- Email: [email protected]
Concept design of ultrafast charge exchange recombination spectroscopy on EAST tokamak
Fusion Engineering and Design 146 (2019) 522–525
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Concept design of ultrafast charge exchange recombination spectroscopy on EAST tokamak
T
⁎
Yingying Lia, , Yixuan Zhoub, Di Jianga, Xiaojun Zhouc, Yi Yub, Jia Fua, Bo Lyua, Yuejiang Shid, Guosheng Xua, Minyou Yeb, Baonian Wana a
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui, 230031, China Department of Engineering and Applied Physics, University of Science and Technology of China, Hefei, Anhui, 230026, China c Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 23001, Hefei, China d Department of Nuclear Engineering, Seoul National University, Seoul, 151-742, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: EAST Ultrafast charge exchange recombination spectroscopy Fluctuation measurement Diagnostic
An ultrafast charge exchange recombination spectroscopy (CXRS) has been constructing for the measurement of the plasma ion temperature and rotation velocity fluctuations on EAST tokamak. The system shares the collection optics with the existing BES. The designed spectrometer operates at the spectral range of 527.5 ± 5 nm, then the emission lines from C VI at 529.059 nm and Ne X at 524.897 nm can be observed simultaneously. The collimating and focusing lenses are specially designed to realize the maximized transmission (˜95% for each lenses) and the optimized image quality. One customized transmission grating sandwiched by two identical BK7 prisms from Wasatch Photonics is used. The clear size of the grating is ˜205 × 100 mm and the grating has very high diffraction efficiency of > 80% for unpolarized light. The Simulation of Spectra code is used to guide the design and evaluate the diagnostic performance. In this paper, an overview of the status and performance of the ultrafast CXRS system will be presented.
1. Introduction It is crucial to measure plasma ion temperature and velocity fluctuations, which is associated with plasma turbulence and transport in present day fusion devices [1,2]. Charge exchange recombination spectroscopy (CXRS) [3,4] is the standard technique to measure the plasma ion temperature and rotation in most of the today’s tokamaks. The conventional CXRS system are normally operated with tens of milliseconds’ integration time, the diagnostic presented in the paper is the high throughput, and high efficiency coupled with fast photonnoise-limited detector to provide measurement of ion temperature and rotation velocity at a turbulence-relevant 1 μs time resolution. EAST (Refs. [5] and [6] and references therein) is a fully superconducting tokamak with major and minor radii of 1.88 m and 0.45 m respectively, plasma current Ip < 1 MA, and toroidal field BT˜2 T. Cocurrent and counter-current directed neutral beam injection (NBI) systems [7] are operated with 2–4 MW beam power and injection energies of 50–80 keV to heat, fuel and rotate the plasma. Each NBI system consists of two beam sources, and the injection of one (tNBI) is more tangential than the other (nNBI). CXRS systems on EAST usually exploit the line (C VI, n = 8→7, 529.059 nm) from the charge exchange
⁎
recombination reaction between fully ionized carbon impurities (C6+) and energetic neutral beams [8,9]. The EAST CXRS system has been successfully recorded a charge exchange emission from Ne10+ ions recently [10], and Ne X (n = n = 11→10, 524.897 nm) lines was found to be a good candidate for the CXRS measurement on EAST tokamak. The paper is organized as follows. In Sec. II, a detailed description of the hardware of the ultrafast CXRS is given. These cover the diagnostic layout and critical optical components of the high throughput spectrometer. In Sec. III, performance evaluation and prediction of the system through simulation of spectra code are described. A summary of the paper and discussion of the future plans are presented in Sec. IV. 2. Ultrafast CXRS system on EAST The ultrafast CXRS system comprises a collection optics located in a port plug with a protective shutter in EAST tokamak, fiber bundles, and one high etendue spectrometer system with one high speed detector. The light emitted from the beam-impurity ions interaction zone is collected by the collection optics and then transferred via the fiber bundles towards the spectrometers behind the EAST hall for the spectral analysis, followed the data acquisition systems and analysis software.
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.fusengdes.2019.01.012 Received 8 October 2018; Received in revised form 6 December 2018; Accepted 3 January 2019 Available online 06 January 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 146 (2019) 522–525
Y. Li et al.
Fig. 1. Layout schematic of the ultrafast CXRS (UF-CXRS) and BES system. The light is separated by a beam splitter in spectrometer side.
