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Deuterium pellet fueling in type-III ELMy H-mode plasmas on EAST superconducting tokamak
Fusion Engineering and Design 145 (2019) 79–86
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Deuterium pellet fueling in type-III ELMy H-mode plasmas on EAST superconducting tokamak ⁎
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Jilei Houa,b, Jiansheng Hua,c, , Yue Chena, , Yumin Wanga, Qing Zanga, Jichan Xua, Haiqing Liua, Kevin Tritzd, Erik Gilsone, Xiaolin Yuana, Zhen Suna,e, Rajesh Maingie, Hailin Zhaoa, Jiangang Lia a
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China University of Science and Technology of China, Hefei, 230029, China c CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Hefei, 230031, China d Johns Hopkins University, Baltimore, MD, 21211, USA e Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ, 08543, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fueling Pellet injection Type-III ELM EAST
Type-III ELMy H-mode plasmas are a valuable operating scenario, as they may simultaneously achieve fusion power gain of Q = 10 and acceptable steady-state and peak power fluxes to the plasma-facing components. It is important to investigate cryogenic deuterium pellet injection fueling in this kind of H-mode plasma since it has been chosen as a main plasma fueling technique for ITER. Recently, pellet fueling experiments have been successfully carried out on EAST, an ITER-like tokamak. A maximum electron density of 0.73 × nGW has been obtained. It is found that ELM behavior was modified immediately and a string of small ELMs were induced when pellets were injected at a low velocity from ˜40 cm above the mid-plane on the low field side, which is different from the fact that pellets trigger signal ELMs in Type-I ELMy H-mode plasmas in other tokamaks. The decreased edge temperature caused by the cryogenic pellet was proposed as the reason for this ELM modification. These results will provide a useful reference for the pellet fueling in a Type-III ELMy H-mode plasma on ITER in the future.
1. Introduction The H-mode, a high confinement operating regime, is characterized by a steep pressure gradient and “pedestal” at the plasma edge, which lead to strong self-driven plasma currents that, in turn, result in an instability known as edge-localized modes (ELMs) [1]. ELMs can cause periodic particle and heat losses from the plasma edge, thereby introducing efficient particle density and impurity control. Several types of ELMs with different characteristics have been identified in various experiments [2]. Based on the ELM frequency dependence on the auxiliary heating power, ELMs were classified into three types. Type I ELMs, of which the repetition frequency fELM increases with heating power, appear as isolated sharp bursts on the Dα signal without a detectable magnetic precursor oscillation. For Type II ELMs, no information on the power dependence of fELM or the MHD precursor activity exists. The repetition frequency of Type III decreases with heating power and a coherent magnetic precursor oscillation is observed on magnetic probes.
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Future fusion research machines, such as ITER, are planned to operate in H-mode, which is the most promising mode for producing fusion energy using the tokamak configuration [3]. One of the most severe problems for fusion reactors is the power load on the plasma facing components from power exhausted by ELMs accompanying the Hmode. Only loads of less than 10 MW/m2 in steady state, and less than 0.5 MJ/m2 during transients caused by ELMs, are acceptable [4]. In order to reduce the power flux to the plasma-facing components in the divertor to manageable levels, the peak heat flux has to be dissipated. A possible solution is the Type-III ELMy H-mode since the transient heat loads due to Type-III ELMs are acceptable with even the most stringent boundary conditions [5]. However, stationary operation with Type-III ELMs is often found to impose an upper limit on the pedestal temperature that is lower than the values reached by Type-I ELMy Hmodes, thereby reducing projected plasma performance [6]. Although Type-III ELMy H-mode plasma operation has some shortcomings, TypeIII ELMy H-mode regime has been extended to higher levels of additional heating by increasing the edge density and/or by increasing the
Corresponding author at: Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China. Corresponding author. E-mail addresses: [email protected] (J. Hu), [email protected] (Y. Chen).
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https://doi.org/10.1016/j.fusengdes.2019.05.038 Received 29 March 2019; Received in revised form 9 May 2019; Accepted 25 May 2019 Available online 03 June 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic drawing of the 10 Hz pellet injection system installed on the EAST. The plasma is operated in upper single null divertor configuration. Only the transition tube in LFS was used in these experiments. The vertical position for POINT edge channel is Z = 42.5 cm and the distance is 8.5 cm between two adjacent channels. The TS line of sight is at R = 1.9 m and the vertical position for RF is Z = 3 cm.
electron cyclotron resonance heating (ECRH) [14].
radiated power with injection of extrinsic impurities. Injection of impurities allows achieving the Type-III ELMy H-mode regime at lower collisionalities compared to increasing the edge density, and the extrapolated ELM size in these conditions are suitable for meeting the requirements for ITER Q = 10 operation [7]. Cryogenic deuterium pellet injection with a well-known high fueling efficiency has been chosen as the main plasma fueling technique for ITER [8]. It is important to carry out pellet fueling experiment in Hmode plasmas in ITER-like tokamak. Although the ITER scenario is based on the Type-I ELMy H-mode regime [9], which has a very broad experimental and theoretical basis, investigation of Type-III ELMy Hmode plasmas fueled by pellet is still valuable as a possible scenario for ITER Q = 10 operation. EAST is an ITER-like tokamak with a flexible configuration and a dedicated pellet injector was successfully developed a few years ago. These have provided a good opportunity to do pellet fueling experiments in all kinds of H-mode plasmas in ITER-like tokamak. Recently, pellet fueling experiment in Type-III ELMy H-mode plasmas has been successfully carried out on EAST and numerous interesting results were obtained. In this paper, we will mainly focus on introducing the experimental results of pellet fueling and its effect on Type-III ELM behavior. The set-up used in the experiments will be described in Section 2. In Section 3, the results of pellet fueling and the modification of Type-III ELM behavior induced by cryogenic pellet injection will be presented. Finally, we will give a discussion and summary of this paper.
