-60SA are provided
in Table.2. In order to assess the exposure doses resulting
from neutrons and γ-rays in the fiber area of concern theneutron and γ-ray flux in the fiber position (R (major
radius) = 4.7 m) were estimated utilizing 3-dimensional
neutronics analysis [11]. The doses during and without thedischarges were considered here. The sum of the doses
estimated as C/kg and as a function of time (operational
phases) is provided in Fig. 6.
As mentioned in the introduction, the fiber used in thisoptimization is radiation resistant. In order to estimate thetransmissivity with various doses the assumption was madethat 10 m would be the effective irradiated length of the
100 m fiber as the fiber bundle cables could get irradiatedfrom the end of the collection optics to the ground floor in
the machine room due to having no shielding because of
the lack of space. The remaining 90 m was treated as
non-damaged fiber (0 C/kg). However, and unfortunately,
the exposure dose actually reaches 6450 C/kg, which is the
maximum exposure dose in Fig.4 11 years after the
beginning of the operations (refer to Fig. 6). This thenmeans that at present radiation damage to the fibers could
not be evaluated across all the extended phases.

)]¦ (n g ′ σ )
(8)
′
′
(
)
(
)
[
(
)
]
−
n
g
σ
g
σ
n
g
g
σ
¦
¦
¦
2

σ n2 =
e

e

j

2

e

j

j

j

2

j

j

e

j

j

2
j

2

,where gj and gj' respectively denote gj(Te) and gj(Te)/ Teand the summation takes place over j=1 – J. Here J denotes
the number of channels in the polychromator. In the
optimization σTe/Te (<σTe/Te>) was averaged over 50 points
within the range of Te = 0.01 - 30 keV on logarithmic scale,the minimum value identified, and the best configurationof the filters in front of the APDs then determined in
identifying the wavelength ranges of the detectors thatshould be measured. All the polychromator channels wereconsidered to cover the entire wavelength range (420 –1065.5 nm) in the calculations. The decision was
therefore made to change the patterns by varying thebordering wavelength points where the detectors lie next
to each other in the wavelength space. One of thedetectors, however, needed to be reserved for theRayleigh scattering calibrations at 1064 nm and hence
one filter of 1063.5 – 1065.5 nm is employed in thecalculations. Optimizing <σTe/Te> was executed withthrough use of the annealing method, which will be
explained in the following section.
4.2 Simulated annealing method in optimization
The simulated annealing method can be used to identify
the minimum values with problems that include many local
minimum values in a multi-dimensional function [10], thus
making it suitable for use in optimizing polychromator inquestion. The basic principle of the method is summarized in
the following expression:§ E − E1 ·(9)¸
p = exp¨¨ − 2
TSA ¸¹©

291

H. Tojo et al., Conceptual Design of Polychromator for Thomson Scattering System in JT-60SA

nm. A less than 10 % relative error was preferable inproviding information on the detailed spatial structure ofphysics targets such as magnetic islands. The radiation
resistant fibers had a slight difference at 0 C/kg to the otherirradiated fibers in the high Te area (Te > 10 keV) andwhich is reflected in both transmissivities in the shortwavelength region. In order to clarify that difference theaverage relative error with respect to time (experimental
phase) was evaluated, the details of which are provided in
Fig.9. In that evaluation the average relative error in therange of Te > 10 keV was used. The exposure dose of thefibers can be transformed into the time in years using therelationship estimated and then provided in Fig. 6. About
1% of the gradual increases in error can be seen with both
Nch = 5 and Nch = 6, suggesting the damage caused by theradiation to be rather insignificant. However, it should be
noted that the error at Nch = 5 approaches 10 % at Te = 30
keV (Fig. 8), which could result in high Te measurements
being rather difficult.
If the fiber used in JT-60U are utilized, the error is muchlarger than with the resistant fibers by a factor of 1.7
actually exceeding 10 % for both Nch = 5 and 6 at Te = 30
keV as revealed in Fig.8. Te > 10 keV results in the 10.2 %and 8.3 % of the average relative errors being estimated for
Nch = 5 and 6, respectively. Moreover, further enhancementof the relative errors can be expected after reuse. The
non-radiation resistant fiber (used fiber) therefore isinappropriate for use with such high Te measurements.
With low Te measurements under the condition of theassumed ne (6.0×1019 m-3) more than 10 % of the relative
errors at 100 eV are inevitable even at Nch = 9 (~ 12 %)with the radiation resistant fiber, and the determining suchlow Te values for use in revealing a detailed structure of theplasma was very difficult.