The aims of the ultrafast CXRS are the measurement of ion temperature and rotation fluctuations at 1 MHz. In order to satisfy the requirement, the performances of all the optics elements are optimized for the required wavelength band of 527.5 ± 5 nm (for the simultaneous measurement of C VI and Ne X lines). Furthermore, the values of the numerical aperture (NA) of collection optics, the transmitted fibers, and the spectrometer are best match for maximized overall optical efficiency. 2.1. Planned diagnostic layout on EAST The planned ultrafast CXRS system shares the collection optics with the existing beam emission spectroscopy (BES) [11] situated at the horizontal P port on EAST. The BES aims at the radial and poloidal measurement of the plasma density fluctuation. The collection optics of BES has been upgraded for the simultaneous measurement of the ultrafast CXRS and BES systems. Fig. 1 shows the layout schematic of the upgraded collection optics. The system is carefully designed to measure 656–661 nm (BES spectra) and 527.5 ± 5 nm (for C VI and Ne X lines) simultaneously. The light from plasma is collected by collected optics, transmitted by fused silica fiber bundles of ˜60 m, and then separated by a beam splitter (BS) in the laboratory. The reflection light from beam splitter (shown in Fig. 1) is refocused on the entrance slit of the spectrometer. The radial and poloidal observation ranges cover the main plasma from R = 2.05–2.31 m and Z= ± 40 mm, respectively. The magnification of the system is ˜5, and the intersection of the optical sightline with the tangential neutral beam is aligned to a magnetic flux surface to achieve good spatial resolution in the radial and poloidal planes (ΔR˜ΔZ˜1–2 cm). Each channel of 2.3 × 2.3 mm is filled with 200/220 μm core/clad diameter fibers with fill factor of ˜0.63 and contains 105 fibers. The numerical aperture (NA) of the used fiber is 0.22, matching with the collection optics.
Fig. 2. Optical diagram of the high etendue spectrometer. (1) Entrance slit; (2) Collimating lens; (3) Grism (prism-grating-prism); (4) Focusing lens; (5) Image plane (Detector).
located at the end of collimating lens is to block the spare light from the horizontal direction. At the first phase, the fibers from the EAST are directly arranged in an array of three rows at the entrance slit, which is corresponding to 0.64 mm width and 210 fibers with 200/220 μm core/ clad diameter can be accommodated with 15.4 mm-long slit, which corresponds to two viewing chords. The nominal curvature radius of the fiber bundle is about 41 mm for the straight image on the detector plane. The collimating and focusing lenses are optimized for the desired spectral band. The clear apertures of the lenses are 90 and 112 mm, the corresponding fields of view (FOV) are 4 and 7.24 ° respectively. The transmission of the two lenses is about 95%. Overall on-axis modulation transfer function (MTF) produced by the combination of collimating and focusing lenses can up to 0.7 at spatial frequency of 150 cycles/ mm. One bandpass filter (90 mm clear diameter) is mounted between the collimated lens and the grism to ensure that undesirable input to the sensor from the stray light will be minimized. The filter bandwidth is 14 nm full width at half maximum (FWHM), and the transmission achieved is up to 95% in the 524–531 nm wavelength range. The half angle of the input light rays on the filter is about 3 °, and the influence on the filter transmission can be ignored for the relative wide FWHM of the filter [14]. The custom-made transmission grism from Wasatch Photonics was designed and optimized for the 524.5–530.5 nm wavelength range. The grating has an active area of ˜205 × 100 mm2 on a substrate of 273 × 110 mm2 and groove frequency of 4871 lines/mm. The grooves of the grating are parallel to short dimension. The diffraction efficiency of the grating is higher than 80% for unpolarized light. Considering the wide image plane (˜18.4 × 15.4 mm2) as well as purpose of assembly and alignment of the spectrometer, one existing Phantom V710 CMOS camera is used to record dispersed lines and placed at the exit position at the first step. The camera has a 1280 × 800 pixel sensor with 20 μm pixels, equaling to 25.6 × 16 mm2
2.2. Hardware of high-etendue spectrometer An optical schematic of the spectrometer is shown in Fig. 2 and the engineering design is shown in Fig. 3. The detailed designed parameters of the spectrometer are listed in Table 1. The spectrometer was constructed using high-throughput collection optics and a high-efficiency volume phase holographic transmission grating sandwiched by two identical prisms (BK7 glass, with anti-reflection coating for the designed wavelength band). The dual prisms and grating combination is called grism. The similar type of spectrometer with commercial camera lens has been well developed by Kaiser Optical Systems [12] and also at DIII-D tokamak [13]. The input light is collimated by a customized lens (focal length: 200 mm, F-number: 2.27, matched with the input fiber bundles), dispersed by the grism, and finally focused by another lens (focal length: 200 mm, F-number: 2.27) to the detector. The effective F-number in the dispersion direction of the collimating and focusing lens is 2.7 and determined by the limited grating size. One rectangular aperture 523
Fusion Engineering and Design 146 (2019) 522–525
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Fig. 3. Engineering design of the spectrometer. (1) Fiber bundle and entrance slit. The other indications are in the agreement with the Fig. 2. Phantom V710 CMOS camera (5) is used for the spectrometer assembly, alignment, and performance tests. (6) Filter. Table 1 Main parameters of the spectrometer. Parameters
Value
Spectral band Focal length Slit width×Height Image dimension F-number Spectral resolution Dispersion Etendue No. of channels Monitored lines
527.5 ± 5 nm 200 mm 0.64 × 15.4 mm2 18.4 × 15.4 mm2 2.7(Horizontal)×2.27(Vertical) ˜[email protected] mm width slit ˜0.36 nm/mm ˜0.63 mm2sr for each channel 2 C VI(529.059 nm); Ne X(524.897 nm)
Fig. 4. Photon flux of C VI (n = 8→7, 529.059 nm) and Ne X (n = 11→10, 524.897 nm) charge exchange lines that reach the image plane of the spectrometer.
