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Optimization of optical dumps for H-alpha spectroscopy in ITER
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Optimization of optical dumps for H-alpha spectroscopy in ITER E.N. Andreenko a,b,∗ , A.G. Alekseev a,b , A.B. Kukushkin a,c , V.S. Neverov a , S.W. Lisgo d , A.A. Morozov a,b a
NRC “Kurchatov Institute”, Moscow, Russia International Fusion Projects Coordinating Centre, Moscow, Russia NRNU “MEPhI”, Moscow, Russia d ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, St. Paul Lez Durance Cedex, 13067, France b c
h i g h l i g h t s • • • • •
Simulation of spectrum of deuterium Balmer-alpha divertor stray light (DSL) in ITER is carried out using Zemax OpticStudio ray tracing software. The DSL collected by H-alpha diagnostics pupil in equatorial port EPP11 is simulated for final reflection from optical dump (OD) and ordinary wall. Accuracy of respective “differential measurement scheme” is evaluated for five designs of OD in ITER blanket module BM4. The scheme is aimed at separating the contribution of the light emitted in the high- and low-field side SOL from the total registered signal. Requested capability of five designs of OD to suppress the DSL and retain DSL spectrum is shown to be very sensitive to OD’s location on the first wall.
a r t i c l e
i n f o
Article history: Received 30 September 2016 Received in revised form 17 February 2017 Accepted 3 April 2017 Available online xxx Keywords: Optical dumps Optical diagnostics ITER
a b s t r a c t The performance of ITER Main Chamber H-alpha (and Visible Light) Spectroscopy diagnostics is challenged by the problem of separating the Balmer-alpha light emitted in the scrape-off-layer (SOL) in the main chamber from the much more intense divertor stray light (DSL) − the light emitted in the divertor in the same spectral line and reflected by the all-metal first wall (FW). The differential measurement scheme with the use of the optical dumps (OD) can address this issue by separating the contribution of DSL to the total signal. However, this scheme is effective only if the OD does not distort the DSL normalized spectrum (the line shape). The modeling of reflection properties of the ODs of various designs was performed using the ray-tracing simulation with “Zemax OpticStudio” software package. The resulting accuracy of recovering the SOL emission contribution to the total measured signal was estimated as a function of the DSL fraction in the total signal for the considered OD designs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction One of the main goals of the ITER Main Chamber H-alpha (and Visible Light) High Resolution Spectroscopy (HRS) is the recovery of the flux of the hydrogen isotopes from the first wall (FW). The diagnostics faced a challenge of separating the useful signal, namely, the Balmer-alpha light emitted in the scrape-off-layer (SOL) in the main chamber, from the divertor stray light (DSL) − the light emitted in the divertor in the same spectral line and reflected by the all-metal FW. The DSL may dominate in the total signal [1,2]. The measurement scheme, which implies the simultaneous observation with a pair of lines of sight (LoS) of the two neighboring
∗ Corresponding author at: NRC “Kurchatov Institute”, Ak. Kurchatova sq. 1, Moscow, Russia. E-mail address: Andreenko [email protected] (E.N. Andreenko).
spots in the FW with significantly different reflectivity, Rw , was proposed to address this issue [1]. This scheme allows one to separate the unknown spectral contribution of the DSL to the total signal, under the following conditions: (i) the spectral intensities of the useful signals are the same on both LoS; (ii) the normalized spectra (the line shapes) of the reflected stray light are the same on both LoS. The remarkable difference of the Rw value can be achieved by using the optical dumps (OD). The OD reflectivity strongly depends on the incident light direction [3]. This, in combination with the strong local inhomogeneity of the plasma emissivity in the divertor and the anisotropy of the light emitted by atoms in a strong magnetic field, may cause a distortion of the DSL line shape measured on the LoS viewing an optical dump. In this work, the spectral and angular distribution of the emission power reflected by the ODs of various designs and locations on the ITER FW was simulated using the “Zemax OpticStudio” software
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Please cite this article in press as: E.N. Andreenko, et al., Optimization of optical dumps for H-alpha spectroscopy in ITER, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.009
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package. The accuracy of the separation of the spectral contribution of the SOL plasma emission near the high-field side of the FW was estimated as a function of the DSL fraction in the total signal for the considered OD designs. 2. Modeling with Zemax Zemax can cast rays only from the light source to the eye (not backward), therefore only a few from the hundreds of thousands of launched rays hit the small diagnostic pupil. To overcome this problem, we: a derive the bidirectional scattering distribution function (BSDF) — dR( in , in , out , out )/d˝out for each OD design modelled in Zemax, b replace the ODs the center of the ITER inner wall blanket modules (BMs) with the artificial detectors in theFW to derive the angular distribution of incident stray light − Iin in , in , , c calculate the angular distributions of the intensity of stray light reflected by the ODs in the direction of diagnostics pupil with Eq. (1):
qout out , out , = Sdump
dR in , in , out , out d˝out
Fig. 1. Design of (a) the central bolt hole, (b) the cylindrical OD with concentric diaphragms, (c) the cylinder with a conical bottom, (d) the cylinder with a conical bottom and carving.
