Design of EAST lower divertor by considering target erosion and tungsten ion transport during the external impurity seeding

To optimize the divertor shape, carbon PFM is assumed at the beginning with the focus on the inherent power radiation capability of the divertor. The systematic examination of different target shapes, target angles and the pump locations has been carried out. Firstly, the horizontal target with outer strike point (OSP) at different positions and the vertical target are compared. P_SOL = 4 MW is fixed unless otherwise stated. Sketches of the divertor geometry/mesh and the main plasma quantities at the outer target as functions of electron density at the separatrix of OMP ${n}_{\mathrm{e},\mathrm{s}\mathrm{e}\mathrm{p}}^{\text{OMP}}$ are shown in figure 2. It can be seen that vertical target and horizontal target with OSP close to corner (called Horiz-near) cases have similar maximum and OSP values of the heat flux density q_dep, while the horizontal target with OSP far from corner (named Horiz-far) has much higher q_dep, indicating Horiz-far case is not applicable for power handling. The controlling of T_e is another important requirement for divertor. Figure 2(d) shows the vertical target can hardly reduce peak T_e compared to horizontal target, and Horiz-near case has the lowest T_e. From requirements for q_dep and T_e reduction, the Horiz-near divertor is preferred. Here we define the rollover of target ion flux at OSP as the indication of detachment onset [46, 52]. The detachment onset density is ${n}_{\mathrm{e},\mathrm{s}\mathrm{e}\mathrm{p}}^{\mathrm{O}\mathrm{M}\mathrm{P},\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{e}\mathrm{t}}=2.1{\times}1{0}^{19}\enspace {\mathrm{m}}^{-3}$, 1.6 × 10¹⁹ m⁻³, 1.9 × 10¹⁹ m⁻³, respectively, for cases figures 2(a)–(c), showing Horiz-near case can promote the occurrence of plasma detachment. This is in agreement with recent EAST experiment observation [53].

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To show the differences among three divertor cases, similar upstream density ${n}_{\mathrm{e},\mathrm{s}\mathrm{e}\mathrm{p}}^{\text{OMP}}\sim 2.65{\times}1{0}^{19}\enspace {\mathrm{m}}^{-3}$ (as marked by the dash line in figures 2(d)–(f)) is selected for comparison. The profiles of main plasma quantities and neutral density along the outer target are shown in figure 3. The divertor shape changes divertor plasma remarkably. For the vertical target, the main features are: (1) T_e and q_dep near the OSP can be well reduced; (2) T_e in the far SOL region is much higher than that of horizontal target due to much lower neutral particle compression there, which may be problematic for resulting in net erosion of W target; (3) q_dep can be controlled in the far SOL region due to low plasma density, and the location of peak q_dep is away from OSP; (4) the plasma wetted area is larger than that of horizontal target, which can lower deposited particle and energy flux density. These are similar to previous DIII-D modeling results [14]. For the horizontal target, when the OSP is close to corner, neutral particle can be well compressed in a small region, resulting in significant reduction of both T_e and q_dep, i.e. the peak T_e and q are ∼1.7 eV and ∼0.8 MW m⁻² compared to ∼4.4 eV and ∼1.5 MW m⁻² of Horiz-far divertor, and it achieves fully detachment. These show the advantages of Horiz-near divertor. The corresponding 2D contours of T_e, molecular D₂ density n_D2t and total energy radiation P_rad are shown in figure 4. A clear 'gas cushion' is created in front of the target of Horiz-near divertor (figure 4(e)), leading to significant falling down of T_e (figure 4(b)), in agreement with previous finding of strong correlation between n_D2t and T_e [5, 54]. n_D2t in the SOL of the vertical target case is very small, thus T_e is high. The essential problem for vertical target is that neutral particles can hardly be compressed in the far SOL region and only partial detachment can be easily obtained.