sensor area. It is allowed to handle 15.4 mm-long image simultaneously. A set of strategies are performed to reduce the stray light and increase the transmission of the system. Anti-reflective (AR) coating is applied to each surface of the lens and prisms of the grism. The two end faces of the fiber bundles will be dealt with RARe Motheye™ technology [15] provided by Fiberguide industries, in this way, the transmission of the fibers can be improved about 6% for the needed wavelength band. All the inner surfaces of the mechanical components of the system and the edge of lens are blackened for elimination of the stray light.
(CX) photon fluxes from C VI and Ne X lines that reach to the image plane of the spectrometer. The photon flux of active CX line is determined by Icx = Ncx ∙T∙ε , where Ncx is the corresponding CX photons emitted from plasma, T is the optical transmission efficiency, and the overall transmission for the whole system including the collection optics, fiber bundle, and the components of the spectrometer is about 0.4. This provides a guidance to determine the desired parameters of the high-speed detector [19]. Fig. 5 shows the predicted errors for the ion temperature from C VI
3. Performance assessment A Simulation of Spectra (SOS) code [16,17] is used to predict the ultrafast CXRS spectra collected by each viewing chord and optimize the instrument performance. Based on the code, the conceptual design of the EAST ultrafast CXRS diagnostic has been accomplished. The plasma parameters used for modeling simulation are as follows: Ti = Te = 2× (1-(r/a) 2) keV, ne = 4•1019× (1-(r/a) 2)0.5 /m3, here, r/a indicates the normalized minor radius. E (D°) = 25 keV/amu and 40 keV/amu, and the corresponding power fractions of the full, half, and third energy components of the beam are about 0.7:0.2:0.1 and 0.8:0.14:0.06 respectively [18]. Concentration of carbon and neon impurity is set as 1% and flat along the minor radius. In the simulation, only the tangential beam of co-current NBI is used. The passive signal emitted from the plasma edge is subtracted by beam modulation in the SOS code. The etendue is defined as ε = π∙S∙NA2 , with S is area of each spatial channel and π∙NA2 is solid angle of the spectrometer. For the designed spectrometer in the paper, ε = 0.63 mm2sr is listed in Table 1. Fig. 4 shows the radial profiles of the calculated charge exchange
Fig. 5. Predicted errors of the ion temperature from C VI and Ne X lines, as well as the intensity of C VI and Ne X lines at two beam energies, t = 1 μs, NBI modulated for background suppression in the simulation. 524
Fusion Engineering and Design 146 (2019) 522–525
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detector’s parameters will be confirmed. Acknowledgments This work was supported by the National Magnetic Confinement Fusion Science Program of China (Nos. 2015GB101002, 2015GB101000, 2015GB103001 and 2017YFE0301300) and the National Natural Science Foundation of China (Nos. 11535013, 11575235). The authors would like to thank Dr. Manfred von Hellermann and Dr. Sandor Zoletnik for their help in the design of the ultrafast CXRS diagnostic on EAST. References [1] R.J. Groebner, et al., Role of edge electric field and poloidal rotation in the L-H transition, Phys. Rev. Lett. 64 (25) (1990) 3015–3018. [2] W. Horton, Drift waves and transport, Rev. Modern Phy. 71 (3) (1999) 735–778. [3] R.C. Isler, An overview of charge-exchange spectroscopy as a plasma diagnostic, Plasma Phys. Control. Fusion 36 (1994) 171–208. [4] R.J. Fonck, et al., Determination of plasma ion velocity distribution via chargeexchange recombination spectroscopy, Phys. Rev. A 29 (1984) 3288–3309. [5] B.N. Wan, et al., Progress of long pulse and H-mode experiments in EAST, Nucl. Fusion 53 (2013) 104006. [6] B.N. Wan, et al., Overview of EAST experiments on the development of high-performance steady-state scenario, Nucl. Fusion 57 (2017) 102019. [7] J. Wang, et al., Initial operation of the first neutral beamline on EAST, Fusion Eng. Des. 101 (2015) 56–60. [8] Y.Y. Li, et al., Development of the charge exchange recombination spectroscopy and beam emission spectroscopy on the EAST tokamak, Rev. Sci. Instrum. 