REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D407 (2014)

Edge profile measurements using Thomson scattering on the KSTAR tokamaka) J. H. Lee,1,2,b) S. Oh,1 W. R. Lee,1 W. H. Ko,1,2 K. P. Kim,1 K. D. Lee,1 Y. M. Jeon,1 S. W. Yoon,1 K. W. Cho,1 K. Narihara,3 I. Yamada,3 R. Yasuhara,3 T. Hatae,4 E. Yatsuka,4 T. Ono,4 and J. H. Hong5 1

National Fusion Research Institute, Daejeon, South Korea Department of Nuclear Fusion and Plasma Science, University of Science and Technology (UST), Daejeon, South Korea 3 National Institute for Fusion Science, Nagoya, Japan 4 Japan Atomic Energy Agency, Naka, Japan 5 Department of Physics, KAIST, South Korea 2

(Presented 3 June 2014; received 29 May 2014; accepted 2 July 2014; published online 1 August 2014) In the KSTAR Tokamak, a “Tangential Thomson Scattering” (TTS) diagnostic system has been designed and installed to measure electron density and temperature profiles. In the edge system, TTS has 12 optical fiber bundles to measure the edge profiles with 10–15 mm spatial resolution. These 12 optical fibers and their spatial resolution are not enough to measure the pedestal width with a high accuracy but allow observations of L-H transition or H-L transitions at the edge. For these measurements, the prototype ITER edge Thomson Nd:YAG laser system manufactured by JAEA in Japan is installed. In this paper, the KSTAR TTS system is briefly described and some TTS edge profiles are presented and compared against the KSTAR Charge Exchange Spectroscopy and other diagnostics. The future upgrade plan of the system is also discussed in this paper. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4890258] I. INTRODUCTION

II. EXPERIMENTS AND DATA

The Nd:YAG Thomson scattering diagnostic system has been widely used for measuring electron temperature and density profiles in fusion plasma researches. Moreover, this system operates independent of tokamak conditions such as magnetic field, heating condition, and so on.1 For these reasons many nuclear fusion research groups operate Thomson scattering diagnostic systems. For the KSTAR (Korea Superconducting Tokamak Advanced Research) tokamak, a multipoint Tangential Thomson Scattering (TTS) diagnostic system has been developed based on the prototype ITER (International Thermonuclear Experimental Reactor) Edge Thomson Nd:YAG laser (1064 nm, 100 Hz, 5 J) leased from JAEA (Japan Atomic Energy Agency) for three years. This system is consisted of two objective lenses to collect the scattered lights from the plasma core and the edge regions. Twenty-five polychromators are installed by NIFS (National Institute of Fusion Science) through the Japan-Korea collaboration program. The core polychromators were modified by NFRI (National Fusion Research Institute) based on the edge polychromator design. The KSTAR TTS system has been installed since 20102 with its first data acquisitioned in 2012. In this paper, some edge profiles measured by the KSTAR TTS diagnostic system are presented along with their comparison against other profile diagnostics.

The KSTAR Thomson scattering system is of a tangential type. Therefore, its configuration, as illustrated in Fig. 1, requires three medium ports inside the KSTAR machine (the medium L-port for the laser beam input, the medium N-port for collection optics,3 and the medium B-port for the beam dump).

a) Contributed paper, published as part of the Proceedings of the 20th

Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected] 0034-6748/2014/85(11)/11D407/3/$30.00

FIG. 1. Layout of the KSTAR tangential Thomson scattering system.

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FIG. 2. Electron temperature and electron density profiles for the KSTAR shot #7082 measured by the KSTAR Thomson scattering system at 1.6 and 2.9 s (blue: L-mode, red: H-mode).

The prototype is a 1064 nm, 5 J, 100 Hz double beam system, thus each of A- and B-line operates at 2.5 J and the frequency of 100 Hz.4 During the 2012 campaign, however, we used the B-line only with the laser energy reduced down to 2 J at 100 Hz because of a minor failure in the optical component test after its delivery from JAEA. Thus, for the 2012 KSTAR campaign, the 2 J laser beam was injected into the KSTAR vacuum vessel through eight mirrors and focused on the targeted region by the convex lenses. The laser beams depart the vessel through the beam dump,

FIG. 3. Time evolution of the edge electron density profiles during an L-H transition (#7082).