Table 2 Research phases and annual neutron yields.
Phase

Duration(year)

Annualneutronrate

Total
input power

Initial
phase

1-2y
2-3y

4×1019

23 MW
33 MW

Integrated
phase

2-3y
> 2y

4×1020
1×1021

37MW

Extended
phase

>5y

1.5×1021

41MW

Fig. 6 Expected time evolution of irradiation dose at R
= 4.7m.
4.4 Optimization and time evolution of the relative error
Optimizing the filters first took place using the annealing
method with different number of the detectors (Nch),including one channel used in the Rayleigh calibration. The
optimized patterns of the filter are revealed in Fig. 5 (at thebottom). A low average relative error was discovered at thelarge Nch = 6 and that it appeared to converge at the highNch as described in Fig. 7. At a high Nch (>7) value the error
increased because some of the APD channels were locatedrelatively far from the input (in the high wavelength
region) to the polychromator and only a small number ofthe photons were detected due to the reflection rate andtransmissivity of the band-pass filters and lenses. The costof the polychromators strongly depends on the Nch used,making a low Nch such as Nch = 5 or Nch = 6 more
preferable. The optimized filter configurations revealed by
the wavelength border of the filters were [420.0, 912.1,
1025.9, 1052.0, 1062.5, 1065.5 nm] at Nch = 5 and [420.0,
866.2, 995.1, 1033.8, 1052.7, 1062.5, 1065.5 nm] at Nch =6 as revealed in Fig. 5. The dependency of the error on Nch
can be seen in the level of the fluctuation and the numberof local minimum values provided by the waveforms.
Large Nch values tend to suppress any such fluctuation.
Figure 8 provides the relative error of the optimized
filter configuration with respect to the Te for various
irradiated radiation resistant fibers and the fibers used in
JT-60U that were treated as non-radiation resistant fibers.
The damaged transmissivity shown in Fig. 1 was utilized
and 0.63 assumed to be transmissivity for > 1000 nm,
which is the average value within the range of 900 – 1000

5. Conclusion and future work
Optimization of the filter configuration, which depends
on the transmissivity spectrum in fibers, in apolychromator was carried out. The time evolution of therelative error in Te which was obtained by estimating
exposure dose up to early on in the extended phase in
JT-60SA was then evaluated. The gradual small increases
in the average error (~ 1 %) in Te was estimated for 11
years of operation, but did not exceed 10% at Nch = 6,
suggesting this number of detectors to be more suitable
than Nch = 5. With low Te measurements more than 10 % of
the relative error at 100 eV are inevitable even at Nch = 9 (~12 %) at ne = 6.0×1019 m-3. Difficulties therefore are be
expected in attempting to understand the detailed spatial
structures of a plasma with runaway electrons (Te ~ 10 eV).
However, that error could be reduced in high intensity
discharges because a strong intensity of TS light can be
expected.
Radiation resistant fiber do need to be used in high Te
values measurements (> 10 keV), otherwise a large relative
errors of more than 10 % with both Nch = 5 and Nch = 6
could result. Note that the fibers used in the JT-60U
experiments were treated as being non-resistant fiber.
Measurement of the transmissivities of the fibers used in292

H. Tojo et al., Conceptual Design of Polychromator for Thomson Scattering System in JT-60SA(http://physics.nist.gov/PhysRefData/ASD/index.html.)

JT-60U revealed the significant losses to have occurred.
The loss was presumably generated by the strong neutronand γ-ray radiation in the short wavelength region. Use of
the resistant fiber, however, does have a disadvantage as
the materials used in them limit the NA value (~ 0.2), thus
requiring the size of the end face of the fiber to be enlarged.
As future work the validity of Bremsstrahlung (Zeff)
measurements using the TS system will be discussed. Inorder to detect photons in a wavelength range not influenced
by the line radiation in the polychromator one channel must
be in the wavelength range of 1045 - 1064 nm because that
range does not include the dominat line components of C, He,
H, O and N (refer to Fig. 10) [12]. The uncertainty in theparameters of JT-60SA must therefore be evaluated indetermining the Zeff profile.

Fig. 8 Relative errors in Te and ne with respect to Te at
Nch =5 (a) and Nch =6 (b). Colors indicate different
operation year (irradiation doses). Broken lines
present the fiber used in JT-60U.

Fig. 7 Averaged relative errors in Te with respect tothe number of detectors in polychromator (Nch).

6. Acknowledgements
The authors would like to express their sincere thanks to
Dr. A. M. Sukegawa, Dr. E. Yatsuka, and Dr. O. Naito for
their valuable discussions. Data on the fibertransmissivities was provided by the courtesy of
Mitsubishi Cable Industries, Ltd.

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Fig. 9 Expected time evolutions of the averaged
relative error in Te at Nch =5 (circle) and Nch =6(square).

Fig. 10 The positions with line impurity emissions on
wavelength space from the Ref. [12]293