- Email: [email protected]
Design of a Thomson scattering diagnostic system for VEST
Fusion Engineering and Design 96–97 (2015) 882–886
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design of a Thomson scattering diagnostic system for VEST Young-Gi Kim a , Jong Ha Lee b , Jeongwon Lee a , YoungHwa An a , Jeong Jeung Dang a , Jungmin Jo a , HyunYeong Lee a , Kyoung-Jae Chung a , Y.S. Hwang a , Yong-Su Na a,∗ a b
Department of Nuclear Engineering, Seoul National University, Seoul 151-744, Republic of Korea National Fusion Research Institute, Gwahangno 113, Daejeon 305-333, Republic of Korea
h i g h l i g h t s • • • • •
A Thomson scattering system for Versatile Experiment Spherical Torus is designed. The system is designed with care for the plasma with a low target electron density. APD of low dark current and enhanced sensitivity for near infrared has been chosen. A collecting optics system will provide a sufficient number of photoelectrons. A designed polychromator is able to measure the electron temperature of 10–1000 eV.
a r t i c l e
i n f o
Article history: Received 5 October 2014 Received in revised form 12 June 2015 Accepted 15 June 2015 Available online 7 July 2015 Keywords: Spherical torus Versatile Experiment Spherical Torus VEST Thomson scattering Electron temperature measurement
a b s t r a c t A Thomson scattering diagnostic system is designed for Versatile Experiment Spherical Torus (VEST) to measure the spatial profiles of the electron temperature. The system is carefully designed to collect a sufficient number of photoelectrons and to reduce the noise sources, since relatively low electron densities and temperatures are expected in VEST due to the limited power capacity at present. The target electron temperature and the density are 10–200 eV and 5 × 1018 m−3 , respectively which are extrapolated from the data of triple Langmuir probes measuring the edge plasma parameters at R = 0.75 m by assuming a parabolic density profile. The collecting optics is designed to have a wide-view angle and low cost by using a commercial photographic lens of low f-number and high transmittance optical fiber bundle. The bandwidths of the interference filters in the polychromator are designed for reliable measurements within the target electron temperature range. As a photo detector which is coupled with the filters, an avalanche photodiode (APD) with a low dark current and an adequate quantum efficiency near the laser wavelength is selected for the high signal-to-noise ratio. The number of photons transferred to the polychromator and the number of photoelectrons in the APD are calculated. At the commissioning phase, an oscilloscope with a high sampling rate will be adopted to check the necessity of the noise reduction by multi-shot signal accumulation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The Thomson scattering (TS) diagnostic system is one of the most important diagnostics to measure the spatial profile of both the electron temperature and density of fusion plasmas. It is widely used in many large to medium size tokamaks such as JET [1], KSTAR [2], MAST [3], and RFX [4] as well as small tokamaks such as TST-2 [5] and Pegasus Toroidal experiment [6]. Versatile Experiment Spherical Torus (VEST) is the first spherical torus in Korea, which has been operational since 2012 [7]. The first plasma experiment was successfully conducted with the maximum
∗ Corresponding author. E-mail address: [email protected] (Y.-S. Na). http://dx.doi.org/10.1016/j.fusengdes.2015.06.062 0920-3796/© 2015 Elsevier B.V. All rights reserved.
plasma current of 70 kA and the pulse duration of ∼15 ms [8]. Its major and minor radius are about 0.4 m and 0.3 m, respectively. To address the issue of relatively small space in the center stack restricting the use of central solenoids in STs, various experiments such as the non-inductive start-up and the current drive study are in progress in VEST. The measurement of the core electron temperature with the TS system in small devices such as VEST is quite challenging due to the low electron density and therefore the low signal level. The target electron density in VEST is estimated to be 5 × 1018 m−3 in the core region, R = 0.4 m. This value is extrapolated from the triple Langmuir probe data measured at R = 0.75 m by assuming a parabolic electron density profile. The target electron temperature range is estimated to be 10–200 eV in the similar way.
