Tungsten control in type-I ELMy H-mode plasmas on EAST Enhanced Publication

Tungsten control in type-I ELMy H-mode plasmas on EAST 1 2 first-author https://orcid.org/0000-0001-5852-0250 3 corresp E-mail:Zhen-Wei Wu zwwu@ipp.ac.cn, E-mail:Zhen-Wei Wu zwwu@ipp.ac.cn, zwwu@ipp.ac.cn https://orcid.org/0000-0002-3141-9969 3 corresp Ling Zhang zhangling@ipp.ac.cn Ling Zhang zhangling@ipp.ac.cn zhangling@ipp.ac.cn 3 4 3 1 2 1 2 3 3 1 2 3 1 2 3 1 2 3 3 3 3 3 3 Advanced Energy Research Center, Shenzhen University, Shenzhen 518060, China Advanced Energy Research Center, Shenzhen University, Shenzhen 518060, China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060,China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060,China Institute of Plasma Physics, Chinese Academy of Science, Hefei 230031, China Institute of Plasma Physics, Chinese Academy of Science, Hefei 230031, China University of Science & Technology of China, Hefei 230031, China University of Science & Technology of China, Hefei 230031, China The first experimental investigation of the tungsten behavior in ELMy H-mode plasmas with co-/counter neutral beam injection (NBI) and unfavorable/favorable B t was performed on EAST. Tungsten was found to accumulate easily in ELMy H-mode plasma with co-NBI heating and unfavorable B t . Thus, in this case the tungsten concentration can exceed 10 −4 , resulting in degradation of the plasma confinement and periodic H-L transitions. To reduce the tungsten concentration in steady-state type-I ELMy H-mode operation, counter-NBI is applied to modify the density and temperature and brake the plasma toroidal rotation. The applied counter-NBI decreases the PHZ+ E r inward pinch velocity and reverses the direction of neoclassical inward convection, thus decreasing the tungsten concentration from ~7 × 10 −5 to ~2 × 10 −5 in type-I ELMy H-mode plasma with favorable B t . A comparison of the effects of different B t directions on the tungsten behavior also shows that favorable B t is beneficial for reducing the tungsten concentration in the core plasma. These results imply that counter-NBI with favorable B t can effectively prevent tungsten accumulation and expand the operating window for exploring steady-state type-I ELMy H-mode operation of EAST. Tungsten accumulation Counter-NBI favorable B t ELMy H-mode EAST 1. Introduction 1 Tungsten is a good candidate for plasma-facing materials in future fusion devices such as ITER and CFETR. Its intrinsic benefits include a high melting point, a low sputtering yield, and a low tritium retention. However, tungsten accumulation in the core plasma can result in serious radiation losses, the degradation of the overall energy confinement [ 1 1 ], magnetohydrodynamic (MHD) instabilities [ 2 2 ], or even disruptions [ 3 3 - 5 5 3-5 ]. For ITER, the tungsten concentration ( C W = n W / n e , where n W and n e are the tungsten and electron densities, respectively) must be less than 10 −4 to avoid unacceptable radiative cooling [ 6 6 ]. It is therefore important to avoid tungsten accumulation in the high-confinement regime in devices with tungsten divertor plates, where tungsten atoms can be sputtered from the divertor plates by giant edge-localized modes (ELMs) [ 7 7 , 8 8 ]. Methods of reducing the stored energy radiated by tungsten in the central plasma is a concern for the economical operation of future commercial reactors [ 9 9 ]. Previous investigations have shown that core impurity accumulation is driven mainly by neoclassical inward convection [ 10 10 ], and the negative radial electric field ( E r ) is responsible for inward impurity transport [ 11 11 , 12 12 ]. In addition, the inward pinch of high- Z impurities (PHZ) attributed to the atomic processes of ionization and recombination also plays a role in core tungsten accumulation in a toroidally rotating plasma [ 12 12 - 14 14 12-14 ]. Thus, to suppress and avoid tungsten accumulation, these inward pinch velocities must be reduced and well controlled. Theoretical predictions and experimental observations have shown that neoclassical convection can be controlled by changing the density and temperature gradients [ 15 15 - 17 17 15-17 ], and the PHZ and E r pinch depend strongly on the toroidal rotation velocity of the background plasma [ 12 12 ]. Turbulent transport due to, for example, the ion temperature gradient (ITG) mode and trapped electron mode (TEM), can also affect tungsten transport [ 18 18 - 20 20 18-20 ], but it is much weaker than neoclassical transport in the central region of a toroidally rotating plasma, as has been found in JET and the ASDEX Upgrade [ 21 21 - 23 23 21-23 ]. Turbulent transport can be enhanced by decreasing the ion-to-electron temperature ratio ( T i / T e ) [ 24 24 , 25 25 ] so as to increase the impurity diffusivity and suppress impurity accumulation. Neutral beam injection (NBI), which can alter the density and temperature and change the toroidal rotation velocity, will be used to study the tung sten behavior in the core of an EAST type-I ELMy H-mode plasma, because the ELMy H-mode with a low ELM frequency ( f ELM ) of dozens of hertz has inevitably been found to result in strong impurity accumulation in EAST [ 26 26 - 28 28 26-28 ]. Here, the effects of neoclassical transport [ 10 10 ] and the PHZ and E r pinch on the tungsten behavior in the type-I ELMy H-mode regime are studied. In addition, the effect of turbulent transport on tungsten transport is briefly discussed. The remainder of the paper is structured as follows. Section 2 describes the NBI systems and relevant diagnostics in EAST, and Sect. 3 reports the experimental investigation of the tungsten behavior in ELMy H-mode plasmas with co-/counter-NBI and different toroidal field (TF) directions. In Sect. 4, a summary and discussion are presented. 