2.2. Diagnostics A variety of high-performance diagnostics have been developed and deployed to measure the key parameters of the plasma. The main diagnostics associated with this work are as following. A reflectometer (RF) with sampling frequency ˜64 MHz, located in the outer mid-plane, was used to measure electron density profiles [15–17]. A Thomson scattering (TS) system [18] was used to measure the profile of electron density and temperature with a temporal resolution of 200 ms. Heat and particle fluxes in the divertor region were acquired by triple Langmuir probes arrays, which are embedded in the upper and lower divertor regions [19]. An 11-channel Polarimeter-Interferometer (POINT) with a temporal resolution of 1 ms, which provided core and edge electron densities [20], was also used during the experiment. Moreover, routine diagnostics such as multi-channel extreme ultraviolet (XUV) arrays, Mirnov probes, and electron cyclotron emission (ECE) were also used. 2.3. Pellet injector At present, a pellet injector with an injection frequency of 10 Hz, which is based on screw extrusion forming ice and pneumatic acceleration, has been developed for plasma fueling on EAST. The system consists of the injector, vacuum system, and pellet transition system. The schematic drawing of the pellet injector, located at Port J of EAST, is shown in Fig. 1. Helium is used for propellant gas and the two-stage differential chamber system is used to exhaust pneumatic gas. When the pellets are injected, the vacuum degree in second differential chamber close to EAST vessel is about 10−2–10−3 Pa and little accelerating gas can fly into plasma. It can inject Φ2 × 2 mm pellets with nominal number of atoms ˜3.8 × 1020 at velocity 100–300 m/s at a maximum frequency of 10 Hz with reliability of over 95% for more than 1000s from either the low field side (LFS) or the high field side (HFS) [21]. The lengths of the transition tubes are ˜10 m and ˜12 m for the LFS and HFS respectively. Pellet velocity is changing during the flight through the pellet-guide. When pellet is transferred in guide tube, the friction and collision between tube and pellet may result in a disintegrated pellet and decreased velocity. In the present experiment, pellets were injected into Type-III ELMy H-mode plasmas from 40 cm above midplane at the LFS with a low average velocity of ˜100 m/s. Generally, there is a considerable number of pellet particles lost (ablated or broken) after being transported for a long-distance. Even though many pellets have been injected with the 10 Hz system in past experiments, the exact number of deuterium atoms that were injected into the EAST tokamak per pellet has never been properly calibrated.
2. Set-up 2.1. Experimental device The EAST tokamak, which was built to demonstrate high power and long pulse operations under fusion-relevant conditions, is a non-circular full superconducting tokamak with major radius R = 1.85 m and minor radius a = 0.45 [10]. The machine can be operated in lower single null (LSN), double null (DN) and upper single null (USN) divertor configurations, which can be flexibly switched during a long-pulse discharge [11]. To improve machine capabilities and power or particle exhaust toward advanced steady-state high-performance operations, the internal plasma facing components (PFCs) were changed from carbon tiles to molybdenum tiles in 2012, except for those near the strike points [12]. In 2013, the upper divertor was replaced by 80 tungsten divertor modules whose design was based on cassette and mono-block technology such as that planned for ITER [13]. The lower graphite divertor will be also replaced with tungsten divertor modules in 2019. The EAST auxiliary heating systems include lower hybrid current drive (LHCD) at both 2.45 GHz and 4.6 GHz, ion cyclotron resonance heating (ICRH), neutral beam injection (NBI, 2 systems, 4 ion sources), and 80
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injection. A modest reduction in the diamagnetic stored energy Wdia was also observed. Occasionally, a transition from H-mode to L-mode (H-L), indicated by vertical black dash lines in Fig. 2, was induced because of plasma temperature over-cooling after pellet injection, which is characterized by an abrupt decay of plasma stored energy and density. In addition, the divertor probe signal measuring the exhausted particles to the divertor also had an obvious change after pellet injections. The fueling efficiency η for each pellet, defined as the total increase in number of plasma electrons divided by the number of input fuel atoms, can be roughly estimated from the equation:
On TUMAN-3M tokamak a method with Dα detectors looking at the pellets has been applied for calibration of deposited particles number [22]. Because these diagnostic devices are lacked for pellet deposited particles during discharges, we roughly calibrated the number of injected particles by measuring changes in the vacuum pressure of EAST before and after pellet injection without a plasma discharge prior to the present experiment. After multiple calibration injections, the number of atoms injected into the vacuum chamber was determined to be Npel ˜ 1.03 × 1020 per pellet. Since these injections were done without plasma, the calibration only measured Npel into the chamber. The number that would get into the plasma might be lower due to ablation in the scrape-off layer (SOL) when a plasma is present.
