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Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST
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Fusion Engineering and Design xxx (2017) xxx–xxx
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Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST YooSung Kim a , Yue-Jiang Shi a , Kihyun Lee a , Soo-Ghee Oh b , Manfred von Hellermann c , Kyoung-Jae Chung a,∗ , Y.S. Hwang a a
Department of Nuclear Engineering, Seoul National University, Seoul 151-744, Republic of Korea Department of Physics, Ajou University, Suwon 442-749, Republic of Korea c Diagnostic Team, ITER Organization, Route Vinon sur Verdon, 13067 St Paul Lez Durance, France b
h i g h l i g h t s • A combined He CES and H BES system is designed to measure He impurity density in VEST. • Transmission gratings are employed to obtain high throughput and spectral resolution. • The estimated CES spectrum has an SNR of 10 with H beam of 40 kV and 0.4 A.
a r t i c l e
i n f o
Article history: Received 4 October 2016 Received in revised form 2 March 2017 Accepted 3 March 2017 Available online xxx Keywords: Beam emission spectroscopy Charge exchange spectroscopy Helium ash Rotation Ion temperature
a b s t r a c t A 468.6-nm HeII charge exchange spectroscopy (CES) system combined with 656.3-nm H beam emission spectroscopy (BES) system is designed to measure helium impurity density as well as toroidal rotation and ion temperature in Versatile Experiment Spherical Torus (VEST). A dichroic beam splitter is used to measure CES and BES spectra simultaneously with sharing the same observation and transmission components which include the line of sight, lenses, and fiber bundles. In the combined system, we employ the transmission gratings with high diffraction efficiency for CES and BES in order to obtain high throughput and spectral resolution. Spectra simulation results show that the signal to noise ratio (SNR) of this diagnostic system is no less than 10 with the injection of a diagnostic neutral hydrogen (beam energy and current are 40 keV and 0.4 A, respectively). © 2017 Elsevier B.V. All rights reserved.
1. Introduction Removal of helium ash as a product of D-T fusion reaction is a critical issue for the realization of ITER and fusion reactor. In order to understand behaviors of helium ash, precise diagnostics of its spatial distribution is of a great importance in burning plasmas like ITER. Until now, it has been noticed that the charge exchange spectroscopy (CES) is the only direct method to measure the helium ash density in the core for ITER [1]. However, it is also recognized that the Helium CES is very challenging due to the presence of new effects such as plume which need not to be considered for the CES with conventionally used impurities of N, O, and C in current magnetic fusion devices [2,3]. Technically, ITER HeII CES system is designed to share the most of optical components with the beam
emission spectroscopy (BES) in order to minimize the uncertainty of neutral beam density along the beam path and avoid errors coming from dis-alignment of optical system by any means [2,4–6]. The above-mentioned importance and uniqueness of the helium ash diagnostics adopted in ITER motivate the research activity in Versatile Experiment Spherical Torus (VEST) at Seoul National University [7]. It is well aligned with recent research direction of VEST toward pursuing advanced operational scenario adaptable to fusion reactor. The VEST device is a university-scale spherical torus with main parameters of R0 = 0.4 m, R0 /a > 1.3, and BT = 0.1 T on axis [7]. In this paper, we present the detailed design of a 468.6-nm HeII CES system combined with 656.3-nm H BES system to measure helium impurity density as well as toroidal rotation and ion temperature in VEST. The expected spectra analyzed with a spectrum analysis code for the designed CES and BES systems are also described.
