Thermographic Studies of Outer Target Heat Fluxes on KSTAR

ARTICLE IN PRESS JID: NME [m5G; December 27, 2016;13:21 ]

Nuclear Materials and Energy 0 0 0 (2016) 1–7

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Nuclear Materials and Energy

journal homepage: www.elsevier.com/locate/nme

Thermographic studies of outer target heat ﬂuxes on KSTAR

∗ H.H. Lee a, , R.A. Pitts b, C.S. Kang a, S.T. Oh a, J.G. Bak a, S.H. Hong a,c,d, H.M. Wi a, Y.S. Kim a,

H.S. Kim a, D.C. Seo a, H.T. Kim a, K.M. Kim a a National Fusion Research Institute, Daejeon, Korea b ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France c Korea University of Science and Technology, Daejeon, Korea d Department of electrical engineering, Hanyang University, Seoul, Korea

a r t i c l e i n f o a b s t r a c t

Article history: A new infra-red (IR) thermography system with high spatial resolution has been installed on KSTAR and Received 15 July 2016 is now mainly applied to measure the outer divertor heat load proﬁle. The ﬁrst measurement results

Revised 14 November 2016 of the outer divertor heat load profiles between ELMs have been applied to characterize the inter-ELMs Accepted 16 December 2016 outer divertor heat loads in KSTAR H-mode plasmas. In particular, the power decay length ( λ ) of the Available online xxx q divertor heat load profile has been determined by fitting the profile to a convolution of an exponential decay and a Gaussian function. The analysis on the power decay length shows a good agreement with

the recent multi-machine λq scaling, which predicts λq of the inter-ELMs divertor heat load to be ∼1 mm under the standard H-mode scenario in ITER. The divertor IR thermography system has also successfully measured the strike point splitting of the outer divertor heat flux during the application of resonant magnetic perturbation (RMP) fields. In addition, it has provided a clear evidence that the strike point splitting pattern depends on the RMP fields configuration. ©2016 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

1. Introduction SOL structure and lead to non-axisymmetric power loading on the targets. The study of this three-dimensional heat deposition under One of most important concerns for burning-plasma devices highly dissipative, H-mode conditions is one of key research areas such as ITER is the power load on the divertor and its power han- for ITER. dling capability. Furthermore, the power load must be below the An accurate measurement of the heat flux profile onto the di- engineering limits set by material constraints (erosion and melt- vertor target is imperative for these studies. Divertor target infra- ing). For example, it has been expected that the maximum inter- red (IR) thermography has now become an established diagnostic ELMs heat load tolerable to the divertor target in ITER is in the for the divertor heat flux profile measurement [10,15–20] . KSTAR 2 range of ∼10 MW / m [1,2] . The power decay length in the scrape- is a medium-sized tokamak with its nominal major radius R 0 = off layer (SOL) above the X-point region, λq , is a critical quan- 1 . 8 m, minor radius a = 0 . 5 m. Dueterium plasma is usually dis- tity for determining the peak heat load on the divertor tile. Al- charged with plasma current I p ≤ 1 MA and toroidal magnetic field though an accurate prediction of λq is strongly demanded for fu- at the magnetic axis B 0 ≤ 3 . 5 T. A neutral beam injection (NBI) ture burning-plasma devices, the prediction has relied upon the is a dominant plasma heating source and mainly used to obtain extrapolation from the empirical scaling laws established from cur- H-mode plasmas with its deliverable power of up to 5 MW. The rent devices [3–10] . graphite divertor target tiles have been designed to accommodate

On the one hand, the application of resonant magnetic per- the heat ﬂux ≤ 4 . 3 MW/m2 [21] . An active cooling system of the turbation (RMP) ﬁelds has been recognized as an effective con- divertor target plates has been installed but not operated yet. Un- trol knob to reduce the inter-ELMs peak heat load onto the di- til 2014, the KSTAR tokamak was equipped only with a midplane vertor tiles [11–14] since it can substantially modify the divertor mounted, tangentially viewing IR camera with views of the inner divertor target. A new, outer target imaging system has now been installed and been used in a variety of physics studies of the outer