85 (2014) 11E428. [9] M.Y. Ye, et al., Toroidal charge exchange recombination spectroscopy on EAST, Fusion Eng. Des. 96-97 (2015) 1017–1020. [10] Y.Y. Li, et al., Simultaneous measurement of CVI, NeX, and LiIII charge exchange lines on EAST, Rev. Sci. Instrum. 89 (2018) 10D119. [11] H.J. Wang, et al., Development of beam emission spectroscopy diagnostic on EAST, Rev. Sci. Instrum. 88 (2017) 083505. [12] R.E. Bell, Exploiting a transmission grating spectrometer, Rev. Sci. Instrum. 75 (2004) 4158. [13] I.U. Uzun-Kaymak, et al., Ultra-fast charge exchange spectroscopy for turbulent ion temperature fluctuation measurements on the DIII-D tokamak, Rev. Sci. Instrum. 83 (2012) 10D526. [14] See https://en.wikipedia.org/wiki/Interference_filter for information of the filter. [15] See https://www.fiberguide.com/product/rare-motheye-fiber/ for the information about the Rare Motheye Fiber. [16] M.G. von Hellermann, Active beam spectroscopy for ITER, nuclear instruments and methods in physics research section a: accelerators, spectrometers, Detectors Assoc. Equip. 623 (2) (2010) 720–725. [17] See http://fusionweb.phys.tue.nl/ sos for information about the Simulation of Spectra code (SOS). [18] Y. Wang, et al., Beam species evolution under long pulse operation for EAST-NBI by Doppler shift spectroscopy diagnostic system, J. Fusion Energy 34 (2015) 615–619. [19] D. Dunai, et al., Avalanche photodiode based detector for beam emission spectroscopy, Rev. Sci. Instrum. 81 (2010) 103503. [20] E.S. Marmar, Active Spectroscopic Diagnostics for ITER Utilizing Neutral Beams in : Diagnostics for Experimental Thermonuclear Fusion Reactors, Plenum Press, New York, 1996, pp. 281–290.
Fig. 6. Radial profiles of the signal-to-noise ratio (SNR) for C VI and Ne X lines. t = 1 μs, NBI modulated for background suppression in the simulation.
and Ne X, as well as the intensity of C VI and Ne X lines at two beam energies. It can be seen that the predicted errors from Ne X line are below 10%, less than the values obtained from C VI line. This indicates that Ne X line is more suitable for the ultrafast CXRS measurement. The increased fitting error of ion temperature and intensity of C VI and Ne X lines at r/a = 0-–0.2 and 0.8–1 comes from lower photon number, as shown in the Fig. 4. The calculated SNR value is shown in Fig. 6. The continuum noise level determines the signal to noise ratio (SNR), and can be calculated by [20] =
Icx ∙ D ∙ texp 2Icont + Icx
, where texp is the integration time, texp = 1 μs in
the modeling simulation, D is the dispersion for each spectral channel. Icont = Ncont ∙T∙ε indicate the photons of continuum signal that reach to the image plane of the spectrometer, Ncont is the corresponding photons from plasma. It can be seen that SNR is larger than 10, which is sufficient for the measurement of ultrafast CXRS system at 1 MHz bandwidth. 4. Conclusions and future work To investigate the ion temperature and rotation velocity fluctuations, the ultrafast CXRS system has been constructing on EAST tokamak. Fabrications of the mechanical part for the spectrometer as well as the design of the collection optics are underway. Assembling, alignment, and laboratory tests of the spectrometer will be made in the next few months of 2018. Tests of total transmission, the image curvature, and dispersion of spectrometer are performed at the first phase, one existing high speed Phantom V710 will be used for the wide image plane. A system will be designed for the stray light measurement of the spectrometer. Base on the test results, a curved entrance fiber bundle will be designed for the straight images of measured spectral lines and
525
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Concept design of ultrafast charge exchange recombination spectroscopy on EAST tokamak
T
⁎
Yingying Lia, , Yixuan Zhoub, Di Jianga, Xiaojun Zhouc, Yi Yub, Jia Fua, Bo Lyua, Yuejiang Shid, Guosheng Xua, Minyou Yeb, Baonian Wana a
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui, 230031, China Department of Engineering and Applied Physics, University of Science and Technology of China, Hefei, Anhui, 230026, China c Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 23001, Hefei, China d Department of Nuclear Engineering, Seoul National University, Seoul, 151-742, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: EAST Ultrafast charge exchange recombination spectroscopy Fluctuation measurement Diagnostic