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (K.-J. Chung). http://dx.doi.org/10.1016/j.fusengdes.2017.03.006 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006
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2. Basic principle of combined CES and BES system Ion temperature and rotation can be obtained from the charge exchange spectral shape as following relations, shift =
v c
FWHM =
0 cos,
(1)
8Ti ln (2) 0 , Mi c 2
(2)
where v is the ion flow velocity 0 is the wavelength at rest, is the angle between the flow direction and line of sight, Ti is the ion temperature, and Mi is the ion mass. HeII line (n = 4–3, 468.6 nm) and main ion H␣ (n = 3–2, 656.3 nm) line are primary candidates for CES in VEST. Helium gas will be puffed to maintain the helium concentration about 1–10%. The relation between the intensity of charge exchange signal and the impurity density is as following [4]: ICES =
1 n 2+ QCES 4 He
nb ds,
(3)
where nHe2+ is the local helium impurity density, QCES is the effective atomic emission rate of CES, and nb is the neutral beam density at the observation point and the line of sight integration across the beam path. The BES signal is mainly generated by collisional excitation between neutral beam and electrons. The BES intensity can be represented by IBES =
1 ne QBES 4
nb ds,
(4)
where QBES is the effective atomic emission rate of BES. As seen in to the neutral Eqs. (3) and (4), those two signals are proportional beam density. By combining them, the terms nb ds can be canceled out and the impurity density is obtained with relative intensity ratio between the CES and BES signals as following: nHe2+ QBES ICES = . ne QCES IBES
is not so sensitive to dis-alignment events such as disruption and robust in relative intensity calibration if CES and BES share the same light collecting system which includes line of sight, lens at machine port and optical fibers.
(5)
In this manner, the combined system can eliminate the devinb ds with a beam stopping model and ation in calculating uncertainties in absolute calibration. Also, this line ratio method
3. Design of combined CES and BES system The whole spectroscopic system is composed of collection optics, optical fibers, and spectrometer. This system is carefully designed to measure 656.3 nm (BES) and 468.6 nm (for helium CES) simultaneously for good temporal resolution and wide spatial range. Fig. 1 shows the schematic diagram of the system. A set of collection optics with toroidal view is installed in the equatorial port. A commercial photographic lens (f/1.8 50 mm) is used to image the light onto the fiber bundles. The total number of fibers (f/2.2) is 140 with a configuration of 4 rows (in vertical direction) × 35 columns (in horizontal direction). The maximum spatial observation points in radial direction are 35. Two rows of fibers are binned for the measurement of one spatial channel. More fibers (up to 4) can be binned to one spatial channel if the light intensity is weak. The observation range covers the main plasma of VEST from the R = 0.45–0.65 m. The diameters of core and cladding of the optical fiber are 400 m and 440 m, respectively. The magnification is 12 and the image diameter is about 6 mm. The spatial resolution is about 0.5–2 cm from the edge to the core as shown in Fig. 1(b). H␣ and HeII lines share the same line of sights and light collecting components. In spectrometer side, the transmitted lights are separated in wavelength and measured by different detectors. Two f/1.8 lenses with focal length of 50 mm are used for collimation and image focusing. A dichroic mirror (Thorlabs, DMLP550L), which transmits H␣ line (efficiency 98%) and reflects HeII line (efficiency 97.5%) in 90◦ direction, can separate the incoming lights for H␣ and HeII spectrometers. Both H␣ and HeII spectrometers employ transmission gratings due to the high efficiency (∼70%) and high throughput (f/1.8). It is well matched to optical fiber (f/2.2) without any etendue matching optics. HDG (Kaiser optical systems, center wavelength 468.6 nm) and VPG (Kaiser optical system, center wavelength 656.3 nm) are selected for gratings. The detailed design of transmission grating system is described elsewhere [8]. Disper-
Fig. 1. (a) The schematic view of CES and BES system. The collection parts are shared with CES and BES. The light is separated by a dichroic splitter in spectrometer side. (b) The spatial resolution along the major radius.
Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006
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Fig. 2. (a) Signal to noise ratio of HeII charge exchange signal as a function of diagnostic neutral beam energy and current at R = 0.64 m. (b) Simulated HeII spectrum with beam energy of 40 keV and beam current of 0.4 A. Electron impact excitation (green) is a dominant passive component compared to passive charge exchange (cyan). These passive components can be subtracted with the beam modulation or multi-spectral fitting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
sions of the spectrometers are 0.89 nm/mm and 2.26 nm/mm for HeII and H␣, respectively. An electron multiplying charge coupled device (EMCCD) is used to record HeII lines and a scientific complementary metal-oxide semiconductor (sCMOS) is used to measure H␣ lines. It is allowed to handle 58 fibers simultaneously with a 100 m double-slit structure [8]. The time resolution is 3.5 ms with binning option and could be improved with reduced channels. 4. Analysis of estimated spectrum The expected spectrum is estimated with Simulation of Spectra (SOS) code [1]. This code is a forward modeling simulation code to predict the active spectra and passive spectra for charge exchange spectroscopy, beam emission spectroscopy, and fast ion spectroscopy. The input parameters are plasma properties, beam properties, geometry, and spectrometer properties. It is assumed that the plasma parameter profiles are shaped as: ne , Te , Ti ∝ (1 − (r/a)2 )␣ in certain reasonable ranges. The rotation is considered to be zero. The input parameters for SOS simulation are summarized in Table 1. The signal to noise ratio (SNR) is closely related to the measurement error in the system. When a beam modulation is assumed to be perfect, noises from photon statistical fluctuation is dominant. Then, the SNR value is calculated from the following equation [1], SNR =
Iactive tint A˝TD
,
(6)
2Ipassive + Iactive tint A˝TD
where Iactive and Ipassive are the intensities of charge exchange signal and passive signal in m−2 s−1 sr−1 nm−1 , tint is the integration time in s, A˝ is the etendue of the system, is the quantum efficiency, T is the transmission efficiency, and D is the dispersion in nm/pixel. In order to have a spectrum within an acceptance level, the SNR larger than 10 is required. Table 1 The input parameters for the Simulation of Spectra (SOS) code. Parameters
Value
Specification
Ti (0) = Te (0) ne (0) nHe2+ /ne rotation fbeam (E,E/2,E/3)
0.2 keV 1019 m−3 10% 0 km/s 3:4:3
˛ = 0.5 ˛ = 0.2 uniform
The SNR of helium CES is calculated as a function of the diagnostic neutral beam parameters as shown in Fig. 2. Fig. 2(a) shows that the SNR gradually increases as the beam energy increases up to 50 keV. For hydrogen beam, the effective charge exchange reaction rate is peaked near the beam energy of 30–40 keV [9]. By considering the beam fraction of fbeam (E, E/2, E/3) = 3:4:3, the signal intensities increase gradually even when the beam energy is higher than 40 keV. The SNR increases with the beam current because the active signal depends on the number of injected beam particles. To attain a sufficient high SNR above 10, the beam energy and current should exceed 40 keV and 0.4 A, respectively. The CES spectrum calculated for the hydrogen beam with 40 keV and 0.4 A is shown in Fig. 2(b). The passive signals including passive charge exchange and electron excitation have a narrow spectral width due to the low ion temperature of edge plasma. The dominant contribution for passive emission in VEST is electron impact excitation. In experiments, the passive signal will be subtracted to give an active signal only using techniques of beam modulation and multiple peak analysis. An example of BES spectrum is shown in Fig. 3(b). The background H␣ is the most dominant emission which is about 104 times stronger than the BES signal. This intense signal can saturate the detector and induce blooming. A block should be installed in front of the detector to reject the intense passive component. The BES spectra are red-shifted. The spectra from the three energy components of (E, E/2, E/3) are shown in Fig. 3(b). These BES spectra can be distinguished by Doppler shift from the bulk emission. The spectra of half energy and third energy components are partially overlapped. Based on the shift from the full energy component, the spectra will be fitted to identify the half and third energy components. Fig. 3(a) shows SNR of the BES signal for the full energy component. It is sufficient to measure the spectrum in beam parameters range in consideration because the bremsstrahlung radiation which is a main contributor of passive signal in BES is very weak compared to the BES signal. Other energy components result are also similar. Total beam emission intensities with the consideration for each energy component are applied to measure the helium impurity density, together with the He CES signal. In addition, the detailed shape of each BES can give the safety factor from the motional Stark effect (MSE). In spherical torus with low BT , it is difficult to use the polarimetric MSE due to the small wavelength splitting. Instead, spectral MSE using and intensity ratio [10] is considered in future.