∗ Corresponding author. divertor heat flux profile since 2014. The KSTAR divertor IRTV has E-mail address: [email protected] (H.H. Lee). been successfully applied to measure the inter-ELMs heat flux pro- http://dx.doi.org/10.1016/j.nme.2016.12.019 2352-1791/© 2016 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Please cite this article as: H.H. Lee et al., Thermographic studies of outer target heat ﬂuxes on KSTAR, Nuclear Materials and Energy (2016), http://dx.doi.org/10.1016/j.nme.2016.12.019 ARTICLE IN PRESS JID: NME [m5G; December 27, 2016;13:21 ]

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Fig. 1. The schematic diagram of the divertor IRTV on KSTAR.

file onto the outer divertor target and characterize the heat flux of the periscope is around 1.2 mm/pixel for the IR camera detec- profile. The system has also found clear evidence for strike point tor pitch of 25 μm. The IR camera (FLIR SC 6101) measures the tile splitting of the divertor heat flux during the application of RMP surface temperature by using the IR emission in the range of 3–5 fields in KSTAR. Then, in this paper, the first measurement results μm. The full frame rate is 125 Hz and the rate can be increased of the divertor heat flux profile by the divertor IRTV in KSTAR are up to few tens kHz by reducing the viewing region. The measur- presented following the detailed description of the diagnostic sys- able range of the temperature is 30–1500 °C. tem. The power decay length of the inter-ELMs divertor heat load In 2015, an add-on 3X zoom lens was developed to increase is characterized and compared to the multi-machine scaling from the spatial resolution up to 0.4 mm/pixel, which is 3 times higher other tokamaks [4,6,9] . Finally, the impacts of the RMP fields on than that of the original periscope. The add-on zoom lens is eas- the divertor heat flux profile are discussed. ily attached to the end of the periscope and can increase up the spatial resolution without any change of the periscope. But, with 2. KSTAR divertor infrared television the add-on zoom lens, the region covered by the IR camera is necessarily reduced. Fig. 2 (a) shows an sample IR image of divertor Since 2014, a divertor infrared television (IRTV) system has target tiles measured by the periscope without the zoom lens. It been installed to monitor the heat flux onto the divertor target in shows that the divertor targets from inboard to outboard sides can the KSTAR tokamak. The KSTAR divertor IRTV ( Fig. 1 ) consists of be monitored by the periscope. By attaching the zoom lens to the vacuum cassette, 3-meter long periscope and IR camera. Supercon- periscope, it is available to monitor some of the outer divertor tar- ducting tokamaks such as KSTAR pose specific problems for such get tiles with high spatial resolution as shown in Fig. 2 (b). The IR systems, since the image must be delivered over long distances image shows the toroidally axisymmetric strike point located on from first mirror to detector. A 3-m long Kepler-type periscope the target tile, which is mainly used to study the divertor heat load consisting of one fused silica mirror and seven CaF2 and ZnSe characteristics in KSTAR. Note that the hot spot detected on the up- lenses [22] is the chosen solution for KSTAR, all mounted in a com- per target tile in the picture mainly comes from the heat flux onto pact vacuum cassette with sapphire viewing window and inserted the leading edge of the tile due to its slight misalignment. Then, in one of the upper lateral KSTAR ports. The system views the the calculation of the heat load onto the divertor target is always ”central divertor” region of the KSTAR lower divertor, an inclined performed outside that region. surface often used for outer strike point impact in lower single The heat flux onto the divertor target can be determined by null configurations. It can cover divertor targets from inboard to solving the heat diffusion equation with the temporal evolution of outboard sides with the solid angle of 18 °. The spatial resolution the tile surface temperature measured by the IR camera. The heat