|nin , ndump | × Iin out , in ,
d˝in
(1)
Here ˝out is the solid angle of reflected light defined by the pair of angles, out and out , ˝in is the solid angle of the incident light defined by in and in , Sdump is the effective area of OD aperture, nin is the incident ray direction, ndump is the normal to the OD’s aper-
Fig. 2. Scheme of BSDF measurements.
ture surface, Iin in , in , is the spectral-angular distribution of incident light, dR( in , in , out , out )/d˝out is the OD’s BSDF. Similar calculations are performed for the optical model of the flat plate to simulate the reflection from the neighboring surface of the ITER FW. One can use the following objects as the OD: (i) the technological holes in the BM (e.g. Central Bolt hole), (ii) special cutouts in the BMs, (iii) the separate specially designed structures installed in the ITER FW apertures. Here we analyze the reflection properties of a single OD of type “i” — the central bolt hole (Ø60 mm) of the BM, three ODs of type “ii” — a simple cylinder, a cylinder with a conical bottom and a cylinder with a conical bottom and carving on the inner surface (all of them share the same dimensions: Ø86 mm, length — 85 mm) and a single OD of type “iii” — cylindrical OD with concentric diaphragms and conical bottom (Ø60 mm for outer surface to fit into the central bolt hole). Fig. 1 shows the design of some of these ODs. During the plasma operation the sputtering of ITER FW surface will cause quite likely the Be deposition on any plasma facing component including the OD’s inner surface. Therefore, the reflection model typical for unpolished metals such as beryllium, tungsten and stainless steel, namely, 50% of absorption, 46% of diffusive reflectivity and 4% of specular reflectivity [3], is set for the modeling of all reflecting surfaces of the ODs and FW. To obtain the BSDF (dR( in , in , out , out )/d˝out ) the OD is illuminated by a parallel light beam at various angles with respect to the OD’s axis. The angular distribution of light reflected by the OD is measured with a hemispherical detector (Fig. 2). All the ODs studied in this work have axial symmetry, and to obtain the BSDF it was enough to vary only the polar angle, in , of the incident light in the range [0–90◦ ]. To reduce the computational time tenfold and perform the calculations on a desktop PC within a reasonable time frame (about
one month per Balmer-alpha spectral line shape) the simplified model of the ITER vacuum chamber from the ITER ENOVIA database was created in Zemax. Assuming the toroidal symmetry of the plasma, for the ray-tracing simulation one can use the model for a ten-degree sector of the vacuum chamber bounded by the two planes with the 100% specular reflectivity of a perfect mirror (Fig. 3). This trick, used in [2], also significantly reduces the computing time. Poloidal profile of D␣ emission is obtained by using the data of predictive modeling of the flat-top stage of the ITER operation inductive mode (with Q = 10), calculated by the SOLPS4.3 (B2EIRENE) code [4–6] (with some modifications described in [7]). It was shown in [2] that the contribution of the DSL to the total signal is maximal for the “low-density L-mode” scenario of the ITER operation. Therefore, only “low- and moderate-density L-mode” scenarios are studied here. Zemax supports only flat square-shaped anisotropic light sources, so the original triangular mesh is transformed to meet the Zemax requirements. The model of the light sources consists of ten identical poloidal planes shifted and tilted 1◦ one to another in the toroidal direction. Each plane consists of ∼150 flat square-shaped elementary light sources (Fig. 4). The total number of the elementary sources in the divertor is close to that in [2] and determined by the balance between the acceptable computational time and the desired accuracy. The elementary sources have different sizes to keep the details close to the original mesh for the brightest and the most heterogeneous areas. 25 points in total are calculated along the wavelength within the range of the D␣ line shape with the Doppler-Zeeman structure. The angular distribution of D␣ emission of the elementary source is different for each wavelength (Fig. 4). Only the divertor light sources are taken into account since ∼98% of D␣ emission power is concentrated in the divertor. To measure the angular distribution of incident light, Iin in , in , , eight detectors are installed in the centers of BMs
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Fig. 5. The (normalized) spectral line shapes of the DSL reflected by the First Wall and the OD with diaphragms installed in BM#1, in the direction of diagnostics pupil.