beam fraction
Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006
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Fig. 3. (a) Signal to noise ratio of beam emission signal as a function of diagnostic neutral beam energy and current at R = 0.64 m. (b) Simulated spectrum H␣ spectrum with beam energy of 40 keV and beam current of 0.4 A. Intense passive component will be blocked. Doppler shifted beam emission spectra are shown in green line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions In this paper, we present the design and analysis of the He CES system combined with H BES system for measuring helium impurity density as well as toroidal rotation and ion temperature in VEST. We find that the sufficient SNR can be obtained if the energy and current of hydrogen neutral beam are larger than 40 keV and 0.4 A, respectively. We expect that the upcoming experiments with the combined CES and BES system in VEST will provide useful experience to verify such diagnostic concept. Acknowledgments This research was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2014M1A7A1A03045374) and by the Ministry of Science, ICT and Future Planning of the Republic of Korea under the Korean ITER project contract. References
[2] A. Kappatou, Investigations of Helium Transport in ASDEX Upgrade Plasmas with Charge Exchange Recombination Spectroscopy, Eindhoven University of Technology, 2014, Ph.D thesis. [3] R.J. Fonck, D.S. Darrow, K.P. Jaehnig, Determination of plasma-ion velocity distribution via charge-exchange recombination spectroscopy, Phys. Rev. A 29 (1984) 3288–3309. [4] M.G. von Hellermann, E. Delabie, R.J.E. Jaspers, W. Biel, O. Marchuk, H.P. Summers, et al., Active beam spectroscopy, AIP Conf. Proc. 988 (2008) 165–176. [5] K. Ida, M. Yoshinuma, B. Wieland, M. Goto, Y. Nakamura, M. Kobayashi, et al., Measurement of radial profiles of density ratio of helium to hydrogen ion using charge exchange spectroscopy with two-wavelength spectrometer, Rev. Sci. Instrum. 86 (2015) 123514. [6] L.A. Klyuchnikov, et al., Charge exchange recombination spectroscopy on the T-10 tokamak, Rev. Sci. Instrum. 87 (2016) 053506. [7] K.J. Chung, Y.H. An, B.K. Jung, H.Y. Lee, C. Sung, Y.S. Na, et al., Design features and commissioning of the Versatile Experiment Spherical torus (VEST) at Seoul National University, Plasma Sci. Technol. 15 (2013) 244–251. [8] R.E. Bell, Exploiting a transmission grating spectrometer, Rev. Sci. Instrum. 75 (2004) 4158–4161. [9] M. von Hellermann, P. Breger, J. Frieling, R. Konig, W. Mandl, A. Maas, et al., Analytical approximation of cross-section effects on charge exchange spectra observed in hot fusion plasmas, Plasma Phys. Controlled Fusion 37 (1995) 71–94. [10] O. Marchuk, Yu. Ralchenko, D.R. Schultz, Non-statistical population distributions for hydrogen beams in fusion plasmas, Plasma Phys. Controlled Fusion 54 (2012) 095010.
[1] 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.
Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006
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Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST YooSung Kim a , Yue-Jiang Shi a , Kihyun Lee a , Soo-Ghee Oh b , Manfred von Hellermann c , Kyoung-Jae Chung a,∗ , Y.S. Hwang a a
Department of Nuclear Engineering, Seoul National University, Seoul 151-744, Republic of Korea Department of Physics, Ajou University, Suwon 442-749, Republic of Korea c Diagnostic Team, ITER Organization, Route Vinon sur Verdon, 13067 St Paul Lez Durance, France b
h i g h l i g h t s • A combined He CES and H BES system is designed to measure He impurity density in VEST. • Transmission gratings are employed to obtain high throughput and spectral resolution. • The estimated CES spectrum has an SNR of 10 with H beam of 40 kV and 0.4 A.