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The benefits of Horiz-near divertor to plasma detachment are confirmed. Next, we want to know the impact of target slant angle on divertor plasma performance. Three angles, i.e. θ = 0°, 10°, and 20°, which is defined by the angle between horizontal plane and target as shown in figures 5(a)–(c), are selected for comparison. By varying the upstream plasma density, the variations of main divertor plasma parameters (maximum and OSP values) with ${n}_{\mathrm{e},\mathrm{s}\mathrm{e}\mathrm{p}}^{\text{OMP}}$ are shown in figures 5(d)–(i). It illustrates that the target slant angle influences the divertor plasma slightly. The reason may lie in that the poloidal incident angle is still large. Therefore, θ = 0° target is preferred to reduce the complexity of engineering construction. It should be noted that the total magnetic field incident angle varies slightly with the target angle, i.e. 1.5°–2.5° during the target angle variation.

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Particle exhaust is another important issue for divertor [46]. The impact of pump opening location at common flux region (CFR) side on the divertor plasma is thus assessed. Four cases, i.e. pump opening at the bottom of CFR side, no pump opening at CFR side, pump opening at CFR side with a baffle height of 1.5 cm and with a baffle height of 2.5 cm, are simulated. Sketches of the divertor target with different pump opening locations and corresponding divertor plasma parameters are shown in figure 6. It can be seen that the pump location has remarkable impact on divertor plasma. Setting the pump opening at the CFR bottom makes it a relative open divertor, while as the baffle height increases, it becomes to a more and more closed divertor. The case without CFR pumping opening is a relative closed divertor. The simulation results demonstrate that the more closed divertor enhances divertor power radiation more distinctly. As the baffle height increases, the detachment onset density reduces. The main reason is that the pump location influences effective particle exhaust. As the pump opening moves further away from OSP, the exhausted particle reduces and the neutral density in the divertor region increases significantly, which in turn influences divertor plasma [46].

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Our previous work confirmed that for a closed divertor with flat target configuration both pump opening at the bottom of CFR side and PFR side are good to improve particle exhaust [46]. The present work finds that making the pump opening at the bottom of both PFR and CFR sides simultaneously can hardly maintain the energy radiation in the divertor region, i.e. divertor 7A in figure 6(a). Therefore, keeping the pump opening at the bottom of PFR, while in the CFR side either setting the pump opening with a certain baffle height (e.g. divertor 8C in figure 6(a)) or leaving no pump opening, is preferred. However, an additional pump to be placed under the PFR should be considered with this design.

After the optimization of the divertor geometry, the horizontal target with OSP close to corner and a pump duct above 4.0 cm height baffle is chosen, which has similar performance to that of the closed one in figure 6. Note that it was not shown in figure 6 to avoid too many similar lines and to make the figure more distinguishable. The sketch of the divertor is shown in figure 7(a), and its performance with W target is further investigated. Since W impurity is not included by the present SOLPS modeling, we first simulate pure deuterium discharge (no impurity included), as shown in figures 7(b)–(g) the maximum and OSP values of T_et, q_dep and Γ_dep, as functions of P_SOL and ${n}_{\mathrm{e},\mathrm{s}\mathrm{e}\mathrm{p}}^{\text{OMP}}$, respectively. It can be seen that the maximum values and OSP values are very similar. As P_SOL increases from 2 MW to 8 MW, while maintaining n_D+,CEI = 6.0 × 10¹⁹ m⁻³, T_e at OSP increases from ∼5.5 eV to ∼177.7 eV and q_dep at OSP raises from ∼1.0 MW m⁻² to ∼17.8 MW m⁻², exceeding the tolerance of W target, i.e. (1) T_et should be below 44 eV, assuming D⁺ accelerated through a sheath drop of 3kT_e with an impact energy of ∼5kT_e [55] and the sputtering threshold for D⁺ incident into W of 220 eV [38]; (2) q_dep should be under 10 MW m⁻² [43, 56]. Fixing P_SOL = 4 MW and varying upstream plasma density, the plasma detachment is not observed even ${n}_{\mathrm{e},\mathrm{s}\mathrm{e}\mathrm{p}}^{\text{OMP}}$ reaches 4.0 × 10¹⁹ m⁻³. These indicate that for W divertor edge power exhaust will be a great challenge during pure D discharge and the external impurity seeding is necessary, which is consistent with JET experiment and modeling [57].

