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Study on divertor heat flux of L-H transition with LHCD and NBI in EAST
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Study on divertor heat flux of L-H transition with LHCD and NBI in EAST Bo Shi a,b,c,∗ , Xianzu Gong a , Weihua Wang a,c,∗ , Bin Zhang a , Zhendong Yang a , Jinhong Yang c , Kaifu Gan a , Rongfei Wang c , Junli Qi c , Hui Zhang c , Lianqing Zhang c a
Institute of Plasma Physics, Hefei Institutes of Physical Sciences, Chinese Academy of Sciences, Hefei 230031, China Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230031, China c Institute of Applied Physics, Army Officer Academy, Hefei 230031, China b
h i g h l i g h t s • It is the first time we studied the peak heat fluxes on the lower outer divertor and heat flux profile during L-H transitions in EAST. • A big change of the heat profile after L-H transition has been observed under LHCD heating scheme. • The results consist with that (neutral beam)NB-heated plasma has a narrower SOL than (radio-frequency) RF-heated plasma in EAST.
a r t i c l e
i n f o
Article history: Received 28 September 2016 Received in revised form 24 April 2017 Accepted 25 April 2017 Available online xxx Keywords: EAST LHCD NBI Heat flux L-H transition
a b s t r a c t An infrared (IR)/visible endoscope system was built on the Experimental Advanced Superconducting Tokamak (EAST) in 2014. The temperature distributions of the lower divertor have been measured. Based on the IR data of EAST, the heat fluxes on the lower outer divertor were calculated with a code named DFLUX developed by ASIPP, aimed to research the heat fluxes on divertor under different auxiliary heating schemes. Peak heat fluxes of the lower outer divertor show a reduction of several hundred kilowatts during L-H transition and an increase of nearly 1 MW/m2 during H-L transition, corresponding with the D␣ signal, electron density and plasma stored energy under LHCD heating scheme(PLHCD ∼ 1.9 MW). The heat flux profile of lower outer divertor was broad before the first L-H transition and narrowed rapidly after L-H transition but without obvious change during the next L-H transitions. Narrower heat flux profile and higher heat flux than LHCD heating scheme were induced by NBI heating scheme (0.9 MW at first and 1.7 MW before L-H transition), with the heat flux profile narrowed after L-H transition. The study results can afford reference for the optimization of H-mode plasma discharge experiments on EAST. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The discovery of H-mode brought great encouragement to the research of tokamak and much effort has been made on the research of L-H transition. The following parameter changes are observed experimentally [1]: – increase in the radial electric field and its shear; – reduction in the intensities of electrostatic and electromagnetic fluctuations;
∗ Corresponding authors at: Institute of Plasma Physics, Hefei Institutes of Physical Sciences, Chinese Academy of Sciences, Hefei 230031, China. E-mail addresses: shibo1982 [email protected] (B. Shi), [email protected] (W. Wang).
– reduction in the intensities of H␣ /D␣ radiation at plasma boundary or divertor; – increase in the plasma stored energy and electron line averaged density; – formation of an improved confinement zone at the plasma periphery, resulting in electron pressure gradient rise; – reduction in the heat load on divertor target plate, and so on. The electric field bifurcation model has been adopted to explain the L-H transition throughout the theoretical and experimental studies [2–7]. However, more research is needed for understanding of the L-H transition phenomena and physics mechanism. At the same time, the statistical and stochastic features of L-H transition must be clarified [8]. The power threshold Pth changes with the divertor or limiter configuration, the deuterium or hydrogen plasmas, the single-null plasmas or double-null plasmas, etc. After the
http://dx.doi.org/10.1016/j.fusengdes.2017.04.110 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: B. Shi, et al., Study on divertor heat flux of L-H transition with LHCD and NBI in EAST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.110
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Fig. 2. Temperature on lower outer divertor and the chosen line in radial direction.
behavior of the unknown deposited layer and dlayer is the layer thickness [12]. The heat fluxes were calculated using [13] Fig. 1. The observation area of IR/visible system.