Fig. 3. Ten-degree sector of ITER vacuum vessel simplified model. 1–ideal mirrors, 2–detectors, 3–H-alpha pupil.
while keff represents how the integral efficiency of DSL reduction by the OD:
##1–8 (that fall in the field of view of the H␣ diagnostics) of the simplified FW model (Fig. 1). Once all the BSDFs are obtained and the values of Iin in , in , are calculated, the sought-for stray light spec-
tra, qout out , out , , may be calculated using Eq. (1) for the ODs and the FW plate. 3. Results and discussion The reflection properties of the OD may be characterized with a pair of integral parameters — it’s distortion ratio, DOD , and optical dumping efficiency, keff . Parameter DOD characterizes the distortion of the DSL spectral line shape by the OD (compared with the DSL spectrum reflected by the FW): DOD = 100% ·
FW |qOD norm () − qnorm () |
(2)
FW/OD
qnorm
() ≡ 1
(3)
1 qFW (i ) N qOD (i ) N
keff =
(4)
i=1
The higher the value of keff and the lower the value DOD are, the better is the accuracy of measurements. The normalized DSL spectra (see Eq. (3)) calculated with Eq. (1) for OD with diaphragms are shown in Fig. 5 for BM#1. These spectra are interpolated with a set of Gaussians. Table 1 shows the calculated reflection properties of all analyzed ODs. The OD models with small apertures have higher dumping efficiency but require higher optical resolution of the diagnostics. The values of keff and DOD are correlated − the OD with a higher dumping efficiency usually has higher a distortion rate too. The angle of reflection from OD to the diagnostic pupil have a strong dependence on the BM location, therefore the optimal design of OD may vary from one BM to another. The estimation of accuracy of the signal/background separation was performed for the ODs of various design, in the differential measurement scheme based on simultaneous observation of the OD and the nearby FW, with two neighboring LoS. To calculate the density of the neutral atoms it is necessary not only to separate the useful signal from the background but also to isolate the spectral contribution of the light emitted in the high-
Fig. 4. Example of the model light sources (left) and their radiation pattern in the strong magnetic field (right).
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Fig. 6. Normalized relative error of the recovered fraction of HFS SOL light in the total signal (Eq. (5)) on the radial LoS observing the BM#4 of FW as a function of the fraction of the DSL in the total signal, for various designs of the OD.