a r t i c l e
i n f o
Article history: Received 4 October 2016 Received in revised form 2 March 2017 Accepted 3 March 2017 Available online xxx Keywords: Beam emission spectroscopy Charge exchange spectroscopy Helium ash Rotation Ion temperature
a b s t r a c t A 468.6-nm HeII charge exchange spectroscopy (CES) system combined with 656.3-nm H beam emission spectroscopy (BES) system is designed to measure helium impurity density as well as toroidal rotation and ion temperature in Versatile Experiment Spherical Torus (VEST). A dichroic beam splitter is used to measure CES and BES spectra simultaneously with sharing the same observation and transmission components which include the line of sight, lenses, and fiber bundles. In the combined system, we employ the transmission gratings with high diffraction efficiency for CES and BES in order to obtain high throughput and spectral resolution. Spectra simulation results show that the signal to noise ratio (SNR) of this diagnostic system is no less than 10 with the injection of a diagnostic neutral hydrogen (beam energy and current are 40 keV and 0.4 A, respectively). © 2017 Elsevier B.V. All rights reserved.
1. Introduction Removal of helium ash as a product of D-T fusion reaction is a critical issue for the realization of ITER and fusion reactor. In order to understand behaviors of helium ash, precise diagnostics of its spatial distribution is of a great importance in burning plasmas like ITER. Until now, it has been noticed that the charge exchange spectroscopy (CES) is the only direct method to measure the helium ash density in the core for ITER [1]. However, it is also recognized that the Helium CES is very challenging due to the presence of new effects such as plume which need not to be considered for the CES with conventionally used impurities of N, O, and C in current magnetic fusion devices [2,3]. Technically, ITER HeII CES system is designed to share the most of optical components with the beam
emission spectroscopy (BES) in order to minimize the uncertainty of neutral beam density along the beam path and avoid errors coming from dis-alignment of optical system by any means [2,4–6]. The above-mentioned importance and uniqueness of the helium ash diagnostics adopted in ITER motivate the research activity in Versatile Experiment Spherical Torus (VEST) at Seoul National University [7]. It is well aligned with recent research direction of VEST toward pursuing advanced operational scenario adaptable to fusion reactor. The VEST device is a university-scale spherical torus with main parameters of R0 = 0.4 m, R0 /a > 1.3, and BT = 0.1 T on axis [7]. In this paper, we present the detailed design of a 468.6-nm HeII CES system combined with 656.3-nm H BES system to measure helium impurity density as well as toroidal rotation and ion temperature in VEST. The expected spectra analyzed with a spectrum analysis code for the designed CES and BES systems are also described.
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (K.-J. Chung). http://dx.doi.org/10.1016/j.fusengdes.2017.03.006 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006
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2. Basic principle of combined CES and BES system Ion temperature and rotation can be obtained from the charge exchange spectral shape as following relations, shift =
v c
FWHM =
0 cos,
(1)
8Ti ln (2) 0 , Mi c 2
(2)
where v is the ion flow velocity 0 is the wavelength at rest, is the angle between the flow direction and line of sight, Ti is the ion temperature, and Mi is the ion mass. HeII line (n = 4–3, 468.6 nm) and main ion H␣ (n = 3–2, 656.3 nm) line are primary candidates for CES in VEST. Helium gas will be puffed to maintain the helium concentration about 1–10%. The relation between the intensity of charge exchange signal and the impurity density is as following [4]: ICES =
1 n 2+ QCES 4 He
nb ds,
(3)
where nHe2+ is the local helium impurity density, QCES is the effective atomic emission rate of CES, and nb is the neutral beam density at the observation point and the line of sight integration across the beam path. The BES signal is mainly generated by collisional excitation between neutral beam and electrons. The BES intensity can be represented by IBES =
1 ne QBES 4
nb ds,
(4)
where QBES is the effective atomic emission rate of BES. As seen in to the neutral Eqs. (3) and (4), those two signals are proportional beam density. By combining them, the terms nb ds can be canceled out and the impurity density is obtained with relative intensity ratio between the CES and BES signals as following: nHe2+ QBES ICES = . ne QCES IBES
is not so sensitive to dis-alignment events such as disruption and robust in relative intensity calibration if CES and BES share the same light collecting system which includes line of sight, lens at machine port and optical fibers.