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Fig. 2. Sample IR images of (a) the divertor target tiles measured by the periscope without the zoom lens and (b) the central divertor target used for the outer divertor heat load study. The toroidally axisymmetric strike point located on the central divertor target tile is clearly observed. The directions of the toroidal magnetic field ( B T ) and plasma current ( I p ) are represented in (a). The white box in (a) corresponds to the viewing region by the zoom lens in (b) . diffusion equation in Cartesian coordinates can be written as ∂ ∂T ∂ ∂T ∂ ∂T ∂T κ + κ + κ = ρC p (1) ∂x ∂x ∂y ∂y ∂z ∂z ∂t where T is the temperature, ρ is the mass density, C p is the specific heat, and κ is the thermal conductivity of the tile. Here, x, y and z correspond to the coordinates along the tile depth, width and length, respectively. Then, the heat flux onto the target, q ⊥ , can be ∂T estimated from q ⊥ = −κ at the tile surface. Since the tile is typ- ∂x ically thermally equilibrated before a plasma discharge begins, the temperature is assumed to be uniform in the tile at the beginning of the plasma discharge. Then, the heat diffusion equation is solved in time with the measured tile surface temperature while estimat- ing the temperature profile. In KSTAR, a heat flux reconstruction Fig. 3. Heat flux profile calculated with the constant values of the tile thermal code (NANTHELOT [23] ) has been developed to solve the heat dif- properties at the temperature of 25 °C (blue dashed) is compared to that with the temperature-dependent values of the thermal properties (red solid). (For inter- fusion equation by applying the finite volume method. NANTHELOT pretation of the references to color in this figure legend, the reader is referred to can provide 1D, 2D and 3D solutions for the heat flux profile. Both the web version of this article.) explicit and implicit schemes are available. The adiabatic condition, which assumes the heat flux at the boundary to be zero, is mainly applied to solve the heat diffusion equation. The boundary condi- 3. Measurement result tion for the radiative circumstance has been implemented as well. The examination of the surface layer effect [24,25] is also possi- The divertor IRTV has been mainly applied to measure the outer ble. Especially, the temperature-dependent thermal properties ( ρ, divertor heat flux profile in KSTAR. Several characteristics of the λ C p , and κ ) of the divertor tile can be taken into account during divertor heat flux profile such as the power decay length, q , can the heat flux calculation. Actually, as shown in Fig. 3 , it has been be determined by fitting the measured heat flux profile with an realized for the KSTAR graphite divertor target tile that the diver- analytic fit function that is set to be applied on the entire profile tor heat load can be overestimated if the thermal properties of the and provide some physics insight [4] . The fit function is given by tile is assumed to be constant regardless of the tile temperature. 2 Then, in KSTAR, the divertor heat load is always calculated consid- q S s¯ S s¯ = 0 − − + ering the temperature-dependent thermal properties of the tile. In q ( s¯ ) exp erfc qBG (2) 2 2 λq λq f x 2 λq Sf x this paper, the 2D solution with the temperature-dependent thermal properties and adiabatic boundary condition has been applied where q is the peak heat flux, s¯ = s − s is the target radial coor- to estimate the heat flux onto the target. The surface layer effect 0 0 dinate (where s o is the position of the strike point), S is the width has not been considered at the moment. of the Gaussian function, f x is the flux expansion from upstream to In KSTAR, the strike point is actively controlled to be located on the divertor, and q is the background heat flux. Here, q o , s , S, λq the divertor target tile of interest in order to take advantage of the BG 0 and q are free parameters for fitting while f x can be determined high spatial resolution of the divertor IRTV for the outer divertor BG from the magnetic flux reconstruction as heat load study. The high spatial resolution divertor IRTV can pro- mp vide very accurate information on the position of strike point on R mp B θ f x ≡ (3) the target. And it has been found that the IRTV measurement and di v Rdi v Bθ the real-time EFIT magnetic flux reconstruction result mostly have

mp di v good agreement on the strike point position within 1 cm along the where Rmp (Rdiv ) and Bθ Bθ are the major radius and poloidal target surface as shown in Fig. 4 . magnetic ﬁeld at the midplane (divertor tile), respectively.

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Fig. 4. (a) Tile surface temperature distribution measured by the divertor IRTV (where t m and t ep are the measurement time and exposure time, respectively) (b) The heat flux profile along the line (A-B) on the central divertor target tile. ‘Sep’ corresponds to the location of the separatrix on the target tile deduced from (c) the magnetic flux surfaces reconstructed by the real-time EFIT at 8.152 s.

flux profile is well described by the fit function. Actually, how well the heat flux profiles measured by the KSTAR divertor IRTV are fitted by the fit function has been examined. It has been shown that, if the fit function describes the measured heat flux profile well, the power decay length determined by the fit function should be close to the relationship [26] ,

λint ∼ λq + 1 . 64 · S (4)

λ where int is the integral width given as [3] q ( s ) − q BG λint ≡ ds . (5) ﬁt range max ( q ( s ) − q BG )

