1 Modeling of EAST Divertor S. Zhu Institute of Plasma Physics, Chinese Academy of Sciences

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3 NominalUpgrade B T0 (T) I p (MA)11.5 ICRH (MW)36 LHCD (MW)3.58 ECRH (MW) NBI (MW)08 Main parameters in the different phases of the operation

4 The EAST divertor should be designed to accommodate (in the I & II phase) : 1.total power load of 7.5MW 2.long duration discharges τ pulse = s During the last decade, some expected benefits of a closed divertor have been confirmed by experiments. To increase the “closure”, the EAST divertor : is deep and consists of vertical target tightly fitting baffle dome in private flux region Structures of Divertor and internal components

5 2.1 SOLPS Modeling 1. Couples a multi-fluid plasma code B2 with a Monte-Carlo neutral code Eirene ( imported from IPP-Garching, David Coster, Andrei Kukushkin ) 2. Simulations done for H/D + C ( physical+Chemical sputtering) 3. Simple Recycling model / neon puffing + pumping R = 1 at all surfaces 4. The anomalous perpendicular transport model: constant in space, with the thermal diffusivities χ i ⊥ = χ e ⊥ 5. P i,cb = P e,cb 2.2 Computational Mesh 122 poloidal 24 radial 2. Simulation Model

6 Toroidal field, B T (T)3.5 Plasma current, I p (MA)1.0 Major radius, R 0 (m)1.94 Minor radius, a (m)0.46 Elongation at separatrix,κ x 1.69 Upper triangularity at separatrix,δ ux 0.32 Lower triangularity at separatrix,δ lx 0.54 Safety factor, q Plasma internal inductance, l i 0.95 Poloidal beta, β p 1.40 Plasma volume, V P (m 3 )~ Major parameters of the EAST SN

7 Toroidal field, B T (T)3.5 Plasma current, I p (MA)1.0 Major radius, R 0 (m)1.94 Minor radius, a (m)0.47 Elongation at separatrix,κ x 1.76 Upper triangularity at separatrix,δ ux 0.56 Lower triangularity at separatrix,δ lx 0.56 Safety factor, q Plasma internal inductance, l i 1.32 Poloidal beta, β p 1.58 Plasma volume, V P (m 3 )~ Major parameters of the EAST CDN

8 Computational & Physical Domain SOL Core Upper Divertor Lower Divertor

9 3.1 Effect of the vertical targets (1) Neutrals produced at the target plates are preferentially reflected towards the separatrix. (2) Hence ionization is enhanced near the vicinity of the separatrix. (a) neutral density and (b) ionization source (H + ions m -3 s -1 ) 3. Results of the SOLPS Prediction

10 (3) As the power is mainly conducted through the region close to the separatrix: the peak heat flux is reduced and the profile is broader. electron density peaks more towards sep. temperature profiles looks “inverted”. Comparison of profiles across the target (a) vertical target (b) target normal to flux surface

Effect of PFR baffle A PFR baffle structure (dome) is introduced to prevent neutrals from escaping back into the bulk and SOL plasma through the PFR to increase the PFR neutral pressure, which favors divertor pumping The gaps between the vertical targets and dome should be optimized, so that they can allow neutral to reach the divertor pumping system but impede their escape back into the plasma. The result of optimization calculations performed for the EAST divertor shows there is an optimum width of the pumping gap for this dome and vertical target configuration.

Effect of divertor topology The poloidal field coil system of EAST allows us to run in SN or DN magnetic configurations for more flexibility in experiments.

13 The heat flux sharing by the divertors will be strongly affected by the variation in the magnetic topology of the divertor. Electron temperature (eV) and total parallel energy flux (W) contours in (a) SN and (b) CDN configurations

14 The figure shows peak heat flux at the outer divertor plates and the Z eff at the separatrix as a function of the separatrix density for the SN and CDN divertor configuration. As can be seen, both q pk0 and Z s are reduced for the CDN configurations, as would be expected.

15 As the configuration transitions from SN to CDN divertor, there exists configurations of disconnected double null DDN. In DDN, if the distance Δsep between both separatrices at the outer midplane is comparable to the SOL width of the parallel heat flux, a significant part of the heat flux can still flow along the outer separatrix to the second divertor. (a) Electron temperature (eV) and (b) total parallel energy flux (W) contours in DDN configuration.

16 Our modeling indicates that, for EAST, CDN has easier access to the detachment regime than SN. This might be partialy due to the fact that in the CDN configuration, the separatrix strike points are closer to the target corners than in the SN, so that an effective "V- shaped target" is formed. Such a configuration reduces the target loads because it helps to confine neutrals around the strike point and this facilitates partial plasma detachment. Particle neutral losses (ionization) contour at CDN (upper) and SN (lower)

Divertor Operational Windows of EAST Greenwald limit allows HT-7U to run safely with the line average densities up to 1.0×10 20 m -3 in Ohmic discharges. But LHCD efficiency requires much lower density Low recycling regime n e ， sep ~ 0.7×10 19 m -3. The profiles show little drops along field lines. Temperature at target is high up to ~120 eV. The peak heat flux exceeds engineering constrain >5 MW/m 2.

High recycling regime Midplane separatrix density is ~1.4×10 19 m -3. Significant gradients along field lines. High density and low temperature at target. Z eff has the ideal value of 1.4. But operational window is narrow. Comparison of profiles at midplane and target (a) low recycling (b) high recycling A high density and low temperature plasma exists close to the target

Detachment regime Transition to power detachment occurs at line average densities ~ 7.8×10 19 m -3. It is about 80% of the Greenwald limit and is much higher than the density limit required by the LHCD efficiency. Consequently, additional approach such as gas puffing or impurity seeding should be adopted to attempt detachment. Inner target Te < 4 eV throughout most of the plate. Te even < 2 eV at the separatrix. Outer target Te > 10 eV in the outer SOL. Te already < 2 eV at the separatrix. Shows: Complete detachment is attained in inner divertor; partial detachment attained in outer divertor; Detachment starts from the separatrix.

20 Low recycling regime: no flow reverse observed due to the lack of strong ionization there. High recycling regime: flow reverse occurs in both divertor. The reverse region is close to the separatrix. Detachment: flow reverse disappears at the inner target. Reverse region shrinks but not disappears at the outer target Flow reverse v.s. Divertor Operational regimes Mach number

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22 4. Summary and Conclusions In order to increase the degree of closure, the EAST divertor is designed to be deep and well baffled. Its vertical target plates preferentially reflect neutrals towards the separatrix and hence are beneficial to improve the power exhaust. The vertical divertor geometry also has effects on the detachment behavior. The heat flux sharing by the divertors will be strongly affected by the variation in the magnetic topology of the divertor. In DDN, if the distance Δ sep between both separatrices at the outer midplane is comparable to the SOL width of the parallel heat flux, a significant part of the heat flux can still flow along the outer separatrix to the second divertor. Performing in the high recycling or detached divertor operating regimes is of particular importance for heat and particle control in steady state. To extend plasma operational space of EAST with LHCD or to attempt to produce detachment for the divertor plasma, additional approach such as gas puffing or impurity seeding should be adopted.