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To raise the power radiation in the W divertor, Ar impurity is seeded from the puffing location shown in figure 7(a). The medium and high input power cases, i.e. P_SOL = 4 MW and 10 MW, are compared by varying the seeding rate. The power radiation can be expressed as P_rad = Vn_e n_imp L_Z, where V is the plasma volume, n_imp is the impurity density and L_Z is the radiative loss factor of impurity. For a specified tokamak with fixed V and n_D+, both n_imp and L_Z are important to the power radiation.

The main plasma quantities at outer target as functions of Ar seeding rate can be seen in figure 8. With the increasing of seeding rate, both T_et and q_dep fall first gradually due to the raising of n_imp, then remarkably until fully detachment is achieved. The sudden drop occurs when T_et ∼ 130 eV for both P_SOL cases, which is caused by more than one order of magnitude increment of L_Z as T_e raises higher than ∼130 eV [58]. Larger P_SOL requires higher seeding rate to dissipate energy. To reduce T_e at OSP below 5 eV, the required Ar seeding rate is 1.5 × 10²⁰ argon atoms s⁻¹ and 3.0 × 10²⁰ argon atoms s⁻¹ for P_SOL = 4 MW and 10 MW, respectively. Plasma detachment can be achieved with remarkable reduction of Γ_dep as seeding rate is adding. The trends of peak and OSP values with seeding rate are similar. This indicates the power dissipation problem of W divertor can be solved by Ar gas seeding with appropriate seeding rate.

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Previous work showed strong influence of the divertor plasma on the main SOL and pedestal electron density, especially during highly dissipative conditions [59]. DIII-D experiment found that the seed impurity leads to increment of pedestal pressure, and a significant amount of the impurity has been found in pedestal [60]. The accumulation of the impurities in the core region causes a harmful effect to the sustaining of the high-performance plasma by radiation cooling and dilution [61]. Therefore, the understanding of SOL and pedestal changes associated with impurity injection is crucial for future reactors. For the designed divertor, it is important to know the core–edge compatibility during the impurity seeding, and the impurity content in the core region should be well controlled.

The effective ion charge Z_eff in the core is usually taken as one of the important parameters to quantify the impurity content and divertor shielding. Z_eff at CEI of OMP during Ar seeding is shown in figure 9, and it is proportional to impurity puffing rate for both P_SOL, i.e. higher seeding rate leads to larger Z_eff in the core region, figure 9(a). The scaling law for Z_eff based on multi-machine database has been estimated by Matthews [62] as:

$$\begin{equation}{Z}_{\text{eff}}=1+\alpha {Z}^{\beta }\frac{{P}_{\text{rad}}}{{\bar{{n}_{\mathrm{e}}}}^{1.95}},\end{equation} \tag{ 1 }$$

where P_rad is power radiation in MW, $\bar{{n}_{\mathrm{e}}}$ is line averaged electron density in unit of 10²⁰ m⁻³, Z is the atomic number of impurity, α and β are fitting factors. The present simulation results can be fitted by specified α and β, and the scaling law becomes to

$$\begin{equation}{Z}_{\text{eff}}=1+4.5{Z}^{-1.2}\bar{{T}_{\mathrm{e}}}\frac{{P}_{\text{rad}}}{{\bar{{n}_{\mathrm{e}}}}^{1.95}}.\end{equation} \tag{ 2 }$$

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Here P_rad is the total radiation in the simulation domain, $\bar{{n}_{\mathrm{e}}}$ is the averaged n_e along OMP, and $\bar{{T}_{\mathrm{e}}}$ is the averaged T_e along OMP with unit of keV. The comparison of fitted Z_eff with simulated value is shown in figure 9(b), indicating the fitting law can roughly describe the dependence of Z_eff. However, the agreement is not very good for some points, due to the simple model omits radiation in different region and different discharge regimes of divertor plasma [30]. Since the line averaged values are taken from upstream region, and divertor plasma is not included, the power radiation in the core region P_rad,core can be used instead of P_rad. The corresponding scaling law becomes to:

$$\begin{equation}{Z}_{\text{eff}}=1+23.5{Z}^{-1.2}\bar{{T}_{\mathrm{e}}}\frac{{P}_{\mathrm{r}\mathrm{a}\mathrm{d},\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}}{{\bar{{n}_{\mathrm{e}}}}^{1.95}}.\end{equation} \tag{ 3 }$$

Figure 9(c) shows that equation (3) scales much better Z_eff than that of equation (2). The scaling relations indicate that in a specified tokamak with fixed upstream density $\bar{{n}_{\mathrm{e}}}$, by seeding impurity to dissipate heat flux to divertor target, larger P_SOL requires higher P_rad, and leading to higher Z_eff.