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Table 1 Design parameters of the TS system for VEST. Design parameter
Unit
Value
Laser wavelength Laser energy per pulse Number of laser photons per pulse Collecting f-number (at R = 0.4 m) Scattering length Major radius viewing range Number of spatial points
nm J
1064 1.5 8.0 × 1018 10.0 50 0.3–0.5 3
mm m
The design focus of the TS system in VEST is to overcome the expected low signal due to the low electron density. As the number of scattered photons by TS is proportional to both the electron density and the input laser power, the system is designed to minimize the loss of laser power and maximize the number of scattered photons to obtain a sufficient signal-to-noise ratio. The TS system of VEST consists of an injection optics system, a scattered light collecting optics system, 3 polychromators, and a data acquisition system (DAQ). The scattered photons from a plasma are collected and transferred to the polychromators by the collecting optics system, when a laser beam is injected into the plasma. Each channel of the polychromator selectively detects the scattered photons in the desired spectral bandwidth to analyze the Doppler broadening which is proportional to the electron temperature. Overall design parameters of the TS system are summarized in Table 1. The specification of the laser and the determination of the beam path are described in Section 2. The collecting optics including a lens, optical fiber bundles, and the expected number of photoelectrons are presented in Section 3. Section 4 depicts the design of polychromator consisted of low dark current photodetectors and interference filters. A short discussion about DAQ systems is covered in Section 5.
Fig. 1. Cross sectional view of the overall TS diagnostic system at the Z = 0 plane. Gray dashed lines indicate the major radius of VEST in 0.1 m interval.
Although the collecting solid angle in the vertical injection case is larger than that in the tangential injection case, the beam cannot pass through the plasma core region in the present design of the VEST ports on the top and the bottom plates.
2. Injection optics system
3. Collecting optics system
A commercial high power Nd:YAG laser is adopted as a light source of TS, which has the energy of 1.5 J per pulse, the pulse duration of 10 ns, the beam diameter of 10 mm, and the pulse repetition rate of 10 Hz. The fundamental wavelength of 1064 nm will be used to avoid loss during the frequency conversion and maximize the laser power. Since the duration of a typical discharge is 10–15 ms in VEST, only a single time point can be measured in each discharge. The system features including the laser beam path and the VEST chamber are illustrated in Fig. 1. The power loss of the laser outside the vacuum vessel is assumed to be negligible due to the short distance between the laser and the beam entrance window. It is noteworthy that the beam is focused by a convex lens with the focal length of 1500 mm in front of the beam entrance. The calculated focused beam waist is ∼0.2 mm at the measurement point of R = 0.4 m. The laser is injected on the mid-plane (Z = 0) after passing through a Brewster window to reduce the power loss and 4 baffles to reduce the noise from stray lights. The beam is aligned as close as possible to the center stack and polarized in the Z-direction (perpendicular to the plane of the sheet) as shown in Fig. 1, to cover the target major radius of R = 0.3–0.5 m, the core region, where the expected number of photoelectrons is relatively higher than the outer region. The beam path outside the vacuum chamber is designed by considering the restrictions from the laboratory room space and heating or diagnostic systems. A beam dump outside the vacuum chamber traps the laser which comes out through the beam exit. The exit window will be mounted on a 0.9 m-long-extension pipe to reduce the damage power. The tangential injection is preferred for the VEST TS system to accommodate the electron temperature profiles of the core region.
The collecting optics are designed with care to cover the major radius of R = 0.3–0.5 m. The scattered photons are collected by a lens, and they are transferred into polychromators by the optical fibers. The number of collected photons increases as the collecting solid angle increases. Since the solid angle of the collecting cone is inversely proportional to the second power of the f-number, the lens with a low f-number is required. A commercial photographic lens with a diameter of an entrance pupil of 71 mm, the focal length of 200 mm, and the f-number of F/2.8 is adopted as the collecting lens. The lens is located outside the port #2MMR (rectangular shaped port, 246 mm × 700 mm) at the position which allows both the maximum collecting solid angle and the maximum angle of view, covering all the target points. For the measurement point on R = 0.4 m, the effective f-number calculated on the object side is about F/10.0 (7.8 msr). The collected photons are focused into the optical fiber bundles by the lens with the effective f-number of F/3.9 with the magnification of 2/5×. Optical fiber bundles transfer the collected photons to the polychromators. An optical fiber has a numerical aperture of 0.2, a core diameter of 200 m, and a transmittance of >70% with 18 m length. About 100 optical fibers are packed in the rectangular case of 0.2 mm × 20 mm with a packing fraction of 70% at the collecting lens side. On the opposite side, they are bundled into a circular case of a diameter of 2.4 mm. The number of photoelectrons in the polychromator is estimated to evaluate the scattered light intensity transmitted by the collecting optics. The number of scattered photons from R = 0.4 m is calculated assuming that the photons from TS have the same wavelength of 1064 nm [9]. The total number of scattered photons
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Fig. 2. Schematic view of the filter polychromator designed by referring the KSTAR polychromator.