2. Experimental setup 1 This work is based on the EAST tokamak device. EAST is a full superconducting device that was designed to operate with major radii R of 1.7–1.9 m, minor radii a of 0.4–0.45 m, a maximum TF strength B t of 3.5 T, a maximum plasma current I p of 1 MA, an elongation k of 1.2–2, and a triangularity δ of < 0.8. Since the 2014 campaign, EAST has been upgraded to facilitate progress in achieving long-pulse stable high-performance plasma operation [pulse length > 1000 s, central electron temperature T e0 > 10 keV, normalized beta β N > 2, and n e ~ n GW , where n GW = I p /π a 2 (10 20 m −3 )]. For these purposes, several auxiliary heating systems have been developed to provide effective heating and current drive for EAST [ 29 29 , 30 30 ]. As the auxiliary heating power is increased, the materials of plasma facing components (PFCs) are replaced at different positions of the inner wall to improve the heat exhaust ability [ 31 31 ]. The main PFC materials in EAST are molybdenum tiles mounted on the first wall of the vacuum chamber, SiC-coated graphite tiles fixed in the lower divertor, and W/Cu mono-blocks installed in the upper divertor [ 32 32 ]. Two sets of NBI systems have been installed on EAST since the 2014 and 2015 campaigns [ 33 33 , 34 34 ]. The NBI system in EAST consists of two sets of neutral beam injectors. The maximum beam power of each injector is 4 MW, and the maximum beam energy ranges from 50 to 80 keV. One injector has two ion sources which with an independent beam channel and can be operated independently. The layout of the EAST NBI system is shown in Fig. 1 Fig. 1 . For anticlockwise I p , the tangential direction of each injector (the co- I p direction is denoted as co-NBI, whereas the counter- I p direction is denoted as counter-NBI) and the unfavorable direction of B t (counter- I p direction) are shown in the figure. To investigate the tungsten behavior in core plasmas with different NBI and B t directions, three newly developed extreme ultraviolet (EUV) spectrometers (EUV_Short, EUV_Long, and EUV_Long2 in Fig. 1 Fig. 1 ) will be used. EUV_Short and EUV_Long [ 35 35 , 36 36 ] operate in wavelength ranges of 20–50 Å and 20–500 Å, respectively, and both have good spectral resolution for measuring the chord-integrated intensity of line emission from tungsten. Other relevant diagnostics include a Thomson scattering system for observing the profiles of the electron temperature and density, a polarimeter-interferometer for monitoring the profiles of the electron density and safety factor, a fast CCD camera for detecting the plasma position [ 37 37 ], and charge exchange recombination spectroscopy for monitoring the ion temperature profile and toroidal rotation velocity of the background plasma. 3. Experimental results 1 3.1 Tungsten accumulation at high toroidal rotation velocity 2 Since 2014, EAST has been used to access advanced regimes with steady-state high-performance plasma in ITER-like tungsten divertor operation [ 38 38 - 41 41 38-41 ]. However, the accumulation of tungsten that is transported from the divertor by regular collisions or large ELMy impacts [ 42 42 , 43 43 ] always degrades the performance of H-mode plasma. In this section, the behavior of tungsten under a combination of NBI and lower hybrid wave (LHW) heating in the type-I ELMy H-mode regime is investigated to explore suitable heating scenarios without high- Z impurity accumulation and to develop the steady-state long-pulse and high-performance regime. Type-I ELMy H-mode discharges with the ITER-like tungsten divertor are selected to investigate the tungsten behavior in the core plasma. In the discharge of shot #63723, type-I ELMs with f ELM ~ 80 Hz were obtained when additional co-NBI was performed during the LHW heating phase, as shown in Fig. 2 Fig. 2 . This discharge was performed in the upper single-null (USN) configuration with an unfavorable B t direction (counter- I p direction, where the direction of <math id="M1"><mrow><msub><mi>B</mi><mtext>t</mtext></msub><mo>×</mo><mo>∇</mo><msub><mi>B</mi><mtext>t</mtext></msub></mrow></math> points away from the divertor X-point) at B t = 2.3 T, I p = 0.5 MA, a line-averaged electron density <math id="M2"><mrow><msub><mover accent="true"><mi>n</mi><mo>¯</mo></mover><mtext>e</mtext></msub><mo> </mo></mrow></math> of ~2.5 × 10 19 m −3 , and a safety factor at 95% of the magnetic flux, q 95 , of 5, where the source power of LHW and co-NBI heating is 1.5 and 4.0 MW, respectively. In this shot, the toroidal rotation velocity of the background plasma can be driven to a quite high level [a central toroidal rotation velocity V t (0) of ~140 km/s] because co-NBI heating is applied. The emission from high ionization stages of Li, C, Fe, Cu, Mo, and W is detected by EUV spectrometers at wavelengths of 