η= 3. Results
Δ (ne l) V /2a Npel
(1)
Here, Δ(nel) is the increment of core line integrated density, V = 10.5 m3 is the plasma volume, and the number of pellet fuel atoms is replaced by Npel. The estimated η may be less than the actual value due to ablation in SOL.The time difference Δt for measuring the Δ(nel) is 1.4–1.8 ms. Fig. 2(g) gives the fueling efficiency calculated from the Eq. (1) for each pellet injection. It can be seen that the fueling efficiency η is 16–30% and 8–13%, respectively during H-mode and L-mode phases. This is different from the general perception that the pellet fueling efficiency in H-mode plasmas is lower than in L-mode [23]. This result is presumably because of some effects of the fueling processes (ablation or plasmoid drift). The pellet injection depth during pellet ablation pulse is about ˜15 cm from the last closed flux surface (LCFS). The pellet injection occasionally induced confinement degradation due to temperature cooling at the edge. As can be seen in Fig. 3(a)–(d), the first pellet caused a dramatic degradation in plasma confinement displaying a decline in the stored energy Wdia, almost taking the plasma into L-mode. The confinement changed to H-mode immediately with a nearly constant electron density after several density fluctuations sustaining for τdec ˜21 ms. The significant temperature cooling is likely responsible for the confinement degradation. About Δt1˜72 ms later, the second pellet reached the plasma. Unlike the first pellet, the second pellet did not cause an obvious decline in Wdia. The core density nel remained constant for τ2 ˜ 93 ms after injection, even though the edge density nel had a modest decline. Both the core and edge density nel started to exhibit a decreasing trend at 5.874 s. Meanwhile, the Mirnov
On EAST, H-mode plasmas are often obtained with lower hybrid wave power in conjunction with wall conditioning by lithium (Li). Before the deuterium pellet injection experiments, multiple lithium granule injections were performed to improve wall conditions. The Type-III ELMy H-mode plasmas were easy to obtain. These target discharges were operated in an upper-single-null configuration, as shown in Fig. 1, with LHCD (PLHCD ˜ 0.8 MW), and NBI (PNBI ˜ 3.6 MW). The basic parameters were set as: plasma current Ip = 0.45 MA, toroidal magnetic field Bt ˜ 1.67 T, vertical elongation κ ˜ 1.63, central-chord average density ne = 4.0 × 1019 m−3, and q95 ˜3.9. And intrinsic ELMs during these discharges exhibited a high frequency ˜250 Hz. 3.1. Pellet fueling Multiple cryogenic deuterium pellets were injected into the target Type-III ELMy H-mode plasmas described in Section 2. As shown in Fig. 2, for a typical shot (shot #80678), deuterium pellets were injected from the LFS with a velocity of ˜100 m/s at a frequency of 10 Hz between t = 5.72–7.0 s, indicated by vertical black lines. Consequently, there were 12 pellets successfully injected into the plasma, where 15 pellet trigger signals were set in advance shown in Fig. 2(g). After each pellet injection, the divertor Dαsignal has an obvious spike. Also, the core and edge line integrated density nel had a distinct rise. Furthermore, the edge temperature decreased, consistent with particle
Fig. 2. Time evolution of the main parameters for shot #80678 during pellet injection. The data from top to bottom are: (a) the core and edge line integrated density nel; (b) the edge electron cyclotron emission (ECE) signal at ρ˜0.83, where ρ (ρ˜r/a) is the normalized radius; (c) diamagnetic stored energy Wdia; (d) the intensity of Dα emission viewing the upper divertor region; (e) upper inner divertor probe signal; (f) pellet fueling efficiency η; and (g) pellet trigger signal. The 8th, 11th and 12th pellets were injected during L mode. The pellet tine-of-flight in the transition tube was about 100 ± 20 ms. It should be noted that the raw data of ECE can only be used for qualitative analysis since LHCD induces super thermal electrons.
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Fig. 3. An expanded view of the pellet injection phase from 5.69 s to 6.82 s.The data from top to bottom are: (a) the core and edge line integrated density nel; (b) the edge ECE signal; (c) diamagnetic stored energy Wdia; and (d) Mirnov probe signal.
the third pellet injection, as shown in Fig. 5. It can be seen that the electron density increased in the region ρ = 0.7–1 after pellet injection by comparing the profiles for 5.92 s and 6.10 s. In other words, the pellet particles were mainly deposited at the edge. Subsequently, the edge particles were exhausted, which can be concluded from the density profiles for 6.10 s and 6.12 s. The density profile for 5.71 s is also displayed for contrast. After the seventh pellet was injected, the system transitioned to L-mode instantly. It must be said that the hollow density profiles can be excluded by ne profiles completeness and comparing with other diagnostic data (e.g. POINT). The pellet particle retention time was τren˜20 ms for L-mode, normalized to the global energy confinement time τren/τE˜0.74, which is defined as the time that is needed for the electron density to recover to the pre-injection level after a pellet
probe signal, representing the ELM instabilities, had an increasing trend, which could be responsible for the density decay. At ˜5.94 s, the core density recovered to its level before the second pellet injection. High density has been obtained by continuous pellet injection. From 6.085 s to 6.8 s, pellets were injected continuously every 100 ± 5 ms for shot #80678, as shown in Fig. 4(a)–(d). During this process, the edge electron temperature, stored energy Wdia, and Mirnov signal had a varied decay after each injection. However, a density increase was induced after every pellet injection and a maximum density of 0.73×nGWl was achieved, where nGW (1020 m−3) = Ip /πa2 (m) is the Greenwald limit. From 6.085 s to 6.45 s, there were 4 pellets injected into the Hmode phase and the core electron density nel had a step-like increase. A typical complete density profile evolution was given before and after
Fig. 4. An expanded view of the pellet injection phase from 6.02 s to 6.82 s. The data from top to bottom are : (a) the core and edge line integrated density nel; (b) the edge ECE signal; (c) diamagnetic stored energy Wdia; (d) and Mirnov probe signal. The L-H transition is indicated by vertical blue solid line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Fig. 7. Electron temperature profiles at 5.71 s and 6.12 s provided from TS system. The electron temperature profiles right after pellet are difficult to achieve due to the low temporal and spatial resolution for TS system.
Fig. 5. Evolution of ne profiles before and after the third pellet injection provided by the RF. The separatrix ρ˜1, i.e. last closed flux surface, is indicated by vertical black dash line. The time points are indicated in Fig. 3. The ne profile for 5.71 s, when the plasma has never been influenced by pellet, is also shown indicated by the pink line.