auxiliary heating power has been injected and before the plasma transits into the H-mode, there is a dwell-time, which depends on the auxiliary heating power and the magnetic configuration [9]. Study of the detailed characteristics of the L-H transition can afford reference for the optimization of H-mode plasma discharge. Reduction in the heat load on divertor target plate is one of the characteristics of L-H transition corresponding to the fluctuation suppression and transport reduction across the separatrix. EAST is a superconducting tokamak with the poloidal field control system to accommodate both single null (SN) and double null (DN) divertor configurations. It achieved its first H-mode plasma with type-III edge localized modes (ELMs) in 2010 [10,11]. Researches on EAST demonstrated that L-H transitions are more easily obtained with Lower Single Null (LSN) configuration with lithium coated wall. It also has been showed that the H-mode threshold power does not depend on the detailed heating scheme [6]. Results of this study, however, show that the heat flux characteristics on lower outer divertor before and after L-H transitions differ with heating schemes. In Section 2 of this paper, the IR/visible endoscope system and calculation method are introduced. The results and discussions are presented in Section 3. The changes of heat fluxes on the lower outer divertor during L-H/H-L transition have been observed and the heat flux profiles were compared under three auxiliary heating schemes. The summary is given in Section 4.
2. The measurement and calculation method of heat flux An IR/visible endoscope system was built on EAST to monitor the lower divertor, the upper divertor and the limiter between P and N windows in 2014, which can acquire the IR and visible data simultaneously. The used IR camera is FlirSC7700BB (2.5 ∼ 5.4 m IR range) with a frame rate from 115 Hz (640 × 512 pixels) to 2.9 kHz (132 × 3 pixels). The spatial resolution of the camera is 4 mm on the divertor plate. The observation area of IR/visible system on EAST is shown in Fig. 1. The temperature distribution of the lower outer divertor was measured by the IR camera, and then the heat flux distribution were calculated with a 2D finite element analysis code named DFLUX, which provides the flexibility to specify the heat transmission coefficient for a loosely bound plasma-facing layer. The heat transmission coefficient which is defined as ␣= klayer /dlayer is used as a boundary condition when calculating with the measured surface temperature, where klayer represents the heat conduction
q = ˛(Tsurf − Tbulk ) where, Tsurf is the surface temperature of divertor target and Tbulk is the bulk temperature. The heat fluxes were two-dimensional results calculated by taking a line in the radial direction on divertor from the IR data as shown in Fig. 2, and toroidal symmetry is assumed. In Fig. 2, r represents the radial direction and represents the toroidal direction. 3. Results and discussions The analyzed discharges were LSN divertor configuration Hmode discharges. The heat fluxes of the lower outer divertor with LHCD or the NBI only as well as with LHCD combined with NBI have been calculated. The results and discussions are presented as follows. 3.1. Peak heat fluxes of the lower outer divertor during L-H/H-L transition Fig. 3 shows (a) the plasma current, (b) plasma stored energy, (c) plasma density, (d) lower divertor D␣ signal, (e) dRsep = Rsep,L –Rsep,U (Rsep,L and Rsep,U are the lower and upper separatrix radii mapped to the low-field-side mid-plane), (f) auxiliary power (LHCD only) and the calculated peak heat flux on lower outer divertor, and (g) the magnification of the peak heat flux of discharge 49722. The dash lines in Fig. 3(b) give the time of L-H transition. It can be seen that there were three times of L-H transition accompanied by the increase of the plasma stored energy, plasma density and the reduction of D␣ radiation on lower outer divertor. After the injection of Low Hybrid Wave(LHW ∼ 1.9 MW), the peak heat flux of lower outer divertor increased rapidly to more than 2.5 MW/m2 and then reduced rapidly. It is regret that the reason is not clear. L-H transition occurred at 2.86 s, and the peak heat flux had a decline during L-H transition, which was the result of the built of the barrier. ELMfree appeared after L-H transition with the heat flux not more than 1 MW/m2 . At about 4.16 s, H-L transition happened accompanied by a sharp increase of the peak heat flux of nearly 1 MW/m2 . The second and third L-H transition occurred at 5.40 s and 7.06 s respectively, also accompanied by several hundred kilowatts decline of the heat fluxes on lower outer divertor. 3.2. Heat flux profiles during L-H transition under different auxiliary heating schemes Fig. 4 shows (a) the temporal and spatial distribution of heat flux on lower outer divertor, and (b) heat flux profiles of lower outer divertor versus radius of discharge 49722. The first L-H transition