In addition, it is required that S /(2 · λq ) < 1 to enable the relationship ( Eq. (4) ) applicable to the dataset [10,26] . Then, we have examined the quality of the measured heat flux profile in KSTAR with the dataset of inter-ELMs divertor heat flux profiles measured in two discharges of very different toroidal magnetic fields but similar plasma currents. The com- Fig. 5. Divertor heat flux profile from the KSTAR discharge #13127 ( I p = 450 kA, λ λ + . · B T = 2 . 0 T and q cyl ∼ 5.39 where q cyl is the cylindrical safety factor) fitted by the parison between int and q 1 64 S represents that the ratio λ / λ + . · analytic formulation (Eq. (2)) . int q 1 64 S values from the dataset are close to one indicat- λ ing satisfactory fit [8,10] although int values are mostly smaller λ + . · λ than q 1 64 S (Fig. 6). Actually, int value can be smaller than The analytic formulation has been applied to extract the charac- λq + 1 . 64 · S when the full profile of the outer heat load is not cap- teristics of the measured inter-ELMs heat flux profile on KSTAR as tured by the measured heat flux profile on the target tile of inter- shown in Fig. 5 . It is clearly seen that the measured divertor heat est. For the present experiment, the length of the target tile for

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Fig. 8. Comparison of the multi-machine scaling and the values of λq from the − . . . − . λ = . 0 8 1 11 0 11 0 13 KSTAR H-mode plasmas. The multi-machine scaling, q 0 86BT qcyl PSOL Rgeo [9] is used for the comparison. Here, the power crossing the separatrix, P SOL is assumed to be 70% of the external heating power and the major radius, R geo = 1 . 8 m. Note that the λq for each shot is a mean value of data set obtained from a dis-

Fig. 6. Comparison between λint and λq + 1 . 64 · S for the present dataset. charge. The error bar corresponds to the one standard deviation of the mean value.

also suspected that the slight radial oscillatory movement of the strike point can attribute to the variation. Since λq is a critical quantity for determining the peak heat load on the divertor target and the heat load should be controlled not to exceed the tolerable range set by engineering and materials constraints, an accurate prediction of λq is strongly demanded for future burning-plasma devices such as ITER. But, the prediction has relied upon the extrapolation from the empirical scaling laws established from current devices. Previously, λq had been predicted to be in the range of 3–5 mm for the ITER standard H-mode scenario [3] . However, recent multi-machine scaling [6,9] yields that λq can be 3 times shorter than the previous prediction, i.e., λq ∼ 1 mm, which is a big concern for the ITER divertor design and operation. It is noted that a recently proposed heuristic drift-based theory [27] supports the multi-machine scaling [9] . We also found that the present KSTAR dataset of inter-ELMs divertor heat load λq agrees well with the multi-machine scaling as shown in Fig. 8 . But, it is still requested to establish an empirical scaling law of λq for the KSTAR H-mode inter-ELMs outer divertor heat load and com- pare the values of λq predicted from the scaling with those from the multi-machine scaling. Then, more data points necessary to de- duce the KSTAR scaling law will be obtained in the 2016 KSTAR experiment campaign. It is well known that the application of resonant magnetic per- Fig. 7. Variation of the Gaussian spreading factor, S , with the power decay length, turbation (RMP) fields can change the divertor heat flux profile λ q . [11,12,14,28,29] . In general, the magnetic field topology modified by the RMP fields is represented as the strike point splitting or three- dimensional pattern of the divertor heat flux. Since the magnitudes the heat flux profile measurement is 14 cm. We found that the of heat or particle fluxes onto the divertor target can be modified full profile of the outer heat load cannot be described by the mea- by the RMP fields, searching for an appropriate RMP fields config- sured heat flux profile on the target when the power decay length uration to suppress/mitigate ELMs and reduce the stationary diver- of the heat flux is large or strike point is located closer to the tile tor peak heat load is one of critical issues for ITER [13,14] . edge in the low magnetic field side. Thus, it is suspected that the Understanding the impacts of the RMP fields on the plasma is λ smaller int values mainly come from this environment. In the near usually based on the divertor heat flux profile measurements. In future, this disadvantage will be removed by replacing the present KSTAR, the divertor IRTV has been successfully applied to measure target tile with a longer target tile. On the one hand, the ratio S /(2 the strike point splitting of the divertor heat load during the ap- · λq ) is always below one, representing that the relationship be- plication of RMP fields as shown in Fig. 9 . In this case, the KSTAR λ λ tween q and int is applicable to the present dataset as shown in-vessel coils (top, middle, and bottom coil sets) are applied with in Fig. 7 . Here, the variation in the λq values for each plasma dis- n = 1, −90 ° phasing (where n is the toroidal mode number and the charge can be due to the varying plasma parameters such as tem- phasing normally corresponds to the phase difference between top perature, density, SOL power flux, etc. during the discharge. It is and bottom coil currents) from 10 s to 11 s during the H-mode