higher, and correspondingly the ELM amplitude became smaller compared to the spontaneous ELMs. In order to understand the specific relationship between the behaviors of Type-III ELMs and edge plasma parameters, a detailed analysis is provided. The evolution of edge temperature trend and some signals which can present the ELMs’ behaviors was shown in Fig. 6. It is noteworthy that the ELMs’ behaviors have a close connection to the edge temperature. Once one pellet was injected into the plasma, the edge temperature suffered from an immediate and dramatic cooling. Correspondingly, the ELMs became smaller and the ELM frequency became higher. When the edge temperature was recovering, the ELM frequency was decreasing. This phenomenon is similar to an ELM frequency modulation due to a change in the edge temperature by the arrival of sawtooth heat pulses in the edge plasma of JET [28]. It can be interpreted by noting the fact that the edge temperature plays a crucial role in the type III ELM behavior due to resistive MHD instabilities [29]. Peeling-ballooning mode has been mentioned as a driver for ELM in
injection, was much shorter than for H-mode. The ninth pellet was injected during the L-H transition phase and the density didn’t increase slowly until the tenth pellet was injected. The maximum core density 0.73 × nGWl ˜4.8 × 1019 m-2 was obtained after the tenth injection. 3.2. Modification of type-III ELM behaviors Pellet particles were deposited in the edge and would have an effect on edge pedestal of H-mode plasma, consequently changing the behavior of ELMs. Pellets could induce a string of small ELMs when they were injected into Type-III ELMy H-mode plasmas, which was different from the fact that pellets trigger signal ELMs in Type-I ELMy H-mode plasmas in ASDEX-U [24], DIII-D [25], JET [26] and MAST [27]. A modification of Type-III ELM behaviors was always happened after pellet injections. After each pellet the ELM frequency fELM became
Fig. 6. Modulation of the type III ELMs due to a change in the edge temperature induced by pellet injection. The data from top to bottom are : (a) ECE channel 5; (b) channel 6; (c) channel 7; (d) Mirnov coil signal; (e) edge XUV. The sample frequency of ECE signal is 1 MHz. 83
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Ti is difficult to acquire in the absence of measurement tool at present. We will continue studying peeling-ballooning mode stability as soon as the complete profile data can be acquired. Unlike Type-I ELMs, the Type-III ELM frequency has a strong inverse correlation with ▽p in the edge, as pointed out in Ref. [28]. Although there was a reduced edge electron temperature, the edge density was enhanced after pellet injection. In order to confirm the edge ▽p, the electron temperature profiles at 5.71 s and 6.12 s were provided by TS, as shown in Fig. 7. The edge pressure profile can be calculated through the equation Pe=neTe, where Pe is the electron pressure, ne is the electron density, and Te is the electron temperature. From the pressure profile in Fig. 8, it can be seen that the edge gradient ▽Pe was really lowered after pellet injection. Therefore, the ELMs had a smaller amplitude and higher frequency after each pellet injection. The transient heat flux on the divertor was reduced after pellet injections in Type-III ELMy H-mode plasmas and this is helpful to sustain a long-term operation for the divertor. An important question for a future fusion reactor environment is whether the power exhausted by ELMs is consistent with divertor lifetime requirements [31]. Thus, how repeated pellet injections influence the heat and particle flux to divertor needs to be studied. Herein, the particle and heat flux to the divertor before and after pellet injections have been measured with a Langmuir probe array and are presented in Figs. 9 and 10. As can be seen in Fig. 9(a), the particle flux after a pellet event, indicated by ion saturated current, became larger than before the pellet injection for the outer
Fig. 8. Pressure profiles at 5.71 s and 6.12 s calculated using the fitted density and temperature profiles as shown in Figs. 5 and 8 according to the equation Pe=neTe.
some published papers [29,30]. ELITE code can be used to calculate the peeling-ballooning mode stability in EAST. When we use this code to calculate the peeling-ballooning mode stability, we need to acquire some precise profile data. But on EAST the peripheral ion temperature
Fig. 9. Evolution of ion saturated current js (A. m−2) on the upper outer (UO) and upper inner (UI) divertor target from 5 s to 7 s for shot 80678. The pellets were indicated by vertical pink arrows and the L mode phases were also indicated between the vertical pink lines. 84
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Fig. 10. Evolution of peak heat flux qt on the UO divertor target and UI divertor target for shot 80678.
divertor target. However, the particle flux became smaller for the inner target despite the strike points moving after pellet injection as shown in Fig. 9(b). Furthermore, it can also be seen that the particle flux for Lmode was larger than for H-mode due to the low confinement and enhanced outward transition. The transient divertor peak heat flux from ELMs was also estimated shown in Fig. 10. The heat flux had an obviously short-time decline after each pellet injection both for inner and outer targets. Therefore, the pellet injection is beneficial to reduce the heat flux on the divertor target and sustain a long-term operation for the divertor. In addition, the fact that the heat flux during the Lmode phase is lower than that in H-mode for the divertor can be also observed. Also, the peak heat power on the outer target was larger than that on the inner target after each pellet event.
Type-III ELMy H-mode can be fueled by pellet injection while maintaining high confinement, which is rarely reported in other tokamaks. A maximum electron density of 0.73×nGW has been obtained. Since the pellets were injected at a low velocity ˜100 m/s from the LFS, the perturbation depth was shallow and the pellet particles were mainly deposited at the edge plasma region, resulting in a low fueling efficiency and edge temperature cooling. The edge temperature plays a crucial role in the Type-III ELM behavior due to resistive effects. The modification of ELM behavior by the pellet injection was also observed, and is primarily due to the change in edge temperature after pellet injection. On the other hand, despite the increase in the density profile, the edge plasma pressure gradient was not as steep compared to prior to the injection because of the temperature cooling after pellet injection. Consequently, the higher frequency small ELMs were obtained due to the fact that the ELM frequency has a strong inverse correlation with the pressure gradient in the edge for Type-III ELMy H-mode plasmas.