custom-designed for the characteristic double beam lines of the prototype ITER edge Thomson laser beam system. The diameter of the beam dump is 4 in. and a knife edge is installed inside. A total of 17 optical fiber bundles are installed to measure plasma profiles; six for the core and eleven for the edge plasma parameters in 2012. For the 2013 campaign, the measuring positions were modified to five points for the core and twelve for the edge in order to improve the edge profile measurement. The spatial resolution for the core is 50 mm–100 mm and 10 mm–30 mm for the edge. The optical fiber bundles are consisted of ∼96 silica-silica type single optical fibers with their diameters of 200 μm. And the polychromator band-pass filter configuration is in Ref. 5. The signal from the polychromator goes into the electric fast amplifier (CAEN N979, ×4) and integrated ADC (Analog to Digital Converter) type digitizer (CAEN V792, V792N).6 For the KSTAR experiments, the KSTAR CCS (Central Control System) sends a soft trigger signal to the Thomson system 60 s before the PF (Poloidal Field) coils blip time and the laser is warmed up for 30 s before measuring the Thomson signals. The Rayleigh scattering method with N2 (Nitrogen) gas is used to calibrate the Thomson scattering system while a Tungsten (W) light and monochromator system is used for the relative calibration. N2 gas is injected from the 0.1 to 20 mbar in vacuum vessel. The Rayleigh scattering signals are detected on the 1064 nm polychromator channels and the measured signals are linearly fitted to evaluate the electron densities. Figure 2 shows the electron density and temperature profiles at the edge for the KSTAR shot #7082. The red points

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FIG. 4. The electron density profile fitted with the modified tangent hyperbolic function for an KSTAR H-mode (during t = 6.0–6.29 s @ #7102)7 from which the pedestal position (Position ) and the pedestal width (Width ) are calculated.

denote an H-mode profile while the blue points are for an L-mode, measured at 1.6 and 2.9 s, respectively. These data sets, averaged for 100 ms, have been processed using the cali-

Rev. Sci. Instrum. 85, 11D407 (2014)

bration lookup tables and the Rayleigh calibration results with the Chi squire method. For the 2013 campaign, the Thomson data were processed with the simple ratio method due to the low SNR (Signal to Noise Ratio). From Fig. 3, we can clearly observe the edge electron density profiles changing from L-mode to H-mode at the edge (L-H transition) (#7082). Figure 4 (#7102) shows an H-mode electron density profile during the 6.0 to 6.29 s period, which is fitted by the modified tangent hyperbolic function.7 All the data points are time-averaged for 300 ms. From this work we can estimate the approximated pedestal position, Position , and the width, Width (∼50 mm). These pedestal widths are compared with other profile diagnostics such as CES (Charge Exchange Spectroscopy) and ECE (Electron Cyclotron Emission). Figure 5 shows a comparison of the pedestal widths against the CES and ECE measurements for the KSTAR shot #9422. This comparison validates the accuracy of the pedestal position and width measurements of the KSTAR Thomson TTS system.

III. CONCLUSION AND FUTURE WORKS

In conclusion, the KSTAR Thomson scattering diagnostic system has been designed and installed based on the prototype ITER edge Thomson laser system and has successfully collected the preliminary Thomson scattering data for the KSTAR tokamak. From these preliminary data, the pedestal width and position are successfully measured and compared with other diagnostics. However, the KSTAR electron temperature and density profile data from Thomson scattering are not routinely being uploaded to the KSTAR data server because of the low SNR in the raw signals. The high amplitude noise is believed to originate from the electronic devices and optical components. Thus, in the next campaign, we will attempt to increase the SNR with the following two approaches: (1) the APD (Avalanched Photo Diode) circuit or the APD itself will be upgraded; and (2) an optical fiber positioner will be added to the edge collection optic module because the beam alignment is very important in measuring signals.

ACKNOWLEDGMENTS

This work was supported by the Ministry of Education Science and Technology of Republic of Korea. The authors would like to thank all the NFRI teams, NIFS Thomson Team, JAEA Thomson Team and Dr. C. Bae. 1 I.

FIG. 5. Qualitative comparison of the pedestal width from Thomson (ne, Te) against ECE (Te) (#9422) and CES (Vφ: toroidal velocity).

H. Hutchinson, Principles of Plasma Diagnostics, 2nd ed. (Cambridge University Press, 2002), p. 273. 2 J. H. Lee, S. T. Oh, and H. M. Wi, Rev. Sci. Instrum. 81, 10D528 (2010). 3 S. Oh and J. H. Lee, Rev. Sci. Instrum. 81, 10D504 (2010). 4 T. Hatae, E. Yatsuka, T. Hayashi, H. Yoshida, T. Ono, and Y. Kusama, Rev. Sci. Instrum. 83, 10E344 (2012). 5 J. H. Lee, S. Oh, H. M. Wi, K. P. Kim, I. Yamada, K. Narihara, and K. Kawahata, JINST 7, 10E344 (2012). 6 W. R. Lee, H. S. Kim, M. K. Park, J. H. Lee, and K. H. Kim, Rev. Sci. Instrum. 83, 093505 (2012). 7 E. R. Arends, Density Gradients in Spherical Tokamak Plasmas (Technische Universiteit Eindhoven, 2003), p. 94.

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Edge profile measurements using Thomson scattering on the KSTAR tokamak.

In the KSTAR Tokamak, a "Tangential Thomson Scattering" (TTS) diagnostic system has been designed and installed to measure electron density and temper...
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