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Fig. 9. Time traces of (a) the plasma energy ( W dia ), D alpha ( D α ) signals and (b) the divertor heat flux profiles. (c) The heat flux profiles before and after the application of n = 1, −90 ° phasing RMP fields. (d) The difference in the splitting pattern of the strike point according to the configuration of the RMP fields. Here, n is the toroidal mode number. The +/ − sign correspond to the directions of the radial magnetic perturbations induced by the RMP fields coils in KSTAR.

phase. The decrease of the plasma energy (Wdia ) mainly comes magnetic islands inside the plasma. Surely, it is very necessary to from the so-called density pump-out induced by RMPs [30,31] . do three-dimensional modeling of the divertor heat load coupled Note that the heating power is constant during the application of with the measurement results to understand the effects of RMP RMPs. As shown in Fig. 9 (b) and (c), we clearly observed an ap- fields on the divertor heat load profile. For the modeling works, parent splitting of the strike point resulting in the double-peaked EMC3-EIRENE has been recently set up for the KSTAR experiment heat flux profile. We also found that the major radius integrated environment but is still to be optimized. Then, the modeling works heat flux during the application of RMPs is larger than that with- of the divertor heat load under the application of RMP fields are out RMPs. It is suspected that the increase of the integrated heat remained for the future. flux is due to the enhanced radial particle and heat transports (as reflected in the decrease of the plasma energy) by the change of 4. Summary the magnetic field topology at the boundary region [31,32] . But, the heat flux profile measurement is now performed at the fixed po- The first divertor IR thermography system has been installed sition in the toroidal direction. The striation pattern or magnitude to investigate the heat flux onto the outer divertor tiles in KSTAR of the heat flux can be different in the toroidal direction. Then, the since 2014. In the last year, the spatial resolution of the thermog- effect of RMPs on the integrated heat flux should be investigated raphy system was significantly improved from 1.2 mm/pixel to by considering a whole 3D structure of the heat flux profile in the 0.4 mm/pixel by utilizing the add-on 3X zoom lens. The high spa- toroidal direction, but this study is still remained for the future. tial resolution divertor IRTV is now a key diagnostic to measure In addition, there is a clear evidence of different strike point the outer divertor heat load profile in the KSTAR tokamak. Care- splitting pattern seen on the target between the two different coil ful analysis of the first inter-ELMs outer divertor heat load pro- perturbations, with a third lobe appearing in the final phase with file measurements in KSTAR shows a very good agreement with only the middle coil set active. It is also observed that the inte- the recent multi-machine λq scaling and close correspondence to grated heat flux increases as the plasma energy decreases. But, the a heat flux profile based on the convolution of an exponential ra- locked mode appears in the plasma immediately after the RMPs dial heat flux with Gaussian spreading in the divertor. The divertor configuration is changed at 11.0 s. From 11.1 s, the divertor IRTV IRTV measurement has also suggested that the divertor heat flux was not able to measure the heat flux profile since the strike point profile can be significantly modified and strike point splitting pat- was located outside the target tile. So, the measurement of the tern can be actively controlled with the RMP fields. triple-peaked heat flux profile was transiently available. Actually, This work was supported by MSIP, the Korean Ministry of Sci- this striation pattern can be not only due to the configuration of ence, ICT and Future Planning, through the KSTAR project. The au- RMPs but also due to the magnetic field topology changed by the thors would like to thank Dr. A. Herrmann for his helpful discus-

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Please cite this article as: H.H. Lee et al., Thermographic studies of outer target heat ﬂuxes on KSTAR, Nuclear Materials and Energy (2016), http://dx.doi.org/10.1016/j.nme.2016.12.019