4. Discussion and summary It is noted that plasma confinement degradation related to increasing edge density was often observed due to the sustained temperature cooling when a Type-III ELMy H-mode was fueled by pellet injection. As a result, an H–L transition was induced. Such a transition is unfavorable for future fusion reactors and must be avoided. A simple solution to this problem is to lower the pellet injection frequency to allow sufficient time for plasma temperature to recover. But most of pellet particles will be exhausted during the inter-pellet time and a high density operation is difficult to obtain. So, the most likely resolution to the pellet cooling may be to increase the auxiliary heating power appropriately once the pellet is injected, which can compensate the temperature over-cooling at the edge. This method has been indirectly demonstrated during radiative Type-III ELMy H-mode plasmas in the all-tungsten ASDEX Upgrade [7]. Besides, faster pellet injection can be also used to possibly solve both fuelling and cooling problem due to its deeper evaporation. Thus, the experiment for pellet fueling of Type-III ELMy H-mode accompanied by auxiliary heating power modulation or fast pellet injection experiments will be considered and performed in the future on EAST. Cryogenic deuterium pellets have been successfully injected into Type-III ELMy H-mode plasmas on EAST. It is demonstrated that the
Acknowledgments This research is funded by National Key Research and Development Program of China (2017YFA0402500, 2017YFE0301100), National Nature Science Foundation of China (11625524, 11775261, 11605246, 11075185 and 11021565). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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Deuterium pellet fueling in type-III ELMy H-mode plasmas on EAST superconducting tokamak ⁎
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Jilei Houa,b, Jiansheng Hua,c, , Yue Chena, , Yumin Wanga, Qing Zanga, Jichan Xua, Haiqing Liua, Kevin Tritzd, Erik Gilsone, Xiaolin Yuana, Zhen Suna,e, Rajesh Maingie, Hailin Zhaoa, Jiangang Lia a
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China University of Science and Technology of China, Hefei, 230029, China c CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Hefei, 230031, China d Johns Hopkins University, Baltimore, MD, 21211, USA e Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ, 08543, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fueling Pellet injection Type-III ELM EAST
Type-III ELMy H-mode plasmas are a valuable operating scenario, as they may simultaneously achieve fusion power gain of Q = 10 and acceptable steady-state and peak power fluxes to the plasma-facing components. It is important to investigate cryogenic deuterium pellet injection fueling in this kind of H-mode plasma since it has been chosen as a main plasma fueling technique for ITER. Recently, pellet fueling experiments have been successfully carried out on EAST, an ITER-like tokamak. A maximum electron density of 0.73 × nGW has been obtained. It is found that ELM behavior was modified immediately and a string of small ELMs were induced when pellets were injected at a low velocity from ˜40 cm above the mid-plane on the low field side, which is different from the fact that pellets trigger signal ELMs in Type-I ELMy H-mode plasmas in other tokamaks. The decreased edge temperature caused by the cryogenic pellet was proposed as the reason for this ELM modification. These results will provide a useful reference for the pellet fueling in a Type-III ELMy H-mode plasma on ITER in the future.
1. Introduction The H-mode, a high confinement operating regime, is characterized by a steep pressure gradient and “pedestal” at the plasma edge, which lead to strong self-driven plasma currents that, in turn, result in an instability known as edge-localized modes (ELMs) [1]. ELMs can cause periodic particle and heat losses from the plasma edge, thereby introducing efficient particle density and impurity control. Several types of ELMs with different characteristics have been identified in various experiments [2]. Based on the ELM frequency dependence on the auxiliary heating power, ELMs were classified into three types. Type I ELMs, of which the repetition frequency fELM increases with heating power, appear as isolated sharp bursts on the Dα signal without a detectable magnetic precursor oscillation. For Type II ELMs, no information on the power dependence of fELM or the MHD precursor activity exists. The repetition frequency of Type III decreases with heating power and a coherent magnetic precursor oscillation is observed on magnetic probes.
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Future fusion research machines, such as ITER, are planned to operate in H-mode, which is the most promising mode for producing fusion energy using the tokamak configuration [3]. One of the most severe problems for fusion reactors is the power load on the plasma facing components from power exhausted by ELMs accompanying the Hmode. Only loads of less than 10 MW/m2 in steady state, and less than 0.5 MJ/m2 during transients caused by ELMs, are acceptable [4]. In order to reduce the power flux to the plasma-facing components in the divertor to manageable levels, the peak heat flux has to be dissipated. A possible solution is the Type-III ELMy H-mode since the transient heat loads due to Type-III ELMs are acceptable with even the most stringent boundary conditions [5]. However, stationary operation with Type-III ELMs is often found to impose an upper limit on the pedestal temperature that is lower than the values reached by Type-I ELMy Hmodes, thereby reducing projected plasma performance [6]. Although Type-III ELMy H-mode plasma operation has some shortcomings, TypeIII ELMy H-mode regime has been extended to higher levels of additional heating by increasing the edge density and/or by increasing the
Corresponding author at: Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China. Corresponding author. E-mail addresses: [email protected] (J. Hu), [email protected] (Y. Chen).
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https://doi.org/10.1016/j.fusengdes.2019.05.038 Received 29 March 2019; Received in revised form 9 May 2019; Accepted 25 May 2019 Available online 03 June 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic drawing of the 10 Hz pellet injection system installed on the EAST. The plasma is operated in upper single null divertor configuration. Only the transition tube in LFS was used in these experiments. The vertical position for POINT edge channel is Z = 42.5 cm and the distance is 8.5 cm between two adjacent channels. The TS line of sight is at R = 1.9 m and the vertical position for RF is Z = 3 cm.
electron cyclotron resonance heating (ECRH) [14].
radiated power with injection of extrinsic impurities. Injection of impurities allows achieving the Type-III ELMy H-mode regime at lower collisionalities compared to increasing the edge density, and the extrapolated ELM size in these conditions are suitable for meeting the requirements for ITER Q = 10 operation [7]. Cryogenic deuterium pellet injection with a well-known high fueling efficiency has been chosen as the main plasma fueling technique for ITER [8]. It is important to carry out pellet fueling experiment in Hmode plasmas in ITER-like tokamak. Although the ITER scenario is based on the Type-I ELMy H-mode regime [9], which has a very broad experimental and theoretical basis, investigation of Type-III ELMy Hmode plasmas fueled by pellet is still valuable as a possible scenario for ITER Q = 10 operation. EAST is an ITER-like tokamak with a flexible configuration and a dedicated pellet injector was successfully developed a few years ago. These have provided a good opportunity to do pellet fueling experiments in all kinds of H-mode plasmas in ITER-like tokamak. Recently, pellet fueling experiment in Type-III ELMy H-mode plasmas has been successfully carried out on EAST and numerous interesting results were obtained. In this paper, we will mainly focus on introducing the experimental results of pellet fueling and its effect on Type-III ELM behavior. The set-up used in the experiments will be described in Section 2. In Section 3, the results of pellet fueling and the modification of Type-III ELM behavior induced by cryogenic pellet injection will be presented. Finally, we will give a discussion and summary of this paper.
