REVIEW OF SCIENTIFIC INSTRUMENTS 81, 10E506 共2010兲
The first experimental results from x-ray imaging crystal spectrometer for KSTARa… S. G. Lee,1,b兲 J. G. Bak,1 U. W. Nam,2 M. K. Moon,3 Y. Shi,4 M. Bitter,5 and K. Hill5
1
National Fusion Research Institute, Daejeon 305-333, South Korea Korea Astronomy and Space Science Institute, Daejeon 305-348, South Korea 3 Korea Atomic Energy Research Institute, Daejeon 305-353, South Korea 4 Institute of Plasma Physics, Chinese Academy of Science, Hefei 230031, China 5 Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA 2
共Presented 20 May 2010; received 12 May 2010; accepted 19 June 2010; published online 5 October 2010兲 The x-ray imaging crystal spectrometer 共XICS兲 for the Korea Superconducting Tokamak Advanced Research has been first applied for the experimental campaign in 2009. The XICS was designed to provide measurements of the profiles of the ion and electron temperatures from the heliumlike argon 共Ar XVII兲 spectra. The basic functions of the XICS are properly working although some satellites lines are not well matched with the expected theoretical values. The initial experimental results from the XICS are briefly described. © 2010 American Institute of Physics. 关doi:10.1063/1.3478631兴
I. INTRODUCTION
The x-ray imaging crystal spectrometer 共XICS兲 for the Korea Superconducting Tokamak Advanced Research 共KSTAR兲 diagnostic system1 has been installed and operated for the second experimental campaign in 2009. The XICS system for KSTAR was well described in previous papers.2–5 The XICS for the KSTAR device was designed to provide measurements of the profiles of the ion and electron temperatures from the heliumlike argon 共Ar XVII兲 spectra. The spectral range extends from the resonance line 共w兲 at 3.9494 Å to the forbidden line 共z兲 at 3.9944 Å. A four segmented multiwire proportional counter 共MWPC兲 and time-to-digital converter based delay-line readout electronics were used for the XICS. In this article, the first experimental results from the XICS are discussed. II. EXPERIMENTAL RESULTS
The XICS was installed at the end of the pumping duct on the KSTAR tokamak as shown in Fig. 1. In Fig. 1, the KSTAR tokamak, pumping duct, and the main components of the XICS are shown. The MWPC was well shielded by 100 mm thick lead bricks as shown in Fig. 1. The XICS was equipped with a spherically bent 110 quartz crystal with a 2D spacing of 4.913 Å. The diameter and radius of curvature of the crystal are 100 and 5294 mm, respectively. Figure 2 shows a typical example of the plasma conditions in the 2009 experimental campaign. The maximum plasma current was above 330 kA and its duration was about 3.7 s. Argon gas was injected into the KSTAR tokamak at 0.1 s. The measured photon count rate from the XICS, as shown in Fig. 2, starts at about 0.15 s and varied with the a兲
Contributed paper published as part of the Proceedings of the 18th Topical Conference on High Temperature Plasma Diagnostics, Wildwood, New Jersey, May 2010. b兲 Electronic mail:
[email protected]. 0034-6748/2010/81共10兲/10E506/3/$30.00
electron temperature and density. Note that the neutral Ar gas penetration and the ionization equilibrium time for the Ar XVII lines take about 50 ms. The Ar XVII is the dominant charge state for the electron temperature in the range from 0.4 to 3.0 keV so that the best time for the XICS starts near the beginning of the plasma discharge based on the electron temperature. The core electron temperature in Fig. 2 was measured by an electron cyclotron emission 共ECE兲 diagnostic, which was not accurately calibrated. Figure 3 shows a time-integrated spatially resolved image of the Ar XVII lines for shot 2136. The wavelength is shown on the x axis and the spatial information is given on the y axis. The Ar XVII resonance line w at 3.9494 Å and forbidden line z at 3.9944 Å are clearly shown. The measured image is divided into four because the MWPC is in four segments. The dark horizontal lines in each detector are the shadows from the supporting metal ribs for the beryllium entrance window of the MWPC. The spectral lines are curved due to the rotational symmetry about the normal of the spherically bent crystal, since the incident and reflected rays form a cone about the crystal.6 Figure 4 shows the raw spectral data binned on the x axis after curvature correction and a fitted spectrum from detectors 2 and 3, which view central parts of the plasma. The spectral features are identified by using Gabriel’s notation.7 The ion and electron temperatures, Ti and Te, can be derived from a least-squares fit of the w and z lines, and resonance and satellite line ratios, respectively. As shown in Fig. 4, the least-squares fits for the w and z lines are reasonable, but the satellites lines are not well matched with the expected theoretical values. This inconsistency is not clearly understood but may be either due to the fact that the spectra are not from equilibrium plasma conditions but from transient conditions, where runaway electrons may exist and where the ionization equilibrium of Ar was not yet established or to an excess amount of Ar puffing. Note that the piezovalve for the Ar
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FIG. 1. 共Color online兲 The experimental arrangement layout of the XICS. The enlarged MWPC shown in the right side was well shielded by thick lead bricks.
puffing was not controlled to inject a measured amount of gas in the 2009 experimental campaign. This may induce a large amount of pulse pile-up rejection on the MWPC. The Ti and Te were derived from the Doppler width of the w line and the intensity ratios of the n ⬎ 3 satellites nearby w. The time interval for this fitting was from 0.2 to 0.7 s. The measured Ti and Te are about 0.26 and 1.0 keV, respectively. Note that the measured Ti includes a correction for the instrumental broadening by the Johann error8 and the spatial resolution of the detector. Although the measured Te contains a large uncertainty since the least-squares fit to the n ⬎ 3 satellites is poor, the Te value is in the same order from the ECE diagnostic. Figure 5 shows temporal behaviors of the raw spectral data binned on the x axis from detectors 2 and 3. The time binning and interval for the spectrum are 0.5 s. It is almost FIG. 2. 共Color online兲 Experimental conditions for shot no. 2136.
FIG. 3. 共Color online兲 A spatially resolved image of the Ar XVII lines.
FIG. 4. 共Color online兲 A raw and a fitted spectrum.
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formed in the second experimental campaign in 2009. The results demonstrated that basic functions of the XICS for the KSTAR tokamak are properly working although some satellites lines are not well matched with the expected theoretical values. This inconsistency will be clarified in the 2010 experimental campaign after installing the second XICS on the KSTAR tokamak. ACKNOWLEDGMENTS
This research was supported by National R&D Program through the National Research Foundation of Korea 共NRF兲, funded by the Ministry of Education, Science and Technology 共Grant No. 2009 0082449兲, and the KSTAR project funded by the Korea Ministry of Education, Science, and Technology. 1
FIG. 5. Temporal behaviors of the raw spectral data binned on the x axis.
impossible to determine the Ti from the least-squares fits of the w and z lines after 1.2 s due to the decrease of the electron temperature shown in Fig. 2. In addition, the background photon intensities are also larger after 1.2 s. As shown in Fig. 4, the best time for the least-squares fits for the w and z lines is 0.2–0.7 s. As a summary, the initial measurements of the KSTAR x-ray imaging crystal spectrometer were successfully per-
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