Table 1 The reflection properties of optical dumps. BM number
Central bolt hole
1 2 3 4 5 6 7 8
Simple cylinder
HFS ıxDSL
norm
=
Cylinder with con. bottom and carving
Cylinder with diaphragms
DOD , %
keff
DOD , %
keff
DOD , %
keff
DOD , %
keff
DOD , %
keff
5.0 6.2 5.4 1.8 2.0 2.2 1.7 2.4
27 19 14 13 9 9 7 11
6.9 6.7 6.2 2.0 2.5 2.4 1.8 1.9
16 11 5 3 5 5 4 3
6.8 5.9 5.4 1.7 2.1 2.3 1.6 0.5
19 13 7 5 6 6 5 5
8.3 8.4 6.6 2.3 3.2 3.2 3.2 3.0
29 16 9 10 7 6 5 11
11.6 10.3 6.1 2.3 4.1 4.1 3.3 4.3
77 24 31 56 13 11 9 63
field side (HFS) SOL from that emitted in the low-field side (LFS) SOL. Let xHFS be the unknown fraction of the contribution of the HFS SOL light to the total signal measured by the LoS observing the FW. The accuracy of the recovery of this value is estimated in the approach called synthetic diagnostics, which allows the direct comparison of the recovered values with the true pre-set ones. For H-alpha diagnostics in ITER the algorithms of synthetic diagnostics under condition of strong DSL are formulated and developed in [8]. The results of calculation of the neutral atoms’ velocity distribution function (VDF) on the radial LoS from the equatorial port carried out in ITER Organization with the EIRENE code were used to simulate the synthetic “experimental” data. Since such calculations were performed only for the 4-th BM, the comparative analysis of the effectiveness of the various ODs was carried out only for this BM. The accuracy of the xHFS can be characterized by its recovery HFS normalized relative error, ıxDSL :
Cylinder with conical bottom
HFS xDSL
−
HFS xDSL=0
−
norm
HFS exp xDSL exp HFS xDSL=0
1 1 − (xDSL )exp
(5)
HFS where (xDSL )exp is the true pre-set values of DSL/signal ratio, xDSL=0
HFS is the value of xHFS without the DSL and xDSL
exp
is the true pre-
set value of xHFS . The brackets <. . .> denote averaging over uniform distribution of pre-set values of xHFS .HFS Fig. 6 shows the behavior of ıxDSL as a function of the norm fraction of DSL in the total signal. The values in the bar chart are given for the ITER operation scenario with moderate plasma density
in the far SOL in the L-mode, while the shown trends are valid for all six analyzed HFS scenarios. For ıxDSL > 2, the OD performance is not good enough to norm provide acceptable measurement accuracy. E.g., the simple cylinder loses its efficiency if the DSL fraction in the total signal is as high as 92%.
4. Conclusions The accuracy of differential measurement scheme with the use of various OD designs to separate the contribution of the light emitted in the high-field side SOL from the total signal registered by H-alpha diagnostics was studied by modeling with Zemax OpticStudio software. For this purpose, the simplified optical models of the ITER vacuum chamber and of the light sources in divertor were developed in Zemax. The model sources take into account the spectral line shape of D␣ emission and the anisotropy of the light emitted by atoms in a strong magnetic field. The results show that the properties of ODs have a strong dependence on its location on the FW. For example, the dumping efficiency (Eq. (4)) varies from 9 to 76, and the distortion ratio (Eq. (2)) varies from 1.6% to 10.5% for the cylindrical OD with concentric diaphragms. So the optimal design of ODs may vary for different blanket modules. The cylinder with diaphragms and the central bolt hole are the most efficient ODs for the BM#4. The simulations of the deuterium VDF are required to find the optimal ODs for other BMs.
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Disclaimer
References
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
[1] A.B. Kukushkin, et al., Proc. 24th IAEA Fusion Energy Conference, San Diego, USA, 8–13 October, 2012, ITR/P 5-44. [2] S. Kajita, et al., Plasma Phys. Contr. Fusion 55 (2013) 085020, http://dx.doi.org/ 10.1088/0741-3335/55/8/085020. [3] E.N. Andreenko, et al., Int. Conf. on Fusion Reactor Diagnostics, Varenna, Italy, September 9–13, in: AIP Conf. Proc., 2014, 2013, p. 171, http://dx.doi.org/10. 1063/1.4894047 (1612). [4] A.S. Kukushkin, et al., Fusion Eng. Des. 86 (2011) 2865, http://dx.doi.org/10. 1016/j.fusengdes.2011.06.009. [5] B.J. Braams, PhD Thesis, Rijksuniversiteit, Utrecht, 1986. [6] D. Reiter, M. Baelmans, P. Börner, Sci. Fusion Technol. 47 (2005) 172, http://dx. doi.org/10.13182/FST47-172. [7] S.W. Lisgo, P. Börner, et al., J. Nucl. Mater. 415 (2011) S965, http://dx.doi.org/ 10.1016/j.jnucmat.2010.11.061. [8] A.B. Kukushkin, V.S. Neverov, et al., Fusion Sci. Technol. 69 (3) (2016) 628, http://dx.doi.org/10.13182/FST15-186.