Please cite this article in press as: B. Shi, et al., Study on divertor heat flux of L-H transition with LHCD and NBI in EAST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.110
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occurred at 2.86 s, and the heat flux profile of lower outer divertor was broad before L-H transition. In the next 0.1 s, the heat flux profile narrowed rapidly, which meant the SOL narrowed rapidly. The flux profile at 2.96 s continues to negative values relates to the inevitable errors in the calculation. In the next two L-H transitions, there were no obvious change of the heat flux profiles. Fig. 5 shows (a) temporal and spatial distribution of heat flux on lower outer divertor, (b) heat flux profiles of lower outer divertor versus radius of discharge 51015. The major parameters of discharge 51015 are: the plasma current Ip ∼ 400 kA, the line-averaged electron density ne ∼ (3–5) × 1019 m −3 , dRsep = −2.2 cm and the toroidal field Bt ∼2.3 T. The auxiliary power of discharge 51015 was NBI only, with 0.9 MW of left source and 0.8 MW of right source. The left source power injection began at about 1 s and then the right source began at about 3.5 s. The heat flux profile was narrow before L-H transition, as (neutral beam) NB-heated plasma has a narrower SOL than (radio-frequency) RF-heated plasma under this condition. Meanwhile, it can be seen that the peak heat flux was located on the inner edge of the outer target plate. L-H transition occurred
at about 3.69 s with the full width at half maximum (FWHM) was 1.36 cm. In the next 0.1 s, the heat flux profile narrowed and the FWHM reduced to 0.78 cm. Fig. 6 shows the heat flux profiles of lower outer divertor versus radius at the beginning of L-H transition and 0.05 s later. Black line and black line plus symbol are under LHCD heating scheme, the first and the second L-H transition respectively, with PLHCD ∼ 1.9 MW. Red line is under NBI heating scheme as Fig. 5 shown. Blue lines are under LHCD combined with NBI heating scheme. About 1.1 MW power of LHCD was injected at about 1 s, without a rapid increase of the heat flux and a broad profile as discharge 49722. Then about 1 MW power of NBI was injected and lead to L-H transition. From Fig. 6, it can be seen that NBI induced narrower heat flux profile and higher heat flux than LHCD, with the position of the peak heat flux in NBI H-mode closer to the inner edge of the outer target plate than LHCD H-mode and LHCD + NBI H-mode. By comparing, the heat flux profile of the first L-H transition under LHCD heating scheme narrowed rapidly, but small change of others.
Please cite this article in press as: B. Shi, et al., Study on divertor heat flux of L-H transition with LHCD and NBI in EAST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.110
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signal, electron density and plasma stored energy under LHCD heating scheme (PLHCD ∼ 1.9 MW). The heat flux profile of lower outer divertor was broad before L-H transition and narrowed rapidly after L-H transition when ELM-free appeared. But in the next L-H transitions, the changes of heat flux profiles were not obvious. In case of the auxiliary power with NBI only (0.9 MW at first and 1.7 MW before L-H transition), narrower heat flux profile and higher heat flux than LHCD heating scheme were induced. The peak heat flux was located on the inner edge of the outer target plate. The heat flux profile narrowed after L-H transition. In case of LHCD combined with NBI (PLHCD ∼ 0.7 MW at first and PLHCD ∼ 0.7 MW + PNBI ∼ 1 MW before L-H transition), heat fluxes induced by LHCD before L-H transition were relatively small and so did the change of the heat flux profiles before and after L-H transition. Fig. 5. Heat flux calculation results of discharge 51015. (a) Temporal and spatial distribution of heat flux on lower outer divertor, (b) Heat flux profiles of lower outer divertor versus radius.