2.2. Diagnostics A variety of high-performance diagnostics have been developed and deployed to measure the key parameters of the plasma. The main diagnostics associated with this work are as following. A reflectometer (RF) with sampling frequency ˜64 MHz, located in the outer mid-plane, was used to measure electron density profiles [15–17]. A Thomson scattering (TS) system [18] was used to measure the profile of electron density and temperature with a temporal resolution of 200 ms. Heat and particle fluxes in the divertor region were acquired by triple Langmuir probes arrays, which are embedded in the upper and lower divertor regions [19]. An 11-channel Polarimeter-Interferometer (POINT) with a temporal resolution of 1 ms, which provided core and edge electron densities [20], was also used during the experiment. Moreover, routine diagnostics such as multi-channel extreme ultraviolet (XUV) arrays, Mirnov probes, and electron cyclotron emission (ECE) were also used. 2.3. Pellet injector At present, a pellet injector with an injection frequency of 10 Hz, which is based on screw extrusion forming ice and pneumatic acceleration, has been developed for plasma fueling on EAST. The system consists of the injector, vacuum system, and pellet transition system. The schematic drawing of the pellet injector, located at Port J of EAST, is shown in Fig. 1. Helium is used for propellant gas and the two-stage differential chamber system is used to exhaust pneumatic gas. When the pellets are injected, the vacuum degree in second differential chamber close to EAST vessel is about 10−2–10−3 Pa and little accelerating gas can fly into plasma. It can inject Φ2 × 2 mm pellets with nominal number of atoms ˜3.8 × 1020 at velocity 100–300 m/s at a maximum frequency of 10 Hz with reliability of over 95% for more than 1000s from either the low field side (LFS) or the high field side (HFS) [21]. The lengths of the transition tubes are ˜10 m and ˜12 m for the LFS and HFS respectively. Pellet velocity is changing during the flight through the pellet-guide. When pellet is transferred in guide tube, the friction and collision between tube and pellet may result in a disintegrated pellet and decreased velocity. In the present experiment, pellets were injected into Type-III ELMy H-mode plasmas from 40 cm above midplane at the LFS with a low average velocity of ˜100 m/s. Generally, there is a considerable number of pellet particles lost (ablated or broken) after being transported for a long-distance. Even though many pellets have been injected with the 10 Hz system in past experiments, the exact number of deuterium atoms that were injected into the EAST tokamak per pellet has never been properly calibrated.
2. Set-up 2.1. Experimental device The EAST tokamak, which was built to demonstrate high power and long pulse operations under fusion-relevant conditions, is a non-circular full superconducting tokamak with major radius R = 1.85 m and minor radius a = 0.45 [10]. The machine can be operated in lower single null (LSN), double null (DN) and upper single null (USN) divertor configurations, which can be flexibly switched during a long-pulse discharge [11]. To improve machine capabilities and power or particle exhaust toward advanced steady-state high-performance operations, the internal plasma facing components (PFCs) were changed from carbon tiles to molybdenum tiles in 2012, except for those near the strike points [12]. In 2013, the upper divertor was replaced by 80 tungsten divertor modules whose design was based on cassette and mono-block technology such as that planned for ITER [13]. The lower graphite divertor will be also replaced with tungsten divertor modules in 2019. The EAST auxiliary heating systems include lower hybrid current drive (LHCD) at both 2.45 GHz and 4.6 GHz, ion cyclotron resonance heating (ICRH), neutral beam injection (NBI, 2 systems, 4 ion sources), and 80
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injection. A modest reduction in the diamagnetic stored energy Wdia was also observed. Occasionally, a transition from H-mode to L-mode (H-L), indicated by vertical black dash lines in Fig. 2, was induced because of plasma temperature over-cooling after pellet injection, which is characterized by an abrupt decay of plasma stored energy and density. In addition, the divertor probe signal measuring the exhausted particles to the divertor also had an obvious change after pellet injections. The fueling efficiency η for each pellet, defined as the total increase in number of plasma electrons divided by the number of input fuel atoms, can be roughly estimated from the equation:
On TUMAN-3M tokamak a method with Dα detectors looking at the pellets has been applied for calibration of deposited particles number [22]. Because these diagnostic devices are lacked for pellet deposited particles during discharges, we roughly calibrated the number of injected particles by measuring changes in the vacuum pressure of EAST before and after pellet injection without a plasma discharge prior to the present experiment. After multiple calibration injections, the number of atoms injected into the vacuum chamber was determined to be Npel ˜ 1.03 × 1020 per pellet. Since these injections were done without plasma, the calibration only measured Npel into the chamber. The number that would get into the plasma might be lower due to ablation in the scrape-off layer (SOL) when a plasma is present.