Acknowledgements This work was partially supported by the RF State Corporation ROSATOM. The authors are grateful to A.V. Gorshkov, S. Kajita, M.G. Levashova, E. Veshchev, I.I. Orlovskiy, D.K. Vukolov, K.Yu. Vukolov for their collaboration in studies on the ITER Main Chamber H-alpha (and Visible Light) Spectroscopy.
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Optimization of optical dumps for H-alpha spectroscopy in ITER E.N. Andreenko a,b,∗ , A.G. Alekseev a,b , A.B. Kukushkin a,c , V.S. Neverov a , S.W. Lisgo d , A.A. Morozov a,b a
NRC “Kurchatov Institute”, Moscow, Russia International Fusion Projects Coordinating Centre, Moscow, Russia NRNU “MEPhI”, Moscow, Russia d ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, St. Paul Lez Durance Cedex, 13067, France b c
h i g h l i g h t s • • • • •
Simulation of spectrum of deuterium Balmer-alpha divertor stray light (DSL) in ITER is carried out using Zemax OpticStudio ray tracing software. The DSL collected by H-alpha diagnostics pupil in equatorial port EPP11 is simulated for final reflection from optical dump (OD) and ordinary wall. Accuracy of respective “differential measurement scheme” is evaluated for five designs of OD in ITER blanket module BM4. The scheme is aimed at separating the contribution of the light emitted in the high- and low-field side SOL from the total registered signal. Requested capability of five designs of OD to suppress the DSL and retain DSL spectrum is shown to be very sensitive to OD’s location on the first wall.
a r t i c l e
i n f o
Article history: Received 30 September 2016 Received in revised form 17 February 2017 Accepted 3 April 2017 Available online xxx Keywords: Optical dumps Optical diagnostics ITER
a b s t r a c t The performance of ITER Main Chamber H-alpha (and Visible Light) Spectroscopy diagnostics is challenged by the problem of separating the Balmer-alpha light emitted in the scrape-off-layer (SOL) in the main chamber from the much more intense divertor stray light (DSL) − the light emitted in the divertor in the same spectral line and reflected by the all-metal first wall (FW). The differential measurement scheme with the use of the optical dumps (OD) can address this issue by separating the contribution of DSL to the total signal. However, this scheme is effective only if the OD does not distort the DSL normalized spectrum (the line shape). The modeling of reflection properties of the ODs of various designs was performed using the ray-tracing simulation with “Zemax OpticStudio” software package. The resulting accuracy of recovering the SOL emission contribution to the total measured signal was estimated as a function of the DSL fraction in the total signal for the considered OD designs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction One of the main goals of the ITER Main Chamber H-alpha (and Visible Light) High Resolution Spectroscopy (HRS) is the recovery of the flux of the hydrogen isotopes from the first wall (FW). The diagnostics faced a challenge of separating the useful signal, namely, the Balmer-alpha light emitted in the scrape-off-layer (SOL) in the main chamber, from the divertor stray light (DSL) − the light emitted in the divertor in the same spectral line and reflected by the all-metal FW. The DSL may dominate in the total signal [1,2]. The measurement scheme, which implies the simultaneous observation with a pair of lines of sight (LoS) of the two neighboring
∗ Corresponding author at: NRC “Kurchatov Institute”, Ak. Kurchatova sq. 1, Moscow, Russia. E-mail address: Andreenko [email protected] (E.N. Andreenko).