from the 1.5 J laser with the scattering length of 50 mm inside the plasma whose electron density of 5 × 1018 m−3 can be estimated as following, NTS
EL · ne · L · TS = 1.3362 × 108 , = hc/
(1)
where EL is the energy of the Nd:YAG laser, ne is the electron density, L is the scattering length along the beam path, and NTS is the number of scattered photons. The total cross-section of TS could be represented by TS = (8/3) × re 2 = 6.65 × 10−29 m−2 , where re is the classical electron radius [10]. The number of scattered photons in the collecting cone of the system is calculated by assuming the isotropic differential crosssection of TS, as the angle between the observing direction and the laser polarization is nearly 90◦ . From the relation between the solid angle and the f-number, the number of photons collected in the cone can be evaluated as follows, Ncoll. = NTS ·
˝ 1 = 8.2491 × 104 , = NTS · 2 4 16(F/#)
(2)
where Ncoll. , ˝, and F/# are the number of collected photons, the solid angle of the collecting cone, and the f-number on the object side, respectively. These photons will be transferred to the polychromator through the vacuum window, the collecting lens, and the optical fiber, in the order mentioned above. Therefore, the number of photons transferred to the polychromator, Npoly is obtained after multiplying their transmittance to Ncoll . NPoly = Ncoll. × (Twindow × Tlens × Tfiber × PF) = 2.1827 × 104 .
(3)
Here, Twindow , Tlens , and Tfiber are transmittance of the window, the lens, and the optical fiber, respectively. PF is the packing fraction of the optical fiber. Finally, in the polychromator, only the half spectrum of photons shifted toward short wavelengths is used. Using 70% as transmittance of the filters, and ∼40% as an avalanche photodiode quantum efficiency near 1064 nm, the total number of photoelectrons generated in APDs of the polychromator is 3056. The photoelectrons are separated to each APD according to the filter bandwidths. The estimation result considering the TS spectrum of various electron temperatures at R = 0.4 m is presented in Section 4. The total number of photoelectrons at R = 0.3 m (ne = 4 × 1018 m−3 , 8.3 msr) and R = 0.5 m (ne = 5 × 1018 m−3 , 7.0 msr) are 2616 and 2752, respectively, along the same lines. In the future, the collecting lens will be optimized; the diameter will be 100 mm, the focal length will be 200 mm, and the solid angle of the collecting cone will be twice of that of the present design. The higher solid angle will allow doubling the spatial resolution while maintaining the same number of collected photoelectrons.
4. Polychromator The electron temperature is deduced from the Doppler broadening of the Thomson-scattered lights. The polychromator with the interference filters is utilized for TS system, which has an advantage of the relatively high intensity but has the limited spectral resolution. The polychromator frame is designed by benchmarking that installed in KSTAR [12]. Fig. 2 presents its schematic view. Each channel of the polychromator detects the photons whose energy is within the bandwidth of each filter, and the electron temperature is deduced by comparing the signals. As one of the major components of the polychromator, photo detectors are selected for the reliable photon measurement by minimizing the sensor noise. The avalanche photodiode (APD), S11519 of Hamamatsu photonics, is chosen as the photo detector. It has a peak quantum efficiency of ∼65% at 960 nm falling to ∼40% at 1064 nm. The APD will be operated at a gain of 100 by setting a bias voltage up to ∼270 V. It typically has a low dark current of 9 nA, and 2 nA at the operation region of the bias voltage at the room temperature of 25 ◦ C. Since the APD is very sensitive to the operation temperature, the temperature compensation or detector cooling will be prepared. To overcome the noise from the dark current, more than ∼100 photoelectrons should be generated during the laser pulse duration. The output signal is amplified by the avalanche amplification in the APD and 100 times multiplied again by the high frequency amplifier circuit. An appropriate bandwidth of each interference filter of the polychromator is designed. Each polychromator is equipped with 5 channels including 4 channels for the electron temperature measurement and 1 channel for the calibration. The specification of interference filters are designed by a numerical code which calculates the number of photoelectrons transferred to each filter. Considering the scattered spectrum of the maximum target electron temperature, 960 nm is picked as the shortest wavelength of the filters. Channel 1 [1063 nm, 1065 nm] is designed for the absolute calibration using Rayleigh scattering. The bandwidths of the 4 filters are; Channel 2 [1058 nm, 1063 nm], channel 3 [1043 nm, 1058 nm], channel 4 [1010 nm, 1043 nm], and channel 5 [960 nm, 1010 nm]. The black solid lines in Fig. 3 indicate the product of the photo sensitivity of the APD [11] and the filter transmittance, namely the responsivity of each channel. The current signal of the APD is the integral of the incident photon power over the bandwidth weighted by the responsivity for each channel. The blue dashed lines are scattered spectra calculated by the Mattioli’s formulation [13]. While the transmittances of the filters are conservatively selected to be 70%, the blocking ratio outside the
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Fig. 3. Ideal spectral responsivity of each channel in the polychromator (black) and calculated scattered power (blue). Fig. 5. The expected APD signals normalized by the signals of channel 3.
