22p104-P
On measurement of ion temperature at the center ofhigh density plasmas of JT-60SA
JT-60SAの高密度プラズマ中心部でのイオン温度測定について	 Tomohide Nakano, Maiko Yoshida, Kiyoshi Itami, Kensaku Kamiya, Kouji Shinohara,
Hiroshi Tojo, Takaki Hatae and Masakatsu Fukumoto
仲野友英, 吉田麻衣子，伊丹潔, 神谷健作, 篠原孝司, 東條寛, 波多江仰紀, 福本正勝
The Japan Atomic Energy Agency
801-1, Mukoyama, Naka, Ibaraki 311-0193, Japan
日本原子力研究開発機構 〒311-0193	  茨城県那珂市向山801-1
Because of significant decay of neutral beam, the C VI emission intensity becomes so low that estimateduncertainties of the ion temperature and Doppler shift do not satisfy the requirements in the JT-60SA
research plan. To satisfy the requirements, it is necessary to increase the law signal by a factor of 5 for theuncertainty of the ion temperature and by a factor of 20 for that of the Doppler shift. This can beaccomplished by increasing the number of bundled optical fibers, or long exposure of a detector although
the spatial and/or time resolution decreases.

2. Simulation
2.1. Target plasma and neutral beam
The electron and ion temperature, and electron
density profile for the present analysis are shownin Fig. 1. The plasma effective charge is assumed
to be 2.2. These parameters are based on Scenario
2 of JT-60SA. A deuterium neutral beam with aninjection power of 1.7 MW is injected from theedge ( the normalized minor radius, ρ = 1) toward

816612-320E049434t10s

4820ne ( 101910m )the center (ρ = 0). The ratio of the beamcomponent with an energy of 85 keV, 42.5 keV
and 28 keV is assumed to be 65 : 20 : 15 [2]. Thebeam deposition, density and emission intensityof C VI (n= 7-8) are calculated one-dimensionallyalong the beam axis with 50 cells.

Te, Ti ( keV )1. Introduction
Ion temperature is one of the fundamental
quantities to determine the fusion reaction rate. In
fusion experimental devices, in order to measure
the ion temperature, beam-aided charge exchange
recombination spectroscopy (CXRS) [1] has been
often applied. With this measurement technique,local ion temperatures along neutral beam can bemeasured. However, the measurement is limited
to the location where sufficient neutrals aresupplied from the neutral beam. Hence it isdifficult to measure the ion temperature at thecenter of high density plasmas due to significant
decay of the neutral beam. This is the case in
JT-60SA. Uncertainty of the ion temperaturedepends on the signal-to-noise ratio of themeasured spectrum. In order to evaluate thesignal-to-noise ratio, it is necessary to consider
the sensitivity of the whole measurement system,
which consists of optics, a spectrometer and adetector, in addition to evaluation of the emission
intensity from the plasma. This evaluation gives
essential information for design of themeasurement system.

400.2

0.4
0.6
0.8
Normalized radius, !

10Fig.1. Electron (Te) and ion (Ti) temperature (Te = Ti)
and electron density (ne) profile.

2-2. Analysis model
The deposition of the neutral beam ndep at a cell i
is calculated with a beam stopping cross-section σST[3] as follows:
ndep (i) = nNB(i) σST vNB ne(i) dt (m-3),
where nNB is a beam density, vNB a beam velocity, dt
a time to penetrate a cell i. Some of beam deuterium
atom is excited by a collision with electron anddeuteron in the plasma. The population of excitedlevel at a principal quantum number of 2 iscalculated with a collisional-radiative model in thecase of electron collision [4] and with a coronalmodel in the case of deuteron collision. Then, thecharge exchange reaction of the ground and theexcited beam deuterium with thermal C6+ produces

3. Results
600
Zeff = 2.2
P NB = 1.7 MW
5 fiber bundled!=0Ti = 8.3 +/- 0.5 keV
V T = 0 +/- 7.3 km/s

Counts500
40030020010000100200300
Pixel

400500

Fig.2. Predicted C VI (n=7-8) spectrum and a curve
fitted with a Gaussian function as a function of pixel
number of CCD. The number of bundled optical fiber isassumed to be 5.

