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Radiative divertor is an effective method for the divertor heat flux control, but excessive core impurity radiation can deteriorate the confinement in tokamaks. In recent EAST experiments, the compatibility of divertor detachment and impurity control with good core confinement (H98,y2 ∼1) has been achieved simultaneously by combining an upstream deuterium (D2) puff and divertor argon (Ar) seeding. Both experimental and SOLPS-ITER modeling reveal that additional D2 puff can further mitigate the target heat load and facilitate the detachment. The SOLPS-ITER results also show that the D2 puff makes a limited contribution to the Ar line radiation but a significant contribution to the neutral radiation in the SOL and divertor regions. The physical mechanism behind the effect of the D2 puff on the Ar retention is also revealed by the modeling. The increase in the Ar velocity is the major reason for the improved Ar retention in the partial detachment state. The increase in vAr is mainly caused by the strengthening of frictional drag of D+, which is because the additional D2 puff increases the pressure gradient force (FPG) and the electric field force (FE) on D+. Further analysis reveals that in the deep detachment state, some other negative effects play a dominant role in determining the Ar retention, such as the shift of the region of the Ar+ ionization source, the decrease in the strength of the Ar+ ionization source, and the net force on the Ar ions near the target. [ABSTRACT FROM AUTHOR]

In the EAST radiative divertor experiments, stable detachment and divertor protection have been achieved with mixed impurity gas seeding. Experiments indicate that neon (Ne) is an appropriate external impurity for plasma detachment in EAST. In this work, the physical mechanism of plasma detachment with Ne seeding in EAST is studied by using the SOLPS-ITER code package. The electron temperature Tet on the upper outer divertor target decreases to below 10 eV with Ne seeding, which is mainly due to the power loss and upstream power drop caused by the impurity radiation. For Tet < 10 eV, the significant drop of the poloidal saturation ion current js,p occurs around the upper outer strike point. The simulation results show that the js,p drop is mainly driven by the rise of the momentum loss and the upstream pressure loss. The increase in the momentum loss is mainly caused by the plasma–neutral interactions in the region from the X-point to the upper outer divertor target. The upstream pressure loss around the X-point is related to the decrease in the upstream electron temperature Teu, which originates from the increased radiation loss with Ne seeding. [ABSTRACT FROM AUTHOR]

A numerical model is developed to study the temporal evolution of helicon discharge. The initiation of helicon discharge is first presented in terms of time-dependent electron density and temperature, electric and magnetic fields, and power deposition; then, their steady-state values are compared with experimental data and previous simulations for benchmark. It is found that the evolutions of electron density and temperature arrive steady state around 100 and $1.5~\mu \text{s}$ , respectively, while electric and magnetic fields become steady prior to 2 $\mu \text{s}$. The spatial distributions of power deposition indicate that RF power is absorbed mostly near the edge, similar to those of electron temperature. Parameter studies show that high background pressure, confining magnetic field strength, and RF power are beneficial to obtaining high-density helicon plasma, while the driving frequency has little effect. These findings are interesting for the design and optimization of helicon plasma source. [ABSTRACT FROM AUTHOR]

Steady-state heat flux deposition on the first wall of the Chinese Fusion Engineering Testing Reactor (CFETR) is investigated for 1 GW fusion power and under the updated design scheme. The computation is accomplished by the modelling of two simulation systems: SOLPS-ITER and PFCFlux. Seeding neon (Ne) gas is considered to be one of the possible methods of protecting the CFETR divertor from high heat flux damage. Simulations with different Ne seeding rates were carried out on a standard divertor with a lower single-null configuration to determine the effect of Ne seeding on heat flux deposition on the first wall. A blob-based transport model was used to investigate the effect of enhanced cross-field transport on the CFETR first wall, as with ITER and EU DEMO simulations. This work establishes a potential method for simulating the complete heat flux deposition on the first wall for the CFETR steady state by SOLPS-ITER and PFCFlux. After modelling and energy analysis, the peak value of complete heat flux on each module of the CFETR first wall is obtained, which may provide the basic reference for the CFETR physics and engineering design team. The CFETR first wall module which receives the highest heat flux has also been identified, and Ne seeding has a positive effect on reducing the heat flux on the CFETR first wall from around 2.5 MW m−2 to 1 MW m−2, which is within the engineering requirement. [ABSTRACT FROM AUTHOR]

To investigate the optimal scenario of impurity seeding to obtain divertor plasma detachment for target protection, experiments with Ar&D2 seeding from two different poloidal locations, the upper outer (UO) divertor target and lower outer (LO) target, were carried out on EAST. Partial energy detachment (the electron temperature near the strike point Te,spt ⩽ 10 eV) were obtained with Ar&D2 mixture puffing from the UO target and, for the first time, from the LO target into H-mode plasmas in the upper single null (USN) configuration. The peak heat flux qt on the UO target was significantly reduced (by ∼80%). The rollover of ion flux density js did not appear probably due to insufficient momentum loss, which is independent of the puff locations. The poloidal asymmetries of particle and heat fluxes on the targets have also been investigated. The UO-dominant asymmetry of particle flux was reversed, while the UO-dominant asymmetry of heat flux was mitigated but not reversed. The plasma confinement dropped by only 14% and 8.2% in the LO-puff case and UO-puff case, respectively, during detachment. The high level of C may contributed greatly to the higher radiation in the bulk plasma region and the greater decline in WMHD in the LO-puff case. [ABSTRACT FROM AUTHOR]

The ITER-like tungsten divertor with toroidally symmetric 1 mm × 1 mm chamfers on monoblock tiles has been installed in EAST in 2014. Hot spots were experimentally observed mostly along the toridial facing gaps between two columns of W/Cu monoblock units, which are often aggravated by installation misalignment. These hot spots can significantly degrade the power handling capability of W divertor and need to be alleviated. A numerical optimization model for tile chamfering design is built based on the finite element method (FEM), in which the numerical experiments are designed by the uniform table. The calculation results in ANSYS for these experiments are then processed employing the code Design and Analysis of Computer Experiments (DACE) in which the Kriging method is adopted to reconstruct a response surface. The optimum geometry can be derived from the minimum point on the surface. The results show that, under 200 MW/m 2 parallel heat flux with an inclination angle of 3° with respect to tile surface, the maximum temperature on W tile with a 0.5 mm misalignment can be decreased to 2084 °C by adopting an optimized single-sided chamfer, 1.8 times lower than 1 mm × 1 mm symmetrically chamfered tile. The optimum chamfering geometry has a strong dependence on the inclination angle of plasma flux to tile surface. As a result, the monoblock tiles in a flat cassette module need to be chamfered differently to adapt to the varied inclination angles. [ABSTRACT FROM AUTHOR]