4. Summary The peak heat fluxes on lower outer divertor during L-H transition and heat flux profiles before and after L-H transition were researched through the IR/visible endoscope system. The change of heat flux width on lower outer divertor can reflect the change of the scrape-off layer (SOL) width during L-H transition. Peak heat fluxes of the lower outer divertor show a reduction of several hundred kilowatts during L-H transition and an increase of nearly 1 MW/m2 during H-L transition, corresponding with the D␣
Acknowledgements This research is supported by the National Natural Science Foundation of China (Nos. 11505290, 51576208), and National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB113004, 2015GB102004). References [1] [2] [3] [4] [5]
I.A. Vojtsekhovich, et al., Nucl. Fusion 35 (6) (1995) 631–640. T.E. Stringer, Nucl. Fusion 33 (9) (1993) 1249–1265. K.C. Shaing, E.C. Crume Jr., Phys. Rev. Lett. 63 (21) (1989) 2369–2372. P.H. Diamond, Y.-M. Liang, Phys. Rev. Lett. 72 (16) (1994) 2565–2568. Eun-jin Kim, et al., Phys. Rev. Lett. 90 (2003) 185006.
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[6] [7] [8] [9] [10]
G.S. Xu, et al., Nucl. Fusion 54 (2014) 103002. T. Kobayashi, et al., Nucl. Fusion 55 (2015) 063009. S.-I. Itoh, K. Itoh, S. Toda, Plasma Phys. Control. Fusion 45 (2003) 823–840. F. Wagner, Plasma Phys. Control. Fusion 49 (2007) B1–B33. Baonian Wan, et al., Nucl. Fusion 53 (2013) 104006.
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[11] Liu Peng, et al., Plasma Sci. Technol. 15 (7) (2013) 619–622. [12] K.F. Gan, et al., J. Nucl. Mater. 438 (2013) S364–S367. [13] T. Eich, P. Andrew, A. Herrmann, et al., Plasma Phys. Control. Fusion 49 (2007) 573–604.
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Study on divertor heat flux of L-H transition with LHCD and NBI in EAST Bo Shi a,b,c,∗ , Xianzu Gong a , Weihua Wang a,c,∗ , Bin Zhang a , Zhendong Yang a , Jinhong Yang c , Kaifu Gan a , Rongfei Wang c , Junli Qi c , Hui Zhang c , Lianqing Zhang c a
Institute of Plasma Physics, Hefei Institutes of Physical Sciences, Chinese Academy of Sciences, Hefei 230031, China Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230031, China c Institute of Applied Physics, Army Officer Academy, Hefei 230031, China b
h i g h l i g h t s • It is the first time we studied the peak heat fluxes on the lower outer divertor and heat flux profile during L-H transitions in EAST. • A big change of the heat profile after L-H transition has been observed under LHCD heating scheme. • The results consist with that (neutral beam)NB-heated plasma has a narrower SOL than (radio-frequency) RF-heated plasma in EAST.