bandwidth of each filter is 105 to block the stray light which can cause serious noise especially in the small ST device. The number of photoelectrons in each APD channel with consideration of the spectral response is estimated as a function of electron temperature at R = 0.4 m (ne = 5 × 1018 m−3 , 7.8 msr) as shown in Fig. 4. Channel 3 (red dashed line) always has more than 500 photoelectrons in the electron temperature range of 10–1000 eV. The number of photoelectrons transferred to channel 2 (black solid line) is also large where the electron temperature is lower than ∼100 eV. Therefore, the electron temperature between 10 and 200 eV can be deduced by the signals of channel 2 and channel 3. Even though the maximum target temperature is quite low, the polychromator is possible to measure the temperature up to 1000 eV by using the signals from channel 4 and channel 5 as shown in Fig. 5. 5. Data acquisition system While TS system for VEST is under preparation to set up and align the injection optics system, the type of data acquisition system has not been determined yet. An oscilloscope with a high sampling rate of up to 10 GSamples/s (TDS7104, Tektronix) is considered to be utilized as a test digitizer for APD signals to check the necessity of the noise reduction by multi-shot signal accumulation.
A permanent digitizer should be discreetly selected after verifying the TS system working within the budget considering the number of required channels between two types, a gated charge integrator and a fast waveform digitizer. The gated charge integrator type data acquisition (DAQ) system is being used in NSTX [14] and KSTAR [2]. Meanwhile, the fast waveform digitizer can be another option for DAQ system as in RFX-mod [4]. The latter has the advantage of signal analysis using known pulse shapes and of noise reduction by accumulation, though the cost is relatively higher than the gated charge integrator. 6. Summary The Thomson scattering diagnostic system has been designed to measure the spatial profiles of the core electron temperature in VEST. The design focus of this system is to overcome the expected low signal due to the low electron density expected in the core plasma of VEST. The system is composed of the injection optics system, the collecting optics system, and the polychromator. The injection optics system is designed to avoid the power loss of the laser beam at the vacuum window and the stray light inside the vacuum vessel. The beam path is determined to cover the target range. The collecting optics system is designed to collect the photons scattered from 3 points at R = 0.3–0.5 m with the interval of 0.1 m. The polychromator has been designed to be suitable for the measurement of the target electron temperature range by a careful design of the bandwidths of the interference filters and the selection of the low noise APD. The electron temperature up to 1 keV is expected to be measured by the designed TS system. DAQ systems will be determined between the gated charge integrator and the fast waveform digitizer. Acknowledgments
Fig. 4. The estimated number of photoelectrons generated in each channel for the electron temperature.
The authors would like to thank Prof. Michiaki Inomoto of the University of Tokyo for the valuable discussions. The authors acknowledge Mr. SeungKyu Kang for the technical advice. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2008-0061900). This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2014M1A7A1A03045368). This research was supported by
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the National Research Foundation of Korea (NRF) (No. NRF2012K2A2A6000505) References [1] L. Frassinetti, et al., Spatial resolution of the JET Thomson scattering system, Rev. Sci. Instrum. 83 (2012) 013506. [2] J.H. Lee, et al., Development of KSTAR Thomson scattering system, Rev. Sci. Instrum. 81 (2010) 10D528J. [3] R. Scannell, et al., Design of a new Nd:YAG Thomson scattering system for MAST, Rev. Sci. Instrum. 72 (2008) 10E730. [4] A. Alfier, R. Pasqualotto, New Thomson scattering diagnostic on RFX-mod, Rev. Sci. Instrum. 78 (2007) 013505. [5] T. Yamaguchi, et al., Development of a Thomson scattering system in the TST-2 Spherical Tokamak, Plamsa Fusion Res. 5 (2010) S2092. [6] D.J. Schlossberg, et al., A Thomson scattering diagnostic on the Pegasus Toroidal experiment, Rev. Sci. Instrum. 83 (2012) 10E335.