20–150 Å [ 35 35 , 36 36 ]. Figure 3 (a)–(c) Figure 3 (a)–(c) shows the EUV spectra and total radiative power ( P rad ) at 3.1, 3.3, and 4.1 s, respectively. The stronger emission lines, such as Li III (134.99 Å), C VI (33.73 Å), and Fe XXIII (132.91 Å), and the first- and second-order tungsten emission in the unresolved transition array (W-UTA) are labeled. C W in core plasma is evaluated as the W-UTA intensity at 45–70 Å, which is composed of W 24+ –W 45+ . The C W calculation method is analyzed and introduced in the literature [ 44 44 ]. The W-UTA intensity is found to increase sharply with increasing P rad . In particular, in Fig. 3(b) and (c) Fig. 3(b) and (c) , when P rad is changed from 1.0 to 1.3 MW, the intensity of the first-order W-UTA becomes saturated, and the second-order W-UTA emission appears and becomes more intense, whereas the intensity of emission from other impurities remains relatively low. This result confirmed that tungsten accumulates easily in the core plasma when co-NBI is performed and makes a large contribution to this important radiation. The process of tungsten accumulation is shown in Fig. 4(a) and (b) Fig. 4(a) and (b) . C W increases continually during each tungsten accumulation phase, and the radiation profile becomes progressively peaked correspondingly [I mean that the degree of peaking of radiation profile increases gradually]. Noticeably, in the first phase of the H mode, because q (0) is slightly less than 1 [ Fig. 5(d) Fig. 5(d) ], core MHD activity ( m / n = 1/1) begins and is observed on the spectrogram of the core soft X-ray emission signals from 3.0 to 3.6 s, as shown in Fig. 4(c) Fig. 4(c) . This core MHD activity may enhance the impurity diffusivity and decrease the neoclassical convection [ 45 45 , 46 46 ], which suppresses further increases in C W in the core plasma. Although this MHD activity vanishes after 3.6 s, C W continues to grow. In this case, tungsten accumulation results in a larger radiated power loss; thus, a transition back to the L-mode phase occurs after approximately one energy confinementtime τ E [ τ E = W MHD / ( P total – d W MHD / d t )]. These sequences of tungsten accumulation and H–L transitions can repeat several times with a period of approximately 3–6 τ Ε . In each tungsten accumulation phase, the effective charge number Z eff increases from ~3 to ~3.5, and C W increases from ~0.23 × 10 −4 to ~1.6 × 10 −4 ; thus, the power radiation fraction P rad / P total increases from ~15% to a peak value of ~50%. The energy confinement factor H 98 and β N are clearly attenuated during each tungsten accumulation process, as shown in Fig. 2 Fig. 2 . Figure 5 (a)–(d) Figure 5 (a)–(d) shows the electron density, electron temperature, ion temperature, and safety factor profiles, respectively, during the L-mode phase (at ~3.1 s, blue line) and before (at ~3.3 s, red line) and during (at ~4.0 s, black line) tungsten accumulation in the H-mode phase of shot #63723. The strong radiative power loss caused by tungsten accumulation changes the profiles and results in an aggravated plasma performance and steady-state operation, as follows. (i) The electron temperature profile inside <math id="M4"><mi>ρ</mi></math> = 0.2 becomes hollow owing to radiative power loss in the tungsten accumulation phase, as shown in Fig. 5(b) Fig. 5(b) . This change in the electron temperature profile in the core region can reduce the turbulent outward pinch driven by the TEM [ 47 47 ], which decreases the outward impurity transport. (ii) The increase in electron density and Z eff during the tungsten accumulation phase can reduce the ITG mode growth rate and particle diffusion [ 48 48 , 49 49 ], producing a peak in ion temperature in the central plasma region, as shown in Fig. 5(c) Fig. 5(c) . (iii) The reason for the increase in the safety factor in the core region [ Fig. 5(d) Fig. 5(d) ] may be that plasma cooling and the increase in Z eff in the accumulation phase alternates the plasma electric conductivity and the current redistribution. Overall, T i / T e increases as co-NBI is added, indicating that the turbulent transport is reduced and tungsten accumulation is accelerated. However, there have been no reports that the turbulent transport exhibits Z dependence [ 50 50 ] (that the impurity accumulation increases with charge number Z [ 51 51 ]); thus, another mechanism needs to be considered to explain the observation in Fig. 3 Fig. 3 . The contribution of neoclassical transport to tungsten accumulation can be indicated by the ratio of the neoclassical radial drift velocity <math id="M5"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> and the diffusion coefficient <math id="M6"><mrow><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> ( <math id="M7"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> ). In this analysis, because the core collisionality is greater than 1, <math id="M8"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> can be obtained using the simplified expression [ 10 10 ] <math display="block" id="M9"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub><mo>=</mo><mrow><mo>(</mo><mrow><msub><mi>v</mi><mrow><mtext>CL</mtext></mrow></msub><mo>+</mo><msub><mi>v</mi><mrow><mtext>PS</mtext></mrow></msub></mrow><mo>)</mo></mrow><mo>/</mo><mo