(5)
In this manner, the combined system can eliminate the devinb ds with a beam stopping model and ation in calculating uncertainties in absolute calibration. Also, this line ratio method
3. Design of combined CES and BES system The whole spectroscopic system is composed of collection optics, optical fibers, and spectrometer. This system is carefully designed to measure 656.3 nm (BES) and 468.6 nm (for helium CES) simultaneously for good temporal resolution and wide spatial range. Fig. 1 shows the schematic diagram of the system. A set of collection optics with toroidal view is installed in the equatorial port. A commercial photographic lens (f/1.8 50 mm) is used to image the light onto the fiber bundles. The total number of fibers (f/2.2) is 140 with a configuration of 4 rows (in vertical direction) × 35 columns (in horizontal direction). The maximum spatial observation points in radial direction are 35. Two rows of fibers are binned for the measurement of one spatial channel. More fibers (up to 4) can be binned to one spatial channel if the light intensity is weak. The observation range covers the main plasma of VEST from the R = 0.45–0.65 m. The diameters of core and cladding of the optical fiber are 400 m and 440 m, respectively. The magnification is 12 and the image diameter is about 6 mm. The spatial resolution is about 0.5–2 cm from the edge to the core as shown in Fig. 1(b). H␣ and HeII lines share the same line of sights and light collecting components. In spectrometer side, the transmitted lights are separated in wavelength and measured by different detectors. Two f/1.8 lenses with focal length of 50 mm are used for collimation and image focusing. A dichroic mirror (Thorlabs, DMLP550L), which transmits H␣ line (efficiency 98%) and reflects HeII line (efficiency 97.5%) in 90◦ direction, can separate the incoming lights for H␣ and HeII spectrometers. Both H␣ and HeII spectrometers employ transmission gratings due to the high efficiency (∼70%) and high throughput (f/1.8). It is well matched to optical fiber (f/2.2) without any etendue matching optics. HDG (Kaiser optical systems, center wavelength 468.6 nm) and VPG (Kaiser optical system, center wavelength 656.3 nm) are selected for gratings. The detailed design of transmission grating system is described elsewhere [8]. Disper-
Fig. 1. (a) The schematic view of CES and BES system. The collection parts are shared with CES and BES. The light is separated by a dichroic splitter in spectrometer side. (b) The spatial resolution along the major radius.
Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006
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Fig. 2. (a) Signal to noise ratio of HeII charge exchange signal as a function of diagnostic neutral beam energy and current at R = 0.64 m. (b) Simulated HeII spectrum with beam energy of 40 keV and beam current of 0.4 A. Electron impact excitation (green) is a dominant passive component compared to passive charge exchange (cyan). These passive components can be subtracted with the beam modulation or multi-spectral fitting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
sions of the spectrometers are 0.89 nm/mm and 2.26 nm/mm for HeII and H␣, respectively. An electron multiplying charge coupled device (EMCCD) is used to record HeII lines and a scientific complementary metal-oxide semiconductor (sCMOS) is used to measure H␣ lines. It is allowed to handle 58 fibers simultaneously with a 100 m double-slit structure [8]. The time resolution is 3.5 ms with binning option and could be improved with reduced channels. 4. Analysis of estimated spectrum The expected spectrum is estimated with Simulation of Spectra (SOS) code [1]. This code is a forward modeling simulation code to predict the active spectra and passive spectra for charge exchange spectroscopy, beam emission spectroscopy, and fast ion spectroscopy. The input parameters are plasma properties, beam properties, geometry, and spectrometer properties. It is assumed that the plasma parameter profiles are shaped as: ne , Te , Ti ∝ (1 − (r/a)2 )␣ in certain reasonable ranges. The rotation is considered to be zero. The input parameters for SOS simulation are summarized in Table 1. The signal to noise ratio (SNR) is closely related to the measurement error in the system. When a beam modulation is assumed to be perfect, noises from photon statistical fluctuation is dominant. Then, the SNR value is calculated from the following equation [1], SNR =
Iactive tint A˝TD
,
(6)
2Ipassive + Iactive tint A˝TD