[7] K.J. Chung, et al., Initial plasma start-up using partial solenoid coils in Versatile Experiment Spherical Torus (VEST), Fusion Eng. Des. 88 (2013) 787. [8] Y. An, et al., Plasma start-up design and first plasma experiment in VEST, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.05.015 [9] L. Yang, et al., Design of Thomson scattering diagnostic system on EAST, Plasma Sci. Technol. 12 (2010) 284. [10] I.H. Hutchinson, Principles of Plasma Diagnostics, 2nd ed., Cambridge University Press, 2005, pp. 277. [11] Hamamatsu photonics “IR-enhanced Si APD” S11519 series datasheet, update: November 2013. [12] J. Lee, et al., Conceptual design of new polychromator on Thomson scattering system to measure Zeff, Rev. Sci. Instrum. 83 (2012) 10E334. [13] M. Mattioli, Incoherent light scattering from high temperature plasmas, EUR-CEA-FC-752, 1974. [14] D.W. Johnson, et al., APD detector electronics for the NSTX Thomson scattering system, Rev. Sci. Instrum. 72 (2001) 1129.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design of a Thomson scattering diagnostic system for VEST Young-Gi Kim a , Jong Ha Lee b , Jeongwon Lee a , YoungHwa An a , Jeong Jeung Dang a , Jungmin Jo a , HyunYeong Lee a , Kyoung-Jae Chung a , Y.S. Hwang a , Yong-Su Na a,∗ a b
Department of Nuclear Engineering, Seoul National University, Seoul 151-744, Republic of Korea National Fusion Research Institute, Gwahangno 113, Daejeon 305-333, Republic of Korea
h i g h l i g h t s • • • • •
A Thomson scattering system for Versatile Experiment Spherical Torus is designed. The system is designed with care for the plasma with a low target electron density. APD of low dark current and enhanced sensitivity for near infrared has been chosen. A collecting optics system will provide a sufficient number of photoelectrons. A designed polychromator is able to measure the electron temperature of 10–1000 eV.
a r t i c l e
i n f o
Article history: Received 5 October 2014 Received in revised form 12 June 2015 Accepted 15 June 2015 Available online 7 July 2015 Keywords: Spherical torus Versatile Experiment Spherical Torus VEST Thomson scattering Electron temperature measurement
a b s t r a c t A Thomson scattering diagnostic system is designed for Versatile Experiment Spherical Torus (VEST) to measure the spatial profiles of the electron temperature. The system is carefully designed to collect a sufficient number of photoelectrons and to reduce the noise sources, since relatively low electron densities and temperatures are expected in VEST due to the limited power capacity at present. The target electron temperature and the density are 10–200 eV and 5 × 1018 m−3 , respectively which are extrapolated from the data of triple Langmuir probes measuring the edge plasma parameters at R = 0.75 m by assuming a parabolic density profile. The collecting optics is designed to have a wide-view angle and low cost by using a commercial photographic lens of low f-number and high transmittance optical fiber bundle. The bandwidths of the interference filters in the polychromator are designed for reliable measurements within the target electron temperature range. As a photo detector which is coupled with the filters, an avalanche photodiode (APD) with a low dark current and an adequate quantum efficiency near the laser wavelength is selected for the high signal-to-noise ratio. The number of photons transferred to the polychromator and the number of photoelectrons in the APD are calculated. At the commissioning phase, an oscilloscope with a high sampling rate will be adopted to check the necessity of the noise reduction by multi-shot signal accumulation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The Thomson scattering (TS) diagnostic system is one of the most important diagnostics to measure the spatial profile of both the electron temperature and density of fusion plasmas. It is widely used in many large to medium size tokamaks such as JET [1], KSTAR [2], MAST [3], and RFX [4] as well as small tokamaks such as TST-2 [5] and Pegasus Toroidal experiment [6]. Versatile Experiment Spherical Torus (VEST) is the first spherical torus in Korea, which has been operational since 2012 [7]. The first plasma experiment was successfully conducted with the maximum
∗ Corresponding author. E-mail address: [email protected] (Y.-S. Na). http://dx.doi.org/10.1016/j.fusengdes.2015.06.062 0920-3796/© 2015 Elsevier B.V. All rights reserved.
plasma current of 70 kA and the pulse duration of ∼15 ms [8]. Its major and minor radius are about 0.4 m and 0.3 m, respectively. To address the issue of relatively small space in the center stack restricting the use of central solenoids in STs, various experiments such as the non-inductive start-up and the current drive study are in progress in VEST. The measurement of the core electron temperature with the TS system in small devices such as VEST is quite challenging due to the low electron density and therefore the low signal level. The target electron density in VEST is estimated to be 5 × 1018 m−3 in the core region, R = 0.4 m. This value is extrapolated from the triple Langmuir probe data measured at R = 0.75 m by assuming a parabolic electron density profile. The target electron temperature range is estimated to be 10–200 eV in the similar way.