stretchy="false">(</mo><msub><mi>D</mi><mrow><mtext>CL</mtext></mrow></msub><mo>+</mo><msub><mi>D</mi><mrow><mtext>PS</mtext></mrow></msub><mo stretchy="false">)</mo><mo>,</mo></mrow></math> where the subscript CL denotes the classical regime, and PS denotes the Pfirsch–Schlüter regime. According to neoclassical theory, in each regime [ 10 10 ], v/D can be predicted using the expression <math display="block" id="M10"><mrow><mi>v</mi><mo>=</mo><mi>D</mi><mfrac><mrow><msub><mi>Z</mi><mi>I</mi></msub></mrow><mrow><msub><mi>Z</mi><mtext>D</mtext></msub></mrow></mfrac><mrow><mo>(</mo><mrow><mfrac><mn>1</mn><mrow><msub><mi>n</mi><mtext>D</mtext></msub></mrow></mfrac><mfrac><mrow><mtext>d</mtext><msub><mi>n</mi><mtext>D</mtext></msub></mrow><mrow><mtext>d</mtext><mi>ρ</mi></mrow></mfrac><mo>−</mo><mi>H</mi><mfrac><mn>1</mn><mrow><msub><mi>T</mi><mi>D</mi></msub></mrow></mfrac><mfrac><mrow><mtext>d</mtext><msub><mi>T</mi><mtext>D</mtext></msub></mrow><mrow><mtext>d</mtext><mi>ρ</mi></mrow></mfrac></mrow><mo>)</mo></mrow><mo>,</mo></mrow></math> Where n D and T D are the main ion density and temperature respectively; Z I is the impurity charge number; and Z D is the main ion charge number. In the CL regime, H = 0.5, and in the PS regime, H is determined by the impurity concentration and collisionality of the main ions. The values of <math id="M11"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> before and during co-NBI in shot #63723 are estimated assuming n D= n e and T D= T i , and are shown in Fig. 6(a) Fig. 6(a) . The value of <math id="M12"><mrow><mrow><mo>|</mo><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow><mo>|</mo></mrow></mrow></math> is clearly smaller during the co-NBI phase than before co-NBI, which does not explain the tungsten accumulation observed in this shot. The PHZ must be taken into account for plasma with a high toroidal rotation velocity [ 12 12 ]. The PHZ velocity is given by the expression <math display="block" id="M13"><mrow><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub><mo>=</mo><mfrac><mrow><msubsup><mi>v</mi><mrow><mi>d</mi><mn>0</mn></mrow><mn>2</mn></msubsup></mrow><mrow><mn>2</mn><msub><mi>Z</mi><mn>0</mn></msub></mrow></mfrac><mfrac><mrow><msub><mi>C</mi><mi>T</mi></msub><msub><mi>C</mi><mrow><mo>∇</mo><mi>T</mi></mrow></msub></mrow><mrow><msubsup><mi>C</mi><mi>Z</mi><mn>2</mn></msubsup><mo>+</mo><msup><mi>ω</mi><mn>2</mn></msup></mrow></mfrac><mo>,</mo></mrow></math> where v d0 is the magnetic drift velocity, <math id="M14"><mrow><msub><mi>v</mi><mrow><mtext>d</mtext><mn>0</mn></mrow></msub><mo>=</mo><mi>m</mi><msubsup><mi>V</mi><mtext>t</mtext><mn>2</mn></msubsup><mo>/</mo><msub><mi>Z</mi><mn>0</mn></msub><mi>e</mi><mi>R</mi><mi>B</mi></mrow></math> ; Z 0 is the impurity ion charge state in ionization equilibrium; ω is the angular frequency of the poloidal motion of impurity ions, <math id="M15"><mrow><mi>ω</mi><mo>=</mo><mrow><mo>(</mo><mrow><msub><mi>V</mi><mtext>t</mtext></msub><mo>−</mo><msub><mi>E</mi><mtext>r</mtext></msub><mo>/</mo><msub><mi>B</mi><mi>θ</mi></msub></mrow><mo>)</mo></mrow><mo>/</mo><mrow><mo>(</mo><mrow><mi>q</mi><msub><mi>R</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow></mrow></math> ; <math id="M16"><mrow><msub><mi>C</mi><mtext>T</mtext></msub><mo>=</mo><msub><mi>n</mi><mtext>e</mtext></msub><mo>∂</mo><mrow><mo>(</mo><mrow><msub><mi>γ</mi><mi>k</mi></msub><mo>−</mo><msub><mi>α</mi><mi>k</mi></msub></mrow><mo>)</mo></mrow><mo>/</mo><mo>∂</mo><msub><mi>T</mi><mtext>e</mtext></msub></mrow></math> ; <math id="M17"><mrow><msub><mi>C</mi><mi>Z</mi></msub><mo>=</mo><msub><mi>n</mi><mtext>e</mtext></msub><mo>∂</mo><mrow><mo>(</mo><mrow><msub><mi>γ</mi><mi>k</mi></msub><mo>−</mo><msub><mi>α</mi><mi>k</mi></msub></mrow><mo>)</mo></mrow><mo>/</mo><mo>∂</mo><mi>Z</mi></mrow></math> ; and <math id="M18"><mrow><msub><mi>C</mi><mrow><mo>∇</mo><mi>T</mi></mrow></msub><mo>=</mo><mtext>d</mtext><mi>T</mi><mo>/</mo><mtext>d</mtext><mi>ρ</mi></mrow></math> . In addition, m is the impurity mass, <math id="M19"><mrow><msub><mi>B</mi><mi>θ</mi></msub></mrow></math> is the poloidal magnetic field, <math id="M20"><mrow><msub><mi>γ</mi><mi>k</mi></msub></mrow></math> is the ionization rate, and <math id="M21"><mrow><msub><mi>α</mi><mi>k</mi></msub></mrow></math> is the recombination rate ( <math id="M22"><mrow><msub><mi>γ</mi><mi>k</mi></msub></mrow></math> and <math id="M23"><mrow><msub><mi>α</mi><mi>k</mi></msub></mrow></math> are obtained from are obtained from [ 52 52 ]). E r is evaluated using the ion radial force balance equation of the background plasma [ 53 53 ]: <math display="block" id="M24"><mrow><mo> </mo><msub><mi>E</mi><mi>r</mi></msub><mo>=</mo><msub><mi>V</mi><mtext>t</mtext></msub><msub><mi>B</mi><mi>θ</mi></msub><mo>−</mo><msub><mi>V</mi><mi>p</mi></msub><msub><mi>B</mi><mi>t</mi></msub><mo>+</mo><mfrac><mrow><mo>∇</mo><msub><mi>P</mi><mi>i</mi></msub></mrow><mrow><msub><mi>Z</mi><mi>i</mi></msub><mi>e</mi><msub><mi>n</mi><mi>i</mi></msub></mrow></mfrac><mo>,</mo></mrow></math> where <math id="M25"><mrow><msub><mi>P</mi><mtext>i</mtext></msub><mo>=</mo><msub><mi>n</mi><mtext>i</mtext></msub><mo>×</mo><msub><mi>T</mi><mtext>i</mtext></msub></mrow></math> ( <math id="M26"><mrow><msub><mi>n</mi><mtext>i</mtext></msub></mrow></math> is the ion density), and V p is the poloidal rotation