a r t i c l e
i n f o
Article history: Received 28 September 2016 Received in revised form 24 April 2017 Accepted 25 April 2017 Available online xxx Keywords: EAST LHCD NBI Heat flux L-H transition
a b s t r a c t An infrared (IR)/visible endoscope system was built on the Experimental Advanced Superconducting Tokamak (EAST) in 2014. The temperature distributions of the lower divertor have been measured. Based on the IR data of EAST, the heat fluxes on the lower outer divertor were calculated with a code named DFLUX developed by ASIPP, aimed to research the heat fluxes on divertor under different auxiliary heating schemes. Peak heat fluxes of the lower outer divertor show a reduction of several hundred kilowatts during L-H transition and an increase of nearly 1 MW/m2 during H-L transition, corresponding with the D␣ signal, electron density and plasma stored energy under LHCD heating scheme(PLHCD ∼ 1.9 MW). The heat flux profile of lower outer divertor was broad before the first L-H transition and narrowed rapidly after L-H transition but without obvious change during the next L-H transitions. Narrower heat flux profile and higher heat flux than LHCD heating scheme were induced by NBI heating scheme (0.9 MW at first and 1.7 MW before L-H transition), with the heat flux profile narrowed after L-H transition. The study results can afford reference for the optimization of H-mode plasma discharge experiments on EAST. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The discovery of H-mode brought great encouragement to the research of tokamak and much effort has been made on the research of L-H transition. The following parameter changes are observed experimentally [1]: – increase in the radial electric field and its shear; – reduction in the intensities of electrostatic and electromagnetic fluctuations;
∗ Corresponding authors at: Institute of Plasma Physics, Hefei Institutes of Physical Sciences, Chinese Academy of Sciences, Hefei 230031, China. E-mail addresses: shibo1982 [email protected] (B. Shi), [email protected] (W. Wang).
– reduction in the intensities of H␣ /D␣ radiation at plasma boundary or divertor; – increase in the plasma stored energy and electron line averaged density; – formation of an improved confinement zone at the plasma periphery, resulting in electron pressure gradient rise; – reduction in the heat load on divertor target plate, and so on. The electric field bifurcation model has been adopted to explain the L-H transition throughout the theoretical and experimental studies [2–7]. However, more research is needed for understanding of the L-H transition phenomena and physics mechanism. At the same time, the statistical and stochastic features of L-H transition must be clarified [8]. The power threshold Pth changes with the divertor or limiter configuration, the deuterium or hydrogen plasmas, the single-null plasmas or double-null plasmas, etc. After the
http://dx.doi.org/10.1016/j.fusengdes.2017.04.110 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: B. Shi, et al., Study on divertor heat flux of L-H transition with LHCD and NBI in EAST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.110
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Fig. 2. Temperature on lower outer divertor and the chosen line in radial direction.
behavior of the unknown deposited layer and dlayer is the layer thickness [12]. The heat fluxes were calculated using [13] Fig. 1. The observation area of IR/visible system.
auxiliary heating power has been injected and before the plasma transits into the H-mode, there is a dwell-time, which depends on the auxiliary heating power and the magnetic configuration [9]. Study of the detailed characteristics of the L-H transition can afford reference for the optimization of H-mode plasma discharge. Reduction in the heat load on divertor target plate is one of the characteristics of L-H transition corresponding to the fluctuation suppression and transport reduction across the separatrix. EAST is a superconducting tokamak with the poloidal field control system to accommodate both single null (SN) and double null (DN) divertor configurations. It achieved its first H-mode plasma with type-III edge localized modes (ELMs) in 2010 [10,11]. Researches on EAST demonstrated that L-H transitions are more easily obtained with Lower Single Null (LSN) configuration with lithium coated wall. It also has been showed that the H-mode threshold power does not depend on the detailed heating scheme [6]. Results of this study, however, show that the heat flux characteristics on lower outer divertor before and after L-H transitions differ with heating schemes. In Section 2 of this paper, the IR/visible endoscope system and calculation method are introduced. The results and discussions are presented in Section 3. The changes of heat fluxes on the lower outer divertor during L-H/H-L transition have been observed and the heat flux profiles were compared under three auxiliary heating schemes. The summary is given in Section 4.