An ultrafast charge exchange recombination spectroscopy (CXRS) has been constructing for the measurement of the plasma ion temperature and rotation velocity fluctuations on EAST tokamak. The system shares the collection optics with the existing BES. The designed spectrometer operates at the spectral range of 527.5 ± 5 nm, then the emission lines from C VI at 529.059 nm and Ne X at 524.897 nm can be observed simultaneously. The collimating and focusing lenses are specially designed to realize the maximized transmission (˜95% for each lenses) and the optimized image quality. One customized transmission grating sandwiched by two identical BK7 prisms from Wasatch Photonics is used. The clear size of the grating is ˜205 × 100 mm and the grating has very high diffraction efficiency of > 80% for unpolarized light. The Simulation of Spectra code is used to guide the design and evaluate the diagnostic performance. In this paper, an overview of the status and performance of the ultrafast CXRS system will be presented.
1. Introduction It is crucial to measure plasma ion temperature and velocity fluctuations, which is associated with plasma turbulence and transport in present day fusion devices [1,2]. Charge exchange recombination spectroscopy (CXRS) [3,4] is the standard technique to measure the plasma ion temperature and rotation in most of the today’s tokamaks. The conventional CXRS system are normally operated with tens of milliseconds’ integration time, the diagnostic presented in the paper is the high throughput, and high efficiency coupled with fast photonnoise-limited detector to provide measurement of ion temperature and rotation velocity at a turbulence-relevant 1 μs time resolution. EAST (Refs. [5] and [6] and references therein) is a fully superconducting tokamak with major and minor radii of 1.88 m and 0.45 m respectively, plasma current Ip < 1 MA, and toroidal field BT˜2 T. Cocurrent and counter-current directed neutral beam injection (NBI) systems [7] are operated with 2–4 MW beam power and injection energies of 50–80 keV to heat, fuel and rotate the plasma. Each NBI system consists of two beam sources, and the injection of one (tNBI) is more tangential than the other (nNBI). CXRS systems on EAST usually exploit the line (C VI, n = 8→7, 529.059 nm) from the charge exchange
⁎
recombination reaction between fully ionized carbon impurities (C6+) and energetic neutral beams [8,9]. The EAST CXRS system has been successfully recorded a charge exchange emission from Ne10+ ions recently [10], and Ne X (n = n = 11→10, 524.897 nm) lines was found to be a good candidate for the CXRS measurement on EAST tokamak. The paper is organized as follows. In Sec. II, a detailed description of the hardware of the ultrafast CXRS is given. These cover the diagnostic layout and critical optical components of the high throughput spectrometer. In Sec. III, performance evaluation and prediction of the system through simulation of spectra code are described. A summary of the paper and discussion of the future plans are presented in Sec. IV. 2. Ultrafast CXRS system on EAST The ultrafast CXRS system comprises a collection optics located in a port plug with a protective shutter in EAST tokamak, fiber bundles, and one high etendue spectrometer system with one high speed detector. The light emitted from the beam-impurity ions interaction zone is collected by the collection optics and then transferred via the fiber bundles towards the spectrometers behind the EAST hall for the spectral analysis, followed the data acquisition systems and analysis software.
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.fusengdes.2019.01.012 Received 8 October 2018; Received in revised form 6 December 2018; Accepted 3 January 2019 Available online 06 January 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 146 (2019) 522–525
Y. Li et al.
Fig. 1. Layout schematic of the ultrafast CXRS (UF-CXRS) and BES system. The light is separated by a beam splitter in spectrometer side.