η= 3. Results
Δ (ne l) V /2a Npel
(1)
Here, Δ(nel) is the increment of core line integrated density, V = 10.5 m3 is the plasma volume, and the number of pellet fuel atoms is replaced by Npel. The estimated η may be less than the actual value due to ablation in SOL.The time difference Δt for measuring the Δ(nel) is 1.4–1.8 ms. Fig. 2(g) gives the fueling efficiency calculated from the Eq. (1) for each pellet injection. It can be seen that the fueling efficiency η is 16–30% and 8–13%, respectively during H-mode and L-mode phases. This is different from the general perception that the pellet fueling efficiency in H-mode plasmas is lower than in L-mode [23]. This result is presumably because of some effects of the fueling processes (ablation or plasmoid drift). The pellet injection depth during pellet ablation pulse is about ˜15 cm from the last closed flux surface (LCFS). The pellet injection occasionally induced confinement degradation due to temperature cooling at the edge. As can be seen in Fig. 3(a)–(d), the first pellet caused a dramatic degradation in plasma confinement displaying a decline in the stored energy Wdia, almost taking the plasma into L-mode. The confinement changed to H-mode immediately with a nearly constant electron density after several density fluctuations sustaining for τdec ˜21 ms. The significant temperature cooling is likely responsible for the confinement degradation. About Δt1˜72 ms later, the second pellet reached the plasma. Unlike the first pellet, the second pellet did not cause an obvious decline in Wdia. The core density nel remained constant for τ2 ˜ 93 ms after injection, even though the edge density nel had a modest decline. Both the core and edge density nel started to exhibit a decreasing trend at 5.874 s. Meanwhile, the Mirnov
On EAST, H-mode plasmas are often obtained with lower hybrid wave power in conjunction with wall conditioning by lithium (Li). Before the deuterium pellet injection experiments, multiple lithium granule injections were performed to improve wall conditions. The Type-III ELMy H-mode plasmas were easy to obtain. These target discharges were operated in an upper-single-null configuration, as shown in Fig. 1, with LHCD (PLHCD ˜ 0.8 MW), and NBI (PNBI ˜ 3.6 MW). The basic parameters were set as: plasma current Ip = 0.45 MA, toroidal magnetic field Bt ˜ 1.67 T, vertical elongation κ ˜ 1.63, central-chord average density ne = 4.0 × 1019 m−3, and q95 ˜3.9. And intrinsic ELMs during these discharges exhibited a high frequency ˜250 Hz. 3.1. Pellet fueling Multiple cryogenic deuterium pellets were injected into the target Type-III ELMy H-mode plasmas described in Section 2. As shown in Fig. 2, for a typical shot (shot #80678), deuterium pellets were injected from the LFS with a velocity of ˜100 m/s at a frequency of 10 Hz between t = 5.72–7.0 s, indicated by vertical black lines. Consequently, there were 12 pellets successfully injected into the plasma, where 15 pellet trigger signals were set in advance shown in Fig. 2(g). After each pellet injection, the divertor Dαsignal has an obvious spike. Also, the core and edge line integrated density nel had a distinct rise. Furthermore, the edge temperature decreased, consistent with particle
Fig. 2. Time evolution of the main parameters for shot #80678 during pellet injection. The data from top to bottom are: (a) the core and edge line integrated density nel; (b) the edge electron cyclotron emission (ECE) signal at ρ˜0.83, where ρ (ρ˜r/a) is the normalized radius; (c) diamagnetic stored energy Wdia; (d) the intensity of Dα emission viewing the upper divertor region; (e) upper inner divertor probe signal; (f) pellet fueling efficiency η; and (g) pellet trigger signal. The 8th, 11th and 12th pellets were injected during L mode. The pellet tine-of-flight in the transition tube was about 100 ± 20 ms. It should be noted that the raw data of ECE can only be used for qualitative analysis since LHCD induces super thermal electrons.
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Fig. 3. An expanded view of the pellet injection phase from 5.69 s to 6.82 s.The data from top to bottom are: (a) the core and edge line integrated density nel; (b) the edge ECE signal; (c) diamagnetic stored energy Wdia; and (d) Mirnov probe signal.
the third pellet injection, as shown in Fig. 5. It can be seen that the electron density increased in the region ρ = 0.7–1 after pellet injection by comparing the profiles for 5.92 s and 6.10 s. In other words, the pellet particles were mainly deposited at the edge. Subsequently, the edge particles were exhausted, which can be concluded from the density profiles for 6.10 s and 6.12 s. The density profile for 5.71 s is also displayed for contrast. After the seventh pellet was injected, the system transitioned to L-mode instantly. It must be said that the hollow density profiles can be excluded by ne profiles completeness and comparing with other diagnostic data (e.g. POINT). The pellet particle retention time was τren˜20 ms for L-mode, normalized to the global energy confinement time τren/τE˜0.74, which is defined as the time that is needed for the electron density to recover to the pre-injection level after a pellet
probe signal, representing the ELM instabilities, had an increasing trend, which could be responsible for the density decay. At ˜5.94 s, the core density recovered to its level before the second pellet injection. High density has been obtained by continuous pellet injection. From 6.085 s to 6.8 s, pellets were injected continuously every 100 ± 5 ms for shot #80678, as shown in Fig. 4(a)–(d). During this process, the edge electron temperature, stored energy Wdia, and Mirnov signal had a varied decay after each injection. However, a density increase was induced after every pellet injection and a maximum density of 0.73×nGWl was achieved, where nGW (1020 m−3) = Ip /πa2 (m) is the Greenwald limit. From 6.085 s to 6.45 s, there were 4 pellets injected into the Hmode phase and the core electron density nel had a step-like increase. A typical complete density profile evolution was given before and after
Fig. 4. An expanded view of the pellet injection phase from 6.02 s to 6.82 s. The data from top to bottom are : (a) the core and edge line integrated density nel; (b) the edge ECE signal; (c) diamagnetic stored energy Wdia; (d) and Mirnov probe signal. The L-H transition is indicated by vertical blue solid line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Fig. 7. Electron temperature profiles at 5.71 s and 6.12 s provided from TS system. The electron temperature profiles right after pellet are difficult to achieve due to the low temporal and spatial resolution for TS system.
Fig. 5. Evolution of ne profiles before and after the third pellet injection provided by the RF. The separatrix ρ˜1, i.e. last closed flux surface, is indicated by vertical black dash line. The time points are indicated in Fig. 3. The ne profile for 5.71 s, when the plasma has never been influenced by pellet, is also shown indicated by the pink line.