Y.-G. Kim et al. / Fusion Engineering and Design 96–97 (2015) 882–886
883
Table 1 Design parameters of the TS system for VEST. Design parameter
Unit
Value
Laser wavelength Laser energy per pulse Number of laser photons per pulse Collecting f-number (at R = 0.4 m) Scattering length Major radius viewing range Number of spatial points
nm J
1064 1.5 8.0 × 1018 10.0 50 0.3–0.5 3
mm m
The design focus of the TS system in VEST is to overcome the expected low signal due to the low electron density. As the number of scattered photons by TS is proportional to both the electron density and the input laser power, the system is designed to minimize the loss of laser power and maximize the number of scattered photons to obtain a sufficient signal-to-noise ratio. The TS system of VEST consists of an injection optics system, a scattered light collecting optics system, 3 polychromators, and a data acquisition system (DAQ). The scattered photons from a plasma are collected and transferred to the polychromators by the collecting optics system, when a laser beam is injected into the plasma. Each channel of the polychromator selectively detects the scattered photons in the desired spectral bandwidth to analyze the Doppler broadening which is proportional to the electron temperature. Overall design parameters of the TS system are summarized in Table 1. The specification of the laser and the determination of the beam path are described in Section 2. The collecting optics including a lens, optical fiber bundles, and the expected number of photoelectrons are presented in Section 3. Section 4 depicts the design of polychromator consisted of low dark current photodetectors and interference filters. A short discussion about DAQ systems is covered in Section 5.
Fig. 1. Cross sectional view of the overall TS diagnostic system at the Z = 0 plane. Gray dashed lines indicate the major radius of VEST in 0.1 m interval.
Although the collecting solid angle in the vertical injection case is larger than that in the tangential injection case, the beam cannot pass through the plasma core region in the present design of the VEST ports on the top and the bottom plates.
2. Injection optics system
3. Collecting optics system
A commercial high power Nd:YAG laser is adopted as a light source of TS, which has the energy of 1.5 J per pulse, the pulse duration of 10 ns, the beam diameter of 10 mm, and the pulse repetition rate of 10 Hz. The fundamental wavelength of 1064 nm will be used to avoid loss during the frequency conversion and maximize the laser power. Since the duration of a typical discharge is 10–15 ms in VEST, only a single time point can be measured in each discharge. The system features including the laser beam path and the VEST chamber are illustrated in Fig. 1. The power loss of the laser outside the vacuum vessel is assumed to be negligible due to the short distance between the laser and the beam entrance window. It is noteworthy that the beam is focused by a convex lens with the focal length of 1500 mm in front of the beam entrance. The calculated focused beam waist is ∼0.2 mm at the measurement point of R = 0.4 m. The laser is injected on the mid-plane (Z = 0) after passing through a Brewster window to reduce the power loss and 4 baffles to reduce the noise from stray lights. The beam is aligned as close as possible to the center stack and polarized in the Z-direction (perpendicular to the plane of the sheet) as shown in Fig. 1, to cover the target major radius of R = 0.3–0.5 m, the core region, where the expected number of photoelectrons is relatively higher than the outer region. The beam path outside the vacuum chamber is designed by considering the restrictions from the laboratory room space and heating or diagnostic systems. A beam dump outside the vacuum chamber traps the laser which comes out through the beam exit. The exit window will be mounted on a 0.9 m-long-extension pipe to reduce the damage power. The tangential injection is preferred for the VEST TS system to accommodate the electron temperature profiles of the core region.
The collecting optics are designed with care to cover the major radius of R = 0.3–0.5 m. The scattered photons are collected by a lens, and they are transferred into polychromators by the optical fibers. The number of collected photons increases as the collecting solid angle increases. Since the solid angle of the collecting cone is inversely proportional to the second power of the f-number, the lens with a low f-number is required. A commercial photographic lens with a diameter of an entrance pupil of 71 mm, the focal length of 200 mm, and the f-number of F/2.8 is adopted as the collecting lens. The lens is located outside the port #2MMR (rectangular shaped port, 246 mm × 700 mm) at the position which allows both the maximum collecting solid angle and the maximum angle of view, covering all the target points. For the measurement point on R = 0.4 m, the effective f-number calculated on the object side is about F/10.0 (7.8 msr). The collected photons are focused into the optical fiber bundles by the lens with the effective f-number of F/3.9 with the magnification of 2/5×. Optical fiber bundles transfer the collected photons to the polychromators. An optical fiber has a numerical aperture of 0.2, a core diameter of 200 m, and a transmittance of >70% with 18 m length. About 100 optical fibers are packed in the rectangular case of 0.2 mm × 20 mm with a packing fraction of 70% at the collecting lens side. On the opposite side, they are bundled into a circular case of a diameter of 2.4 mm. The number of photoelectrons in the polychromator is estimated to evaluate the scattered light intensity transmitted by the collecting optics. The number of scattered photons from R = 0.4 m is calculated assuming that the photons from TS have the same wavelength of 1064 nm [9]. The total number of scattered photons
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Fig. 2. Schematic view of the filter polychromator designed by referring the KSTAR polychromator.