A pair of similar OD plasma condition cases, i.e. maximum T_et ∼ 5 eV, is chosen with seeding rate of 1.5 × 10²⁰ and 3.0 × 10²⁰ argon atoms s⁻¹ for P_SOL = 4 MW and 10 MW (as labeled by the dash lines in figure 8), respectively. The corresponding power radiation is P_rad = 1.3 MW and 3.2 MW, and the fractional power radiation are similar f_rad = P_rad/P_SOL ∼ 0.32. It can be seen from figure 10 that the profiles trends of different quantities are similar for both P_SOL cases, and q_dep of P_SOL = 10 MW case is much larger than that of P_SOL = 4 MW case due to larger incident particle flux and higher plasma density. Z_eff at CEI of OMP is 1.62 and 3.98, respectively.

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By the comparison between different input power, the simulation results demonstrate that higher P_SOL requires larger seeding rate to achieve same f_rad, while it results in much higher impurity content in the core region and the corresponding core radiation is accordingly increased, scaled well by equation (3).

Ne has been proposed as the main seed impurity for ITER [5, 63]. In this work, the implantation of Ne seeding to EAST is also carried out and is compared to Ar seeding. Figure 11 shows the main plasma quantities at OSP as functions of the puffing rate of Ar and Ne, respectively. With the increase of seeding rate, significant energy loss is found for both impurities, and plasma detachment can be achieved. The rollover of ${{\Gamma}}_{\text{dep}}^{\text{OSP}}$ occurs with seeding rate of 1.1 × 10²⁰ Ar atoms s⁻¹ and 2.6 × 10²⁰ Ne atoms s⁻¹, respectively, showing that detachment requires a factor of 2.4 higher Ne seeding rate than Ar. The main reason can be attributed to that Ar has much higher radiative loss factor L_Z than that of Ne [64]. This indicates power dissipation problem can be solved and Ar is more efficient than Ne. The remain question is what is the difference of two seed impurities to the core plasma? We turn to that next.

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From the power radiation point of view, the difference between Ar and Ne lies in the seeding amount. Since the required divertor plasma condition can be achieved by sufficient seed impurity, it is important to know the difference of the upstream plasma under similar plasma condition. We use ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$ and radiated power in the divertor region P_rad,div to represent divertor condition, respectively, and Z_eff is used to represent the influence on the core plasma. The Z_eff scaling law equations (2) and (3) also fit Ne seeding case well, figures 9(b) and (c). Z_eff at CEI of OMP as functions of ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$ and P_rad,div are shown in figure 12. Smaller ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$ corresponds to larger Z_eff. When ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$ < 20 eV, Ne seeding leads to larger Z_eff than Ar seeding with same ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$; while ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$ > 20 eV, Z_eff is similar with same ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$. The correlation between Z_eff and P_rad,div is the inverse of that between Z_eff and ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$, due to that larger P_rad,div leads to lower ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$. It should be noted that there is a reversal of P_rad,div vs Z_eff when P_rad,div reaches the highest value, figure 12(b). It is attributed to that after detachment the energy radiation front moves from divertor region to upstream, leading to the reduction of P_rad,div [59], thus impurity screening by divertor becomes worse.