spots in the FW with significantly different reflectivity, Rw , was proposed to address this issue [1]. This scheme allows one to separate the unknown spectral contribution of the DSL to the total signal, under the following conditions: (i) the spectral intensities of the useful signals are the same on both LoS; (ii) the normalized spectra (the line shapes) of the reflected stray light are the same on both LoS. The remarkable difference of the Rw value can be achieved by using the optical dumps (OD). The OD reflectivity strongly depends on the incident light direction [3]. This, in combination with the strong local inhomogeneity of the plasma emissivity in the divertor and the anisotropy of the light emitted by atoms in a strong magnetic field, may cause a distortion of the DSL line shape measured on the LoS viewing an optical dump. In this work, the spectral and angular distribution of the emission power reflected by the ODs of various designs and locations on the ITER FW was simulated using the “Zemax OpticStudio” software
http://dx.doi.org/10.1016/j.fusengdes.2017.04.009 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: E.N. Andreenko, et al., Optimization of optical dumps for H-alpha spectroscopy in ITER, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.009
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package. The accuracy of the separation of the spectral contribution of the SOL plasma emission near the high-field side of the FW was estimated as a function of the DSL fraction in the total signal for the considered OD designs. 2. Modeling with Zemax Zemax can cast rays only from the light source to the eye (not backward), therefore only a few from the hundreds of thousands of launched rays hit the small diagnostic pupil. To overcome this problem, we: a derive the bidirectional scattering distribution function (BSDF) — dR( in , in , out , out )/d˝out for each OD design modelled in Zemax, b replace the ODs the center of the ITER inner wall blanket modules (BMs) with the artificial detectors in theFW to derive the angular distribution of incident stray light − Iin in , in , , c calculate the angular distributions of the intensity of stray light reflected by the ODs in the direction of diagnostics pupil with Eq. (1):
qout out , out , = Sdump
dR in , in , out , out d˝out
Fig. 1. Design of (a) the central bolt hole, (b) the cylindrical OD with concentric diaphragms, (c) the cylinder with a conical bottom, (d) the cylinder with a conical bottom and carving.
|nin , ndump | × Iin out , in ,
d˝in
(1)
Here ˝out is the solid angle of reflected light defined by the pair of angles, out and out , ˝in is the solid angle of the incident light defined by in and in , Sdump is the effective area of OD aperture, nin is the incident ray direction, ndump is the normal to the OD’s aper-
Fig. 2. Scheme of BSDF measurements.
ture surface, Iin in , in , is the spectral-angular distribution of incident light, dR( in , in , out , out )/d˝out is the OD’s BSDF. Similar calculations are performed for the optical model of the flat plate to simulate the reflection from the neighboring surface of the ITER FW. One can use the following objects as the OD: (i) the technological holes in the BM (e.g. Central Bolt hole), (ii) special cutouts in the BMs, (iii) the separate specially designed structures installed in the ITER FW apertures. Here we analyze the reflection properties of a single OD of type “i” — the central bolt hole (Ø60 mm) of the BM, three ODs of type “ii” — a simple cylinder, a cylinder with a conical bottom and a cylinder with a conical bottom and carving on the inner surface (all of them share the same dimensions: Ø86 mm, length — 85 mm) and a single OD of type “iii” — cylindrical OD with concentric diaphragms and conical bottom (Ø60 mm for outer surface to fit into the central bolt hole). Fig. 1 shows the design of some of these ODs. During the plasma operation the sputtering of ITER FW surface will cause quite likely the Be deposition on any plasma facing component including the OD’s inner surface. Therefore, the reflection model typical for unpolished metals such as beryllium, tungsten and stainless steel, namely, 50% of absorption, 46% of diffusive reflectivity and 4% of specular reflectivity [3], is set for the modeling of all reflecting surfaces of the ODs and FW. To obtain the BSDF (dR( in , in , out , out )/d˝out ) the OD is illuminated by a parallel light beam at various angles with respect to the OD’s axis. The angular distribution of light reflected by the OD is measured with a hemispherical detector (Fig. 2). All the ODs studied in this work have axial symmetry, and to obtain the BSDF it was enough to vary only the polar angle, in , of the incident light in the range [0–90◦ ]. To reduce the computational time tenfold and perform the calculations on a desktop PC within a reasonable time frame (about