velocity. Because E r is a function of V t , the E r pinch due to the effect of E r resulting from Coulomb collisions [ 12 12 ] is also considered in this analysis. The E r pinch is given by the equation <math display="block" id="M27"><mrow><mo> </mo><mo> </mo><msub><mi>v</mi><mrow><mi>E</mi><mi>r</mi></mrow></msub><mo>=</mo><mi>Δ</mi><mi>r</mi><mfrac><mrow><msub><mi>v</mi><mtext>c</mtext></msub></mrow><mrow><mn>1</mn><mo>+</mo><msup><mrow><mrow><mo>(</mo><mrow><msub><mi>v</mi><mtext>c</mtext></msub><mo>/</mo><mi>ω</mi></mrow><mo>)</mo></mrow></mrow><mn>2</mn></msup></mrow></mfrac><mo>,</mo></mrow></math> Where Δ r is the change in the minor radial position and is expressed as <math id="M28"><mrow><mi>Δ</mi><mi>r</mi><mo>=</mo><mfrac><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><mn>2</mn><mi>α</mi></mrow><mo>)</mo></mrow><mi>k</mi><msubsup><mi>Δ</mi><mn>0</mn><mn>2</mn></msubsup></mrow><mrow><mn>2</mn><msup><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><mi>α</mi></mrow><mo>)</mo></mrow></mrow><mn>3</mn></msup></mrow></mfrac></mrow></math> ( <math id="M29"><mrow><mi>α</mi><mo>=</mo><msub><mi>E</mi><mtext>r</mtext></msub><mo>/</mo><msub><mi>V</mi><mtext>t</mtext></msub><msub><mi>B</mi><mi>θ</mi></msub></mrow></math> , <math id="M30"><mrow><mi>k</mi><mo>=</mo><mi>Z</mi><mi>e</mi><msub><mi>E</mi><mtext>r</mtext></msub><mo>/</mo><mi>m</mi><msubsup><mi>V</mi><mtext>t</mtext><mn>2</mn></msubsup></mrow></math> , <math id="M31"><mrow><msub><mi>Δ</mi><mn>0</mn></msub><mo>=</mo><msub><mi>v</mi><mrow><mtext>d</mtext><mn>0</mn></mrow></msub><mo>/</mo><msub><mi>ω</mi><mn>0</mn></msub></mrow></math> ), and <math id="M32"><mrow><msub><mi>v</mi><mtext>c</mtext></msub></mrow></math> is the collision frequency of impurity ions with the background plasma. The value of <math id="M33"><mrow><msub><mi>v</mi><mtext>c</mtext></msub></mrow></math> for tungsten can be estimated using the expression <math id="M34"><mrow><msub><mi>v</mi><mtext>c</mtext></msub><mo> </mo><mo>~</mo><msubsup><mi>Z</mi><mi>W</mi><mn>2</mn></msubsup><msup><mrow><mrow><mo>(</mo><mrow><msub><mi>m</mi><mi>D</mi></msub><mo>/</mo><msub><mi>m</mi><mi>W</mi></msub></mrow><mo>)</mo></mrow></mrow><mrow><mn>0.5</mn></mrow></msup><msubsup><mi>v</mi><mi>e</mi><mo>*</mo></msubsup></mrow></math> , where the electron collisionality is <math id="M35"><mrow><msubsup><mi>v</mi><mi>e</mi><mo>*</mo></msubsup><mo>~</mo><msub><mi>v</mi><mrow><mtext>eff</mtext></mrow></msub><mo>/</mo><msup><mi>ε</mi><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow></msup><mo>~</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>14</mn></mrow></msup><msub><mi>n</mi><mtext>e</mtext></msub><msub><mi>Z</mi><mrow><mtext>eff</mtext></mrow></msub><msubsup><mi>T</mi><mtext>e</mtext><mrow><mo>−</mo><mn>2</mn></mrow></msubsup><mi>R</mi><mo>/</mo><msup><mi>ε</mi><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow></msup></mrow></math> ( <math id="M36"><mi>ε</mi></math> is the inverse aspect ratio). In this work, <math id="M37"><mrow><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub><mo>+</mo><msub><mi>v</mi><mrow><mi>E</mi><mtext>r</mtext></mrow></msub></mrow></math> is used to evaluate the effect of plasma toroidal rotation velocity on tungsten transport. To simplify the estimation of <math id="M41"><mrow><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub><mo>+</mo><msub><mi>v</mi><mrow><mi>E</mi><mtext>r</mtext></mrow></msub></mrow></math> before and during co-NBI, V t (0) = <math id="M42"><mtext> </mtext></math> 20 km/s before co-NBI and V t ( r ) = V t (0) e − r/a throughout the plasma radial region are assumed. In the ion radial force balance equation, the second term is not included because V p is small in the core region, and the third term can be simplified to <math id="M43"><mrow><mo>∇</mo><mrow><mo>(</mo><mrow><msub><mi>n</mi><mtext>e</mtext></msub><mo>×</mo><msub><mi>T</mi><mtext>i</mtext></msub></mrow><mo>)</mo></mrow><mo>/</mo><mi>e</mi><msub><mi>n</mi><mtext>e</mtext></msub></mrow></math> for bulk plasma. As the toroidal rotation velocity increases from 20 to 140 km/s, the estimated <math id="M44"><mrow><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub><mo>+</mo><msub><mi>v</mi><mrow><mi>E</mi><mtext>r</mtext></mrow></msub></mrow></math> ( ρ ~ 0.5) is found to increase from approximately –0.02 m/s to approximately −1.0 m/s, as shown in Fig. 6(b) Fig. 6(b) , which results in increased inward transport of tungsten and causes tungsten accumulation. Because the PHZ velocity depends strongly on the impurity mass, the increase in inward pinch velocity <math id="M45"><mrow><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub></mrow></math> (from approximately −0.017 m/s to approximately −0.8 m/s around ρ = 0.5) can explain why tungsten accumulates more easily than other impurities ( Fig. 3 Fig. 3 ). 