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The plasma state with similar energy radiation in the simulation domain P_rad ∼ 2.1 MW is chosen for further comparison. The seeding rate is 1.8 × 10²⁰ Ar atoms s⁻¹ and 3.0 × 10²⁰ Ne atoms s⁻¹, respectively, as labeled in figure 11 by the dash lines. Profiles along the outer target are shown in figure 13. It can be seen that peak T_e is similar ∼3 eV, while peak q_dep of Ne seeding case is about two times of Ar seeding case. More Ar accumulates in the vicinity the divertor target than that of Ne impurity, figure 13(e). The corresponding 2D contours of impurity density are shown in figure 14. It is clear that the Ne density in the core region is much higher than that of the Ar density, resulting in higher Z_eff at the CEI 2.4 vs 2.0. The energy radiation distribution in the physical and computational regions are shown in figure 15. Ne impurity has larger radiation area, while Ar impurity focuses on radiating energy along the separatrix in the divertor region. The total radiated power in the divertor region is similar for two cases ∼1.2 MW. However, it should be noticed that power radiation in the core region by Ar is even higher than that by Ne, 0.71 MW vs 0.56 MW, as shown in figure 16, due to that L_Z of Ar is much larger than that of Ne [64]. Again, it demonstrates that heavier impurities are more efficient seed species for core radiation, which should be limited to avoid the degradation of the energy confinement. The ionization potential of the Ne atom is 21.6 eV, larger than that of Ar atom (15.8 eV). This leads to a deeper penetration of recycling Ne from divertor targets and correspondingly weaker screening for the Ne ions. Furthermore, it may also lead to a larger residence time of neon impurity, which is unfavorable for radiation [30].

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The simulation results show the advantage of Ar impurity on the power radiation efficiency and divertor impurity screening, compared to Ne impurity. The potential disadvantage of Ar is the strong core radiation, which is fatal for ITER due to the power flux across the separatrix has to stay above the H–L threshold [65]. The successful application of Ar requires further compression in the divertor region, which can be obtained by 'puff-pump' as realized in DIII-D experiment [66]. Our previous work found that the seed impurity plays a dominant role in the W erosion [38], which is another critical issue since the acceptable central concentration of W is below 10⁻⁵ in ITER [67, 68]. The last question is which seed impurity is better from aspects of the W target erosion and W impurity accumulation.

The erosion of the W target during the Ar and Ne seeding is calculated, respectively. The eroded W flux Γ_W can be expressed by

$$\begin{equation*}{{\Gamma}}_{\mathrm{W}}={{\Gamma}}_{\text{Imp}}{Y}_{\mathrm{P}\mathrm{h}\mathrm{y},\mathrm{W}},\end{equation*}$$

where Γ_Imp is the incident impurity flux density and Y_Phy,W is physical sputtering yield. Y_Phy,W depends on incident species, incident energy and angle. It increases as the incident energy increases once the energy exceeds energy threshold. Figure 17 shows the correlations between the peak Γ_W at outer target and puffing rate, ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$, and P_rad,OD, respectively. With the increasing of seeding rate, Γ_W first increases, then decreases until to be neglectable. The reason has been discussed in reference [38]. Similarly, higher T_et corresponds to lower puffing rate (figure 11(a)), and the competition between reduction of Γ_Imp and the increment of Y_Phy,W makes Γ_W first increase and then decrease with the enhancement of ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$, which is similar to JET experiment with nitrogen seeding [69]. Heavier incident species (e.g. Ar) have lower energy threshold and higher Y_Phy,W [49]. This explains the reason of much larger Γ_W with Ar seeding than that with Ne seeding with the same ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$ or P_rad,OD. ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$ should be controlled below 10 eV with Ne seeding or below 5 eV with Ar seeding to effectively eliminate the eroded W source, as shown in figure 17(b).

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The eroded W then transports in the plasma. The contamination of the core plasma has been observed with Ar seeding owing to the sputtering of upper W divertor on EAST experiment [70]. Thus, it is important to know the corresponding W impurity accumulation in the core region. To this end, the transport of W impurity is simulated by the coupling of DIVIMP and SOLPS modeling. The eroded W flux, i.e. figure 17, is taken as W source, the corresponding plasma state is used as plasma background, and the W impurities are tracked by DIVIMP. Figure 18 shows the dependence of W density at CEI on impurity seeding rate and ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$, respectively. It can be seen that Ar seeding leads to higher W density in the core region than that of Ne seeding with insufficient seeding rate (i.e. <1.6 × 10²⁰ argon atoms s⁻¹). For the same ${T}_{\mathrm{e}\mathrm{t}}^{\text{OSP}}$, Ar seeding leads to more W impurity accumulated in the core plasma region, which can be explained by the eroded W source (figure 17(b)).