higher, and correspondingly the ELM amplitude became smaller compared to the spontaneous ELMs. In order to understand the specific relationship between the behaviors of Type-III ELMs and edge plasma parameters, a detailed analysis is provided. The evolution of edge temperature trend and some signals which can present the ELMs’ behaviors was shown in Fig. 6. It is noteworthy that the ELMs’ behaviors have a close connection to the edge temperature. Once one pellet was injected into the plasma, the edge temperature suffered from an immediate and dramatic cooling. Correspondingly, the ELMs became smaller and the ELM frequency became higher. When the edge temperature was recovering, the ELM frequency was decreasing. This phenomenon is similar to an ELM frequency modulation due to a change in the edge temperature by the arrival of sawtooth heat pulses in the edge plasma of JET [28]. It can be interpreted by noting the fact that the edge temperature plays a crucial role in the type III ELM behavior due to resistive MHD instabilities [29]. Peeling-ballooning mode has been mentioned as a driver for ELM in
injection, was much shorter than for H-mode. The ninth pellet was injected during the L-H transition phase and the density didn’t increase slowly until the tenth pellet was injected. The maximum core density 0.73 × nGWl ˜4.8 × 1019 m-2 was obtained after the tenth injection. 3.2. Modification of type-III ELM behaviors Pellet particles were deposited in the edge and would have an effect on edge pedestal of H-mode plasma, consequently changing the behavior of ELMs. Pellets could induce a string of small ELMs when they were injected into Type-III ELMy H-mode plasmas, which was different from the fact that pellets trigger signal ELMs in Type-I ELMy H-mode plasmas in ASDEX-U [24], DIII-D [25], JET [26] and MAST [27]. A modification of Type-III ELM behaviors was always happened after pellet injections. After each pellet the ELM frequency fELM became
Fig. 6. Modulation of the type III ELMs due to a change in the edge temperature induced by pellet injection. The data from top to bottom are : (a) ECE channel 5; (b) channel 6; (c) channel 7; (d) Mirnov coil signal; (e) edge XUV. The sample frequency of ECE signal is 1 MHz. 83
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Ti is difficult to acquire in the absence of measurement tool at present. We will continue studying peeling-ballooning mode stability as soon as the complete profile data can be acquired. Unlike Type-I ELMs, the Type-III ELM frequency has a strong inverse correlation with ▽p in the edge, as pointed out in Ref. [28]. Although there was a reduced edge electron temperature, the edge density was enhanced after pellet injection. In order to confirm the edge ▽p, the electron temperature profiles at 5.71 s and 6.12 s were provided by TS, as shown in Fig. 7. The edge pressure profile can be calculated through the equation Pe=neTe, where Pe is the electron pressure, ne is the electron density, and Te is the electron temperature. From the pressure profile in Fig. 8, it can be seen that the edge gradient ▽Pe was really lowered after pellet injection. Therefore, the ELMs had a smaller amplitude and higher frequency after each pellet injection. The transient heat flux on the divertor was reduced after pellet injections in Type-III ELMy H-mode plasmas and this is helpful to sustain a long-term operation for the divertor. An important question for a future fusion reactor environment is whether the power exhausted by ELMs is consistent with divertor lifetime requirements [31]. Thus, how repeated pellet injections influence the heat and particle flux to divertor needs to be studied. Herein, the particle and heat flux to the divertor before and after pellet injections have been measured with a Langmuir probe array and are presented in Figs. 9 and 10. As can be seen in Fig. 9(a), the particle flux after a pellet event, indicated by ion saturated current, became larger than before the pellet injection for the outer
Fig. 8. Pressure profiles at 5.71 s and 6.12 s calculated using the fitted density and temperature profiles as shown in Figs. 5 and 8 according to the equation Pe=neTe.
some published papers [29,30]. ELITE code can be used to calculate the peeling-ballooning mode stability in EAST. When we use this code to calculate the peeling-ballooning mode stability, we need to acquire some precise profile data. But on EAST the peripheral ion temperature
Fig. 9. Evolution of ion saturated current js (A. m−2) on the upper outer (UO) and upper inner (UI) divertor target from 5 s to 7 s for shot 80678. The pellets were indicated by vertical pink arrows and the L mode phases were also indicated between the vertical pink lines. 84
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Fig. 10. Evolution of peak heat flux qt on the UO divertor target and UI divertor target for shot 80678.
divertor target. However, the particle flux became smaller for the inner target despite the strike points moving after pellet injection as shown in Fig. 9(b). Furthermore, it can also be seen that the particle flux for Lmode was larger than for H-mode due to the low confinement and enhanced outward transition. The transient divertor peak heat flux from ELMs was also estimated shown in Fig. 10. The heat flux had an obviously short-time decline after each pellet injection both for inner and outer targets. Therefore, the pellet injection is beneficial to reduce the heat flux on the divertor target and sustain a long-term operation for the divertor. In addition, the fact that the heat flux during the Lmode phase is lower than that in H-mode for the divertor can be also observed. Also, the peak heat power on the outer target was larger than that on the inner target after each pellet event.
Type-III ELMy H-mode can be fueled by pellet injection while maintaining high confinement, which is rarely reported in other tokamaks. A maximum electron density of 0.73×nGW has been obtained. Since the pellets were injected at a low velocity ˜100 m/s from the LFS, the perturbation depth was shallow and the pellet particles were mainly deposited at the edge plasma region, resulting in a low fueling efficiency and edge temperature cooling. The edge temperature plays a crucial role in the Type-III ELM behavior due to resistive effects. The modification of ELM behavior by the pellet injection was also observed, and is primarily due to the change in edge temperature after pellet injection. On the other hand, despite the increase in the density profile, the edge plasma pressure gradient was not as steep compared to prior to the injection because of the temperature cooling after pellet injection. Consequently, the higher frequency small ELMs were obtained due to the fact that the ELM frequency has a strong inverse correlation with the pressure gradient in the edge for Type-III ELMy H-mode plasmas.
4. Discussion and summary It is noted that plasma confinement degradation related to increasing edge density was often observed due to the sustained temperature cooling when a Type-III ELMy H-mode was fueled by pellet injection. As a result, an H–L transition was induced. Such a transition is unfavorable for future fusion reactors and must be avoided. A simple solution to this problem is to lower the pellet injection frequency to allow sufficient time for plasma temperature to recover. But most of pellet particles will be exhausted during the inter-pellet time and a high density operation is difficult to obtain. So, the most likely resolution to the pellet cooling may be to increase the auxiliary heating power appropriately once the pellet is injected, which can compensate the temperature over-cooling at the edge. This method has been indirectly demonstrated during radiative Type-III ELMy H-mode plasmas in the all-tungsten ASDEX Upgrade [7]. Besides, faster pellet injection can be also used to possibly solve both fuelling and cooling problem due to its deeper evaporation. Thus, the experiment for pellet fueling of Type-III ELMy H-mode accompanied by auxiliary heating power modulation or fast pellet injection experiments will be considered and performed in the future on EAST. Cryogenic deuterium pellets have been successfully injected into Type-III ELMy H-mode plasmas on EAST. It is demonstrated that the
Acknowledgments This research is funded by National Key Research and Development Program of China (2017YFA0402500, 2017YFE0301100), National Nature Science Foundation of China (11625524, 11775261, 11605246, 11075185 and 11021565). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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