from the 1.5 J laser with the scattering length of 50 mm inside the plasma whose electron density of 5 × 1018 m−3 can be estimated as following, NTS
EL · ne · L · TS = 1.3362 × 108 , = hc/
(1)
where EL is the energy of the Nd:YAG laser, ne is the electron density, L is the scattering length along the beam path, and NTS is the number of scattered photons. The total cross-section of TS could be represented by TS = (8/3) × re 2 = 6.65 × 10−29 m−2 , where re is the classical electron radius [10]. The number of scattered photons in the collecting cone of the system is calculated by assuming the isotropic differential crosssection of TS, as the angle between the observing direction and the laser polarization is nearly 90◦ . From the relation between the solid angle and the f-number, the number of photons collected in the cone can be evaluated as follows, Ncoll. = NTS ·
˝ 1 = 8.2491 × 104 , = NTS · 2 4 16(F/#)
(2)
where Ncoll. , ˝, and F/# are the number of collected photons, the solid angle of the collecting cone, and the f-number on the object side, respectively. These photons will be transferred to the polychromator through the vacuum window, the collecting lens, and the optical fiber, in the order mentioned above. Therefore, the number of photons transferred to the polychromator, Npoly is obtained after multiplying their transmittance to Ncoll . NPoly = Ncoll. × (Twindow × Tlens × Tfiber × PF) = 2.1827 × 104 .
(3)
Here, Twindow , Tlens , and Tfiber are transmittance of the window, the lens, and the optical fiber, respectively. PF is the packing fraction of the optical fiber. Finally, in the polychromator, only the half spectrum of photons shifted toward short wavelengths is used. Using 70% as transmittance of the filters, and ∼40% as an avalanche photodiode quantum efficiency near 1064 nm, the total number of photoelectrons generated in APDs of the polychromator is 3056. The photoelectrons are separated to each APD according to the filter bandwidths. The estimation result considering the TS spectrum of various electron temperatures at R = 0.4 m is presented in Section 4. The total number of photoelectrons at R = 0.3 m (ne = 4 × 1018 m−3 , 8.3 msr) and R = 0.5 m (ne = 5 × 1018 m−3 , 7.0 msr) are 2616 and 2752, respectively, along the same lines. In the future, the collecting lens will be optimized; the diameter will be 100 mm, the focal length will be 200 mm, and the solid angle of the collecting cone will be twice of that of the present design. The higher solid angle will allow doubling the spatial resolution while maintaining the same number of collected photoelectrons.
4. Polychromator The electron temperature is deduced from the Doppler broadening of the Thomson-scattered lights. The polychromator with the interference filters is utilized for TS system, which has an advantage of the relatively high intensity but has the limited spectral resolution. The polychromator frame is designed by benchmarking that installed in KSTAR [12]. Fig. 2 presents its schematic view. Each channel of the polychromator detects the photons whose energy is within the bandwidth of each filter, and the electron temperature is deduced by comparing the signals. As one of the major components of the polychromator, photo detectors are selected for the reliable photon measurement by minimizing the sensor noise. The avalanche photodiode (APD), S11519 of Hamamatsu photonics, is chosen as the photo detector. It has a peak quantum efficiency of ∼65% at 960 nm falling to ∼40% at 1064 nm. The APD will be operated at a gain of 100 by setting a bias voltage up to ∼270 V. It typically has a low dark current of 9 nA, and 2 nA at the operation region of the bias voltage at the room temperature of 25 ◦ C. Since the APD is very sensitive to the operation temperature, the temperature compensation or detector cooling will be prepared. To overcome the noise from the dark current, more than ∼100 photoelectrons should be generated during the laser pulse duration. The output signal is amplified by the avalanche amplification in the APD and 100 times multiplied again by the high frequency amplifier circuit. An appropriate bandwidth of each interference filter of the polychromator is designed. Each polychromator is equipped with 5 channels including 4 channels for the electron temperature measurement and 1 channel for the calibration. The specification of interference filters are designed by a numerical code which calculates the number of photoelectrons transferred to each filter. Considering the scattered spectrum of the maximum target electron temperature, 960 nm is picked as the shortest wavelength of the filters. Channel 1 [1063 nm, 1065 nm] is designed for the absolute calibration using Rayleigh scattering. The bandwidths of the 4 filters are; Channel 2 [1058 nm, 1063 nm], channel 3 [1043 nm, 1058 nm], channel 4 [1010 nm, 1043 nm], and channel 5 [960 nm, 1010 nm]. The black solid lines in Fig. 3 indicate the product of the photo sensitivity of the APD [11] and the filter transmittance, namely the responsivity of each channel. The current signal of the APD is the integral of the incident photon power over the bandwidth weighted by the responsivity for each channel. The blue dashed lines are scattered spectra calculated by the Mattioli’s formulation [13]. While the transmittances of the filters are conservatively selected to be 70%, the blocking ratio outside the
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Fig. 3. Ideal spectral responsivity of each channel in the polychromator (black) and calculated scattered power (blue). Fig. 5. The expected APD signals normalized by the signals of channel 3.