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To further investigate the difference of W impurity distribution between Ne and Ar seeding, the seeding rate of 1.8 × 10²⁰ Ar atoms s⁻¹ and 3.0 × 10²⁰ Ne atoms s⁻¹ (with similar energy radiation in the simulation domain, the same cases to figures 13–16) are selected for comparison. From figure 13(b) we can see that both peak T_e at outer target is smaller than 3 eV, therefore the erosion of outer target is well suppressed as shown in figure 17(a). However, there is still W impurity in the CEI of the two cases. The reason is that the ID is still in partial detached, i.e. T_e at the far SOL is above 10 eV as shown in the inset graph of figure 19(a). As a result, the erosion of inner target is still remarkable. Ar seeding also causes stronger sputtered W flux than that of Ne seeding at inner target, which leads to higher W density at CEI as shown in figure 19(b). The total W impurity density distributions are demonstrated in figures 19(c) and (d). The highest density appears in the divertor region, where the W impurity is produced. This indicates the prompt redeposition is the dominant processes for W impurity during transport.

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The ID usually shows general more pronounced detachment compared to the OD and it is thus noncritical for most of tokamaks operated with normal B_t direction [71], even without the consideration of drifts [59]. Therefore, we prefer to use as simple shape of the ID target as possible, as shown in figure 1. One should be noted that in the present modeling, the drifts are switched off by default since it is very time consuming to obtain a steady-state solution due to numerical difficulty. For the normal B_t direction, the drifts drive the ion flux from outboard divertor to inboard divertor, resulting in the in–out asymmetry [72–74]. The EAST lower divertor is designed to have an open vertical inner target, plus the related closed outer horizontal target. This will naturally lead to much weaker power radiation capability of the ID than that of the OD. Therefore, T_e is higher in inner target than that of outer target when drifts are neglected, figure 19(a) vs figure 13(b).

To fully evaluate the performance of both ID and OD, SOLPS-ITER [75] with new version of B2.5 code [76], which can better handle drifts, is applied to model the drift case with Ar as the seed gas. The in–out asymmetry of T_e has been offset by introducing drifts as shown in figure 20. It can be explained by the poloidal (E_r × B) and radial (E_p × B) drift flows. Figure 21 demonstrates the Ar impurity distributions. In the outer target the radial drift flow drives Ar impurity from SOL towards the separatrix and into PFR, while in the ID it drives the impurity from PFR across the separatrix into the SOL. The poloidal drift pushes the impurity from OD to ID. As a result, Ar impurity accumulates in the inner SOL region with drifts as shown in figure 21, leading to the reduction of T_e even in the far SOL region. In addition, drifts lead to the impurity flow reversal due to the competition with fluxes by thermal forces, parallel electric fields and the friction coupling between impurities and main plasma ions [77]. More ionized Ar appears upon the stagnation point with drifts, leading to the increment of impurity leakage [78]. Therefore, the Ar density at OMP is higher with drifts than that of without drifts. However, the increment of impurity core accumulation by drifts may also depend on the impurity species, divertor plasma states, divertor magnetic field configurations, or divertor shapes, which is required to be further studied. It should be noted that on EAST the drifts also lead to the formation of high field side high density, please see the detailed explanation in reference [78].

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In general, with the consideration of drifts, the ion flow to inner target driven by drifts offsets the particle leakage in the inner open vertical divertor, resulting in the more symmetrical in–out divertor. This is one of the advantages of the designed divertor, which assists the W impurity control. It should be noted that the drifts change the absolute plasma values of the OD. However, the main conclusions of sections 3.1–3.3 do not change.

The designed OD target has enough area to swing the OSP for the 'fishtail divertor' operation. It is also compatible with the advanced magnetic configuration. A divertor coil (DC) with current |I_DC| = 0–10 KA/turn is planned to be added under the lower divertor with the aim to create QSF divertor configuration. Our simulation results show that with impurity seeding, QSF divertor can significantly increase the divertor power dissipation, thus promoting the achievement of the plasma detachment compared to standard divertor [45]. Finally, to make it more flexible, the pumping opening in the CFR side can also be removed without remarkable influence of the divertor performance.