The aims of the ultrafast CXRS are the measurement of ion temperature and rotation fluctuations at 1 MHz. In order to satisfy the requirement, the performances of all the optics elements are optimized for the required wavelength band of 527.5 ± 5 nm (for the simultaneous measurement of C VI and Ne X lines). Furthermore, the values of the numerical aperture (NA) of collection optics, the transmitted fibers, and the spectrometer are best match for maximized overall optical efficiency. 2.1. Planned diagnostic layout on EAST The planned ultrafast CXRS system shares the collection optics with the existing beam emission spectroscopy (BES) [11] situated at the horizontal P port on EAST. The BES aims at the radial and poloidal measurement of the plasma density fluctuation. The collection optics of BES has been upgraded for the simultaneous measurement of the ultrafast CXRS and BES systems. Fig. 1 shows the layout schematic of the upgraded collection optics. The system is carefully designed to measure 656–661 nm (BES spectra) and 527.5 ± 5 nm (for C VI and Ne X lines) simultaneously. The light from plasma is collected by collected optics, transmitted by fused silica fiber bundles of ˜60 m, and then separated by a beam splitter (BS) in the laboratory. The reflection light from beam splitter (shown in Fig. 1) is refocused on the entrance slit of the spectrometer. The radial and poloidal observation ranges cover the main plasma from R = 2.05–2.31 m and Z= ± 40 mm, respectively. The magnification of the system is ˜5, and the intersection of the optical sightline with the tangential neutral beam is aligned to a magnetic flux surface to achieve good spatial resolution in the radial and poloidal planes (ΔR˜ΔZ˜1–2 cm). Each channel of 2.3 × 2.3 mm is filled with 200/220 μm core/clad diameter fibers with fill factor of ˜0.63 and contains 105 fibers. The numerical aperture (NA) of the used fiber is 0.22, matching with the collection optics.
Fig. 2. Optical diagram of the high etendue spectrometer. (1) Entrance slit; (2) Collimating lens; (3) Grism (prism-grating-prism); (4) Focusing lens; (5) Image plane (Detector).
located at the end of collimating lens is to block the spare light from the horizontal direction. At the first phase, the fibers from the EAST are directly arranged in an array of three rows at the entrance slit, which is corresponding to 0.64 mm width and 210 fibers with 200/220 μm core/ clad diameter can be accommodated with 15.4 mm-long slit, which corresponds to two viewing chords. The nominal curvature radius of the fiber bundle is about 41 mm for the straight image on the detector plane. The collimating and focusing lenses are optimized for the desired spectral band. The clear apertures of the lenses are 90 and 112 mm, the corresponding fields of view (FOV) are 4 and 7.24 ° respectively. The transmission of the two lenses is about 95%. Overall on-axis modulation transfer function (MTF) produced by the combination of collimating and focusing lenses can up to 0.7 at spatial frequency of 150 cycles/ mm. One bandpass filter (90 mm clear diameter) is mounted between the collimated lens and the grism to ensure that undesirable input to the sensor from the stray light will be minimized. The filter bandwidth is 14 nm full width at half maximum (FWHM), and the transmission achieved is up to 95% in the 524–531 nm wavelength range. The half angle of the input light rays on the filter is about 3 °, and the influence on the filter transmission can be ignored for the relative wide FWHM of the filter [14]. The custom-made transmission grism from Wasatch Photonics was designed and optimized for the 524.5–530.5 nm wavelength range. The grating has an active area of ˜205 × 100 mm2 on a substrate of 273 × 110 mm2 and groove frequency of 4871 lines/mm. The grooves of the grating are parallel to short dimension. The diffraction efficiency of the grating is higher than 80% for unpolarized light. Considering the wide image plane (˜18.4 × 15.4 mm2) as well as purpose of assembly and alignment of the spectrometer, one existing Phantom V710 CMOS camera is used to record dispersed lines and placed at the exit position at the first step. The camera has a 1280 × 800 pixel sensor with 20 μm pixels, equaling to 25.6 × 16 mm2
2.2. Hardware of high-etendue spectrometer An optical schematic of the spectrometer is shown in Fig. 2 and the engineering design is shown in Fig. 3. The detailed designed parameters of the spectrometer are listed in Table 1. The spectrometer was constructed using high-throughput collection optics and a high-efficiency volume phase holographic transmission grating sandwiched by two identical prisms (BK7 glass, with anti-reflection coating for the designed wavelength band). The dual prisms and grating combination is called grism. The similar type of spectrometer with commercial camera lens has been well developed by Kaiser Optical Systems [12] and also at DIII-D tokamak [13]. The input light is collimated by a customized lens (focal length: 200 mm, F-number: 2.27, matched with the input fiber bundles), dispersed by the grism, and finally focused by another lens (focal length: 200 mm, F-number: 2.27) to the detector. The effective F-number in the dispersion direction of the collimating and focusing lens is 2.7 and determined by the limited grating size. One rectangular aperture 523
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Fig. 3. Engineering design of the spectrometer. (1) Fiber bundle and entrance slit. The other indications are in the agreement with the Fig. 2. Phantom V710 CMOS camera (5) is used for the spectrometer assembly, alignment, and performance tests. (6) Filter. Table 1 Main parameters of the spectrometer. Parameters
Value
Spectral band Focal length Slit width×Height Image dimension F-number Spectral resolution Dispersion Etendue No. of channels Monitored lines
527.5 ± 5 nm 200 mm 0.64 × 15.4 mm2 18.4 × 15.4 mm2 2.7(Horizontal)×2.27(Vertical) ˜[email protected] mm width slit ˜0.36 nm/mm ˜0.63 mm2sr for each channel 2 C VI(529.059 nm); Ne X(524.897 nm)
Fig. 4. Photon flux of C VI (n = 8→7, 529.059 nm) and Ne X (n = 11→10, 524.897 nm) charge exchange lines that reach the image plane of the spectrometer.