3.2 Tungsten control using counter-NBI and favorable B t 2 To suppress or avoid tungsten accumulation in NBI-heated ELMy H-mode plasmas, the values of T i /T e , <math id="M46"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> , and <math id="M47"><mrow><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub><mo>+</mo><msub><mi>v</mi><mrow><mi>E</mi><mtext>r</mtext></mrow></msub><mo> </mo></mrow></math> should be decreased. For this purpose, counter-NBI is applied to modify the density and temperature and brake the plasma toroidal rotation. Normally, the counter-NBI is not good for plasma confinement other than the co-NBI because counter-NBI introduces impurities into the main plasma and degrades the confinement [ 54 54 ]. To rule out the possibility that the decreased tungsten content is due to the loss of confinement, another mechanism is needed for reducing the penetration of impurities into the main plasma and compensating for plasma confinement. It has been found that a favorable B t (in the co- I p direction, where the direction of <math id="M48"><mrow><msub><mi>B</mi><mtext>t</mtext></msub><mo>×</mo><mo>∇</mo><msub><mi>B</mi><mtext>t</mtext></msub></mrow></math> points toward the X-point) can improve plasma confinement [ 55 55 ] (compared with an unfavorable B t , a favorable B t can improve the H 98 by ~ 20% in shots with similar plasma parameters). In addition, favorable B t is found to have a positive effect on tungsten control, which will be discussed in the next section. For these reasons, the tungsten behavior was investigated using counter-NBI with a favorable B t direction, as in shot #71605. As shown in Fig. 7 Fig. 7 , type-I ELMs with f ELM ~ 40 Hz were obtained when additional co-NBI was performed during the LHW heating phase. This shot was the last shot before lithium wall conditioning and was performed in the USN configuration with B t = 2.4 T, I p = 0.6 MA, <math id="M49"><mrow><msub><mover accent="true"><mi>n</mi><mo>¯</mo></mover><mtext>e</mtext></msub></mrow></math> ~4.2 × 10 19 m −3 , q 95 = 4.8, and maximum source powers of LHW and NBI of 1.6 and 4.8 MW, respectively. The central radiation (as indicated by the central XUVemission intensity) clearly decreases rapidly when counter-NBI is performed, although counter-NBI slightly enhances the tungsten source (as indicated by the intensity of W I line emission at 400.9 nm) near the upper divertor region. Further analysis shows that the evolution of C W is directly correlated with the control of V t (0) by co- and counter-NBI. As shown in Fig. 7(d) Fig. 7(d) , when co-NBI heating with a 2 MW source power is applied at 3.0 s, V t (0) begins to increase and reaches a peak value of approximately 150 km/s. As shown in Fig. 8(a) and (b) Fig. 8(a) and (b) , the increase in plasma toroidal rotation velocity results in a rapid increase in C W and a gradual peak in the W-UTA emission intensity measured by EUV_Long2. EUV_Long2 [ 56 56 ] is a newly developed space-resolved EUV spectrometer that can provide the radial profile of impurity line emission covering a wide vertical range of −0.085 m ≤ Z ≤ 0.4 m [ Fig. 12(b) Fig. 12(b) ]. These rapid variations in C W and the W-UTA profile in the core region suggest tungsten accumulation. However, C W decreases and the W-UTA emission intensity in the core plasma suddenly dissipates when counter-NBI heating with a 1.4 MW source power is superimposed at 4.0 s to reduce V t (0) to 45 km/s. When the counter-NBI source power is increased to 2.8 MW, V t (0) can be further reduced and finally maintained at ~20 km/s. When the counter-NBI source power decreases and vanishes, C W increases again with the increase in V t (0). In addition, additional peaks in the W-UTA emission intensity in the core region indicate that tungsten accumulation again occurs. In this process, when counter-NBI with a maximum power of 2.8 MW is added during the co-NBI phase, V t (0) changes from 150 to 20 km/s. Consequently, C W decreases from ~7 × 10 −5 to ~2 × 10 −5 ; thus the power radiation fraction P rad / P total is decreased from ~32% to ~8%. In the ELMy H-mode phase, the superimposed counter-NBI also changes the characteristics of ELMs [ 57 57 ] (such as frequency f ELM and amplitude A ELM ), as shown in Fig. 8(c) and (d) Fig. 8(c) and (d) . It has been found that high-frequency ELMs can provide continuous transport of impurities in the edge plasma region, which prevents the accumulation of impurities [ 58 58 - 60 60 58-60 ]. This effect of ELMs on the elimination of impurities is called impurity flushing. The correlation analysis of V t (0), f ELM , and C W from 4 to 4.55 s shows that C W is strongly dependent on V t (0) but weakly dependent on f ELM . This weak dependence of C W on f ELM results from the fact that the low-frequency ELMs (which increase from ~50 to ~65 Hz during that time period) are insufficiently effective in flushing out impurities [ 58 58 ]. Note also that no core MHD activity is observed on the spectrogram of the central chord of the soft X-ray emission signals, and the energy confinement factor H 98 remains nearly constant during the counter-NBI heating phase. This result implies that the counter-NBI added here does not degrade the plasma confinement and can effectively suppress tungsten accumulation in the type-I ELMy H-mode on EAST. However, the decrease in C W during the added counter-NBI phase is attributed not only to the reduced toroidal rotation velocity, but also to the variations in density and temperature and their gradients. Figure 9 (a)–(c) Figure 9 (a)–(c) shows the density and temperature profiles at time points before (at 3.5 s, red line), during (at ~5.0 s, blue line), and after (at ~7.0 s, black line) counter-NBI in shot #71605. Under the assumption described above, the estimated <math id="M52"><mrow><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub><mo>+</mo><msub><mi>v</mi><mrow><mi>E</mi><mtext>r</mtext></mrow></msub></mrow></math> ( <math id="M53"><mi>ρ</mi></math> ~ 0.4) decreases from approximately −0.57 m/s to approximately −0.02 m/s when the counter-NBI changes V t (0) from ~80 to ~20 km/s, as shown in Fig. 10(b) Fig. 10(b) . The figure also shows that the applied counter-NBI increases the density and flattens its profile in the region 0.3 < <math id="M54"><mi>ρ</mi></math> < 0.9 and clearly enhances the ion temperature and its gradient inside <math id="M55"><mi>ρ</mi></math> ~ 0.7. These changes will increase the impurity diffusivity owing to the enhanced collisionality [ 61 61 ] and may decrease the pinch velocity of impurities in the central region. The values of <math id="M56"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> in the co-NBI and counter-NBI phases in shot #71605 were also estimated and are shown in Fig. 10(a) Fig. 10(a) . When counter-NBI is added during the co-NBI phase, <math id="M57"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> outside of ρ ~ 0.2 can be reduced, and the direction of <math id="M58"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> can even be reversed from inward to outward around ρ = 0.6. The reversed direction is determined by the ratio of the normalized temperature gradient to the normalized main ion density gradient, <math id="M59"><mrow><msub><mi>η</mi><mtext>D</mtext></msub><mo>=</mo><mfrac><mn>1</mn><mrow><msub><mi>T</mi><mtext>D</mtext></msub></mrow></mfrac><mfrac><mrow><mtext>d</mtext><msub><mi>T</mi><mtext>D</mtext></msub></mrow><mrow><mtext>d</mtext><mi>r</mi></mrow></mfrac><mo>/</mo><mfrac><mn>1</mn><mrow><msub><mi>n</mi><mi>D</mi></msub></mrow></mfrac><mfrac><mrow><mtext>d</mtext><msub><mi>n</mi><mtext>D</mtext></msub></mrow><mrow><mtext>d</mtext><mi>r</mi></mrow></mfrac></mrow></math> . It is inferred that the added counter-NBI in this shot reduces the PHZ+ E r inward pinch velocity and weakens the contribution of neoclassical inward transport. Note, however, that T i / T e increases slightly when counter-NBI is added, which should reduce the turbulent impurity transport and increase tungsten accumulation. Thus, it can be concluded that the decrease in C W during the counter-NBI phase is not caused by turbulent transport. 3.3 Effect of favorable B t on tungsten control 2 In this section, the effect of B t direction on tungsten control is discussed. It was found that the impurity confinement time ( τ imp ) was significantly reduced in favorable B t experiments in recent EAST campaigns. Typically, τ imp can be reduced by nearly half for favorable B t H-mode plasmas (e.g., τ imp ~ 112.0 ms for unfavorable B t in shot #79062 and τ imp ~ 61.2 ms for favorable B t in shot #80303). Shots #79062 and #80303 are similar shots that have opposite B t directions [ 55 55 ]. The effect was further studied by investigating C W and the tungsten flux in the divertor. To decrease the effect of toroidal rotation velocity on tungsten transport, the shots with low V t (0) ( < 30 km/s) are chosen for comparison. C W versus V t (0) in steady-state ELMy H-mode discharges with unfavorable and favorable B t directions is compared in Fig. 11 Fig. 11 . The selected shots have the following parameters: I p = 0.45–0.55 MA, <math id="M63"><mrow><msub><mover accent="true"><mi>n</mi><mo>¯</mo></mover><mtext>e</mtext></msub></mrow></math> = 2.8–4.5 × 10 19 m −3 , and the total heating powers are similar. This figure shows that C W can be easily controlled below 8 × 10 5 in the ELMy H-mode plasmas with reduced toroidal rotation velocity [ V t (0) < 30 km/s], where C W is generally lower in the shots with favorable B t direction than in those with unfavorable B t direction. These two B t directions produce E r × B drift with opposite poloidal directions, as shown in Fig. 11(c) Fig. 11(c) . For the unfavorable B t direction, the divertor E r × B drift increases the backflow (PS flow) of impurity particles from the upper outer divertor to the upstream region. For the favorable B t direction, the backflow of impurity particles is directed to the lower divertor and can be exhausted by the closed divertor structure [ 62 62 ]. The lower divertor structure is closed because it is W-shaped with a dome. This effect partially explains the lower C W in the shots with the favorable B t direction. In addition, the E r × B drift in the favorable B t direction increases the tungsten flux on the inboard side [as indicated by the increased I In / I Out shown in Fig. 11(b) Fig. 11(b) ], which may affect tungsten transport in the core region owing to the poloidal asymmetry of the tungsten density when the toroidal rotation velocity is low. The in-out asymmetry effect is beneficial for removing the tungsten from the core plasma [ 63 63 - 65 65 63-65 ], for example, by reversing the direction of <math id="M64"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> from inward to outward. The W-UTA emission intensity profiles in ELMy H-mode plasmas with different <math id="M66"><mrow><msub><mi>B</mi><mtext>t</mtext></msub><mo>×</mo><mo>∇</mo><msub><mi>B</mi><mtext>t</mtext></msub></mrow></math> drift directions are presented in Fig. 12(a) Fig. 12(a) . To eliminate the effect of electron temperature on tungsten transport, two shots with similar magnetic axes (near Z = 0.024 m) and central electron temperatures are compared. Figure 12 (a) Figure 12 (a) shows that the W-UTA emission intensity profile is flatter for the favorable B t direction than for the unfavorable B t direction. The flatter W-UTA emission intensity profile suggests that the favorable B t direction is beneficial for avoiding tungsten accumulation and can decrease the risk of intense radiated power in the core region. From these facts, it can be deduced that lower C W with favorable B t will expand the operating window toward steady-state type-I ELMy H-mode operation. 