one month per Balmer-alpha spectral line shape) the simplified model of the ITER vacuum chamber from the ITER ENOVIA database was created in Zemax. Assuming the toroidal symmetry of the plasma, for the ray-tracing simulation one can use the model for a ten-degree sector of the vacuum chamber bounded by the two planes with the 100% specular reflectivity of a perfect mirror (Fig. 3). This trick, used in [2], also significantly reduces the computing time. Poloidal profile of D␣ emission is obtained by using the data of predictive modeling of the flat-top stage of the ITER operation inductive mode (with Q = 10), calculated by the SOLPS4.3 (B2EIRENE) code [4–6] (with some modifications described in [7]). It was shown in [2] that the contribution of the DSL to the total signal is maximal for the “low-density L-mode” scenario of the ITER operation. Therefore, only “low- and moderate-density L-mode” scenarios are studied here. Zemax supports only flat square-shaped anisotropic light sources, so the original triangular mesh is transformed to meet the Zemax requirements. The model of the light sources consists of ten identical poloidal planes shifted and tilted 1◦ one to another in the toroidal direction. Each plane consists of ∼150 flat square-shaped elementary light sources (Fig. 4). The total number of the elementary sources in the divertor is close to that in [2] and determined by the balance between the acceptable computational time and the desired accuracy. The elementary sources have different sizes to keep the details close to the original mesh for the brightest and the most heterogeneous areas. 25 points in total are calculated along the wavelength within the range of the D␣ line shape with the Doppler-Zeeman structure. The angular distribution of D␣ emission of the elementary source is different for each wavelength (Fig. 4). Only the divertor light sources are taken into account since ∼98% of D␣ emission power is concentrated in the divertor. To measure the angular distribution of incident light, Iin in , in , , eight detectors are installed in the centers of BMs
Please cite this article in press as: E.N. Andreenko, et al., Optimization of optical dumps for H-alpha spectroscopy in ITER, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.009
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Fig. 5. The (normalized) spectral line shapes of the DSL reflected by the First Wall and the OD with diaphragms installed in BM#1, in the direction of diagnostics pupil.
Fig. 3. Ten-degree sector of ITER vacuum vessel simplified model. 1–ideal mirrors, 2–detectors, 3–H-alpha pupil.
while keff represents how the integral efficiency of DSL reduction by the OD:
##1–8 (that fall in the field of view of the H␣ diagnostics) of the simplified FW model (Fig. 1). Once all the BSDFs are obtained and the values of Iin in , in , are calculated, the sought-for stray light spec-
tra, qout out , out , , may be calculated using Eq. (1) for the ODs and the FW plate. 3. Results and discussion The reflection properties of the OD may be characterized with a pair of integral parameters — it’s distortion ratio, DOD , and optical dumping efficiency, keff . Parameter DOD characterizes the distortion of the DSL spectral line shape by the OD (compared with the DSL spectrum reflected by the FW): DOD = 100% ·
FW |qOD norm () − qnorm () |
(2)
FW/OD
qnorm
() ≡ 1
(3)
1 qFW (i ) N qOD (i ) N
keff =
(4)
i=1
The higher the value of keff and the lower the value DOD are, the better is the accuracy of measurements. The normalized DSL spectra (see Eq. (3)) calculated with Eq. (1) for OD with diaphragms are shown in Fig. 5 for BM#1. These spectra are interpolated with a set of Gaussians. Table 1 shows the calculated reflection properties of all analyzed ODs. The OD models with small apertures have higher dumping efficiency but require higher optical resolution of the diagnostics. The values of keff and DOD are correlated − the OD with a higher dumping efficiency usually has higher a distortion rate too. The angle of reflection from OD to the diagnostic pupil have a strong dependence on the BM location, therefore the optimal design of OD may vary from one BM to another. The estimation of accuracy of the signal/background separation was performed for the ODs of various design, in the differential measurement scheme based on simultaneous observation of the OD and the nearby FW, with two neighboring LoS. To calculate the density of the neutral atoms it is necessary not only to separate the useful signal from the background but also to isolate the spectral contribution of the light emitted in the high-
Fig. 4. Example of the model light sources (left) and their radiation pattern in the strong magnetic field (right).
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Fig. 6. Normalized relative error of the recovered fraction of HFS SOL light in the total signal (Eq. (5)) on the radial LoS observing the BM#4 of FW as a function of the fraction of the DSL in the total signal, for various designs of the OD.