sensor area. It is allowed to handle 15.4 mm-long image simultaneously. A set of strategies are performed to reduce the stray light and increase the transmission of the system. Anti-reflective (AR) coating is applied to each surface of the lens and prisms of the grism. The two end faces of the fiber bundles will be dealt with RARe Motheye™ technology [15] provided by Fiberguide industries, in this way, the transmission of the fibers can be improved about 6% for the needed wavelength band. All the inner surfaces of the mechanical components of the system and the edge of lens are blackened for elimination of the stray light.
(CX) photon fluxes from C VI and Ne X lines that reach to the image plane of the spectrometer. The photon flux of active CX line is determined by Icx = Ncx ∙T∙ε , where Ncx is the corresponding CX photons emitted from plasma, T is the optical transmission efficiency, and the overall transmission for the whole system including the collection optics, fiber bundle, and the components of the spectrometer is about 0.4. This provides a guidance to determine the desired parameters of the high-speed detector [19]. Fig. 5 shows the predicted errors for the ion temperature from C VI
3. Performance assessment A Simulation of Spectra (SOS) code [16,17] is used to predict the ultrafast CXRS spectra collected by each viewing chord and optimize the instrument performance. Based on the code, the conceptual design of the EAST ultrafast CXRS diagnostic has been accomplished. The plasma parameters used for modeling simulation are as follows: Ti = Te = 2× (1-(r/a) 2) keV, ne = 4•1019× (1-(r/a) 2)0.5 /m3, here, r/a indicates the normalized minor radius. E (D°) = 25 keV/amu and 40 keV/amu, and the corresponding power fractions of the full, half, and third energy components of the beam are about 0.7:0.2:0.1 and 0.8:0.14:0.06 respectively [18]. Concentration of carbon and neon impurity is set as 1% and flat along the minor radius. In the simulation, only the tangential beam of co-current NBI is used. The passive signal emitted from the plasma edge is subtracted by beam modulation in the SOS code. The etendue is defined as ε = π∙S∙NA2 , with S is area of each spatial channel and π∙NA2 is solid angle of the spectrometer. For the designed spectrometer in the paper, ε = 0.63 mm2sr is listed in Table 1. Fig. 4 shows the radial profiles of the calculated charge exchange
Fig. 5. Predicted errors of the ion temperature from C VI and Ne X lines, as well as the intensity of C VI and Ne X lines at two beam energies, t = 1 μs, NBI modulated for background suppression in the simulation. 524
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detector’s parameters will be confirmed. Acknowledgments This work was supported by the National Magnetic Confinement Fusion Science Program of China (Nos. 2015GB101002, 2015GB101000, 2015GB103001 and 2017YFE0301300) and the National Natural Science Foundation of China (Nos. 11535013, 11575235). The authors would like to thank Dr. Manfred von Hellermann and Dr. Sandor Zoletnik for their help in the design of the ultrafast CXRS diagnostic on EAST. References [1] R.J. Groebner, et al., Role of edge electric field and poloidal rotation in the L-H transition, Phys. Rev. Lett. 64 (25) (1990) 3015–3018. [2] W. Horton, Drift waves and transport, Rev. Modern Phy. 71 (3) (1999) 735–778. [3] R.C. Isler, An overview of charge-exchange spectroscopy as a plasma diagnostic, Plasma Phys. Control. Fusion 36 (1994) 171–208. [4] R.J. Fonck, et al., Determination of plasma ion velocity distribution via chargeexchange recombination spectroscopy, Phys. Rev. A 29 (1984) 3288–3309. [5] B.N. Wan, et al., Progress of long pulse and H-mode experiments in EAST, Nucl. Fusion 53 (2013) 104006. [6] B.N. Wan, et al., Overview of EAST experiments on the development of high-performance steady-state scenario, Nucl. Fusion 57 (2017) 102019. [7] J. Wang, et al., Initial operation of the first neutral beamline on EAST, Fusion Eng. Des. 101 (2015) 56–60. [8] Y.Y. Li, et al., Development of the charge exchange recombination spectroscopy and beam emission spectroscopy on the EAST tokamak, Rev. Sci. Instrum. 