Figure 2 shows the spectrum synthesized with thepresent model. The evaluated uncertainty of the ion
temperature (0.5 keV) is satisfied with therequirement, 10% of the ion temperature, i.e., 0.8keV, according the JT-60SA research plan [6].
However the uncertainty of the Doppler shift islarger than the requirement, 5 km/s.
In order to reduce the uncertainty of the Doppler
shift, it is necessary to improve a signal-to-noise
ratio by increasing collected number of photons.
Here this is investigated by increasing the numberof bundled optical fibers. Figure 3 shows theuncertainties of the ion temperature and the Doppler

2.020(a)Zeff = 2.2

1.515

1.010!Ti = 0.8 keV!v T = 5 km / s

0.50.00102030
4050
Number of bundled optical fibers

5!v T ( km / s )2-3. Synthesis of spectrum
The C VI emission is collected with lenses,
transmitted through optical fibers into aspectrometer, and detected with a CCD camera. The
sensitivity of the optical system is assumed to be
the same as that of the optical system used inJT-60U. With this sensitivity, the C VI emission isconverted to the number of photo-electrons NPH ineach CCD pixel. The noise for the photo-electrons
expressed as σPH = NPH0.5 is added. Then, thenumber of photo-electrons is multiplied by a factorof 200 (this is a simulation of a gain function suchas EM gain), and converted to digital counts bydividing with a analog-to-digital ratio of 8.7.
Finally the spectrum of C VI ( n=7-8 ) is
synthesized from the counts and the width
determined from the ion temperature at the center ofthe plasma (8 keV).
The synthesized spectrum is fitted with aGaussian function in order to evaluate uncertainties
of ion temperature and Doppler shift.

shift as a function of the number of bundled opticalfibers. The uncertainty of the Doppler shift
decreases with increasing number of optical fibers.
The requirement of the uncertainty of 5 km/s issatisfied in the case that 20 fibers are bundled.

!Ti ( keV )excited C5+ at n=8, resulting in C VI ( n=7-8 )emission, which intensity is calculated withemission rate coefficients from ADAS [5].

0Fig.3. Estimated uncertainties of the ion temperature

σTi and the Doppler shift σvT as a function of bundled
optical fibers.

4. Discussion
To improve the signal-to-noise ratio, longexposure of the CCD camera is an alternative way.
However, long exposure time results in poor timeresolution while increasing the number of bundled
optical fiber results in poor spatial resolution.
Hence the best solution, which satisfies therequirements of the time and the spatial resolution,
should be determined. This depends on anexperimental purpose.
Other factors that cause uncertainties, forinstance, background emission of C VI ( n=7-8 )and bremsstrahlung emission, will be considered infuture analysis. Because both of the above factors
decrease the signal-to-noise ratio, further increaseof the number of bundled optical fiber is required tomeet the requirements for the uncertainties.
5. Conclusions
The requirement of uncertainty of the ion
temperature from the JT-60SA research plan can besatisfied if 5 optical fibers are bundled. However, abundle of 20 optical fibers is needed to meet therequirement of uncertainty of the Doppler shift.
References[1] R. J. Fonck, D. S. Darrow and K. P. Jaehning: Phys.
Rev. A 29 (1984) 3288.
[2] T. Nakano, Y. Koide, et al, J. Plasma Fusion Res. 81(2005) 708.
[3] S. Suzuki, T. Shirai, et al, Plasma Phys. Control.
Fusion 40 (1998) 2097.
[4] K. Sawada and T. Fujimoto, Phys. Rev. E 49 (1994)
5565.
[5] H.P. Summers, JET-IR 06 (1994),[6] JT-60SA research plan.