Table 1 The reflection properties of optical dumps. BM number
Central bolt hole
1 2 3 4 5 6 7 8
Simple cylinder
HFS ıxDSL
norm
=
Cylinder with con. bottom and carving
Cylinder with diaphragms
DOD , %
keff
DOD , %
keff
DOD , %
keff
DOD , %
keff
DOD , %
keff
5.0 6.2 5.4 1.8 2.0 2.2 1.7 2.4
27 19 14 13 9 9 7 11
6.9 6.7 6.2 2.0 2.5 2.4 1.8 1.9
16 11 5 3 5 5 4 3
6.8 5.9 5.4 1.7 2.1 2.3 1.6 0.5
19 13 7 5 6 6 5 5
8.3 8.4 6.6 2.3 3.2 3.2 3.2 3.0
29 16 9 10 7 6 5 11
11.6 10.3 6.1 2.3 4.1 4.1 3.3 4.3
77 24 31 56 13 11 9 63
field side (HFS) SOL from that emitted in the low-field side (LFS) SOL. Let xHFS be the unknown fraction of the contribution of the HFS SOL light to the total signal measured by the LoS observing the FW. The accuracy of the recovery of this value is estimated in the approach called synthetic diagnostics, which allows the direct comparison of the recovered values with the true pre-set ones. For H-alpha diagnostics in ITER the algorithms of synthetic diagnostics under condition of strong DSL are formulated and developed in [8]. The results of calculation of the neutral atoms’ velocity distribution function (VDF) on the radial LoS from the equatorial port carried out in ITER Organization with the EIRENE code were used to simulate the synthetic “experimental” data. Since such calculations were performed only for the 4-th BM, the comparative analysis of the effectiveness of the various ODs was carried out only for this BM. The accuracy of the xHFS can be characterized by its recovery HFS normalized relative error, ıxDSL :
Cylinder with conical bottom
HFS xDSL
−
HFS xDSL=0
−
norm
HFS exp xDSL exp HFS xDSL=0
1 1 − (xDSL )exp
(5)
HFS where (xDSL )exp is the true pre-set values of DSL/signal ratio, xDSL=0
HFS is the value of xHFS without the DSL and xDSL
exp
is the true pre-
set value of xHFS . The brackets <. . .> denote averaging over uniform distribution of pre-set values of xHFS .HFS Fig. 6 shows the behavior of ıxDSL as a function of the norm fraction of DSL in the total signal. The values in the bar chart are given for the ITER operation scenario with moderate plasma density
in the far SOL in the L-mode, while the shown trends are valid for all six analyzed HFS scenarios. For ıxDSL > 2, the OD performance is not good enough to norm provide acceptable measurement accuracy. E.g., the simple cylinder loses its efficiency if the DSL fraction in the total signal is as high as 92%.
4. Conclusions The accuracy of differential measurement scheme with the use of various OD designs to separate the contribution of the light emitted in the high-field side SOL from the total signal registered by H-alpha diagnostics was studied by modeling with Zemax OpticStudio software. For this purpose, the simplified optical models of the ITER vacuum chamber and of the light sources in divertor were developed in Zemax. The model sources take into account the spectral line shape of D␣ emission and the anisotropy of the light emitted by atoms in a strong magnetic field. The results show that the properties of ODs have a strong dependence on its location on the FW. For example, the dumping efficiency (Eq. (4)) varies from 9 to 76, and the distortion ratio (Eq. (2)) varies from 1.6% to 10.5% for the cylindrical OD with concentric diaphragms. So the optimal design of ODs may vary for different blanket modules. The cylinder with diaphragms and the central bolt hole are the most efficient ODs for the BM#4. The simulations of the deuterium VDF are required to find the optimal ODs for other BMs.
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Disclaimer
References
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
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Acknowledgements This work was partially supported by the RF State Corporation ROSATOM. The authors are grateful to A.V. Gorshkov, S. Kajita, M.G. Levashova, E. Veshchev, I.I. Orlovskiy, D.K. Vukolov, K.Yu. Vukolov for their collaboration in studies on the ITER Main Chamber H-alpha (and Visible Light) Spectroscopy.
Please cite this article in press as: E.N. Andreenko, et al., Optimization of optical dumps for H-alpha spectroscopy in ITER, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.009