2. The measurement and calculation method of heat flux An IR/visible endoscope system was built on EAST to monitor the lower divertor, the upper divertor and the limiter between P and N windows in 2014, which can acquire the IR and visible data simultaneously. The used IR camera is FlirSC7700BB (2.5 ∼ 5.4 m IR range) with a frame rate from 115 Hz (640 × 512 pixels) to 2.9 kHz (132 × 3 pixels). The spatial resolution of the camera is 4 mm on the divertor plate. The observation area of IR/visible system on EAST is shown in Fig. 1. The temperature distribution of the lower outer divertor was measured by the IR camera, and then the heat flux distribution were calculated with a 2D finite element analysis code named DFLUX, which provides the flexibility to specify the heat transmission coefficient for a loosely bound plasma-facing layer. The heat transmission coefficient which is defined as ␣= klayer /dlayer is used as a boundary condition when calculating with the measured surface temperature, where klayer represents the heat conduction
q = ˛(Tsurf − Tbulk ) where, Tsurf is the surface temperature of divertor target and Tbulk is the bulk temperature. The heat fluxes were two-dimensional results calculated by taking a line in the radial direction on divertor from the IR data as shown in Fig. 2, and toroidal symmetry is assumed. In Fig. 2, r represents the radial direction and represents the toroidal direction. 3. Results and discussions The analyzed discharges were LSN divertor configuration Hmode discharges. The heat fluxes of the lower outer divertor with LHCD or the NBI only as well as with LHCD combined with NBI have been calculated. The results and discussions are presented as follows. 3.1. Peak heat fluxes of the lower outer divertor during L-H/H-L transition Fig. 3 shows (a) the plasma current, (b) plasma stored energy, (c) plasma density, (d) lower divertor D␣ signal, (e) dRsep = Rsep,L –Rsep,U (Rsep,L and Rsep,U are the lower and upper separatrix radii mapped to the low-field-side mid-plane), (f) auxiliary power (LHCD only) and the calculated peak heat flux on lower outer divertor, and (g) the magnification of the peak heat flux of discharge 49722. The dash lines in Fig. 3(b) give the time of L-H transition. It can be seen that there were three times of L-H transition accompanied by the increase of the plasma stored energy, plasma density and the reduction of D␣ radiation on lower outer divertor. After the injection of Low Hybrid Wave(LHW ∼ 1.9 MW), the peak heat flux of lower outer divertor increased rapidly to more than 2.5 MW/m2 and then reduced rapidly. It is regret that the reason is not clear. L-H transition occurred at 2.86 s, and the peak heat flux had a decline during L-H transition, which was the result of the built of the barrier. ELMfree appeared after L-H transition with the heat flux not more than 1 MW/m2 . At about 4.16 s, H-L transition happened accompanied by a sharp increase of the peak heat flux of nearly 1 MW/m2 . The second and third L-H transition occurred at 5.40 s and 7.06 s respectively, also accompanied by several hundred kilowatts decline of the heat fluxes on lower outer divertor. 3.2. Heat flux profiles during L-H transition under different auxiliary heating schemes Fig. 4 shows (a) the temporal and spatial distribution of heat flux on lower outer divertor, and (b) heat flux profiles of lower outer divertor versus radius of discharge 49722. The first L-H transition
Please cite this article in press as: B. Shi, et al., Study on divertor heat flux of L-H transition with LHCD and NBI in EAST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.110
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Time (s) Fig 3. The major parameters and peak heat flux on lower outer divertor of discharge 49722. (a) Plasma current, (b) Plasma stored energy, (c) Plasma density, (d) Lower divertor D␣ signal, (e) dRsep , (f) Injected power of LHCD (black line) and the calculated peak heat flux on lower outer divertor (red line), and (g) the magnification of (f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