where Iactive and Ipassive are the intensities of charge exchange signal and passive signal in m−2 s−1 sr−1 nm−1 , tint is the integration time in s, A˝ is the etendue of the system, is the quantum efficiency, T is the transmission efficiency, and D is the dispersion in nm/pixel. In order to have a spectrum within an acceptance level, the SNR larger than 10 is required. Table 1 The input parameters for the Simulation of Spectra (SOS) code. Parameters
Value
Specification
Ti (0) = Te (0) ne (0) nHe2+ /ne rotation fbeam (E,E/2,E/3)
0.2 keV 1019 m−3 10% 0 km/s 3:4:3
˛ = 0.5 ˛ = 0.2 uniform
The SNR of helium CES is calculated as a function of the diagnostic neutral beam parameters as shown in Fig. 2. Fig. 2(a) shows that the SNR gradually increases as the beam energy increases up to 50 keV. For hydrogen beam, the effective charge exchange reaction rate is peaked near the beam energy of 30–40 keV [9]. By considering the beam fraction of fbeam (E, E/2, E/3) = 3:4:3, the signal intensities increase gradually even when the beam energy is higher than 40 keV. The SNR increases with the beam current because the active signal depends on the number of injected beam particles. To attain a sufficient high SNR above 10, the beam energy and current should exceed 40 keV and 0.4 A, respectively. The CES spectrum calculated for the hydrogen beam with 40 keV and 0.4 A is shown in Fig. 2(b). The passive signals including passive charge exchange and electron excitation have a narrow spectral width due to the low ion temperature of edge plasma. The dominant contribution for passive emission in VEST is electron impact excitation. In experiments, the passive signal will be subtracted to give an active signal only using techniques of beam modulation and multiple peak analysis. An example of BES spectrum is shown in Fig. 3(b). The background H␣ is the most dominant emission which is about 104 times stronger than the BES signal. This intense signal can saturate the detector and induce blooming. A block should be installed in front of the detector to reject the intense passive component. The BES spectra are red-shifted. The spectra from the three energy components of (E, E/2, E/3) are shown in Fig. 3(b). These BES spectra can be distinguished by Doppler shift from the bulk emission. The spectra of half energy and third energy components are partially overlapped. Based on the shift from the full energy component, the spectra will be fitted to identify the half and third energy components. Fig. 3(a) shows SNR of the BES signal for the full energy component. It is sufficient to measure the spectrum in beam parameters range in consideration because the bremsstrahlung radiation which is a main contributor of passive signal in BES is very weak compared to the BES signal. Other energy components result are also similar. Total beam emission intensities with the consideration for each energy component are applied to measure the helium impurity density, together with the He CES signal. In addition, the detailed shape of each BES can give the safety factor from the motional Stark effect (MSE). In spherical torus with low BT , it is difficult to use the polarimetric MSE due to the small wavelength splitting. Instead, spectral MSE using and intensity ratio [10] is considered in future.
beam fraction
Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006
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Fig. 3. (a) Signal to noise ratio of beam emission signal as a function of diagnostic neutral beam energy and current at R = 0.64 m. (b) Simulated spectrum H␣ spectrum with beam energy of 40 keV and beam current of 0.4 A. Intense passive component will be blocked. Doppler shifted beam emission spectra are shown in green line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions In this paper, we present the design and analysis of the He CES system combined with H BES system for measuring helium impurity density as well as toroidal rotation and ion temperature in VEST. We find that the sufficient SNR can be obtained if the energy and current of hydrogen neutral beam are larger than 40 keV and 0.4 A, respectively. We expect that the upcoming experiments with the combined CES and BES system in VEST will provide useful experience to verify such diagnostic concept. Acknowledgments This research was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2014M1A7A1A03045374) and by the Ministry of Science, ICT and Future Planning of the Republic of Korea under the Korean ITER project contract. References
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Please cite this article in press as: Y. Kim, et al., Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.006