85 (2014) 11E428. [9] M.Y. Ye, et al., Toroidal charge exchange recombination spectroscopy on EAST, Fusion Eng. Des. 96-97 (2015) 1017–1020. [10] Y.Y. Li, et al., Simultaneous measurement of CVI, NeX, and LiIII charge exchange lines on EAST, Rev. Sci. Instrum. 89 (2018) 10D119. [11] H.J. Wang, et al., Development of beam emission spectroscopy diagnostic on EAST, Rev. Sci. Instrum. 88 (2017) 083505. [12] R.E. Bell, Exploiting a transmission grating spectrometer, Rev. Sci. Instrum. 75 (2004) 4158. [13] I.U. Uzun-Kaymak, et al., Ultra-fast charge exchange spectroscopy for turbulent ion temperature fluctuation measurements on the DIII-D tokamak, Rev. Sci. Instrum. 83 (2012) 10D526. [14] See https://en.wikipedia.org/wiki/Interference_filter for information of the filter. [15] See https://www.fiberguide.com/product/rare-motheye-fiber/ for the information about the Rare Motheye Fiber. [16] M.G. von Hellermann, Active beam spectroscopy for ITER, nuclear instruments and methods in physics research section a: accelerators, spectrometers, Detectors Assoc. Equip. 623 (2) (2010) 720–725. [17] See http://fusionweb.phys.tue.nl/ sos for information about the Simulation of Spectra code (SOS). [18] Y. Wang, et al., Beam species evolution under long pulse operation for EAST-NBI by Doppler shift spectroscopy diagnostic system, J. Fusion Energy 34 (2015) 615–619. [19] D. Dunai, et al., Avalanche photodiode based detector for beam emission spectroscopy, Rev. Sci. Instrum. 81 (2010) 103503. [20] E.S. Marmar, Active Spectroscopic Diagnostics for ITER Utilizing Neutral Beams in : Diagnostics for Experimental Thermonuclear Fusion Reactors, Plenum Press, New York, 1996, pp. 281–290.
Fig. 6. Radial profiles of the signal-to-noise ratio (SNR) for C VI and Ne X lines. t = 1 μs, NBI modulated for background suppression in the simulation.
and Ne X, as well as the intensity of C VI and Ne X lines at two beam energies. It can be seen that the predicted errors from Ne X line are below 10%, less than the values obtained from C VI line. This indicates that Ne X line is more suitable for the ultrafast CXRS measurement. The increased fitting error of ion temperature and intensity of C VI and Ne X lines at r/a = 0-–0.2 and 0.8–1 comes from lower photon number, as shown in the Fig. 4. The calculated SNR value is shown in Fig. 6. The continuum noise level determines the signal to noise ratio (SNR), and can be calculated by [20] =
Icx ∙ D ∙ texp 2Icont + Icx
, where texp is the integration time, texp = 1 μs in
the modeling simulation, D is the dispersion for each spectral channel. Icont = Ncont ∙T∙ε indicate the photons of continuum signal that reach to the image plane of the spectrometer, Ncont is the corresponding photons from plasma. It can be seen that SNR is larger than 10, which is sufficient for the measurement of ultrafast CXRS system at 1 MHz bandwidth. 4. Conclusions and future work To investigate the ion temperature and rotation velocity fluctuations, the ultrafast CXRS system has been constructing on EAST tokamak. Fabrications of the mechanical part for the spectrometer as well as the design of the collection optics are underway. Assembling, alignment, and laboratory tests of the spectrometer will be made in the next few months of 2018. Tests of total transmission, the image curvature, and dispersion of spectrometer are performed at the first phase, one existing high speed Phantom V710 will be used for the wide image plane. A system will be designed for the stray light measurement of the spectrometer. Base on the test results, a curved entrance fiber bundle will be designed for the straight images of measured spectral lines and
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