occurred at 2.86 s, and the heat flux profile of lower outer divertor was broad before L-H transition. In the next 0.1 s, the heat flux profile narrowed rapidly, which meant the SOL narrowed rapidly. The flux profile at 2.96 s continues to negative values relates to the inevitable errors in the calculation. In the next two L-H transitions, there were no obvious change of the heat flux profiles. Fig. 5 shows (a) temporal and spatial distribution of heat flux on lower outer divertor, (b) heat flux profiles of lower outer divertor versus radius of discharge 51015. The major parameters of discharge 51015 are: the plasma current Ip ∼ 400 kA, the line-averaged electron density ne ∼ (3–5) × 1019 m −3 , dRsep = −2.2 cm and the toroidal field Bt ∼2.3 T. The auxiliary power of discharge 51015 was NBI only, with 0.9 MW of left source and 0.8 MW of right source. The left source power injection began at about 1 s and then the right source began at about 3.5 s. The heat flux profile was narrow before L-H transition, as (neutral beam) NB-heated plasma has a narrower SOL than (radio-frequency) RF-heated plasma under this condition. Meanwhile, it can be seen that the peak heat flux was located on the inner edge of the outer target plate. L-H transition occurred
at about 3.69 s with the full width at half maximum (FWHM) was 1.36 cm. In the next 0.1 s, the heat flux profile narrowed and the FWHM reduced to 0.78 cm. Fig. 6 shows the heat flux profiles of lower outer divertor versus radius at the beginning of L-H transition and 0.05 s later. Black line and black line plus symbol are under LHCD heating scheme, the first and the second L-H transition respectively, with PLHCD ∼ 1.9 MW. Red line is under NBI heating scheme as Fig. 5 shown. Blue lines are under LHCD combined with NBI heating scheme. About 1.1 MW power of LHCD was injected at about 1 s, without a rapid increase of the heat flux and a broad profile as discharge 49722. Then about 1 MW power of NBI was injected and lead to L-H transition. From Fig. 6, it can be seen that NBI induced narrower heat flux profile and higher heat flux than LHCD, with the position of the peak heat flux in NBI H-mode closer to the inner edge of the outer target plate than LHCD H-mode and LHCD + NBI H-mode. By comparing, the heat flux profile of the first L-H transition under LHCD heating scheme narrowed rapidly, but small change of others.
Please cite this article in press as: B. Shi, et al., Study on divertor heat flux of L-H transition with LHCD and NBI in EAST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.110
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Fig. 4. Heat flux calculation results of discharge 49722. (a) Temporal and spatial distribution of heat flux on lower outer divertor, (b) Heat flux profiles of lower outer divertor versus radius.
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signal, electron density and plasma stored energy under LHCD heating scheme (PLHCD ∼ 1.9 MW). The heat flux profile of lower outer divertor was broad before L-H transition and narrowed rapidly after L-H transition when ELM-free appeared. But in the next L-H transitions, the changes of heat flux profiles were not obvious. In case of the auxiliary power with NBI only (0.9 MW at first and 1.7 MW before L-H transition), narrower heat flux profile and higher heat flux than LHCD heating scheme were induced. The peak heat flux was located on the inner edge of the outer target plate. The heat flux profile narrowed after L-H transition. In case of LHCD combined with NBI (PLHCD ∼ 0.7 MW at first and PLHCD ∼ 0.7 MW + PNBI ∼ 1 MW before L-H transition), heat fluxes induced by LHCD before L-H transition were relatively small and so did the change of the heat flux profiles before and after L-H transition. Fig. 5. Heat flux calculation results of discharge 51015. (a) Temporal and spatial distribution of heat flux on lower outer divertor, (b) Heat flux profiles of lower outer divertor versus radius.
4. Summary The peak heat fluxes on lower outer divertor during L-H transition and heat flux profiles before and after L-H transition were researched through the IR/visible endoscope system. The change of heat flux width on lower outer divertor can reflect the change of the scrape-off layer (SOL) width during L-H transition. Peak heat fluxes of the lower outer divertor show a reduction of several hundred kilowatts during L-H transition and an increase of nearly 1 MW/m2 during H-L transition, corresponding with the D␣
Acknowledgements This research is supported by the National Natural Science Foundation of China (Nos. 11505290, 51576208), and National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB113004, 2015GB102004). References [1] [2] [3] [4] [5]
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Please cite this article in press as: B. Shi, et al., Study on divertor heat flux of L-H transition with LHCD and NBI in EAST, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.04.110