4 Summary and discussion 1 The tungsten behavior in ELMy H-mode plasmas with co-/counter-NBI and unfavorable/favorable B t on EAST was investigated. The results showed that tungsten always accumulates in ELMy H-mode plasma with co-NBI owing to the increased inward pinch velocity <math id="M67"><mrow><msub><mi>v</mi><mrow><mtext>PHZ</mtext></mrow></msub><mo>+</mo><msub><mi>v</mi><mrow><mi>E</mi><mtext>r</mtext></mrow></msub></mrow></math> and decreased turbulent transport. C W can exceed 10 −4 in the type-I ELMy H-mode with V t (0) > 120 km/s and unfavorable B t , ultimately resulting in the degradation of plasma confinement and periodic H–L transitions. Thus, C W must be maintained below 10 −4 for future steady-state type-I ELMy H-mode operation on EAST. To suppress and avoid tungsten accumulation in ELMy H-mode plasmas, both the neoclassical inward convection and PHZ inward pinch velocities should be decreased, whereas turbulent transport needs to be increased. For this purpose, counter-NBI was applied to modify the density and temperature and brake the plasma toroidal rotation. The result shows that C W can be decreased from ~7×10 −5 to ~2 × 10 −5 in the type-I ELMy H-mode plasma with favorable B t when counter-NBI is superimposed during the co-NBI heating phase. The analysis indicated that the added counter-NBI not only decreases the PHZ+ E r inward pinch velocity but also reverses the <math id="M68"><mrow><msub><mi>v</mi><mrow><mtext>neo</mtext></mrow></msub><mo>/</mo><msub><mi>D</mi><mrow><mtext>neo</mtext></mrow></msub></mrow></math> direction from inward to outward; both of these changes contribute to the suppression of tungsten accumulation. It was preliminarily found that, because NBI directly heats ions, turbulent transport in the core region driven by co-/counter-NBI is disadvantageous for the suppression of tungsten accumulation. Increasing the turbulent transport during the co/counter-NBI phases by increasing the power of LHW heating or adopting on-axis electron cyclotron resonance heating are required. Further detailed studies remain as future work. It was also found that C W is lower in ELMy H-mode plasmas with favorable B t than in similar discharges with unfavorable B t . Possible reasons for the effect of B t direction on C W were discussed. On one hand, in discharges with favorable B t in the USN configuration, the backflow of impurity particles is directed to the lower divertor and can be exhausted by the closed divertor structure. On the other hand, in low-rotation plasmas, the E r × B drift in plasmas with favorable B t increases the tungsten flux on the inboard side, which can weaken the contribution of neoclassical inward convection because of in-out asymmetry. These results demonstrate the potential of counter-NBI in the favorable B t direction for suppressing tungsten accumulation in ELMy H-mode plasma. The application of counter-NBI to suppress tungsten accumulation and avoid plasma confinement degradation must ensure that V t and I p have the same direction ( V t > 0), that V t is sufficiently low, and that B t is favorable. 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Catto , Pedestal radial flux measuring method to prevent impurity accumulation . Phys. Plasmas 24 , 055904 ( 2017 ). doi: 10.1063/1.4978886 http://doi.org/10.1063/1.4978886 Pedestal radial flux measuring method to prevent impurity accumulation All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Zong Xu, Zhen-Wei Wu, Ling Zhang, Yun-Xing Cheng, Wei Gao, Xiao-Dong Lin, Xiang Gao, Ying-Jie Chen, Yue-Heng Huang, Juan Huang, Qing Zang, Yin-Xian Jie, Yan-Min Duan, Ying-Ying Li, Hai-Qing Liu and Guo-Sheng Xu. The first draft of the manuscript was written by Zong Xu and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. 10.1007/s41365-021-00929-4 National Key R&D Program of China (Nos. 2018YFE0311100 and 2017YFE0301205), National Natural Science Foundation of China (Nos. 11905146, 11775269, 11575244, 11575249, 11575235, 11422546, 11805133, and U19A20113), Users with Excellence Program of Hefei Science Center, CAS (No. 2019HSC-UE014), National Magnetic Confinement Fusion Science Program of China (Nos. 2015GB110005, 2015GB103003, 2015GB101002, and 2015GB103000), Key Research Program of Frontier Sciences, CAS (No. QYZDB-SSWSLH001), CASHIPS Director’s Fund (No. BJPY2019A01), and Shenzhen Clean Energy Research Institute. 2021-02-01 2021-06-23 2021-06-23 2021-09-08 2021-09-01 2022-10-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2021 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-021-00929-4 http://dx.doi.org/10.1007/s41365-021-00929-4 . 2021 Nuclear Science and Techniques 09 32