bandwidth of each filter is 105 to block the stray light which can cause serious noise especially in the small ST device. The number of photoelectrons in each APD channel with consideration of the spectral response is estimated as a function of electron temperature at R = 0.4 m (ne = 5 × 1018 m−3 , 7.8 msr) as shown in Fig. 4. Channel 3 (red dashed line) always has more than 500 photoelectrons in the electron temperature range of 10–1000 eV. The number of photoelectrons transferred to channel 2 (black solid line) is also large where the electron temperature is lower than ∼100 eV. Therefore, the electron temperature between 10 and 200 eV can be deduced by the signals of channel 2 and channel 3. Even though the maximum target temperature is quite low, the polychromator is possible to measure the temperature up to 1000 eV by using the signals from channel 4 and channel 5 as shown in Fig. 5. 5. Data acquisition system While TS system for VEST is under preparation to set up and align the injection optics system, the type of data acquisition system has not been determined yet. An oscilloscope with a high sampling rate of up to 10 GSamples/s (TDS7104, Tektronix) is considered to be utilized as a test digitizer for APD signals to check the necessity of the noise reduction by multi-shot signal accumulation.
A permanent digitizer should be discreetly selected after verifying the TS system working within the budget considering the number of required channels between two types, a gated charge integrator and a fast waveform digitizer. The gated charge integrator type data acquisition (DAQ) system is being used in NSTX [14] and KSTAR [2]. Meanwhile, the fast waveform digitizer can be another option for DAQ system as in RFX-mod [4]. The latter has the advantage of signal analysis using known pulse shapes and of noise reduction by accumulation, though the cost is relatively higher than the gated charge integrator. 6. Summary The Thomson scattering diagnostic system has been designed to measure the spatial profiles of the core electron temperature in VEST. The design focus of this system is to overcome the expected low signal due to the low electron density expected in the core plasma of VEST. The system is composed of the injection optics system, the collecting optics system, and the polychromator. The injection optics system is designed to avoid the power loss of the laser beam at the vacuum window and the stray light inside the vacuum vessel. The beam path is determined to cover the target range. The collecting optics system is designed to collect the photons scattered from 3 points at R = 0.3–0.5 m with the interval of 0.1 m. The polychromator has been designed to be suitable for the measurement of the target electron temperature range by a careful design of the bandwidths of the interference filters and the selection of the low noise APD. The electron temperature up to 1 keV is expected to be measured by the designed TS system. DAQ systems will be determined between the gated charge integrator and the fast waveform digitizer. Acknowledgments
Fig. 4. The estimated number of photoelectrons generated in each channel for the electron temperature.
The authors would like to thank Prof. Michiaki Inomoto of the University of Tokyo for the valuable discussions. The authors acknowledge Mr. SeungKyu Kang for the technical advice. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2008-0061900). This research was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2014M1A7A1A03045368). This research was supported by
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the National Research Foundation of Korea (NRF) (No. NRF2012K2A2A6000505) References [1] L. Frassinetti, et al., Spatial resolution of the JET Thomson scattering system, Rev. Sci. Instrum. 83 (2012) 013506. [2] J.H. Lee, et al., Development of KSTAR Thomson scattering system, Rev. Sci. Instrum. 81 (2010) 10D528J. [3] R. Scannell, et al., Design of a new Nd:YAG Thomson scattering system for MAST, Rev. Sci. Instrum. 72 (2008) 10E730. [4] A. Alfier, R. Pasqualotto, New Thomson scattering diagnostic on RFX-mod, Rev. Sci. Instrum. 78 (2007) 013505. [5] T. Yamaguchi, et al., Development of a Thomson scattering system in the TST-2 Spherical Tokamak, Plamsa Fusion Res. 5 (2010) S2092. [6] D.J. Schlossberg, et al., A Thomson scattering diagnostic on the Pegasus Toroidal experiment, Rev. Sci. Instrum. 83 (2012) 10E335.
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